[{"command":"openDialog","selector":"#drupal-modal","settings":null,"data":"\u003Cdiv id=\u0022republish_modal_form\u0022\u003E\u003Cform class=\u0022modal-form-example-modal-form ecl-form\u0022 data-drupal-selector=\u0022modal-form-example-modal-form\u0022 action=\u0022\/en\/article\/modal\/6910\u0022 method=\u0022post\u0022 id=\u0022modal-form-example-modal-form\u0022 accept-charset=\u0022UTF-8\u0022\u003E\u003Cp\u003EHorizon articles can be republished for free under the Creative Commons Attribution 4.0 International (CC BY 4.0) licence.\u003C\/p\u003E\n \u003Cp\u003EYou must give appropriate credit. We ask you to do this by:\u003Cbr \/\u003E\n 1) Using the original journalist\u0027s byline\u003Cbr \/\u003E\n 2) Linking back to our original story\u003Cbr \/\u003E\n 3) Using the following text in the footer: This article was originally published in \u003Ca href=\u0027#\u0027\u003EHorizon, the EU Research and Innovation magazine\u003C\/a\u003E\u003C\/p\u003E\n \u003Cp\u003ESee our full republication guidelines \u003Ca href=\u0027\/horizon-magazine\/republish-our-stories\u0027\u003Ehere\u003C\/a\u003E\u003C\/p\u003E\n \u003Cp\u003EHTML for this article, including the attribution and page view counter, is below:\u003C\/p\u003E\u003Cdiv class=\u0022js-form-item form-item js-form-type-textarea form-item-body-content js-form-item-body-content ecl-form-group ecl-form-group--text-area form-no-label ecl-u-mv-m\u0022\u003E\n \n\u003Cdiv\u003E\n \u003Ctextarea data-drupal-selector=\u0022edit-body-content\u0022 aria-describedby=\u0022edit-body-content--description\u0022 id=\u0022edit-body-content\u0022 name=\u0022body_content\u0022 rows=\u00225\u0022 cols=\u002260\u0022 class=\u0022form-textarea ecl-text-area\u0022\u003E\u003Ch2\u003EDark energy is the biggest mystery in cosmology, but it may not exist at all - leading physicist\u003C\/h2\u003E\u003Cp\u003E\u003Cstrong\u003EIn the late 1990s, astronomers found evidence from supernovae that the universe has been expanding faster and faster as it gets older. Having no explanation for what was driving it, they dubbed this accelerating expansion \u2018dark energy\u2019. What did you think about these findings at the time?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018I was sceptical from the beginning. I\u2019m unusual in the cosmology community in that I\u2019ve had experience working in experiments as well as theory, and I didn\u2019t think the astronomers were taking full account of the systematic uncertainties in their data. But for a long time, the data were not made available for checking. That changed in 2014 when an international collaboration \u003Ca href=\u0022https:\/\/www.aanda.org\/articles\/aa\/abs\/2014\/08\/aa23413-14\/aa23413-14.html\u0022 target=\u0022_blank\u0022 rel=\u0022noopener noreferrer\u0022\u003Epublished a catalogue\u003C\/a\u003E\u0026nbsp;of all the relevant supernovae \u2013 publicly, for the first time.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EHow do supernovae tell us about the expansion of the universe?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018Supernovae are the incredibly bright explosions that occur when big stars die. They come in different types. One type, Ia, are known commonly as \u0022standard candles\u0022 because they emit roughly the same amount of light as one another. That means that if we measure how bright they appear to us here on Earth, we can work out how far away they are \u2013 closer ones appear brighter, and farther ones appear dimmer.\u003C\/p\u003E\u003Cp\u003E\u2018But the key parameter here isn\u2019t distance itself. The light from more distant supernovae has taken longer to reach us, which means that these distant ones are a record of what the universe was like a long time ago. If we also measure the red-shift of their light spectra, we can work out how fast all the supernovae are receding away from us, and therefore how fast the universe is \u2013 or was \u2013 expanding.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EAnd this is where the concept of dark energy came from - that the older, more distant supernovae are not receding as quickly as expected from observations of the younger, closer ones.\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018That\u2019s what the astronomers Saul Perlmutter, Brian P Schmidt, Adam Riess and others concluded in the late 1990s. Various other measurements seemed to support their idea, too, leading to their being awarded the Nobel Prize in Physics in 2011. Today, most cosmologists believe about two-thirds of the universe is made of dark energy.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EWhere do you believe they\u2019ve gone wrong?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018Just as not all trees are alike, not all supernovae are alike \u2013 even the so-called standard candles. In fact, their absolute brightness varies by about a factor of ten.\u003C\/p\u003E\u003Cp\u003E\u2018In 2015, following the publication of the supernovae data, my colleagues and I performed an independent analysis using industry-standard statistical tools. We found that the evidence for cosmic acceleration had a statistical significance of only three standard deviations, or three sigma, away from zero.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EWhat does that mean?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018The sigma of a result tells you how unlikely it is to be a fluke. For astronomers, a three-sigma result may be interesting, but for a particle physicist like me it\u2019s not worth getting out of bed for. I\u2019ve seen many three-sigma results come and go.\u003C\/p\u003E\u003Cp\u003E\u2018Just two years ago, a much more significant four-sigma result at the Large Hadron Collider, the world\u2019s most powerful particle-physics experiment, turned out to be nothing but a fluke. You should not get too excited until you have at least a five-sigma result \u2013 certainly when it concerns fundamental physics.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cblockquote class=\u0022tw-text-center tw-text-blue tw-font-bold tw-text-2xl lg:tw-w-1\/2 tw-border-2 tw-border-blue tw-p-12 tw-my-8 lg:tw-m-12 lg:tw--ml-16 tw-float-left\u0022\u003E\n \u003Cspan class=\u0022tw-text-5xl tw-rotate-180\u0022\u003E\u201c\u003C\/span\u003E\n \u003Cp class=\u0022tw-font-serif tw-italic\u0022\u003E\u2018For astronomers, a three-sigma result may be interesting, but for a particle physicist like me it\u2019s not worth getting out of bed for.\u2019\u003C\/p\u003E\n \u003Cfooter\u003E\n \u003Ccite class=\u0022tw-not-italic tw-font-normal tw-text-sm tw-text-black\u0022\u003EProf. Subir Sarkar, University of Oxford, UK\u003C\/cite\u003E\n \u003C\/footer\u003E\n\u003C\/blockquote\u003E\n\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003ESo you\u2019re saying evidence for dark energy might peter out too, in time?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018Yes, but not just because of uncertainty in supernovae brightness. In our \u003Ca href=\u0022https:\/\/arxiv.org\/abs\/1808.04597\u0022\u003Elatest work\u003C\/a\u003E, published in August this year, we went one step further by accounting for the inhomogeneity of the universe \u2013 that is, the uneven distribution of matter. Because of this, expansion is not quite the same in all directions. In fact, the Milky Way is being pulled in a particular direction at over 600 kilometres per second. What\u2019s more, this flow peculiar to our local region of space extends surprisingly far out, to about a billion light years. It turns out that most of the supernovae studied so far are within this unusual region \u2013 the net result being that the evidence for dark energy, previously three sigma, drops even further to two sigma. Even an astronomer would baulk at saying two sigma is significant.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EDoes all this rule out dark energy?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018It\u2019s a bit like being in a car, and recording other cars accelerating away from you. Well, maybe they\u2019re not accelerating \u2013 maybe you\u2019re decelerating. The point is, we\u2019re not ideal observers. We are moving relative to other objects in the universe, and that biases our measurements.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cfigure role=\u0022group\u0022 class=\u0022@alignleft@\u0022\u003E\n\u003Cimg alt=\u0022The standard model of a universe expanding at an accelerated rate is based on a result that could turn out to be a fluke, according to Prof. Subir Sarkar. Image credit - NASA\/WMAP Science Team\u0022 height=\u00221188\u0022 src=\u0022\/research-and-innovation\/sites\/default\/files\/hm\/IMCEUpload\/2048px-CMB_Timeline300_no_WMAP.jpg\u0022 title=\u0022The standard model of a universe expanding at an accelerated rate is based on a result that could turn out to be a fluke, according to Prof. Subir Sarkar. Image credit - NASA\/WMAP Science Team\u0022 width=\u00221800\u0022\u003E\n\u003Cfigcaption class=\u0022tw-italic tw-mb-4\u0022\u003EThe standard model of a universe expanding at an accelerated rate is based on a result that could turn out to be a fluke, according to Prof. Subir Sarkar. Image credit - NASA\/WMAP Science Team\u003C\/figcaption\u003E\n\u003C\/figure\u003E\n\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EYour view that evidence for dark energy is on shaky ground is very much a minority one within cosmology. Why have you persuaded so few of your colleagues?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018I believe a lot of cosmological results that support the consensus view came about only because the authors knew beforehand which lamppost to look under. In other words, they may be suffering from confirmation bias.\u003C\/p\u003E\u003Cp\u003E\u2018It would be much better if analysis were done blind, so that all of the data goes into a black box, out pops an answer, and if it doesn\u2019t agree with the consensus view, so be it.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EIs there any chance of this happening?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018I\u2019m hopeful, because some of the next generation of experiments will be done by a new generation (of scientists). For example, several of the key people behind the \u003Ca href=\u0022https:\/\/www.lsst.org\/\u0022 target=\u0022_blank\u0022 rel=\u0022noopener noreferrer\u0022\u003ELarge Synoptic Survey Telescope\u003C\/a\u003E under construction in Chile are particle physicists. I\u2019m not saying they are better physicists, but I do believe they have more experience of working with full scrutiny and transparency in large collaborations, and have tried-and-tested ways of avoiding biased results. Also\u0026nbsp;ESO\u2019s (European Southern Observatory) Extremely Large Telescope will do real-time cosmology by measuring over several years the tiny change of red-shifts with time \u2013 thus directly testing if the expansion rate is accelerating.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EIf you are right, and dark energy doesn\u2019t exist, how long will it take cosmologists as a whole to change tack?\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u2018The trouble is that people think our standard model of cosmology is simple and it fits the data. The Ancient Greeks thought the same about Aristotle\u2019s model of the universe, in which the sun and the planets revolve around the Earth. But we need to be open to different possibilities. Let\u2019s just hope that it doesn\u2019t take as long to replace our standard model as it did Aristotle\u2019s \u2013 2,000 years.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cem\u003E\u003Cstrong\u003EThis interview has been edited for length and clarity.\u003C\/strong\u003E\u003C\/em\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cem\u003EProf. Sarkar\u2019s \u003Ca href=\u0022https:\/\/cordis.europa.eu\/project\/rcn\/82448_en.html\u0022 target=\u0022_blank\u0022 rel=\u0022noopener noreferrer\u0022\u003EUniversenet \u003C\/a\u003Eproject, which concluded in 2010, received EU funding. 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