Cosmic map generated by DESI
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Until relatively recently only about two million galaxies had ever been imaged via our astronomical telescopes. That radically increased in April with up to 6 million now numbered in the galactic census. This thanks to the Dark Energy Spectroscopic Instrument (DESI), an Observatory in Kitt Peak, AZ, funded by the U.S. Department of Energy. The 3d map produced by DESI now offers much more elaborate clues to the behavior of dark energy, which I've written about before, e.g.
Regarding dark energy , precision measurements of the cosmic microwave background (CMB), including data from the Wilkinson Microwave Anisotropy Probe (WMAP), have already provided solid evidence for it. The same is true of data from two extensive projects charting the large-scale distribution of galaxies - the Two-Degree Field (2DF) and Sloan Digital Sky Survey (SDSS).
The curves from other data - with corrected apparent magnitude v. redshift (z) - give different combinations of W dark to W matter over the range. However, only one of the graph combinations bests fits the data:
Wdark = 0.65 and Wmatter = 0.35
These correspond to an expansion accelerating for the last 6 million years- with much more dark energy involved (0.65) than ordinary matter.
When the predictions of the different theoretical models are combined with the best measurements of the cosmic microwave background, galaxy clustering and supernova distances, we find that:
0.62 < W dark < 0.76,
where Wdark = r dark/ rc,
The energy density of dark energy is denoted in the numerator while the critical density is the denominator.
By way of update the DESI project has capped off the first seven months of its survey run by smashing through all previous records for three-dimensional galaxy surveys, creating the largest and most detailed map of the universe ever. Yet it’s only about 10% of the way through its five-year mission. Once completed, that phenomenally detailed 3D map will yield a better understanding of dark energy, and thereby give physicists and astronomers a better understanding of the past – and future – of the universe. Meanwhile, the impressive technical performance and literally cosmic achievements of the survey thus far are helping scientists reveal the secrets of the most powerful sources of light in the universe.
DESI is an international science collaboration managed by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) with primary funding for construction and operations from DOE’s Office of Science. DESI scientists are presenting the performance of the instrument, and some early astrophysics results, this week at a Berkeley Lab-hosted webinar called CosmoPalooza
Which will also feature updates from other leading cosmology experiments. In the words of Berkeley Lab scientist Julien Guy, one of the presenters:
“There is a lot of beauty to it. In the distribution of the galaxies in the 3D map, there are huge clusters, filaments, and voids. They’re the biggest structures in the universe. But within them, you find an imprint of the very early universe, and the history of its expansion since then.”
DESI has come a long way to reach this point. Originally proposed over a decade ago, construction on the instrument started in 2015. It was installed at the Nicholas U. Mayall 4-meter telescope at Kitt Peak National Observatory near Tucson, Arizona. Kitt Peak National Observatory is a program of the National Science Foundation’s (NSF) NOIRLab, which the Department of Energy contracts with to operate the Mayall Telescope for the DESI survey.
This unique instrument saw its beginnings in late 2019. Alas, during its validation phase the coronavirus hit shutting down the telescope for several months. Then, by December 2020, DESI turned its eyes to the sky again, testing hardware and software. By May 2021 it was ready to start its science survey.
But work on DESI itself didn’t end once the survey started. In the words of physicist Klaus Honscheid of Ohio State University, co-Instrument Scientist on the project:
“It’s constant work that goes on to make this instrument perform. The feedback I get from the night observers is that the shifts are boring, which I take as a compliment,”
In fact that monotonous productivity requires incredibly detailed control over each of the 5000 cutting-edge robots that position optical fibers on the DESI instrument, ensuring their positions are accurate to within 10 microns. According to Honscheid:
“Ten microns is tiny. It’s less than the thickness of a human hair. And you have to position each robot to collect the light from galaxies billions of light-years away. Every time I think about this system, I wonder how could we possibly pull that off? The success of DESI as an instrument is something to be very proud of.”
That level of accuracy is needed to accomplish the primary task of the survey: collecting detailed color spectrum images of millions of galaxies across more than a third of the entire sky. By breaking down the light from each galaxy into its spectrum of colors, DESI can determine how much the light has been redshifted – stretched out toward the red end of the spectrum by the expansion of the universe during the billions of years it traveled before reaching Earth. It is those redshifts that let DESI see the depth of the sky.
The more redshifted a galaxy’s spectrum is, in general, the farther away it is. With a 3D map of the cosmos in hand, physicists can chart clusters and superclusters of galaxies. Those structures carry echoes of their initial formation, when they were just ripples in the infant cosmos. By teasing out those echoes, physicists can use DESI’s data to determine the expansion history of the universe. In the words of another DESI Collaborator:
“Our science goal is to measure the imprint of waves in the primordial plasma. It’s astounding that we can actually detect the effect of these waves billions of years later, and so soon in our survey.”
Understanding the expansion history is crucial, with nothing less than the fate of the entire universe at stake. Today, about 70% of the content of the universe is dark energy, a mysterious form of energy driving the expansion of the universe ever faster. As the universe expands, more dark energy pops into existence, which speeds up the expansion more, in a cycle that is driving the fraction of dark energy in the universe ever upwards. Dark energy will ultimately determine the destiny of the universe:
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