The Dark Energy Spectrographic Instrument (DESI) has just completed the first seven months of its sky observation mission. It makes it possible to show a three-dimensional map – still unfinished – of the Universe, the largest and most detailed ever created. It is already helping astronomers reveal the secrets of the most powerful light sources in the Universe, and will allow a better understanding of its history later.
This is a 3D map of the cosmos showing clusters, filaments and voids, provided by the Kitt Peak Observatory in the United States. Since May 2021, the Dark Energy Spectrographic Instrument (DESI) has been mapping the sky, installed on the Mayall telescope (4 meters). It should contain more than 35 million galaxies in its catalog by the end of its operation in 2026, allowing scientists to launch numerous researches in cosmology and astrophysics.
It is important to note that 70% of the energy density of the Universe seems to be due to “dark energy” of an unknown nature, which is only known to be accelerating the expansion of the Universe. While gravity should be holding back the expansion of the Universe, the rate of this expansion is actually accelerating more and more, revealing more dark energy. The latter has never been observed directly, although its effects are measured.
3D cartography of the Universe produced by DESI: each colored point represents a galaxy, which itself is made up of hundreds of billions of stars. The Earth is in the lower left, with a view of more than 5 billionlight years towards the constellation Virgo. As the animation progresses, the perspective widens towards the Botes constellation. © D. Schlegel/Berkeley Lab/DESI data.A mission that reveals the past and future of the Universe
The observation mission took a long time to start. Built in 2015, the DESI spectrograph opened its 5000 “eyes” in 2019, even if the health crisis delayed its launch… Fiber optic eyes positioned by robots, with an accuracy of 10 microns. ” It’s less than the thickness of a human hair », said physicist Klaus Honscheid of Ohio State University, co-scientist of the instrument, who will present the first paper of the DESI session. ” And you must position each robot to collect light from galaxies billions of light-years away. “, he adds.
In five years, the spectrograph should help astronomers understand what role dark energy plays in the expansion of the Universe, by studying its past, and what future awaits it. For this, the measurement of the distance of 35 million galaxies and 2.4 million quasars – a variety of particularly bright galaxies – over more than a third of the entire sky is planned. Scientists will be able to measure the variation in the rate of expansion of the Universe with unprecedented precision. This level of precision is needed to collect detailed images of the color spectrum of millions of galaxies.
DESI breaks down the light from each galaxy into its color spectrum and determines how much the light has been redshifted. Indeed, we know that the more the spectrum of the light of a galaxy is shifted towards the red, the further this galaxy is from us. These redshifts allow DESI to see the depth of the sky and, by mapping galaxy clusters and superclusters, to determine its expansion history. Clusters and superclusters carry echoes of their original formation, when they were but ripples in the nascent cosmos.
The study has already cataloged more than 7.5 million galaxies and is adding more than a million per month. In November 2021 alone, DESI cataloged the redshifts of 2.5 million galaxies.
Black holes and bright galaxies
In addition, DESI data is currently being used to understand the behavior of black holes intermediate mass in small galaxies. Indeed, a black hole is by definition very difficult to find, unless it attracts enough matter to form an active galactic nucleus (NGA): gas, dust and other materials.
In large galaxies, NGAs are among the brightest objects, but in small galaxies, NAGs can be much harder to distinguish from nascent stars. DESI will, however, yield more information regarding the nuclei of small galaxies, which will give scientists clues as to how bright NGAs formed in the very early Universe.
The instrument will also provide a better understanding of the evolution of quasars, the most luminous objects in our universe. ” I like to think of them as streetlights looking back into the history of the Universe. said Victoria Fawcett, an astronomy student at Durham University, UK. Quasars, whose evolution has never been tested before, are excellent probes of the early Universe because of their sheer power. Large samples of rare objects may also be studied in the years to come.