Ohio State University scientists are collaborating on an international effort to produce a 3D map of the universe in unprecedented detail, allowing physicists and astronomers a clearer understanding of dark energy and yielding more insights into the universe’s past and future.
Though it’s only about 10% through its five-year mission, the Dark Energy Spectroscopic Instrument (DESI) 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 map of the universe ever.
The survey’s technical performance and cosmic achievements are also helping scientists reveal the secrets of the most powerful sources of light in the universe. DESI scientists will present on the instrument’s performance and early astrophysics results this week at a Berkeley Lab-hosted webinar, CosmoPalooza, which will also feature updates from other leading cosmology experiments.
“The goal of DESI is to understand one of the greatest mysteries in science today: Why is the expansion of the universe accelerating?” said Paul Martini, former DESI instrument scientist and professor of astronomy at Ohio State. “We cannot explain cosmic acceleration by the known laws of physics, so the answer to this question will be incredibly exciting, no matter what it is.”
DESI is an international science collaboration managed by the Department of Energy’s Lawrence Berkeley National Laboratory with primary funding for construction and operations from DOE’s Office of Science. Housed at Kitt Peak National Observatory in Arizona, DESI’s array of 5,000 fiber-optic robots point at 35 million galaxies throughout the universe to gather and analyze their light to precisely map the galaxies’ distance from Earth.
Ohio State astronomers played a major role in DESI’s development and continue to contribute significantly throughout its operation. During its creation, Martini managed the testing and integration of the various pieces of equipment used to construct DESI while Honscheid designed the software that keeps the instrument operating.
Additionally, researchers and graduate students from Ohio State’s Center for Cosmology and AstroParticle Physics created software, analyzed data, worked on the fiber-optic positioning system and helped design spectrograph components.
Honscheid and his team ensure the instrument runs smoothly and automatically, ideally without any input during a night’s observing.
“It’s constant work that goes on to make this instrument perform,” Honscheid said. “The feedback I get from the night observers is that the shifts are boring, which I take as a compliment.”
That monotonous productivity requires detailed control over each of DESI’s 5,000 robots, ensuring their positions are accurate to within 10 microns.
“Ten microns is tiny,” Honscheid said. “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 survey’s primary task: 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.
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.
“There is a lot of beauty to it,” says Berkeley Lab scientist Julien Guy, a co-project scientist for DESI who also serves as a member of the DESI leadership team.
“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.”
Understanding the expansion history is crucial to learning more about how dark energy will influence the fate of the universe. Today, about 70% of the content of the universe is dark energy, a mysterious form of energy that accelerates the expansion of the universe.
As the universe expands, more dark energy pops into existence, which speeds up the expansion more. This acceleration, researchers believe, is critical in impacting the future of the universe. Finding the answers means learning more about how dark energy’s past behavior – and that’s exactly what DESI is designed to do.
Understanding the fate of the universe will have to wait until DESI has completed more of its survey. In the meantime, DESI is already driving breakthroughs in understanding the distant past, more than 10 billion years ago when galaxies were still young.
DESI scientists are using its data to understand the behavior of intermediate-mass black holes in small galaxies. Enormous black holes are thought to inhabit the cores of nearly every large galaxy, like our own Milky Way. But whether small galaxies always contain their own (smaller) black holes at their cores remains unknown.
When gas, dust and other material falling into the black hole heats up, an active galactic nucleus (AGN) forms. In large galaxies, AGNs are among the brightest objects in the known universe. But in smaller galaxies, AGNs can be much fainter.
The spectra taken by DESI can help solve this problem – and its reach across the sky will yield more information about the cores of small galaxies than ever before. Those cores, in turn, will give scientists clues about how bright AGNs formed in the very early universe.
Quasars – a particularly bright variety of galaxies – are among the brightest and most distant objects known, and DESI data can shed light on their evolution. Scientists theorize that quasars start out surrounded by an envelope of dust, which reddens the light they give off. As they age, they drive off this dust and become bluer. DESI can test this theory by finding more quasars than any prior survey, with an estimated 2.4 million quasars expected in the final survey data.
DESI has already cataloged over 7.5 million galaxies and is adding more at a rate of over a million a month. In November 2021 alone, DESI cataloged redshifts from 2.5 million galaxies. By the end of its run in 2026, it is expected to have over 35 million galaxies in its catalog, enabling an enormous variety of cosmology and astrophysics research.
DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the U.S. National Science Foundation, the Science and Technologies Facilities Council of the United Kingdom, the Gordon and Betty Moore Foundation, the Heising-Simons Foundation, the French Alternative Energies and Atomic Energy Commission (CEA), the National Council of Science and Technology of Mexico, the Ministry of Economy of Spain and the DESI member institutions.
The DESI collaboration has permission to conduct scientific research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.