tag:theconversation.com,2011:/africa/topics/dark-matter-95/articlesDark matter – The Conversation2024-03-21T18:01:43Ztag:theconversation.com,2011:article/2262622024-03-21T18:01:43Z2024-03-21T18:01:43Z‘Dark stars’: dark matter may form exploding stars – and observing the damage could help reveal what it’s made of<figure><img src="https://images.theconversation.com/files/583424/original/file-20240321-23-mbtrm1.jpeg?ixlib=rb-1.1.0&rect=0%2C262%2C1280%2C597&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">We wouldn't be able to see them directly, but they could be out there.</span> <span class="attribution"><a class="source" href="https://esawebb.org/images/potm2301a/">ESA/Webb, NASA & CSA, A. Martel</a></span></figcaption></figure><p>Dark matter is a ghostly substance that astronomers have failed to detect for decades, yet which we know has an enormous influence on normal matter in the universe, such as stars and galaxies. Through the massive gravitational pull it exerts on galaxies, it spins them up, gives them an extra push along their orbits, or even rips them apart. </p>
<p>Like a cosmic carnival mirror, it also bends the light from distant objects to create distorted or multiple images, a process which is called <a href="https://theconversation.com/uk/topics/gravitational-lensing-112201">gravitational lensing</a>. </p>
<p>And <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.109.043018">recent research</a> suggests it may create even more drama than this, by producing stars that explode.</p>
<p>For all the havoc it plays with galaxies, not much is known about whether dark matter can interact with itself, other than through gravity. If it experiences other forces, they must be very weak, otherwise they would have been measured.</p>
<p>A possible candidate for a dark matter particle, made up of a hypothetical class of weakly interacting massive particles (or <a href="https://www.symmetrymagazine.org/article/july-2015/miraculous-wimps?language_content_entity=und">WIMPs</a>), has been studied intensely, so far with no observational evidence.</p>
<p>Recently, other types of particles, also weakly interacting but extremely light, have become the focus of attention. These particles, called <a href="https://bigthink.com/starts-with-a-bang/axions-dark-matter/">axions</a>, were first <a href="https://www.symmetrymagazine.org/article/the-other-dark-matter-candidate?language_content_entity=und">proposed in late 1970s</a> to <a href="https://www.forbes.com/sites/startswithabang/2019/11/19/the-strong-cp-problem-is-the-most-underrated-puzzle-in-all-of-physics/">solve a quantum problem</a>, but they may also fit the bill for dark matter. </p>
<p>Unlike WIMPs, which cannot “stick” together to form small objects, axions can do so. Because they are so light, a huge number of axions would have to account for all the dark matter, which means they would have to be crammed together. But because they are a type of subatomic particle known as a <a href="https://www.science.org/doi/10.1126/sciadv.abj3618">boson</a>, they don’t mind.</p>
<p>In fact, calculations show axions could be packed so closely that they start behaving strangely – collectively acting like a wave – according to the rules of quantum mechanics, the theory which governs the microworld of atoms and particles. This state is called a <a href="https://www.pbs.org/wgbh/nova/article/ultracold-atoms/">Bose-Einstein condensate</a>, and it may, unexpectedly, <a href="https://www.livescience.com/63977-axion-stars-form-quickly.html">allow axions to form “stars”</a> of their own. </p>
<p>This would happen when the wave moves on its own, forming what physicists call a “soliton”, which is a localised lump of energy that can move without being distorted or dispersed. This is often seen on Earth in vortexes and whirlpools, or the bubble rings that <a href="https://thekidshouldseethis.com/post/what-will-dolphins-make-of-these-underwater-bubble-rings">dolphins enjoy underwater</a>.</p>
<p>The <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.109.043018">new study</a> provides calculations which show that such solitons would end up growing in size, becoming a star, similar in size to, or larger than, a normal star. But finally, they become unstable and explode. </p>
<p>The energy released from one such explosion (dubbed a “bosenova”) would rival that of a supernova (an exploding normal star). Given that dark matter far outweighs the visible matter in the universe, this would surely leave a sign in our observations of the sky. We have yet to find such scars, but the new study gives us something to look for.</p>
<h2>An observational test</h2>
<p>The <a href="https://phys.org/news/2024-02-explosive-axion-stars-dark.html">researchers behind the study</a> say that the surrounding gas, made of normal matter, would absorb this extra energy from the explosion and emit some of it back. Since most of this gas is made of hydrogen, we know this light should be in radio frequencies. </p>
<p>Excitingly, future observations with the <a href="https://theconversation.com/in-australia-and-south-africa-construction-has-started-on-the-biggest-radio-observatory-in-earths-history-195818">Square Kilometre Array</a> radio telescope may be able to pick it up.</p>
<figure class="align-center ">
<img alt="Artist's impression of the SKA telescope." src="https://images.theconversation.com/files/583420/original/file-20240321-20-b3sckt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/583420/original/file-20240321-20-b3sckt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=328&fit=crop&dpr=1 600w, https://images.theconversation.com/files/583420/original/file-20240321-20-b3sckt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=328&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/583420/original/file-20240321-20-b3sckt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=328&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/583420/original/file-20240321-20-b3sckt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=412&fit=crop&dpr=1 754w, https://images.theconversation.com/files/583420/original/file-20240321-20-b3sckt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=412&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/583420/original/file-20240321-20-b3sckt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=412&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 SKA telescope.</span>
<span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>So, while the fireworks from dark star explosions may be hidden from our view, we might be able to find their aftermath in the visible matter. What’s great about this is that such a discovery would help us work out what dark matter is actually made of – in this case, most likely axions.</p>
<p>What if observations will not detect the predicted signal? That probably won’t rule out this theory completely, as other “axion-like” particles are still possible. A failure of detection may indicate, however, that the masses of these particles are very different, or that they do not couple with radiation as strongly as we thought.</p>
<p>In fact, this has happened before. Originally, it was thought that axions would couple so strongly that they would be able to <a href="https://www.youtube.com/watch?v=3EjezghXkaw">cool the gas inside stars</a>. But since models of star cooling showed stars were just fine without this mechanism, the axion coupling strength had to be lower than originally assumed.</p>
<p>Of course, there is no guarantee that dark matter is made of axions. WIMPs are still contenders in this race, and <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">there are others too</a>. </p>
<p>Incidentally, some studies suggest that WIMP-like dark matter <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.100.051101">may also form “dark stars”</a>. In this case, the stars would still be normal (made of hydrogen and helium), with dark matter just powering them. </p>
<p>These WIMP-powered dark stars are predicted to be supermassive and to live only for a short time in the early universe. But they could be observed by the James Webb space telescope. A recent study has claimed <a href="https://www.scientificamerican.com/article/jwst-might-have-spotted-the-first-dark-matter-stars/">three such discoveries</a>, although the jury is still out on whether that’s really the case.</p>
<p>Nevertheless, the excitement about axions is growing, and there are many plans to detect them. For example, axions are expected <a href="https://theconversation.com/this-australian-experiment-is-on-the-hunt-for-an-elusive-particle-that-could-help-unlock-the-mystery-of-dark-matter-187014">to convert into photons</a> when they pass through a magnetic field, so observations of photons with a certain energy are targeting stars with magnetic fields, such as neutron stars, or even <a href="https://home.cern/science/experiments/cast">the Sun</a>.</p>
<p>On the theoretical front, there are efforts to refine the predictions for what the universe would look like with different types of dark matter. For example, axions may be distinguished from WIMPs <a href="https://theconversation.com/new-look-at-einstein-rings-around-distant-galaxies-just-got-us-closer-to-solving-the-dark-matter-debate-204109">by the way they bend the light</a> through gravitational lensing.</p>
<p>With better observations and theory, we are hoping that the mystery of dark matter will soon be unlocked.</p><img src="https://counter.theconversation.com/content/226262/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>We may be able to find traces of dark matter star explosions.Andreea Font, Reader in Theoretical Astrophysics, Liverpool John Moores UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2259062024-03-20T17:15:37Z2024-03-20T17:15:37ZHow a balloon-borne experiment can do the job of the Hubble space telescope<figure><img src="https://images.theconversation.com/files/583085/original/file-20240320-20-8uzvny.jpg?ixlib=rb-1.1.0&rect=45%2C34%2C3788%2C2121&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">SuperBIT waiting for launch while its giant helium balloon is inflated.</span> <span class="attribution"><span class="source">Bill Rodman/NASA</span></span></figcaption></figure><p>An astronomical telescope designed to complement the ageing <a href="https://esahubble.org/">Hubble Space Telescope</a> lifted off from New Zealand’s south island on April 16 2023. But as a sphere the size of a football stadium rose silently and slowly over the Tauhinukorokio mountains, calls started coming in from residents. </p>
<p>Local police and radio stations, however, had been briefed by Nasa that the giant helium balloon would lift the two-ton <a href="https://sites.physics.utoronto.ca/bit">SuperBIT</a> telescope to 40km above sea level, over the next three hours. The mission, in which we were involved, was to test whether a balloon-borne telescope could capture <a href="https://blogs.nasa.gov/superpressureballoon/category/2023-campaign/superbit/">deep space images</a> with high enough resolution to study the unknown substance, dubbed <a href="https://www.esa.int/Science_Exploration/Space_Science/What_are_dark_matter_and_dark_energy">dark matter</a>, that is 85% of all material in the universe.</p>
<p>The observations and subsequent data analysis have proved that balloon-borne experiments can be just as useful as those launched by rockets, but are much cheaper. It is now up to scientists, government agencies and private companies to make the most of them.</p>
<p>For the next month, <a href="https://earth.nullschool.net/#2023/05/01/2300Z/wind/isobaric/10hPa/orthographic=-213.50,-71.03,550">polar stratospheric winds</a> carried SuperBIT <a href="https://www.csbf.nasa.gov/map/balloon10/Google728NT.htm">around the world every eight days</a>, mainly over the Antarctic ocean but clipping the tip of South America. It went where the wind carried it, but could look in any direction. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Flight path of SuperBIT, five and a half times around the Southern ocean." src="https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/582186/original/file-20240315-24-1009uq.png?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"></a>
<figcaption>
<span class="caption">Flight path of SuperBIT.</span>
<span class="attribution"><span class="source">Nasa</span></span>
</figcaption>
</figure>
<p>Each day, solar panels recharged its batteries. At night, it photographed the sky, including the <a href="https://apod.nasa.gov/apod/ap230427.html">Tarantula nebula</a>, a light source 160,000 light years away, and clusters of galaxies 20,000 times farther. </p>
<p>Without a tripod, SuperBIT used gyroscopes to stabilise any swinging (we discovered that the stratosphere is remarkably calm … except in turbulence above the Andes, where SuperBIT once dropped 1,000 feet). It was the first balloon-borne telescope to achieve <a href="https://pubs.aip.org/aip/rsi/article/91/3/034501/1032358/Robust-diffraction-limited-near-infrared-to-near">Hubble-like performance</a> for the short wavelengths of light that are visible to a human eye.</p>
<p>The balloon and the telescope continued to work perfectly, but satellite communication links gradually failed. We think radiation damaged SuperBIT’s <a href="https://www.starlink.com">antennae</a>. We could still download data by <a href="https://www.mdpi.com/2226-4310/10/11/960">dropping hard drives</a>, attached to the telescope, to the ground. But ultimately, Nasa wanted their balloon back, so we brought the telescope down by parachute to Argentina. </p>
<p>This was SuperBIT’s <a href="https://arxiv.org/abs/1807.02887">fifth flight</a>, building on ten years of graft. </p>
<h2>Balloon benefits</h2>
<p>Unlike orbital missions, if balloon payloads don’t work first time, they can be fixed and relaunched. This fosters simple, creative design. Components now proven to <a href="https://www.nasa.gov/directorates/somd/space-communications-navigation-program/technology-readiness-levels/">work in space</a> include hair gel (to hold things), chicken roasting bags (to keep them warm), and parts of bows used by Olympic archers (to let them go).</p>
<p>Failure and success are both opportunities to learn. After each flight, we make-do-and-mend, or improve the technology. For example, since cameras have rapidly got better and cheaper, we have fitted SuperBIT with a new sensor each year. All this reduces costs.</p>
<p>Most of the cost of traditional spaceflight is to mitigate the risk of failure. Compromises are always needed between safety, protecting expensive equipment and getting data. </p>
<p>If a balloon mission goes wrong, it usually matters less, because we get the equipment back. SuperBIT was built mainly by <a href="https://sites.physics.utoronto.ca/barthnetterfield">Canadian PhD students</a>, who have already spun-out a new <a href="https://www.starspectechnologies.com">tech company</a>.</p>
<p>Risk management is different for balloons, and Nasa doesn’t always get the balance right. Waiting for “perfect” weather and the perfectly designed balloon <a href="https://ntrs.nasa.gov/api/citations/20210017816/downloads/2021%20Balloon%20Technology%20Presentation%20-%20Overview%20of%20the%20NASA%20Scientific%20Balloon%20Activities%20-%20Fairbrother.pptx.pdf">grounded all launches from Texas in 2017</a>. Physically impossible calculations of risk, such as a balloon bursting three times, nearly tanked the 2023 programme. </p>
<p>A balloon can only burst once. But <a href="https://cnes.fr/en">France’s</a> and <a href="https://www.asc-csa.gc.ca/eng">Canada’s</a> space agencies, the US <a href="https://ncar.ucar.edu/">National Center for Atmospheric Research</a> and the UK Science Research Council have all proved that a balloon can be relaunched every few days. Risk assessment can be more realistic. Balloon teams can continually test, play around with and improve the process. For rocket launches, there is one chance only.</p>
<h2>Growing international interest</h2>
<p>Geography is important in developing a successful national balloon programme. Countries with expansive landmass can carry out short flights within their own airspace, such as Canada and the US. Northern European countries can use <a href="https://earth.nullschool.net/#2023/07/02/1500Z/wind/isobaric/10hPa/orthographic=-52.92,30.22,533">stable and reliable summer winds</a> to extend flights across the Atlantic ocean, for example from Scotland to Canada.</p>
<p>Countries can also launch from the territory of partner nations around the world, such as the UK <a href="https://www.gov.uk/government/news/space-bridge-across-the-world-will-help-uk-and-australia-get-ahead-in-global-space-race">launching from Australia</a>. </p>
<p>Geopolitics also influences the choice of flight path: a lesson well learned from the <a href="https://www.bbc.co.uk/news/world-us-canada-66062562">rogue Chinese balloon</a> that flew over the US in 2023 and was ultimately shot down. Crossing any country’s airspace requires permission, and we avoid war zones or areas of conflict where the balloon could be mistaken for a hostile target. This is one reason we launched from New Zealand.</p>
<figure class="align-center ">
<img alt="SuperBIT held by a crane for final checks." src="https://images.theconversation.com/files/582942/original/file-20240319-28-a2dkvq.jpeg?ixlib=rb-1.1.0&rect=217%2C0%2C3814%2C2933&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/582942/original/file-20240319-28-a2dkvq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/582942/original/file-20240319-28-a2dkvq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/582942/original/file-20240319-28-a2dkvq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/582942/original/file-20240319-28-a2dkvq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/582942/original/file-20240319-28-a2dkvq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/582942/original/file-20240319-28-a2dkvq.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">
<figcaption>
<span class="caption">SuperBIT held by a crane for final checks.</span>
<span class="attribution"><span class="source">Richard Massey</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Government interest in national balloon programmes is increasing, as new material science and manufacturing techniques have created balloons that retain helium, lengthening flights from days to months. The US reaffirmed their interest in a <a href="https://www.whitehouse.gov/briefing-room/statements-releases/2023/12/20/fact-sheet-strengthening-u-s-international-space-partnerships/">2023 government paper</a> and <a href="https://www.asc-csa.gc.ca/eng/sciences/balloons/about-stratospheric-balloons.asp">Canada</a>, <a href="https://cnes.fr/en/how-stratospheric-balloons-work%20and%20https://www.hemera-h2020.eu/facilities-2/cnes-balloons/">France</a> and <a href="https://sscspace.com/esrange/">Sweden</a> have long-established balloon programmes. </p>
<p>The UK ran a world-leading balloon programme until the 1990s. Abandoning it lost an opportunity to train scientists and engineers into leadership roles. British teams are still often invited to join French or US satellite missions, but we no longer lead or decide what gets built. We foresee few technical, geographic or political barriers to the UK restarting a balloon programme in parallel to its nascent rocket launches.</p>
<h2>Balloons are high enough</h2>
<p>Officially, space begins 100km above sea level. But there is no magic line, and <a href="https://iopscience.iop.org/article/10.3847/1538-3881/abbffb">precious little atmosphere above 40km</a>. There, stars stop twinkling and the sky is black. Long exposure astronomical photographs become pin sharp and reveal faint, distant objects that are blurred to astronomers on the ground.</p>
<figure class="align-center ">
<img alt="SuperBIT shrouded in early-morning mist before launch." src="https://images.theconversation.com/files/582920/original/file-20240319-28-8sm5fb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/582920/original/file-20240319-28-8sm5fb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/582920/original/file-20240319-28-8sm5fb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/582920/original/file-20240319-28-8sm5fb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/582920/original/file-20240319-28-8sm5fb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/582920/original/file-20240319-28-8sm5fb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/582920/original/file-20240319-28-8sm5fb.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">SuperBIT shrouded in early-morning mist before launch.</span>
<span class="attribution"><span class="source">Steven Benton</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Balloon cameras or spectrographs can also look down, and are high enough to capture Earth observations just like those from satellites. They can also take atmospheric measurements around them, including of the ozone layer in the stratosphere.</p>
<p>Balloons won’t replace all rockets, as they can’t travel higher than 40km.
And even though helium is a <a href="https://www.npr.org/2019/11/01/775554343/the-world-is-constantly-running-out-of-helium-heres-why-it-matters">finite resource</a>, balloons are more “environmentally-friendly”. They require no rocket fuels during launch, don’t add to increasing space debris in orbit – and at the end of their working life, they aren’t <a href="https://theconversation.com/satellites-are-burning-up-in-the-upper-atmosphere-and-we-still-dont-know-what-impact-this-will-have-on-the-earths-climate-223618">burnt up in the atmosphere</a>. What’s not to like?</p><img src="https://counter.theconversation.com/content/225906/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Richard Massey has received funding for SuperBIT from the Royal Society and from UKRI's Science and Technology Facilities Council. </span></em></p><p class="fine-print"><em><span>Fionagh Thomson has carried out consultancy work for the UK space agency. She is an elected member of the sustainability committees for the Royal Astronomical Society and the European Astronomical society.</span></em></p>Giant helium balloons are a cheap, more environmentally friendly alternative to rocket launches – and you get the satellite back.Richard Massey, Professor of extragalactic astrophysics (dark matter and cosmology), Durham UniversityFionagh Thomson, Senior Research Fellow in Disruptive Technologies, Space/Environmental Ethics, Visual ethnographer, Durham UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2204232024-01-08T21:23:31Z2024-01-08T21:23:31ZWhy is the universe ripping itself apart? A new study of exploding stars shows dark energy may be more complicated than we thought<figure><img src="https://images.theconversation.com/files/568137/original/file-20240107-25-i5il3j.jpg?ixlib=rb-1.1.0&rect=5%2C14%2C938%2C930&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The remains of a Type Ia supernova – a kind of exploding star used to measure distances in the universe.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-article/exploded-star-blooms-like-cosmic-flower/">NASA / CXC / U.Texas</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>What is the universe made of? This question has driven astronomers for hundreds of years. </p>
<p>For the past quarter of a century, scientists have believed “normal” stuff like atoms and molecules that make up you, me, Earth, and nearly everything we can see only accounts for 5% of the universe. Another 25% is “dark matter”, an unknown substance we can’t see but which we can detect through how it affects normal matter via gravity. </p>
<p>The remaining 70% of the cosmos is made of “dark energy”. Discovered in 1998, this is an unknown form of energy believed to be making the universe expand at an ever-increasing rate. </p>
<p>In <a href="https://arxiv.org/abs/2401.02929">a new study</a> soon to be published in the Astronomical Journal, we have measured the properties of dark energy in more detail than ever before. Our results show it may be a hypothetical vacuum energy first proposed by Einstein – or it may be something stranger and more complicated that changes over time. </p>
<h2>What is dark energy?</h2>
<p>When Einstein developed the General Theory of Relativity over a century ago, he realised his equations showed the universe should either be expanding or shrinking. This seemed wrong to him, so he added a “cosmological constant” – a kind of energy inherent in empty space – to balance out the force of gravity and keep the universe static. </p>
<p>Later, when the work of Henrietta Swan Leavitt and Edwin Hubble showed the universe was indeed expanding, Einstein did away with the cosmological constant, calling it his “greatest mistake”.</p>
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Read more:
<a href="https://theconversation.com/more-than-70-of-the-universe-is-made-of-dark-energy-the-mysterious-stuff-even-stranger-than-dark-matter-131569">More than 70% of the Universe is made of 'dark energy', the mysterious stuff even stranger than dark matter</a>
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<p>However, in 1998, two teams of researchers found the expansion of the universe was actually accelerating. This implies that something quite similar to Einstein’s cosmological constant may exist after all – something we now call dark energy.</p>
<p>Since those initial measurements, we’ve been using supernovae and other probes to measure the nature of dark energy. Until now, these results have shown the density of dark energy in the universe appears to be constant.</p>
<p>This means the strength of dark energy remains the same, even as the universe grows – it doesn’t seem to be spread more thinly as the universe gets bigger. We measure this with a number called <em>w</em>. Einstein’s cosmological constant in effect set <em>w</em> to –1, and earlier observations have suggested this was about right.</p>
<h2>Exploding stars as cosmic measuring sticks</h2>
<p>How do we measure what is in the universe and how fast it is growing? We don’t have enormous tape measures or giant scales, so instead we use “standard candles”: objects in space whose brightness we know. </p>
<p>Imagine it is night and you are standing on a long road with a few light poles. These poles all have the same light bulb, but the poles further away are fainter than the nearby ones. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A small star slurping material from a much larger one." src="https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=506&fit=crop&dpr=1 600w, https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=506&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=506&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=635&fit=crop&dpr=1 754w, https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=635&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/568131/original/file-20240107-28-pe9gc9.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=635&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">In a Type Ia supernova, a white dwarf slowly pulls mass from a neighboring star before exploding.</span>
<span class="attribution"><a class="source" href="https://exoplanets.nasa.gov/resources/2172/type-ia-supernova/">NASA / JPL-Caltech</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>This is because light fades proportionately to distance. If we know the power of the bulb, and can measure how bright the bulb appears to be, we can calculate the distance to the light pole. </p>
<p>For astronomers, a common cosmic light bulb is a kind of exploding star called a Type Ia supernova. These are white dwarf stars which often suck in matter from a neighbouring star and grow until they reach 1.44 times the mass of our Sun, at which point they explode. By measuring how quickly the explosion fades, we can determine how bright it was and hence how far away from us.</p>
<h2>The Dark Energy Survey</h2>
<p>The <a href="https://www.darkenergysurvey.org/">Dark Energy Survey</a> is the largest effort yet to measure dark energy. More than 400 scientists across multiple continents work together for nearly a decade to repeatedly observe parts of the southern sky. </p>
<p>Repeated observations let us look for changes, like new exploding stars. The more often you observe, the better you can measure these changes, and the larger the area you search, the more supernovae you can find.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo of a red-lit observatory building with the starry sky in the background." src="https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=398&fit=crop&dpr=1 600w, https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=398&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=398&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=500&fit=crop&dpr=1 754w, https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=500&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/568132/original/file-20240107-15-yxxsh1.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=500&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 Cerro Tololo Inter-American Observatory 4-metre telescope which was used by and the Dark Energy Survey.</span>
<span class="attribution"><a class="source" href="https://vms.fnal.gov/asset/detail?recid=1814576">Reidar Hahn / Fermilab</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
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</figure>
<p>The first results indicating the existence of dark energy used only a couple of dozen supernovae. The latest results from the Dark Energy Survey use around 1,500 exploding stars, giving much greater precision.</p>
<p>Using a specially built camera installed on the 4-metre Blanco Telescope at the Cerro-Tololo Inter-American Observatory in Chile, the survey found thousands of supernovae of different types. To work out which ones were Type Ia (the kind we need for measuring distances), we used the 4-metre Anglo Australian Telescope at Siding Spring Observatory in New South Wales. </p>
<hr>
<p>
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<strong>
Read more:
<a href="https://theconversation.com/relax-the-expansion-of-the-universe-is-still-accelerating-67691">Relax, the expansion of the universe is still accelerating</a>
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<p>The Anglo Australian Telescope took measurements which broke up the colours of light from the supernovae. This lets us see a “fingerprint” of the individual elements in the explosion. </p>
<p>Type Ia supernovae have some unique features, like containing no hydrogen and silicon. And with enough supernovae, machine learning allowed us to classify thousands of supernovae efficiently. </p>
<h2>More complicated than the cosmological constant</h2>
<p>Finally, after more than a decade of work and studying around 1,500 Type Ia supernovae, the Dark Energy Survey has produced a new best measurement of <em>w</em>. We found <em>w</em> = –0.80 ± 0.18, so it’s somewhere between –0.62 and –0.98.</p>
<p>This is a very interesting result. It is close to –1, but not quite exactly there. To be the cosmological constant, or the energy of empty space, it would need to be exactly –1. </p>
<p>Where does this leave us? With the idea that a more complex model of dark energy may be needed, perhaps one in which this mysterious energy has changed over the life of the universe.</p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/from-dark-gravity-to-phantom-energy-whats-driving-the-expansion-of-the-universe-60433">From dark gravity to phantom energy: what's driving the expansion of the universe?</a>
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<img src="https://counter.theconversation.com/content/220423/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Brad E Tucker receives funding from the Australian Research Council and ACT Government. </span></em></p>After a decade studying thousands of supernovae, astronomers are still perplexed by the enigma that led Einstein to his ‘greatest mistake’.Brad E Tucker, Astrophysicist/Cosmologist, Australian National UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2202402024-01-08T20:01:06Z2024-01-08T20:01:06ZDark energy is one of the biggest puzzles in science and we’re now a step closer to understanding it<p>Over ten years ago, the <a href="https://www.darkenergysurvey.org/">Dark Energy Survey (DES)</a> began mapping the universe to find evidence that could help us understand the nature of the mysterious phenomenon known as dark energy. I’m one of more than 100 contributing scientists that have helped produce the final <a href="https://arxiv.org/pdf/2401.02929.pdf">DES measurement</a>, which has just been released at the <a href="https://aas.org/meetings/aas243">243rd American Astronomical Society meeting</a> in New Orleans.</p>
<p><a href="https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/">Dark energy</a> is estimated to make up nearly 70% of the observable universe, yet we still don’t understand what it is. While its nature remains mysterious, the impact of dark energy is felt on grand scales. Its primary effect is to drive the <a href="https://www.nobelprize.org/uploads/2018/06/advanced-physicsprize2011.pdf">accelerating expansion of the universe</a>.</p>
<p>The announcement in New Orleans may take us closer to a better understanding of this form of energy. Among other things, it gives us the opportunity to test our observations against an idea called the <a href="https://map.gsfc.nasa.gov/universe/uni_accel.html">cosmological constant</a> that was introduced by Albert Einstein in 1917 as a way of counteracting the effects of gravity in his equations to achieve a universe that was neither expanding nor contracting. Einstein later removed it from his calculations.</p>
<p>However, cosmologists later discovered that not only was the universe expanding, but the expansion was accelerating. This observation was attributed to the mysterious quantity called dark energy. Einstein’s concept of the cosmological constant could actually explain dark energy if it had a positive value (allowing it to conform to the accelerating expansion of the cosmos).</p>
<p>The DES results are the culmination of decades of work by researchers around the globe and provide one of the best measurements yet of an elusive parameter called “w”, which stands for the <a href="https://www.grc.nasa.gov/www/k-12/airplane/eqstat.html">“equation of state</a>” of dark energy. Since the discovery of dark energy in 1998, the value of its equation of state has been a fundamental question.</p>
<p>This state describes the ratio of pressure over energy density for a substance. Everything in the universe has an equation of state. </p>
<p>Its value tells you whether a substance is gas-like, relativistic (described by Einstein’s theory of relativity) or not, or if it behaves like a fluid. Working out this figure is the first step to really understanding the true nature of dark energy.</p>
<p>Our best theory for w predicts that it should be exactly minus one (w=-1). This prediction also assumes that dark energy is the cosmological constant proposed by Einstein.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-euclid-spacecraft-will-transform-how-we-view-the-dark-universe-204245">The Euclid spacecraft will transform how we view the 'dark universe'</a>
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<h2>Subverting expectations</h2>
<p>An equation of state of minus one tells us that as the energy density of dark energy increases, so the negative pressure also increases. The more energy density in the universe, the more repulsion there is – in other words, matter pushes against other matter. This leads to an ever-expanding accelerating universe. It might sound a bit bizarre, as it is counterintuitive to everything we experience on Earth.</p>
<p>The work uses the most direct probe we have on the expansion history of the universe: <a href="https://newscenter.lbl.gov/2014/03/03/standard-candle-supernovae/">Type Ia supernovae</a>. These are a type of star explosion and they act as a kind of cosmic yardstick, allowing us to measure staggeringly large distances far into the universe. These distances can then be compared to our expectations. This is the same technique that was used to detect the existence of dark energy 25 years ago.</p>
<p>The difference now is in the size and quality of our sample of supernovae. Using new techniques, the DES team has 20 times more data, over a wide range of distances. This allows for one of the most precise ever measurements of w, giving a value of -0.8</p>
<figure class="align-center ">
<img alt="Vera Rubin Observatory." src="https://images.theconversation.com/files/568424/original/file-20240109-25-st53oq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/568424/original/file-20240109-25-st53oq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/568424/original/file-20240109-25-st53oq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/568424/original/file-20240109-25-st53oq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/568424/original/file-20240109-25-st53oq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/568424/original/file-20240109-25-st53oq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/568424/original/file-20240109-25-st53oq.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">
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<span class="caption">Facilities such as the Vera Rubin Observatory will make further measurements.</span>
<span class="attribution"><a class="source" href="https://rubin.canto.com/v/gallery/album/HDSNU?display=curatedView&viewIndex=2&column=image&id=hfgkvecufl6krfopg1oq7bbv5g">H. Stockebrand/Rubin/NSF/AURARubinObs/NSF/AURA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>At first sight, this is not the precise minus one value that we predicted. This might indicate that it is not the cosmological constant. However, the uncertainty on this measurement is large enough to allow minus one at a 5% chance, or betting odds of only 20 to 1. This level of uncertainty is not good enough yet to say either way, but it’s an excellent start.</p>
<p>The detection of the Higgs Boson subatomic particle in 2012 at the Large Hadron Collider required odds of a million to one chance of being wrong. However, this measurement may signal <a href="https://www.wired.co.uk/article/big-rip-end-of-the-universe">the end of “Big Rip” models</a> which have equations of state that are more negative than one. In such models the universe would expand indefinitely at a faster and faster rate – eventually pulling apart galaxies, planetary systems and even space-time itself. That’s a relief.</p>
<p>As usual, scientists want more data and those plans are already well underway. The DES results suggest that our new techniques will work for future supernova experiments with <a href="https://www.esa.int/Science_Exploration/Space_Science/Euclid">ESA’s Euclid mission</a> (launched July 2023) and the new Vera Rubin Observatory in Chile. This observatory should soon use its telescope to take a first image of the sky following construction, giving a glimpse into its capabilities. </p>
<p>These next-generation telescopes could find thousands more supernovae, helping us make new measurements of the equation of state and shedding even more light on the nature of dark energy.</p><img src="https://counter.theconversation.com/content/220240/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Robert Nichol is a member of the Dark Energy Survey collaboration.</span></em></p>The nature of dark energy remains one of the biggest puzzles in cosmology.Robert Nichol, Pro Vice-Chancellor and Executive Dean, University of SurreyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2170122023-11-08T16:41:22Z2023-11-08T16:41:22ZHow we’re building the world’s biggest optical telescope to crack some of the greatest puzzles in science<figure><img src="https://images.theconversation.com/files/557968/original/file-20231107-25-fl42fz.jpg?ixlib=rb-1.1.0&rect=237%2C89%2C5185%2C3533&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">ESO’s Extremely Large Telescope.</span> <span class="attribution"><span class="source">ESO/wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Astronomers get to ask some of the most fundamental questions there are, ranging from whether we’re alone in the cosmos to what the nature of the mysterious dark energy and dark matter making up most of the universe is.</p>
<p>Now a large group of astronomers from all over the world is building the biggest optical telescope ever – the <a href="https://elt.eso.org/">Extremely Large Telescope (ELT)</a> — in Chile. Once construction is completed in 2028, it could provide answers that transform our knowledge of the universe. </p>
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<iframe id="noa-web-audio-player" style="border: none" src="https://embed-player.newsoveraudio.com/v4?key=x84olp&id=https://theconversation.com/how-were-building-the-worlds-biggest-optical-telescope-to-crack-some-of-the-greatest-puzzles-in-science-217012&bgColor=F5F5F5&color=D8352A&playColor=D8352A" width="100%" height="110px"></iframe>
<p><em>You can listen to more articles from The Conversation <a href="https://theconversation.com/us/topics/audio-narrated-99682">narrated by Noa</a>.</em></p>
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<p>With its 39-metre diameter primary mirror, the ELT will contain the largest, most perfect reflecting surface ever made. Its light-collecting power will exceed that of all other large telescopes combined, enabling it to detect objects millions of times fainter than <a href="https://supernova.eso.org/exhibition/0805/">the human eye can see</a>.</p>
<p>There are several reasons why we need such a telescope. Its incredible sensitivity will let it image some of the first galaxies ever formed, with light that has travelled for 13 billion years to reach the telescope. Observations of such distant objects may allow us to refine our understanding of cosmology and the nature of <a href="https://theconversation.com/dark-matter-should-we-be-so-sure-it-exists-heres-how-philosophy-can-help-184109">dark matter</a> and <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a>.</p>
<h2>Alien life</h2>
<p>The ELT may also offer an answer to the most fundamental question of all: are we alone in the universe? The ELT is expected to be the first telescope to track down Earth-like exoplanets — planets that orbit other stars but have a similar mass, orbit and proximity to their host as Earth. </p>
<p>Occupying the so-called Goldilocks zone, these Earth-like planets will orbit their star at just the right distance for water to neither boil nor freeze – providing the conditions for life to exist.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Size comparison between the ELT and other telescope domes." src="https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=113&fit=crop&dpr=1 600w, https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=113&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=113&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=142&fit=crop&dpr=1 754w, https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=142&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/557768/original/file-20231106-15-g3l1kl.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=142&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">Size comparison between the ELT and other telescope domes.</span>
<span class="attribution"><span class="source">ESO/wikipeida</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The ELT’s camera will have six times better resolution than that of the <a href="https://webb.nasa.gov/">James Webb Space Telescope</a>, allowing it to take the clearest images yet of exoplanets. But fascinating as these pictures will be, they will not tell the whole story.</p>
<p>To learn if life is likely to exist on an exoplanet, astronomers must complement imaging with spectroscopy. While images reveal shape, size and structure, spectra tell us about the speed, temperature and even the chemistry of astronomical objects.</p>
<p>The ELT will contain not one, but four spectrographs — instruments that disperse light into its constituent colours, much like the iconic prism on the Pink Floyd’s The <a href="https://theconversation.com/the-dark-side-of-the-moon-at-50-an-album-artwork-expert-on-pink-floyds-music-marketing-revolution-200932">Dark Side of the Moon</a> album cover.</p>
<p>Each about the size of a minibus, and carefully environmentally controlled for stability, these spectrographs underpin all of the ELT’s key science cases. For giant exoplanets, the <a href="https://elt.eso.org/instrument/HARMONI/">Harmoni instrument</a> will analyse light that has travelled through their atmospheres, looking for the signs of water, oxygen, methane, carbon dioxide and other gases that indicate the existence of life.</p>
<p>To detect much smaller Earth-like exoplanets, the more specialised <a href="https://elt.eso.org/instrument/ANDES/">Andes instrument</a> will be needed. With a cost of around €35 million (£30 million), Andes will be able to detect tiny changes in the wavelength of light.</p>
<p>From previous satellite missions, astronomers already have a good idea of where to look in the sky for exoplanets. Indeed, there have been several thousand confirmed or “candidate” exoplanets detected using <a href="https://theconversation.com/explainer-how-do-you-find-exoplanets-24153">the “transit method”</a>. Here, a space telescope stares at a patch of sky containing thousands of stars and looks for tiny, periodic dips in their intensities, caused when an orbiting planet passes in front of its star.</p>
<p>But Andes will use a different method to hunt for other Earths. As an exoplanet orbits its host star, <a href="https://www.eso.org/public/unitedkingdom/videos/eso1035g/">its gravity tugs on the star, making it wobble</a>. This movement is incredibly small; Earth’s orbit causes the Sun to oscillate at just 10 centimetres per second — the walking speed of a tortoise. </p>
<p>Just as the pitch of an ambulance siren rises and falls as it travels towards and away from us, the wavelength of light observed from a wobbling star increases and decreases as the planet traces out its orbit.</p>
<figure class="align-center ">
<img alt="Artist's impression of ELT." src="https://images.theconversation.com/files/557969/original/file-20231107-19-l8m44m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/557969/original/file-20231107-19-l8m44m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=426&fit=crop&dpr=1 600w, https://images.theconversation.com/files/557969/original/file-20231107-19-l8m44m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=426&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/557969/original/file-20231107-19-l8m44m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=426&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/557969/original/file-20231107-19-l8m44m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=535&fit=crop&dpr=1 754w, https://images.theconversation.com/files/557969/original/file-20231107-19-l8m44m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=535&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/557969/original/file-20231107-19-l8m44m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=535&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">Artist’s impression of ELT.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Extremely_Large_Telescope#/media/File:ELT_concept.jpg">ESO/L. Calçada/wikipedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Remarkably, Andes will be able to detect this minuscule change in the light’s colour. Starlight, while essentially continuous (“white”) from the ultraviolet to the infrared, contains bands where atoms in the outer region of the star absorb specific wavelengths as the light escapes, appearing dark in the spectra. </p>
<p>Tiny shifts in the positions of these features — around 1/10,000th of a pixel on the Andes sensor — may, over months and years, reveal the periodic wobbles. This could ultimately help us to find an Earth 2.0.</p>
<p>At Heriot-Watt University, we are piloting <a href="https://www.hw.ac.uk/news/articles/2023/planet-hunting-systems-for-the-extremely.htm">the development of a laser system</a> known as a frequency comb, that will enable Andes to reach such exquisite precision. Like the millimetre ticks on a ruler, the laser will calibrate the Andes spectrograph by providing a spectrum of light structured as thousands of regularly spaced wavelengths.</p>
<p>This scale will remain constant over decades, mitigating the measurement errors that occur from environmental changes in temperature and pressure.</p>
<p>With the ELT’s construction cost coming in at €1.45 billion, some will question the value of the project. But astronomy has a significance that spans millennia and transcends cultures and national borders. It is only by looking far outside our Solar System that we can gain a perspective beyond the here and now.</p><img src="https://counter.theconversation.com/content/217012/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Derryck Reid receives funding from the UK Science and Technology Facilities Council (STFC).</span></em></p>From improving our understanding of dark matter to revealing the location of Earth 2.0, the Extremely Large Telescope promises answers to some of the biggest scientific questions of our time.Derryck Telford Reid, Professor of Physics, Heriot-Watt UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2117272023-10-04T19:53:51Z2023-10-04T19:53:51ZGravitational distortion of time helps tell modified gravity apart from a dark force<figure><img src="https://images.theconversation.com/files/550733/original/file-20230927-15-pmj07x.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1200%2C878&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/gpb/gpb_012.html">(NASA)</a></span></figcaption></figure><iframe style="width: 100%; height: 100px; border: none; position: relative; z-index: 1;" allowtransparency="" allow="clipboard-read; clipboard-write" src="https://narrations.ad-auris.com/widget/the-conversation-canada/gravitational-distortion-of-time-helps-tell-modified-gravity-apart-from-a-dark-force" width="100%" height="400"></iframe>
<p>With his theory of General Relativity in 1915, Albert Einstein revolutionized how we think about our universe. Rather than the cosmos simply providing the room for the planets and stars to orbit each other, space and time themselves were now dynamical entities in one ever-evolving play with matter and light. </p>
<p>Einstein’s equations described how <a href="https://www.space.com/17661-theory-general-relativity.html">stars, galaxies and all other matter curve or warp space and time</a>. The galaxies and the light rays then travel in this distorted space-time according to <a href="https://www.britannica.com/biography/Leonhard-Euler">the equation provided by the 18th-century Swiss mathematician Leonhard Euler</a>.</p>
<p>With the help of modern telescopes, we can watch this dance and compare it to the choreography scripted by the two giants of science, Einstein and Euler. But can we differentiate a universe where Einstein’s equations were violated from a universe where Euler’s equation were modified? In other words, if what we observed with telescopes disagreed with what Einstein and Euler prescribed, would we be able to tell which one of the two was wrong?</p>
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Read more:
<a href="https://theconversation.com/rippling-space-time-how-to-catch-einsteins-gravitational-waves-7058">Rippling space-time: how to catch Einstein's gravitational waves</a>
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<h2>A universe filled with unknowns</h2>
<p>You may wonder why one would want to doubt Einstein or Euler in the first place. After all, existing observations have beautifully confirmed the validity of their theories. The reason to put those to the test comes from the fact that our universe is filled with unknowns. </p>
<p>In the 1930s, the Swiss-American astrophysicist Fritz Zwicky observed that there was five times more matter in the universe than we can detect with our telescopes. He called this new matter “dark matter.” </p>
<p>Nearly 100 years later, <a href="https://doi.org/10.1038/509560a">we still don’t know what dark matter is</a>: we have never detected a particle of dark matter and we don’t know how it moves. It is therefore legitimate to question if it behaves as ordinary matter and obeys Euler’s law. Could it be affected by other forces and interactions, which would change the Euler equation? </p>
<p>Then, in 1998, two groups of astrophysicists observed that <a href="https://doi.org/10.1063/1.3232196">the expansion of our universe</a> <a href="https://doi.org/10.1086/300499">is accelerating</a>, contrary to the deceleration expected because of the gravitational attraction between galaxies. </p>
<p>As of today, we don’t know what causes this strange behaviour: is it due to the presence of yet another “dark” substance that has repulsive gravity? Or is it due to gravity itself, meaning Einstein’s predictions of how it behaves over very large distances would be wrong? Testing Einstein’s and Euler’s equations is therefore the logical consequence of the mysteries we face.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="multicoloured galaxies against a black background" src="https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=386&fit=crop&dpr=1 600w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=386&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=386&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=485&fit=crop&dpr=1 754w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=485&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/551584/original/file-20231003-17-8j13mm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=485&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">A composite image showing the distribution of dark matter, galaxies and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters.</span>
<span class="attribution"><a class="source" href="https://images.nasa.gov/details/GSFC_20171208_Archive_e001774">(NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University))</a></span>
</figcaption>
</figure>
<h2>Vast distances of the universe</h2>
<p>Checking if Einstein’s gravity works over the vast distances of the universe has become an active field of research. Theoreticians propose new ideas for how gravity could work differently, while astronomers use increasingly advanced facilities to provide the data needed to test them. </p>
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Read more:
<a href="https://theconversation.com/we-tested-einsteins-theory-of-gravity-on-the-scale-of-the-universe-heres-what-we-found-194118">We tested Einstein's theory of gravity on the scale of the universe – here's what we found</a>
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<p>Researchers have identified a particular “smoking gun” signature of modified gravity known as the “<a href="https://doi.org/10.1103/PhysRevD.77.103513">gravitational slip</a>.” General Relativity predicts that the pathways of light and matter should bend in the same way when travelling through the same distorted space-time. </p>
<p>This is much like the fact that different objects fall at the same rate in Earth’s gravity (if the air resistance could be neglected) — <a href="https://www.sciencenews.org/article/galileo-gravity-experiment-atoms-general-relativity-einstein">something Galileo famously demonstrated from the tower of Pisa</a>. By comparing the way galaxies fall into gravitational wells to how the light from these galaxies is deflected by gravitational lensing, one can deduce if they feel the same gravity. </p>
<p>If one finds them to be different, we would say there was a <a href="https://doi.org/10.48550/arXiv.0802.1068">gravitational slip</a>. Measuring the slip is one of the main targets of Euclid, <a href="https://www.esa.int/Science_Exploration/Space_Science/Euclid">a wide-angle space telescope launched by the European Space Agency on a Space-X rocket</a>. </p>
<p>But what if Euclid found that there was a slip? Could we be certain that it occurs due to a modification of gravity, or could it also be due to a modification of Euler’s equation? The latter would be different if, for example, the dark matter in the galaxies were subject to a new force.</p>
<h2>Gravitational slips</h2>
<p>The two of us approached this question from different perspectives: one involved developing tests of <a href="https://www.sfu.ca/physics/cosmology/TestingGravity2023/">modified gravity</a>, while the other investigated the subtle corrections General Relativity adds to <a href="https://doi.org/10.1103/PhysRevD.84.063505">what we measure with galaxy surveys</a>. </p>
<p>To our surprise, while both of us came into this thinking that the answer would be obvious, our initial answers were opposite to each other. After intensive discussion, we eventually came to an agreement, <a href="https://doi.org/10.1038/s41550-023-02003-y">resulting in a paper published in <em>Nature Astronomy</em></a>. </p>
<p>Our conclusion was that, despite the common expectation, measuring the gravitational slip would not allow one to distinguish a modification of Einstein’s laws from a modification of Euler’s equation. </p>
<p>However, the distinction may be possible if one could measure the effect called “gravitational redshift,” which should be possible with telescopes such as the <a href="https://www.desi.lbl.gov/">Dark Energy Spectroscopic Instrument</a> and the upcoming <a href="https://www.skao.int/en">Square Kilometer Array</a>.</p>
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Read more:
<a href="https://theconversation.com/canadas-participation-in-the-worlds-largest-radio-telescope-means-new-opportunities-in-research-and-innovation-201341">Canada's participation in the world's largest radio telescope means new opportunities in research and innovation</a>
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<p>One of our key realizations was that to determine if the measured gravitational slip signals a breakdown of General Relativity, one would need to measure the velocity of normal matter when it is not confined to a galaxy. In practice, however, we can only observe the light from stars that reside in galaxies, and hence move together with the dark matter. </p>
<p>Telescopes can only measure the collective motion of a galaxy that contains both normal matter and dark matter. So, if a galaxy were to fall into a gravitational potential in a way that was not consistent with our expectations, we would be unable to tell if it’s because the dark matter is doing something, or because gravity was modified. </p>
<h2>Light and gravity</h2>
<p>There <em>is</em> a way to probe the gravitational potential directly through the way it distorts time via gravitational redshift.</p>
<p>The time kept by a clock <a href="https://www.washingtonpost.com/national/health-science/an-atomic-clock-is-used-to-measure-not-time-but-the-height-of-mountains/2018/02/23/5a845166-11c3-11e8-9570-29c9830535e5_story.html">that’s on top of a tall mountain is different from that of a clock at the sea level</a>. These differences are extremely tiny but are, in fact, very important in the design of satellite navigation systems. </p>
<p>When the light from a galaxy escapes the gravitational potential it is falling into, its colour shifts closer to red. This gravitational redshift is solely due to time distortion. Gravitational lensing, which differs from redshift, is due to both space and time distortions, as opposed to just time. </p>
<p>We need to have both lensing and redshift in order to isolate the gravitational slip. It is this ability to separate the distortion of space and time from the distortion of time alone that is key to measuring true gravitational slip.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/-3oFwAd08yc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Swiss cosmologists explain how the distortion of time, an effect predicted by General Relativity, can be measured.</span></figcaption>
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<p>A measurement of the gravitational redshift is impossible if one cannot easily keep track if a pair of galaxies swapped their positions. While it’s not that hard to tell any two galaxies measured by a telescope apart, when running a statistical analysis on a catalogue of millions of galaxies, you can quickly lose the ability to assign any identity to the galaxies; at some point they are all treated as points on the sky. </p>
<p>Techniques have, however, been developed to split galaxies into different populations and <a href="https://doi.org/10.48550/arXiv.1309.1321">keep track of swaps between them</a>. In time, new technologies will be able to detect the tiny effects of gravitational redshift, and consequently distinguish a modification of Euler’s equation for dark matter from a modification of gravity.</p><img src="https://counter.theconversation.com/content/211727/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Levon Pogosian receives funding from the Natural Sciences and Engineering Research Council of Canada.</span></em></p><p class="fine-print"><em><span>Camille Bonvin receives funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program and from the Swiss National Science Foundation.</span></em></p>The gravitational field can affect space and time: the stronger gravity is, the slower time moves. This prediction of General Relativity can be used to reveal hidden forces acting on dark matter.Levon Pogosian, Professor of Physics, Simon Fraser UniversityCamille Bonvin, Associate professor, Cosmology and Astroparticle Physics, Université de GenèveLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2110672023-09-24T12:10:23Z2023-09-24T12:10:23ZWhy Einstein must be wrong: In search of the theory of gravity<figure><img src="https://images.theconversation.com/files/548379/original/file-20230914-27-fu6fow.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C6000%2C2497&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">As new and powerful telescopes gather new data about the universe, they reveal the limits of older theories.</span> <span class="attribution"><span class="source">(Shutterstock)</span></span></figcaption></figure><iframe style="width: 100%; height: 100px; border: none; position: relative; z-index: 1;" allowtransparency="" allow="clipboard-read; clipboard-write" src="https://narrations.ad-auris.com/widget/the-conversation-canada/why-einstein-must-be-wrong-in-search-of-the-theory-of-gravity" width="100%" height="400"></iframe>
<p>Einstein’s theory of gravity — <a href="https://www.space.com/17661-theory-general-relativity.html">general relativity</a> — has been very successful for more than a century. However, it has theoretical shortcomings. This is not surprising: the theory predicts its own failure at spacetime singularities inside black holes — and the <a href="https://www.einstein-online.info/en/spotlight/avoiding_the_big_bang/">Big Bang itself</a>. </p>
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<a href="https://theconversation.com/our-understanding-of-black-holes-has-changed-over-time-172816">Our understanding of black holes has changed over time</a>
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<p>Unlike physical theories describing the other three fundamental forces in physics — the electromagnetic and the strong and weak nuclear interactions — the general theory of relativity has only been tested in weak gravity. </p>
<p>Deviations of gravity from general relativity are by no means excluded nor tested everywhere <a href="https://doi.org/10.1038/s41550-022-01808-7">in the universe</a>. And, according to theoretical physicists, deviation must happen.</p>
<h2>Deviations and quantum mechanics</h2>
<p>According to Einstein, our universe originated in a Big Bang. Other singularities hide inside black holes: Space and time cease to have meaning there, while quantities such as energy density and pressure become infinite. These signal that Einstein’s theory is failing there and must be replaced with a more fundamental one.</p>
<p>Naively, spacetime singularities should be resolved by quantum mechanics, which apply at very small scales.</p>
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Read more:
<a href="https://theconversation.com/will-we-have-to-rewrite-einsteins-theory-of-general-relativity-50057">Will we have to rewrite Einstein's theory of general relativity?</a>
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<p>Quantum physics relies on two simple ideas: <a href="https://www.quantamagazine.org/what-is-a-particle-20201112/">point particles</a> make no sense; and the <a href="https://theconversation.com/explainer-heisenbergs-uncertainty-principle-7512">Heisenberg uncertainty principle</a>, which states that one can never know the value of certain pairs of quantities with absolute precision — for example, the position and velocity of a particle. This is because particles should not be thought of as points but as waves; at small scales they behave as waves of matter.</p>
<p>This is enough to understand that a theory that embraces both general relativity and quantum physics should be free of such pathologies. However, all attempts to blend general relativity and quantum physics necessarily introduce deviations from Einstein’s theory. </p>
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<a href="https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="a black circle surrounded with a ring of light" src="https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=358&fit=crop&dpr=1 600w, https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=358&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=358&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=450&fit=crop&dpr=1 754w, https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=450&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/548820/original/file-20230918-27-exi2b0.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>
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<span class="caption">A photo of the 1919 complete solar eclipse.</span>
<span class="attribution"><a class="source" href="https://doi.org/10.1098/rsta.1920.0009">(Arthur Eddington/Philosophical Transactions of the Royal Society)</a></span>
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<p>Therefore, Einstein’s gravity cannot be the ultimate theory of gravity. Indeed, it was not long after the introduction of general relativity by Einstein in 1915 that Arthur Eddington, best known for verifying this theory in the <a href="https://doi.org/10.1098/rsnr.2020.0040">1919 solar eclipse</a>, started searching for alternatives just to see how things could be different. </p>
<p>Einstein’s theory has survived all tests to date, accurately predicting various results from the <a href="https://doi.org/10.12942/lrr-2014-4">precession of Mercury’s orbit to the existence of gravitational waves</a>. So, where are these deviations from general relativity hiding?</p>
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Read more:
<a href="https://theconversation.com/gravitational-waves-discovered-top-scientists-respond-53956">Gravitational waves discovered: top scientists respond</a>
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<h2>Cosmology matters</h2>
<p>A century of research has given us the standard model of cosmology known as the Λ-Cold Dark Matter <a href="https://lambda.gsfc.nasa.gov/education/graphic_history/univ_evol.html">(ΛCDM) model</a>. Here, Λ stands for either Einstein’s famous cosmological constant or a mysterious dark energy with similar properties. </p>
<p>Dark energy was introduced ad hoc by astronomers to explain the <a href="https://doi.org/10.1103/RevModPhys.75.559">acceleration of the cosmic expansion</a>. Despite fitting cosmological data extremely well until recently, the ΛCDM model is spectacularly incomplete and unsatisfactory from the theoretical point of view. </p>
<p>In the past five years, it has also faced severe <a href="https://doi.org/10.1088/1361-6382/ac086d">observational tensions</a>. The Hubble constant, which determines the age and the distance scale in the universe, can be measured in the early universe using the cosmic microwave background and in the late universe using supernovae as standard candles. </p>
<p>These two measurements give <a href="https://doi.org/10.1088/1361-6382/ac086d">incompatible results</a>. Even more important, the nature of the main ingredients of the ΛCDM model — <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a>, <a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">dark matter</a> and the field driving early universe <a href="https://www.newscientist.com/definition/cosmic-inflation/">inflation</a> (a very brief period of extremely fast expansion originating the seeds for galaxies and galaxy clusters) — remains a mystery.</p>
<p>From the observational point of view, the most compelling motivation for modified gravity is the acceleration of the universe discovered in 1998 with <a href="https://doi.org/10.1086/307221">Type Ia supernovae</a>, whose luminosity is dimmed by this acceleration. The ΛCDM model based on general relativity postulates an extremely exotic dark energy with negative pressure permeating the universe. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="eight bright circles in a dark sky" src="https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/548822/original/file-20230918-27-jr07hx.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">Type Ia supernovae were discovered in 1998, and revealed more about the rate of the universe’s acceleration.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/jpl/galex/pia18929/after-the-explosion-investigating-supernova-sites">(Sloan Digital Sky Survey/NASA)</a></span>
</figcaption>
</figure>
<p>Problem is, this dark energy has no physical justification. Its nature is completely unknown, although a <a href="https://doi.org/10.1142/S0219887807001928">plethora of models</a> has been proposed. The proposed alternative to dark energy is a cosmological constant Λ which, according to quantum-mechanical <a href="https://doi.org/10.1103/RevModPhys.61.1">back-of-the-envelope (but questionable) calculations</a>, should be huge. </p>
<p>However, Λ must instead be incredibly fine-tuned to a tiny value to fit the cosmological observations. If dark energy exists, our ignorance of its nature is deeply troubling.</p>
<h2>Alternatives to Einstein’s theory</h2>
<p>Could it be that troubles arise, instead, from wrongly trying to fit the cosmological observations into general relativity, like fitting a person into a pair of trousers that are too small? That we are observing the first deviations from general relativity while the mysterious dark energy simply does not exist? </p>
<p>This idea, <a href="https://doi.org/10.1142/S0218271802002025">first proposed</a> by researchers at the University of Naples, has gained tremendous popularity while the contending dark energy camp remains vigorous. </p>
<p>How can we tell? Deviations from Einstein gravity are <a href="https://doi.org/10.12942/lrr-2014-4">constrained by solar system experiments</a>, the recent observations of <a href="https://doi.org/10.1103/PhysRevLett.116.061102">gravitational waves</a> and the <a href="https://doi.org/10.3847/2041-8213/ab0ec7">near-horizon images of black holes</a>.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/say-hello-to-sagittarius-a-the-black-hole-at-the-center-of-the-milky-way-galaxy-183008">Say hello to Sagittarius A*, the black hole at the center of the Milky Way galaxy</a>
</strong>
</em>
</p>
<hr>
<p>There is now a <a href="https://doi.org/10.1103/RevModPhys.82.451">large literature</a> on theories of gravity alternative to general relativity, going back to Eddington’s 1923 early investigations. A very popular class of alternatives is the so-called scalar-tensor gravity. It is conceptually very simple since it only introduces one additional ingredient (a scalar field corresponding to the simplest, spinless, particle) to Einstein’s geometric description of gravity. </p>
<p>The consequences of this program, however, are far from trivial. A striking phenomenon is the “<a href="https://doi.org/10.1007/s41114-018-0011-x">chameleon effect</a>,” consisting of the fact that these theories can disguise themselves as general relativity in high-density environments (such as in stars or in the solar system) while deviating strongly from it in the low-density environment of cosmology.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-search-for-dark-matter-and-dark-energy-just-got-interesting-46422">The search for 'dark matter' and 'dark energy' just got interesting</a>
</strong>
</em>
</p>
<hr>
<p>As a result, the extra (gravitational) field is effectively absent in the first type of systems, disguising itself as a chameleon does, and is felt only at the largest (cosmological) scales.</p>
<h2>The current situation</h2>
<p>Nowadays the spectrum of alternatives to Einstein gravity has widened dramatically. Even adding a single massive scalar excitation (namely, a spin-zero particle) to Einstein gravity —and keeping the resulting equations “simple” to avoid some known fatal instabilities — has resulted in the much wider class of <a href="https://doi.org/10.1142/S0218271819420069">Horndeski theories</a>, and subsequent generalizations. </p>
<p>Theorists have spent the last decade extracting physical consequences from these theories. The recent detections of <a href="https://doi.org/10.1103/PhysRevLett.116.061102">gravitational waves</a> have provided a way to <a href="https://doi.org/10.1103/PhysRevD.95.084029">constrain the physical class of modifications</a> of Einstein gravity allowed.</p>
<p>However, much work still needs to be done, with the hope that future advances in <a href="https://www.nature.com/articles/s42254-019-0101-z">multi-messenger astronomy</a> lead to discovering modifications of general relativity where gravity is extremely strong.</p><img src="https://counter.theconversation.com/content/211067/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Valerio Faraoni receives funding from the Natural Sciences and Engineering Research Council of Canada.</span></em></p><p class="fine-print"><em><span>Andrea Giusti received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Actions (grant agreement No. 895648). </span></em></p>Einstein’s theory of general relativity suggests that our universe originated in a Big Bang. But black holes, and their gravitational forces, challenge the limits of Einstein’s work.Valerio Faraoni, Professor, Physics & Astronomy, Bishop's UniversityAndrea Giusti, Postdoctoral fellow, Swiss Federal Institute of Technology ZurichLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2110752023-08-09T12:31:45Z2023-08-09T12:31:45ZResearchers dig deep underground in hopes of finally observing dark matter<figure><img src="https://images.theconversation.com/files/541255/original/file-20230804-21123-c0m9ny.jpeg?ixlib=rb-1.1.0&rect=11%2C0%2C1280%2C831&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The inside of the LZ outer detector. The LZ is a super sensitive machine that may one day detect a dark matter particle. </span> <span class="attribution"><span class="source">Matt Kapust, SURF</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Physicists like me don’t fully understand what makes up about <a href="https://doi.org/10.1051/0004-6361/201833910">83% of the matter of the universe</a> — something we call “<a href="https://theconversation.com/dark-matter-the-mystery-substance-physics-still-cant-identify-that-makes-up-the-majority-of-our-universe-85808">dark matter</a>.” But with a <a href="https://sanfordlab.org/experiment/lux-zeplin">tank full of xenon</a> buried nearly a mile under South Dakota, we might one day be able to measure what dark matter really is.</p>
<p>In the typical model, dark matter accounts for most of the gravitational attraction in the universe, providing the glue that allows structures like galaxies, including our own Milky Way, <a href="https://www.esa.int/Science_Exploration/Space_Science/Planck/History_of_cosmic_structure_formation">to form</a>. As the solar system orbits around the center of the Milky Way, Earth moves through a <a href="https://theconversation.com/dark-matter-our-method-for-catching-ghostly-haloes-could-help-unveil-what-its-made-of-147953">dark matter halo</a>, which makes up most of the matter in our galaxy. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing the Milky Way galaxy, with a blurrred region or 'halo' around it indicating dark matter." src="https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=601&fit=crop&dpr=1 600w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=601&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=601&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=755&fit=crop&dpr=1 754w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=755&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?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">An artist’s rendition of the halo of dark matter surrounding the central spiral disk of the Milky Way.</span>
<span class="attribution"><span class="source">NASA/ESA/A Feild STSci</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>I’m a <a href="http://hep.ucsb.edu/people/hugh/">physicist</a> interested in understanding the nature of dark matter. One popular guess is that dark matter is a new type of particle, the <a href="https://www.britannica.com/science/weakly-interacting-massive-particle">Weakly Interacting Massive Particle</a>, or WIMP. “WIMP” captures the particle’s essence quite nicely – it has mass, meaning it interacts gravitationally, but it otherwise interacts very weakly – or rarely – with normal matter. WIMPs in the Milky Way theoretically fly through us on Earth all the time, but because they interact weakly, they just don’t hit anything.</p>
<h2>Searching for WIMPs</h2>
<p>Over the past 30 years, scientists have developed <a href="https://doi.org/10.48550/arXiv.2209.07426">an experimental program</a> to try to detect the rare interactions between WIMPs and regular atoms. On Earth, however, we are constantly surrounded by low, nondangerous levels of radioactivity coming from trace elements – mainly uranium and thorium – in the environment, as well as cosmic rays from space. The goal in hunting for dark matter is to build as sensitive a detector as possible, so it can see the dark matter, and to put it in as quiet a place as possible, so the dark matter signal can be seen over the background radioactivity. </p>
<p>With <a href="https://doi.org/10.1103/PhysRevLett.131.041002">results published in July 2023</a>, the <a href="https://sanfordlab.org/experiment/lux-zeplin">LUX-ZEPLIN</a>, or LZ, collaboration has done just that, building the largest dark matter detector to date and operating it 4,850 feet (1,478 meters) underground in the <a href="https://sanfordlab.org/">Sanford Underground Research Facility</a> in Lead, South Dakota. </p>
<p>At the center of LZ rests <a href="https://sanfordlab.org/feature/searching-dark-matter">10 metric tons (10,000 kilograms) of liquid xenon</a>. When particles pass through the detector, they may collide with xenon atoms, leading to a flash of light and the release of electrons.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing a particle interacting and releasing an electron, which registers in the detector" src="https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=627&fit=crop&dpr=1 600w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=627&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=627&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=788&fit=crop&dpr=1 754w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=788&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=788&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Particles interact with xenon in the LZ, releasing light that is detected by two light-sensing arrays at top and bottom.</span>
<span class="attribution"><span class="source">SLAC/LZ</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In LZ, two massive electrical grids apply an electric field across the volume of liquid, which pushes these released electrons to the liquid’s surface. When they breach the surface, they are pulled into the space above the liquid, which is filled with xenon gas, and accelerated by another electric field to create a second flash of light. Two large arrays of light sensors collect these two flashes of light, and together they allow researchers to reconstruct the position, energy and type of interaction that took place. </p>
<h2>Reducing radioactivity</h2>
<p>All materials on Earth, including those used in WIMP detector construction, <a href="https://www.world-nuclear.org/information-library/safety-and-security/radiation-and-health/naturally-occurring-radioactive-materials-norm.aspx">emit some radiation</a> that could potentially mask dark matter interactions. Scientists therefore build dark matter detectors using the most “radiopure” materials – that is, free of radioactive contaminants – they can find, both inside and outside the detector. </p>
<p>For example, by working with metal foundries, LZ was able to use the <a href="https://doi.org/10.1016/j.astropartphys.2017.09.002">cleanest titanium on Earth</a> to build the central cylinder – or cryostat – that holds the liquid xenon. Using this special titanium reduces the radioactivity in LZ, creating a clear space to see any dark matter interactions. Furthermore, liquid xenon is so dense that it actually acts as a radiation shield, and it is easy to purify the xenon of radioactive contaminants that might sneak in. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An LZ worker wearing a white hazard suit stands by a tall white cylinder." src="https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?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"></a>
<figcaption>
<span class="caption">In the inner detector of LZ, two light-sensing arrays at top and bottom view a central cylinder that will be filled with liquid xenon.</span>
<span class="attribution"><span class="source">Matt Kapust, SURF</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In LZ, the central xenon detector lives inside two other detectors, called the xenon skin and the outer detector. These supporting layers catch radioactivity on the way in or out of the central xenon chamber. Because dark matter interactions are so rare, a dark matter particle will only ever interact one time in the entire apparatus. Thus, if we observe an event with multiple interactions in the xenon or the outer detector, we can assume it’s not being caused by a WIMP. </p>
<p>All of these objects, including the central detector, the cryostat and the outer detector, live in a large water tank nearly a mile underground. The water tank shields the detectors from the cavern, and the underground environment shields the water tank from cosmic rays, or charged particles that are constantly hitting the Earth’s atmosphere.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/dwoFeiqiNe0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The LZ lives underground to block out cosmic radiation. But in order to get it down there, SURF engineers had to figure out a way to transport all the machinery and equipment.</span></figcaption>
</figure>
<h2>The hunt continues</h2>
<p>In the result <a href="https://doi.org/10.1103/PhysRevLett.131.041002">just published</a>, using 60 days of data, LZ recorded about five events per day in the detector. That’s about a trillion fewer events than a typical particle detector on the surface would record in a day. By looking at the characteristics of these events, researchers can safely say that no interaction so far has been caused by dark matter. The result is, alas, not a discovery of new physics – but we can set limits on exactly how weakly dark matter must interact, as it remains unseen by LZ.</p>
<p>These limits help to tell physicists what dark matter is not – and LZ does that better than any experiment in the world. Meanwhile, there’s hope for what comes next in the search for dark matter. LZ is collecting more data now, and we expect to take more than 15 times more data over the next few years. A WIMP interaction may already be in that data set, just waiting to be revealed in the next round of analysis.</p><img src="https://counter.theconversation.com/content/211075/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Hugh Lippincott receives funding from the US Department of Energy Office of Science. </span></em></p>To detect dark matter, you need to build an ultra-sensitive detector and put it somewhere ultra-quiet. For one physics collaboration, that place is almost a mile under Lead, S.D.Hugh Lippincott, Associate Professor of Physics, University of California, Santa BarbaraLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2083202023-06-27T15:49:23Z2023-06-27T15:49:23ZEuclid space mission is set for launch – here’s how it will test alternative theories of gravity<figure><img src="https://images.theconversation.com/files/533759/original/file-20230623-27-zpv5wg.jpeg?ixlib=rb-1.1.0&rect=0%2C1%2C769%2C429&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>The European Space Agency’s (Esa) <a href="https://www.esa.int/Science_Exploration/Space_Science/Euclid_overview">Euclid mission</a> will launch into space on a Falcon9 rocket from SpaceX on July 1, or soon after. Many of us who have worked on it will be in Florida to watch the nail-biting event.</p>
<p>The mission is specifically <a href="https://theconversation.com/the-euclid-spacecraft-will-transform-how-we-view-the-dark-universe-204245">designed to study</a> the dark universe, probing both “<a href="https://theconversation.com/dark-matter-should-we-be-so-sure-it-exists-heres-how-philosophy-can-help-184109">dark matter</a>” and “<a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a>” – unknown substances thought to make up 95% of the energy density of the universe.</p>
<p>But it will also be able to test some strange, <a href="https://theconversation.com/new-theory-of-general-relativity-casts-doubt-on-dark-matter-16446">alternative models</a> of gravity – potentially challenging Albert Einstein’s great <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">theory of general relativity</a>.</p>
<p>Scientists have known about the existence of dark matter for nearly a century now. It was proposed after astronomers noted that galaxies in clusters had mysteriously high speeds. Such speeds should cause the clusters to evaporate unless there was some extra mass holding them together. As this matter wasn’t shining in the same way as the visible galaxies, it was dubbed dark matter. </p>
<p>Gravitational lensing is <a href="https://theconversation.com/method-to-weigh-galaxy-clusters-could-help-us-understand-mysterious-dark-matter-structures-85023">a new tool</a> to see this dark material. This effect relies on our understanding of general relativity. As light travels to us from distant galaxies, its path is bent by large clumps of matter (dark or bright) in the foreground – changing their appearance (and location). </p>
<p>This change is easily seen near the cores of massive clusters (see image below) – with galaxies stretched into arcs, appearing to be long, thin and curved. We can use this warping to determine the amount of matter in the foreground cluster. And that confirms again that much of the mass in these clusters is indeed dark.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Gravitational lensing in the galaxy cluster Abell 1689." src="https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=623&fit=crop&dpr=1 600w, https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=623&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=623&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=783&fit=crop&dpr=1 754w, https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=783&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/533742/original/file-20230623-25-xfx4d6.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=783&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Gravitational lensing in the galaxy cluster Abell 1689.</span>
<span class="attribution"><span class="source">NASA/CXC/MIT/E.-H Peng et al; Optical: NASA/STScI</span></span>
</figcaption>
</figure>
<p>But what <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">could it be made of</a>? Many physicists believe it is an unknown elementary particle. A popular candidate, which is yet to be detected, is axions, which were originally introduced to explain why certain fundamental symmetries of nature appear to be broken.</p>
<p>However, there are other possibilities. Rather than postulating the need for dark matter, one can probe gravity. The strength of gravity may become weaker than predicted on the scale of galaxies and beyond. On these scales, there are some alternative models of gravity that <a href="https://theconversation.com/dark-matter-our-review-suggests-its-time-to-ditch-it-in-favour-of-a-new-theory-of-gravity-186344">can explain galaxy rotation curves</a> without assuming there’s any dark matter. The challenge for any of these alternatives is to do so consistently on all scales. </p>
<p>While there are several Earth-based searches for dark matter particles, they have so far not found significant evidence. Therefore, astronomical observations of galaxy clusters remain our best option for testing the various theories that can explain dark matter. This is where Euclid will excel due to its outstanding resolution, providing a sharpness similar to the Hubble space telescope (see image) across a third of the sky. By comparison, Hubble has observed only 5% of the whole sky. </p>
<p>The number of images we will obtain of clusters will increase a hundred-fold with Euclid, allowing us to study in detail the distribution of dark matter within such clusters to high precision. How the dark matter is distributed may be key to its origin and mass, ruling out a range of possible candidate particles and gravity theories along the way. </p>
<h2>Dark energy and gravity</h2>
<p>Dark matter is potentially easy to understand compared to <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a>, which was proposed to explain the discovery that the expansion of the universe is accelerating – at odds with the prediction from Einstein’s theory of gravity. This strange substance is vexing to physicists and cosmologists, with the simplest idea being that dark energy is just <a href="https://theconversation.com/what-is-nothing-martin-rees-qanda-101498">the energy of empty space</a> (“vacuum energy”). </p>
<p>Essentially, as we gain more space in an expanding universe, we gain more vacuum energy, which then drives the observed acceleration. </p>
<p>This simple explanation is reasonable except for the uncomfortable truth that the observed density of dark energy is many orders of magnitude lower than predicted by quantum theory, which rules the universe on the smallest of scales. In short, this simple explanation asks more questions than it answers.</p>
<p>As with dark matter, an alternative explanation for dark energy is that it isn’t really a substance or form of energy at all, but again a sign that gravity is behaving differently on the largest scales. </p>
<p>This has led to a flurry of new ideas that extend our theory of gravity beyond general relativity. For example, could gravity exist in <a href="https://arxiv.org/abs/hep-th/0005016">more than the four dimensions</a> (three spatial dimensions plus time) that the rest of the universe experiences? Are there <a href="https://arxiv.org/abs/1901.07183">new fundamental fields</a> that we don’t know about yet, which interact with gravity? </p>
<p>Or perhaps Einstein’s theory is valid for the weak gravitational fields we experience on Earth, but becomes radically <a href="https://iopscience.iop.org/article/10.1088/0264-9381/32/24/243001">different in extremely strong gravitational fields</a>, like those near the event horizons of black holes.</p>
<p>The challenge for all these alternative gravity models is to work together, for both dark matter and dark energy. Ideally, they should work on all scales and masses, as a single theory. Physicists believe strongly in Occam’s razor – that the best theories have the least number of assumptions.</p>
<p>Euclid will help us test these exotic gravity models by mapping the positions of millions of galaxies over vast regions of the universe. This allows us to trace the “<a href="https://theconversation.com/scientists-start-mapping-the-hidden-web-that-scaffolds-the-universe-124616">cosmic web</a>”, a sponge-like structure of filaments and voids in space. These seem to be laid down first in dark matter and then sprinkled with galaxies. </p>
<p>This cosmic web is formed by billions of years of gravitational collapse, meaning its structure and statistics are sensitive to the laws of gravity at work on cosmological scales. By measuring its properties, we can determine whether a new theory of gravity would fit the data better than Einstein’s theory. </p>
<p>As we return to Earth, there is much excitement in the astrophysics community about what Euclid will do. This is the first time we’ve had a <a href="https://theconversation.com/the-euclid-spacecraft-will-transform-how-we-view-the-dark-universe-204245">satellite dedicated to</a> mapping dark matter and dark energy. </p>
<p>The Euclid data will last a lifetime and generations of cosmologists will spend their careers studying it. As we watch Euclid launch into the Florida sky, we will be one step closer to answering some of the most fundamental questions in science.</p><img src="https://counter.theconversation.com/content/208320/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Robert Nichol has received funding in the past for his work on Euclid and remains a member of the Euclid Consortium Board.</span></em></p><p class="fine-print"><em><span>Tessa Baker receives funding from the Royal Society and the European Research Council. </span></em></p>The Euclid mission is preparing to launch on July 1.Robert Nichol, Pro Vice-Chancellor and Executive Dean, University of SurreyTessa Baker, Royal Society University Research Fellow, Reader in Cosmology, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2042452023-05-05T17:04:27Z2023-05-05T17:04:27ZThe Euclid spacecraft will transform how we view the ‘dark universe’<figure><img src="https://images.theconversation.com/files/524112/original/file-20230503-26-6f6as6.jpg?ixlib=rb-1.1.0&rect=17%2C8%2C5973%2C2964&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Euclid is set to launch this year on a rocket built by SpaceX.</span> <span class="attribution"><a class="source" href="https://www.esa.int/ESA_Multimedia/Search?SearchText=euclid&result_type=images">Work performed by ATG under contract for ESA</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The European Space Agency’s (ESA) <a href="https://www.esa.int/Science_Exploration/Space_Science/Euclid_overview">Euclid satellite</a> completed the first part of its long journey into space on May 1 2023, when it <a href="https://www.esa.int/Science_Exploration/Space_Science/Euclid/Euclid_arrives_at_launch_site">arrived in Florida on a boat from Italy</a>. It is scheduled to lift off on a Falcon 9 rocket, built by SpaceX, from Cape Canaveral in early July.</p>
<p>Euclid is designed to provide us with a better understanding of the “mysterious” components of our universe, known as dark matter and dark energy. </p>
<p>Unlike the normal matter we experience here on Earth, <a href="https://www.nasa.gov/audience/forstudents/9-12/features/what-is-dark-matter.html">dark matter</a> neither reflects nor emits light. It binds galaxies together and is thought to make up about 80% of all the mass in the universe. We’ve known about it for a century, but its true nature remains an enigma. </p>
<p><a href="https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy">Dark energy</a> is similarly puzzling. Astronomers have shown that the expansion of the universe over the last five billion years has been <a href="https://iopscience.iop.org/article/10.1086/300499/fulltext/">accelerating faster than expected</a>. Many believe <a href="https://iopscience.iop.org/article/10.1086/307221/meta">this acceleration</a> is driven by an unseen force, which has been dubbed dark energy. This makes up about 70% of the energy in the universe. </p>
<p>Euclid will map this “dark universe”, using a suite of scientific instruments to shed light on different aspects of dark energy and dark matter. </p>
<h2>A light in the dark</h2>
<p>After launch, Euclid will undertake a month-long journey to a region in space called the <a href="https://solarsystem.nasa.gov/resources/754/what-is-a-lagrange-point/">second Earth-Sun Lagrangian point</a>, which is five times further from us than the Moon. It’s where the gravitational pull of the Sun and the Earth balance out and provides a stable vantage point for Euclid to observe the universe. Euclid will join the <a href="https://webb.nasa.gov">James Webb Space Telescope (JWST)</a> at this point and will be the perfect companion to that amazing space observatory.</p>
<p>My involvement in Euclid began in 2007 when I was invited by ESA to participate in an independent concept advisory team to assess two competing mission proposals called SPACE and DUNE. </p>
<p>Both used different techniques, and therefore different instruments, to study the dark universe, and ESA was struggling to decide between them. Both were compelling concepts and our team decided that both had merit, especially to provide a vital cross-check between them. Euclid was thus <a href="https://sci.esa.int/web/cosmic-vision/-/42437-study-missions">born from the best of both concepts</a>.</p>
<p>Euclid is designed to study the whole universe so needs instruments with wide fields of view. The wider the field of view of the imaging instrument, the more of the universe it can observe. To do this, Euclid uses a relatively small telescope compared to JWST. In size, Euclid is roughly the size of a truck compared to the aircraft-sized JWST. But Euclid also carries some of the biggest digital cameras deployed in space with fields of view hundreds of times greater than JWST’s. </p>
<h2>Shapes and colours</h2>
<p>The <a href="https://arxiv.org/pdf/1608.08603.pdf">Euclid VIS (or visible) instrument</a>, built mostly in the UK, is designed to measure the positions and shapes of as many galaxies as possible to look for subtle correlations in this data caused by the gravitational lensing of the light, as it travels to us through the intervening dark matter. This gravitational lensing effect is weak, only one part in a hundred thousand for most galaxies, thus requiring lots of galaxies to see the effect in high definition. Thus VIS will produce Hubble telescope-like image quality over a third of the night sky. </p>
<p>VIS, however, can’t measure the colours of objects. This is needed to measure their distance through the <a href="https://www.esa.int/Science_Exploration/Space_Science/What_is_red_shift">redshift effect</a>, where light from those objects is shifted to longer, or redder, wavelengths in a way that relates to their distance from us. Some of this data will need to come from existing and planned ground-based observatories, but Euclid also carries the <a href="https://arxiv.org/pdf/2203.01650.pdf">NISP (Near-Infra Spectrometer and Photometer)</a> instrument which is specifically designed to measure the infrared colours and spectra, and therefore redshifts, for the most distant galaxies that Euclid will see. </p>
<p>To measure dark energy, NISP will exploit a relative new technique called <a href="https://svs.gsfc.nasa.gov/13768">Baryon Acoustic Oscillations (BAO)</a> that provides an accurate measurement of the expansion history of the universe over its last 10 billion years. That history is vital for testing possible models of dark energy including suggested modifications to Einstien’s Theory of General Relativity. </p>
<figure class="align-center ">
<img alt="The Whirlpool Galaxy, known as M51, and a companion galaxy." src="https://images.theconversation.com/files/524144/original/file-20230503-26-56rt5t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/524144/original/file-20230503-26-56rt5t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=416&fit=crop&dpr=1 600w, https://images.theconversation.com/files/524144/original/file-20230503-26-56rt5t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=416&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/524144/original/file-20230503-26-56rt5t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=416&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/524144/original/file-20230503-26-56rt5t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=523&fit=crop&dpr=1 754w, https://images.theconversation.com/files/524144/original/file-20230503-26-56rt5t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=523&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/524144/original/file-20230503-26-56rt5t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=523&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Euclid will gather information on the shapes and other properties of galaxies in the sky.</span>
<span class="attribution"><a class="source" href="https://esahubble.org/images/heic0506a/">NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA)</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Treasure trove</h2>
<p>Such an experiment takes an army of scientists and not everyone is solely working on dark matter and dark energy. Like JWST, Euclid will be a treasure-trove of new discoveries in many areas of astronomy. The Euclid consortium needs hundreds of people to help develop the sophisticated software needed to merge the space data with the ground-based data, and extract, to high accuracy, the shapes and colours of billions of galaxies. </p>
<p>This software has also been checked and verified using some of the largest simulations of the universe that have ever been constructed. After arriving at L2, Euclid will undergo several months of testing, validation and calibration to ensure the instruments and telescope are working as expected. We are all familiar with such nervous waiting after the recent JWST launch. </p>
<p>Once ready, Euclid will embark on a five-year survey of 15,000 square degrees of the sky with about 2,000 scientists from across the world collecting results along the way. However, the true power of Euclid will only be realised once we have all this data together and analysed carefully. That could take another five years, taking us well into next decade before we have our final dark answers. The SpaceX launch therefore only feels like the half-way point in the Euclid story.</p>
<p>I will travel to Florida this summer to see the launch of Euclid. I will be joined by hundreds of my colleagues who have dedicated their careers to building this amazing telescope and experiment. Seeing the project come together in this way makes me proud to call myself a “Euclidian”.</p><img src="https://counter.theconversation.com/content/204245/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Bob Nichol previously received funding from UKSA as part of his leadership roles in the Euclid Consortium. He has not received any funding from UKSA since 2020.</span></em></p>A spacecraft set to launch this year will throw a spotlight on the mysterious ‘dark side’ of the universe.Robert Nichol, Pro Vice-Chancellor and Executive Dean, University of SurreyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2041092023-04-20T20:01:55Z2023-04-20T20:01:55ZNew look at ‘Einstein rings’ around distant galaxies just got us closer to solving the dark matter debate<figure><img src="https://images.theconversation.com/files/521998/original/file-20230420-3121-axsfat.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C2000%2C1320&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">ESA / Hubble & NASA</span></span></figcaption></figure><p>Physicists believe most of the matter in the universe is made up of an invisible substance that we only know about by its indirect effects on the stars and galaxies we can see.</p>
<p>We’re not crazy! Without this “dark matter”, the universe as we see it would make no sense. </p>
<p>But the nature of dark matter is a longstanding puzzle. However, <a href="https://www.nature.com/articles/s41550-023-01943-9">a new study</a> by Alfred Amruth at the University of Hong Kong and colleagues, published in Nature Astronomy, uses the gravitational bending of light to bring us a step closer to understanding. </p>
<h2>Invisible but omnipresent</h2>
<p>The reason we think dark matter exists is that we can see the effects of its gravity in the behaviour of galaxies. Specifically, dark matter seems to make up about 85% of the universe’s mass, and most of the distant galaxies we can see appear to be surrounded by a halo of the mystery substance.</p>
<p>But it’s called dark matter because it doesn’t give off light, or absorb or reflect it, which makes it incredibly difficult to detect. </p>
<p>So what is this stuff? We think it must be some kind of unknown fundamental particle, but beyond that we’re not sure. All attempts to detect dark matter particles in laboratory experiments so far have failed, and physicists have been debating its nature for decades.</p>
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<strong>
Read more:
<a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">Why do astronomers believe in dark matter?</a>
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<p>Scientists have proposed two leading hypothetical candidates for dark matter: relatively heavy characters called weakly interacting massive particles (or WIMPs), and extremely lightweight particles called axions. In theory, WIMPs would behave like discrete particles, while axions would behave a lot more like waves due to quantum interference. </p>
<p>It has been difficult to distinguish between these two possibilities – but now light bent around distant galaxies has offered a clue.</p>
<h2>Gravitational lensing and Einstein rings</h2>
<p>When light travelling through the universe passes a massive object like a galaxy, its path is bent because – according to Albert Einstein’s theory of general relativity – the gravity of the massive object distorts space and time around itself.</p>
<p>As a result, sometimes when we look at a distant galaxy we can see distorted images of other galaxies behind it. And if things line up perfectly, the light from the background galaxy will be smeared out into a circle around the closer galaxy. </p>
<p>This distortion of light is called “gravitational lensing”, and the circles it can create are called “Einstein rings”.</p>
<p>By studying how the rings or other lensed images are distorted, astronomers can learn about the properties of the dark matter halo surrounding the closer galaxy. </p>
<h2>Axions vs WIMPs</h2>
<p>And that’s exactly what Amruth and his team have done in their new study. They looked at several systems where multiple copies of the same background object were visible around the foreground lensing galaxy, with a special focus on one called HS 0810+2554.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=589&fit=crop&dpr=1 600w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=589&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=589&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=740&fit=crop&dpr=1 754w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=740&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=740&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Multiple images of a background image created by gravitational lensing can be seen in the system HS 0810+2554.</span>
<span class="attribution"><a class="source" href="https://hubblesite.org/contents/media/images/2020/05/4613-Image?news=true">Hubble Space Telescope / NASA / ESA</a></span>
</figcaption>
</figure>
<p>Using detailed modelling, they worked out how the images would be distorted if dark matter were made of WIMPs vs how they would if dark matter were made of axions. The WIMP model didn’t look much like the real thing, but the axion model accurately reproduced all features of the system.</p>
<p>The result suggests axions are a more probable candidate for dark matter, and their ability to explain lensing anomalies and other astrophysical observations has scientists buzzing with excitement. </p>
<h2>Particles and galaxies</h2>
<p>The new research builds on previous studies that have also pointed towards axions as the more likely form of dark matter. For example, <a href="https://doi.org/10.1093/mnras/sty271">one study</a> looked at the effects of axion dark matter on the cosmic microwave background, while <a href="https://doi.org/10.1093/mnras/stx1941">another</a> examined the behaviour of dark matter in dwarf galaxies. </p>
<p>Although this research won’t yet end the scientific debate over the nature of dark matter, it does open new avenues for testing and experiment. For example, future gravitational lensing observations could be used to probe the wave-like nature of axions and potentially measure their mass.</p>
<p>A better understanding of dark matter will have implications for what we know about particle physics and the early universe. It could also help us to understand better how galaxies form and change over time. </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>
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<img src="https://counter.theconversation.com/content/204109/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rossana Ruggeri 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>For decades physicists have argued over the nature of the elusive dark matter that pervades the Universe. A clever new study uses gravitational lensing to bring new evidence to the debate.Rossana Ruggeri, Research Fellow in Cosmology, The University of QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1917722023-04-06T07:04:07Z2023-04-06T07:04:07ZA ‘next-generation’ gamma-ray observatory is underway to probe the extreme Universe<figure><img src="https://images.theconversation.com/files/512377/original/file-20230227-4042-xvl5v5.jpg?ixlib=rb-1.1.0&rect=17%2C0%2C5716%2C3837&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">LST-1 prototype in La Palma, Spain.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/cta_observatory/50018704248/in/album-72157671493684827/">Tomohiro Inada/CTA</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span></figcaption></figure><p>Long gone are the days when astronomers only studied the skies with simple optical telescopes. Today, unveiling the mysteries of the Universe involves ever-larger and more complex facilities that detect things like gravitational waves and different forms of electromagnetic radiation – the spectrum of energy that includes visible light and X-rays.</p>
<p>One particularly specialised branch of astronomy is gamma-ray astronomy. It does what is says on the tin, searching for <a href="https://www.space.com/gamma-rays-explained">gamma rays</a>, which are the most energetic photons (light particles) on the electromagnetic spectrum. In fact, they are <a href="https://www.britannica.com/science/electromagnetic-radiation/Gamma-rays">millions of times more energetic</a> than the light we can see.</p>
<p>In astronomy, gamma rays are produced by some of the hottest, most energetic events in the universe, such as star explosions and <a href="https://theconversation.com/like-a-spinning-top-wobbling-jets-from-a-black-hole-thats-feeding-on-a-companion-star-116067">black holes violently “feeding” on surrounding matter</a>. While gamma rays are now linked to dozens of different types of sources, in many cases we still don’t know conclusively what kinds of energetic particles are creating these rays.</p>
<p>Excitingly, gamma-ray astronomy is due to get a massive leg up with a new facility. Once the globally distributed <a href="https://www.cta-observatory.org">Cherenkov Telescope Array</a> (CTA) is complete, it will view the gamma-ray sky with ten times more sensitivity than what’s currently possible.</p>
<p>With more than 60 telescopes, the CTA is expected to provide deep insight into the nature of dark matter – an invisible, hypothetical type of matter making up about 85% of the mass of the Universe. The array could also help solve one of the longest-running mysteries in astronomy: where cosmic ray particles (energetic nuclei and electrons in our galaxy and beyond) come from. Gamma rays are linked to these particles, providing a means to trace them.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">Why do astronomers believe in dark matter?</a>
</strong>
</em>
</p>
<hr>
<h2>Flashes from outer space</h2>
<p>Gamma-ray astronomy was born <a href="https://imagine.gsfc.nasa.gov/science/toolbox/gamma_ray_astronomy1.html">in the early 1960s</a> as space-based satellites were developed to look for energetic radiation from outer space.</p>
<p>NASA’s Fermi mission, launched in 2008 to a low-Earth orbit, has so far catalogued <a href="https://fermi.gsfc.nasa.gov/ssc/data/access/lat/10yr_catalog/">several thousand gamma-ray sources</a>. The Fermi spacecraft continues to provide 24-hour live coverage of the sky, measuring gamma rays with energies reaching several 1,000 giga-electron volts in energy. That’s about one trillion times the energy of visible light.</p>
<p>To study gamma rays with even higher energies, we need to use ground-based methods. Although Earth’s atmosphere shields us against radiation from outer space, we can still detect the secondary effects of this shielding on the ground. </p>
<p>That’s because when a gamma ray interacts with Earth’s atmosphere, it sparks an electromagnetic cascade or “air shower” of more than a billion secondary particles. These particles are mostly electrons and their anti-matter partners, called positrons. These air showers contribute about 30-50% of the natural radiation we experience in our lives.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A chart illustrating how gamma rays produce Cherenkov light when hitting the atmosphere" src="https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=435&fit=crop&dpr=1 600w, https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=435&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=435&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=547&fit=crop&dpr=1 754w, https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=547&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/512372/original/file-20230227-1701-jolgvd.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=547&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">CTA won’t be detecting gamma rays directly. It will pick up Cherenkov light, the blue flash of light resulting from gamma rays interacting with Earth’s atmosphere.</span>
<span class="attribution"><a class="source" href="https://www.eso.org/public/australia/images/eso1841x/">CTAO/ESO</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Making the invisible visible</h2>
<p>While nothing can go faster than the speed of light in a vacuum, charged particles such as electrons and positrons (anti-electrons) can actually move faster than light when moving through air. </p>
<p>When this happens, a shockwave is created as a flash of blue and ultraviolet light. This flash, called Cherenkov radiation, is named after Soviet physicist Pavel Cherenkov who first detected the phenomenon in 1934 (and received the <a href="https://www.nobelprize.org/prizes/physics/1958/cerenkov/facts/">1958 Nobel Prize in Physics</a> for it alongside two colleagues). The blue glow of Cherenkov radiation can be seen in water cooling ponds surrounding nuclear power reactors.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A concrete room with a circular hole in the middle surrounded with railings, with blue glowing water inside" src="https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/512371/original/file-20230227-2341-a0usyn.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">The blue glow seen in the water cooling the core of a nuclear reactor is known as Cherenkov radiation.</span>
<span class="attribution"><span class="source">Parilov/Shutterstock</span></span>
</figcaption>
</figure>
<p>At ground level, telescopes with large mirrors and sensitive cameras can detect the Cherenkov light produced by a gamma ray striking our atmosphere. These cameras need just about ten nanoseconds to capture a Cherenkov flash against the bright background of starlight and moonlight. </p>
<p>The first Cherenkov telescopes were developed in the 1960s. After many variants, it was the Whipple Telescope in the United States that in <a href="https://ui.adsabs.harvard.edu/abs/1989ApJ...342..379W/abstract">1989 discovered gamma-ray photons</a> coming from the Crab Nebula.</p>
<p>This was the first time gamma rays with energies of more than 1,000 giga-electron volts (or 1 tera-electron-volt, TeV) were detected. Thus, tera-electron-volt gamma-ray astronomy was born.</p>
<h2>Searching for the extremes</h2>
<p>Today, all three of the world’s best TeV gamma-ray facilities – <a href="https://www.mpi-hd.mpg.de/hfm/HESS/">HESS</a> in Namibia, <a href="https://www.mpp.mpg.de/forschung/magic">MAGIC</a> in La Palma, Spain and <a href="https://veritas.sao.arizona.edu/">VERITAS</a> in Arizona – have discovered more than 200 TeV <a href="http://tevcat.uchicago.edu/">gamma-ray sources</a>. These powerful rays are linked to cosmic regions of particle acceleration, such as pulsars, supernova remnants, massive star clusters, and supermassive black holes in the Milky Way and other galaxies. </p>
<p>HESS has shown our Milky Way galaxy is rich in TeV gamma-ray “light”, including <a href="https://theconversation.com/supermassive-black-holes-could-be-a-source-of-mysterious-cosmic-rays-56357">in the centre of the galaxy</a>.</p>
<p>TeV gamma-rays are also seen from <a href="https://theconversation.com/a-collapsing-star-in-a-distant-galaxy-fired-out-some-of-the-most-energetic-gamma-rays-ever-seen-127114">mysterious gamma-ray bursts</a> and other fleeting, transient events. These are now informing our understanding of the extreme conditions in which gamma rays are created.</p>
<p>The next-generation CTA will use the lessons learnt from HESS, VERITAS and MAGIC, by extending the number of telescopes deployed on the ground to over 60 telescopes. CTA will also use a combination of three different telescope sizes optimised for three gamma-ray energy bands, providing unprecedented performance and “sharpness”.</p>
<p>It will have arrays at two sites on the ground: one in Paranal, Chile (51 telescopes) in the Southern Hemisphere, and one in La Palma (13 telescopes) in the Northern Hemisphere.</p>
<p>CTA has attracted membership from more than 1,000 scientists, including Australian scientists from seven universities. It’s progressing well, with the first northern telescope already detecting gamma rays from the Crab Nebula and several gamma-ray flares from <a href="https://astronomerstelegram.org/?read=14783">active galaxies powered by supermassive black holes</a>.</p>
<p>Within a few years we expect to see the first southern telescopes also detecting gamma rays, yielding many more discoveries. With CTA, we will have new insights into where extreme particle acceleration is taking place in our Milky Way.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/new-era-of-astronomy-uncovers-clues-about-the-cosmos-100155">New era of astronomy uncovers clues about the cosmos</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/191772/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gavin Rowell receives funding from the Australian Research Council to support the Cherenkov Telescope Array.</span></em></p>The most energetic events in the universe shower us with unbelievably energetic particles of light. Capturing these can help us to solve some enticing cosmic mysteries.Gavin Rowell, Professor in High Energy Astrophyics, University of AdelaideLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1998312023-02-15T17:16:17Z2023-02-15T17:16:17ZBlack holes may be the source of mysterious dark energy that makes up most of the universe<figure><img src="https://images.theconversation.com/files/509870/original/file-20230213-4443-xsmxpu.jpg?ixlib=rb-1.1.0&rect=313%2C0%2C2251%2C1483&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">black hole</span> </figcaption></figure><p><a href="https://www.nasa.gov/vision/universe/starsgalaxies/black_hole_description.html">Black holes</a> could explain a mysterious form of energy that makes up most of the universe, according to astronomers. The existence of <a href="https://en.wikipedia.org/wiki/Dark_energy">“dark energy”</a> has been inferred from observations of stars and galaxies, but no one has been able to explain what it is, or where it comes from.</p>
<p>The stuff, or matter, that makes up the familiar world around us is just 5% of everything in the universe. Another 27% is <a href="https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy">dark matter</a>, a shadowy counterpart of ordinary matter which does not emit, reflect or absorb light. However, the majority of the cosmos – around 68% – is dark energy.</p>
<p>The new evidence that black holes could be the source of dark energy is described in <a href="https://iopscience.iop.org/article/10.3847/2041-8213/acb704">a scientific paper</a> published in The Astrophysical Journal Letters. The study is the work of 17 astronomers in nine countries and was led by the University of Hawaii. The collaboration included researchers in the UK, based at STFC RAL Space, The Open University, and Imperial College London.</p>
<p>Searching through data spanning nine billion years of cosmic history, the astronomers have uncovered the first evidence of <a href="https://physicsworld.com/a/cosmological-coupling-is-making-black-holes-bigger-study-suggests/">“cosmological coupling”</a>, which would mean that the growth of black holes over time is linked to the expansion of the universe itself.</p>
<p>The idea that black holes might contain something called <a href="https://en.wikipedia.org/wiki/Vacuum_energy">vacuum energy</a> (a manifestation of dark energy) is not particularly new and in fact was discussed theoretically as far back as the 1960s. But this latest work assumes this energy (and therefore the mass of the black holes) would increase with time as the universe expands as a result of cosmological coupling.</p>
<p>The team calculated how much of the dark energy in the universe could be attributed to this process. They found that black holes could potentially explain the total amount of dark energy we measure in the universe today. The result could solve one of the most fundamental problems in modern cosmology.</p>
<h2>Rapid expansion</h2>
<p><a href="https://hubblesite.org/contents/articles/the-big-bang">Our universe began in a Big Bang</a> around 13.7 billion years ago. The energy from this explosion of space and time caused the universe to expand rapidly, with all the galaxies flying away from each other. However, we expect that this expansion would gradually slow down because of the effect of gravity on all the stuff in the cosmos.</p>
<p>This is the version of the universe we thought we lived in until the late 1990s, when the Hubble space telescope discovered something strange. Observations of distant exploding stars showed that, in the past, the universe <a href="https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe">was actually expanding more slowly than it is today</a>. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/gjwxnoPoEHQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The new discovery is explained by Chris Pearson of RAL Space and The Open University.</span></figcaption>
</figure>
<p>So the expansion of the universe has not been slowing due to gravity, as everyone thought, but instead has been accelerating. This was highly unexpected and astronomers struggled to explain it.</p>
<p>To account for this, it was proposed that a “dark energy” was responsible for pushing things apart more strongly than gravity pulled things together. The concept of dark energy was very similar to a mathematical construct Einstein had proposed but later discarded – a <a href="https://en.wikipedia.org/wiki/Cosmological_constant">“cosmological constant”</a> that opposed gravity and kept the universe from collapsing.</p>
<h2>Stellar explosions</h2>
<p>But what is dark energy? The solution, it seems, might lie with another cosmic mystery: black holes. Black holes are commonly born when <a href="https://public.nrao.edu/ask/when-does-a-neutron-star-or-black-hole-form-after-a-supernova/">massive stars explode and die at the ends of their lives</a>. The gravity and pressure in these violent explosions compresses vast amounts of material into a small space. For instance, a star about the same mass as our sun would be squashed into a space of just a few tens of kilometres. </p>
<p>A black hole’s gravitational pull is so strong that not even light can escape it – everything gets sucked in. At the centre of the black hole is a place called a <a href="https://bigthink.com/starts-with-a-bang/singularity-black-hole/">singularity</a>, where matter is crushed into a point of infinite density. The problem is that singularities are a mathematical construct that should not exist.</p>
<figure class="align-center ">
<img alt="The Andromeda galaxy" src="https://images.theconversation.com/files/509866/original/file-20230213-14-3kjqam.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/509866/original/file-20230213-14-3kjqam.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/509866/original/file-20230213-14-3kjqam.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/509866/original/file-20230213-14-3kjqam.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/509866/original/file-20230213-14-3kjqam.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/509866/original/file-20230213-14-3kjqam.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/509866/original/file-20230213-14-3kjqam.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">Dark energy explains why the expansion of the universe is speeding up.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/galex/pia15416.html">NASA/JPL-Caltech</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The black holes nestled at the centres of galaxies are much heftier than those born when stars die violently. These galactic “supermassive” black holes can weigh millions to billions of times the mass of our Sun.</p>
<p>All black holes increase in size by accumulating matter, by swallowing stars that get too close, or by merging with other black holes. So we expect them to get bigger as the universe gets older.</p>
<p>In the latest paper, the team looked at supermassive black holes in the centres of galaxies and found that these black holes gain mass over billions of years. </p>
<h2>Radical rethink</h2>
<p>The team compared observations of <a href="https://en.wikipedia.org/wiki/Elliptical_galaxy">elliptical galaxies</a>, which lack star formation, in the past and in the present day. These dead galaxies have used up all their fuel so any increase in their black hole mass over this time cannot be ascribed to the normal processes by which black holes grow by accumulating matter.</p>
<p>Instead, the team proposed that these black holes actually contain vacuum energy and that they are “coupled” to the expansion of the universe, so that they increase in mass as the universe expands. </p>
<figure class="align-center ">
<img alt="Visualisation of a black hole" src="https://images.theconversation.com/files/509864/original/file-20230213-18-s6s06q.jpeg?ixlib=rb-1.1.0&rect=17%2C34%2C3782%2C2098&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/509864/original/file-20230213-18-s6s06q.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/509864/original/file-20230213-18-s6s06q.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/509864/original/file-20230213-18-s6s06q.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/509864/original/file-20230213-18-s6s06q.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/509864/original/file-20230213-18-s6s06q.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/509864/original/file-20230213-18-s6s06q.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">
<figcaption>
<span class="caption">A visualisation of a black hole, which could play a role in dark energy.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/feature/goddard/2019/nasa-visualization-shows-a-black-hole-s-warped-world">NASA’s Goddard Space Flight Center/Jeremy Schnittman</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>This model neatly provides a possible origin for the dark energy in the universe. It also circumvents the mathematical problems that affect some studies of black holes, because it avoids the need for a singularity at the centre.</p>
<p>The team also calculated how much of the dark energy in the universe could be attributed to this process of coupling. They concluded that it would be possible for black holes to provide the necessary amount of vacuum energy to account for all the dark energy that we measure in the universe today. </p>
<p>This would not only explain the origin of dark energy in the universe but would also make us radically rethink our understanding of black holes and their role in the cosmos.</p>
<p>Much more work needs to be done to test and confirm this idea, both from observations of the sky and from theory. But we may at last be seeing a new way to solve the problem of dark energy.</p><img src="https://counter.theconversation.com/content/199831/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Pearson receives funding from STFC and is head of astronomy at STFC RAL Space and a visiting fellow at the Open University </span></em></p><p class="fine-print"><em><span>Dave Clements receives funding from STFC and the UKSA and works at Imperial College London.</span></em></p>Astronomers have found that mysterious dark energy may originate in black holes.Chris Pearson, Astronomy Group Lead, Space Operations Division at RAL Space, and Visiting Fellow, The Open UniversityDave Clements, Reader in Astrophysics, Imperial College LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1948392022-12-09T15:49:18Z2022-12-09T15:49:18ZArrakhis: the tiny satellite aiming to reveal what dark matter is made of<figure><img src="https://images.theconversation.com/files/498940/original/file-20221205-25-h665s4.jpg?ixlib=rb-1.1.0&rect=239%2C0%2C5427%2C3494&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Milky Way over sand dunes in Cervantes, Australia.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/reverse-arch-milky-way-on-sand-1072692071">Nik Coli/Shutterstock</a></span></figcaption></figure><p>The European Space Agency (Esa) recently announced a new mission of its <a href="https://www.esa.int/About_Us/Business_with_ESA/Business_Opportunities/Science_Programme">science programme</a>: a small telescope orbiting the Earth dubbed <a href="https://www.cosmos.esa.int/web/call-for-missions-2021/selection-of-f2">Arrakhis</a>. But although its name is inspired by the sci-fi novel <a href="https://en.wikipedia.org/wiki/Dune_(novel)">Dune</a>, it will not be looking for sandworms or “spice” on a desert planet. </p>
<p>Instead, this nimble satellite will punch hugely above its weight and try to track down one of the most elusive and mysterious substances in the universe: <a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">dark matter</a>. This is the term given to the hypothetical invisible matter that is thought to be more abundant than normal matter and have a similar gravitational effect on its surroundings.</p>
<p>The mission is classified as fast (F), which means it is smaller, more focused and has a quicker turnaround (less than ten years to launch) than other types of Esa missions. The agency’s previous F-mission, selected in 2019, is called the <a href="https://www.cometinterceptor.space">Comet Interceptor</a>. Already parked at a stable point in the Solar System, this probe is waiting for a comet to show up and fly by it, something that’s due to happen around the time that Arrakhis launches in the early 2030s.</p>
<h2>Follow the light</h2>
<p>Since dark matter <a href="https://theconversation.com/dark-matter-the-mystery-substance-physics-still-cant-identify-that-makes-up-the-majority-of-our-universe-85808">still eludes detection</a>, the mission will target sources of light that are sensitive to it. We expect normal matter – the stuff that actually emits light, such as stars in galaxies – to move primarily under the influence of dark matter, which is more abundant. </p>
<p>We believe entire galaxies are moved to and fro by the underlying dark matter, like beacons spread across an invisible ocean. Their sailing is bumpy though, as dark matter is thought to be distributed unevenly across the universe, forming a <a href="https://theconversation.com/scientists-start-mapping-the-hidden-web-that-scaffolds-the-universe-124616">“cosmic web”</a> over vast distances, and having a more clumpy appearance on galaxy scales. Some of these clumps should be populated with small galaxies called <a href="https://esahubble.org/wordbank/dwarf-galaxy/">dwarf galaxies</a>, while others would be made up entirely of dark matter.</p>
<p>There is also debris left over from those dwarf galaxies that venture too close to the host galaxies they orbit. As the surrounding dark matter rips these galaxies apart through gravitational tides, they start to unravel into long streams of stars that wrap around vast swathes of space. These thin veils of light are another connection with the unseen. By counting and measuring their shapes, we can infer what type of particle dark matter is made of – and ultimately which cosmological model is the most accurate.</p>
<figure class="align-center ">
<img alt="Image of NGC 5907, a galaxy which hosts faint streams of stars that wrap all around it." src="https://images.theconversation.com/files/496311/original/file-20221120-26-cl1krw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/496311/original/file-20221120-26-cl1krw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/496311/original/file-20221120-26-cl1krw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/496311/original/file-20221120-26-cl1krw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/496311/original/file-20221120-26-cl1krw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/496311/original/file-20221120-26-cl1krw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/496311/original/file-20221120-26-cl1krw.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">Like the Milky Way, the NGC 5907 galaxy hosts faint streams of stars that wrap all around it.</span>
<span class="attribution"><span class="source">wikipedia/R Jay Gabany (Blackbird Observatory) - collaboration; D.Martinez-Delgado(IAC, MPIA), J.Penarrubia (U.Victoria) I. Trujillo (IAC) S.Majewski (U.Virginia), M.Pohlen (Cardiff)</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The clumpiness in space is a robust prediction of our cosmological models, as it simply represents the outcome of gravity acting on matter. However, our models give conflicting predictions about the number of these clumps, which could be higher or lower depending on <a href="https://www.symmetrymagazine.org/article/is-dark-matter-cold-warm-or-hot">what type of particle or particles</a> we assume dark matter to be made up of. </p>
<p>In the “standard” model of cosmology, dark matter particles are assumed to be <a href="https://astronomynow.com/2020/01/12/hubble-finds-evidence-for-widely-held-cold-dark-matter-theory/">“cold”</a>, meaning they are heavy and slow moving (an example would be “weakly interacting massive particles”, <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">or Wimps</a>). This implies that our Milky Way will contain hundreds of dark matter clumps, some of which will contain dwarf galaxies. But the problem is that we only see a few dozen dwarf galaxies around us, which is very puzzling. It could mean that most of these clumps are made of dark matter.</p>
<p>Cosmologists have other viable ideas though. For example, if dark matter is <a href="https://www.symmetrymagazine.org/article/is-dark-matter-cold-warm-or-hot">“warm”</a> - meaning that particles are much lighter and faster, such as <a href="https://www.symmetrymagazine.org/article/the-search-for-the-sterile-neutrino">sterile neutrinos</a> - there would be far fewer clumps to begin with. Observations can give us the final clue as to which model is right, but to get there, we first need an accurate <a href="https://www.cosmotography.com/images/dwarf_galaxy_dark_matter.html">census of dwarf galaxies</a> orbiting the Milky Way. </p>
<h2>The tip of the iceberg</h2>
<p>There are strong indications that the dwarf galaxies discovered so far near the Milky Way or other large galaxies are just the tip of the iceberg, and that <a href="https://news.fnal.gov/2020/04/the-milky-ways-satellites-help-reveal-link-between-dark-matter-halos-and-galaxy-formation/">many more remain hidden </a> behind the light of their hosts. Arrakhis will be able to discover this missing population even at large distances from us.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Figure of a galaxy simulated with cold dark matter (left) versus warm dark matter (right). There are many more clumps of cold dark matter that can host dwarf galaxies than warm dark matter ones." src="https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=298&fit=crop&dpr=1 600w, https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=298&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=298&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=374&fit=crop&dpr=1 754w, https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=374&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/496320/original/file-20221120-48207-2cj8xd.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=374&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 galaxy simulated with cold dark matter (left) versus warm dark matter (right). There are many more clumps of cold dark matter that can host dwarf galaxies than warm dark matter ones.</span>
<span class="attribution"><span class="source">Aquarius/Virgo/ICC Durham University.</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>Observing this faint starlight has proven to be challenging even for the largest telescopes on Earth, since it requires very deep imaging and surveying of large portions of the sky. Besides, the Earth’s atmosphere is a hindrance. Arrakhis will observe from space, with an innovative camera that probes deeper in both the optical and near-infrared part of the spectrum, and with a much wider field of view. (Incidentally, this type of camera can also <a href="https://satlantis.com/isim-170-has-successfully-arrived-at-the-international-space-station/">look back at Earth</a> with excellent resolution.)</p>
<p>The hundred or so Milky Way-like systems that will be observed are about 100 million light-years away, where <a href="https://earthsky.org/space/milky-way-normal-galaxy-outlier-saga/">only a few dwarf galaxies</a> have been discovered so far, and no stellar streams yet. When we know the number of soon-to-be discovered dwarf galaxies and <a href="https://theconversation.com/dance-of-galaxies-challenges-current-thinking-on-cosmology-91097">how they will be seen distributed in space</a>, we should be able to pin down the correct cosmological model. </p>
<p>Arrakhis will find many of the missing pieces in the puzzle that dark matter provides, complementing what we already know from the nearby universe and what we will learn in the future from other upcoming telescopes, such as <a href="https://www.esa.int/Science_Exploration/Space_Science/Euclid_overview">Euclid</a> or the <a href="https://www.lsst.org">Vera Rubin Observatory</a>.</p>
<p>The hope is that these detailed, combined observations will finally reveal the dark matter mystery, and help us understand what makes up the majority of matter in the cosmos.</p><img src="https://counter.theconversation.com/content/194839/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>By spotting and counting tiny galaxies, we can work out how much dark matter is hiding in the cosmos.Andreea Font, Reader in Theoretical Astrophysics, Liverpool John Moores UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1897842022-09-05T20:03:35Z2022-09-05T20:03:35ZGamma rays from a dwarf galaxy solve an astronomical puzzle<figure><img src="https://images.theconversation.com/files/482639/original/file-20220905-22-g5344a.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2874%2C1612&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/GLAST/news/new-structure.html">NASA's Goddard Space Flight Center</a></span></figcaption></figure><p>A glowing blob known as “the cocoon”, which appears to be inside one of the enormous gamma-ray emanations from the centre of our galaxy dubbed the “Fermi bubbles”, has puzzled astronomers since it was discovered in 2012. </p>
<p>In <a href="https://www.nature.com/articles/s41550-022-01777-x">new research</a> published in Nature Astronomy, we show the cocoon is caused by gamma rays emitted by fast-spinning extreme stars called “millisecond pulsars” located in the Sagittarius dwarf galaxy, which orbits the Milky Way. While our results clear up the mystery of the cocoon, they also cast a pall over attempts to search for dark matter in any gamma-ray glow it may emit.</p>
<h2>Seeing with gamma rays</h2>
<p>Thankfully for life on Earth, our atmosphere blocks gamma rays. These are particles of light with energies more than a million times higher than the photons we detect with our eyes. </p>
<p>Because our ground-level view is obscured, scientists had no idea of the richness of the gamma-ray sky until instruments were lofted into space. But, starting with the serendipitous discoveries made by the Vela satellites (put into orbit in the 1960s to monitor the Nuclear Test Ban), more and more of this richness has been revealed.</p>
<p>The state-of-the-art gamma-ray instrument operating today is the Fermi Gamma Ray Space Telescope, a large NASA mission in orbit for more than a decade. Fermi’s ability to resolve fine detail and detect faint sources has uncovered a number of surprises about our Milky Way and the wider cosmos.</p>
<h2>Mysterious bubbles</h2>
<p>One of these surprises <a href="https://www.nasa.gov/mission_pages/GLAST/news/new-structure.html">emerged in 2010</a>, soon after Fermi’s launch: something in the Milky Way’s centre is blowing what look like a pair of giant, gamma-ray-emitting bubbles. These completely unanticipated “Fermi bubbles” cover fully 10% of the sky. </p>
<p>A prime suspect for the source of the bubbles is the Galaxy’s resident supermassive black hole. This behemoth, 4 million times more massive than the Sun, lurks in the galactic nucleus, the region from which the bubbles emanate.</p>
<p>Most galaxies host such giant black holes in their centres. In some, these black holes are actively gulping down matter. Thus fed, they simultaneously spew out giant, outflowing “jets” visible across the electromagnetic spectrum.</p>
<p>Thus a question researchers asked after the discovery of the bubbles: can we find a smoking gun tying them to our Galaxy’s supermassive black hole? Soon, tentative evidence did emerge: there was a <a href="https://ui.adsabs.harvard.edu/abs/2012ApJ...753...61S/abstract">hint</a>, inside each bubble, of a thin gamma-ray jet pointing back towards the galactic centre. </p>
<p>With time and further data, this picture became muddied, however. While the jet-like feature in one of the bubbles was confirmed, the apparent jet in the other seemed to <a href="https://ui.adsabs.harvard.edu/abs/2014ApJ...793...64A/abstract">evaporate under scrutiny</a>. </p>
<p>The bubbles looked strangely lopsided: one contained an elongated bright spot – the “cocoon” – with no counterpart in the other bubble.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/astronomers-have-detected-one-of-the-biggest-black-hole-jets-in-the-sky-188357">Astronomers have detected one of the biggest black hole jets in the sky</a>
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<h2>The cocoon and where it comes from</h2>
<p>Our <a href="https://www.nature.com/articles/s41550-022-01777-x">recent work</a> in Nature Astronomy is a deep examination of the nature of the “cocoon”. Remarkably, we found this structure has nothing to do with the Fermi bubbles or, indeed, the Galaxy’s supermassive black hole. </p>
<p>Rather, we found the cocoon is actually something else entirely: gamma rays from the Sagittarius dwarf galaxy, which happens to be behind the southern bubble as seen from the position of Earth. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=538&fit=crop&dpr=1 600w, https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=538&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=538&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=677&fit=crop&dpr=1 754w, https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=677&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/482582/original/file-20220902-18-6lvt9s.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=677&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Schematic showing the Milky Way, the gamma-ray-emitting Fermi Bubbles (pink), and the Sagittarius dwarf galaxy and its tails (yellow/green). From the position of the Sun, we view the Sagittarius dwarf through the southern Fermi Bubble.</span>
<span class="attribution"><span class="source">Aya Tsuboi, Kavli IPMU</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The Sagittarius dwarf, so called because its sky position is in the constellation of Sagittarius, is a “satellite” galaxy orbiting the Milky Way. It is the remnant of a much larger galaxy that the Milky Way’s strong gravitational field has literally ripped apart. Indeed, stars pulled out of the Sagittarius dwarf can be found in “tails” that wrap around the entire sky.</p>
<h2>What’s making the gamma rays?</h2>
<p>In the Milky Way, the main source of gamma rays is when high-energy particles, called cosmic rays, collide with the very tenuous gas between the stars. </p>
<p>However, this process cannot explain the gamma rays emitted from the Sagittarius dwarf. It long ago lost its gas to the same gravitational harassment that pulled away so many of its stars. </p>
<p>So where do the gamma rays come from? </p>
<p>We considered several possibilities, including the exciting prospect they are a signature of dark matter, the invisible substance known only by its gravitational effects which astronomers believe makes up much of the universe. Unfortunately, the shape of the cocoon closely matches the distribution of visible stars, which rules out dark matter as the origin. </p>
<p>One way or another, the stars were responsible for the gamma rays. And yet: the stars of the Saggitarius dwarf are old and quiescent. What type of source amongst such a population produces gamma rays? </p>
<h2>Millisecond pulsars</h2>
<p>We are satisfied there is only one possibility: rapidly spinning objects called “millisecond pulsars”. These are the remnants of particular stars, significantly more massive than the Sun, that are also closely orbiting another star.</p>
<p>Under just the right circumstances, such binary systems produce a neutron star – an object about as heavy as the Sun but only about 20km across – that rotates hundreds of times per second. </p>
<p>Because of their rapid rotation and strong magnetic field, these neutron stars act as natural particle accelerators: they launch particles at extremely high energy into space. </p>
<p>These particles then emit gamma rays. Millisecond pulsars in the Sagittarius dwarf were the ultimate source of the mysterious cocoon, we found.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/this-newly-discovered-neutron-star-might-light-the-way-for-a-whole-new-class-of-stellar-object-184050">This newly discovered neutron star might light the way for a whole new class of stellar object</a>
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</p>
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<h2>The hunt for dark matter</h2>
<p>Our findings shed new light – pun intended – on millisecond pulsars as sources of gamma rays in other old stellar systems. </p>
<p>At the same time, they also cast a pall over efforts to find evidence for dark matter via observations of other satellite galaxies of the Milky Way; unfortunately, there is a stronger “background” of gamma rays from millisecond pulsars in these systems than previously realised. </p>
<p>Thus, any signal they produce might not be unambiguously interpreted as due to dark matter. </p>
<p>The hunt for dark matter signals goes on.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/we-dont-know-if-dark-matter-exists-so-why-do-astronomers-keep-looking-187656">We don't know if dark matter exists. So why do astronomers keep looking?</a>
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<img src="https://counter.theconversation.com/content/189784/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roland Crocker receives funding from the Australian Research Council. </span></em></p>A glowing blob in the sky known as “the cocoon” may be caused by pulsars in the Sagittarius dwarf galaxy.Roland Crocker, Associate Professor of Astronomy, Australian National UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1876562022-08-30T13:44:48Z2022-08-30T13:44:48ZWe don’t know if dark matter exists. So why do astronomers keep looking?<figure><img src="https://images.theconversation.com/files/476134/original/file-20220726-32571-v7qdc9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Spiral galaxies like M100, pictured here, may hold answers about the nature of dark matter.</span> <span class="attribution"><span class="source">NASA Spitzer Space Telescope/NASA/JPL-Caltech</span></span></figcaption></figure><p>Scientists know very little about the matter that makes up the galaxies in the Universe. About 20% of the matter in galaxies is visible or <a href="https://astronomy.swin.edu.au/cosmos/b/Baryonic+Matter">baryonic</a>: subatomic particles like protons, neutrons and electrons. The other 80%, referred to as “dark matter”, remains mysterious and unseen. </p>
<p>In fact, it may not exist at all. “Dark matter” is just a hypothesis. Physicists and astronomers may be chasing a phantom – but that doesn’t stop us from looking. Why? Because if dark matter isn’t real, then the behaviour of the stars, planets and galaxies makes little sense. </p>
<iframe id="noa-web-audio-player" style="border: none" src="https://embed-player.newsoveraudio.com/v4?key=x84olp&id=https://theconversation.com/we-dont-know-if-dark-matter-exists-so-why-do-astronomers-keep-looking-187656&bgColor=F5F5F5&color=D8352A&playColor=D8352A" width="100%" height="110px"></iframe>
<p>Today dark matter – and its cousin, dark energy – are the main pillars of a cosmological model called Lambda Cold Dark Matter, or <a href="https://www.universetoday.com/tag/lambda-cdm/">Lambda-CDM</a>. This model stresses that dark matter affects baryonic matter only via gravity. It does not interact with the electromagnetic force, meaning that it does not absorb, reflect or emit light.</p>
<p>In a recent study, published in <a href="https://www.aanda.org/articles/aa/abs/2021/09/aa40532-21/aa40532-21.html">Astronomy and Astrophysics</a>, we provide further evidence to support the existence of dark matter halos around early galaxies (when the Universe was half of its current age). We also challenge some assumptions about it. This is a way to deepen our understanding of the Universe and its galaxies.</p>
<h2>Origins of the theory</h2>
<p>In the 1970s, astronomers Vera Rubin and Kent Ford unveiled <a href="https://www.space.com/vera-rubin.html">the theory of dark matter</a>. They weren’t just taking a shot in the dark: there had long been a debate about why stars, planets and galaxies behaved in certain ways. For instance, why aren’t stars and gases constantly flung far and wide into outer space? What sort of glue keeps galaxies intact, exerting a <a href="https://www.astro.princeton.edu/%7Eburrows/classes/250/dark_matter.html">gravitational effect</a> on everyday baryonic particles?</p>
<p>Scientists also wondered why objects far beyond the centre of a galaxy orbit at much the same speeds (or velocities) as objects closer to the centre. This flies in the face of <a href="https://www.britannica.com/science/Newtons-laws-of-motion">Newton’s law</a>, which suggests that stars and gas should be slowing down the further they are from a galaxy’s centre. The greater abundance of stars and gases near the core should provide the necessary gravitational force that speeds up the stars and gas. The more thinly distributed they are at the edges of the galaxy, the less the gravitational force – and so, stars and gas should slow down. But observations suggest that they don’t.</p>
<p>To account for these discrepancies, Rubin and Ford argued that every galaxy is engulfed by a large halo of dark matter, providing the unaccounted-for mass. Dark matter, they claimed, provides around 85% of the matter within any one galaxy. Its <a href="https://www.sciencedaily.com/releases/2022/02/220211102628.htm">dominant presence</a> throughout galaxies arises from the fact that the stars and hydrogen gas are moving as if governed by an invisible element.</p>
<p>Their theory hasn’t been universally embraced. Some scientists have argued that <a href="https://iopscience.iop.org/article/10.3847/1538-4357/abbb96">dark matter doesn’t exist</a>.</p>
<p>But we and many others agree with Rubin and Ford. Dark matter exists because it explains so much. As one writer <a href="https://www.science.org/content/article/explain-away-dark-matter-gravity-would-have-be-really-weird-cosmologists-say">put it</a>:</p>
<blockquote>
<p>many [physicists] would happily dismiss the idea – if it didn’t work so well.</p>
</blockquote>
<h2>Looking way back into the Universe</h2>
<p>For <a href="https://www.aanda.org/articles/aa/abs/2021/09/aa40532-21/aa40532-21.html">our new study</a> we observed about 260 spiral-shaped star-forming galaxies some seven billion light years away. This is essentially a glimpse into the past. It is estimated that these galaxies existed when the Universe was half its present age of around 13.8 billion years. They appear to us now as no more than light signals. Spiral galaxies, of which our Milky Way is one, are characterised by the distinctive coiled, spiral arms of stars and gas clouds.</p>
<p>Our aim was to observe and determine, and then compare, the distribution of mass in these distant spiral galaxies with more recent, nearer galaxies of more or less the same characteristics. </p>
<p>Some recent studies have suggested that the earlier star-forming galaxies appear to be <a href="https://www.mpg.de/11170451/early-galaxies-dark-matter">deficient in dark matter</a> when compared to more recent or local ones. This has led <a href="https://www.mpg.de/11170451/early-galaxies-dark-matter">some researchers</a> to assert that dark matter plays a much smaller role in early star systems than in today’s galaxies. Our findings refute this suggestion. </p>
<p>We were able to confirm that the earlier galaxies we studied have the trademark halos of dark matter that build up from the centre and maintain a constant density up to a certain radius. This is largely in keeping with the standard scenario of dark matter observed in the galaxies of the local Universe. A surprise finding, however, was that these halos are <a href="https://www.aanda.org/articles/aa/full_html/2022/03/aa41822-21/aa41822-21.html">much more compact</a> than those galaxies closer to our Milky Way. This suggests that distribution of dark matter within a galaxy expand slowly over time. But how is this process powered?</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/481594/original/file-20220829-9177-u3jh7b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/481594/original/file-20220829-9177-u3jh7b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=236&fit=crop&dpr=1 600w, https://images.theconversation.com/files/481594/original/file-20220829-9177-u3jh7b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=236&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/481594/original/file-20220829-9177-u3jh7b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=236&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/481594/original/file-20220829-9177-u3jh7b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=297&fit=crop&dpr=1 754w, https://images.theconversation.com/files/481594/original/file-20220829-9177-u3jh7b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=297&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/481594/original/file-20220829-9177-u3jh7b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=297&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p>Our conclusion is that this phenomenon illustrates a direct interaction between dark matter particles and the everyday baryonic particles. This alters the density of the halos – and in doing so, goes beyond just the textbook gravitational relationship.</p>
<p>These findings don’t provide all the answers to all or even a few of the questions that exist about dark matter. But it certainly narrows the long search for dark matter particles. </p>
<p>It also provides some direction to the identification of dark matter particles, based on what they are capable of. That, in turn, opens up the discussion to other theories of dark matter, such as warm dark matter, self-interacting dark matter, and ultra light dark matter. All of these are much more interactive than cold dark matter. </p>
<h2>A deeper look</h2>
<p>For those of us equally mesmerised and confounded by dark matter, there may well be a light at the end of the tunnel. New technology is helping us to better understand the Universe and its dynamics.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/james-webb-space-telescope-an-astronomer-explains-the-stunning-newly-released-first-images-186800">James Webb Space Telescope: An astronomer explains the stunning, newly released first images</a>
</strong>
</em>
</p>
<hr>
<p>Finding answers will mean peering ever deeper into the centre of these earlier and “younger” galaxies. The new James Webb Space Telescope, launched at the end of 2021 and now orbiting some 1,500,000 km beyond Earth’s orbit around the Sun, may help in this regard. </p>
<p>So, too, will the new <a href="https://www.newscientist.com/article/2327468-worlds-most-sensitive-dark-matter-detector-tested-for-the-first-time/">LUX-ZEPLIN</a> dark matter detector, <a href="https://www.space.com/dark-matter-most-sensitive-detector-first-results">touted as</a> the “world’s most sensitive dark matter detector” and located around 1.5km underground in the US.</p>
<p><em>Professor Paolo Salucci (SISSA, Italy) and Professor Glenn van de Ven (UniVie, Austria) co-authored this article.</em></p><img src="https://counter.theconversation.com/content/187656/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gauri Sharma receives funding from SARAO and NRF. She is affiliated with University of the Western Cape, South Africa, SISSA, IFPU and INFN Trieste (Italy)</span></em></p>A comparison of star-forming galaxies suggests, surprisingly, that dark matter and visible matter do interact – taking us closer to understanding what keeps the galaxies together.Gauri Sharma, SARAO Postdoctoral Fellow, University of the Western CapeLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1870142022-07-25T20:01:55Z2022-07-25T20:01:55ZThis Australian experiment is on the hunt for an elusive particle that could help unlock the mystery of dark matter<figure><img src="https://images.theconversation.com/files/475787/original/file-20220725-22-e2kyjx.jpeg?ixlib=rb-1.1.0&rect=150%2C25%2C5440%2C4166&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>Australian scientists are making strides towards solving one of the greatest mysteries of the universe: the nature of invisible “dark matter”.</p>
<p>The ORGAN Experiment, Australia’s first major dark matter detector, recently completed a search for a hypothetical particle called an axion – a popular candidate among theories that try to explain dark matter.</p>
<p>ORGAN has placed new limits on the possible characteristics of axions and thus helped narrow the search for them. But before we get ahead of ourselves …</p>
<h2>Let’s start with a story</h2>
<p>About 14 billion years ago, all the little pieces of matter – the fundamental particles that would later become you, the planet and the galaxy – were compressed into one very dense, hot region.</p>
<p>Then the Big Bang happened and everything flew apart. The particles combined into atoms, which eventually clumped together to make stars, which exploded and created all kinds of exotic matter. </p>
<p>After a few billion years came Earth, which was eventually crawling with little things called humans. Cool story, right? Turns out it’s not the whole story; it’s not even half.</p>
<p>People, planets, stars and galaxies are all made of “regular matter”. But we know regular matter makes up just one-sixth of all the matter in the universe. </p>
<p>The rest is made of what we call “dark matter”. Its name tells you almost everything we know about it. It doesn’t emit light (so we call it “dark”) and it has mass (so we call it “matter”).</p>
<h2>If it’s invisible, how do we know it’s there?</h2>
<p>When we observe the way things move in space, we find time and again that we can’t explain our observations if we consider only what we can see. </p>
<p>Spinning galaxies are a great example. Most galaxies spin at speeds that can’t be explained by the gravitational pull from visible matter alone. </p>
<p>So there must be dark matter in these galaxies, providing extra gravity and allowing them to spin faster – without parts being flung off into space. We think dark matter literally holds galaxies together.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Cluster of galaxies displayed in hues of pink and purple against a black cosmic background." src="https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=434&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=434&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=434&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=545&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=545&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=545&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 ‘Bullet Cluster’ is a massive cluster of galaxies which has been interpreted as being strong evidence for the existence of dark matter.</span>
<span class="attribution"><a class="source" href="https://science.nasa.gov/matter-bullet-cluster">NASA</a></span>
</figcaption>
</figure>
<p>So there must be an enormous amount of dark matter in the universe, pulling on all the things we can see. It’s passing through you, too, like some kind of cosmic ghost. You just can’t feel it.</p>
<h2>How could we detect it?</h2>
<p>Many scientists believe dark matter could be composed of hypothetical particles called axions. Axions were originally proposed as part of a solution to another major problem in particle physics called the “strong CP problem” (which we could write a whole article about). </p>
<p>Anyway, after the axion was proposed, scientists realised the particle could also make up dark matter under certain conditions. That’s because axions are expected to have very weak interactions with regular matter, but still have some mass: the two conditions needed for dark matter.</p>
<p>So how do you go about searching for axions? </p>
<p>Well, since dark matter is thought to be all around us, we can build detectors right here on Earth. And, luckily, the theory that predicts axions also predicts that axions can convert into photons (particles of light) under the right conditions.</p>
<p>This is good news, because we’re great at detecting photons. And this is exactly what ORGAN does. It engineers the correct conditions for axion–photon conversion and looks for weak photon signals – little flashes of light generated by dark matter passing through the detector. </p>
<p>This kind of experiment is called an axion haloscope and was first proposed in the <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.51.1415">1980s</a>. There are a few in the world today, each one slightly different in important ways.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.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 ORGAN Experiment’s main detector. A small copper cylinder called a ‘resonant cavity’ traps photons generated during dark matter conversion. The cylinder is bolted to a ‘dilution refrigerator’ which cools the experiment to very low temperatures.</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Shining a light on dark matter</h2>
<p>An axion is believed to convert into a photon in the presence of a strong magnetic field. In a typical haloscope, we generate this magnetic field using a big electromagnet called a “superconducting solenoid”.</p>
<p>Inside the magnetic field we place one or several hollow chambers of metal, which are meant to trap the photons and cause them to bounce around inside, making them easier to detect.</p>
<p>However, there is one hiccup. Everything that has a temperature constantly emits small random flashes of light (which is why thermal imaging cameras work). These random emissions, or “noise”, make it harder to detect the faint dark matter signals we’re looking for. </p>
<p>To work around this, we’ve placed our resonator in a “dilution refrigerator”. This fancy fridge cools the experiment to cryogenic temperatures, about −273°C, which greatly reduces the noise. </p>
<p>The colder the experiment is, the better we can “listen” for faint photons produced during dark matter conversion.</p>
<h2>Targeting mass regions</h2>
<p>An axion of a certain mass will convert into a photon of a certain frequency, or colour. But since the mass of axions is unknown, experiments must target their search to different regions, focusing on those where dark matter is considered more likely to exist.</p>
<p>If no dark matter signal is found, then either the experiment is not sensitive enough to hear the signal above the noise, or there’s no dark matter in the corresponding axion mass region. </p>
<p>When this happens, we set an “exclusion limit” – which is just a way of saying “we didn’t find any dark matter in this mass range, to this level of sensitivity”. This tells the rest of the dark matter research community to direct their searches elsewhere.</p>
<p>ORGAN is the most sensitive experiment in its targeted frequency range. Its recent run detected no dark matter signals. This result has set an important exclusion limit on the possible characteristics <a href="https://www.science.org/doi/10.1126/sciadv.abq3765">of axions</a>.</p>
<p>This is the first phase of a multi-year plan to search for axions. We’re currently preparing the next experiment, which will be more sensitive and target a new, as-yet-unexplored mass range. </p>
<h2>But why does dark matter matter?</h2>
<p>Well, for one, we know from history that when we invest in fundamental physics, we end up developing important technologies. For instance, all modern computing relies on our understanding of quantum mechanics.</p>
<p>We never would have discovered electricity, or radio waves, if we didn’t pursue things that, at the time, appeared to be strange physical phenomena beyond our understanding. Dark matter is the same.</p>
<p>Consider everything humans have accomplished by understanding just one-sixth of the matter in the universe – and imagine what we could do if we unlocked the rest.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-search-for-dark-matter-gets-a-speed-boost-from-quantum-technology-153604">The search for dark matter gets a speed boost from quantum technology</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/187014/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ben McAllister works for The University of Western Australia. The work referenced in this article is funded by the Australian Research Council.</span></em></p>Regular matter makes up just one-sixth of all the matter in the universe. What would it mean to finally understand what makes up the rest?Ben McAllister, Research Fellow, Department of Physics, The University of Western AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1863442022-07-07T16:47:33Z2022-07-07T16:47:33ZDark matter: our review suggests it’s time to ditch it in favour of a new theory of gravity<figure><img src="https://images.theconversation.com/files/472844/original/file-20220706-25-fxgof1.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1022%2C873&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The barred spiral galaxy UGC 12158.</span> <span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Barred_spiral_galaxy#/media/File:UGC_12158.jpg">Wikimedia </a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>We can model the motions of planets in the Solar System quite accurately using Newton’s laws of physics. But in the early 1970s, scientists noticed that <a href="https://www.sciencedirect.com/topics/physics-and-astronomy/disk-galaxies">this didn’t work for</a> <a href="https://astronomy.swin.edu.au/cosmos/d/Disk+Galaxies">disc galaxies</a> – stars at their outer edges, far from the gravitational force of all the matter at their centre – were moving much faster than Newton’s theory predicted. </p>
<p>This made physicists propose that an invisible substance called “dark matter” was providing extra gravitational pull, causing the stars to speed up – a theory that’s become hugely popular. However, in a <a href="https://www.mdpi.com/2073-8994/14/7/1331">recent review</a> my colleagues and I suggest that observations across a vast range of scales are much better explained in an alternative theory of gravity proposed by Israeli physicist Mordehai Milgrom in 1982 called Milgromian dynamics or <a href="https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics">Mond</a> – requiring no invisible matter.</p>
<p>Mond’s main postulate is that when gravity becomes very weak, as occurs at the edge of galaxies, it starts behaving differently from Newtonian physics. In this way, it is possible to <a href="https://doi.org/10.1051/0004-6361/201732547">explain</a> why stars, planets and gas in the outskirts of over 150 galaxies rotate faster than expected based on just their visible mass. But Mond doesn’t merely <em>explain</em> such rotation curves, in many cases, it <em>predicts</em> them.</p>
<p>Philosophers of science <a href="https://www.cambridge.org/core/books/philosophical-approach-to-mond/9E770E2F021E79EE639C9A750143C589">have argued</a> that this power of prediction makes Mond superior to the standard cosmological model, which proposes there is more dark matter in the universe than visible matter. This is because, according to this model, galaxies have a highly uncertain amount of dark matter that depends on details of how the galaxy formed – which we don’t always know. This makes it impossible to predict how quickly galaxies should rotate. But such predictions are routinely made with Mond, and so far these have been confirmed.</p>
<p>Imagine that we know the distribution of visible mass in a galaxy but do not yet know its rotation speed. In the standard cosmological model, it would only be possible to say with some confidence that the rotation speed will come out between 100km/s and 300km/s on the outskirts. Mond makes a more definite prediction that the rotation speed must be in the range 180-190km/s.</p>
<p>If observations later reveal a rotation speed of 188km/s, then this is consistent with both theories – but clearly, Mond is preferred. This is a modern version of <a href="https://en.wikipedia.org/wiki/Occam%27s_razor">Occam’s razor</a> – that the simplest solution is preferable to more complex ones, in this case that we should explain observations with as few “free parameters” as possible. Free parameters are constants - certain numbers that we must plug into equations to make them work. But they are not given by the theory itself – there’s no reason they should have any particular value – so we have to measure them observationally. An example is the gravitation constant, G, in Newton’s gravity theory or the amount of dark matter in galaxies within the standard cosmological model.</p>
<p>We introduced a concept known as “theoretical flexibility” to capture the underlying idea of Occam’s razor that a theory with more free parameters is consistent with a wider range of data – making it more complex. In our review, we used this concept when testing the standard cosmological model and Mond against various astronomical observations, such as the rotation of galaxies and the motions within galaxy clusters.</p>
<p>Each time, we gave a theoretical flexibility score between –2 and +2. A score of –2 indicates that a model makes a clear, precise prediction without peeking at the data. Conversely, +2 implies “anything goes” – theorists would have been able to fit almost any plausible observational result (because there are so many free parameters). We also rated how well each model matches the observations, with +2 indicating excellent agreement and –2 reserved for observations that clearly show the theory is wrong. We then subtract the theoretical flexibility score from that for the agreement with observations, since matching the data well is good – but being able to fit anything is bad.</p>
<p>A good theory would make clear predictions which are later confirmed, ideally getting a combined score of +4 in many different tests (+2 -(-2) = +4). A bad theory would get a score between 0 and -4 (-2 -(+2)= -4). Precise predictions would fail in this case – these are unlikely to work with the wrong physics.</p>
<p>We found an average score for the standard cosmological model of –0.25 across 32 tests, while Mond achieved an average of +1.69 across 29 tests. The scores for each theory in many different tests are shown in figures 1 and 2 below for the standard cosmological model and Mond, respectively.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Comparing the standard cosmological model with observations" src="https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=502&fit=crop&dpr=1 600w, https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=502&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=502&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=631&fit=crop&dpr=1 754w, https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=631&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/472829/original/file-20220706-4568-wcqncw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=631&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Comparison of the standard cosmological model with observations based on how well the data matches the theory (improving bottom to top) and how much flexibility it had in the fit (rising left to right). The hollow circle is not counted in our assessment, as that data was used to set free parameters. Reproduced from table 3 of our review.</span>
<span class="attribution"><span class="source">Arxiv</span></span>
</figcaption>
</figure>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Comparing MOND with observations" src="https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=361&fit=crop&dpr=1 600w, https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=361&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=361&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=454&fit=crop&dpr=1 754w, https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=454&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/472828/original/file-20220706-14-a9kfqm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=454&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Similar to Figure 1, but for Mond with hypothetical particles that only interact via gravity called sterile neutrinos. Notice the lack of clear falsifications. Reproduced from Table 4 of our review.</span>
<span class="attribution"><span class="source">Arxiv</span></span>
</figcaption>
</figure>
<p>It is immediately apparent that no major problems were identified for Mond, which at least plausibly agrees with all the data (notice that the bottom two rows denoting falsifications are blank in figure 2).</p>
<h2>The problems with dark matter</h2>
<p>One of the most striking failures of the standard cosmological model relates to “galaxy bars” – rod-shaped bright regions made of stars – that spiral galaxies often have in their central regions (see lead image). The bars rotate over time. If galaxies were embedded in massive halos of dark matter, their bars would slow down. However, most, if not all, observed galaxy bars are fast. This <a href="https://astrobites.org/2022/06/04/the-standard-model-fails-to-pass-the-bar/">falsifies</a> the standard cosmological model with very high confidence.</p>
<p>Another problem is that the <a href="https://ui.adsabs.harvard.edu/abs/1973ApJ...186..467O">original models</a> that suggested galaxies have dark matter halos made a big mistake – they assumed that the dark matter particles provided gravity to the matter around it, but were not affected by the gravitational pull of the normal matter. This simplified the calculations, but it doesn’t reflect reality. When this was taken into account in <a href="https://doi.org/10.1093/mnras/stz1145">subsequent simulations</a> it was clear that dark matter halos around galaxies do not reliably explain their properties.</p>
<p>There are many other failures of the standard cosmological model that we investigated in our review, with Mond often able to <a href="https://doi.org/10.3847/1538-4357/abc623">naturally explain</a> the observations. The reason the standard cosmological model is nevertheless so popular could be down to computational mistakes or limited knowledge about its failures, some of which were discovered quite recently. It could also be due to people’s reluctance to tweak a gravity theory that has been so successful in many other areas of physics.</p>
<p>The huge lead of Mond over the standard cosmological model in our study led us to conclude that Mond is strongly favoured by the available observations. While we do not claim that Mond is perfect, we still think it gets the big picture correct – galaxies really do lack dark matter.</p><img src="https://counter.theconversation.com/content/186344/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Indranil Banik is paid from a grant awarded by the STFC whose primary objective is to test MOND using wide binary stars in the Solar neighbourhood.</span></em></p>Recent results cast doubt on dark matter.Indranil Banik, Postdoctoral Research Fellow of Astrophysics, University of St AndrewsLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1841092022-06-06T12:51:02Z2022-06-06T12:51:02ZDark matter: should we be so sure it exists? Here’s how philosophy can help<figure><img src="https://images.theconversation.com/files/467159/original/file-20220606-18-6986u0.jpg?ixlib=rb-1.1.0&rect=0%2C19%2C1276%2C797&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The galaxy cluster Abell 520, with suspected dark matter highlighted in blue.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>It has been more than 50 years since astronomers first proposed “dark matter”, which is thought to be the most common form of matter in the universe. Despite this, <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">we have no idea</a> what it is – nobody has directly seen it or produced it in the lab. </p>
<p>So how can scientists be so sure it exists? Should they be? It turns out philosophy can help us answer these questions.</p>
<p>Back in the 1970s, a seminal study by astronomers Vera Rubin and Kent Ford of how our neighbour galaxy Andromeda rotates revealed a <a href="https://www.space.com/vera-rubin.html">surprising inconsistency</a> between theory and observation. According to our best gravitational theory for these scales – <a href="https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion/">Newton’s laws</a> – stars and gas in a galaxy should rotate slower and slower the further away they are from the galaxy’s centre. That’s because most of the stars will be near the centre, creating a strong gravitational force there.</p>
<p>Rubin and Ford’s result showed that this wasn’t the case. Stars on the outer edge of the galaxy moved about as fast as the stars around its centre. The idea that the galaxy must be embedded in a large halo of dark matter was basically proposed to explain this anomaly (though others had suggested it previously). This invisible mass interacts with the outer stars through gravity to boost their velocities.</p>
<p>This is only one example of several anomalies in cosmological observations. However, most of these can be equally explained by tweaking the current gravitational laws of Newtonian dynamics and Einstein’s <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">theory of general relativity</a>. Perhaps nature behaves slightly differently on certain scales than these theories predict?</p>
<p>One of the first such theories, developed by Israeli physicist Mordehai Milgrom in 1983, <a href="https://ui.adsabs.harvard.edu/abs/1983ApJ...270..365M/abstract">suggested that</a> Newtonian laws may work slightly differently when there’s extremely small acceleration involved, such as at the edge of galaxies. This tweak was perfectly compatible with the observed galactic rotation. Nevertheless, physicists today overwhelmingly favour the dark matter hypothesis incorporated in the so-called <a href="https://lambda.gsfc.nasa.gov/education/graphic_history/univ_evol.html">ΛCDM model</a> (Lambda Cold Dark Matter). </p>
<p>This view is so strongly entrenched in physics that is widely referred to as the “standard model of cosmology”. However, if the two competing theories of dark matter and modified gravity can equally explain galactic rotation and other anomalies, one might wonder whether we have good reasons to prefer one over another. </p>
<figure class="align-center ">
<img alt="A diagram showing rotation velocities in a spiral galaxy." src="https://images.theconversation.com/files/467158/original/file-20220606-12-9uc1zh.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/467158/original/file-20220606-12-9uc1zh.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/467158/original/file-20220606-12-9uc1zh.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/467158/original/file-20220606-12-9uc1zh.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/467158/original/file-20220606-12-9uc1zh.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/467158/original/file-20220606-12-9uc1zh.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/467158/original/file-20220606-12-9uc1zh.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 graph shows the rotation velocity of stars and gas (vertical axis) as a function of their distance from the centre (horizontal axis). Theory suggests we should get the graph marked ‘expected by the visible disk’, but reality is different.</span>
<span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Why does the scientific community have a strong preference for the dark mater explanation over modified gravity? And how can we ever decide which of the two explanations is the correct one? </p>
<h2>Philosophy to the rescue</h2>
<p>This is an example of what philosophers call “<a href="https://plato.stanford.edu/entries/scientific-underdetermination/">underdetermination of scientific theory</a>” by the available evidence. This describes any situation in which the available evidence may be insufficient to determine what beliefs we should hold in response to it. It is a problem that has puzzled philosophers of science for a long time. </p>
<p>In the case of the strange rotation in galaxies, the data alone cannot determine whether the observed velocities are due to the presence of additional unobservable matter or due to the fact that our current gravitational laws are incorrect.</p>
<p>Scientists therefore look for additional data in other contexts that will eventually settle the question. One such example in favour of dark matter comes from <a href="https://chandra.harvard.edu/graphics/resources/handouts/lithos/bullet_lithos.pdf">the observations</a> of how matter is distributed in the Bullet cluster of galaxies, which is made up of two colliding galaxies about 3.8 billion light years from Earth. Another is <a href="https://physicsworld.com/a/dark-energy-spotted-in-the-cosmic-microwave-background/">based on measurements</a> of how light is deflected by (invisible) matter in the cosmic microwave background, the light left over from the big bang. These are often seen as indisputable evidence in favour of dark matter because due Milgrom’s initial theory can’t explain them. </p>
<p>However, following the publication of these results, further theories of modified gravity <a href="https://theconversation.com/dark-matter-may-not-actually-exist-and-our-alternative-theory-can-be-put-to-the-test-110238">have been developed</a> during the last decades in order to account for all the observational evidence for dark matter, <a href="https://arxiv.org/pdf/2007.00082.pdf">sometimes with great success</a>. Yet, the dark matter hypothesis still remains the favourite explanation of physicists. Why? </p>
<p>One way to understand it is to employ the philosophical tools of <a href="https://citeseerx.ist.psu.edu/viewdoc/downloaddoi=10.1.1.384.3542&rep=rep1&type=pdf">Bayesian confirmation theory</a>. This is a probabilistic framework for estimating the degree to which hypotheses are supported by the available evidence in various contexts. </p>
<p>In the case of two competing hypotheses, what determines the final probability of each hypothesis is the product of the ratio between the initial probabilities of the two hypotheses (before evidence) and the ratio of the probabilities that the evidence appears in each case (called the likelihood ratio). </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/467143/original/file-20220606-22-nzo91r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/467143/original/file-20220606-22-nzo91r.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=82&fit=crop&dpr=1 600w, https://images.theconversation.com/files/467143/original/file-20220606-22-nzo91r.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=82&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/467143/original/file-20220606-22-nzo91r.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=82&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/467143/original/file-20220606-22-nzo91r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=103&fit=crop&dpr=1 754w, https://images.theconversation.com/files/467143/original/file-20220606-22-nzo91r.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=103&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/467143/original/file-20220606-22-nzo91r.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=103&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Dark matter vs modified gravity, where Pr means probability.</span>
<span class="attribution"><span class="source">Author provided</span></span>
</figcaption>
</figure>
<p>If we accept that the most sophisticated versions of modified gravity and dark matter theory are equally supported by the evidence, then the likelihood ratio is equal to one. That means the final decision depends on the initial probabilities of these two hypotheses. </p>
<p>Determining what exactly counts as the “initial probability” of a hypothesis, and the possible ways in which such probabilities can be determined, remains one of the most difficult challenges in Bayesian confirmation theory. And it is here where philosophical analysis turns out to be useful.</p>
<p>At the heart of the <a href="https://research-information.bris.ac.uk/ws/portalfiles/portal/286405282/Full_text_PDF_final_published_version_.pdf">philosophical literature</a> on this topic lies the question of whether the initial probabilities of scientific hypotheses should be objectively determined based solely on probabilistic laws and rational constraints. Alternatively, they could involve a number of additional factors, such as psychological considerations (whether scientists are favouring a particular hypothesis based on interest or for sociological or political reasons), background knowledge, the success of a scientific theory in other domains, and so on.</p>
<p>Identifying these factors will ultimately help us understand the reasons why dark matter theory is overwhelmingly favoured by the physics community.</p>
<p>Philosophy cannot ultimately tell us whether astronomers are right or wrong about the existence of dark matter. But it can tell us whether astronomers do indeed have good reasons to believe in it, what these reasons are, and what it would take for modified gravity to become as popular as dark matter. </p>
<p>We still don’t know the exact answers to these questions, but we are working on it. More research in philosophy of science will give us a better verdict.</p><img src="https://counter.theconversation.com/content/184109/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Antonis Antoniou receives funding from the UK Arts and Humanities Research Council via the South, West and Wales Doctoral Training Partnership. </span></em></p>There’s a perfectly good alternative to dark matter, but most scientists aren’t keen on the idea.Antonis Antoniou, PhD candidate in Philosophy of Science, University of BristolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1749402022-01-24T15:10:28Z2022-01-24T15:10:28ZCurious Kids: will time ever stop?<figure><img src="https://images.theconversation.com/files/441751/original/file-20220120-8326-t062wt.jpg?ixlib=rb-1.1.0&rect=8%2C8%2C5742%2C3819&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/childrens-palms-old-clock-conceptual-photography-1448333879">Olga_Kuzmina/Shutterstock</a></span></figcaption></figure><p><strong>Will time ever stop? – Casandra, aged 11, Epsom, UK</strong></p>
<p>Time began when the universe did. How – and if – the universe ends will determine whether time will end as well. </p>
<p>We think the universe started out squeezed into an infinitely small space. For some reason we do not yet understand, the universe immediately started to expand – to get bigger and bigger. This idea, or “model”, of the beginning of the universe is called the <a href="https://www.esa.int/kids/en/learn/Our_Universe/Story_of_the_Universe/The_Big_Bang">Big Bang</a>.</p>
<p><a href="https://hubblesite.org/hubble-30th-anniversary/hubbles-exciting-universe/discovering-dark-energy">In 1998</a>, scientists learned that the universe is expanding faster and faster, but we still don’t know why this is happening. </p>
<h2>Dark energy</h2>
<p>It might have something to do with the energy of the vacuum of space. It might be a new type of energy field. Or, it might be some completely new form of physics. To symbolise our lack of understanding, we call this new phenomenon “<a href="https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy">dark energy</a>”. </p>
<hr>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p><em><a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a> is a series by <a href="https://theconversation.com/uk">The Conversation</a> that gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskids@theconversation.com">curiouskids@theconversation.com</a> and make sure you include the asker’s first name, age and town or city. We won’t be able to answer every question, but we’ll do our very best.</em></p>
<hr>
<p>Even though we are still trying to work out what dark energy is, we can already use it to predict different ways in which the universe might end. </p>
<p>If dark energy is not too strong, it will take an infinite amount of time for the universe to expand to an infinitely large size. Infinite means never-ending, and so in this case, time will never end. </p>
<p>But, if dark energy is too strong, it will cause the universe to expand so fast that everything in it – even the <a href="https://www.bbc.co.uk/bitesize/topics/zstp34j/articles/zc86m39">tiny atoms</a> that are the building blocks for every single thing in existence – will be ripped apart. In this <a href="https://www.wired.co.uk/article/big-rip-end-of-the-universe">Big Rip</a> scenario, the universe will not expand forever. </p>
<p>Instead, it will expand so fast that it will reach an infinitely large size at a specific moment in time. That moment, when the universe is infinitely large and all matter has been ripped apart, will be the last. The universe will cease to exist, and time will come to an end.</p>
<figure class="align-center ">
<img alt="small light galaxies on black background" src="https://images.theconversation.com/files/441777/original/file-20220120-9349-1w4wfat.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/441777/original/file-20220120-9349-1w4wfat.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/441777/original/file-20220120-9349-1w4wfat.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/441777/original/file-20220120-9349-1w4wfat.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/441777/original/file-20220120-9349-1w4wfat.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/441777/original/file-20220120-9349-1w4wfat.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/441777/original/file-20220120-9349-1w4wfat.jpeg?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">An image of galaxies in the Virgo galaxy cluster taken from the Nasa Galaxy Evolution Explorer space telescope.</span>
<span class="attribution"><a class="source" href="https://images.nasa.gov/details-PIA07906">NASA/JPL-Caltech/SSC</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>There is another way that the universe might end. This is called the <a href="https://hubblesite.org/contents/media/videos/2004/12/421-Video.html?news=true">Big Crunch</a>. In this scenario, the universe will at some point stop expanding and start shrinking again. </p>
<p>The universe will get smaller and smaller, galaxies will collide with each other, and all the matter in the universe will be scrunched up together. When the universe will once again be squeezed into an infinitely small space, time will end.</p>
<h2>The Big Bounce</h2>
<p>Some physicists think that a Big Crunch may not be the end of the universe, but merely the middle of its existence. According to this way of thinking, the universe starts out infinitely large, then shrinks for an infinitely long time until it is squeezed into the smallest size possible. When that happens, instead of ending, there is a Big Bang and the universe begins to expand. </p>
<p>In this <a href="https://www.wired.com/story/what-if-the-big-bang-was-actually-a-big-bounce/">Big Bounce</a> scenario, there is an infinite amount of time before the universe becomes scrunched up into the smallest possible space, and an infinite amount of time as it expands afterwards. Time has no beginning and no end. </p>
<p>In some Big Bounce models, the universe only bounces once. In others it goes through an infinite number of bounces, constantly expanding and contracting, like an accordion that never stops playing.</p>
<p>All of these scenarios show us what is possible, not necessarily what is true. For one thing, we still need to figure out what dark energy is. More importantly, there is no guarantee that our current understanding of how the universe works is correct. </p>
<p>One day, maybe 100 years or just a few weeks from now, someone (perhaps you?) will come up with a better theory to describe the workings of the universe. Maybe then we will know whether time ever comes to and end. Then again, perhaps the new theory will have a wildly different concept of time, or even do away with it altogether.</p><img src="https://counter.theconversation.com/content/174940/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Or Graur receives funding from the UKRI Science and Technology Facilities Council. </span></em></p>Time ends when the universe does.Or Graur, Senior Lecturer in Astrophysics, University of PortsmouthLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1728162021-12-02T23:16:47Z2021-12-02T23:16:47ZOur understanding of black holes has changed over time<figure><img src="https://images.theconversation.com/files/435167/original/file-20211201-23-jknx6e.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C3600%2C3600&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">New technologies have enabled us to learn more about black holes.</span> <span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/images/ngc-3627-revealing-hidden-black-holes">(NASA)</a></span></figcaption></figure><p>It took <a href="https://www.pbs.org/wgbh/aso/databank/entries/dp15ei.html">Albert Einstein 10 years to find the equations of general relativity</a>, but <a href="https://arxiv.org/abs/physics/9912033">German astrophysicist Karl Schwarzschild only needed a few months to solve them</a>. Schwarzschild’s solution describes the gravity of an isolated, spherical and unchanging object — the enigmatic black hole — but it was not understood for many years. </p>
<p>Black holes helped to explain new astronomical discoveries, becoming essential ingredients of astrophysics. Science regarded <a href="https://science.nasa.gov/astrophysics/focus-areas/black-holes/">black holes as abstractions until the 1960s</a>. The recent experimental discovery of gravitational waves has changed our understanding of what black holes are.</p>
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<em>
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Read more:
<a href="https://theconversation.com/a-brief-history-of-black-holes-107298">A brief history of black holes</a>
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</em>
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<p>In 2016, the LIGO-Virgo collaboration detected <a href="https://theconversation.com/gravitational-waves-discovered-the-universe-has-spoken-54237">gravitational waves generated by two merging black holes</a>, opening a new era of astronomy celebrated by the 2017 Nobel Prize in physics. </p>
<p>In 2019, the <a href="https://theconversation.com/first-black-hole-photo-confirms-einsteins-theory-of-relativity-115167">Event Horizon Telescope</a> released an image of the supermassive black hole in the nearby galaxy M87. The following year, the Nobel Prize in physics recognized <a href="https://www.nobelprize.org/prizes/physics/2020/press-release/">the trailblazing theoretical black hole studies of Roger Penrose and the observational ones by Andrea Ghez and Reinhard Genzel</a>. </p>
<h2>What is a black hole?</h2>
<p>The notion of black hole reflected in popular science hinges on the idea of event horizon — this is when the velocity needed to escape the gravitational pull of the black hole exceeds the speed of light. Whatever falls into the event horizon is lost forever.</p>
<p>The Schwarzschild radius is the radius of the event horizon, <a href="https://astronomy.swin.edu.au/cosmos/S/Schwarzschild+Radius">and is proportional to the mass of the black hole</a>. But Schwarzschild’s definition has a pitfall: it requires us to know that nothing will emerge from the black hole. This means that the black hole must be monitored forever to know that nothing exits. In practice, this is impossible.</p>
<p>Another mathematical solution to Einstein’s equations <a href="https://doi.org/10.1139/p05-063">describes the formation of a black hole through the collapse of a spherical shell of light</a>. An event horizon forms at its centre, expands outwards, <a href="https://doi.org/10.1103/PhysRevD.52.7053">meets the infalling shell of light at the Schwarzschild radius where it stops</a> — <em>et voilà!</em> — a black hole is formed. </p>
<h2>New black holes</h2>
<p>Perfectly isolated or unchanging black holes do not exist. Real-world black holes are surrounded by disks orbiting them, stellar winds and dark matter, all of which produce infalling matter that increases their masses.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/cutting-through-the-spin-on-supermassive-black-holes-12528">Cutting through the spin on supermassive black holes </a>
</strong>
</em>
</p>
<hr>
<p>Black holes often exist in pairs, spiralling closer and closer to each other and emitting gravitational waves until they merge into a larger black hole, the horizon changing in time, dramatically so at the merger. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two black holes surrounded by yellow and orange flames spiralling into each other." src="https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/435166/original/file-20211201-15-1deuifg.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">An artist’s rendition of two black holes spiralling into each other.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/images/two-black-holes-on-way-to-becoming-one-artists-concept">(NASA)</a></span>
</figcaption>
</figure>
<p>The <a href="https://doi.org/10.1103/PhysRevX.6.041015">2016 LIGO/Virgo gravitational waves</a> originated in the spectacular merger of two black holes. By the time these waves reached Earth, they were weaker than the ambient noise and could only be identified by matching theoretical templates of the expected signal to the data. </p>
<p>Large banks of templates are generated in computer simulations that obviously cannot run forever, as would be necessary if the black hole was characterized by the eternal event horizon. Instead, simulations use the <a href="https://theconversation.com/grey-is-the-new-black-hole-is-stephen-hawking-right-22481">apparent horizon</a>, characterized by the property that nothing can escape from it <em>now</em>.</p>
<p>Apparent horizons have played a crucial role in the newly born <a href="https://doi.org/10.1103/PhysRevLett.116.241103">gravitational wave astronomy</a>, but are surprisingly little known. </p>
<p>Black holes change because they live in an <a href="https://theconversation.com/explainer-the-mysterious-dark-energy-that-speeds-the-universes-rate-of-expansion-40224">expanding universe</a>. Theoretical physicist Stephen Hawking predicted that all black holes <a href="https://www.vox.com/science-and-health/2018/3/14/17119320/stephen-hawking-hawking-radiation-explained">radiate energy due to quantum mechanics</a>, which makes them shrink. Although negligible for practical purposes, this radiation is unavoidable in principle.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/experiments-simultaneously-detect-gravitational-waves-and-help-open-up-a-new-era-of-astronomy-84818">Experiments simultaneously detect gravitational waves – and help open up a new era of astronomy</a>
</strong>
</em>
</p>
<hr>
<h2>New understandings</h2>
<p>Our understanding of black holes is based on the mathematical definition of horizon. The apparent horizon <a href="https://arxiv.org/abs/gr-qc/0508107v2">is defined by the behaviour of light rays in its vicinity</a>: rays cannot escape (and since nothing moves faster than light, nothing escapes) <em>at the present moment</em>.</p>
<p>But how light rays behave depends on the observer describing them using mathematical simulations. Since, in relativity, time and space depend on the observer, the location where rays stop and the present time are different for different observers. So, the apparent horizon itself depends on the observer. </p>
<p>The very existence of the black hole has come to depend on the observer, while the old event horizon was universal. </p>
<p>The mathematics expressing the new black hole concept tells us that, even in Schwarzschild’s case, <a href="https://arxiv.org/abs/gr-qc/0511017v3">certain observers exist</a> according to whom there is no apparent horizon and, therefore, no black hole! Admittedly, these mathematical observers are very artificial. All natural observations (those that occur through observing the actual behaviour of a black hole) that perceive the Schwarzschild geometry as spherical, agree on <a href="https://arxiv.org/abs/1610.05822v1">the existence and location of the apparent horizon</a>.</p>
<p>Scientists have finally detected gravitational waves from black holes but had to change the way they understand them. The essence of black hole theory is now different.</p><img src="https://counter.theconversation.com/content/172816/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Valerio Faraoni receives funding from the Natural Sciences & Engineering Research Council of Canada.</span></em></p>Advanced technologies and the information they collect have revealed how black holes form and behave.Valerio Faraoni, Professor, Physics & Astronomy, Bishop's UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1696032021-10-12T19:31:56Z2021-10-12T19:31:56ZThe most powerful space telescope ever built will look back in time to the Dark Ages of the universe<figure><img src="https://images.theconversation.com/files/425795/original/file-20211011-27-1pyo32m.jpeg?ixlib=rb-1.1.0&rect=35%2C72%2C951%2C833&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Hubble took pictures of the oldest galaxies it could – seen here – but the James Webb Space Telescope can go back much farther in time.</span> <span class="attribution"><a class="source" href="https://svs.gsfc.nasa.gov/vis/a030000/a030900/a030946/hudf-hst-6200x6200_print.jpg">NASA</a></span></figcaption></figure><p>Some have called NASA’s James Webb Space Telescope the “<a href="https://doi.org/10.1038/4671028a">telescope that ate astronomy</a>.” It is the <a href="https://www.jwst.nasa.gov/">most powerful space telescope</a> ever built and a complex piece of mechanical origami that has pushed the limits of human engineering. On Dec. 25, 2021, after years of delays and billions of dollars in cost overruns, the telescope is <a href="https://theconversation.com/james-webb-space-telescope-an-astronomer-on-the-team-explains-how-to-send-a-giant-telescope-to-space-and-why-167516">launched into space</a> to usher in the next era of astronomy.</p>
<p>I’m an <a href="https://scholar.google.com/citations?user=OrRLRQ4AAAAJ&hl=en">astronomer</a> with a specialty in observational cosmology – I’ve been studying distant galaxies for 30 years. Some of the biggest unanswered questions about the universe relate to its early years just after the Big Bang. When did the first stars and galaxies form? Which came first, and why? I am incredibly excited that astronomers may soon uncover the story of how galaxies started because James Webb was built specifically to answer these very questions. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A graphic showing the progression of the Universe through time." src="https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/425594/original/file-20211010-23-1ff1ae6.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 Universe went through a period of time known as the Dark Ages before stars or galaxies emitted any light.</span>
<span class="attribution"><a class="source" href="https://webbtelescope.org/contents/media/images/4352-Image">Space Telescope Institute</a></span>
</figcaption>
</figure>
<h2>The ‘Dark Ages’ of the universe</h2>
<p>Excellent evidence shows that the universe started with an event called the <a href="https://www.space.com/40370-why-should-we-believe-big-bang.html">Big Bang</a> 13.8 billion years ago, which left it in an ultra-hot, ultra-dense state. The universe immediately began expanding after the Big Bang, cooling as it did so. One second after the Big Bang, the universe was a hundred trillion miles across with an average temperature of an incredible 18 billion F (10 billion C). Around 400,000 years after the Big Bang, the universe was 10 million light years across and the <a href="http://www.astro.ucla.edu/%7Ewright/BBhistory.html">temperature had cooled</a> to 5,500 F (3,000 C). If anyone had been there to see it at this point, the universe would have been glowing dull red like a giant heat lamp.</p>
<p>Throughout this time, space was filled with a smooth soup of high energy particles, radiation, hydrogen and helium. There was no structure. As the expanding universe became bigger and colder, the soup thinned out and everything faded to black. This was the start of what astronomers call the <a href="https://astronomy.com/magazine/news/2021/01/the-beginning-to-the-end-of-the-universe-the-cosmic-dark-ages">Dark Ages</a> of the universe.</p>
<p>The soup of the Dark Ages was <a href="https://wmap.gsfc.nasa.gov/universe/bb_cosmo_fluct.html">not perfectly uniform</a> and due to gravity, tiny areas of gas began to clump together and become more dense. The smooth universe became lumpy and these small clumps of denser gas were seeds for the eventual formation of stars, galaxies and everything else in the universe. </p>
<p>Although there was nothing to see, the Dark Ages were an important phase in the evolution of the universe.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing different wavelengths of light compared to size of normal objects." src="https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=236&fit=crop&dpr=1 600w, https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=236&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=236&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=296&fit=crop&dpr=1 754w, https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=296&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/425792/original/file-20211011-17-126iwpp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=296&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Light from the early universe is in the infrared wavelength – meaning longer than red light – when it reaches Earth.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:EM_Spectrum_Properties_edit.svg#/media/File:EM_Spectrum_Properties_edit.svg">Inductiveload/NASA via Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Looking for the first light</h2>
<p>The Dark Ages ended when gravity formed the first stars and galaxies that eventually began to emit the first light. Although astronomers don’t know when first light happened, the best guess is that it was <a href="https://earthsky.org/space/cosmic-dark-ages-lyman-alpha-galaxies-lager/">several hundred million years</a> after the Big Bang. Astronomers also don’t know whether stars or galaxies formed first. </p>
<p><a href="https://astronomy.com/magazine/greatest-mysteries/2019/07/20-did-stars-galaxies-or-black-holes-come-first">Current theories</a> based on how gravity forms structure in a universe dominated by dark matter suggest that small objects – like stars and star clusters – likely formed first and then later grew into dwarf galaxies and then larger galaxies like the Milky Way. These first stars in the universe were extreme objects compared to stars of today. They were <a href="https://webbtelescope.org/resource-gallery/articles/pagecontent/filter-articles/what-were-the-first-stars-like?filterUUID=a776e097-0c60-421c-baec-1d8ad049bfb0">a million times brighter</a> but they lived very short lives. They burned hot and bright and when they died, they left behind <a href="https://astronomy.com/magazine/greatest-mysteries/2019/07/20-did-stars-galaxies-or-black-holes-come-first">black holes</a> up to a hundred times the Sun’s mass, which might have <a href="https://astronomynow.com/2020/03/24/how-to-seed-supermassive-black-holes-in-the-early-universe/">acted as the seeds for galaxy formation</a>. </p>
<p>Astronomers would love to study this fascinating and important era of the universe, but detecting first light is incredibly challenging. Compared to massive, bright galaxies of today, the first objects were very small and due to the constant expansion of the universe, they’re now tens of billions of light years away from Earth. Also, the earliest stars were surrounded by gas left over from their formation and this gas acted like fog that absorbed most of the light. It took several hundred million years for <a href="https://www.quantamagazine.org/how-the-cosmic-dark-ages-snuffed-out-all-light-20200302/">radiation to blast away the fog</a>. This early light is very faint by the time it gets to Earth. </p>
<p>But this is not the only challenge.</p>
<p>As the universe expands, it continuously stretches the wavelength of light traveling through it. This is called <a href="https://www.esa.int/Science_Exploration/Space_Science/What_is_red_shift#:%7E:text=Ever%20since%201929%2C%20when%20Edwin,is%20'red%2Dshifted">redshift</a> because it shifts light of shorter wavelengths – like blue or white light – to longer wavelengths like red or infrared light. Though not a perfect analogy, it is similar to how when a car drives past you, the pitch of any sounds it is making drops noticeably. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/8WgSQlRymwE?wmode=transparent&start=35" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Similar to how a pitch of a sound drops if the source is moving away from you, the wavelength of light stretches due to the expansion of the universe.</span></figcaption>
</figure>
<p>By the time light emitted by an early star or galaxy 13 billion years ago reaches any telescope on Earth, it has been stretched by a factor of 10 by the expansion of the universe. It arrives as infrared light, meaning it has a wavelength longer than that of red light. To see first light, you have to be looking for infrared light.</p>
<p>[<em>The Conversation’s science, health and technology editors pick their favorite stories.</em> <a href="https://theconversation.com/us/newsletters/science-editors-picks-71/?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=science-favorite">Weekly on Wednesdays</a>.]</p>
<h2>Telescope as a time machine</h2>
<p>Enter the James Webb Space Telescope. </p>
<p>Telescopes are like time machines. If an object is 10,000 light-years away, that means the light takes 10,000 years to reach Earth. So the further out in space astronomers look, the <a href="https://astronomy.swin.edu.au/cosmos/l/lookback+time">further back in time we are looking</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A large golden colored disc with a sensor in the middle and scientists standing below." src="https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=493&fit=crop&dpr=1 754w, https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=493&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/425798/original/file-20211011-25-fi5m9g.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=493&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 James Webb Space Telescope was specifically designed to detect the oldest galaxies in the universe.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/mission_pages/webb/images/index.html">NASA/JPL-Caltech</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Engineers <a href="https://www.jwst.nasa.gov/content/science/firstLight.html">optimized James Webb</a> for specifically detecting the faint infrared light of the earliest stars or galaxies. Compared to the Hubble Space Telescope, <a href="https://www.jwst.nasa.gov/content/about/comparisonWebbVsHubble.html">James Webb has a 15 times wider field of view on its camera</a>, collects six times more light and its sensors are tuned to be most sensitive to infrared light.</p>
<p>The strategy will be to <a href="https://www.nasa.gov/feature/goddard/2021/mapping-the-universes-earliest-structures-with-cosmos-webb">stare deeply at one patch of sky for a long time</a>, collecting as much light and information from the most distant and oldest galaxies as possible. With this data, it may be possible to answer when and how the Dark Ages ended, but there are many other important discoveries to be made. For example, unraveling this story may also <a href="https://doi.org/10.1093/mnras/stz1924">help explain the nature of dark matter</a>, the mysterious form of matter that makes up about <a href="https://www.space.com/20930-dark-matter.html">80% of the mass of the universe</a>. </p>
<p>James Webb is the <a href="https://futurism.com/james-webb-telescope-budget-delay">most technically difficult mission</a> NASA has ever attempted. But I think the scientific questions it may help answer will be worth every ounce of effort. I and other astronomers are waiting excitedly for the data to start coming back sometime in 2022.</p>
<p><em>This article was updated with the launch.</em></p><img src="https://counter.theconversation.com/content/169603/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Impey receives funding from the National Science Foundation and the Hearst Foundation.</span></em></p>The James Webb Space Telescope is set to launch into orbit in December 2021. Its mission is to search for the first light to ever shine in the universe.Chris Impey, University Distinguished Professor of Astronomy, University of ArizonaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1623332021-06-21T12:20:40Z2021-06-21T12:20:40ZDoes outer space end – or go on forever?<figure><img src="https://images.theconversation.com/files/405990/original/file-20210611-13-pcdwbd.jpg?ixlib=rb-1.1.0&rect=321%2C214%2C5770%2C4271&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">It can stretch your mind to ponder what's really out there.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/silhouette-man-sitting-on-rock-against-royalty-free-image/615314285">Stijn Dijkstra/EyeEm via Getty Images</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&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"></span>
</figcaption>
</figure>
<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
<hr>
<blockquote>
<p><strong>What is beyond outer space? – Siah, age 11, Fremont, California</strong></p>
</blockquote>
<hr>
<p>Right above you is the sky – or as scientists would call it, the atmosphere. It extends about <a href="https://www.nationalgeographic.org/encyclopedia/atmosphere/">20 miles (32 kilometers) above the Earth</a>. Floating around the atmosphere is a <a href="https://kids.britannica.com/kids/article/molecule/353479">mixture of molecules</a> – tiny bits of air so small you take in billions of them every time you breathe.</p>
<p>Above the atmosphere is space. It’s called that because it has far fewer molecules, with lots of empty space between them. </p>
<p>Have you ever wondered what it would be like to travel to outer space – and then keep going? What would you find? <a href="https://ui.adsabs.harvard.edu/search/q=%20author%3A%22singal%2C%20jack%22&sort=date%20desc%2C%20bibcode%20desc&p_=0">Scientists like me</a> are able to explain a lot of what you’d see. But there are some things we don’t know yet, like whether space just goes on forever. </p>
<h2>Planets, stars and galaxies</h2>
<p>At the beginning of your trip through space, you might recognize some of the sights. The <a href="https://www.solarsystemscope.com/">Earth is part of a group of planets</a> that all orbit the Sun – with some orbiting asteroids and comets mixed in, too.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram of the solar system, showing the sun and its orbiting planets." src="https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/405719/original/file-20210610-15-1eygla0.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>
<figcaption>
<span class="caption">A familiar neighborhood.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/solar-system-artwork-royalty-free-image/529831132">Mark Garlick/Science Photo Library via Getty Images</a></span>
</figcaption>
</figure>
<p>You might know that the Sun is actually just an average star, and looks bigger and brighter than the other stars <a href="https://apod.nasa.gov/apod/ap011018.html">only because it is closer</a>. To get to the next nearest star, you would have to travel through trillions of miles of space. If you could ride on the fastest space probe NASA has ever made, it would still take you thousands of years to get there. </p>
<p>If stars are like houses, then galaxies are like cities full of houses. Scientists estimate there are <a href="https://en.wikipedia.org/wiki/Milky_Way">100 billion stars in Earth’s galaxy</a>. If you could zoom out, way beyond Earth’s galaxy, those 100 billion stars would blend together – the way lights of city buildings do when viewed from an airplane. </p>
<p>Recently astronomers have learned that <a href="https://exoplanets.nasa.gov/">many or even most stars have their own orbiting planets</a>. Some are even like Earth, so it’s possible they might be home to other beings also wondering what’s out there. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An image showing detail of one galaxy, but visually implying there are many more." src="https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=579&fit=crop&dpr=1 600w, https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=579&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=579&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=727&fit=crop&dpr=1 754w, https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=727&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/405722/original/file-20210610-10377-8e930k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=727&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 galaxy among many other galaxies.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/messier-106-a-spiral-galaxy-in-the-constellation-royalty-free-image/495835787">Michael Miller/Stocktrek Images via Getty Images</a></span>
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</figure>
<p>You would have to travel through <a href="https://imagine.gsfc.nasa.gov/features/cosmic/local_group_info.html">millions of trillions more miles of space just to reach another galaxy</a>. Most of that space is almost completely empty, with only some stray molecules and tiny mysterious <a href="https://home.cern/science/physics/dark-matter">invisible particles scientists call “dark matter</a>.”</p>
<p>Using big telescopes, <a href="https://hubblesite.org/contents/articles/hubble-deep-fields">astronomers see millions of galaxies</a> out there – and they just keep going, in every direction. </p>
<p>If you could watch for long enough, over millions of years, it would look like new <a href="https://astronomy.swin.edu.au/cosmos/H/Hubble+Flow">space is gradually being added between all the galaxies</a>. You can visualize this by imagining tiny dots on a deflated balloon and then thinking about blowing it up. The dots would keep moving farther apart, just like the galaxies are.</p>
<h2>Is there an end?</h2>
<p>If you could keep going out, as far as you wanted, would you just keep passing by galaxies forever? Are there an infinite number of galaxies in every direction? Or does the whole thing eventually end? And if it does end, what does it end with? </p>
<p>These are questions scientists don’t have definite answers to yet. Many think it’s likely you would just <a href="https://doi.org/10.1103/PhysRevD.64.043511">keep passing galaxies in every direction, forever</a>. In that case, the universe would be infinite, with no end.</p>
<p>Some scientists think it’s possible the <a href="https://www.quantamagazine.org/what-shape-is-the-universe-closed-or-flat-20191104/">universe might eventually wrap back around on itself</a> – so if you could just keep going out, you would someday come back around to where you started, from the other direction.</p>
<p>One way to think about this is to picture a globe, and imagine that you are a creature that can move only on the surface. If you start walking any direction, east for example, and just keep going, eventually you would come back to where you began. If this were the case for the universe, it would mean it is not infinitely big – although it would still be bigger than you can imagine.</p>
<p>In either case, you could never get to the end of the universe or space. Scientists now consider it unlikely the universe has an end – a region where the galaxies stop or where there would be a barrier of some kind marking the end of space. </p>
<p>But nobody knows for sure. How to answer this question will need to be figured out by a future scientist.</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
<p><em>And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.</em></p>
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<p><em>This article has been updated to correct the distances to the nearest star and galaxy.</em></p><img src="https://counter.theconversation.com/content/162333/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jack Singal 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>Astronomers know a lot about what’s in outer space – and think it’s possible it never ends.Jack Singal, Associate Professor of Physics, University of RichmondLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1559272021-02-25T12:04:31Z2021-02-25T12:04:31ZLeaving Hong Kong after China’s clampdown: where are people thinking of going and why? – The Conversation Weekly podcast<p>In this week’s episode of <a href="https://theconversation.com/uk/topics/the-conversation-weekly-98901">The Conversation Weekly</a> podcast, three experts explain why more people are thinking of leaving Hong Kong after China’s clampdown on dissent – and the choices they face about where to go. And we hear about new research that found a new way to speed up the search for one of the universe’s most elusive enigmas: dark matter. </p>
<iframe src="https://embed.acast.com/60087127b9687759d637bade/6036629da9f63074a33de340?cover=true&ga=false" frameborder="0" allow="autoplay" width="100%" height="110"></iframe>
<p><iframe id="tc-infographic-561" class="tc-infographic" height="100" src="https://cdn.theconversation.com/infographics/561/4fbbd099d631750693d02bac632430b71b37cd5f/site/index.html" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>Since China imposed a new National Security Law on Hong Kong in mid-2020, the situation for political protesters has become much more dangerous. Many of those involved in recent pro-democracy protests are being <a href="https://apnews.com/article/legislature-primary-elections-democracy-hong-kong-elections-25a66f7dd38e6606c9f8cce84106d916">rounded up and arrested</a>. </p>
<p>Many Hong Kongers are now thinking about leaving – and in this episode we hear from experts researching what is influencing these decisions. Sui-Ting Kong, assistant professor in sociology at Durham University, who has been interviewing Hong Kongers about the way political participation affects their everyday lives, sets out three different ways people have described the decision to migrate. For one group – those who say they are “fleeing from disaster” – she says: “They find it really difficult to reconcile the idea of moving away from Hong Kong while they have invested so much energy, sacrificed so much in the movement, to make Hong Kong a better place for themselves.”</p>
<p>At the end of January, the UK government opened up a new visa route for those who qualify for British National Overseas (BNO) status. We hear from Peter Walsh, a researcher at the Migration Observatory at the University of Oxford, who explains the history of BNO status, how the new visa route will work and why the UK government has no clear idea about how many people will apply. </p>
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Read more:
<a href="https://theconversation.com/hong-kong-china-crackdown-is-likely-to-boost-migration-to-uk-152766">Hong Kong: China crackdown is likely to boost migration to UK</a>
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<p>But it’s Taiwan, not the UK, which is the most <a href="https://foreignpolicy.com/2020/07/08/hong-kong-exile-taiwan-first-choice/">attractive migration destination</a> for Hong Kongers. Tsungyi Michelle Huang, professor of geography at National Taiwan University, tells us about her research on how Hong Kongers’ attitudes towards Taiwan have shifted in recent years. But she says that there is some suspicion emerging in Taiwan about a recent <a href="https://www.bloomberg.com/news/articles/2021-02-03/hong-kongers-move-to-taiwan-in-record-numbers-amid-turmoil">increase in migration</a> from Hong Kong. “Taiwanese are worried whether many of the Hong Kongers who have immigrated to Taiwan are actually mainlanders,” she tells us, “because of this distrust of the Chinese.”</p>
<p>In our second story, we’re joined by Benjamin Brubaker, a physicist at the University of Colorado, Boulder, who is on the hunt for dark matter. Dark matter is invisible, but it accounts for 85% of the matter in the universe. You can’t detect it and it’s matter, so, “dark matter”. Based on a bunch of clues – from how galaxies move to how light bends in space – we know dark matter has to exist, but no one has ever figured out what it actually is. </p>
<p>Physicists around the world are running special detectors to try and find evidence of a dark matter particle. But this is an incredibly slow process. Brubaker has worked on the Haystac detector that looks for one of the potential dark matter particles called an axion. He <a href="https://theconversation.com/the-search-for-dark-matter-gets-a-speed-boost-from-quantum-technology-153604">just published a paper</a> explaining how he and his colleagues used technology from the quantum computing world to speed up the search for dark matter. </p>
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<p>
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<strong>
Read more:
<a href="https://theconversation.com/the-search-for-dark-matter-gets-a-speed-boost-from-quantum-technology-153604">The search for dark matter gets a speed boost from quantum technology</a>
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<p>And to end this week’s episode, Luthfi Dzulfikar, associate editor at The Conversation in Jakarta, recommends a couple of stories by academics in Indonesia. </p>
<p>The Conversation Weekly is produced by Mend Mariwany and Gemma Ware, with sound design by Eloise Stevens. Our theme music is by Neeta Sarl.</p>
<p>News clips in this episode from <a href="https://www.youtube.com/watch?v=dvDMjlguBDk">CBC News</a>, <a href="https://www.youtube.com/watch?v=FBXgKblIFRg">France 24</a>, <a href="https://www.youtube.com/watch?v=pZtfpxXMTwk">BBC</a> <a href="https://www.youtube.com/watch?v=pZtfpxXMTwk">News</a>, <a href="https://www.youtube.com/watch?v=gHfWuUhrKQg&has_verified=1">NYT</a>, <a href="https://www.youtube.com/watch?v=GBeD3ntGOlw">Global News</a>, <a href="https://www.youtube.com/watch?v=pWm_3_saGvI">ViuTV News</a>, <a href="https://www.youtube.com/watch?v=zQG3cheC1gE">AJE</a>, <a href="https://www.youtube.com/watch?v=79mcI7TGjgY">CNA</a>, <a href="https://www.youtube.com/watch?v=rMaAfiye-X0">NBC News</a> and <a href="https://www.youtube.com/watch?v=VF0I_3howHk">CNN</a>. </p>
<p>A transcript of this episode is <a href="https://theconversation.com/hong-kong-political-turmoil-provokes-difficult-decisions-about-whether-to-leave-155994">available here</a>. </p>
<p><em>You can listen to The Conversation Weekly via any of the apps listed above, our <a href="https://feeds.acast.com/public/shows/60087127b9687759d637bade">RSS feed</a>, or find out how else to <a href="https://theconversation.com/how-to-listen-to-the-conversations-podcasts-154131">listen here</a>.</em></p><img src="https://counter.theconversation.com/content/155927/count.gif" alt="The Conversation" width="1" height="1" />
Plus new research finds a way to speed up the search for dark matter. Listen to episode 4 of The Conversation Weekly.Gemma Ware, Head of AudioDaniel Merino, Associate Breaking News Editor and Co-Host of The Conversation Weekly PodcastLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1543812021-02-11T04:35:53Z2021-02-11T04:35:53ZA brief history: what we know so far about fast radio bursts across the universe<figure><img src="https://images.theconversation.com/files/383653/original/file-20210211-14-1qn1hrd.jpg?ixlib=rb-1.1.0&rect=0%2C25%2C2480%2C1770&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.scienceimage.csiro.au/image/249/parkes-radio-telescope/">CSIRO/John Masterson</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p><a href="https://theconversation.com/au/topics/fast-radio-bursts-6352">Fast radio bursts</a> are one of the great mysteries of the universe. Since their discovery, we have learned a great deal about these intense millisecond-duration pulses.</p>
<p>But we still have much to learn, such as what causes them. </p>
<p>We know the intense bursts originate in galaxies billions of light years away. We have also used these bursts (called <a href="https://astronomy.swin.edu.au/cosmos/F/Fast+Radio+Bursts">FRB</a>s) to <a href="https://theconversation.com/half-the-matter-in-the-universe-was-missing-we-found-it-hiding-in-the-cosmos-138569">find missing matter</a> that couldn’t be found otherwise.</p>
<p>With teams of astronomers around the world racing to understand their enigma, how did we get to where we are now? </p>
<h2>The first burst</h2>
<p>The first FRB was discovered in 2007 by a team led by British-American astronomer <a href="https://physics.wvu.edu/faculty-and-staff/faculty/duncan-lorimer">Duncan Lorimer</a> using <a href="https://blog.csiro.au/parkes-telescope-indigenous-name/">Murriyang</a>, the traditional Indigenous name for the iconic Parkes radio telescope (image, top).</p>
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Read more:
<a href="https://theconversation.com/silence-please-why-radio-astronomers-need-things-quiet-in-the-middle-of-a-wa-desert-118922">Silence please! Why radio astronomers need things quiet in the middle of a WA desert</a>
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<p>The team found an incredibly bright pulse — so bright that many astronomers did not believe it to be real. But there was yet more intrigue. </p>
<p>Radio pulses provide a tremendous gift to astronomers. By measuring when a burst arrives at the telescope at different frequencies, astronomers can tell the total amount of gas that it passed through on its journey to Earth.</p>
<figure class="align-center ">
<img alt="A curved graph, starting high top left and curving down low to bottom right." src="https://images.theconversation.com/files/381639/original/file-20210201-19-15wjt3o.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/381639/original/file-20210201-19-15wjt3o.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=366&fit=crop&dpr=1 600w, https://images.theconversation.com/files/381639/original/file-20210201-19-15wjt3o.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=366&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/381639/original/file-20210201-19-15wjt3o.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=366&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/381639/original/file-20210201-19-15wjt3o.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=459&fit=crop&dpr=1 754w, https://images.theconversation.com/files/381639/original/file-20210201-19-15wjt3o.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=459&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/381639/original/file-20210201-19-15wjt3o.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=459&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A typical Fast Radio Burst. The burst arrives first at high frequencies and is delayed by as much as several seconds at the lower frequencies. This tell-tale curve is what astronomers are looking for.</span>
<span class="attribution"><span class="source">Ryan Shannon and Vikram Ravi</span></span>
</figcaption>
</figure>
<p>The Lorimer burst had travelled through far too much gas to have originated in our galaxy, the Milky Way. The team concluded it came from a galaxy billions of light years away.</p>
<p>To be visible from so far away, whatever produced it must have released an enormous amount of energy. In just a millisecond it released as much energy as our Sun would in 80 years.</p>
<p>Lorimer’s team could only guess which galaxy their FRB had come from. Murriyang can’t pinpoint FRB locations very accurately. It would take several years for another team to make the breakthrough.</p>
<h2>Locating FRBs</h2>
<p>To pinpoint a burst location, we need to detect an FRB with a radio interferometer — an array of antennas spread out over at least a few kilometres.</p>
<p>When signals from the telescopes are combined, they produce an image of an FRB with enough detail not only to see in which galaxy the burst originated, but in some cases to tell where within the galaxy it was produced. </p>
<p>The first FRB localised was from a source that emitted many bursts. The first burst was discovered in 2012 with the giant <a href="http://www.naic.edu/">Arecibo telescope</a> in Puerto Rico.</p>
<p>Subsequent bursts were detected by the <a href="https://public.nrao.edu/telescopes/vla/">Very Large Array</a>, in New Mexico, and found to be coming from a tiny galaxy about 3 billion light years away.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Several dish-shipped antenna in the desert, all pointing up towards the sky in daylight." src="https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/386907/original/file-20210301-23-1k5kkjz.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">Several of the ASKAP radio telescope antennas in WA.</span>
<span class="attribution"><span class="source">CSIRO</span>, <span class="license">Author provided</span></span>
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</figure>
<p>In 2018, using the Australian Square Kilometre Array Pathfinder Telescope (<a href="https://www.csiro.au/en/Research/Facilities/ATNF/ASKAP">ASKAP</a>) in Western Australia, <a href="https://theconversation.com/how-we-closed-in-on-the-location-of-a-fast-radio-burst-in-a-galaxy-far-far-away-119177">our team identified the second FRB host galaxy</a>.</p>
<p>In stark contrast to the previous galaxy, this galaxy was very ordinary. But our <a href="https://science.sciencemag.org/content/365/6453/565" title="A single fast radio burst localized to a massive galaxy at cosmological distance">published discovery</a> was this month <a href="https://www.aaas.org/news/astronomical-discovery-earns-2020-aaas-newcomb-cleveland-prize">awarded a prize by the American Association for the Advancement of Science</a>. </p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1359616899945013256"}"></div></p>
<p>Teams including ours have now localised roughly a dozen more bursts from a wide range of galaxies, large and small, young and old. The fact FRBs can come from such a wide range of galaxies remains a puzzle. </p>
<h2>A burst from close to home</h2>
<p>On April 28, 2020, a flurry of X-rays suddenly bashed into the <a href="https://swift.gsfc.nasa.gov/">Swift</a> telescope orbiting Earth.</p>
<p>The satellite telescope dutifully noted the rays had come from a very magnetic and erratic neutron star in our own Milky Way. This star has form: it goes into fits every few years.</p>
<p>Two telescopes, <a href="https://chime-experiment.ca/en">CHIME</a> in Canada and the STARE2 array in the United States, detected a very bright radio burst within milliseconds of the X-rays and in the direction of that star. This demonstrated such neutron stars could be a source of the FRBs we see in galaxies far away.</p>
<p>The simultaneous release of X-rays and radio waves gave astrophysicists important clues to how nature can produce such bright bursts. But we still don’t know for certain if this is the cause of FRBs.</p>
<h2>So what’s next?</h2>
<p>While 2020 was the year of the local FRB, we expect 2021 will be the year of the the far-flung FRB, even further than already observed.</p>
<p>The CHIME telescope has collected by far the largest sample of bursts and is compiling a meticulous catalogue that should be available to other astronomers soon.</p>
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Read more:
<a href="https://theconversation.com/how-we-closed-in-on-the-location-of-a-fast-radio-burst-in-a-galaxy-far-far-away-119177">How we closed in on the location of a fast radio burst in a galaxy far, far away</a>
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<p>A team at Caltech is building an <a href="https://www.deepsynoptic.org/">array</a> specifically dedicated to finding FRBs.</p>
<p>There’s plenty of action in Australia too. We are developing a new burst-detection supercomputer for ASKAP that will find FRBs at a faster rate and find more distant sources.</p>
<p>It will effectively turn ASKAP into a high-speed, high-definition video camera, and make a movie of the universe at 40 trillion pixels per second.</p>
<p>By finding more bursts, and more distant bursts, we will be able to better study and understand what causes these mysteriously intense bursts of energy. </p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/0t0KoVhqz3Y?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">For the localisation of the first ‘one-off’ FRB, our team was awarded the 2020 Newcomb Cleveland Prize from the American Association for the Advancement of Science.</span></figcaption>
</figure><img src="https://counter.theconversation.com/content/154381/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ryan Shannon receives funding from the Australian Research Council</span></em></p><p class="fine-print"><em><span>Keith Bannister receives funding from CSIRO and the Australian Research Council.</span></em></p>Australian astronomers are part of a prize-winning team that was the first to pinpoint the location of a fast radio burst. But there is much we still don’t know about these mysterious bursts.Ryan Shannon, Associate Professor, Swinburne University of Technology, Swinburne University of TechnologyKeith Bannister, Astronomer, CSIROLicensed as Creative Commons – attribution, no derivatives.