tag:theconversation.com,2011:/id/topics/neutrinos-21505/articlesNeutrinos – The Conversation2023-07-26T12:15:06Ztag:theconversation.com,2011:article/2058912023-07-26T12:15:06Z2023-07-26T12:15:06ZMeasuring helium in distant galaxies may give physicists insight into why the universe exists<figure><img src="https://images.theconversation.com/files/537554/original/file-20230714-21948-g2t785.jpg?ixlib=rb-1.1.0&rect=60%2C0%2C6698%2C4489&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">New measurements from Japan's Subaru telescope have helped researchers study the matter-antimatter asymmetry problem. </span> <span class="attribution"><a class="source" href="https://media.gettyimages.com/id/1335056886/photo/andromeda-galaxy-surrounded-by-stars.jpg?s=612x612&w=0&k=20&c=yhgVDZmt3gODQx_vm9nzfweVT8-WzwwOpxJehbnynrI=">Javier Zayas Photography/Moment via Getty</a></span></figcaption></figure><p>When theoretical physicists like myself say that we’re studying why the universe exists, we sound like philosophers. But new data collected by researchers using Japan’s <a href="https://subarutelescope.org/en/">Subaru telescope</a> has revealed insights into that very question.</p>
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<a href="https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A cylindrical building sitting on a cliff overlooking a sunset." src="https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/538069/original/file-20230718-7668-yp1ts1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Japan’s Subaru telescope, located on Mauna Kea in Hawaii.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Subaru_Telescope._Mauna_Kea_Summit_-_panoramio.jpg">Panoramio/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p><a href="https://science.nasa.gov/astrophysics/focus-areas/what-powered-the-big-bang">The Big Bang</a> <a href="https://theconversation.com/how-could-an-explosive-big-bang-be-the-birth-of-our-universe-128430">kick-started the universe</a> as we know it 13.8 billion years ago. <a href="https://www.slac.stanford.edu/pubs/beamline/26/1/26-1-sather.pdf">Many theories</a> in particle physics suggest that for all the matter created at the universe’s conception, an equal amount of antimatter should have been created alongside it. Antimatter, like matter, has mass and takes up space. However, antimatter particles exhibit the opposite properties of their corresponding matter particles. </p>
<p>When pieces of matter and antimatter collide, they <a href="https://home.cern/science/physics/matter-antimatter-asymmetry-problem">annihilate each other in a powerful explosion</a>, leaving behind only energy. The puzzling thing about theories that predict the creation of an equal balance of matter and antimatter is that if they were true, the two would have totally annihilated each other, leaving the universe empty. So there must have been more matter than antimatter at the birth of the universe, because the universe isn’t empty – it’s full of stuff that’s made of matter like galaxies, stars and planets. A little bit of antimatter <a href="https://www.energy.gov/science/doe-explainsantimatter">exists around us</a>, but it is very rare. </p>
<p>As a <a href="https://inspirehep.net/authors/2064347">physicist working on Subaru data</a>, I’m interested in this so-called <a href="https://home.cern/science/physics/matter-antimatter-asymmetry-problem">matter-antimatter asymmetry problem</a>. In our <a href="https://doi.org/10.1103/PhysRevLett.130.131001">recent study</a>, my collaborators and I found that the telescope’s new measurement of the amount and type of helium in faraway galaxies may offer a solution to this long-standing mystery.</p>
<h2>After the Big Bang</h2>
<p>In the first milliseconds after the Big Bang, the universe was hot, dense and full of elementary particles like protons, neutrons and electrons <a href="https://www.space.com/25126-big-bang-theory.html">swimming around in a plasma</a>. Also present in this pool of particles were <a href="https://theconversation.com/explainer-the-elusive-neutrino-431">neutrinos</a>, which are very tiny, weakly interacting particles, and antineutrinos, their antimatter counterparts.</p>
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<a href="https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An image showing a burst of light and color against black space and stars." src="https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/537553/original/file-20230714-27-ymcvpp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The Big Bang created fundamental particles that make up other particles like protons and neutrons. Neutrinos are another type of fundamental particle.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/big-bang-conceptual-image-royalty-free-illustration/639549057?phrase=the%20big%20bang">Alfred Pasieka/Science Photo Library via Getty Images</a></span>
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<p>Physicists believe that just one second after the Big Bang, the nuclei of light <a href="https://theconversation.com/after-our-universes-cosmic-dawn-what-happened-to-all-its-original-hydrogen-65527">elements like hydrogen</a> and helium began to form. This process is known as <a href="https://w.astro.berkeley.edu/%7Emwhite/darkmatter/bbn.html">Big Bang Nucleosynthesis</a>. The nuclei formed were about <a href="https://science.howstuffworks.com/dictionary/astronomy-terms/big-bang-theory5.htm">75% hydrogen nuclei and 24% helium nuclei</a>, plus small amounts of heavier nuclei. </p>
<p>The physics community’s <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/bbnuc.html">most widely accepted theory</a> on the formation of these nuclei tells us that neutrinos and antineutrinos played a fundamental role in the creation of, in particular, helium nuclei. </p>
<p>Helium creation in the early universe happened in a two-step process. First, neutrons and protons converted from one to the other in a <a href="https://ned.ipac.caltech.edu/level5/March04/Steigman3/Steigman2.html">series of processes</a> involving neutrinos and antineutrinos. As the universe cooled, these processes stopped and the <a href="https://ned.ipac.caltech.edu/level5/March04/Steigman3/Steigman2.html">ratio of protons to neutrons was set</a>. </p>
<p>As theoretical physicists, we can create models to test how the ratio of protons to neutrons depends on the relative number of neutrinos and antineutrinos in the early universe. If <a href="https://doi.org/10.1103/PhysRevLett.130.131001">more neutrinos were present</a>, then our models show more protons and fewer neutrons would exist as a result. </p>
<p>As the universe cooled, hydrogen, helium and other elements <a href="https://ned.ipac.caltech.edu/level5/March04/Steigman3/Steigman2.html">formed from these protons and neutrons</a>. Helium is made up of two protons and two neutrons, and hydrogen is just one proton and no neutrons. So the fewer the neutrons available in the early universe, the less helium would be produced.</p>
<p>Because the nuclei formed during Big Bang Nucleosynthesis <a href="https://doi.org/10.1103/PhysRevLett.130.131001">can still be observed today</a>, scientists can infer how many neutrinos and antineutrinos were present during the early universe. They do this by looking specifically at galaxies that are rich in light elements like hydrogen and helium.</p>
<figure class="align-center ">
<img alt="A diagram showing how protons and neutrons form helium atoms." src="https://images.theconversation.com/files/537555/original/file-20230714-25-rbf648.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/537555/original/file-20230714-25-rbf648.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/537555/original/file-20230714-25-rbf648.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/537555/original/file-20230714-25-rbf648.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/537555/original/file-20230714-25-rbf648.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/537555/original/file-20230714-25-rbf648.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/537555/original/file-20230714-25-rbf648.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">In a series of high-energy particle collisions, elements like helium are formed in the early universe. Here, D stands for deuterium, an isotope of hydrogen with one proton and one neutron, and γ stands for photons, or light particles. In the series of chain reactions shown, protons and neutrons fuse to form deuterium, then these deuterium nuclei fuse to form helium nuclei.</span>
<span class="attribution"><span class="source">Anne-Katherine Burns</span></span>
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<h2>A clue in helium</h2>
<p>Last year, the Subaru Collaboration – a group of Japanese scientists working on the Subaru telescope – released data on <a href="https://doi.org/10.3847/1538-4357/ac9ea1">10 galaxies</a> far outside of our own that are almost exclusively made up of hydrogen and helium. </p>
<p>Using a technique that allows researchers to distinguish different elements from one another <a href="https://theconversation.com/explainer-seeing-the-universe-through-spectroscopic-eyes-37759">based on the wavelengths of light</a> observed in the telescope, the Subaru scientists determined exactly how much helium exists in each of these 10 galaxies. Importantly, they found less helium than the previously accepted theory predicted. </p>
<p>With this new result, my collaborators and I worked backward to find the <a href="https://doi.org/10.1103/PhysRevLett.130.131001">number of neutrinos and antineutrinos</a> necessary to produce the helium abundance found in the data. Think back to your ninth grade math class when you were asked to solve for “X” in an equation. What my team did was essentially the more sophisticated version of that, where our “X” was the number of neutrinos or antineutrinos.</p>
<p>The previously accepted theory predicted that there should be the same number of neutrinos and antineutrinos in the early universe. However, when we tweaked this theory to give us a prediction that matched the new data set, <a href="https://doi.org/10.1103/PhysRevLett.130.131001">we found that</a> the number of neutrinos was greater than the number of antineutrinos.</p>
<h2>What does it all mean?</h2>
<p>This analysis of new helium-rich galaxy data has a far-reaching consequence – it can be used to explain the asymmetry between matter and antimatter. The Subaru data points us directly to a source for that imbalance: neutrinos. In this study, my collaborators and I proved that this new measurement of helium is consistent with there being more neutrinos then antineutrinos in the early universe. Through <a href="https://doi.org/10.1103/PhysRevLett.130.131001">known and likely particle physics processes</a>, the asymmetry in the neutrinos could propagate into an asymmetry in all matter. </p>
<p>The result of our study is a common type of result in the theoretical physics world. Basically, we discovered a viable way in which the matter-antimatter asymmetry could have been produced, but that doesn’t mean it definitely was produced in that way. The fact that the data fits with our theory is a hint that the theory we’ve proposed might be the correct one, but this fact alone doesn’t mean that it is. </p>
<p>So, are these tiny little neutrinos the key to answering the age old question, “Why does anything exist?” According to this new research, they just might be.</p><img src="https://counter.theconversation.com/content/205891/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Anne-Katherine Burns does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The way particles interacted while the universe was forming seconds after the Big Bang could explain why the universe exists the way it does – a physicist explains matter-antimatter asymmetry.Anne-Katherine Burns, Ph.D. Candidate in Theoretical Particle Physics, University of California, IrvineLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2088792023-06-30T16:33:17Z2023-06-30T16:33:17ZFirst ever view of the Milky Way seen through the lens of neutrino particles<figure><img src="https://images.theconversation.com/files/535016/original/file-20230630-29-2zlq8e.jpg?ixlib=rb-1.1.0&rect=39%2C9%2C6557%2C3712&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Milky Way, as seen with neutrino particles.</span> <span class="attribution"><a class="source" href="https://icecube.wisc.edu/gallery/high-energy-neutrinos-from-the-galactic-plane/#modulagallery-10913-11962">IceCube Collaboration / US National Science Foundation (Lily Le and Shawn Johnson) / ESO (S. Brunier)</a></span></figcaption></figure><p>Data collected by an observatory in Antarctica has produced our first view of the Milky Way galaxy through the lens of neutrino particles. It’s the first time we have seen our galaxy “painted” with a particle, rather than in different wavelengths of light.</p>
<p>The result, <a href="https://www.science.org/doi/10.1126/science.adc9818">published in Science</a>, provides researchers with a new window on the cosmos. The neutrinos are thought to be produced, in part, by high-energy, charged particles called cosmic rays colliding with other matter. Because of the limits of our detection equipment, there’s much we still don’t know about cosmic rays. Therefore, neutrinos are another way of studying them. </p>
<p>It has been speculated since antiquity that the Milky Way we see arching across the night sky consists of stars like our Sun. In the 18th century, it was recognised to be a flattened slab of stars that we are viewing from within. It is only 100 years since we learnt that the Milky Way is in fact a galaxy, or “island universe”, one among a hundred billion others.</p>
<p>In 1923, the American astronomer <a href="https://www.nasa.gov/content/about-story-edwin-hubble">Edwin Hubble</a> identified a type of pulsating star called a “Cepheid variable” in what was then known as the Andromeda “nebula” (a giant cloud of dust and gas). Thanks to the prior work of Henrietta Swan Leavitt, this provided a measure of the distance from Earth to Andromeda. </p>
<p>This demonstrated that Andromeda is a far away galaxy like our own, settling a long-running debate and completely transforming our notion of our place in the universe.</p>
<h2>Opening windows</h2>
<p>Subsequently, as new astronomical windows have opened on to the sky, we have seen our galactic home in many different wavelengths of light –- in radio waves, in various infrared bands, in X-rays and in gamma-rays. Now, we can see our cosmic abode in neutrino particles, which have very low mass and only interact very weakly with other matter – hence their nickname of “ghost particles”. </p>
<p>Neutrinos are emitted from our galaxy when cosmic rays collide with interstellar matter. However, neutrinos are also produced by stars like the Sun, some exploding stars, or supernovas, and probably by most high-energy phenomena that we observe in the universe such as gamma-ray bursts and quasars. Hence, they can provide us an unprecedented view of highly energetic processes in our galaxy – a view that we can’t get from using light alone.</p>
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<img alt="Digital Operating Module." src="https://images.theconversation.com/files/535040/original/file-20230630-21-lpttam.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/535040/original/file-20230630-21-lpttam.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/535040/original/file-20230630-21-lpttam.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/535040/original/file-20230630-21-lpttam.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/535040/original/file-20230630-21-lpttam.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/535040/original/file-20230630-21-lpttam.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/535040/original/file-20230630-21-lpttam.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A digital operating module, part of the IceCube observatory, being lowered into the ice.</span>
<span class="attribution"><a class="source" href="https://icecube.wisc.edu/gallery/high-energy-neutrinos-from-the-galactic-plane/#modulagallery-10913-2055">Mark Krasberg, IceCube/NSF</a>, <span class="license">Author provided</span></span>
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<p>The new breakthrough detection required a rather strange “telescope” that is buried several kilometres deep in the Antarctic ice cap, under the South Pole. The <a href="https://icecube.wisc.edu/">IceCube Neutrino Observatory</a> uses a gigatonne of the ultra-transparent ice under huge pressures to detect a form of energy called Cherenkov radiation. </p>
<p>This faint radiation is emitted by charged particles, which, in ice, can travel faster than light (but not in a vacuum). The particles are created by incoming neutrinos, which come from cosmic ray collisions in the galaxy, hitting the atoms in the ice.</p>
<p>Cosmic rays are mainly proton particles (these make up the atomic nucleus along with neutrons), together with a few heavy nuclei and electrons. About a century ago, these were discovered to be raining down on the Earth uniformly from all directions. We do not yet definitively know all their sources, as their travel directions are scrambled by magnetic fields that exist in the space between stars.</p>
<h2>Deep in the ice</h2>
<p>Neutrinos can act as unique tracers of cosmic ray interactions deep in the Milky Way. However, the ghostly particles are also generated when cosmic rays hit the Earth’s atmosphere. So the researchers using the IceCube data needed a way to distinguish between the neutrinos of “astrophysical” origin – those originating from extraterrestrial sources – and those created from cosmic ray collisions within our atmosphere.</p>
<p>The researchers focused on a type of neutrino interaction in the ice called a cascade. These result in roughly spherical showers of light and give the researchers a better level of sensitivity to the astrophysical neutrinos from the Milky Way. This is because a cascade provides a better measurement of a neutrino’s energy than other types of interactions, even though they are harder to reconstruct.</p>
<figure class="align-center ">
<img alt="IceCube Observatory" src="https://images.theconversation.com/files/535014/original/file-20230630-25-7iub59.jpeg?ixlib=rb-1.1.0&rect=0%2C6%2C4319%2C2892&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/535014/original/file-20230630-25-7iub59.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/535014/original/file-20230630-25-7iub59.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/535014/original/file-20230630-25-7iub59.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/535014/original/file-20230630-25-7iub59.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/535014/original/file-20230630-25-7iub59.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/535014/original/file-20230630-25-7iub59.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">The IceCube Observatory is located at the South Pole.</span>
<span class="attribution"><a class="source" href="https://icecube.wisc.edu/gallery/nsf-renews-icecube-maintenance-and-operations-contract-2/#modulagallery-7041-1987">Erik Beiser, IceCube/NSF</a>, <span class="license">Author provided</span></span>
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<p>Analysis of ten years of IceCube data using sophisticated machine learning techniques yielded nearly 60,000 neutrino events with an energy above 500 gigaelectronvolts (GeV). Of these, only about 7% were of astrophysical origin, with the rest being due to the “background” source of neutrinos that are generated in the Earth’s atmosphere. </p>
<p>The hypothesis that all the neutrino events could be due to cosmic rays hitting the Earth’s atmosphere was definitively rejected at a level of statistical significance known as 4.5 sigma. Put another way, our result has only about a 1 in 150,000 chance of being a fluke. </p>
<p>This falls a little short of the conventional 5 sigma standard for claiming a discovery in particle physics. However, such emission from the Milky Way is expected on sound astrophysical grounds.</p>
<p>With the upcoming enlargement of the experiment – <a href="https://icecube.wisc.edu/science/beyond/">IceCube-Gen2</a> will be ten times bigger – we will acquire many more neutrino events and the current blurry picture will turn into a detailed view of our galaxy, one that we have never had before.</p><img src="https://counter.theconversation.com/content/208879/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Subir Sarkar received funding from the University of Oxford to support his participation in IceCube. </span></em></p>An observatory called IceCube was used to produce a view of our galaxy in particles rather than light.Subir Sarkar, Emeritus professor, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2087432023-06-29T20:01:55Z2023-06-29T20:01:55ZIceCube neutrino detector in Antarctica spots first high-energy neutrinos emitted in our own Milky Way galaxy<figure><img src="https://images.theconversation.com/files/534884/original/file-20230629-25340-vu0a05.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C6667%2C3750&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Scientists have detected the first neutrinos from our galaxy.</span> <span class="attribution"><span class="source">NSF/IceCube</span></span></figcaption></figure><p>The South Pole <a href="https://icecube.wisc.edu/">IceCube Neutrino Observatory</a> – the biggest and strangest telescope in the world – has detected the first neutrino emissions from within the Milky Way, an achievement that will shape how astronomers view our galaxy. </p>
<p><a href="https://theconversation.com/how-neutrinos-which-barely-exist-just-ran-off-with-another-nobel-prize-48726">Neutrinos</a> are tiny, electrically neutral particles that pass through most matter undetected. They are created in extreme environments like those surrounding massive black holes, and they <a href="https://icecube.wisc.edu/outreach/neutrinos/">travel unhindered through space and matter</a> in a straight path.</p>
<p>Because black holes and exploding stars are too far away to visit, and too extreme to reproduce in a laboratory, scientists rely on cosmic messengers – like visible light from stars – to study them. Neutrinos are another type of cosmic messenger, but they’re too small to be seen with our eyes, or even most types of telescopes. </p>
<p>That’s where <a href="https://icecube.wisc.edu/science/icecube/">IceCube comes in</a>. The observatory, based in Antarctica, is made of up of <a href="https://theconversation.com/scientist-at-work-searching-for-tiny-neutrinos-in-the-south-poles-thick-ice-49979">a billion tons of ice</a> equipped with a grid of frozen-in sensors. The sensors light up when they detect a neutrino passing through, and, based on the sensors’ arrangement, researchers can determine the energy and direction of the neutrino that created the flash.</p>
<p>From there, researchers can use the energy and direction to try to figure out <a href="https://theconversation.com/the-icecube-observatory-detects-neutrino-and-discovers-a-blazar-as-its-source-99720">where in the universe</a> the neutrino came from. </p>
<p>As the interim director of the <a href="https://wipac.wisc.edu/">Wisconsin IceCube Particle Astrophysics Center</a>, I ensure we have the people and resources needed to <a href="https://icecube.wisc.edu/about-us/overview/">help researchers succeed in using the IceCube observatory</a>.</p>
<h2>Detecting neutrinos using ice</h2>
<p>Identifying the flashes of light from neutrino interactions on IceCube’s sensors can be a challenge. IceCube <a href="https://icecube.wisc.edu/about-us/facts/">records about 2,600 events each second</a>, though most of these events come from high-energy particles called <a href="https://home.cern/science/physics/cosmic-rays-particles-outer-space">cosmic rays</a>, which also produce a steady rain of neutrinos upon hitting the Earth’s atmosphere. <a href="https://icecube.wisc.edu/science/research/">Only a few hundred</a> of the hundred thousand neutrinos seen each year are from galactic or extragalactic sources rather than cosmic rays.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/2DDQYHIbL3Q?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A neutrino interacts with ice in the IceCube detector, producing light recorded by IceCube sensors and indicating its direction and energy. IceCube.</span></figcaption>
</figure>
<p>Finding the neutrinos from outer space, rather than those from cosmic rays, is like trying to see a faint feature in a portrait covered by many layers of paint – you have to be careful not to remove what you’re trying to uncover.</p>
<p>Surprisingly, the first two neutrino sources that IceCube researchers previously identified came from outside the Milky Way – <a href="https://theconversation.com/the-icecube-observatory-detects-neutrino-and-discovers-a-blazar-as-its-source-99720">one of which</a> was a very bright galactic object called a blazar. These neutrinos were quite distant, but higher-energy than any sources from within the Milky Way. </p>
<p>Finding fainter Milky Way neutrinos required some clever work by IceCube collaborators at Drexel University and Dortmund University. Their work on IceCube’s detection of the first Milky Way neutrinos was <a href="https://doi.org/10.1126/science.adc9818">published in Science on June 29, 2023</a>.</p>
<p>Scientists can use a few tricks to filter neutrinos from outer space from cosmic ray neutrinos and other cosmic ray noise. We can <a href="https://doi.org/10.1126/science.1242856">sort by energy</a>, with the higher-energy neutrinos being more likely to be from outer space. Researchers can also look for <a href="https://doi.org/10.1126/science.abg3395">clusters of neutrinos</a>, because neutrinos from outside our galaxy tend to clump together in one location. Lastly, researchers can look for neutrinos <a href="https://doi.org/10.1126/science.aat1378">from transient, astrophysical events</a> like <a href="https://doi.org/10.1126/science.aat2890">black holes</a> that have already been detected by other telescopes.</p>
<p>In 2013, IceCube published the <a href="https://doi.org/10.1126/science.1242856">first evidence of astrophysical neutrinos</a> identified based on their energy. These were isolated single neutrinos – so researchers couldn’t tell exactly where they were coming from.</p>
<h2>Searching for a neutrino’s source</h2>
<p>Even though scientists figured out that these most recently discovered neutrinos came from within our own galaxy, they don’t have a clear enough map of the Milky Way to identify the individual location where the newly uncovered neutrinos originated. Improving the analysis to determine the specific location of neutrino emission is the next step. </p>
<p>There are a few ways to improve the hunt for the sources. First, the longer scientists look and the more data they collect, the more likely they are to pinpoint a neutrino’s source – but to improve by a factor of 10 takes 100 times more data. So being clever has a better return than being patient. </p>
<p>Here are some ways to be more clever. First, researchers can <a href="https://doi.org/10.1088/1748-0221/17/11/P11003">improve the event selection</a> by choosing which cosmic events to zero in on, so that more potential neutrino candidates are in the sample. They can also <a href="https://icecube.wisc.edu/news/research/2022/11/machine-learning-method-improves-reconstruction-and-classification-of-low-energy-icecube-events/">better reconstruct the neutrinos’ path</a> – this is like revisiting a museum with new glasses to see with more clarity. Lastly, they can try to find a way to reduce the background, sort of like looking for a region where the portrait is covered by fewer layers of paint. </p>
<p>It took using all these tricks to <a href="https://doi.org/10.1126/science.adc9818">see faint Milky Way neutrinos</a>. Our team found ways to improve the sample size, and we used machine learning to improve the event reconstruction. This reduced the background enough to trace our neutrinos back to the Milky Way.</p>
<p>For most forms of cosmic light emission we study, light from sources within the Milky Way shine the brightest because they’re the closest. But for neutrinos, this isn’t the case – Galaxy NGC1068, tens of millions of light-years away, <a href="https://doi.org/10.1126/science.adc9818">emits more high-energy neutrinos</a> than the Milky Way. This tells us not all galaxies have the same ability to produce high-energy particles, but also that we need to to find and study more galaxies that emit neutrinos to understand the Milky Way’s cosmic quirks.</p>
<p>IceCube is planning a high-energy upgrade that would make the detector array <a href="https://icecube.wisc.edu/science/beyond/">about eight times larger</a>. Once the upgrade finishes in the 2030s, scientists will be able to continue their search for neutrinos with improved technology.</p><img src="https://counter.theconversation.com/content/208743/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jim Madsen works for the University of Wisconsin-Madison where is the interim director of the Wisconsin IceCube Particle Astrophysics Center (WIPAC). He receives funding from National Science Foundation to support IceCube. </span></em></p>New data from the IceCube collaboration shows neutrino emissions from within our Milky Way galaxy – but figuring out where exactly these ghost particles come from is harder than it seems.Jim Madsen, Executive Director, Wisconsin IceCube Particle Astrophysics Center, University of Wisconsin-MadisonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2086222023-06-29T20:01:42Z2023-06-29T20:01:42ZA neutrino portrait of our galaxy reveals high-energy particles from within the Milky Way<figure><img src="https://images.theconversation.com/files/534726/original/file-20230629-23-u6xkg.jpg?ixlib=rb-1.1.0&rect=643%2C0%2C1211%2C850&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">IceCube Collaboration/Science Communication Lab for CRC 1491</span></span></figcaption></figure><p>Our Milky Way galaxy is an awe-inspiring feature of the night sky, viewable with the naked eye as a hazy band of stars stretching from horizon to horizon.</p>
<p>For the first time, the IceCube Neutrino Observatory in Antarctica has produced an image of the Milky Way using neutrinos – tiny, ghost-like astronomical messengers. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo of the band of the Milky Way with extra shading in blue." src="https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A portrait of the Milky Way combining visible light and neutrino emissions (in blue).</span>
<span class="attribution"><span class="source">IceCube Collaboration/US National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier)</span></span>
</figcaption>
</figure>
<p>In <a href="http://dx.doi.org/10.1126/science.adc9818">research published today</a> in the journal Science, the IceCube Collaboration – an international group of more than 350 scientists – presents evidence of high-energy neutrino emission coming from the Milky Way.</p>
<p>We have not yet figured out exactly where in our galaxy these particles are coming from. But today’s result brings us closer to finding some of the galaxy’s most extreme environments.</p>
<h2>Neutrino astronomy</h2>
<p>Neutrinos offer a unique view of the cosmos as they can travel directly from places no other radiation or particles can escape from. This makes them very interesting to astronomers, because neutrinos offer a window into the extreme cosmic environments that create another kind of particle called cosmic rays.</p>
<p>Cosmic rays are high-energy particles that permeate our Universe, but their origins are difficult to pin down. Cosmic rays are electrically charged, which means their path through space is scrambled by magnetic fields, and by the time one arrives at Earth there is no way to tell where it came from. </p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/spotting-astrophysical-neutrinos-is-just-the-tip-of-the-icecube-20499">Spotting astrophysical neutrinos is just the tip of the IceCube</a>
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<p>However, the environments that accelerate cosmic rays to extraordinary energies also produce neutrinos – and neutrinos have no electric charge, so they travel in nice straight lines. So if we can detect the path of neutrinos arriving at Earth, this will point back to where the neutrinos were created. </p>
<p>But detecting those neutrinos is not so easy. </p>
<h2>How to hunt neutrinos</h2>
<p>The IceCube Neutrino Observatory is not far from the South Pole. It uses more than 5,000 light sensors arrayed throughout a cubic kilometre of pristine Antarctic ice to search for signs of high-energy neutrinos from our galaxy and beyond. </p>
<p>Vast numbers of neutrinos are streaming through Earth all the time, but only a tiny fraction of them bump into anything on their way through.</p>
<p>Each neutrino interaction makes a tiny flash of light – and those tiny flashes are what the IceCube sensors look out for. The direction and energy of the neutrino can be determined from the amount and pattern of light detected.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption"></span>
<span class="attribution"><span class="source">IceCube Collaboration</span></span>
</figcaption>
</figure>
<p>IceCube has previously detected high-energy neutrinos coming from outside the Milky Way. However, it has been more challenging to isolate the lower-energy neutrinos coming from within our galaxy.</p>
<p>This is because some flashes IceCube detected can be traced to cosmic rays hitting Earth’s atmosphere, which create neutrinos and other particles called muons. To filter out these flashes, IceCube researchers have developed ways to distinguish particles created in the atmosphere and those from further afield by the shape of the light patterns they create in the ice. </p>
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Read more:
<a href="https://theconversation.com/an-antarctic-neutrino-telescope-has-detected-a-signal-from-the-heart-of-a-nearby-active-galaxy-193845">An Antarctic neutrino telescope has detected a signal from the heart of a nearby active galaxy</a>
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<p>Filtering out the unwanted detections has made IceCube more sensitive to astrophysical neutrinos. The final breakthrough that allowed the creation of a neutrino image of the Milky Way came from machine-learning methods that improve the identification of cascades of light produced by neutrinos, as well as the determination of the neutrino’s direction and energy.</p>
<h2>Closing in on cosmic rays</h2>
<p>The new neutrino lens on our galaxy will help reveal where the most powerful accelerators of galactic cosmic rays are located. We hope to learn how energetic these particles can get, and the inner workings of these high-energy galactic engines.</p>
<p>However, we are yet to pinpoint these accelerators within the Milky Way. The new IceCube analysis found evidence for neutrinos coming from broad regions of the galaxy, but was not able to discern individual sources.</p>
<p>Our team, at the University of Canterbury in New Zealand and the University of Adelaide in Australia, has a plan to realise that next step.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=342&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=342&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=342&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=430&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=430&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=430&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Five views of the Milky Way: the top two bands show visible light and gamma rays, while the lower three show expected and real neutrino results, plus a measure of the significance of neutrino events detected by IceCube.</span>
<span class="attribution"><span class="source">IceCube Collaboration</span></span>
</figcaption>
</figure>
<p>We are making models to predict the neutrino signal close to likely particle accelerators so we can target our searches for neutrinos. </p>
<p>Undergraduate student Rhia Hewett and PhD student Ryan Burley are examining pairs of accelerator candidates and molecular dust clouds. They plan to estimate the flux of neutrinos produced by cosmic rays interacting in the clouds, after the neutrinos travel from the accelerators. </p>
<p>They will use their results to enable a focused search of IceCube data for the sources of neutrino emissions. We believe this will provide the key to using IceCube to unlock the secrets of the most energetic processes in the Milky Way.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=2067&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=2067&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=2067&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=2597&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=2597&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=2597&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A timeline of neutrino astronomy.</span>
<span class="attribution"><span class="source">IceCube Collaboration</span></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/208622/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jenni Adams has received funding from the Marsden Fund Council from New Zealand Government funding, managed by the Royal Society Te Apārangi. </span></em></p>Neutrinos are some of nature’s most elusive particles, but new research has used them to create an image of our own galaxy.Jenni Adams, Professor, Physics and Astronomy, University of CanterburyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1938452022-11-04T02:44:13Z2022-11-04T02:44:13ZAn Antarctic neutrino telescope has detected a signal from the heart of a nearby active galaxy<figure><img src="https://images.theconversation.com/files/493415/original/file-20221104-17-ohcw3y.jpg?ixlib=rb-1.1.0&rect=4%2C0%2C2939%2C2321&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://esahubble.org/news/heic1305/">NASA / ESA / A. van der Hoeven</a></span></figcaption></figure><p>An enormous neutrino observatory buried deep in the Antarctic ice has discovered only the second extra-galactic source of the elusive particles ever found.</p>
<p>In results <a href="https://www.science.org/doi/10.1126/science.abg3395">published today in Science</a>, the IceCube collaboration reports the detection of neutrinos from an “active galaxy” called NGC 1068, which lies some 47 million light-years from Earth. </p>
<h2>How to spot a neutrino</h2>
<p>Neutrinos are very shy fundamental particles that don’t often interact with anything else. When they were first detected in the 1950s, physicists soon realised they would in some ways be ideal for astronomy. </p>
<p>Because neutrinos so rarely have anything to do with other particles, they can travel unimpeded across the Universe. However, their shyness also makes them difficult to detect. To catch enough to be useful, you need a very big detector.</p>
<p>That’s where IceCube comes in. Over the course of seven summers from 2005 to 2011, scientists at America’s Amundsen–Scott South Pole Station bored 86 holes in the ice with a hot-water drill. Each hole is almost 2.5 kilometres deep, about 60 centimetres wide, and contains 60 basketball-sized light detectors attached to a long stretch of cable.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing the arrangement of detectors in the IceCube neutrino observatory." src="https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/493417/original/file-20221104-22-e3mlth.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The IceCube neutrino observatory has more than 5,000 detectors buried deep in the Antarctic ice.</span>
<span class="attribution"><a class="source" href="https://icecube.wisc.edu/gallery/neutrino-emission-from-ngc-1068/">NSF/IceCube</a></span>
</figcaption>
</figure>
<p>How does this help us detect neutrinos? Occasionally, a neutrino will bump into a proton or neutron in the ice near a detector. The collision produces a much heavier particle called a muon, travelling so fast it emits a blue glow, which the light detectors can pick up.</p>
<p>By measuring when this light arrives at different detectors, the direction the muon (and neutrino) came from can be calculated. Looking at the particle energies, it turns out most of the neutrinos IceCube detects are created in Earth’s atmosphere.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/spotting-astrophysical-neutrinos-is-just-the-tip-of-the-icecube-20499">Spotting astrophysical neutrinos is just the tip of the IceCube</a>
</strong>
</em>
</p>
<hr>
<p>However, a small fraction of the neutrinos do come from outer space. As of 2022, thousands of neutrinos from somewhere in the distant Universe have been identified.</p>
<h2>Where do neutrinos come from?</h2>
<p>They appear to come fairly uniformly from all directions, without any obvious bright spots showing up. This means there must be a lot of sources of neutrinos out there. </p>
<p>But what are these sources? There are plenty of candidates, exotic-sounding objects like active galaxies, quasars, blazars and gamma-ray bursts.</p>
<p>In 2018, IceCube announced the discovery of the first identified high-energy neutrino emitter – a blazar, which is a particular kind of galaxy that happens to be firing a jet of high-energy particles in Earth’s direction. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/scientists-discover-a-new-source-of-neutrinos-in-space-opening-up-another-window-into-the-universe-99785">Scientists discover a new source of neutrinos in space – opening up another window into the universe</a>
</strong>
</em>
</p>
<hr>
<p>Known as TXS 0506+056, the blazar was identified after IceCube saw a single high-energy neutrino and sent out an urgent astronomer’s telegram. Other telescopes scrambled to take a look at TXS 0506+056, and discovered it was also emitting a lot of gamma rays at the same time. </p>
<p>This makes sense, because we think blazars work by boosting protons to extreme speeds – and these high-energy protons then interact with other gas and radiation to produce both gamma rays and neutrinos.</p>
<h2>An active galaxy</h2>
<p>The blazar was the first extra-galactic source ever discovered. In this new study, IceCube identified the second.</p>
<p>The IceCube scientists re-examined the first decade of data they had collected, applying fancy new methods to pull out sharper measurements of neutrino directions and energy.</p>
<p>As a result, an already interesting bright spot in the background neutrino glow came into sharper focus. About 80 neutrinos had come from a fairly nearby, well-studied galaxy called NGC 1068 (also known as M77, as it is the 77th entry in the famous 18th-century catalogue of interesting astronomical objects created by the French astronomer Charles Messier).</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/QkBAL3yvXBg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The neutrinos offer a glimpse into the heart of the active galaxy NGC 1068.</span></figcaption>
</figure>
<p>Located about 47 million light-years from Earth, NGC 1068 is a known “active galaxy” – a galaxy with an extremely bright core. It is about 100 times closer than the blazar TXS 0506+056, and its angle relative to us means gamma rays from its core are obscured from our view by dust. However, neutrinos happily zoom straight through the dust and into space.</p>
<p>This new discovery will provide a wealth of information to astrophysicists and astronomers about what exactly is going on inside NGC 1068. There are already hundreds of papers attempting to explaining how the galaxy’s inner core works, and the new IceCube data add some information about neutrinos that will help to refine these models.</p><img src="https://counter.theconversation.com/content/193845/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gary Hill is a member of the IceCube collaboration. His IceCube research at the University of Adelaide is funded by the Australian Research Council.</span></em></p>The IceCube telescope near the South Pole has received a first glimpse into the core of the galaxy NGC 1068.Gary Hill, Associate Professor, Astrophysics and Dark Matter Researcher, University of AdelaideLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1731322021-12-22T13:12:07Z2021-12-22T13:12:07Z2021: a year physicists asked, ‘What lies beyond the Standard Model?’<figure><img src="https://images.theconversation.com/files/438717/original/file-20211221-48250-esf86c.jpg?ixlib=rb-1.1.0&rect=45%2C21%2C1566%2C1027&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Experiments at the Large Hadron Collider in Europe, like the ATLAS calorimeter seen here, are providing more accurate measurements of fundamental particles.</span> <span class="attribution"><a class="source" href="https://cds.cern.ch/record/910381">Maximilien Brice</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>If you ask a physicist like me to explain how the world works, my lazy answer might be: “It follows the Standard Model.”</p>
<p><a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">The Standard Model</a> explains the fundamental physics of how the universe works. It has endured over 50 trips around the Sun despite experimental physicists constantly probing for cracks in the model’s foundations. </p>
<p>With few exceptions, it has stood up to this scrutiny, passing experimental test after experimental test with flying colors. But this wildly successful model has conceptual gaps that suggest there is a bit more to be learned about how the universe works.</p>
<p>I am a <a href="https://scholar.google.com/citations?user=N_cqAjYAAAAJ&hl=en&oi=sra">neutrino physicist</a>. <a href="https://neutrinos.fnal.gov/whats-a-neutrino/">Neutrinos</a> represent three of the <a href="https://www.iop.org/explore-physics/physics-stepping-stones/standard-model">17 fundamental particles in the Standard Model</a>. They zip through every person on Earth at all times of day. I study the properties of interactions between <a href="https://neutrinos.fnal.gov/whats-a-neutrino/">neutrinos</a> and normal matter particles.</p>
<p>In 2021, physicists around the world ran a number of experiments that probed the Standard Model. Teams measured basic parameters of the model more precisely than ever before. Others investigated the fringes of knowledge where the best experimental measurements don’t quite match the predictions made by the Standard Model. And finally, groups built more powerful technologies designed to push the model to its limits and potentially discover new particles and fields. If these efforts pan out, they could lead to a more complete theory of the universe in the future.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A chart showing the particles of the Standard Model." src="https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Standard Model of physics allows scientists to make incredibly accurate predictions about how the world works, but it doesn’t explain everything.</span>
<span class="attribution"><a class="source" href="https://cds.cern.ch/images/OPEN-PHO-CHART-2015-001-1/">CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<h2>Filling holes in Standard Model</h2>
<p>In 1897, J.J. Thomson discovered the first fundamental particle, the electron, using nothing more than <a href="https://www.britannica.com/science/atom/Discovery-of-electrons">glass vacuum tubes and wires</a>. More than 100 years later, physicists are still discovering new pieces of the Standard Model.</p>
<p><a href="https://www.energy.gov/science/doe-explainsthe-standard-model-particle-physics">The Standard Model</a> is a <a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">predictive framework</a> that does two things. First, it explains what the basic particles of matter are. These are things like electrons and the quarks that make up protons and neutrons. Second, it predicts how these matter particles interact with each other using “messenger particles.” These are called bosons – they include photons and the famous Higgs boson – and they communicate the basic forces of nature. The Higgs boson wasn’t <a href="https://atlas.cern/updates/feature/higgs-boson">discovered until 2012</a> after decades of work at CERN, the huge particle collider in Europe.</p>
<p>The Standard Model is incredibly good at predicting many aspects of how the world works, but it does have some holes.</p>
<p>Notably, it does not include any description of gravity. While Einstein’s theory of <a href="https://theconversation.com/why-does-gravity-pull-us-down-and-not-up-162141">General Relativity describes how gravity works</a>, physicists have not yet discovered a particle that conveys the force of gravity. A proper “Theory of Everything” would do everything the Standard Model can, but also include the messenger particles that communicate how gravity interacts with other particles.</p>
<p>Another thing the Standard Model can’t do is explain why any particle has a certain mass – physicists must measure the mass of particles directly using experiments. Only after experiments give physicists these exact masses can they be used for predictions. The better the measurements, the better the predictions that can be made.</p>
<p>Recently, physicists on a team at CERN measured <a href="https://atlas.cern/updates/briefing/twice-higgs-twice-challenge">how strongly the Higgs boson feels itself</a>. Another CERN team also measured the Higgs boson’s mass <a href="https://cms.cern/news/cms-precisely-measures-mass-higgs-boson">more precisely than ever before</a>. And finally, there was also progress on measuring the mass of neutrinos. Physicists know neutrinos have more than zero mass but less than the amount currently detectable. A team in Germany has continued to refine the techniques that could allow them to <a href="https://www.katrin.kit.edu/index.php">directly measure the mass of neutrinos</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A blue circular particle acellerator." src="https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Projects like the Muon g-2 experiment highlight discrepancies between experimental measurements and predictions of the Standard Model that point to problems somewhere in the physics.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Fermilab_g-2_(E989)_ring.jpg#/media/File:Fermilab_g-2_(E989)_ring.jpg">Reidar Hahn/WikimediaCommons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Hints of new forces or particles</h2>
<p>In April 2021, members of the <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics/">Muon g-2 experiment at Fermilab announced</a> their first <a href="https://theconversation.com/how-we-found-hints-of-new-particles-or-forces-of-nature-and-why-it-could-change-physics-158564">measurement of the magnetic moment of the muon</a>. The muon is one of the fundamental particles in the Standard Model, and this measurement of one of its properties is the most accurate to date. The reason this experiment was important was because the measurement didn’t perfectly match the Standard Model prediction of the magnetic moment. Basically, muons don’t behave as they should. This finding could point to <a href="https://news.uchicago.edu/story/what-muon-g-2-results-mean-how-we-understand-universe">undiscovered particles that interact with muons</a>.</p>
<p>But simultaneously, in April 2021, physicist Zoltan Fodor and his colleagues showed how they used a mathematical method called Lattice QCD to <a href="https://theconversation.com/proof-of-new-physics-from-the-muons-magnetic-moment-maybe-not-according-to-a-new-theoretical-calculation-157829">precisely calculate the muon’s magnetic moment</a>. Their theoretical prediction is different from old predictions, still works within the Standard Model and, importantly, matches experimental measurements of the muon.</p>
<p>The disagreement between the previously accepted predictions, this new result and the new prediction must be reconciled before physicists will know if the experimental result is truly beyond the Standard Model.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A spinning galaxy in space." src="https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=436&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=436&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=436&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=548&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=548&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=548&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">New tools will help physicists search for dark matter and other things that could help explain mysteries of the universe.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/dark-matter-halo-surrounding-galaxy-royalty-free-illustration/932730112?adppopup=true">Mark Garlick/Science Photo Library via Getty Images</a></span>
</figcaption>
</figure>
<h2>Upgrading the tools of physics</h2>
<p>Physicists must swing between crafting the mind-bending ideas about reality that make up theories and advancing technologies to the point where new experiments can test those theories. 2021 was a big year for advancing the experimental tools of physics. </p>
<p>First, the world’s largest particle accelerator, the <a href="https://theconversation.com/ten-years-of-large-hadron-collider-discoveries-are-just-the-start-of-decoding-the-universe-102331">Large Hadron Collider at CERN</a>, was shut down and underwent some substantial upgrades. Physicists just restarted the facility in October, and they plan to begin the <a href="https://cerncourier.com/a/protons-back-with-a-splash/">next data collection run in May 2022</a>. The upgrades have boosted the power of the collider so that it can <a href="https://www.universetoday.com/140769/the-large-hadron-collider-has-been-shut-down-and-will-stay-down-for-two-years-while-they-perform-major-upgrades/">produce collisions at 14 TeV</a>, up from the previous limit of 13 TeV. This means the batches of tiny protons that travel in beams around the circular accelerator together carry the same amount of energy as an 800,000-pound (360,000-kilogram) passenger train traveling at 100 mph (160 kph). At these incredible energies, physicists may discover new particles that were too heavy to see at lower energies.</p>
<p>Some other technological advancements were made to help the search for dark matter. Many astrophysicists believe that dark matter particles, which don’t currently fit into the Standard Model, could answer some outstanding questions regarding the way gravity bends around stars – called <a href="https://www.nasa.gov/content/discoveries-highlights-shining-a-light-on-dark-matter">gravitational lensing</a> – as well as the <a href="https://phys.org/news/2019-10-dark-massive-spiral-galaxies-breakneck.html">speed at which stars rotate in spiral galaxies</a>. Projects like the Cryogenic Dark Matter Search have yet to find dark matter particles, but the teams are <a href="https://supercdms.slac.stanford.edu/overview">developing larger and more sensitive detectors</a> to be deployed in the near future. </p>
<p>[<em>Over 140,000 readers rely on The Conversation’s newsletters to understand the world.</em> <a href="https://memberservices.theconversation.com/newsletters/?source=inline-140ksignup">Sign up today</a>.]</p>
<p>Particularly relevant to my work with neutrinos is the development of immense new detectors like <a href="http://www.hyper-k.org/en/">Hyper-Kamiokande</a> and <a href="https://lbnf-dune.fnal.gov/">DUNE</a>. Using these detectors, scientists will hopefully be able to answer questions about a <a href="https://cerncourier.com/a/why-does-cp-violation-matter-to-the-universe/">fundamental asymmetry in how neutrinos oscillate</a>. They will also be used to watch for proton decay, a proposed phenomenon that certain theories predict should occur. </p>
<p>2021 highlighted some of the ways the Standard Model fails to explain every mystery of the universe. But new measurements and new technology are helping physicists move forward in the search for the Theory of Everything.</p><img src="https://counter.theconversation.com/content/173132/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Aaron McGowan has received funding in the past from the U.S. Department of Energy. </span></em></p>Physicists know a lot about the most fundamental properties of the universe, but they certainly don’t know everything. 2021 was a big year for physics – what was learned and what’s coming next?Aaron McGowan, Principal Lecturer in Physics and Astronomy, Rochester Institute of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1703692021-10-31T19:06:07Z2021-10-31T19:06:07ZThe hunt for ‘sterile neutrinos’: a new experiment has dashed hopes of an undiscovered particle<figure><img src="https://images.theconversation.com/files/429011/original/file-20211028-15-2e1m3q.jpg?ixlib=rb-1.1.0&rect=25%2C34%2C5695%2C5421&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://vms.fnal.gov/asset/detail?recid=1961196">Reidar Hahn / Fermilab</a></span></figcaption></figure><p>Physicists searching for evidence of a “light sterile neutrino”, a hypothetical particle that could give clues to cosmic puzzles such as the nature of dark matter and why the Universe is made of matter at all, have announced their hunt has <a href="https://news.fnal.gov/2021/10/microboone-experiments-first-results-show-no-hint-of-a-sterile-neutrino/">come back empty-handed</a>. </p>
<p>The MicroBooNE experiment at Fermilab was designed to follow up on earlier hints of neutrinos behaving oddly, but the negative result deals a blow to the idea of such a new elementary particle. </p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-the-elusive-neutrino-431">Explainer: the elusive neutrino</a>
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</em>
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<p>Neutrinos are lightweight, elusive subatomic particles, and current theories recognise three different types. In 1995, however, the Liquid Scintillator Neutrino Detector (LSND) experiment in Los Alamos detected more of one type than anyone expected. </p>
<p>Most attempts to explain the anomaly proposed the existence of a fourth kind of neutrino that barely interacts with normal matter at all: a so-called “sterile” neutrino. </p>
<p><a href="https://en.wikipedia.org/wiki/MiniBooNE">More</a> <a href="https://arxiv.org/abs/2005.05301">recent</a> <a href="https://en.wikipedia.org/wiki/Baksan_Neutrino_Observatory">experiments</a> have also reported results broadly consistent with the sterile neutrino hypothesis, but the MicroBooNE result casts the whole idea into doubt.</p>
<h2>What is a sterile neutrino?</h2>
<p>Neutrinos are subatomic particles postulated by Austrian physicist Wolfgang Pauli in 1930 to explain how some radioactive atoms fire out electrons. </p>
<p>Their existence wasn’t confirmed until 1956, when Americans Clyde Cowan and Frederick Reines observed tiny flashes of light made by neutrinos crashing into the atoms in a tank of water. </p>
<p>Today, neutrinos are an integral part of the “Standard Model of particle physics”. This is our best theory of the Universe’s particles, describing the 17 known elementary particles and how they interact via three fundamental forces (electromagnetism and the strong and weak forces).</p>
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<strong>
Read more:
<a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Explainer: Standard Model of Particle Physics</a>
</strong>
</em>
</p>
<hr>
<p>The Standard Model divides the 17 particles into two basic groups: 12 fermions, which make up matter, and five bosons, which carry the forces.</p>
<p>Not all fermions interact with all the forces. For example, neutrinos are only affected by the weak force (and gravity, which doesn’t fit into the Standard Model).</p>
<p>The fermions are split into three families, each of which has a neutrino: the electron, muon, and tau neutrinos.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=574&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=574&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=574&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=722&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=722&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=722&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The 17 particles of the Standard Model. The 12 fermions on the left make up matter, while the 5 bosons on the right carry forces. The three known neutrinos are on the bottom row.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg">MissMJ / Wikimedia</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>All of these neutrinos are “left-handed” with respect to the weak force. It’s hard to explain simply what that means, but suffice to say left- and right-handed particles are mirror images of one another, and they are affected differently by the weak force.</p>
<p>All other known fermions come in both left- and right-handed versions. This encourages us to think that right-handed neutrinos should also exist in nature. </p>
<p>Being right-handed, these hypothetical neutrinos are blind even to the weak force and are in this sense “sterile”. </p>
<p>But like all known particles, they should still feel gravity. Sterile neutrinos are also predicted by “grand unified theories” that try to combine the three forces into one.</p>
<h2>Hunting for sterile neutrinos</h2>
<p>If sterile neutrinos exist, how would we find them? One way is to use a process called “neutrino oscillation”, in which the three known kinds of neutrinos can transform into one another. </p>
<p>Experiments measuring these oscillations usually look at either how many of a given kind of neutrino appear in some situation, or how many disappear.</p>
<p>The LSND experiment which originally inspired the sterile neutrino hypothesis was an “appearance” experiment, as are MicroBooNE (which produced the new negative result) and its predecessor MiniBooNE.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=447&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=447&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=447&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=561&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=561&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=561&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Results from the MiniBooNE experiment hinted at the possible existence of a light sterile neutrino.</span>
<span class="attribution"><a class="source" href="https://www.hep.princeton.edu//~meyers/boone_pub/pss_install.html">Fred Ullrich / Fermilab</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>They fire a beam of muon-neutrinos over a relatively short distance (between 30 and 500 metres) and measure how many <em>electron</em>-neutrinos are detected at the other end.</p>
<p>At LSND and MiniBooNE, they saw more electron-neutrinos than expected. We know from other experiments that muon-neutrinos cannot oscillate directly into electron-neutrinos over these distances. </p>
<p>But if some of muon-neutrinos turn into very light sterile neutrinos and <em>then</em> into electron-neutrinos, it could explain how those extra electron-neutrinos appeared.</p>
<p>This is the sterile neutrino hypothesis.</p>
<h2>What if there are sterile neutrinos?</h2>
<p>If experiments did confirm the existence of a light sterile neutrino, there would be a good chance that heavier sterile neutrinos exist as well. </p>
<p>These heavier cousins could answer several major puzzles in particle physics, such as the nature of the “dark matter” that seems to make up most of the Universe, why neutrinos have any mass at all, and why the Universe contains so much more matter than antimatter.</p>
<p>There is but one problem. The light sterile neutrino we started with is a headache for cosmologists. </p>
<p>If it exists, we should be able to observe traces of sterile neutrinos formed just after the Big Bang.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=377&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">If sterile neutrinos exist, they should have left traces in the cosmic microwave background, a faint afterglow of radiation from the dawn of the Universe that pervades the sky.</span>
<span class="attribution"><a class="source" href="https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB">ESA / Planck Collaboration</a></span>
</figcaption>
</figure>
<p>However, no recent surveys of the cosmic microwave background radiation or the distribution of galaxies and light elements in between them show any sign these sterile neutrinos existed.</p>
<p>This could mean the sterile neutrino hypothesis is incorrect. But it is also possible that something else in our understanding of the Universe is amiss.</p>
<h2>MicroBooNE and the global picture</h2>
<p>MicroBooNE analysed its results in four different ways, and none of them turned up signs of extra electron-neutrinos. This is disappointing for the researchers behind the LSND and MiniBooNE collaborations, and for proponents of the sterile neutrino hypothesis.</p>
<p>It also raises the question of exactly what caused the results observed by the earlier experiments. Further analysis of the MicroBooNE data may help unravel this mystery.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=358&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=358&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=358&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=450&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=450&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=450&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The IceCube experiment in Antarctica has found no evidence in favour of sterile neutrinos.</span>
<span class="attribution"><a class="source" href="https://icecube.wisc.edu/gallery/landscapes-and-skyscapes/#modulagallery-6276-2088">Emanuel Jacobi / IceCube / NSF</a></span>
</figcaption>
</figure>
<p>Globally, however, MicroBooNE’s latest result is in line with the findings of two large “disappearance” experiments, MINOS+ and IceCube. Neither of these saw evidence of disappearing muon-neutrinos as the sterile neutrino hypothesis predicts.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/spotting-astrophysical-neutrinos-is-just-the-tip-of-the-icecube-20499">Spotting astrophysical neutrinos is just the tip of the IceCube</a>
</strong>
</em>
</p>
<hr>
<p>Elsewhere, there have been claims of disappearing neutrinos in nuclear reactor experiments. However, calculating how many neutrinos a nuclear reactor will emit is notoriously difficult, so these claims are best taken with a grain of salt.</p>
<h2>Future searches</h2>
<p>The MicroBooNE collaboration has so far analysed only half of its collected data. </p>
<p>Some have also questioned whether no excess of electron-neutrinos necessarily means no neutrino oscillations, given the measurement has been made at only one distance. Technically, we need measurements at two distances or more to definitively establish oscillations or otherwise.</p>
<p>These measurements are likely to come in the next few years, when Fermilab switches on two more detectors as part of the <a href="https://sbn.fnal.gov/">Fermilab Short Baseline Neutrino program</a>. The trio of detectors will test for disappearance of muon-neutrinos and appearance of electron-neutrinos using a single beam of source neutrinos.</p>
<p>The prospects for a final verdict on light sterile neutrinos in the next decade are therefore good.</p><img src="https://counter.theconversation.com/content/170369/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Yvonne Wong receives funding from the Australian Research Council. </span></em></p><p class="fine-print"><em><span>Michael Schmidt receives funding from the Australian Research Council.</span></em></p>A new result from the MicroBooNE neutrino experiment has dashed hopes for a neat resolution to several puzzles for physicists.Yvonne Wong, Associate professor of physics and ARC Future Fellow, UNSW SydneyMichael Schmidt, Senior lecturer in physics, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1517882020-12-15T19:16:52Z2020-12-15T19:16:52ZWhere does the Earth’s heat come from?<figure><img src="https://images.theconversation.com/files/373907/original/file-20201209-15-1i85cih.jpg?ixlib=rb-1.1.0&rect=3%2C79%2C2040%2C1311&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Piton de la Fournaise in eruption, 2015.</span> <span class="attribution"><span class="source">Greg de Serra/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Earth generates heat. The deeper you go, the higher the temperature. At 25km down, temperatures rise as high as 750°C; at the core, it is said to be 4,000°C. Humans have been making use of hot springs as far back as antiquity, and today we use geothermal technology to heat our apartments. Volcanic eruptions, geysers and earthquakes are all signs of the Earth’s internal powerhouse.</p>
<p>The average heat flow from the earth’s surface is 87mW/m<sup>2</sup> – that is, 1/10,000th of the energy received from the sun, meaning the earth emits a total of <a href="https://unt.univ-cotedazur.fr/uved/bouillante/cours/i.-la-geothermie-manifestations-quantification-origine-et-utilisations-de-la-chaleur-interne-du-globe/2.-comprendre-et-modeliser-les-transferts-de-chaleur-pour-determiner-l2019origine-de-la-chaleur-interne-du-globe/2.3-origine-de-la-chaleur-interne-du-globe.html">47 terawatts</a>, the equivalent of several thousand nuclear power plants. The source of the earth’s heat has long remained a mystery, but we now know that most of it is the result of radioactivity.</p>
<h2>The birth of atoms</h2>
<p>To understand where all this heat is coming from, we have to go back to the birth of the atomic elements.</p>
<p>The <a href="https://theconversation.com/us/topics/big-bang-470">Big Bang</a> produced matter in the form of protons, neutrons, electrons, and neutrinos. It took around 370,000 years for the first atoms to form – protons attracted electrons, producing hydrogen. Other, heavier nuclei, like deuterium and helium, formed at the same time, in a process called <a href="https://fr.wikipedia.org/wiki/Nucl%C3%A9osynth%C3%A8se_primordiale">Big Bang nucleosynthesis</a>.</p>
<p>The creation of heavy elements was far more arduous. First, stars were born and heavy nuclei formed via accretion in their fiery crucible. This process, called <a href="https://fr.wikipedia.org/wiki/Nucl%C3%A9osynth%C3%A8se_stellaire">stellar nucleosynthesis</a>, took billions of years. Then, when the stars died, these elements spread out across space to be captured in the form of planets.</p>
<p>The earth’s composition is therefore highly complex. Luckily for us, and our existence, it includes all the natural elements, from the simplest atom, hydrogen, to heavy atoms such as uranium, and everything in between, carbon, iron – the entire periodic table. Inside the bowels of the earth is an entire panoply of elements, arranged within various onion-like layers.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/374908/original/file-20201214-15-1ylfnmj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/374908/original/file-20201214-15-1ylfnmj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=382&fit=crop&dpr=1 600w, https://images.theconversation.com/files/374908/original/file-20201214-15-1ylfnmj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=382&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/374908/original/file-20201214-15-1ylfnmj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=382&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/374908/original/file-20201214-15-1ylfnmj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=480&fit=crop&dpr=1 754w, https://images.theconversation.com/files/374908/original/file-20201214-15-1ylfnmj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=480&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/374908/original/file-20201214-15-1ylfnmj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=480&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Our planet contains all the elements of the periodic table.</span>
<span class="attribution"><span class="source">Sandbh/Wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>We know little about the inside of our planet. The deepest mines reach down 10km at the most, while the earth has a radius of 6,500km. Scientific knowledge of deeper levels has been obtained through seismic measurements. Using this data, geologist divided the earth’s structure into various strata, with the core at the center, solid on the inside and liquid on the outside, followed by the lower and upper mantles and, finally, the crust. The earth is made up of heavy, unstable elements and is therefore radioactive, meaning there is another way to find out about its depths and understand the source of its heat.</p>
<h2>What is radioactivity?</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=601&fit=crop&dpr=1 600w, https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=601&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=601&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=755&fit=crop&dpr=1 754w, https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=755&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/371006/original/file-20201124-21-6w5mly.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=755&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Drugs and cosmetics containing a small dose of radium, early 20th century.</span>
<span class="attribution"><a class="source" href="https://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Tho-Radia-IMG_1228.JPG/1023px-Tho-Radia-IMG_1228.JPG">Rama/Wikimedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Radioactivity is a common and inescapable natural phenomenon. Everything on earth is radioactive – that is to say, everything spontaneously produces elementary particles (humans emit a few thousand per second). In Marie Curie’s day, no one was afraid of radioactivity. </p>
<p>On the contrary, it was said to have beneficial effects: beauty creams were certified radioactive and contemporary literature extolled the radioactive properties of mineral water. Maurice Leblanc wrote of a thermal spring saving his protagonist Arsène Lupin during one of his adventures:</p>
<blockquote>
<p>“The water contained such energy and power as to make it a veritable fountain of youth, properties arising from its incredible radioactivity.” (Maurice Leblanc, <a href="https://fr.wikipedia.org/wiki/La_Demoiselle_aux_yeux_verts">“La demoiselle aux yeux verts”</a>, 1927)</p>
</blockquote>
<p>There are various kinds of radioactivity, each involving the spontaneous release of particles and emitting energy that can be detected in the form of heat deposits. Here, we will be talking about “beta” decay, where an election and a neutrino are emitted. The electron is absorbed as soon as it is produced, but the neutrino has the surprising ability to penetrate a wide range of materials. The whole of the Earth is transparent to neutrinos, so detecting neutrinos generated by radioactive decay within the Earth should give us an idea of what is happening at its deepest levels.</p>
<p>These kinds of particles are called <a href="https://neutrino-history.in2p3.fr/the-earth-seen-through-neutrinos/">geoneutrinos</a>, and they provide an original way to investigate the depths of the Earth. Although detecting them is no easy matter, since neutrinos interact little with matter, some detectors are substantial enough to perform this kind of research.</p>
<p>Geoneutrinos mainly arise from heavy elements with very long half-lives, whose properties are now thoroughly understood through lab studies: chiefly uranium, thorium and potassium. The decay of one uranium-238 nucleus, for example, releases an average of 6 neutrinos, and 52 megaelectronvolts of energy carried by the released particles that then lodge in matter and deposit heat. Each neutrino carries around two megaelectronvolts of energy. According to standardized measures, one megaelectronvolt is equivalent to 1.6 10<sup>-13</sup> joules, so it would take around 10<sup>25</sup> decays per second to reach the earth’s total heat. The question is, can these neutrinos be detected?</p>
<h2>Detecting geoneutrinos</h2>
<p>In practice, we have to take aggregate measurements at the detection site of flows coming from all directions. It is difficult to ascertain the exact source of the flows, since we cannot measure their direction. We have to use models to create computer simulations. Knowing the energy spectrum of each decay mode and modeling the density and position of the various geological strata affecting the final result, we get an overall spectrum of expected neutrinos which we then deduct from the number of events predicted for a given detector. This number is always very low – only a handful of events per kiloton of detector per year.</p>
<p>Two recent experiments have added to the research: <a href="https://www.sciencedirect.com/science/article/pii/S0550321316300529">KamLAND</a>, a detector weighing 1,000 metric tons underneath a Japanese mountain, and <a href="https://physicsworld.com/a/borexino-spots-solar-neutrinos-from-elusive-fusion-cycle/">Borexino</a>, which is located in a tunnel under the Gran Sasso mountain in Italy and weighs 280 metric tons. Both use “liquid scintillators”. To detect neutrinos from the earth or <a href="https://www.futura-sciences.com/sciences/actualites/physique-neutrinos-cosmiques-naissent-eruptions-quasars-50447/">the cosmos</a>, you need a detection method that is effective at low energies; this means exciting atoms in a scintillating liquid. Neutrinos interact with protons, and the resulting particles emitted produce observable light.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/371009/original/file-20201124-23-cplwt9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/371009/original/file-20201124-23-cplwt9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/371009/original/file-20201124-23-cplwt9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/371009/original/file-20201124-23-cplwt9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/371009/original/file-20201124-23-cplwt9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/371009/original/file-20201124-23-cplwt9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/371009/original/file-20201124-23-cplwt9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Sno+ experiment uses the SnoLab detector in Canada, to detect geoneutrinos, among other things.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/138424555@N03/23753317224/in/photolist-CbZXE9-CGoBWC-D9vPxZ-Cc82tr-D2dSae-Cc82va-CGoBSj-D2dSct-CYWVvC">SNOLAB</a></span>
</figcaption>
</figure>
<p>KamLAND has announced more than 100 events and Borexino around 20 that could be attributed to geoneutrinos, with an uncertainty factor of 20-30%. We cannot pinpoint their source, but this overall measurement – while fairly rough – is in line with the predictions of the simulations, within the limits of the low statistics obtained.</p>
<p>Therefore, the <a href="https://link.springer.com/chapter/10.1007/978-0-387-70771-6_4">traditional hypothesis</a> of a kind of nuclear reactor at the center of the earth, consisting of a ball of fissioning uranium like those in nuclear power plants, has now been excluded. Fission is not a spontaneous radioactivity but is stimulated by slow neutrons in a chain reaction.</p>
<p>There are now new, more effective detectors being developed: <a href="https://en.wikipedia.org/wiki/SNO%2B">Canada's SNO+</a>, and <a href="https://www.scmp.com/news/china-insider/article/1456878/guangdong-races-ahead-global-effort-measure-elusive-neutrinos">China's Juno</a>, which will improve our knowledge of geoneutrinos.</p>
<blockquote>
<p>“Far from diminishing it, adding the invisible to the visible only enriches the latter, gives it meaning, completes it.” (Paul Claudel, <a href="http://www.gallimard.fr/Catalogue/GALLIMARD/Blanche/Positions-et-propositions">“Positions et propositions”</a>, 1928)</p>
</blockquote>
<hr>
<p><em>Translated from the French by Alice Heathwood for <a href="http://www.fastforword.fr/en">Fast ForWord</a>.</em></p><img src="https://counter.theconversation.com/content/151788/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>François Vannucci ne travaille pas, ne conseille pas, ne possède pas de parts, ne reçoit pas de fonds d'une organisation qui pourrait tirer profit de cet article, et n'a déclaré aucune autre affiliation que son organisme de recherche.</span></em></p>The study of neutrinos produced within the Earth’s interior provides a better understanding of the radioactivity of our planet.François Vannucci, Professeur émérite, chercheur en physique des particules, spécialiste des neutrinos, Université Paris CitéLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1479532020-10-22T11:21:36Z2020-10-22T11:21:36ZDark matter: our method for catching ghostly haloes could help unveil what it’s made of<figure><img src="https://images.theconversation.com/files/364959/original/file-20201022-13-1xjvrhm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's conception of the Milky Way galaxy, which should contain dark matter haloes.</span> <span class="attribution"><span class="source">Nick Risinger/NASA</span></span></figcaption></figure><p>The search for <a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">dark matter</a> – an unknown and invisible substance thought to make up the vast majority of matter in the universe – is at a crossroads. Although it was proposed <a href="https://www.britannica.com/video/186454/Fritz-Zwicky-inference-dark-matter-existence">nearly 70 years ago</a> and has been searched for intensely - with large particle colliders, detectors deep underground and even instruments in space – it is still nowhere to be found. </p>
<p>But astronomers have promised <a href="https://arxiv.org/pdf/1810.01668.pdf">to leave “no stone unturned”</a> and have started to cast their net wider out into the galaxy. The idea is to extract information from astrophysical objects that may have witnessed chunks of it as they were passing by. We have just proposed <a href="https://arxiv.org/abs/2006.06741">a new method of doing so</a> by tracing galactic gas – and it may help tell us what it’s actually made of. </p>
<p>Physicists believe that dark matter has a propensity to structure itself into a hierarchy of haloes and subhaloes, via gravity. The masses of these clumps fall on a spectrum, with lower mass ones expected to be more numerous. Is there a limit to how light they could be? It depends on the nature of the dark matter particles. </p>
<h2>Warm versus cold</h2>
<p>Dark matter cannot be seen directly. We know it exists because we can see the gravitational effects it has on surrounding matter. There are <a href="https://phys.org/news/2016-08-dark-matterhot.html">different theories</a> about what dark matter may actually be. The standard model suggests it is cold, meaning it moves very slowly and only interacts with other matter through the force of gravity. This would be consistent with it being made up of particles <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">known as axions or WIMPS</a>. Another theory, however, suggests it is warm, meaning it moves at higher speeds. One such particle candidate is the <a href="https://www.symmetrymagazine.org/article/what-could-dark-matter-be">sterile neutrino</a>.</p>
<figure class="align-center ">
<img alt="Image of the Milky Way galaxy with a dark matter halo around it." src="https://images.theconversation.com/files/364278/original/file-20201019-13-1c7seek.jpg?ixlib=rb-1.1.0&rect=11%2C0%2C3982%2C2250&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/364278/original/file-20201019-13-1c7seek.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/364278/original/file-20201019-13-1c7seek.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/364278/original/file-20201019-13-1c7seek.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/364278/original/file-20201019-13-1c7seek.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/364278/original/file-20201019-13-1c7seek.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/364278/original/file-20201019-13-1c7seek.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Artist’s impression of the expected dark matter distribution around the Milky Way, seen as a blue halo.</span>
<span class="attribution"><span class="source">ESO/L. Calçada</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>If dark matter is cold, a Milky Way-type galaxy could harbour one or two subhaloes weighing as much as 10<sup>10</sup> Suns, and most likely hundreds with masses of around 10<sup>8</sup> Suns. If dark matter is warm, haloes lighter than around 10<sup>8</sup> Suns cannot form easily. So tallying light mass dark haloes can tell us something about the nature of dark matter.</p>
<h2>Halo imprints</h2>
<p>We believe that the existence of lower mass haloes can be revealed by carefully planned observations. Astronomers have already got pretty good at this game of hide and seek with dark matter haloes and have devised observations to pick up the damage they leave behind. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/364281/original/file-20201019-13-f98uh4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/364281/original/file-20201019-13-f98uh4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/364281/original/file-20201019-13-f98uh4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/364281/original/file-20201019-13-f98uh4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/364281/original/file-20201019-13-f98uh4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/364281/original/file-20201019-13-f98uh4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/364281/original/file-20201019-13-f98uh4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A galaxy cluster with dark matter mapped in blue and bright X-rays in pink.</span>
<span class="attribution"><span class="source">Smithsonian/wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>To date, observations have targeted mostly the changes in the distribution of stars in the Milky Way. For example, the Large Magellanic Cloud, a smaller galaxy orbiting ours, seems to have a dark matter halo which is massive enough to <a href="https://aasnova.org/2019/11/13/hunting-for-a-dark-matter-wake/">trigger an enormous wake</a> – driving the stars from across vast regions to move in unison. </p>
<p>A few of the smaller dark matter haloes thought to be whizzing inside the Milky Way may occasionally pierce through large stellar features, such as globular clusters (spherical collection of stars), leaving <a href="https://astrobites.org/2018/05/08/stellar-streams-the-nature-of-dark-matter/">tell-tale gaps</a> in them. Dark matter haloes can also affect how light bends around astrophysical objects in a process called <a href="https://academic.oup.com/mnras/article/363/4/1136/1044360">gravitational lensing</a>.</p>
<p>But the signals left in the stellar distributions are weak and prone to confusion with the stars’ own motions. Another way to probe the effect of haloes is by looking at the galactic gas it affects. Galaxies have <a href="https://www.nasa.gov/mission_pages/chandra/news/H-12-331.html">plenty of hot gas</a> (with a temperature of around 10<sup>6</sup> degrees Kelvin) which extends out to their edge, providing a wide net for catching these dark matter haloes.</p>
<p>Using a combination of analytical calculations and computer simulations, we have shown that dark haloes heavier than 10<sup>8</sup> solar masses can compress the hot gas through which they are moving. These will create local spikes in the density of the gas, which can be picked up by X-ray telescopes. These are predicted to be minute, of the order of a few per cent, but they will be within the reach of the upcoming <a href="https://wwwastro.msfc.nasa.gov/lynx/">Lynx</a> and <a href="https://www.the-athena-x-ray-observatory.eu/">Athena</a> telescopes.</p>
<p>Our models also predict that the spikes in the density of the <a href="https://www.nasa.gov/feature/goddard/2020/hubble-maps-giant-halo-around-andromeda-galaxy">cooler galactic gas</a> (with temperature of around 10<sup>5</sup> K) will be even more significant. This means that the cooler gas can record the passage of dark matter haloes even more sensitively than the hot gas.</p>
<p>Another promising way of observing the dark-matter-induced fluctuations in the gas is via the photons (light particles) from the cosmic microwave background – the light left over from the Big Bang. This light <a href="https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zeldovich_effect">scatters off</a> the highly energetic electrons in the hot gas in a way that we can detect, providing a complementary approach to the other studies. </p>
<p>Over the next few years, this new method can be used to test models of dark matter. Regardless of whether dark matter haloes below 10<sup>8</sup> solar masses are found in the numbers predicted or not, we will learn something useful. If the numbers match up, the standard cosmological model would have passed an important test. If they are missing, or are far fewer than expected, the standard model would be ruled out and we’ll have to find a more viable alternative. </p>
<p>Dark matter remains a mystery, but there’s a huge amount of work going into solving it. Whether the answer will come from instruments on Earth or astrophysical probes, it will no doubt be one of the most important discoveries of the century.</p><img src="https://counter.theconversation.com/content/147953/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andreea Font does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A new method suggests we should aim to detect dark matter haloes by tracing galactic gas.Andreea Font, Astrophysicist, Liverpool John Moores UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1314152020-02-11T17:58:12Z2020-02-11T17:58:12ZHow a ‘muon accelerator’ could unravel some of the universe’s greatest mysteries<figure><img src="https://images.theconversation.com/files/314757/original/file-20200211-146674-fyj6w5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The supernova remnant Cassiopeia A.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>The fact that we are here at all is one of the greatest puzzles of physics. We are made from normal particles such as electrons, but every such particle also has an antimatter companion that is virtually identical to itself, but with the opposite charge. When matter and antimatter come into contact, they annihilate each other in a flash of light.</p>
<p>Physics suggests that matter and antimatter <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">were created in almost equal quantities</a> in the Big Bang. So how come there’s almost only matter left today – why didn’t the matter and antimatter annihilate each other to render the universe lifeless? Our new research has enabled scientists <a href="http://www.natureasia.com/en/research/highlight/13209">to build a new type of accelerator</a>, based on particles called muons, that could help us find out.</p>
<p>A muon <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">is almost identical</a> to an electron, except that it is 207 times heavier. What’s more, it decays into other particles in two millionths of a second. This lifetime, although short for us, is quite a long time for an unstable fundamental particle, and this explains why muons make up almost all the charged cosmic rays which reach the Earth’s surface.</p>
<p>Much of our ability to investigate the structure of matter at the shortest distances depends on creating beams of particles and accelerate them to high energies. However there are only four stable particles which can be used in this way, the electron and its anti-particle (positron), and the proton and its antiparticle (antiproton).</p>
<p>Particle beams consisting of those have been used for many years, yet both of these pairs have drawbacks. The electron and its partner are very light – when we try to accelerate them, they radiate electromagnetic energy. This is can be useful for applications such as TV but makes it hard to reach the sort of energies which we need to improve our understanding of the universe. </p>
<p>Unlike electrons, the proton and the anti-proton are made up of more fundamental particles – quarks and gluons. In a collision between a proton and anti-proton it is these fundamental particles which actually collide, resulting in a lower energy crash than you would have got if protons had been truly fundamental particles.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=443&fit=crop&dpr=1 600w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=443&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=443&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=557&fit=crop&dpr=1 754w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=557&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=557&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Each particle also has an antiparticle, not included here.</span>
<span class="attribution"><a class="source" href="https://www.publicdomainpictures.net/en/view-image.php?image=35144&picture=fundamental-particles">Publicdomainpictures.net</a></span>
</figcaption>
</figure>
<p>The muons are heavy enough that they radiate much less energy,
but fundamental (not made up of smaller particles) so that all their energy is available for the investigation. When scientists created the <a href="https://theconversation.com/could-the-higgs-nobel-be-the-end-of-particle-physics-18978">Nobel-prize winning Higgs particle</a> with protons, they needed a machine 10km in diameter: the <a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">Large Hadron Collider</a>. A muon machine, however, could achieve this with a circumference of just 200 metres.</p>
<p>The disadvantage of muons is that, unlike electrons and protons, they are unstable and need to be produced and then rapidly used before they all decay. We can produce muons by taking a narrow, high-intensity beam of protons and running it into a target made of a metal, such as titanium. This produces a beam of another fundamental particle called the pion. </p>
<p><a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/hadron.html">Pions</a> form a beam which fans out. If the original proton beam looks like a laser pointer, the pion beam looks more like a torch beam – with intensity dropping rapidly with distance. The pions then decay to produce the muons, meaning the beam spreads even more – resembling a light bulb. </p>
<p>We cannot accelerate such a beam in a machine like the LHC, so we need to create a beam which spreads out much less. This is challenging given that we have only have two millionths of a second to produce, accelerate and collide it.</p>
<p>But our team of physicists and engineers, from around the world – known as the <a href="http://mice.iit.edu/">Muon Ionisation Cooling Experiment</a> (MICE) – has now shown that it is possible. We used a process known as cooling to help compress the beam. This involves passing the muons through a container with liquid hydrogen at -250°C, slowing the particles down. We then passed them through an electromagnetic cavity, which caused the beam to accelerate in the required direction.</p>
<p>By repeating this several times it is possible to create a beam which spreads out much less and has a dense core. This beam can be injected into a particle accelerator to produce a high-energy muon beam. Such a beam can either be collided or left to circulate until the muons decay into an intense beam of neutrinos – far in excess of any neutrino beam which can currently be produced. </p>
<h2>Probing the universe</h2>
<p>In fact, a neutrino beam created from muons is part of a planned <a href="https://stfc.ukri.org/research/accelerator-science/neutrino-factory/">Neutrino Factory</a>, which would allow us to answer many questions related to the origin and evolution of the universe – such as the mysterious imbalance between matter and antimatter. </p>
<p>Neutrinos could also help us understand the details of how life-essential elements such as oxygen, carbon and silicon, which are formed in stars, spread across universe. These heavier elements were not produced in the Big Bang and yet are responsible for the planet we live on and all the life around us. We know that bursts of neutrinos, which are released in star explosions (supernovas), are responsible. </p>
<p>We could also collide two beams of muons in the same way as we collide protons at the LHC. The muons, being simpler than the protons, would allow for more precise determination of the properties of the Higgs particle, for example.</p>
<p>The properties of the muon also makes it an invaluable tool in the field of material physics. The ability to create more closely focused beams may improve the current measurements and open up new diagnostic methods.</p>
<p>Our method may also be used to help increase the intensity of any other charged particle beams. It has been a long project lasting over a dozen years, but it has been worth the effort when we consider what a powerful tool we have created.</p><img src="https://counter.theconversation.com/content/131415/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paul Kyberd receives funding from STFC.</span></em></p>When scientists created the Higgs particle with protons, they needed the 10km-wide Large Hadron Collider. A muon machine could achieve it with a diameter of just 200 metres.Paul Kyberd, Senior Lecturer in Particle Physics Informatics, Brunel University LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1222302019-08-23T10:37:26Z2019-08-23T10:37:26ZGhost particles: how galaxies helped us weigh the lightest neutrino – and why it matters<figure><img src="https://images.theconversation.com/files/289201/original/file-20190823-170931-99idv2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Crab Nebula is a remnant of a supernova, a source of neutrinos.</span> <span class="attribution"><span class="source">NASA, ESA, J. Hester and A. Loll (Arizona State University)</span></span></figcaption></figure><p>Even when you close your eyes at night, 100 billion neutrinos produced in the sun will pass through them – <a href="https://theconversation.com/benefits-of-knowing-more-about-neutrinos-which-pass-through-our-bodies-unnoticed-49025">travelling close to the speed of light, but never hitting anything</a>. Neutrinos are extremely elusive and only weakly interact with matter around them: nature’s true ghosts. Until very recently, these tiny particles were believed to be massless.</p>
<p>In the late 1990s, <a href="https://theconversation.com/how-the-neutrino-could-solve-great-cosmic-mysteries-and-win-its-next-nobel-prize-48789">researchers demonstrated</a> that neutrinos constantly change between three different types (flavours or species), which affects how they interact with matter. This is something they can only do if they have mass – a discovery that was <a href="https://theconversation.com/physics-duo-wins-the-nobel-prize-for-solving-longstanding-neutrino-puzzle-48702">granted the Nobel Prize in 2015</a>. From these particle physics experiments, we know that at least two of the three neutrino species have mass. </p>
<p>Yet little has been known about the mass of the lightest species – until now. Our new study, <a href="https://www.ucl.ac.uk/news/2019/aug/maximum-mass-lightest-neutrino-revealed-using-astronomical-big-data">published in Physical Review Letters</a>, shows that the lightest neutrino is at least 6m times lighter than the mass of an electron, at 0.086 electron volts (a unit of energy). Our technique may give exact masses for each neutrino in the future.</p>
<p>Neutrinos are peculiar. Thanks to the <a href="https://theconversation.com/physicists-prove-quantum-spookiness-and-start-chasing-schrodingers-cat-48190">strange rules of quantum mechanics</a>, the relationships between the flavours and their masses are complicated. In any beam of neutrinos, the three masses are always present but in different ratios. Each flavour of neutrino has a combination of the three masses and each neutrino mass has a combination of the three flavours.</p>
<h2>Massive significance</h2>
<p>Neutrinos matter to understanding space. Back to the 1940s, <a href="https://journals.aps.org/pr/pdf/10.1103/PhysRev.58.1117">a letter to Physical Review</a> by physicists <a href="https://cosmosmagazine.com/space/this-week-in-science-history-big-bang-theorist-george-gamow-dies">George Gamow</a> and <a href="https://en.wikipedia.org/wiki/M%C3%A1rio_Schenberg">Mario Schoenberg</a> (a personal hero of mine) suggested that neutrinos played a major role in stellar evolution and supernovas (exploding stars). This <a href="https://www.nasa.gov/feature/goddard/2017/the-dawn-of-a-new-era-for-supernova-1987a/">was confirmed</a> when scientists detected the first neutrinos from a supernova in 1987, providing a better understanding of supernovas and neutrinos alike.</p>
<p>On a cosmological scale, however, because these ghostly particles have mass, they tend to drag a little bit of matter with them thanks to gravity. And so the more massive the neutrinos are, the “fuzzier” the distribution of galaxies around us will be. This means that by observing galaxies around us, we can infer the mass of neutrinos. It is amazing to think that the largest structures of galaxies in the universe are sensitive to the tiniest particles known by physics. </p>
<p>The reason scientists are so keen to find out their mass, is that it matters to our ultimate understanding of reality. <a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">The Standard Model of Particle Physics</a> is one of the most precise theories of fundamental particles that we have so far. However, this theory predicts that neutrinos should be massless.</p>
<p>Understanding neutrino masses is a key point to move ahead towards a new and improved theory of particle physics. It is quite possible that by doing so, other mysteries in physics, which also cannot be explained by the standard model, such as the nature of <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a> and <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">dark matter</a>, would also be solved.</p>
<h2>Two camps</h2>
<p>The way we made our discovery matters, too. Our international team of researchers from the UK and Brazil combined cosmological data and particle physics experiments.</p>
<p>Each approach has its limitations. When cosmologists determine neutrino masses from observations of galaxy distributions, they can only determine a maximum mass for the sum of the three neutrinos. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=415&fit=crop&dpr=1 600w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=415&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=415&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=522&fit=crop&dpr=1 754w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=522&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=522&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Hercules galaxy cluster.</span>
<span class="attribution"><span class="source">ESO/INAF-VST/OmegaCAM</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Particle physics experiments can directly study neutrinos, for example by <a href="https://www.symmetrymagazine.org/article/november-2012/how-to-make-a-neutrino-beam">creating a beam</a> of them in the lab or detecting neutrinos from space underground. But most of them are based on detecting how neutrino flavours mix. This doesn’t tell you the absolute mass for each particle, but only <a href="https://www.sciencedirect.com/science/article/pii/S0550321316000602?via%3Dihub">the difference in mass</a> for two out of three neutrino species. It also doesn’t tell us which of the two is the heaviest neutrino.</p>
<p>Sadly, many cosmologists often ignore results from particle physics. And some particle physicists are sceptical of cosmologists’ statistical techniques, claiming their way of using prior knowledge, such as estimating that the mass cannot be negative, influences their results.</p>
<p>We combined the two by creating a mathematical model to calculate the sum of the neutrino masses by studying the large scale structure of more than a million galaxies from the <a href="http://www.sdss3.org/surveys/boss.php">Baryon Oscillation Spectroscopic Survey (BOSS)</a>. We also had to include many other parameters that affect galaxy distribution such as dark matter and dark energy. We then fed in what we know from particle physics experiments, which told us in very exact terms what the relationships between the masses of the neutrinos should be. This allowed us to calculate an upper limit of the mass of the lightest particle.</p>
<p>In order to achieve these results, we had to push the boundaries of big data analysis in cosmology to their limits, using more than half a million computing hours to process the data. Luckily, we had a <a href="https://www.ucl.ac.uk/research-it-services/grace-launch">super computer</a> to help us. </p>
<p>Although current available data is not powerful enough to detect a lower-bound for the mass of the lightest neutrino – it could still be massless– this work demonstrates that applying a combined methodology is the way forward. </p>
<p><a href="https://www.desi.lbl.gov">The Dark Energy Spectroscopic Instrument (DESI)</a>, which will analyse ten times more galaxies than we had access to, will give a much more reliable estimate of the total mass for all the neutrinos – and that may make it possible to work out a minimum mass. Excitingly, this might just launch a new era in neutrino physics.</p><img src="https://counter.theconversation.com/content/122230/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Arthur Loureiro received funding from the National Council for Science and Technology (CNPq), Brazil.</span></em></p>If we want an improved theory of particle physics, understanding neutrino masses is key.Arthur Loureiro, Research Assistant in Cosmology and Astroparticle Physics, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1001552018-08-28T21:06:19Z2018-08-28T21:06:19ZNew era of astronomy uncovers clues about the cosmos<figure><img src="https://images.theconversation.com/files/231953/original/file-20180814-2894-1tzyen8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">An illustration of two neutron stars spinning around each other while merging.</span> <span class="attribution"><span class="source"> NASA/CXC/Trinity University/D. Pooley et al.</span></span></figcaption></figure><p>Astronomers have had a blockbuster year. </p>
<p>In addition to tracking down <a href="https://theconversation.com/the-icecube-observatory-detects-neutrino-and-discovers-a-blazar-as-its-source-99720">a cosmic source of neutrinos</a>, they have detected the merger of <a href="https://theconversation.com/at-last-weve-found-gravitational-waves-from-a-collapsing-pair-of-neutron-stars-85528">two city-sized neutron stars, each more massive than the sun</a>. </p>
<p>The <a href="https://www.ligo.org/science/Publication-GW170817MMA/">discoveries were heralded</a> as evidence that a “<a href="https://www.ligo.org/science/Publication-GW170817MMA/">new era of multimessenger astronomy</a>” had arrived. </p>
<p>But what is multimessenger astronomy? </p>
<p>In our daily lives, we interpret the world around us based on different signals, such as sound waves, light (a type of electromagnetic wave) and skin pressure. Each of these signals may be carried by a different “messenger.” New messengers lead to new insights. So astronomers have eagerly welcomed a new set of messengers to their science.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/230974/original/file-20180807-191044-7ega78.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/230974/original/file-20180807-191044-7ega78.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=432&fit=crop&dpr=1 600w, https://images.theconversation.com/files/230974/original/file-20180807-191044-7ega78.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=432&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/230974/original/file-20180807-191044-7ega78.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=432&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/230974/original/file-20180807-191044-7ega78.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=543&fit=crop&dpr=1 754w, https://images.theconversation.com/files/230974/original/file-20180807-191044-7ega78.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=543&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/230974/original/file-20180807-191044-7ega78.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=543&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Twenty-seven radio antennas make up the Karl G. Very Large Array in New Mexico. The VLA is an important tool for studying cosmic radio waves.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<h2>Many messengers</h2>
<p>For most of the history of astronomy, scientists primarily studied signals transmitted by one messenger, electromagnetic radiation. These waves, which move through space and time, are described by their wavelengths or the amount of energy found in their particles, the photons.</p>
<p>Radio waves have photons with the lowest amount of energy and the longest wavelengths, followed by infrared and optical light at intermediate energies and wavelengths. X-rays and gamma-rays have the shortest wavelengths and the highest energy. </p>
<p>But scientists study others messengers too: </p>
<ul>
<li>Cosmic rays: charged atomic particles and nuclei travelling near the speed of light.</li>
<li>Neutrinos: uncharged particles that see most of the universe as transparent.</li>
<li>Gravitational waves: wrinkles in the very fabric of space and time.</li>
</ul>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/230992/original/file-20180807-160647-1ql3xqb.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/230992/original/file-20180807-160647-1ql3xqb.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/230992/original/file-20180807-160647-1ql3xqb.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/230992/original/file-20180807-160647-1ql3xqb.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/230992/original/file-20180807-160647-1ql3xqb.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/230992/original/file-20180807-160647-1ql3xqb.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/230992/original/file-20180807-160647-1ql3xqb.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The four messengers of astronomy.</span>
<span class="attribution"><span class="source">Adapted from IceCube Collaboration</span></span>
</figcaption>
</figure>
<p>And while some fields in astronomy have explored these messengers for years, astronomers have only recently observed events from well beyond the Milky Way with more than one messenger at the same time. In just a few months, the number of sources where astronomers can piece together the signals from different messengers doubled.</p>
<h2>Like a walk on the beach</h2>
<p>Multimessenger astronomy is a natural evolution of astronomy. Scientists need more data to put together a complete picture of the objects they study and match the theories they develop with their observations. </p>
<p>Astronomers have combined different wavelengths of photons to piece together some of the mysteries of the universe. For example, the combination of radio and optical data played a major role in determining that the Milky Way is a spiral galaxy in 1951.</p>
<p>And astronomy continues to reveal great results about our universe using just one messenger, photons. So if multimessenger astronomy is just an evolutionary step of an incredible history of successes, does that mean it’s just a new buzzword?</p>
<p>We don’t think so.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/228950/original/file-20180724-189313-s1tiw7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/228950/original/file-20180724-189313-s1tiw7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=316&fit=crop&dpr=1 600w, https://images.theconversation.com/files/228950/original/file-20180724-189313-s1tiw7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=316&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/228950/original/file-20180724-189313-s1tiw7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=316&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/228950/original/file-20180724-189313-s1tiw7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=398&fit=crop&dpr=1 754w, https://images.theconversation.com/files/228950/original/file-20180724-189313-s1tiw7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=398&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/228950/original/file-20180724-189313-s1tiw7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=398&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">An artistic rendition of NASA’s Chandra X-ray Observatory. This space satellite produces the most detailed X-ray images of high energy astrophysical phenomena.</span>
<span class="attribution"><span class="source">NGST</span></span>
</figcaption>
</figure>
<p>Imagine you are walking along an ocean beach. You are enjoying the sight of an incredible sunset, hearing the rolling waves, feeling the sand beneath your feet and smelling the salty air. Your combined senses form a more complete experience. </p>
<p>With multimessenger astronomy, we hope to learn more from the universe by combining multiple messengers, just as we combine sight, hearing, touch and smell.</p>
<h2>But it’s not always a picnic</h2>
<p>The cultures of astronomers and particle physicists represent different approaches to science. In multimessenger astronomy, these cultures collide.</p>
<p>Astronomy is an observational field and not an experiment. We study astronomical objects that change over time (time-domain astronomy), which means we often have only one chance to observe a transient astronomical event.</p>
<p>Until recently, most time-domain astronomers worked in small teams, on many projects at once. We use resources like <a href="http://www.astronomerstelegram.org/">The Astronomer’s Telegram</a> or the <a href="https://gcn.gsfc.nasa.gov/">Gamma-ray Coordination Network</a> to rapidly communicate results, even before submitting scientific papers.</p>
<p>Since most of the expected sources of multimessenger signals are transient astronomical events, it’s a huge effort to capture the messengers besides photons.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-icecube-observatory-detects-neutrino-and-discovers-a-blazar-as-its-source-99720">The IceCube observatory detects neutrino and discovers a blazar as its source</a>
</strong>
</em>
</p>
<hr>
<p>Particle physicists have led the way in creating large international collaborations to tackle their hardest problems, including the <a href="https://home.cern/topics/large-hadron-collider">Large Hadron Collider</a>, the <a href="https://icecube.wisc.edu/">IceCube Neutrino Observatory</a> and the <a href="https://www.ligo.caltech.edu/">Laser Interferometer Gravitational-Wave Observatory (LIGO)</a>. Corralling hundreds to thousands of researchers to work towards common goals requires comprehensive identification of roles, strict communication guidelines and many teleconferences.</p>
<p>The need to respond to rapid changes in a multimessenger source and the huge effort to capture multimessenger signals means astronomy and particle physics must merge towards one another to elicit the best of both cultures.</p>
<h2>The benefits of multimessenger astronomy</h2>
<p>While multimessenger astronomy is an evolution of what astronomers and particle physicists have done for decades, the combined results are intriguing.</p>
<p>The detection of gravitational waves from merging neutron stars confirmed that <a href="https://theconversation.com/cosmic-alchemy-colliding-neutron-stars-show-us-how-the-universe-creates-gold-86104">these collisions made a large fraction of the gold and platinum</a> on Earth (and throughout the universe). It also showed how these collisions give rise to (at least some) <a href="https://theconversation.com/how-we-discovered-gravitational-waves-from-neutron-stars-and-why-its-such-a-huge-deal-85647">short gamma-ray bursts</a> — the origin of these explosive events has been a huge open question in astronomy. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/228936/original/file-20180723-189310-18799bp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/228936/original/file-20180723-189310-18799bp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/228936/original/file-20180723-189310-18799bp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/228936/original/file-20180723-189310-18799bp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/228936/original/file-20180723-189310-18799bp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/228936/original/file-20180723-189310-18799bp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/228936/original/file-20180723-189310-18799bp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The IceCube Neutrino Observatory used a cubic kilometre of crystal-clear Antarctic ice to capture the signal of a rare neutrino that helped pinpoint a galaxy four billion light years away with a supermassive black hole launching a jet of photons and near light-speed particles directly at our Solar System.</span>
<span class="attribution"><span class="source">IceCube Collaboration/NSF</span></span>
</figcaption>
</figure>
<p>The first association of a neutrino with a single astronomical source provided a glimpse into how the universe makes its most energetic particles. Multimessenger astronomy is revealing details about some of the most extreme conditions in our universe.</p>
<p>The multimessenger perspective is already yielding more than the sum of its parts — and we can expect to see more surprising discoveries in the future. Elite teams across Canada are already contributing to the growth of this young field, and multimessenger astronomy promises to play a major role in our next decade of astronomical research in Canada — and across the world.</p><img src="https://counter.theconversation.com/content/100155/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gregory Sivakoff receives funding from Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation (CFI), and Alberta Economic Development and Trade (EDT). Gregory Sivakoff is a member of several professional societies for astronomy and physics, including the Canadian Astronomical Society (CASCA), Canadian Association of Physicists (CAP), American Astronomical Society (AAS), and the AAS High Energy Astrophysics Division (HEAD). Gregory Sivakoff also serves on the Council of the American Association of Variable Star Observers (AAVSO), a non-profit citizen astronomy organization.
</span></em></p><p class="fine-print"><em><span>Daryl Haggard receives funding from the Canadian Institute for Advanced Research (CIFAR) Azrieli Global Scholars Program, Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de Recherche du Québec – Nature et technologies (FRQNT). Daryl Haggard is a member of several professional societies for astronomy and physics, including the Canadian Astronomical Society (CASCA), Canadian Association of Physicists (CAP), American Astronomical Society (AAS), and the AAS High Energy Astrophysics Division (HEAD). Daryl Haggard also serves on the Laser Interferometer Gravitational-Wave Observatory (LIGO) Program Advisory Committee.</span></em></p>Astronomers are now able to detect a host of signals streaming through the universe. This newfound ability is like gaining new senses and it’s opening the door to understanding the cosmos.Gregory Sivakoff, Associate Professor, University of AlbertaDaryl Haggard, Assistant Professor of Physics, McGill UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/997202018-07-12T15:08:26Z2018-07-12T15:08:26ZThe IceCube observatory detects neutrino and discovers a blazar as its source<figure><img src="https://images.theconversation.com/files/227226/original/file-20180711-27039-129zs7l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">heic a</span> </figcaption></figure><p>About four billion years ago, when the planet Earth was still in its infancy, the axis of a black hole about one billion times more massive than the sun happened to be pointing right to where our planet was going to be on September 22, 2017. </p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/227227/original/file-20180711-27015-xp3xne.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/227227/original/file-20180711-27015-xp3xne.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=776&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227227/original/file-20180711-27015-xp3xne.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=776&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227227/original/file-20180711-27015-xp3xne.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=776&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227227/original/file-20180711-27015-xp3xne.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=976&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227227/original/file-20180711-27015-xp3xne.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=976&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227227/original/file-20180711-27015-xp3xne.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=976&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Blazar shoots neutrinos and gamma rays to Earth: Blazars are a type of active galactic nucleus with one of its jets pointing toward us. In this artistic rendering, a blazar emits both neutrinos and gamma rays that could be detected by the IceCube Neutrino Observatory as well as by other telescopes on Earth and in space.</span>
<span class="attribution"><span class="source">IceCube/NASA</span></span>
</figcaption>
</figure>
<p>Along the axis, a high-energy jet of particles sent photons and neutrinos racing in our direction at or near the speed of light. The IceCube Neutrino Observatory at the South Pole detected one of these subatomic particles – the IceCube-170922A neutrino – and traced it back to a small patch of sky in the constellation Orion and pinpointed the cosmic source: a flaring black hole the size of a billion suns, 3.7 billion light years from Earth, known as blazar TXS 0506+056. Blazars have been known about for some time. What wasn’t clear was that they could produce <a href="http://doi.org/10.1126/science.aat2890">high-energy neutrinos</a>. Even more exciting was such neutrinos had never before been traced to its source. </p>
<p>Finding the cosmic source of high-energy neutrinos for the first time, announced on July 12, 2018 by the National Science Foundation, marks the dawn of a new era of neutrino astronomy. Pursued in fits and starts since 1976, when pioneering physicists first tried to build a <a href="https://www.phys.hawaii.edu/%7Edumand/">large-scale high-energy neutrino detector off the Hawaiian coast</a>, IceCube’s discovery marks the triumphant conclusion of a long and difficult campaign by many hundreds of scientists and engineers – and simultaneously the birth of a completely new branch of astronomy.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=749&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=749&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=749&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=941&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=941&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227255/original/file-20180711-27015-1t34tjo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=941&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The constellation of Orion, with a bullseye on the location of the blazar.</span>
<span class="attribution"><span class="source">Silvia Bravo Gallart/ Project_WIPAC_Communications</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The <a href="http://doi.org/10.1126/science.aat1378">detection of two distinct astronomical messengers</a> -– neutrinos and light –- is a powerful demonstration of how so-called multimessenger astronomy can provide the leverage we need to identify and understand some of the most energetic phenomena in the universe. Since its discovery as a neutrino source less than a year ago, blazar TXS 0506+056 has been the subject of intensive scrutiny. Its associated stream of neutrinos continues to provide deep insights into the physical processes at work near the black hole and its powerful jet of particles and radiation, beamed almost directly toward Earth from its location just off the shoulder of Orion. </p>
<p>As three scientists in a global team of physicists and astronomers involved in this remarkable discovery, we were drawn to participate in this experiment for its sheer audacity, for the physical and emotional challenge of working long shifts at in a brutally cold location while inserting expensive, sensitive equipment into holes drilled 1.5 miles deep in the ice and making it all work. And, of course, for the thrilling opportunity to be the first people to peer into a brand new kind of telescope and see what it reveals about the heavens.</p>
<h2>A remote, frigid neutrino detector</h2>
<p>At an altitude exceeding 9,000 feet and with average summertime temperatures rarely breaking a frigid -30 Celsius, the South Pole may not strike you as the ideal place to do anything, aside from bragging about visiting a place that is so sunny and bright you need sunscreen for your nostrils. On the other hand, once you realize that the altitude is due to a thick coat of ultrapure ice made from several hundred thousand years of pristine snowfall and that the low temperatures have kept it all nicely frozen, then it might not surprise you that for neutrino telescope builders, the scientific advantages outweigh the forbidding environment. The South Pole is now the home of the world’s largest neutrino detector, <a href="https://icecube.wisc.edu">IceCube</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227258/original/file-20180711-27021-mf828x.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">March 2015: The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers that collect raw data from the detector. Due to satellite bandwidth allocations, the first level of reconstruction and event filtering happens in near real time in this lab.</span>
<span class="attribution"><a class="source" href="https://icecube.wisc.edu/gallery/press/view/2085">Erik Beiser, IceCube/NSF</a></span>
</figcaption>
</figure>
<p>It may seem odd that we need such an elaborate detector given that about 100 billion of these fundamental particles sashay right through your thumbnail each second and glide effortlessly through the entire Earth without interacting with a single earthly atom. </p>
<p>In fact, neutrinos are the second most ubiquitous particles, second only to the cosmic microwave background photons left over from the Big Bang. They comprise one-quarter of known fundamental particles. Yet, because they barely interact with other matter, they are arguably the least well understood. </p>
<p>To catch a handful of these elusive particles, and to discover their sources, physicists need big – kilometer-wide – detectors made of an optically clear material – like ice. Fortunately Mother Nature provided this pristine slab of clear ice where we could build our detector. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=410&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=410&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=410&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=515&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=515&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227257/original/file-20180711-27015-15151as.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=515&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The IceCube Neutrino Observatory instruments a volume of roughly one cubic kilometer of clear Antarctic ice with 5,160 digital optical modules (DOMs) at depths between 1,450 and 2,450 meters. The observatory includes a densely instrumented subdetector, DeepCore, and a surface air shower array, IceTop.</span>
<span class="attribution"><a class="source" href="http://gallery.icecube.wisc.edu/web/var/albums/WWW_GALLERY/IceCube-Breakthrough/ArrayWSeasonsLabelsAmanda.jpg?m=1386800062">Felipe Pedreros, IceCube/NSF</a></span>
</figcaption>
</figure>
<p>At the South Pole several hundred scientists and engineers have constructed and deployed over 5,000 individual photosensors in 86 separate 1.5-mile-deep holes melted in the polar ice cap with a custom-designed hot-water drill. Over the course of seven austral summer seasons we installed all the sensors. The IceCube array was fully installed in early 2011 and has been taking data continuously since.</p>
<p>This array of ice-bound detectors can sense with great precision when a neutrino flies through and interacts with a few Earthly particles that generate dim patterns of bluish Cherenkov light, given off when charged particles move through a medium like ice at close to light speed.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/OOWNI0iNGo0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Blazar emission reaches Earth: Gamma rays (magenta), the most energetic form of light, and elusive particles called neutrinos (gray) formed in the jet of an active galactic nucleus far, far away. The radiation traveled for about 4 billion years before reaching Earth. The IceCube Neutrino Observatory at the South Pole detected the arrival of neutrino IC170922 entering Antarctica on Sept. 22, 2017. After the interaction with a molecule of ice, a secondary high-energy particle – a muon – enters IceCube, leaving a trace of blue light behind it. Credit: NASA’s Goddard Space Flight Center/CI Lab/Nicolle R. Fuller/NSF/IceCube.</span></figcaption>
</figure>
<h2>Neutrinos from the cosmos</h2>
<p>The Achilles’ heel of neutrino detectors is that other particles, originating in the nearby atmosphere, can also trigger these patterns of bluish Cherenkov light. To eliminate these false signals, the detectors are buried deep in the ice to filter out interference before it can reach the sensitive detector. But in spite of being under nearly a mile of solid ice, IceCube still faces an onslaught of about 2,500 such particles every second, each of which could plausibly have been due to a neutrino. </p>
<p>With the expected rate of interesting, real astrophysical neutrino interactions (like incoming neutrinos from a black hole) hovering at about one per month, we were faced with a daunting needle-in-a-haystack problem.</p>
<p>The IceCube strategy is to look only at events with such high energy that they are exceedingly unlikely to be atmospheric in origin. With these selection criteria and several years of data, IceCube discovered the astrophysical neutrinos it had long been seeking, but it could not identify any individual sources – such as active galactic nuclei or gamma-ray bursts – among the several dozen high-energy neutrinos it had captured. </p>
<p>To tease out actual sources, IceCube began distributing neutrino arrival alerts in April 2016 with help from the <a href="http://www.amon.psu.edu/">Astrophysical Multimessenger Observatory Network</a> at Penn State. Over the course of the next 16 months, 11 IceCube-AMON neutrino alerts were distributed via AMON and the Gamma-ray Coordinates Network, just minutes or seconds after being detected at the South Pole.</p>
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<a href="https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=336&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=336&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=336&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=423&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=423&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227223/original/file-20180711-27042-bxmmo1.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=423&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">On Sept. 22, 2017, IceCube alerted the international astronomy community about the detection of a high-energy neutrino. About 20 observatories on Earth and in space made follow-up observations, which allowed identification of what scientists deem to be a source of very high energy neutrinos and, thus, of cosmic rays. Besides neutrinos, the observations made across the electromagnetic spectrum included gamma-rays, X-rays, and optical and radio radiation. These observatories are run by international teams with a total of more than 1,000 scientists supported by funding agencies in countries around the world.</span>
<span class="attribution"><span class="source">Nicolle R. Fuller/NSF/IceCube</span></span>
</figcaption>
</figure>
<h2>A new window on the universe</h2>
<p>The alerts triggered an automated sequence of X-ray and ultraviolet observations with NASA’s <a href="https://swift.gsfc.nasa.gov">Neil Gehrels Swift Observatory</a> and led to further studies with NASA’s <a href="https://fermi.gsfc.nasa.gov">Fermi Gamma-Ray Space Telescope</a> and <a href="https://www.nasa.gov/mission_pages/nustar/main/index.html">Nuclear Spectroscopic Telescope Array</a>, and 13 other observatories around the world.</p>
<p>Swift was the first facility to identify the flaring blazar TXS 0506+056 as a possible source of the neutrino event. The <a href="https://www-glast.stanford.edu">Fermi Large Area Telescope</a> then reported that the blazar was in a flaring state, emitting many more gamma-rays than it had in the past. As the news spread, other observatories enthusiastically jumped on the bandwagon and a broad range of observations ensued. The MAGIC ground-based telescope noted our neutrino came from a region producing very high-energy gamma-rays (each about ten million times more energetic than an X-ray), the first time such a coincidence has ever been observed. Other optical observations completed the puzzle by measuring the distance to blazar TXS 0506+056: about four billion light years from Earth.</p>
<p>With the first-ever identification of a cosmic source of high-energy neutrinos, a new branch on the astronomy tree has sprouted. As high-energy neutrino astronomy grows with more data, improved inter-observatory coordination, and more sensitive detectors, we will be able to map the neutrino sky with better and better precision. </p>
<p>And we expect exciting new breakthroughs in our understanding of the universe to follow suit, such as: solving the century-old mystery of the origin of astoundingly energetic cosmic rays; testing if spacetime itself is foamy, with quantum fluctuations at very small distance scales, as predicted by certain theories of quantum gravity; and figuring out exactly how cosmic accelerators, like those around the TXS 0506+056 black hole, manage to accelerate particles to such breathtakingly high energies.</p>
<p>For 20 years, the IceCube Collaboration had a dream to identify the sources of high-energy cosmic neutrinos – and this dream is now a reality.</p><img src="https://counter.theconversation.com/content/99720/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Doug Cowen receives funding from the National Science Foundation, which also supports the IceCube experiment. </span></em></p><p class="fine-print"><em><span>Derek Fox receives funding from the National Science Foundation, which also supports the IceCube experiment. </span></em></p><p class="fine-print"><em><span>Azadeh Keivani does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A detector buried under more than a mile of ice in Antarctica has detected a high-energy subatomic neutrino and traced it to its origin, a blazar – a gargantuan black hole more than a billion times more massive than the sun.Doug Cowen, Professor of Physics and Professor of Astronomy & Astrophysics, Penn StateAzadeh Keivani, Frontiers of Science Fellow, Columbia UniversityDerek Fox, Associate Professor of Astronomy and Astrophysics, Penn StateLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/997852018-07-12T15:03:53Z2018-07-12T15:03:53ZScientists discover a new source of neutrinos in space – opening up another window into the universe<figure><img src="https://images.theconversation.com/files/227377/original/file-20180712-27015-1c0dy1f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's impression based on real picture of Icecube lab.</span> <span class="attribution"><span class="source">IceCube/NSF</span></span></figcaption></figure><p>Neutrinos – extremely light, ghostly particles that barely interact with matter – have so far only been observed originating from supernovae (exploding stars) and the sun. Now a giant detector at the South Pole has discovered that a “blazar”, a galaxy with a supermassive black hole at its centre, also produces neutrinos.</p>
<p>This is the first time a source of neutrinos in space has been discovered in more than 30 years. What’s more, it’s the first time scientists have observed a neutrino particle with high energy associated with an astrophysical event. This is really exciting news. The observation, <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aat2890">just published in Science</a>, opens a completely new chapter in neutrino astronomy.</p>
<p>Neutrinos are fundamental matter particles. The ordinary matter that we are all familiar with is made out of electrons and quarks. We do not observe neutrinos in daily life as they are extremely hard to detect. Theoretical physicist <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1945/pauli-bio.html">Wolfgang Pauli</a> suggested their existence in 1930, but it took until 1956 before they were first seen by experimental physicists <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1995/illpres/neutrino.html">coming from a nuclear reactor</a>.</p>
<p>The reason that they are so hard to detect is because they only weakly interact with ordinary matter. Most neutrinos fly straight through the Earth: they do not interact at all. They do, however, play a very important role in the universe. </p>
<p>For example, when a heavy star explodes at the end of its life, it is known as a supernova – as it shows up as an extremely bright and seemingly new star in the sky. We now know that supernovae emit many more neutrinos than photons (light particles), which we cannot see by eye. Scientists detected the first <a href="https://www.nasa.gov/feature/goddard/2017/the-dawn-of-a-new-era-for-supernova-1987a">neutrinos from a supernova in 1987</a> when a star collapsed just outside our Milky Way.
This unique observation has given us a better understanding of supernovae, as well as the properties of neutrinos themselves. </p>
<p>This event marked the birth of what we call neutrino astronomy. Powerful neutrino telescopes were built soon after. One of them was the <a href="https://www.sno.phy.queensu.ca/">Sudbury Neutrino Observatory (SNO)</a>. Physicist Art McDonald <a href="https://theconversation.com/how-the-neutrino-could-solve-great-cosmic-mysteries-and-win-its-next-nobel-prize-48789">received the Nobel Prize for Physics in 2015</a> for the detailed studies of solar neutrinos that he and his team did using this observatory, and the insights this gave us into the properties of the neutrino particles. </p>
<h2>Arctic analysis</h2>
<p>Another telescope has now grown to be the largest of them all. <a href="https://icecube.wisc.edu/">The IceCube experiment at the South Pole</a> is a cubic kilometre in size and uses deep arctic ice as a target for the neutrinos. Although neutrinos typically don’t interact with anything, they can produce a charged particle when they occasionally do interact with the fundamental particles that make up ice. In IceCube, this resulting particle travels through the ice and produces a trail of faint light. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/227379/original/file-20180712-27021-1p7f1nz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227379/original/file-20180712-27021-1p7f1nz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=159&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227379/original/file-20180712-27021-1p7f1nz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=159&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227379/original/file-20180712-27021-1p7f1nz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=159&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227379/original/file-20180712-27021-1p7f1nz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=199&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227379/original/file-20180712-27021-1p7f1nz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=199&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227379/original/file-20180712-27021-1p7f1nz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=199&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">IceCube drilling tower and hose reel in December 2009.</span>
<span class="attribution"><span class="source">Amble/wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>This trail is captured by a large array of sensitive photodetectors that are mounted up to three kilometres deep into the ice. With this information, IceCube can detect high energy neutrinos, measure their energy and determine where they came from. Other cosmic particles only travel a few kilometres through the Earth, at most. So, if the particle is seen to come up from below, it must have been produced by a neutrino interaction, as it is the only particle that can travel such a large distance through the planet.</p>
<p>The neutrino that was observed by IceCube in September 2017 is very special. This neutrino must have had an extremely high energy – IceCube scientists estimate between 183 and 290 trillion electron volts (a unit of energy). That is 28-45 times more energy than the particles in the beam of the <a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">Large Hadron Collider </a> at CERN, the world’s most powerful particle accelerator. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/227380/original/file-20180712-27012-1m957qz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227380/original/file-20180712-27012-1m957qz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227380/original/file-20180712-27012-1m957qz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227380/original/file-20180712-27012-1m957qz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227380/original/file-20180712-27012-1m957qz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227380/original/file-20180712-27012-1m957qz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227380/original/file-20180712-27012-1m957qz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Sensors below the ice detected the neutrino, which was registered by computers in the IceCube building.</span>
<span class="attribution"><span class="source">IceCube/NSF</span></span>
</figcaption>
</figure>
<p>However, neutrinos with even higher energies have been observed by IceCube before. The exciting thing about the new discovery is that it has been shown to come from a blazar, which has been observed by other experiments, such as the <a href="https://www-glast.stanford.edu/instrument.html">FermiLAT satellite</a> and the <a href="https://magic.mpp.mpg.de/">MAGIC telescope</a>. In a <a href="https://www.youtube.com/watch?time_continue=2&v=DK9TMnC7n8E">blazar</a>, it is thought that the supermassive black hole at the centre absorbs matter to produce two extremely powerful jets of radiation. These jets could act as powerful particle accelerators. </p>
<p>Blazars were long suspected as a possible source of very high energy neutrinos in the universe, but we now have firm evidence. Together with IceCube, observations of this blazar have been made using telescopes that are sensitive to different types of electromagnetic radiation: radio, optical, gamma ray, and X-ray. </p>
<p>With this observation, IceCube has made a significant step forward in neutrino astronomy. Its neutrino adds new information to the observation of the blazar, helping us to understand these fascinating objects better. It can tell us about the mechanism of particle acceleration in blazars and more about how blazars produce such tremendous amounts of energy. We may even learn something new about the universe, or neutrinos, that we didn’t expect.</p><img src="https://counter.theconversation.com/content/99785/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Simon Peeters does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A giant detector at the South Pole has observed a neutrino from a black hole in a distant galaxy for the first time.Simon Peeters, Reader (Physics and Astronomy), University of SussexLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/947002018-05-23T10:39:19Z2018-05-23T10:39:19ZThe Standard Model of particle physics: The absolutely amazing theory of almost everything<figure><img src="https://images.theconversation.com/files/219824/original/file-20180521-14978-36nv6i.jpg?ixlib=rb-1.1.0&rect=174%2C0%2C977%2C649&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How does our world work on a subatomic level?</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Varsha_ys.jpg">Varsha Y S</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The Standard Model. What a dull name for the most accurate scientific theory known to human beings.</p>
<p>More than a quarter of the Nobel Prizes in physics of the last century are direct inputs to or direct results of the Standard Model. Yet its name suggests that if you can afford a few extra dollars a month you should buy the upgrade. <a href="https://scholar.google.com/citations?user=eQiX0m4AAAAJ&hl=en&oi=ao">As a theoretical physicist</a>, I’d prefer The Absolutely Amazing Theory of Almost Everything. That’s what the Standard Model really is.</p>
<p>Many recall the excitement among scientists and media over the 2012 <a href="https://home.cern/topics/higgs-boson">discovery of the Higgs boson</a>. But that much-ballyhooed event didn’t come out of the blue – it capped a five-decade undefeated streak for the Standard Model. Every fundamental force but gravity is included in it. Every attempt to overturn it to demonstrate in the laboratory that it must be substantially reworked – and there have been many over the past 50 years – has failed. </p>
<p>In short, the <a href="https://home.cern/about/physics/standard-model">Standard Model</a> answers this question: What is everything made of, and how does it hold together?</p>
<h2>The smallest building blocks</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">But these elements can be broken down further.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Periodic_table_vectorial.png">Rubén Vera Koster</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>You know, of course, that the world around us is made of molecules, and molecules are made of atoms. Chemist <a href="https://www.famousscientists.org/dmitri-mendeleev/">Dmitri Mendeleev</a> figured out in the 1860s how to organize all atoms – that is, the elements – into the periodic table that you probably studied in middle school. But there are 118 different chemical elements. There’s antimony, arsenic, aluminum, selenium … and 114 more.</p>
<p>Physicists like things simple. We want to boil things down to their essence, a few basic building blocks. Over a hundred chemical elements is not simple. The ancients believed that everything is made of just five elements – <a href="https://en.wikipedia.org/wiki/Classical_element">earth, water, fire, air and aether</a>. Five is much simpler than 118. It’s also wrong. </p>
<p>By 1932, scientists knew that all those atoms are made of just three particles – neutrons, protons and electrons. The neutrons and protons are bound together tightly into the nucleus. The electrons, thousands of times lighter, whirl around the nucleus at speeds approaching that of light. Physicists <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1918/planck-bio.html">Planck</a>, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-bio.html">Bohr</a>, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1933/schrodinger-bio.html">Schroedinger</a>, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1932/heisenberg-bio.html">Heisenberg</a> and friends had invented a new science – <a href="https://en.wikipedia.org/wiki/Quantum_mechanics">quantum mechanics</a> – to explain this motion.</p>
<p>That would have been a satisfying place to stop. Just three particles. Three is even simpler than five. But held together how? The negatively charged electrons and positively charged protons are bound together by <a href="https://en.wikipedia.org/wiki/Electromagnetism">electromagnetism</a>. But the protons are all huddled together in the nucleus and their positive charges should be pushing them powerfully apart. The neutral neutrons can’t help. </p>
<p>What binds these protons and neutrons together? “Divine intervention” a man on a Toronto street corner told me; he had a pamphlet, I could read all about it. But this scenario seemed like a lot of trouble even for a divine being – keeping tabs on every single one of the universe’s 10⁸⁰ protons and neutrons and bending them to its will. </p>
<h2>Expanding the zoo of particles</h2>
<p>Meanwhile, nature cruelly declined to keep its zoo of particles to just three. Really four, because we should count the <a href="https://en.wikipedia.org/wiki/Photon">photon</a>, the particle of light that <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-bio.html">Einstein</a> described. Four grew to five when <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1936/anderson-bio.html">Anderson</a> measured electrons with positive charge – positrons – striking the Earth from outer space. At least <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1933/dirac-bio.html">Dirac</a> had predicted these first anti-matter particles. Five became six when the pion, which <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1949/yukawa-bio.html">Yukawa</a> predicted would hold the nucleus together, was found. </p>
<p>Then came the muon – 200 times heavier than the electron, but otherwise a twin. “Who ordered that?” <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1944/rabi-bio.html">I.I. Rabi</a> quipped. That sums it up. Number seven. Not only not simple, redundant.</p>
<p>By the 1960s there were hundreds of “fundamental” particles. In place of the well-organized periodic table, there were just long lists of baryons (heavy particles like protons and neutrons), mesons (like <a href="https://en.wikipedia.org/wiki/Hideki_Yukawa">Yukawa</a>’s pions) and leptons (light particles like the electron, and the elusive neutrinos) – with no organization and no guiding principles.</p>
<p>Into this breach sidled the Standard Model. It was not an overnight flash of brilliance. No Archimedes leapt out of a bathtub shouting “eureka.” Instead, there was a series of crucial insights by a few key individuals in the mid-1960s that transformed this quagmire into a simple theory, and then five decades of experimental verification and theoretical elaboration. </p>
<p><a href="https://home.cern/about/updates/2014/01/fifty-years-quarks">Quarks</a>. They come in six varieties we call flavors. Like ice cream, except not as tasty. Instead of vanilla, chocolate and so on, we have up, down, strange, charm, bottom and top. In 1964, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1969/gell-mann-bio.html">Gell-Mann</a> and <a href="https://www.macfound.org/fellows/113/">Zweig</a> taught us the recipes: Mix and match any three quarks to get a baryon. Protons are two ups and a down quark bound together; neutrons are two downs and an up. Choose one quark and one antiquark to get a meson. A pion is an up or a down quark bound to an anti-up or an anti-down. All the material of our daily lives is made of just up and down quarks and anti-quarks and electrons.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=536&fit=crop&dpr=1 600w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=536&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=536&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=673&fit=crop&dpr=1 754w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=673&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=673&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Standard Model of elementary particles provides an ingredients list for everything around us.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_From_Fermi_Lab.jpg">Fermi National Accelerator Laboratory</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Simple. Well, simple-ish, because keeping those quarks bound is a feat. They are tied to one another so tightly that you never ever find a quark or anti-quark on its own. The theory of that binding, and the particles called gluons (chuckle) that are responsible, is called <a href="https://en.wikipedia.org/wiki/Quantum_chromodynamics">quantum chromodynamics</a>. It’s a vital piece of the Standard Model, but mathematically difficult, even posing an unsolved problem of basic mathematics. We physicists do our best to calculate with it, but we’re still learning how.</p>
<p>The other aspect of the Standard Model is “<a href="https://doi.org/10.1103/PhysRevLett.19.1264">A Model of Leptons</a>.” That’s the name of the landmark 1967 paper by <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1979/weinberg-bio.html">Steven Weinberg</a> that pulled together quantum mechanics with the vital pieces of knowledge of how particles interact and organized the two into a single theory. It incorporated the familiar electromagnetism, joined it with what physicists called “the weak force” that causes certain radioactive decays, and explained that they were different aspects of the same force. It incorporated <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/2013/higgs-facts.html">the Higgs mechanism</a> for giving mass to fundamental particles. </p>
<p>Since then, the Standard Model has predicted the results of experiment after experiment, including the discovery of several varieties of quarks and of the <a href="https://en.wikipedia.org/wiki/W_and_Z_bosons">W and Z bosons</a> – heavy particles that are for weak interactions what the photon is for electromagnetism. The possibility that <a href="https://en.wikipedia.org/wiki/Neutrino#Mass">neutrinos aren’t massless</a> was overlooked in the 1960s, but slipped easily into the Standard Model in the 1990s, a few decades late to the party.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=385&fit=crop&dpr=1 600w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=385&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=385&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=484&fit=crop&dpr=1 754w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=484&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=484&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">3D view of an event recorded at the CERN particle accelerator showing characteristics expected from the decay of the SM Higgs boson to a pair of photons (dashed yellow lines and green towers).</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:3D_view_of_an_event_recorded_with_the_CMS_detector_in_2012_at_a_proton-proton_centre_of_mass_energy_of_8_TeV.png">McCauley, Thomas; Taylor, Lucas; for the CMS Collaboration CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Discovering the Higgs boson in 2012, long predicted by the Standard Model and long sought after, was a thrill but not a surprise. It was yet another crucial victory for the Standard Model over the dark forces that particle physicists have repeatedly warned loomed over the horizon. Concerned that the Standard Model didn’t adequately embody their expectations of simplicity, worried about its mathematical self-consistency, or looking ahead to the eventual necessity to bring the force of gravity into the fold, physicists have made numerous proposals for theories beyond the Standard Model. These bear exciting names like <a href="https://en.wikipedia.org/wiki/Grand_Unified_Theory">Grand Unified Theories</a>, <a href="https://en.wikipedia.org/wiki/Supersymmetry">Supersymmetry</a>, <a href="https://en.wikipedia.org/wiki/Technicolor_(physics)">Technicolor</a>, and <a href="https://en.wikipedia.org/wiki/String_theory">String Theory</a>. </p>
<p>Sadly, at least for their proponents, beyond-the-Standard-Model theories have not yet successfully predicted any new experimental phenomenon or any experimental discrepancy with the Standard Model.</p>
<p>After five decades, far from requiring an upgrade, the Standard Model is <a href="http://artsci.case.edu/smat50/">worthy of celebration</a> as the Absolutely Amazing Theory of Almost Everything.</p><img src="https://counter.theconversation.com/content/94700/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Glenn Starkman receives funding from the Office of Science of the US Department of Energy. He is affiliated with Case Western Reserve University. </span></em></p>A particle physicist explains just what this keystone theory includes. After 50 years, it’s the best we’ve got to answer what everything in the universe is made of and how it all holds together.Glenn Starkman, Distinguished University Professor of Physics, Case Western Reserve UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/940512018-04-05T11:07:39Z2018-04-05T11:07:39ZOur study suggests the elusive ‘neutrino’ could make up a significant part of dark matter<figure><img src="https://images.theconversation.com/files/212659/original/file-20180329-189824-awj0i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Galaxy cluster with dark matter denoted in blue.</span> <span class="attribution"><span class="source">Smithsonian Institution @ Flickr Commons</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Physicists trying to understand the fundamental structure of nature rely on consistent theoretical frameworks that can explain what we see and simultaneously make predictions that we can test. On the smallest scale of <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">elementary particles</a>, the standard model of particle physics provides the basis of our understanding. </p>
<p>On the scale of the cosmos, much of our understanding is based on “<a href="http://cosmology.berkeley.edu/Education/CosmologyEssays/The_Standard_Cosmology.html">standard model of cosmology</a>”. Informed by Einstein’s theory of general relativity, it posits that the most of the mass and energy in the universe is made up of mysterious, invisible substances known as <a href="https://theconversation.com/method-to-weigh-galaxy-clusters-could-help-us-understand-mysterious-dark-matter-structures-85023">dark matter</a> (making up 80% of the matter in the universe) and <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a>. </p>
<p>Over the past few decades, this model has been remarkably successful at explaining a wide range of observations of our universe. Yet we still don’t know what makes up dark matter – we only know it exists because of the gravitational pull it has on galaxy clusters and other structures. A number of particles <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">have been proposed</a> as candidates, but we can’t say for sure which one or several particles make up dark matter. </p>
<p>Now <a href="https://academic.oup.com/mnras/article-abstract/476/3/2999/4855951?redirectedFrom=fulltext">our new study</a> – which hints that extremely light particles called neutrinos are likely to make up some of the dark matter – challenges our current understanding of its composition.</p>
<h2>Hot versus cold</h2>
<p>The standard model holds that dark matter is “<a href="http://science.sciencemag.org/content/322/5907/1476.full">cold</a>”. That means it consists of relatively heavy particles that initially had sluggish motions. As a consequence, it is very easy for neighbouring particles to get together to form objects bound by gravity. The model therefore predicts that the universe should be filled with small dark matter “haloes”, some of which will merge and form progressively more massive systems – making the cosmos “lumpy”.</p>
<p>However, it is not impossible that at least some dark matter <a href="http://curious.astro.cornell.edu/disclaimer/108-the-universe/cosmology-and-the-big-bang/dark-matter/661-what-is-hot-dark-matter-theory-intermediate">is “hot”</a>. This would comprise relatively light particles that have quite high velocities – meaning the particles could easily escape from dense regions such as galaxies. This would slow the accumulation of new matter and lead to a universe where the formation of structure is suppressed (less lumpy).</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/212830/original/file-20180402-189821-fqcqxs.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/212830/original/file-20180402-189821-fqcqxs.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=385&fit=crop&dpr=1 600w, https://images.theconversation.com/files/212830/original/file-20180402-189821-fqcqxs.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=385&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/212830/original/file-20180402-189821-fqcqxs.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=385&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/212830/original/file-20180402-189821-fqcqxs.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=484&fit=crop&dpr=1 754w, https://images.theconversation.com/files/212830/original/file-20180402-189821-fqcqxs.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=484&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/212830/original/file-20180402-189821-fqcqxs.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=484&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="attribution"><span class="source">ESO/L. Calçada</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Neutrinos, which whizz around at extremely high velocities, are a good candidate for hot dark matter. In particular, they do not emit or absorb light – making them appear “dark”. It was long assumed that neutrinos, which come in three different species, don’t have mass. But experiments have demonstrated that they can change (oscillate) from one species to another. Importantly, scientists have shown that this changing <a href="https://theconversation.com/physics-duo-wins-the-nobel-prize-for-solving-longstanding-neutrino-puzzle-48702">requires them to have mass</a> – making them a legitimate candidate for hot dark matter. </p>
<p>Over the past few decades, however, both particle physics experiments and various astrophysical lines of argument have ruled out neutrinos as making up most of the dark matter in the universe. What’s more, the standard model assumes that neutrinos (and hot dark matter in general) have so little mass that their contribution to dark matter can be ignored completely (in most cases assumed to be 0%). And, <a href="https://theconversation.com/dance-of-galaxies-challenges-current-thinking-on-cosmology-91097">until very recently</a>, this model has reproduced a wide variety of cosmological observations quite well. </p>
<h2>Changing picture</h2>
<p>In the past few years, the quantity and quality of cosmological observations has shot up enormously. One of the most prominent examples of this has been the emergence of “gravitational lensing observations”. General relativity tells us that matter curves spacetime so that light from distant galaxies can be deflected by massive objects that lie between us and the galaxies. Astronomers can measure such deflection to estimate the growth of structure (the “lumpiness”) in the universe over cosmic time. </p>
<p>These new data sets have presented cosmologists with a number of ways to test in detail the predictions of the standard model. A picture that is beginning to emerge <a href="http://www.cfhtlens.org/">from these comparisons</a> is that the mass distribution in the universe <a href="http://kids.strw.leidenuniv.nl/">appears to be less lumpy</a> than it ought to be if the dark matter is entirely cold. </p>
<p>However, making comparisons between the standard model and the new data sets may not be as straightforward as first thought. In particular, researchers have shown that the apparent lumpiness of the universe is not just affected by dark matter, <a href="https://academic.oup.com/mnras/article/415/4/3649/1749254">but also by complex processes that affect normal matter</a> (protons and neutrons). Previous comparisons assumed that normal matter, which “feels” both gravity and pressure forces, is distributed like dark matter, which only feels gravity.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/212658/original/file-20180329-189824-1fp6vhi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/212658/original/file-20180329-189824-1fp6vhi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/212658/original/file-20180329-189824-1fp6vhi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/212658/original/file-20180329-189824-1fp6vhi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/212658/original/file-20180329-189824-1fp6vhi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/212658/original/file-20180329-189824-1fp6vhi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/212658/original/file-20180329-189824-1fp6vhi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=377&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="attribution"><span class="license">Author provided</span></span>
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<p>Now our new study has produced the largest suite of cosmological computer simulations of normal and dark matter to date (called <a href="http://www.astro.ljmu.ac.uk/%7Eigm/BAHAMAS/">BAHAMAS</a>). We have also made careful comparisons with a wide range of recent observations. We conclude that the discrepancy between the new observational data sets and the standard cold dark matter model is even larger than previously claimed.</p>
<p>We looked at the effects of neutrinos and their motions in great detail. As expected, when neutrinos were included in the model, the structure formation in the cosmos was washed out, making the universe less lumpy. Our results suggest that neutrinos make up between 3% and 5% of the total dark matter mass. This is sufficient to consistently reproduce a wide variety of observations – including the new gravitational lensing measurements. If a larger fraction of the dark matter is “hot”, the growth of structure in the universe is suppressed too much.</p>
<p>The research may also help us solve the mystery of what the mass of an individual neutrino is. From various experiments, particle physicists have calculated that the the sum of the three neutrino species should be <a href="https://www.sciencedirect.com/science/article/pii/S0370157306001359?via%3Dihub">at least 0.06 electron Volts</a> (a unit of energy, similar to joules). You can convert this into an estimate of the total neutrino contribution to dark matter, and it works out to be 0.5%. Given that we have found it is actually six to ten times larger than this, we can deduce that the neutrino mass should be about 0.3-0.5 eV instead.</p>
<p>This is tantalisingly close to values that can actually be measured by <a href="https://www.katrin.kit.edu/">upcoming particle physics experiments</a>. If these measurements corroborate the masses we found in our simulations, this would be very reassuring – giving us a consistent picture of the role of neutrinos as dark matter from the largest cosmological scales to the tiniest particle physics realm.</p><img src="https://counter.theconversation.com/content/94051/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ian G. McCarthy works for Liverpool John Moores University. He receives funding from the Science and Technology Facilities Council (STFC) and the European Research Council (ERC). </span></em></p>A new study challenges the established view of what dark matter is.Ian G. McCarthy, Reader of Astrophysics, Liverpool John Moores UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/862792017-11-28T23:20:34Z2017-11-28T23:20:34ZHow scientists unlock secrets of the universe from deep underground<figure><img src="https://images.theconversation.com/files/196809/original/file-20171128-28846-p18fk0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Looking up in the main chamber at SNOLAB's facility in the Vale Creighton nickel mine in Sudbury, Ont., a giant spherical neutrino sensor array the size of a 10 storey building is used to detect subatomic particles that pass through the earth.</span> <span class="attribution"><span class="source">(Handout)</span></span></figcaption></figure><p>What do a big chunk of ice at the South Pole, a mine in northern Ontario and a mountain in Italy have in common? They’re all home to extreme underground environments but they’re connected in another, more unexpected way. All three are host to large physics experiments searching to understand and answer the most basic questions about the world around us.</p>
<p>As a research scientist at <a href="http://snolab.ca">SNOLAB</a> in Sudbury, Ont., I get the chance to talk to a lot of different people about the work we do. The question often comes up: Why are we doing astrophysics — the study of space and the cosmos — from deep underground?</p>
<p>In particle physics, we long ago answered all of the questions that could be answered through tabletop experiments run by small groups of scientists in small laboratory spaces. <a href="https://www.britannica.com/science/Michelson-Morley-experiment">Albert Michelson and Edward Morley</a> showed that “luminiferous aether” didn’t exist, using a light source and mirrors on a bench-top stand. <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1903/marie-curie-bio.html">Marie Skłodowska Curie</a> discovered polonium and radium in a shed next to the school of chemistry and physics at the École Normale Supérieure in Paris. </p>
<p>Fortunately, or unfortunately, we’re now asking bigger questions — we are looking for the nature of the universe: What gives particles their mass? What kind of particle is the <a href="https://www.scientificamerican.com/article/what-is-a-neutrino/">neutrino</a>? What is the rest of space made of? What is <a href="https://www.space.com/20930-dark-matter.html">dark matter</a>? Bigger questions require bigger experiments, which need bigger groups of people making them function.</p>
<p>The way to answer these questions is not just to build bigger detectors, but also to think carefully about where we put these detectors. So, scientists build detectors deep underground. We use kilometres of the earth as a dense shield to stop the particles we don’t want to detect. </p>
<p>The neutrinos and dark matter that we are looking for are unlikely to be stopped by the rock, so they are free to keep travelling to our detectors. The deeper we go, the more of this shielding we have.</p>
<h2>Antarctica</h2>
<p>Take for example <a href="http://icecube.wisc.edu/">IceCube</a>, the experiment built directly into the South Pole. Looking for the highest energy neutrinos requires a cubic kilometre of ice to be buried under another 1.5 kilometres of ice. </p>
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<img alt="" src="https://images.theconversation.com/files/196807/original/file-20171128-28856-dqwfbu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196807/original/file-20171128-28856-dqwfbu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196807/original/file-20171128-28856-dqwfbu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196807/original/file-20171128-28856-dqwfbu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196807/original/file-20171128-28856-dqwfbu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196807/original/file-20171128-28856-dqwfbu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196807/original/file-20171128-28856-dqwfbu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The world’s largest neutrino observatory deep under the Antarctic ice, IceCube Laboratory at Amundsen-Scott South Pole Station, is backlit by the Aurora Australis.</span>
<span class="attribution"><span class="source">(IceCube/NSF/Ian Rees)</span></span>
</figcaption>
</figure>
<p>If you’re standing on the South Pole, the bottom of the detector is 30 city blocks below you! There are over 5,000 light sensors distributed through this volume, with cables stretching to a surface control building from which the experiment is run.</p>
<p>This project requires over 300 people across 12 countries. Professors, students and technicians are all working on building electronics prototypes, testing techniques to drill into the oldest ice on the planet and writing computer code to analyze all that data. There is no one country, much less one single university, that could supply all the necessary resources.</p>
<h2>Canada</h2>
<p><a href="http://snolab.ca/">SNOLAB</a>, Canada’s deep underground astroparticle physics laboratory, is in an active, working nickel mine over two kilometres underground. There is a rich history of particle physics experiments operating in mines, beginning in the 1960s.</p>
<p>Compared to experiments hosted in shuttered mines, working in an active one makes some aspects easier, because scientists don’t need to worry about operating the mine elevator (called the cage) or pumping groundwater out of the mine. But the size of the cage, and the mine tunnels (called drifts) limit the size of items that can be transported into the lab. Overall, the detectors’ increased sensitivity from all of the shielding is worth the effort.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196808/original/file-20171128-28849-14fbf80.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">SNOLAB’s giant spherical neutrino sensor dominates the lab 1.5 kilometres underground in Sudbury, Ont.</span>
<span class="attribution"><span class="source">(Handout)</span></span>
</figcaption>
</figure>
<h2>Italy</h2>
<p><a href="https://www.lngs.infn.it/en">LNGS</a>, the National Laboratory of Grand Sasso, is a 17.8 sq. kilometres lab in the heart of Italy built underneath a mountain, within a national park, adjacent to a freeway tunnel. This provides an underground space without the logistical challenges sometimes posed by working in a mine. </p>
<p>The hours of access aren’t governed by someone else’s mining schedule, and being able to drive your detector straight into the lab is a definite bonus. But its space is limited, and the 1.4 kilometres of mountain rock above provides less shielding, which ultimately limits the capabilities of the 16 experiments it hosts.</p>
<p>Being in these unusual locations can be difficult enough, but working in large groups provides interesting challenges and international collaborations are the norm for most experiments. </p>
<h2>Global collaboration challenges</h2>
<p>SNOLAB is home to eight particle physics experiments (<a href="https://www.snolab.ca/science/experiments/damic">DAMIC</a>, <a href="http://deap3600.ca/">DEAP-3600</a>, <a href="https://www.snolab.ca/halo/">HALO</a>, <a href="http://deapclean.org/">MiniCLEAN</a>, <a href="https://news-g.org/">NEWS</a>, <a href="http://www.picoexperiment.com/">PICO</a>, <a href="https://snoplus.phy.queensu.ca/Home.html">SNO+</a>, <a href="https://cdms.phy.queensu.ca/">SuperCDMS</a>) that are managed and run by scientists from 79 institutions in 14 countries around the world. Beyond the physical logistics of doing science, we have the human logistics of working in large groups and across borders.</p>
<p>We have to keep each other updated on the day to day workings of making science happen. This means conference calls, and their coordination. </p>
<p>As collaborations span the globe, Doodle meeting scheduler polls span time zones, searching for the hours that will inconvenience the fewest people. Audio and video conferencing tools that physicists use to communicate with each other abound, including <a href="https://www.vrvs.org/">EVO</a>, <a href="https://www.uberconference.com/">Uberconference</a>, <a href="https://www.vidyo.com/">Vidyo</a>, and <a href="https://zoom.us/">Zoom</a>. My calendar is peppered with at least two to four calls a day — every… single… week! This can be tedious, difficult, and time-consuming but phone meetings are much better suited for making decisions than infinite email chains.</p>
<h2>Beneath the earth and across the globe</h2>
<p>Another big challenge is in-person meetings. The majority of collaboration projects get together a few times a year to catch everyone up on what’s been going on. This means that at any given time, most people involved in a project must travel. </p>
<p>A large portion of a group leader’s grant will be spent on travel funds for herself or himself and their postdoctoral researchers and students. Depending on what stage the experiment is in, the meetings might be held at the experiment’s site, or hosted by different collaborating institutions. </p>
<p>These trips can be great opportunities to actually see the project for which you might only be analyzing the data. They also offer a chance to explore another country’s culture while making professional connections with colleagues.</p>
<p>Regardless of what you are doing in particle physics, filling up your passport is a given in this field. And depending which experiment you work on, you might not just travel across the globe, but into it as well.</p><img src="https://counter.theconversation.com/content/86279/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Erica Caden does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Deep underground, scientists research subatomic particles from space in a bid to understand the building blocks of our universe.Erica Caden, Research scientist at SNOLAB, Adjunct faculty at Laurentian University, Laurentian UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/880392017-11-24T15:03:45Z2017-11-24T15:03:45ZHow the SuperNEMO experiment could help solve the mystery of the origin of matter in the universe<figure><img src="https://images.theconversation.com/files/196204/original/file-20171123-18021-v7id4y.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Justin Evans, the author, creating a grid of fine steel wire, now sitting inside the SuperNEMO detector.</span> <span class="attribution"><span class="license">Author provided</span></span></figcaption></figure><p>The Savoy region of France is best known for its fir-lined ski slopes and picturesque Alpine villages. Less known is the fact that, deep beneath some of these slopes, scientists are investigating one of the greatest mysteries in physics: the origin of matter.</p>
<p>The Fréjus road tunnel in the region carries traffic between the French town of Modane and the Italian town of Bardonecchia. Take a drive through the tunnel, and you might just notice – at the mid-point – an unassuming green door in the tunnel wall. This sturdy metal door separates the stifling, diesel-infused air of the road tunnel from the clean, controlled atmosphere of the <a href="http://www-lsm.in2p3.fr/">Laboratoire Souterraine de Modane</a>, Europe’s deepest underground laboratory which is home to a particle physics experiment called <a href="http://supernemo.org/">SuperNEMO</a>.</p>
<p>The SuperNEMO detector, around six metres long, four metres high and three metres wide, sits in a tightly controlled clean room to protect it from contamination by the minute amounts of natural radioactivity present in dirt and dust. The mountain itself provides protection from the cosmic rays that continuously bombard the surface of our planet. Such protection is needed, since the job of SuperNEMO is to watch over seven kilograms of <a href="http://selenium.atomistry.com/isotopes.html">selenium</a> and search for one of the rarest forms of radioactivity there is: double-beta decay. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/196206/original/file-20171123-17975-1xdnz8c.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196206/original/file-20171123-17975-1xdnz8c.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196206/original/file-20171123-17975-1xdnz8c.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196206/original/file-20171123-17975-1xdnz8c.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196206/original/file-20171123-17975-1xdnz8c.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196206/original/file-20171123-17975-1xdnz8c.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196206/original/file-20171123-17975-1xdnz8c.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Physicists waiting to cross road in Fréjus road tunnel.</span>
</figcaption>
</figure>
<p>All radioactive elements are unstable and decay (split up) to a stable state due to changes in the atomic nucleus (which consists of protons and neutrons). <a href="https://warwick.ac.uk/study/csde/gsp/eportfolio/directory/crs/phsgbu/research/phdresearch/theory/betadecay/double/">Double-beta decay</a> is a process by which two neutrons in a selenium nucleus simultaneously decay into protons, while emitting two electrons and two particles called <a href="https://theconversation.com/how-the-neutrino-could-solve-great-cosmic-mysteries-and-win-its-next-nobel-prize-48789">antineutrinos</a>.</p>
<p>Antineutrinos are an example of “<a href="https://theconversation.com/explainer-what-is-antimatter-53414">antimatter</a>”. All matter particles have antiparticle versions of themselves – nearly identical but with opposite charge. When a particle and an antiparticle meet, they annihilate in a flash of energy. </p>
<h2>Enigmatic particles</h2>
<p>Antineutrinos are puzzling. Take the way they spin, for instance. Many particles spin as they travel, but neutrinos only seem to spin one way. All neutrinos spin anticlockwise as they travel – and all antineutrinos spin clockwise. We have no idea why this is the case.</p>
<p>Then there is their mass: neutrinos are many, many times lighter than any other particle with mass – so much lighter that we have not yet been able to directly measure their tiny mass. The neutrino is an outlier among particles – and when scientists see outliers, we can’t help but suspect there is some deeper meaning behind the inconsistency which could reveal profound truth about the laws of nature. The seeds of a theory to explain the neutrino’s many eccentricities lie in a relatively mundane observation: unlike other particles, the neutrino has no electric charge. </p>
<p>But with no electric charge, how does the antineutrino differ from the neutrino? There definitely is some difference. The kinds of neutrinos and antineutrinos SuperNEMO is looking at are of the so-called electron type. When the neutrinos interact with matter they produce negatively charged electrons, but when the antineutrinos interact with matter they produce positively charged positrons, the electron’s antiparticle. But before the neutrino or antineutrino interacts, how does it know which one it is?</p>
<p>This profound question led the Italian physicist <a href="http://www.science20.com/quantum_diaries_survivor/ettore_majorana_mystery_might_be_solved-79823">Ettore Majorana</a> to consider whether the neutrino and the antineutrino could in fact be exactly the same particle, just spinning in opposite directions.</p>
<p>If the antineutrinos created in the double-beta decay that SuperNEMO is looking for have the ability to behave like neutrinos, then just occasionally one of them might do that. That would mean you had an antineutrino and a neutrino next to each other – which would mean they could annihilate each other. Should that happen, the two electrons produced in the double-beta decay would get an extra kick of energy from the annihilation – and that is what SuperNEMO is looking for: a tiny kick of energy that would require us to rethink how matter and antimatter are related.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/196205/original/file-20171123-17982-e7ll2b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196205/original/file-20171123-17982-e7ll2b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196205/original/file-20171123-17982-e7ll2b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196205/original/file-20171123-17982-e7ll2b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196205/original/file-20171123-17982-e7ll2b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196205/original/file-20171123-17982-e7ll2b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196205/original/file-20171123-17982-e7ll2b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Inside the Laboratoire Souterraine de Modane. The plastic walls of a cleanroom protect the SuperNEMO detector from dirt and dust.</span>
</figcaption>
</figure>
<p>Patience is the key to this search. The half-life of double-beta decay in selenium – that’s the time you’d have to wait before one atom has a 50% chance of having decayed – is 10<sup>20</sup> years. That’s a 1 with 20 zeros after it: take the lifetime of the universe and add another ten zeros. And even when a double-beta decay happens, the chance that the two antineutrinos annihilate is tiny – if it even happens at all. We make up for that by having a lot of selenium atoms in our detector, but still we are looking out for only one or two such decays every year.</p>
<h2>The origin of matter</h2>
<p>If we observe such a radioactive decay we would have to rewrite the successful <a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Standard Model of Particle Physics</a>. This would be a huge discovery in itself. The Standard Model contains strict rules, called conservation laws, about what can and cannot happen in particle decays and interactions. If our two antineutrinos annihilate (because one of them behaved like a neutrino at the time), then the double-beta decay would produce two matter-like electrons and no antimatter to balance them out. That’s not allowed in the Standard Model, which requires that matter and antimatter are always produced in equal amounts.</p>
<p>This brings us to one of the most profound questions of physics: why is there more matter than antimatter in the universe? You might think we already know the answer to that: the Big Bang produced all the matter. Well, yes it did, but it should have also produced an equal amount of antimatter. So why did all the matter and antimatter not annihilate each other to leave nothing but a sea of light? </p>
<p>If the neutrino and antineutrino are indeed the same particle, the resulting revised Standard Model would allow you to add more of these neutrino-like particles into your model. Some of these neutrino-like particles might be heavy rather than light; and I mean very heavy – so heavy that the Large Hadron Collider hasn’t been able to produce them, and so heavy in fact that they were only common in the hot, dense conditions of the very early universe.</p>
<p>Since this revised Standard Model has a mechanism to break the symmetry between matter and antimatter, these super-heavy neutrinos also have the ability to “choose” to decay into matter over antimatter, providing the early universe with the extra matter we now see. If it didn’t, all the matter and antimatter would have annihilated each other and there would be no stars, the planets, and us.</p>
<p>So if you are ever in the Savoy region of France, enjoying some aprés-ski after a day on the slopes, spare a thought for the SuperNEMO detector – and the particle physicists like me, deep below you, waiting patiently for that radioactive decay that just might explain how you got to be there.</p><img src="https://counter.theconversation.com/content/88039/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Justin Evans receives funding from the Science and Technology Facilities Council (STFC). </span></em></p>Deep beneath the Alpine ski slopes, patient scientists are waiting to observe a rare radioactive decay that would make us rewrite the Standard Model of Particle Physics.Justin Evans, Senior Lecturer in Physics, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/858082017-10-25T13:34:11Z2017-10-25T13:34:11ZDark matter: The mystery substance physics still can’t identify that makes up the majority of our universe<figure><img src="https://images.theconversation.com/files/191475/original/file-20171023-1695-1xeghxr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Map of all matter – most of which is invisible dark matter – between Earth and the edge of the observable universe.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/planck/multimedia/pia16875.html#.We5FQkzMzdc">ESA/NASA/JPL-Caltech</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The past few decades have ushered in an amazing era in the science of cosmology. A diverse array of high-precision measurements has allowed us to reconstruct our universe’s history in remarkable detail.</p>
<p>And when we compare different measurements – of the <a href="http://earthsky.org/space/video-hubble-constant-rate-expansion-universe">expansion rate of the universe</a>, the patterns of light released in the <a href="http://planck.cf.ac.uk/science/cmb">formation of the first atoms</a>, the <a href="https://www.e-education.psu.edu/astro801/content/l10_p6.html">distributions in space of galaxies and galaxy clusters</a> and the <a href="http://w.astro.berkeley.edu/%7Emwhite/darkmatter/bbn.html">abundances of various chemical species</a> – we find that they all tell the same story, and all support the same series of events.</p>
<p>This line of research has, frankly, been more successful than I think we had any right to have hoped. We know more about the origin and history of our universe today than almost anyone a few decades ago would have guessed that we would learn in such a short time.</p>
<p>But despite these very considerable successes, there remains much more to be learned. And in some ways, the discoveries made in recent decades have raised as many new questions as they have answered.</p>
<p>One of the most vexing gets at the heart of what our universe is actually made of. Cosmological observations have determined the <a href="https://doi.org/10.1051/0004-6361/201525830">average density of matter in our universe</a> to very high precision. But this density turns out to be much greater than can be accounted for with ordinary atoms.</p>
<p>After decades of measurements and debate, we are now confident that the overwhelming majority of our universe’s matter – about 84 percent – is not made up of atoms, or of any other known substance. Although we can feel the gravitational pull of this other matter, and clearly tell that it’s there, we simply do not know what it is. This mysterious stuff is invisible, or at least nearly so. For lack of a better name, we call it “dark matter.” But naming something is very different from understanding it.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&rect=79%2C517%2C2119%2C1562&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&rect=79%2C517%2C2119%2C1562&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=750&fit=crop&dpr=1 600w, https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=750&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=750&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=943&fit=crop&dpr=1 754w, https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=943&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/191472/original/file-20171023-1722-1ei7aqm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=943&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Astronomers map dark matter indirectly, via its gravitational pull on other objects.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/hubble/science/dark-matter-map.html">NASA, ESA, and D. Coe (NASA JPL/Caltech and STScI)</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>For almost as long as we’ve known that dark matter exists, physicists and astronomers have been devising ways to try to learn what it’s made of. They’ve built <a href="https://pandax.sjtu.edu.cn/">ultra-sensitive detectors</a>, <a href="http://lux.brown.edu/LUX_dark_matter/Home.html">deployed in</a> <a href="http://www.xenon1t.org/">deep underground mines</a>, in an effort to measure the gentle impacts of individual dark matter particles colliding with atoms.</p>
<p>They’ve built exotic telescopes – sensitive not to optical light but <a href="https://fermi.gsfc.nasa.gov/">to less familiar gamma rays</a>, <a href="http://www.ams02.org/">cosmic rays</a> and <a href="http://icecube.wisc.edu/">neutrinos</a> – to search for the high-energy radiation that is thought to be generated through the interactions of dark matter particles.</p>
<p>And we have searched for signs of dark matter using incredible machines which accelerate beams of particles – typically protons or electrons – up to the highest speeds possible, and then smash them into one another in an effort to <a href="http://www.tedxnaperville.com/talks/dan-hooper/">convert their energy into matter</a>. The idea is these collisions could create new and exotic substances, perhaps including the kinds of particles that make up the dark matter of our universe.</p>
<p>As recently as a decade ago, most cosmologists – including myself – were reasonably confident that we would soon begin to solve the puzzle of dark matter. After all, there was an ambitious experimental program on the horizon, which we anticipated would enable us to identify the nature of this substance and to begin to measure its properties. This program included the world’s most powerful particle accelerator – <a href="https://home.cern/topics/large-hadron-collider">the Large Hadron Collider</a> – as well as an array of other new experiments and powerful telescopes.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/191469/original/file-20171023-1717-1lamup8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Experiments at CERN are trying to zero in on dark matter – but so far no dice.</span>
<span class="attribution"><a class="source" href="http://cds.cern.ch/record/2229237">CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>But things did not play out the way that we expected them to. Although these experiments and observations have been carried out as well as or better than we could have hoped, the discoveries did not come.</p>
<p>Over the past 15 years, for example, experiments designed to detect individual particles of dark matter have become a million times more sensitive, and yet no signs of these elusive particles have appeared. And although the Large Hadron Collider has by all technical standards performed beautifully, with the exception of the <a href="https://home.cern/topics/higgs-boson">Higgs boson</a>, no new particles or other phenomena have been discovered.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=902&fit=crop&dpr=1 600w, https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=902&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=902&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1133&fit=crop&dpr=1 754w, https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1133&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/191628/original/file-20171024-30596-1rj833x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1133&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">At Fermilab, the Cryogenic Dark Matter Search uses towers of disks made from silicon and germanium to search for particle interactions from dark matter.</span>
<span class="attribution"><a class="source" href="http://vms.fnal.gov/asset/detail?recid=1783766">Reidar Hahn/Fermilab</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The stubborn elusiveness of dark matter has left many scientists both surprised and confused. We had what seemed like very good reasons to expect particles of dark matter to be discovered by now. And yet the hunt continues, and the mystery deepens.</p>
<p>In many ways, we have only more open questions now than we did a decade or two ago. And at times, it can seem that the more precisely we measure our universe, the less we understand it. Throughout the second half of the 20th century, theoretical particle physicists were often very successful at predicting the kinds of particles that would be discovered as accelerators became increasingly powerful. It was a truly impressive run.</p>
<p>But our prescience seems to have come to an end – the long-predicted particles associated with our favorite and most well-motivated theories have stubbornly refused to appear. Perhaps the discoveries of such particles are right around the corner, and our confidence will soon be restored. But right now, there seems to be little support for such optimism.</p>
<p>In response, droves of physicists are going back to their chalkboards, revisiting and revising their assumptions. With bruised egos and a bit more humility, we are desperately attempting to find a new way to make sense of our world.</p><img src="https://counter.theconversation.com/content/85808/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dan Hooper does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Cosmologists are heading back to their chalkboards as the experiments designed to figure out what this unknown 84 percent of our universe actually is come up empty.Dan Hooper, Associate Scientist in Theoretical Astrophysics at Fermi National Accelerator Laboratory and Associate Professor of Astronomy and Astrophysics, University of ChicagoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/856472017-10-16T14:02:45Z2017-10-16T14:02:45ZHow we discovered gravitational waves from ‘neutron stars’ – and why it’s such a huge deal<figure><img src="https://images.theconversation.com/files/190387/original/file-20171016-31010-1rr1trx.jpg?ixlib=rb-1.1.0&rect=0%2C243%2C1710%2C1324&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's illustration of two merging neutron stars.</span> <span class="attribution"><span class="source">National Science Foundation/LIGO/Sonoma State University/A. Simonnet.</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Rumours have been <a href="https://www.scientificamerican.com/article/rumors-swell-over-new-kind-of-gravitational-wave-sighting/">swirling for weeks</a> that scientists have detected <a href="https://theconversation.com/explainer-gravitational-waves-and-why-their-discovery-is-such-a-big-deal-53239">gravitational waves</a> – tiny ripples in space and time – from a source other than colliding black holes. Now we can finally confirm that we’ve observed such waves produced by the violent collision of two massive, ultra-dense stars more than 100m light years from the Earth. </p>
<p>The discovery was made on August 17 by the <a href="https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510">global network of advanced gravitational-wave interferometers</a> – comprising the twin LIGO detectors in the US and their European cousin, Virgo, in Italy. It is hugely important, not least because it helps solve some big mysteries in astrophysics – including the cause of bright flashes of light known as “<a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma ray bursts</a>” and perhaps even the origins of heavy elements such as gold.</p>
<p>As a member of the LIGO scientific collaboration, I was immediately in raptures as soon as I saw the initial data. And the period that followed was definitely the most intense and sleep deprived, but also incredibly exciting, two months of my career. </p>
<p>The announcement comes just weeks after three scientists <a href="https://theconversation.com/scientists-behind-the-discovery-of-gravitational-waves-win-the-2017-nobel-prize-for-physics-66457">were awarded the Nobel Prize in Physics</a> for their foundational work leading to the discovery of gravitational waves, first announced in February 2016. Since then, detecting gravitational waves from colliding black holes has started to feel like familiar territory – <a href="https://theconversation.com/experiments-simultaneously-detect-gravitational-waves-and-help-open-up-a-new-era-of-astronomy-84818">with four further such events detected</a>. But as far as we know, colliding black holes offer purely a window on the dark side of the universe. We haven’t been able to register light from these events with any other instruments.</p>
<p>But GW170817 – the catchy title for the event of August 17 — changes all that. That’s because the source of the waves this time was two “<a href="https://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html">neutron stars</a>” – incredibly dense stellar remnants the size of a city, each weighing more than the sun. These stars whizzed around each other at a sizeable fraction of the speed of light before merging in a cataclysmic collision that we’ve now seen shake the very fabric of space and time.</p>
<h2>Mysteries solved</h2>
<p>The cosmic concerto was just beginning, however. Astronomers have long suspected that the merger of two neutron stars could be the overture to a short <a href="https://theconversation.com/flash-aah-aah-could-a-gamma-ray-burst-eradicate-all-life-on-earth-5291">gamma ray burst</a> – an intense flash of gamma-ray light that releases more energy in a fraction of a second than the sun will pump out in ten billion years. For several decades we have observed these gamma ray bursts, but without knowing for sure what causes them.</p>
<p>However, just 1.7 seconds after the gravitational waves from GW170817 arrived at the Earth, <a href="https://www.nasa.gov/content/fermi-gamma-ray-space-telescope/">NASA’s Fermi satellite</a> observed a short burst of gamma rays in the same general region of the sky. LIGO and Virgo had found the smoking gun, and the link between neutron star collisions and short gamma ray bursts was finally and clearly established.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/190392/original/file-20171016-30993-1uxep11.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Many hands make light (and gravity) work. NASA’s Fermi satellite was instrumental in the discovery.</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>The combination of gravitational-wave and gamma-ray observations allowed the position of the cosmic explosion to be pinpointed to less than 30 square degrees on the sky – or about 100 times the size of the full moon. This, in turn, allowed a whole barrage of astronomical telescopes sensitive to light across the entire electromagnetic spectrum to search this small patch of sky for the aftermath of the explosion. And sure enough this was found – in an unfashionable backwater towards the edge of a fairly <a href="https://en.wikipedia.org/wiki/NGC_4993">unassuming galaxy called NGC4993</a>, in the constellation of Hydra. </p>
<p>Over the next few days and weeks astronomers watched agog as the embers from the explosion glowed brightly and faded, beautifully matching the pattern expected for <a href="http://theconversation.com/we-beat-a-cyber-attack-to-see-the-kilonova-glow-from-a-collapsing-pair-of-neutron-stars-85660">a so-called “kilonova”</a>. This is produced when material rich in subatomic particles known as neutrons from the initial merger is ejected at great speed by the gamma ray burst. This ploughs into the surrounding region of space, triggering the production of heavy radioactive elements. </p>
<p>These unstable elements typically split up (decay) to a stable state by emitting radiation. This is what causes the glow of the kilonova, which we have now confirmed by mapping it out in exquisite detail. Our observations also strongly support the theory that the stable end-products of these chains of reactions include copious amounts of precious metals like gold and platinum. While we’ve suspected neutron stars to be key to <a href="https://www.smithsonianmag.com/science-nature/all-the-gold-in-the-universe-could-come-from-the-collisions-of-neutron-stars-13474145/">producing these elements in space</a>, that hypothesis now looks a whole lot more convincing. Indeed, the kilonova that formed from the embers of GW170817 could have produced as much gold as the entire mass of the Earth – that is 1,000 trillion tonnes.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"919936738893602816"}"></div></p>
<p>By observing a kilonova “up close and personal” for the very first time, and seeing how well it fits into the unfolding astronomical storyboard that began with the neutron star merger, astronomers have taken a huge leap forward in our understanding of these violent cosmic events. </p>
<p>The idea that we are all made of stardust is increasingly appreciated in popular culture – in everything from documentaries to song lyrics. But the mind-blowing concept that the gold in our wedding rings and Rolex watches is made of neutron stardust is about to catch on. Perhaps even more exciting, however, is the enormous potential now unlocked by this radical, new approach to studying the cosmos.</p>
<p>By working together collaboratively – using instruments that operate not just across the entire spectrum of light but are sensitive to gravitational waves and even neutrinos too – astronomers are poised to fully open a completely new “multi-messenger” window on the universe, with many further discoveries to be made and cosmic mysteries to be solved. For example, we have already used our observations to make the first ever joint measurement of the expansion rate of the universe, using both gravitational waves and light. Our paper will appear in Nature on October 16.</p>
<p>More results will also surely follow soon. The exciting new era of multi-messenger astronomy just started with a bang.</p><img src="https://counter.theconversation.com/content/85647/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Hendry is a member of the LIGO scientific collaboration.</span></em></p>The discovery of tiny ripples in space from the violent collision of dense stars could help solve many mysteries – including where the gold in our jewellery comes from.Martin Hendry, Professor of Gravitational Astrophysics and Cosmology, University of GlasgowLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/664572017-10-03T10:12:00Z2017-10-03T10:12:00ZScientists behind the discovery of gravitational waves win the 2017 Nobel Prize for Physics<figure><img src="https://images.theconversation.com/files/188580/original/file-20171003-30864-3m2a82.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">This year's winners</span> <span class="attribution"><span class="source"> Illustration by N. Elmehed. NobelPrize.org</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The Swedish Academy of Sciences <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/2017/press.html">has announced</a> that the 2017 Nobel prize in Physics goes to three scientists for their foundational work leading to the discovery of ripples in the fabric of space and time known as <a href="https://theconversation.com/explainer-gravitational-waves-and-why-their-discovery-is-such-a-big-deal-53239">gravitational waves</a>. </p>
<p>Half of the £825,000 prize sum will go to <a href="http://web.mit.edu/physics/people/faculty/weiss_rainer.html">Rainer Weiss</a> of Massachusetts Institute of Technology, and the other half will be be shared by <a href="https://www.its.caltech.edu/%7Ekip/index.html/">Kip Thorne</a> of Caltech and <a href="https://labcit.ligo.caltech.edu/%7EBCBAct/">Barry C Barish</a>, also at Caltech. The scientists, all from the LIGO/VIRGO collaboration, conceived and played major roles in realising the Laser Interferometer Gravitational-Wave Observatory, which first detected the waves in September 2015. I’m pleased to see this achievement recognised on behalf of the thousands of scientists who work on LIGO, including <a href="https://www.shef.ac.uk/physics/research/pppa/gwrg">the University of Sheffield group</a>. I also know the recipients personally, in particular Weiss, who is a friend as well as a colleague.</p>
<p>Gravitational waves, predicted by Einstein in 1916, travel across our universe at the speed of light – stretching space in one direction and shrinking it in the direction that is at right angles. LIGO <a href="https://theconversation.com/gravitational-waves-discovered-how-did-the-experiment-at-ligo-actually-work-54510">measures these fluctuations</a> by monitoring two light beams travelling between pairs of mirrors down pipes running in different directions. </p>
<p>The source of the first detected signals was a pair of black holes, each being about 30 times the mass of the sun. These bodies once collided and converted in to one large spinning black hole – emitting three sun masses worth of pure energy in about a tenth of a second. For that short time, the source outshines the rest of the energy sources in the observable universe – combined! It’s quite something to try and imagine. Despite being such a violent event, it is so far away that the effects on our local fabric of space and time here on Earth are very subtle – which is why a sophisticated detector like LIGO was needed to make the first detection.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/140280/original/image-20161004-20228-1v5siz3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/140280/original/image-20161004-20228-1v5siz3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=598&fit=crop&dpr=1 600w, https://images.theconversation.com/files/140280/original/image-20161004-20228-1v5siz3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=598&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/140280/original/image-20161004-20228-1v5siz3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=598&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/140280/original/image-20161004-20228-1v5siz3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=751&fit=crop&dpr=1 754w, https://images.theconversation.com/files/140280/original/image-20161004-20228-1v5siz3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=751&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/140280/original/image-20161004-20228-1v5siz3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=751&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Aerial view of the facility.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/23925401@N06/24342686634">Kanijoman/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Several more binary black hole signals have been detected by the LIGO detectors since, and one announced just days ago was detected by the Virgo detector in Italy as well. Now that we know these signals exist and can be detected, a new field of gravitational wave astronomy will grow up, enabling us to probe the dark and puzzling universe – phenomena in the cosmos that don’t emit much light but have a lot of mass. It’s an exciting time.</p>
<h2>Unconventional, sharp and fun</h2>
<p>Those of us at LIGO who know Weiss will agree he is an unconventional fellow in the best sense of that description who has inspired a generation of experimental physicists, myself included. </p>
<p>The first time I met Weiss properly was when he interviewed me for my first postdoc, at MIT. I was in my only smart suit, he walked in wearing a woolly hat, baggy sweater and jeans. I had to reassure him that this was the last time he’d see me dressed up that way. He looked relieved. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/188562/original/file-20171003-12163-gttxkz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/188562/original/file-20171003-12163-gttxkz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=535&fit=crop&dpr=1 600w, https://images.theconversation.com/files/188562/original/file-20171003-12163-gttxkz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=535&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/188562/original/file-20171003-12163-gttxkz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=535&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/188562/original/file-20171003-12163-gttxkz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=672&fit=crop&dpr=1 754w, https://images.theconversation.com/files/188562/original/file-20171003-12163-gttxkz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=672&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/188562/original/file-20171003-12163-gttxkz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=672&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Rainer Weiss.</span>
<span class="attribution"><span class="source">Michael Hauser/wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Weiss has a refreshingly informal approach to physics, which is particularly helpful in encouraging others in their work, especially the young. But this informality and enthusiasm only just conceals his razor-sharp instinct for physics, particularly for sources of background noise and for electronics. </p>
<p>And, because he is what I would call “scientifically sociable”, Weiss naturally tends to learn things quickly by talking to people. When I was working at the LIGO lab at Livingston, I did an early systematic comparison of seismic noise between the two LIGO sites in a key frequency range. The tough thing back then was just gathering enough data from the seismometers to be able to make a meaningful comparison between the noise levels. </p>
<p>I’d just made a graph of the results, and I was in the control room staring at it when Weiss walked in. He walked out a few minutes later with a copy of that plot, and the next thing I knew, he was using it in talks to the National Science Foundation when arguing for an upgrade to LIGO Livingston’s seismic isolation system. That’s Weiss in a nutshell. He’s quick on the uptake, good at spotting the key points and problems, and authoritative enough to get others – physicists, engineers and funders on his side. </p>
<p>We also share a love of music. Once when I was invited to dinner at his house, I was asked to bring my cello and had to sight-read several cello sonata movements (rather shakily) with Weiss at the piano. He also showed up to a particularly memorable “hoodoo party night” at a club called Tabby’s blues box in Baton Rouge, Louisiana, where I was playing in a band. He brought along Gaby Gonzalez, who until recently was chairperson of the LIGO scientific collaboration and Peter Saulson, a professor of physics and thermal noise pioneer from Syracuse. A more unlikely crowd on the dance floor at Tabby’s has probably not been seen before or since. They had a great time.</p>
<p>The future of gravitational wave physics is now intimately tied up with the future of astronomy. The field is set to expand rapidly, with more sensitive instruments needed to sense smaller signals and larger scale instruments needed to probe lower frequencies where many of the astronomical signals lie. We also need observers of the heavens, both to interpret the signals we measure, and to make the link between gravitational waves and other sources of information, such as gamma ray and neutrino bursts, and visible transients. We are hoping to continue to play an important role in the research here at Sheffield.</p>
<p>But, for now, it’s time to enjoy the moment of a very well deserved Nobel prize for a great group of physicists. They have played a long game; the project started in 1972, and I didn’t even join until 1997. It’s a lesson to us all to keep both eyes on the science, to be prepared for a protracted struggle with Mother Nature, but ready in the end to step back and admire the edifice we have constructed, and go on to apply the tools we have created to achieving an ever expanding knowledge of our universe.</p><img src="https://counter.theconversation.com/content/66457/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ed Daw works for the University of Sheffield. He receives funding from the Science and Technology Facilities Council (STFC). </span></em></p>Razor-sharp, unconventional and fun on the dance floor. A colleague paints a colourful portrait of one of this year’s Nobel Laureates in physics.Ed Daw, Reader in Physics, University of SheffieldLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/844542017-09-25T13:21:18Z2017-09-25T13:21:18ZWe’re building a 1,300km-long underground science experiment to study the world’s most elusive particles<figure><img src="https://images.theconversation.com/files/187358/original/file-20170925-17437-fryw9r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>In an abandoned gold mine close to Deadwood, South Dakota, construction has started on what is arguably the world’s largest science experiment. I’m part of an international team of around 1,000 scientists assembled to design and run this project – the <a href="http://www.dunescience.org/">Deep Underground Neutrino Experiment</a> (DUNE) – in order to study the most abundant yet elusive matter particle in the universe.</p>
<p>In doing so, we may move a step closer to understanding the origins of matter and to completing science’s model of how the universe works. That’s why the UK government has now <a href="http://www.bbc.co.uk/news/science-environment-41340971">committed £65m</a> to the project, making the UK the second largest contributor to the project after the US.</p>
<p>Particle physicists like me are fascinated by neutrinos because of their unusual properties, which may be directly linked to phenomena that could explain the structure of the universe. Neutrinos are one of the <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">fundamental particles</a> that can’t be broken down into anything else. They are everywhere but are enormously difficult to catch as they have very nearly no mass, are not charged and rarely interact with other particles.</p>
<p>About 100 billion of them travel through our fingertips every second but almost all of them go through the Earth without leaving any trace. Most of these neutrinos originate from nuclear reactions powering the sun. Neutrinos also come from cosmic rays hitting the atmosphere, or exploding stars. They were also abundantly produced just after the birth of the universe. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/187215/original/file-20170922-17294-1cf49y4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/187215/original/file-20170922-17294-1cf49y4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=369&fit=crop&dpr=1 600w, https://images.theconversation.com/files/187215/original/file-20170922-17294-1cf49y4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=369&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/187215/original/file-20170922-17294-1cf49y4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=369&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/187215/original/file-20170922-17294-1cf49y4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=464&fit=crop&dpr=1 754w, https://images.theconversation.com/files/187215/original/file-20170922-17294-1cf49y4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=464&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/187215/original/file-20170922-17294-1cf49y4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=464&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The 1,300km experiment.</span>
<span class="attribution"><span class="source">DUNE</span></span>
</figcaption>
</figure>
<p>That means by studying neutrinos and comparing them with their counterpart “antineutrinos”, we might be able to work out what happened at the start of the universe that meant it would be mostly made of matter and not antimatter. Experiments built to detect neutrinos could also help us find out whether protons decay, a key piece of evidence for proving some scientists’ ideas about how most of the forces in physics can all be explained using a “<a href="http://www.symmetrymagazine.org/article/a-gut-feeling-about-physics">grand unified theory</a>”.</p>
<p>To do this, DUNE will fire beams of neutrinos from the <a href="http://www.fnal.gov/">Fermi National Accelerator Laboratory</a> in Illinois, US, along a 1,300km underground trajectory to the <a href="https://www.sanfordlab.org/">Sanford Underground Research Facility</a> in South Dakota. By comparison, the circular <a href="https://home.cern/topics/large-hadron-collider">Large Hadron Collider</a> particle accelerator used to find the Higgs Boson is just 27km in circumference, although DUNE’s particles will travel through the ground rather than a specially constructed tunnel.</p>
<h2>Detecting the neutrinos</h2>
<p>Neutrinos come in three types or “flavours” as they’re called: electron-neutrinos, muon-neutrinos, and tau-neutrinos. The neutrinos leaving Fermilab will be mostly muon flavour, but they could change or “<a href="http://www.ps.uci.edu/%7Esuperk/oscillation.html">oscillate</a>” as they travel. Detecting these oscillations is what will provide definite answers to the questions about the neutrino’s nature and its role in the universe.</p>
<p>Neutrinos can be detected by recording the light, charge and type of particle they produce when they come into contact with certain liquids. When each neutrino arrives, it will create a particle that corresponds to its flavour. An electron-neutrino, for example, will produce an electron while a muon-neutrino will produce a muon. If we can detect electrons then we know that muon-neutrinos changed their flavour as they travelled.</p>
<p>DUNE will use four large tanks, each containing 10,000 tonnes of liquid argon held at a temperature of -186°C, to <a href="http://www.dunescience.org/neutrino-detectors/">detect the neutrinos</a> with much greater precision than previous experiments that were smaller or used <a href="http://www-sk.icrr.u-tokyo.ac.jp/sk/index-e.html">tanks full of water</a>. The experiment has to take place about one mile underground to protect the detectors from being overwhelmed by fake neutrino signals from the cosmic radiation that bombards the Earth.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/187220/original/file-20170922-17290-1uugkzc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/187220/original/file-20170922-17290-1uugkzc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=360&fit=crop&dpr=1 600w, https://images.theconversation.com/files/187220/original/file-20170922-17290-1uugkzc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=360&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/187220/original/file-20170922-17290-1uugkzc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=360&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/187220/original/file-20170922-17290-1uugkzc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=452&fit=crop&dpr=1 754w, https://images.theconversation.com/files/187220/original/file-20170922-17290-1uugkzc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=452&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/187220/original/file-20170922-17290-1uugkzc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=452&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Neutrino event in liquid argon.</span>
<span class="attribution"><span class="source">MicroBooNE</span></span>
</figcaption>
</figure>
<p>The enormous sensitivity produced by using this method will also help detect neutrino bursts from space. For example, in 1987 a nearby exploding star (supernova) resulted in all the neutrino detectors in the world recording 25 neutrino events in total. DUNE would be able to observe thousands of neutrino scatterings within a period of about ten seconds for a similar supernova. Analysing the composition and time structure of such a neutrino pulse would revolutionise our understanding of supernovae and of neutrino properties.</p>
<h2>Solving the antimatter mystery</h2>
<p>All this should help us answer several key questions about neutrinos, for example about their mass. Neutrinos are so tiny that their mass is probably not created by the Higgs Boson, <a href="https://theconversation.com/definitely-maybe-evidence-grows-for-positive-id-of-higgs-boson-12790">recently discovered</a> by the Large Hadron Collider, in the same way as most other elementary particles. Instead, their mass may come from very heavy partner neutrinos that decay very quickly after formation.</p>
<p>These partner neutrinos would have played a key role in the early evolution of the universe and could also help explain why
there is so much <a href="https://theconversation.com/explainer-what-is-antimatter-53414">more matter than antimatter</a> in the universe. DUNE will also help us work out whether neutrinos and their anti-matter equivalent, anti-neutrinos, behave identically, providing further evidence for matter’s dominance.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/nv13DswIKr8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>Since the large quantities of argon in the detector contain many protons, DUNE is also an ideal experiment to search for proton decay. Under our current “<a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">standard model</a>” of physics that describes all the fundamental particles, it’s impossible for protons to decay. But many of the grand unified theories scientists are putting together to explain all the forces in the universe (except gravity) predict that protons do decay, just very slowly.</p>
<p>So far we have no evidence for proton decay but, if it does occur, then DUNE should be able to detect and locate it within the liquid argon with millimetre precision. This could help prove whether any of the grand unified theories are correct, and again could provide more clues about matter’s dominance over antimatter.</p>
<p>The new funding, <a href="http://www.stfc.ac.uk/news/uk-signs-65m-science-partnership-agreement-with-us/">together with the combined efforts</a> of scientists from across the globe, will put us on track to record the first events in DUNE in 2024. That means within the next decade we could have solved some of the universe’s biggest mysteries.</p><img src="https://counter.theconversation.com/content/84454/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Stefan Soldner-Rembold receives funding from STFC and the Royal Society. </span></em></p>The Deep Underground Neutrino Experiment (DUNE) could help unravel the mysteries of antimatter and complete scientists’ next model of the universe.Stefan Söldner-Rembold, Professor of Particle Physics, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/815242017-08-04T10:31:16Z2017-08-04T10:31:16ZThe source of up to half of the Earth’s internal heat is completely unknown – here’s how to hunt for it<figure><img src="https://images.theconversation.com/files/180610/original/file-20170801-29610-1o62xaa.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">pixabay</span></span></figcaption></figure><p>It may not be obvious while lying in the sun on a hot summer’s day, but a considerable amount of heat is also coming from below you – emanating from deep within the Earth. This heat is equivalent to more than three times the total power consumption of the entire world and drives important geological processes, such as the movement of tectonic plates and the flow of magma near the surface of the Earth. But despite this, where exactly up to half of this heat actually comes from is a mystery.</p>
<p>It is thought that a type of neutrinos – <a href="https://theconversation.com/how-the-neutrino-could-solve-great-cosmic-mysteries-and-win-its-next-nobel-prize-48789">particles</a> with extremely low mass – emitted by radioactive processes in the Earth’s interior may provide important clues to solving this mystery. The problem is that they are nearly impossible to catch. But in a new paper, <a href="https://www.nature.com/articles/ncomms15989">published in the journal Nature Communications</a>, we have set out a way to do just that.</p>
<p>The known sources of heat from the Earth’s interior are radioactive decays, and residual heat from when our planet was first formed. The amount of heating from radioactivity, estimated based on measurements of the composition of rock samples, is highly uncertain – accounting for anywhere from 25-90% of the total heat flow. </p>
<h2>Elusive particles</h2>
<p>Atoms in radioactive materials have unstable nuclei, meaning they can split up (decay to a stable state) by giving off nuclear radiation – some of which gets converted to heat. This radiation consists of various particles with specific energies – depending on what material emitted them – including neutrinos. When the radioactive elements decay within the Earth’s crust and mantle, they emit “geo-neutrinos”. In fact, each second, the Earth radiates more than a trillion trillion such particles to space. Measuring their energy can tell researchers about what material produced them and therefore the composition of the Earth’s hidden interior.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/180913/original/file-20170803-7693-a3e55b.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/180913/original/file-20170803-7693-a3e55b.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=371&fit=crop&dpr=1 600w, https://images.theconversation.com/files/180913/original/file-20170803-7693-a3e55b.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=371&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/180913/original/file-20170803-7693-a3e55b.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=371&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/180913/original/file-20170803-7693-a3e55b.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=466&fit=crop&dpr=1 754w, https://images.theconversation.com/files/180913/original/file-20170803-7693-a3e55b.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=466&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/180913/original/file-20170803-7693-a3e55b.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=466&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Earth’s core.</span>
</figcaption>
</figure>
<p>The main known sources of radioactivity within the Earth are unstable types of uranium, thorium and potassium – something we know based on samples of rock up to 200km below the surface. What lurks beneath that depth is uncertain. We know that the geo-neutrinos emitted when uranium decays have more energy than those emitted when potassium splits up. So by measuring the energy of geo-neutrinos, we can know what type of radioactive material they come from. In fact, this is a much easier way to figure out what’s inside the Earth than drilling tens of kilometres down below the surface. </p>
<p>Unfortunately, geo-neutrinos are notoriously difficult to detect. Rather than interacting with ordinary matter such as that inside detectors, they tend to just whizz right through them. That’s why it took a huge underground detector filled with with about 1,000 tonnes of liquid <a href="https://www.nature.com/nature/journal/v436/n7050/full/nature03980.html">to make the first observation of geo-neutrinos</a>, in 2003. These detectors measure neutrinos by registering their collision with atoms in the liquid.</p>
<p>Since then, only one other experiment has managed <a href="http://www.sciencedirect.com/science/article/pii/S0370269310003722?via%3Dihub">to observe geo-neutrinos</a>, using a similar technology. Both measurements imply that approximately half of the Earth’s heat caused by radioactivity (20 terawatts) can be explained by decays of uranium and thorium. The source of the remaining 50% is an open question. </p>
<p>However, measurements so far have been unable to measure the contribution from potassium decays – the neutrinos emitted in this process have too low an energy. So it could be that the rest of the heat comes from potassium decay.</p>
<h2>New technology</h2>
<p>Our new research suggests we can make a map of the heat flow from inside the Earth by measuring the direction the geo-neutrino comes from, as well as its energy. This sounds simple, but the technological challenge is formidable, requiring new particle detection technology. </p>
<p>We propose using gas-filled “time projection chamber detectors”. Such detectors work by making a 3D picture of a geo-neutrino colliding with the gas inside it – knocking off an electron from a gas atom. The movement of this electron can then be tracked over time to reconstruct one dimension of the process (time). High-resolution imaging technology can then reconstruct the two spatial dimensions of its movement. In the liquid detectors currently used, the particles that get knocked off in collisions travel such a short distance (because they are in a liquid) that the direction is impossible to resolve. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/180903/original/file-20170803-17289-1o0gxw3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/180903/original/file-20170803-17289-1o0gxw3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/180903/original/file-20170803-17289-1o0gxw3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/180903/original/file-20170803-17289-1o0gxw3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/180903/original/file-20170803-17289-1o0gxw3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/180903/original/file-20170803-17289-1o0gxw3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/180903/original/file-20170803-17289-1o0gxw3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Earth heat flow map.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#/media/File:Earth_heat_flow.jpg">wikipedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Similar detectors, on a smaller scale, are currently used to make precision measurements of neutrino interactions, and to search for dark matter. We calculated that the size of the detector needed to discover the geo-neutrinos from radioactive potassium would be 20 tonnes. To properly map the mantle composition for the first time, it would need to be 10 times more massive. We have built a <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.95.122002">prototype</a> for such a detector, and are working on scaling up.</p>
<p>Measuring geo-neutrinos in this way could help map the heat flow in the Earth’s interior. This would help us to understand the evolution of the inner core by assessing the concentration of radioactive elements. It could also help unravel the longstanding mystery of what source of heat powers the convection (transfer of heat by movement of fluids) in the outer core that generates the Earth’s geomagnetic field. This field is vital for retaining our atmosphere which protects life on Earth from the sun’s harmful radiation. </p>
<p>It’s strange that we know so little about what’s going on under the ground that we walk on. That makes it exciting to think about how these measurements could finally allow the pioneering exploration of the veiled inner workings of the Earth.</p><img src="https://counter.theconversation.com/content/81524/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors of this work are Jocelyn Monroe, Michael Leyton and Stephen Dye. Jocelyn Monroe receives funding from the European Research Council and the UK Science and Technology Facilities Council. Stephen Dye appreciates past support from the Cooperative Studies of the Earth’s Deep Interior (CSEDI) and the Cooperative Institute for Dynamic Earth Research (CIDER) programs funded by the US National Science Foundation. </span></em></p><p class="fine-print"><em><span>Michael Leyton receives funding from the Marie Skłodowska-Curie Fellowship program, Ministerio de Economia, Industria y Competitividad (MINECO), Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER).</span></em></p>A new detector could work out what’s causing a heat flow from the Earth’s interior. It may even solve the mystery of what powers the Earth’s magnetic field.Jocelyn Monroe, Professor of Physics, Royal Holloway University of LondonMichael Leyton, Postdoctoral Researcher in Physics, Royal Holloway University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/604332016-06-06T20:05:38Z2016-06-06T20:05:38ZFrom dark gravity to phantom energy: what’s driving the expansion of the universe?<figure><img src="https://images.theconversation.com/files/125319/original/image-20160606-25988-17yaz00.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">There are two broad ways to measure the expansion of the universe. One is based on the cosmic microwave background, shown here, along with our own galaxy viewed in microwave wavelengths.</span> <span class="attribution"><span class="source">ESA, HFI & LFI consortia (2010)</span></span></figcaption></figure><p>There is something strange happening in the local universe, with galaxies moving away from each other <a href="http://www.science20.com/news_articles/the_universe_is_expanding_even_faster_than_believed-173999">faster than expected</a>. </p>
<p>What is driving this extra expansion, and what does it mean for the cosmos? To explore this, let’s start with the observations.</p>
<p>The rate of cosmic expansion is encapsulated in the “<a href="http://hyperphysics.phy-astr.gsu.edu/hbase/astro/hubble.html">Hubble constant</a>”, although don’t let the name fool you, as it’s not a constant and changes as the universe expands. </p>
<p>To determine this constant, astronomers must relate the distances to galaxies to the velocity they’re travelling away from us. But measuring astronomical distances has always proven difficult. This is because we lack convenient signposts, known as <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/astro/stdcand.html">standard candles</a> and <a href="http://philosophy-of-cosmology.ox.ac.uk/standard-rulers.html">rulers</a>, to chart the heavens. </p>
<p>So astronomers have built up cosmic distances through a series of steps, using overlapping methods to span the heavens. But each step in this <a href="https://terrytao.files.wordpress.com/2010/10/cosmic-distance-ladder.pdf">cosmological distance ladder</a> has its own quirks and uncertainties, and extraordinary effort over many decades has been expended to calibrate the various methods.</p>
<p>A new <a href="https://arxiv.org/pdf/1604.01424.pdf">paper</a> has pushed this calibration even harder, using a number of methods to tie down the Hubble constant to an accuracy of 2.4% within a few hundred million light years (which is local by cosmic standards). </p>
<p>A great success! But there’s a problem. </p>
<p>We can also determine the universal expansion from observations of the <a href="http://astronomy.swin.edu.au/cosmos/C/Cosmic+Microwave+Background">cosmic microwave background</a>, which is the radiation leftover from the <a href="http://www.universetoday.com/54756/what-is-the-big-bang-theory/">Big Bang</a>. </p>
<p>Unlike local observations, this reveals the global expansion of the universe. And this is where the problems begin, as this global expansion is 9% slower than that seen in the local universe. In both measurements, the astronomers have worked hard to reduce the uncertainties, and so are confident this difference is valid.</p>
<p>So what can explain this tension in cosmic measurement? Here are a few of the contenders. </p>
<h2>Cosmic contenders</h2>
<h3>Dark matter</h3>
<p>The first potential culprit is <a href="http://www.space.com/20930-dark-matter.html">dark matter</a>, the dominant <em>mass</em> in the universe. We know it is not smoothly spread through space, so perhaps the lumps and bumps, like the galaxies and clusters of galaxies, are exacting less gravitational pull in the local universe. </p>
<p>Perhaps we are in a <a href="http://www.universetoday.com/15719/the-cosmic-void-could-we-be-in-the-middle-of-it/">cosmic void</a>, a region whose density is below the universal average. </p>
<p>If this were the case, we would have to be inhabiting a strange corner of the universe, sitting at the centre of immense emptiness not very unlike anything expected in our <a href="http://astronomy.swin.edu.au/cosmos/L/large-scale+structure">cosmological ideas</a>. </p>
<h3>Dark energy</h3>
<p>And then there is <a href="http://hetdex.org/dark_energy/dark_matter.php">dark energy</a>, the dominant <em>energy</em> in the universe. This component is responsible for accelerating the cosmic expansion, but is assumed to have a very simple form, eternal and unchanging over all of history. </p>
<p>But what if dark energy is dynamic and evolving, changing its properties as the universe expands? If it changed quite recently (in cosmic terms), the additional expansion could be imprinted on the local universe, but have not yet impacted the global expansion. </p>
<p>If this is the case, the universe has something to worry about, as this new form of dark energy would be a “<a href="https://www.physics.rutgers.edu/%7Esaurabh/690/Mar27-Zhang-phantom.pdf">phantom</a>”, driving universal expansion faster and faster into a “<a href="http://www.telegraph.co.uk/news/science/science-news/11715091/Big-Rip-will-end-the-universe-scientists-claim.html">big rip</a>”, which is more dramatic than it sounds.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=417&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=417&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=417&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=524&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=524&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125320/original/image-20160606-25985-3ass74.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=524&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A diagram representing the evolution of the universe, starting with the Big Bang to present day. The red arrow marks the flow of time. New research suggests it’s expanding even faster than shown here.</span>
<span class="attribution"><span class="source">NASA/GSFC</span></span>
</figcaption>
</figure>
<h3>Dark radiation</h3>
<p>Another potential solution is “dark radiation”, which consists of hyper-fast particles that zipped around in the early universe. </p>
<p>While there is no single definition on what constitutes dark radiation, a favoured candidate is a new member of the <a href="http://www.ps.uci.edu/%7Esuperk/neutrino.html">neutrino family</a>, affectionately known as <a href="http://www.quantumdiaries.org/2014/07/27/sterile-neutrinos/">sterile neutrinos</a>. </p>
<p>While dark radiation is theoretical, there is little observational evidence for its existence. But if it had been present in the early universe, it would have influenced the early expansion of the universe, which would still be imprinted on the global value of the Hubble constant, but would now be washed out of the local value. </p>
<h3>Dark gravity</h3>
<p>The potential solutions so far have considered modifying the properties of components in the universe, but there is the more drastic alternative: <a href="http://arxiv.org/abs/0711.0077">dark gravity</a>. </p>
<p>This suggests that we don’t fully understand the fundamental nature of the universe, and that gravity does not follow the rules laid out by <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-bio.html">Albert Einstein</a> in his <a href="https://www.newscientist.com/round-up/instant-expert-general-relativity/">general theory of relativity</a>. </p>
<p>Such theories of <a href="http://arxiv.org/pdf/1106.2476v3.pdf">modified gravity</a> have existed for a long time, and come in many forms, and it is not clear how we deduce the impact of such gravity on the universal expansion. </p>
<h2>Dark speculations</h2>
<p>So there are several alternatives that could potentially explain the discrepancy between the local and global measurements of the Hubble constant. Which one is correct? </p>
<p>At the moment, the observations are rather raw and do not discriminate between the possibilities. And so we enter the realm of theoretical speculation, where ideas are tried and discarded until viable explanations are discovered.</p>
<p>At the same time, astronomers will seek more data, and will continue to tie down calibrations and methods. This brings us to our final possibility.</p>
<p>No observations are perfect, and much of science is about understanding the uncertainties of measurements. Scientists can generally wrangle <a href="https://explorable.com/random-error">random errors</a> and understand how uncertainties in measurement impact uncertainties in results. </p>
<p>But there is another uncertainty: the <a href="http://www.physics.umd.edu/courses/Phys276/Hill/Information/Notes/ErrorAnalysis.html">systematic error</a>, which can strike fear into a researcher. Instead of scattering results, systematic errors shift all results one way or another.</p>
<p>Systematic errors can also influence astronomical distance measures. And if they propagate through the distance ladder, they could potentially shift the local measurement of the Hubble constant away from the global value. </p>
<p>With new data and methods, this tension may evaporate. Some astronomers are already suggesting that this is a <a href="https://arxiv.org/pdf/1606.00634.pdf">“more reasonable explanation”</a>.</p><img src="https://counter.theconversation.com/content/60433/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Geraint Lewis receives funding from the Australian Research Council.</span></em></p>The universe is expanding faster than expected, but we don’t know what’s driving it. Here are a few of the possible explanations, from dark energy to a modification of general relativity.Geraint Lewis, Professor of Astrophysics, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/534142016-01-25T16:05:00Z2016-01-25T16:05:00ZExplainer: what is antimatter?<figure><img src="https://images.theconversation.com/files/109128/original/image-20160125-19660-nhp87q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">There's evidence that antimatter is produced in thunderstorms.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/computerhotline/20391956185/in/photolist-x4Y78t-p3YpHj-yKqDv-x5vJt8-tVoe6u-hBQj3T-o6SerG-yfsaB7-srUHRn-3FYBz-d8ySSU-feHppD-fB8TmL-7bc9YM-uCcZSS-35qj8A-cP1jnh-u4mgU-8sVyZB-9x843c-weQMvR-8s63Mj-8s1gUn-3DKMD-uU1BXG-vD97Bj-d8yTDL-f2Qm9-agNSLn-w8CJWj-6SHKPh-8cQwop-bWZ5MH-7beGV3-gmM1uT-8cEhoV-pejAy7-8m7YyG-m5dPPp-4Wz3yf-9x84hi-fJhdfL-x1P9Zk-afs3XS-gmMatW-ou7f1V-8cHAsE-6NsZQP-9x83WF-ozbPNN">Thomas Bresson/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Antimatter was one of the most exciting physics discoveries of the 20th century. Picked up by fiction writers <a href="http://angelsanddemons.web.cern.ch/antimatter">such as Dan Brown</a>, many people think of it as an “out there” theoretical idea – unaware that it is actually being produced every day. What’s more, <a href="https://theconversation.com/antimatter-measured-for-the-first-time-5782">research on antimatter</a> is actually helping us to <a href="https://theconversation.com/how-we-recreated-the-early-universe-in-the-laboratory-41399">understand</a> how the universe works.</p>
<p>Antimatter is a material composed of so-called antiparticles. It is believed that <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">every particle we know of</a> has an antimatter companion that is virtually identical to itself, but with the opposite charge. For example, an electron has a negative charge. But its antiparticle, called a positron, has the same mass but a positive charge. When a particle and its antiparticle meet, they annihilate each other – disappearing in a burst of light. </p>
<p>Such particles were first predicted by British physicist Paul Dirac when he was trying to combine the <a href="https://theconversation.com/will-we-have-to-rewrite-einsteins-theory-of-general-relativity-50057">two great ideas of early modern physics</a>: relativity and quantum mechanics. Previously, scientists were stumped by the fact that it seemed to predict that particles could have energies lower than when they were at “rest” (ie pretty much doing nothing). This seemed impossible at the time, as it meant that energies could be negative.</p>
<p>Dirac, however, accepted that the equations were telling him that particles are really filling a whole “sea” of these lower energies – a sea that had so far been invisible to physicists as they were only looking “above the surface”. He envisioned that all of the “normal” energy levels that exist are accounted for by “normal” particles. However, when a particle jumps up from a lower energy state, it appears as a normal particle but leaves a “hole”, which appears to us as a strange, mirror-image particle – antimatter.</p>
<p>Despite initial scepticism, examples of these particle-antiparticle pairs were soon found. For example, they are produced when <a href="http://home.cern/about/physics/cosmic-rays-particles-outer-space">cosmic rays hit the Earth’s atmosphere</a>. There is even evidence that the energy in thunderstorms produces <a href="http://example.com/">anti-electrons</a>, called positrons. These are also produced in some radioactive decays, a process used in many hospitals in Positron Emission Tomography (PET) scanners, which allow precise imaging within human bodies. Nowadays, experiments at the Large Hadron Collider (LHC) can produce matter and antimatter, too.</p>
<h2>Matter-antimatter mystery</h2>
<p>Physics predicts that matter and antimatter must be created in almost equal quantities, and that this would have been the case during the Big Bang. What’s more, it is predicted that the laws of physics should be the same if a particle is interchanged with its antiparticle – a relationship <a href="http://www.symmetrymagazine.org/article/october-2005/explain-it-in-60-seconds">known as CP symmetry</a>. However, the universe we see doesn’t seem to obey these rules. It is almost entirely made of matter, so where did all the antimatter go? It is one of the biggest mysteries in physics to date.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/108716/original/image-20160120-26120-yta5eg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/108716/original/image-20160120-26120-yta5eg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=202&fit=crop&dpr=1 600w, https://images.theconversation.com/files/108716/original/image-20160120-26120-yta5eg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=202&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/108716/original/image-20160120-26120-yta5eg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=202&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/108716/original/image-20160120-26120-yta5eg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=253&fit=crop&dpr=1 754w, https://images.theconversation.com/files/108716/original/image-20160120-26120-yta5eg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=253&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/108716/original/image-20160120-26120-yta5eg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=253&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Experimental area at CERN including the alpha experiment.</span>
<span class="attribution"><span class="source">Mikkel D. Lund/wikimeda</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Experiments have shown that some <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13.138">radioactive decay processes</a> do not produce an equal amount of antiparticles and particles. But it is not enough to explain the disparity between amounts of matter and antimatter in the universe. Consequently, physicists such as myself at the LHC, on <a href="http://atlas.ch/">ATLAS</a>, <a href="http://cms.web.cern.ch/">CMS</a> and <a href="http://lhcb-public.web.cern.ch/lhcb-public/">LHCb</a>, and others doing experiments with neutrinos such as <a href="http://t2k-experiment.org/">T2K</a> in Japan, are looking for other processes that could explain the puzzle. </p>
<p>Other groups of physicists such as the <a href="http://alpha.web.cern.ch/">Alpha Collaboration</a> at CERN are working at much lower energies to see if the properties of antimatter really are the mirror of their matter partners. Their <a href="http://nature.com/articles/doi:10.1038/nature16491">latest results</a> show that an anti-hydrogen atom (made up of an anti-proton and an anti-electron, or positron) is electrically neutral to an accuracy of less than one billionth of the charge of an electron. Combined with other measurements, this implies that the positron is equal and opposite to the charge of the electron to better than one part in a billion – confirming what is expected of antimatter. </p>
<p>However, a great many mysteries remain. Experiments are also investigating whether <a href="http://www.universetoday.com/101893/will-antimatter-obey-gravitys-pull/">gravity affects antimatter</a> in the same way that it affects matter. If these exact symmetries are shown to be broken, it will require a fundamental revision of our ideas about physics, affecting not only particle physics but also our understanding of gravity and relativity. </p>
<p>In this way, antimatter experiments are allowing us to put our understanding of the fundamental workings of the universe to new and exciting tests. Who knows what we will find?</p><img src="https://counter.theconversation.com/content/53414/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roger Jones receives STFC.</span></em></p>Antimatter is at the heart of one of the biggest conundrums in physics. Here’s why.Roger Jones, Professor of Physics, Head of Department, Lancaster UniversityLicensed as Creative Commons – attribution, no derivatives.