tag:theconversation.com,2011:/uk/topics/cosmological-constant-16316/articlesCosmological constant – The Conversation2024-01-08T20:01:06Ztag: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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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>
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<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>
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<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/2154142023-11-15T13:21:33Z2023-11-15T13:21:33ZThe universe is expanding faster than theory predicts – physicists are searching for new ideas that might explain the mismatch<figure><img src="https://images.theconversation.com/files/559383/original/file-20231114-23-g88npv.png?ixlib=rb-1.1.0&rect=8%2C7%2C1189%2C1210&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The James Webb Space Telescope's deep field image shows a universe full of sparkling galaxies.</span> <span class="attribution"><a class="source" href="https://webbtelescope.org/contents/media/images/2022/038/01G7JGTH21B5GN9VCYAHBXKSD1?news=true">NASA/STScI</a></span></figcaption></figure><p>Astronomers have known for decades that the universe is expanding. When they use telescopes to observe faraway galaxies, they see that these <a href="https://theconversation.com/explainer-the-mysterious-dark-energy-that-speeds-the-universes-rate-of-expansion-40224">galaxies are moving away</a> from Earth.</p>
<p>To astronomers, the wavelength of light a galaxy emits is longer the faster the galaxy is moving away from us. The farther away the galaxy is, the more its light has shifted toward the longer wavelengths on the red side of the spectrum – so the higher the “redshift.”</p>
<p>Because the speed of light is finite, fast, but not infinitely fast, seeing something far away means we’re looking at the thing how it looked in the past. With distant, high-redshift galaxies, we’re <a href="https://theconversation.com/looking-back-toward-cosmic-dawn-astronomers-confirm-the-faintest-galaxy-ever-seen-207602">seeing the galaxy</a> when the universe was in a younger state. So “high redshift” corresponds to the early times in the universe, and “low redshift” corresponds to the late times in the universe. </p>
<p>But as astronomers have studied these distances, they’ve learned that the universe is not just expanding – its rate of expansion is accelerating. And that expansion rate is even faster than the leading theory predicts it should be, leaving <a href="https://rekeeley.github.io/">cosmologists like me</a> puzzled and looking for new explanations. </p>
<h2>Dark energy and a cosmological constant</h2>
<p>Scientists call the source of this acceleration <a href="https://theconversation.com/dark-energy-map-gives-clue-about-what-it-is-but-deepens-dispute-about-the-cosmic-expansion-rate-143200">dark energy</a>. We’re not quite sure what drives dark energy or how it works, but we think its behavior could be explained by <a href="https://doi.org/10.1086/300499">a cosmological constant</a>, which is a <a href="https://doi.org/10.1038/d41586-018-05095-z">property of spacetime</a> that contributes to the expansion of the universe. </p>
<p>Albert Einstein originally came up with this constant – he marked it with a lambda in his theory of <a href="https://theconversation.com/why-does-gravity-pull-us-down-and-not-up-162141">general relativity</a>. With a <a href="https://www.livescience.com/cosmological-constant.html">cosmological constant</a>, as the universe expands, the energy density of the cosmological constant stays the same.</p>
<p>Imagine a box full of particles. If the volume of the box increases, the density of particles would decrease as they spread out to take up all the space in the box. Now imagine the same box, but as the volume increases, the density of the particles stays the same. </p>
<p>It doesn’t seem intuitive, right? That the energy density of the cosmological constant does not decrease as the universe expands is, of course, very weird, but this property helps explain the accelerating universe.</p>
<h2>A standard model of cosmology</h2>
<p>Right now, the leading theory, or standard model, of cosmology is <a href="https://lambda.gsfc.nasa.gov/education/graphic_history/univ_evol.html">called “Lambda CDM</a>.” Lambda denotes the cosmological constant describing dark energy, and CDM stands for cold dark matter. This model describes both the acceleration of the universe in its late stages as well as the expansion rate in its early days.</p>
<p>Specifically, the Lambda CDM explains observations of the cosmic microwave background, which is the afterglow of microwave radiation from when the universe <a href="https://doi.org/10.1051/0004-6361/201833910">was in a “hot, dense state</a>” about 300,000 years after the Big Bang. Observations using the <a href="https://www.esa.int/Enabling_Support/Operations/Planck">Planck satellite</a>, which measures the <a href="https://www.esa.int/Science_Exploration/Space_Science/Herschel/Cosmic_Microwave_Background_CMB_radiation">cosmic microwave background</a>, led scientists to create the Lambda CDM model. </p>
<p>Fitting the Lambda CDM model to the cosmic microwave background allows physicists to predict the value of the <a href="https://news.uchicago.edu/explainer/hubble-constant-explained">Hubble constant</a>, which isn’t actually a constant but a measurement describing the universe’s current expansion rate. </p>
<p>But the Lambda CDM model isn’t perfect. The expansion rate scientists have calculated by measuring distances to galaxies, and the expansion rate as described in Lambda CDM using <a href="https://doi.org/10.3847/2041-8213/ac5c5b">observations of the cosmic microwave background</a>, don’t line up. Astrophysicists call that disagreement the Hubble tension.</p>
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<a href="https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An illustration showing the progression of the Universe's expansion after the Big Bang. The Universe is depicted as a cylindrical funnel with labels along the bottom showing the first stars, the development of planets, and now the dark energy acceleration" src="https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=390&fit=crop&dpr=1 600w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=390&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=390&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=490&fit=crop&dpr=1 754w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=490&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=490&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 universe is expanding faster than predicted by popular models in cosmology.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/infographics/the-big-bang-and-expansion-of-the-universe">NASA</a></span>
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<h2>The Hubble tension</h2>
<p>Over the past few years, I’ve been <a href="https://doi.org/10.1103/PhysRevLett.131.111002">researching ways</a> to explain this Hubble tension. The tension may be indicating that the Lambda CDM model is incomplete and physicists should modify their model, or it could indicate that it’s time for researchers to come up with new ideas about how the universe works. And new ideas are always the most exciting things for a physicist.</p>
<p>One way to explain the Hubble tension is to modify the Lambda CDM model by changing the expansion rate at low redshift, at late times in the universe. Modifying the model like this can help physicists predict what sort of physical phenomena might be causing the Hubble tension. </p>
<p>For instance, maybe dark energy is not a cosmological constant but instead the result of gravity working in new ways. If this is the case, dark energy would evolve as the universe expands – and the cosmic microwave background, which shows what the universe looked like only a few years after its creation, would have a different prediction for the Hubble constant. </p>
<p>But, <a href="https://doi.org/10.1103/PhysRevLett.131.111002">my team’s latest research</a> has found that physicists can’t explain the Hubble tension just by changing the expansion rate in the late universe – this whole class of solutions falls short.</p>
<h2>Developing new models</h2>
<p>To study what types of solutions could explain the Hubble tension, we <a href="https://doi.org/10.1103/PhysRevLett.131.111002">developed statistical tools</a> that enabled us to test the viability of the entire class of models that change the expansion rate in the late universe. These statistical tools are very flexible, and we used them to match or mimic different models that could potentially fit observations of the universe’s expansion rate and might offer a solution to the Hubble tension.</p>
<p>The models we tested include evolving dark energy models, where dark energy acts differently at different times in the universe. We also tested interacting dark energy-dark matter models, where dark energy interacts with dark matter, and modified gravity models, where gravity acts differently at different times in the universe. </p>
<p>But none of these could fully explain the Hubble tension. These results suggest that physicists should study the early universe to understand the source of the tension.</p><img src="https://counter.theconversation.com/content/215414/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ryan Keeley does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The universe is expanding faster than physicists would expect. To figure out what processes underlie this fast expansion rate, some researchers are first trying to rule out what processes can’t.Ryan Keeley, Postdoctoral Scholar in Physics, University of California, MercedLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2041902023-06-28T20:01:43Z2023-06-28T20:01:43ZCosmological models are built on a simple, century-old idea – but new observations demand a radical rethink<figure><img src="https://images.theconversation.com/files/534263/original/file-20230627-21-puyg9.png?ixlib=rb-1.1.0&rect=114%2C161%2C2184%2C1293&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://doi.org/10.1093/mnras/stab1193">Fosalba & Gaztañaga 2021, MNRAS</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Our ideas about the Universe are based on a century-old simplification known as the cosmological principle. It suggests that when averaged on large scales, the Cosmos is homogeneous and matter is distributed evenly throughout.</p>
<p>This allows a <a href="https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric">mathematical description of space-time</a> that simplifies the application of Einstein’s general theory of relativity to the Universe as a whole. </p>
<p>Our cosmological models are based on this assumption. But as new telescopes, both on Earth and in space, deliver ever more precise images, and astronomers discover massive objects such as the <a href="https://www.bbc.com/future/article/20230302-the-giant-arcs-that-may-dwarf-everything-in-the-cosmos">giant arc of quasars</a>, this foundation is increasingly challenged.</p>
<p>In <a href="https://doi.org/10.1088/1361-6382/acbefc">our recent review</a>, we discuss how these new discoveries force us to radically re-examine our assumptions and change our understanding of the Universe. </p>
<h2>Einstein’s legacy</h2>
<p>Albert Einstein faced huge dilemmas 106 years ago when he first applied his equations for gravity to the Universe as a whole. No physicist had ever attempted something so bold, but it was a natural consequence of his key idea. As a <a href="https://pubs.aip.org/physicstoday/online/41279/Gravitation-s-attraction-50-years-later">50-year-old textbook</a> reminds us: </p>
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<p>Matter tells space how to curve, and space tells matter how to move. </p>
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<p>Data were almost completely lacking in 1917 and the idea that galaxies were objects at vast distances was a minority view among astronomers. </p>
<p>The conventional viewpoint, accepted by Einstein, was that the whole Universe looked like the inside of our galaxy. This suggested stars should be treated as pressure-less fluids, distributed randomly but with a well defined average density – the same, or homogeneous, anywhere in space. </p>
<p>Based on the idea that the Universe is the same everywhere, Einstein introduced his cosmological constant Λ, now known as “dark energy”.</p>
<p>On small scales, Einstein’s equations tell us that space never stands still. But forcing this on the Universe on a large scale was unnatural. Einstein was therefore relieved by the discovery of the <a href="https://en.wikipedia.org/wiki/Cosmological_constant">expanding universe</a> in the late 1920s. He even described Λ as his <a href="https://arxiv.org/abs/1804.06768">biggest blunder</a>.</p>
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Read more:
<a href="https://theconversation.com/dark-matter-our-review-suggests-its-time-to-ditch-it-in-favour-of-a-new-theory-of-gravity-186344">Dark matter: our review suggests it's time to ditch it in favour of a new theory of gravity</a>
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<h2>Ideas about matter have evolved, but not geometry</h2>
<p>We now have amazingly detailed models of the physics of stars and galaxies embedded in the evolving Universe. We can trace the astrophysics of “stuff” from tiny seed ripples in the primordial fireball all the way to complex structures today.</p>
<p>Our telescopes are wonderful time machines. They look back all the way to when the first atoms formed, and the Universe first became transparent.</p>
<p>Beyond is the primordial plasma, opaque like the interior and surface of the Sun. The light that left the Universe’s “<a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background">surface of last scattering</a>” was very hot back then, about 2,700°C.</p>
<p>We receive that same light today, but cooled to minus 270°C and diluted by the expansion of the Universe. This is the cosmic microwave background and it is remarkably uniform in all directions.</p>
<p>This is strong evidence the Universe was very close to spatially uniform when it was a fireball. But there is no direct evidence for such uniformity today. </p>
<h2>A ‘lumpy’ Universe</h2>
<p>Far back in time, our telescopes reveal small merging galaxies, growing into ever larger structures until the present day.</p>
<p>The expansion of the Universe has been halted entirely within the largest matter concentrations known as galaxy clusters. Where space is expanding, the clusters are stretched in filaments and sheets that thread and surround vast empty voids, all growing with time but at different rates. Rather than being smooth, matter forms a “<a href="https://en.wikipedia.org/wiki/Observable_universe#Large-scale_structure">cosmic web</a>”.</p>
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<img alt="A simulation of the cosmic web" src="https://images.theconversation.com/files/534197/original/file-20230626-25-x6xeh5.jpg?ixlib=rb-1.1.0&rect=6%2C24%2C670%2C335&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534197/original/file-20230626-25-x6xeh5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534197/original/file-20230626-25-x6xeh5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534197/original/file-20230626-25-x6xeh5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534197/original/file-20230626-25-x6xeh5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534197/original/file-20230626-25-x6xeh5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534197/original/file-20230626-25-x6xeh5.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">
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<span class="caption">The Universe was uniform long ago, but develops a cosmic web as structures grow. Computer models using simple geometry, as shown, will now be tested against more complex ones.</span>
<span class="attribution"><span class="source">Wikimedia Commons</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p>But the idea that the Universe is spatially homogeneous endures. </p>
<p>There would be a gross inconsistency between the observed cosmic web and an average curved geometry of space if all we see is all there is. Evidence for missing matter has been around since the first observations of <a href="https://en.wikipedia.org/wiki/Galaxy_groups_and_clusters">galaxy clusters</a> in <a href="https://en.wikipedia.org/wiki/Dark_matter">1933</a>. </p>
<p>Our first observations of the cosmic microwave background radiation and its ripples in the decade from 1965 changed that idea. </p>
<p>Our models of nuclear physics are wonderfully predictive. But they are only consistent with observations if the missing mass in galaxy clusters is something like neutrinos that cannot emit light. Thus we invented cold dark matter, which makes gravity stronger within galaxy clusters. </p>
<p>Billions have been spent trying to directly detect dark matter, but decades of such efforts have yielded no definitive detection of what makes up 80% of all matter and 20% of all the energy in the Universe today.</p>
<h2>An anomalous sky</h2>
<p>The cosmic microwave background radiation is not perfectly uniform. Superimposed on it are fluctuations, one of which is abnormally large and has the shape of a <a href="https://science.nasa.gov/cmb-dipole-speeding-through-universe">dipole</a>: a yin-yang diagram covering the whole sky.</p>
<p>We can interpret this as an effect due to relative motion, provided we define the cosmic microwave background radiation as the rest frame of the Universe. If we didn’t do this, we would need a physical explanation for the large dipole.</p>
<p>Much of the puzzle boils down to a power asymmetry – a lopsided Universe. The temperatures of the hemispheres above and below the plane of the Milky Way are slightly different to expectation. </p>
<p>These anomalies have long been explained as a result of unaccounted physical processes in modelling microwave emissions from the Milky Way.</p>
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Read more:
<a href="https://theconversation.com/the-largest-structures-in-the-universe-are-still-glowing-with-the-shock-of-their-creation-199785">The largest structures in the Universe are still glowing with the shock of their creation</a>
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<img alt="An artist's impression of the Euclid satellite mission in space." src="https://images.theconversation.com/files/534202/original/file-20230626-25-wrvgn1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534202/original/file-20230626-25-wrvgn1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534202/original/file-20230626-25-wrvgn1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534202/original/file-20230626-25-wrvgn1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534202/original/file-20230626-25-wrvgn1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534202/original/file-20230626-25-wrvgn1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534202/original/file-20230626-25-wrvgn1.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">
<figcaption>
<span class="caption">The European Space Agency will launch the Euclid satellite on July 1 2023 to look far and wide, answering some of the most fundamental questions about our Universe.</span>
<span class="attribution"><span class="source">ESA/ATG</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Matter within the sky</h2>
<p>The cosmic microwave background radiation is not the only all-sky observation to show a dipole. Last year, researchers used observations of 1.36 million distant quasars and 1.7 million radio sources to <a href="https://www.physics.ox.ac.uk/news/lopsided-universe-could-mean-revision-standard-cosmological-model">test the cosmological principle</a>. They found that matter, too, is unevenly distributed.</p>
<p>Another even more widely discussed mystery is the “<a href="https://www.quantamagazine.org/cosmologists-debate-how-fast-the-universe-is-expanding-20190808/">Hubble tension</a>”. Conventionally, we assume that an all-sky average of the Universe’s present expansion rate gives one well defined value: the Hubble constant. But the measured value differs from expectation, given a standard expansion history based on the cosmic microwave background radiation. If we allowed for inhomogeneous cosmologies, this problem may simply disappear.</p>
<p>Using cosmic microwave background data from individual opposing hemispheres, a standard expansion history implies different Hubble “constants” on each side of the sky today.</p>
<p>These puzzles are <a href="https://www.quantamagazine.org/giant-arc-of-galaxies-puts-basic-cosmology-under-scrutiny-20211213/">compounded</a> by an <a href="https://arxiv.org/abs/2208.05018">ever-growing list</a> of unexpected discoveries: a vast <a href="https://www.bbc.com/future/article/20230302-the-giant-arcs-that-may-dwarf-everything-in-the-cosmos">giant arc of quasars</a> and a complex, bright and element-filled <a href="https://theconversation.com/how-the-james-webb-space-telescope-has-revealed-a-surprisingly-bright-complex-and-element-filled-early-universe-podcast-196649">early Universe</a> unveiled by the James Webb Space Telescope.</p>
<p>If matter is much more varied and interesting than expected, then maybe the geometry is too.</p>
<p>Models which abandon the cosmological principle do exist and <a href="https://theconversation.com/can-we-ditch-dark-energy-by-better-understanding-general-relativity-76777">make predictions</a>. They are simply less studied than standard cosmology. The European Space Agency’s Euclid satellite will be launched this year. Will Euclid reveal that on average space is not Euclidean? If so, then a fundamental revolution in physics might be around the corner.</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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<img src="https://counter.theconversation.com/content/204190/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Wiltshire received funding from the Royal Society of New Zealand, most recently a Catalyst Seeding Grant. With previous Marsden funding, he developed the first detailed observational tests of the non-standard timescape cosmology. These will include tests by the Euclid satellite mission.</span></em></p><p class="fine-print"><em><span>Eoin Ó Colgáin most recently received funding from the National Research Foundation of Korea (2020-2022) to conduct research into cosmological tensions. </span></em></p><p class="fine-print"><em><span>Jenny Wagner received funding from the German Science Foundation to investigate the biasing model-dependencies and degeneracies of strong gravitational lensing as a cosmological probe, for instance to constrain the Hubble constant.</span></em></p><p class="fine-print"><em><span>Shahin Sheikh-Jabbari receives funding from IPM, Tehran and ICTP, Trieste, Italy, which has been used to investigate cosmological tensions and non-standard alternatives such as the dipole cosmology.</span></em></p>New deep-space discoveries suggest the Universe is lumpy and lopsided. But if matter is distributed unevenly, we’ll have to rethink the simple geometry used in cosmological models.David Wiltshire, Professor of Theoretical Physics, University of CanterburyEoin O Colgain, Assistant Lecturer in Physical Sciences, Atlantic Technological UniversityJenny Wagner, Research Scientist in Cosmology, Bahamas Advanced Study Institute & ConferencesShahin Sheikh-Jabbari, Professor in Physics, Institute for Research in Fundamental Sciences Licensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/881812017-11-30T13:00:44Z2017-11-30T13:00:44ZStudy finds ‘dark matter’ and ‘dark energy’ may not exist – here’s what to make of it<figure><img src="https://images.theconversation.com/files/196765/original/file-20171128-28849-1yo8g18.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Image showing where scientists believe dark matter resides in the galaxy cluster Abell 520
– near the hot gas in the middle, coloured green.</span> <span class="attribution"><span class="source">Chandra X-ray Observatory Center</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Dark matter and dark energy are mysterious, unknown substances that are thought to make up more than 96% of the universe. While we may have never directly seen them, they beautifully explain how stars and galaxies move and how the universe is expanding. But a new study, <a href="https://arxiv.org/abs/1710.11425">published in The Astrophysical Journal</a>, suggests they may not exist after all. </p>
<p>I have taught astronomy and astrophysics classes at various universities around the globe, and few topics spark as much excitement as <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">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>
<p>An introductory lecture on the subject will typically begin by looking at a pie chart of the mass and energy density of the universe. This demonstrates that, at the present day, regular atoms like hydrogen and helium make up only a few per cent of the universe. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/196478/original/file-20171127-2021-1n8io53.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196478/original/file-20171127-2021-1n8io53.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=240&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196478/original/file-20171127-2021-1n8io53.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=240&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196478/original/file-20171127-2021-1n8io53.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=240&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196478/original/file-20171127-2021-1n8io53.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=302&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196478/original/file-20171127-2021-1n8io53.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=302&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196478/original/file-20171127-2021-1n8io53.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=302&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Cosmological Composition Pie Chart (CC-BY-3.0).</span>
<span class="attribution"><span class="source">Ben Finney</span></span>
</figcaption>
</figure>
<p>Approximately one quarter of the pie chart is dark matter. In brief, this is a form of matter that only seems to interact through gravitation. Several strands of evidence lend support to the existence of this form of matter. For example, we can see a gravitational pull of galaxy clusters and other structures in the universe. We know that the matter in such structures isn’t enough to hold them together by gravity alone, meaning some additional invisible matter must be present to make them spin at the speeds observed. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=459&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=459&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=459&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=577&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=577&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196485/original/file-20171127-2046-8ajo2t.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=577&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Dark matter is likely to be causing the arcs seen around the central galaxies in this image, making a smiling face.</span>
<span class="attribution"><span class="source">NASA/ESA</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Embarrassingly, the largest part of the pie chart is dark energy. This is a type of energy, or field, that causes an anti-gravity force on the universe itself – pushing it apart. Another way of thinking about it is that dark energy exerts a negative pressure. Rather than the universe just expanding at a constant rate, the expansion is accelerating as a consequence of dark energy.</p>
<p>We can tell that the universe is expanding in this way by looking at how fast galaxies move away from each other. And I have personally been involved in tests of dark energy, such as the <a href="http://wigglez.swin.edu.au/site/">WiggleZ survey</a>. In this, our team measured the effect of dark energy using a novel <a href="https://en.wikipedia.org/wiki/Standard_ruler">“standard ruler” technique</a>, which can estimate distances in the cosmos. This lead to the conclusion that dark energy is real.</p>
<h2>Another way out?</h2>
<p>Suppose for a moment that both dark energy and dark matter are too strange a pill to swallow. What would the alternatives be? One way out would be to suppose that our understanding of the universe is at fault. Perhaps gravity and general relativity do not work in quite the way that we think they do. </p>
<p>In the same way that Newton’s laws – which we long thought told the whole story about movement – are a <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">simplification of the more complicated theory of relativity</a>, perhaps our understanding of relativity is a simplification of something else? More fundamentally, perhaps we have made some error of judgement about the assumptions that underpin the equations we deal with? Maybe we need to modify the equations of gravitation?</p>
<p>The new study by André Maeder, an honorary astronomy professor at the University of Geneva, centres around something called “<a href="https://en.wikipedia.org/wiki/Scale_invariance">scale invariance</a>”. Scale invariance means that the properties of a given law of physics (or set of physical objects) do not change, even if we were to multiply their lengths or energies by some number. They are the same, independent of scale. As an example of a very specific type of scale invariance, think of a fractal. Even if we zoom in by some set magnification, the shape of the fractal will remain the same.</p>
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<a href="https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/196523/original/file-20171127-2066-gpv8sf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Fractal, where the same shape is repeated.</span>
<span class="attribution"><span class="source">Kevin Wong / Flickr</span></span>
</figcaption>
</figure>
<p>We could treat empty space itself as having this quality. In other words, empty space still has the properties of empty space, even if we zoom in or out – and its properties do not change if it’s squashed or stretched. What Maeder finds is that if we treat empty space as being scale invariant, there may be no need for dark matter or dark energy at all. </p>
<p>This is different from what Einstein suggested – that empty space operates based on what’s known as the <a href="https://en.wikipedia.org/wiki/Cosmological_constant">cosmological constant</a>, which is a form of dark energy. Space under these circumstances cannot be scale invariant in the same way. Many scientists implicitly and explicitly assume this is the correct description. Yet this means that dark energy and dark matter are still required and therefore new physics is needed to explain them.</p>
<h2>Putting theory to the test</h2>
<p>Excitingly, Maeder’s theory is testable. For example, we can observationally determine the rotation speed of galaxies and compare the data to the predictions made by his model of empty space. We can even examine the motion of galaxies inside clusters of galaxies to test if there is agreement with the model proposed. </p>
<p>Scientists only proposed dark matter to explain how galaxies and galaxy clusters move due to a gravitational pull. But what if space itself could make them move in this way? Thus far, the tests that Maeder describes are in agreement with the observations made. </p>
<p>However, there are many more tests that need to be run, Maeder has only investigated two galaxy clusters. And let’s not forget the huge body of work suggesting that dark matter and dark energy do exist. Yet it is tantalising that, if the hypotheses that Maeder has put forward are correct, then it points to a large revision of our ideas about cosmology. </p>
<p>While we are not there yet, ultimately, the pie chart of mass and energy density of the entire universe may need to be revisited to scrub out the two biggest parts! It’s an exciting time to be a cosmologist.</p><img src="https://counter.theconversation.com/content/88181/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kevin Pimbblet receives funding from STFC.</span></em></p>Controversial new study challenges contemporary thinking about what the universe is made of.Kevin Pimbblet, Senior Lecturer in Physics, University of HullLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/440242015-06-29T16:02:26Z2015-06-29T16:02:26ZGalaxy survey to probe why the universe is accelerating<figure><img src="https://images.theconversation.com/files/86704/original/image-20150629-9090-195l14k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Understanding how galaxies are arranged could be the key to figuring what causes the expansion of the universe.</span> <span class="attribution"><a class="source" href="http://www.esa.int/spaceinimages/Images/2014/12/This_is_no_supermodel_spiral"> ESA/Hubble, NASA and S. Smartt (Queen's University Belfast)</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>We know that our universe is expanding at an accelerating rate, but what causes this growth remains a mystery. The most likely explanation is that a strange force dubbed “dark energy” is driving it. Now a new astronomical instrument, called the <a href="http://www.pausurvey.org/camera.html">Physics of the Accelerating Universe Camera (PAUCam)</a>, will look for answers by mapping the universe in an innovative way. </p>
<p>The camera, which will record the positions of around 50,000 galaxies at once, could also shed light on what <a href="http://home.web.cern.ch/about/physics/dark-matter">dark matter</a> is and how the cosmos evolved.</p>
<p>In the 1990s, astronomers studying exploding stars – supernovae – in galaxies far away <a href="http://link.springer.com/article/10.1007%2FBF00644200">discovered that the universe’s expansion was accelerating</a>. This came as surprise, as scientists at the time thought <a href="http://discovermagazine.com/2013/may/12-what-does-dark-energy-mean-for-the-fate-of-the-universe">it was slowing down</a>. With no obvious solution at hand, scientists argued that there must be some sort of mysterious force – dark energy – pulling the universe apart. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/86703/original/image-20150629-9090-u28vp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/86703/original/image-20150629-9090-u28vp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/86703/original/image-20150629-9090-u28vp8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/86703/original/image-20150629-9090-u28vp8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/86703/original/image-20150629-9090-u28vp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=425&fit=crop&dpr=1 754w, https://images.theconversation.com/files/86703/original/image-20150629-9090-u28vp8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=425&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/86703/original/image-20150629-9090-u28vp8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=425&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Timeline of the universe, assuming a cosmological constant.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Lambda-CDM_model#/media/File:Lambda-Cold_Dark_Matter,_Accelerated_Expansion_of_the_Universe,_Big_Bang-Inflation.jpg">Coldcreation/wikimedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Fast forward about two decades and we still don’t know what dark energy is, thought to make <a href="http://iopscience.iop.org/0004-637X/686/2/749/">up 71% of all the energy in the universe</a>. One theory says it can be explained by an abandoned version of Einstein’s theory of gravity – known as the <a href="http://arxiv.org/abs/astro-ph/0207347">“cosmological constant”</a> – which is a measure of the energy density of the vacuum of space. Another argues that it is caused by enigmatic scalar fields, which can vary in time and space. Some scientists even believe that a <a href="http://www.sciencedirect.com/science/article/pii/S0370269306008276">weird “energy fluid”</a> that fills space could be driving the expansion.</p>
<h2>Mapping the sky</h2>
<p>Of course, the only way to find out is through observation. After spending six years under design and construction by a consortium of Spanish research institutions, PAUCam was successfully tested out for the first time this month – <a href="http://www.ing.iac.es/PR/press/pau.html">seeing “first light”</a> on the 4.2 metre William Herschel Telescope on La Palma in the Canary Islands. </p>
<p>Using the information captured by PAUCam, an international team, including researchers from <a href="http://www.icc.dur.ac.uk/">Durham University’s Institute for Computational Cosmology</a>, is being set up to build a unique map of how galaxies are arranged in the universe. </p>
<p>Such a map will contain detailed new information about the basic numbers which govern the fate of the universe; its expansion and about how the galaxies themselves were made. The map will reveal the extent of structures in the distribution of galaxies. These structures grow due to gravity – if the expansion of the universe is speeding up, then it is harder for gravity to pull matter together in order build these structures. Knowing the strength of gravity and measuring the size of structures in the galaxy distribution can therefore help us deduce the expansion history of the universe. </p>
<p>Astronomers can map the positions of galaxies on the sky by taking images or photographs. These are projected positions and so do not tell us the distance to a galaxy from the Earth. A galaxy could appear to be very faint because it is at a large distance from us or simply because it is nearby, but is intrinsically faint with few bright stars. </p>
<p>Traditionally, astronomers have used spectroscopy to measure the distance to a galaxy. This technique works by capturing <a href="http://loke.as.arizona.edu/%7Eckulesa/camp/spectroscopy_examples.html">the light from the galaxy and spreading it out into a spectrum according to its wavelengths</a>. In this way, they can investigate the pattern of lines emitted by the different elements in the stars that make up the galaxy. The further away the galaxy is, the more the expansion of the universe shifts these lines to appear at longer wavelengths and lower frequencies than they would appear in a laboratory here on Earth. The size of this so-called “redshift” therefore <a href="http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/origins/redshiftrev3.shtml">gives the distance to the galaxy</a>. </p>
<p><a href="https://en.wikipedia.org/wiki/CfA_Redshift_Survey">Early surveys</a> of galaxy positions painstakingly measured such spectra one galaxy at a time, pointing the telescope at each galaxy in turn. Modern surveys can now record up to a few thousand galaxy spectra in a single exposure. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/86715/original/image-20150629-9062-s757b7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/86715/original/image-20150629-9062-s757b7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/86715/original/image-20150629-9062-s757b7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/86715/original/image-20150629-9062-s757b7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/86715/original/image-20150629-9062-s757b7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/86715/original/image-20150629-9062-s757b7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/86715/original/image-20150629-9062-s757b7.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">The camera has been tested using the William Herschel Telescope.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:William_Herschel_Telescope_at_twilight.jpg">wikimedia commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>PAUcam will revolutionise survey astronomy by measuring the distances to tens of thousands of galaxies it can see each time it looks at the sky. It does this by taking 40 photographs or images using special filters that isolate a portion of the light emitted by a galaxy. This allows a quick spectrum to be built up for each galaxy at a fraction of the traditional cost. This spectrum also acts like a DNA for each galaxy, encoding information about how many stars it contains and how quickly new stars are being added. </p>
<h2>Looking for answers</h2>
<p>My team here at Durham will build computer models of the evolution of the universe, which aim to describe how structures like galaxies have developed over 13.7 billion years of cosmic history. The cosmologist’s universe is <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/astro/darmat.html">mostly made up of an unknown substance called dark matter</a>, with a small amount of “normal matter”. </p>
<p>PAUCam will allow cosmologists to test their models for building galaxies by measuring the lumpiness of the galaxy distribution in the new map. This is important because it tells us about the distribution of the dark matter, which we cannot see directly.</p>
<p>We know from previous observations that galaxy clusters contain dark matter. By counting the number of galaxies in a cluster, astronomers can <a href="http://www.cfhtlens.org/public/what-dark-matter">estimate the total amount of (visible) matter in the cluster</a>. By also measuring the velocities of the galaxies, they find that some are moving so fast that they should escape the gravitational pull of the cluster. The reason they don’t is because huge amounts of invisible dark matter is increasing the gravitational pull. If the galaxies are very clustered – or their distribution is lumpy – then the computer simulations show that this means the galaxies live inside more massive dark matter structures. </p>
<p>PAUCam will allow us to learn more about an effect called <a href="http://cosmology.berkeley.edu/Education/CosmologyEssays/Gravitational_Lensing.html">gravitational lensing</a>, in which the mass in the universe bends the light from distant galaxies, causing their images to appear distorted. Scientists can study the distortions to calculate how massive the patch of the universe really is – including the dark matter. This is one of the key probes of dark energy that is planned for the <a href="http://sci.esa.int/euclid/">European Space Agency’s Euclid mission</a>, which is scheduled for launch in 2020. </p>
<p>The lensing distortion depends on the lumpiness of the dark matter, which is turn is determined by how fast the universe is expanding. If the universe expands at a fast rate, then it is harder for gravity to pull structures together to make bigger ones. PAUCam will help us to disentangle the signal from gravitational lensing from simple alignments between the orientations of galaxies which develop as they form. </p>
<p>A galaxy survey like PAUCam has never been attempted on this scale before. The resulting map will be a unique resource to help us learn more about how galaxies are made and why the expansion of the universe seems to be speeding up. We hope to have the answer once the PAUCam survey is finished by around 2020.</p><img src="https://counter.theconversation.com/content/44024/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Carlton Baugh receives funding from the Science and Technology Facilities Council and the Royal Society.</span></em></p>A unique map of the galaxies in the sky could shed light on the mysteries of the universe – including dark energy and dark matter.Carlton Baugh, Professor of physics, Durham UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/402242015-04-23T09:58:01Z2015-04-23T09:58:01ZExplainer: the mysterious dark energy that speeds the universe’s rate of expansion<figure><img src="https://images.theconversation.com/files/78991/original/image-20150422-1844-8zabh0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How do we think about something we can't see and don't experience in our everyday lives, but seems to be pushing our universe apart ever faster?</span> <span class="attribution"><a class="source" href="http://hubblesite.org/gallery/album/the_universe/hubble_ultra_deep_field/pr2012037a/">NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The nature of dark energy is one of the most important unsolved problems in all of science. But what, exactly, is dark energy, and why do we even believe that it exists?</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=902&fit=crop&dpr=1 600w, https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=902&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=902&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1133&fit=crop&dpr=1 754w, https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1133&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/78969/original/image-20150422-1837-rgb3kt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1133&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">What goes up must come down… right?</span>
<span class="attribution"><a class="source" href="http://www.shutterstock.com/pic.mhtml?id=52648960&src=lb-29877982">Ball image via www.shutterstock.com.</a></span>
</figcaption>
</figure>
<p>Step back a minute and consider a more familiar experience: what happens when you toss a ball straight up into the air? It gradually slows down as gravity tugs on it, finally stopping in mid-air and falling back to the ground. Of course, if you threw the ball hard enough (about 25,000 miles per hour) it would actually escape from the Earth entirely and shoot into space, never to return. But even in that case, gravity would continue to pull feebly on the ball, slowing its speed as it escaped the clutches of the Earth.</p>
<p>But now imagine something completely different. Suppose that you tossed a ball into the air, and instead of being attracted back to the ground, the ball was repelled by the Earth and blasted faster and faster into the sky. This would be an astonishing event, but it’s exactly what astronomers have observed happening to the entire universe!</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=663&fit=crop&dpr=1 600w, https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=663&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=663&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=833&fit=crop&dpr=1 754w, https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=833&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/78970/original/image-20150422-1848-w2itcj.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=833&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This illustration shows abstracted ‘slices’ of space at different points in time as the universe expands.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Big_crunch_to_the_right.png">Ævar Arnfjörð Bjarmason</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Scientists have known for almost a century that the <a href="http://map.gsfc.nasa.gov/universe/uni_expansion.html">universe is expanding</a>, with all of the galaxies flying apart from each other. And until recently, scientists believed that there were only two possible options for the universe in the future. It could expand forever (like the ball that you tossed upward at 25,000 miles an hour), but with the expansion slowing down as gravity pulled all of the galaxies toward each other. Or gravity might win out in the end and bring the expansion of the universe to a halt, finally collapsing it back down in a “big crunch,” just like your ball plunging back to the ground. </p>
<p>So imagine scientists’ surprise when two different <a href="http://dx.doi.org/10.1086/307221">teams</a> of <a href="http://dx.doi.org/10.1086/300499">astronomers</a> discovered, back in 1998, that neither of these behaviors was correct. These astronomers were measuring how fast the universe was expanding when it was much younger than today. But how could they do this without building a time machine?</p>
<p>Luckily, a telescope <em>is</em> a time machine. When you look up at the stars at night, you aren’t seeing what they look like today – you’re seeing light that left the stars a long time ago – often many hundreds of years. By looking at distant supernovae, which are tremendously bright exploding stars, astronomers can look back hundreds of millions of years. They can then measure the expansion rate back then by comparing the distance to these far-off supernovae with the speed at which they are flying away from us. And by comparing how fast the universe was expanding hundreds of millions of years ago to its rate of expansion today, these astronomers discovered that the expansion is actually <em>speeding up</em> instead of slowing down as everyone had expected.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/78989/original/image-20150422-1918-bcfljk.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">What pushes galaxies like these in the Hubble deep field apart?</span>
<span class="attribution"><a class="source" href="http://hubblesite.org/newscenter/archive/releases/2001/09/image/b/">NASA and A. Riess (STScI)</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Instead of pulling the galaxies in the universe together, gravity seems to be driving them apart. But how can gravity be repulsive, when our everyday experience shows that it’s attractive? Einstein’s theory of gravity in fact predicts that gravity can repel as well as attract, but only under very special circumstances.</p>
<p>Repulsive gravity requires a new form of energy, dubbed “dark energy,” with very weird properties. Unlike ordinary matter, dark energy has <em>negative</em> pressure, and it’s this negative pressure that makes gravity repulsive. (For ordinary matter, gravity is always attractive). Dark energy appears to be smoothly smeared out through the entire universe, and it interacts with ordinary matter only through the action of gravity, making it nearly impossible to test in the laboratory.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=539&fit=crop&dpr=1 600w, https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=539&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=539&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=677&fit=crop&dpr=1 754w, https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=677&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/78981/original/image-20150422-1867-yhb2nf.jpg?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">Scientists used to think that the expansion of the universe was described by the yellow, green, or blue curves. But surprise, it’s actually the red curve instead.</span>
</figcaption>
</figure>
<p>The simplest form of dark energy goes by two different names: a cosmological constant or vacuum energy. Vacuum energy has another strange property. Imagine a box that expands as the universe expands. The amount of matter in the box stays the same as the box expands, but the volume of the box goes up, so the density of matter in the box goes down. In fact, the density of <em>everything</em> goes down as the universe expands. Except for vacuum energy - its density stays exactly the same. (Yes, that’s as bizarre as it sounds. It’s like stretching a string of taffy and discovering that it never gets any thinner).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/78985/original/image-20150422-1907-fhjwag.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Astronomers continue to probe the skies, looking for finer details that can build on what we suspect about dark energy.</span>
<span class="attribution"><a class="source" href="http://vms-db-srv.fnal.gov/fmi/xsl/VMS_Site_2/000Return/photography/r_online_mrdetail.xsl?-db=VMS_Frames&-lay=WWWBrowse&-recid=202806&-find=">Reidar Hahn</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Since dark energy can’t be isolated or probed in the laboratory, how can we hope to understand exactly what it’s made of? Different theories for dark energy predict small differences in the way that the expansion of the universe changes with time, so our best hope of probing dark energy seems to come from ever more accurate measurements of the acceleration of the universe, building on that first discovery 17 years ago. Different groups of scientists are currently undertaking a wide range of these measurements. For example, the <a href="http://www.darkenergysurvey.org/">Dark Energy Survey</a> is mapping out the distribution of galaxies in the universe to help resolve this puzzle.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=778&fit=crop&dpr=1 600w, https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=778&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=778&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=977&fit=crop&dpr=1 754w, https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=977&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/78980/original/image-20150422-1848-1q5848o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=977&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Could Einstein’s theory need work?</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Einstein-formal_portrait-35.jpg">Sophie Delar</a></span>
</figcaption>
</figure>
<p>There is one other possibility: maybe scientists have been barking up the wrong tree. Maybe there is no dark energy, and our measurements actually mean that Einstein’s theory of gravity is wrong and needs to be fixed. This would be a daunting undertaking, since Einstein’s theory works exceptionally well when we test it in the solar system. (Let’s face it, Einstein really knew what he was doing). So far, no one has produced a convincing improvement on Einstein’s theory that predicts the correct expansion for the universe and yet agrees with Einstein’s theory inside the solar system. I’ll leave that as a homework problem for the reader.</p><img src="https://counter.theconversation.com/content/40224/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Robert Scherrer receives funding from the United States Department of Energy.</span></em></p>Einstein’s theory of gravity says dark energy must be out there, accelerating the expansion of our universe. But what is it and how can we try to figure out more about it?Robert Scherrer, Professor and Chair of Physics and Astronomy, Vanderbilt UniversityLicensed as Creative Commons – attribution, no derivatives.