Despite millennia of observations devoted to the cosmos and a well-established Standard Model of particle physics that classifies all the known elementary particles and explains with remarkable precision three of the fundamental forces that relates them, the composition of the Universe has been resisting scientists for almost a century. The model that best describes cosmological observations requires that only 5% of the Universe be in the form of ordinary matter, 25% in the form of dark matter of unknown nature, and 70% in a form dubbed \u2018dark energy\u2019 that behaves like nothing else and causes the unexpected \u2013 and otherwise unexplainable \u2013 acceleration of the expansion rate of the Universe. No wonder, then, that understanding the dark Universe has become one of the top-priority challenges that cosmologists face today! Physicists at the French Alternative Energies<\/a> and Atomic Energy Commission (CEA) are playing a major role in this quest.<\/p>\n
Among the various forms that dark matter could take, one of particular interest is neutrinos. In the standard theory, there are three neutrinos whose masses are known to be non-zero but so small that no experiment has yet been able to measure them.<\/p>\n
It has taken billions of years for the hydrogen in the Universe<\/a> to spread out and then to coalesce onto gravitational potential wells and form the structures we can observe today: filaments, galaxies, and clusters of galaxies, from the sparsest to the clumpiest. If most of the dark matter in the Universe is typically as massive as the known elementary particles, then it is responsible for the primordial potential wells. Dominating the mass budget of the Universe, the gravitational behaviour of dark matter rules, and any seed of dark matter over-density in the early Universe will collapse and form the cosmic web, i.e. the clumpy structure of today\u2019s Universe.<\/p>\n
The story does not end there. With its grid of numerical simulations, the CEA group can also address extensions to the Standard Model<\/a>, focusing in particular on hypothetical particles called sterile neutrinos. Although heavier and less abundant than standard neutrinos, these particles are still sufficiently light to smooth out the smaller structures in a similar manner. Trying to explain what the bulk of the dark matter could consist of, the group could exclude various theories of sterile neutrinos. To go beyond what has been achieved so far requires not only more refined simulations, to better take into account the details of what occurs at the same time on large cosmological distances as well as on galactic scales, but also new data that will provide the physicists with a more accurate map of the cosmic web.<\/p>\n
To further improve our understanding of the dark Universe, new-generation experiments are required. Among these, DESI is one of the first to start its science survey. DESI is a spectroscopic instrument located at Kitt Peak observatory<\/a>, Arizona, USA. It possesses a focal plane equipped with 5,000 robotically-positioned fibres that capture the light of distant galaxies. Planned to run for five years, DESI will measure the spectra of 35 million galaxies, ten times more than its predecessor SDSS, with which it will build the largest and most accurate three-dimensional map of the Universe.<\/p>\n
Nathalie Palanque-Delabrouille<\/strong>
\nCo-Spokesperson of DESI<\/strong>
\nnathalie.palanque-delabrouille@cea.fr<\/a> <\/strong>
\nTweet @CEAIrfu<\/a><\/strong>
\nhttp:\/\/irfu.cea.fr<\/a><\/strong><\/p>\n
Please note, this article will also appear in the third edition of our <\/strong><\/em>new quarterly publication<\/em><\/strong><\/a>. <\/strong><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"