Editor’s note: Below is a press release issued by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory.
The U.S. Department of Energy’s Fermi National Accelerator Laboratory contributed key elements to the Dark Energy Spectroscopic Instrument. Fermilab contributions include the online databases used for data acquisition and the software that will ensure that each of the 5,000 robotic positioners are precisely pointing to their celestial targets to within a tenth of the width of a human hair. Fermilab also contributed the corrector barrel, hexapod and cage. The corrector barrel holds DESI’s six large lenses in precise alignment. The hexapod, designed and built with partners in Italy, focuses the DESI images by moving the barrel-lens system. Both the barrel and hexapod are housed in the cage, which provides the attachment to the telescope structure. In addition, Fermilab carried out the testing and packaging of DESI’s charge-coupled devices, or CCDs. The CCDs convert the light passing through the lenses from distant galaxies into digital information that can then be analyzed by the collaboration. Fermilab scientists continue to participate in data-taking and analysis.
“Back when I started in astronomy, it would have taken years to collect the same number of redshifts that we now obtain in as little as 15 minutes,” said Stephen Kent, Fermilab scientist who is responsible for the software that determines each fiber’s position. “The leap forward in capability provided by DESI is simply amazing.”
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 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.
“There is a lot of beauty to it,” says Berkeley Lab scientist Julien Guy, one of the speakers. “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.”
A new quasar discovered using DESI gives a glimpse of the universe as it was nearly 13 billion years ago, less than a billion years after the Big Bang. This is the most distant quasar discovered with DESI to date, from a DESI very high-redshift quasar selection. The background shows this quasar and its surroundings in the DESI Legacy imaging surveys. (Credit: Jinyi Yang, Steward Observatory/University of Arizona)
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, Ariz. 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. The instrument saw first light in late 2019. Then, during its validation phase, the coronavirus pandemic hit, shutting down the telescope for several months, though some work continued remotely. In December 2020, DESI turned its eyes to the sky again, testing out its hardware and software, and by May 2021 it was ready to start its science survey.
But work on DESI itself didn’t end once the survey started. “It’s constant work that goes on to make this instrument perform,” says physicist Klaus Honscheid of Ohio State University, an Instrument Scientist on the project, who will deliver the first paper of the special AAS DESI session. Honscheid and his team ensure the instrument runs smoothly and automatically, ideally without any input during a night’s observing. “The feedback I get from the night observers is that the shifts are boring, which I take as a compliment.” But 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. “10 microns is tiny,” says Honscheid. “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.”
Seeing Dark Energy’s True Colors
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.
DESI’s three-dimensional “CT scan” of the Universe. The earth is in the lower left, looking out past 5 billion light years in the direction of the constellation Virgo. As the video progresses, the perspective sweeps toward the constellation Bootes. Each colored point represents a galaxy, which in turn is composed of hundreds of billions of stars. Gravity has pulled the galaxies into a “cosmic web” of dense clusters, filaments and voids. (Credit: D. Schlegel/Berkeley Lab using data from DESI)
“Our science goal is to measure the imprint of waves in the primordial plasma,” says Guy. “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: will it expand forever? Will it collapse onto itself again, in a Big Bang in reverse? Or will it rip itself apart? Answering these questions means learning more about how dark energy has behaved in the past — and that’s exactly what DESI is designed to do. And by comparing the expansion history with the growth history, cosmologists can check whether Einstein’s general relativity holds over these immense spans of space and time.
Black Holes and Bright Galaxies
But 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 our understanding of the distant past, more than ten billion years ago when galaxies were still young.
“It’s pretty amazing,” says Ragadeepika Pucha, a graduate student in astronomy at the University of Arizona working on DESI. “DESI will tell us more about the physics of galaxy formation and evolution.”
Pucha and her colleagues are using DESI 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 is still not known. Black holes on their own can be nearly impossible to find — but if they attract enough material, they become easier to spot. When gas, dust, and other material falling into the black hole heats up (to temperatures hotter than the core of a star) on its way in, an active galactic nucleus (AGN) is formed. In large galaxies, AGNs are among the brightest objects in the known universe. But in smaller galaxies, AGNs can be much fainter, and harder to distinguish from newborn stars. The spectra taken by DESI can help solve this problem — and its wide 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. “I like to think of them as lampposts, looking back in time into the history of the universe,” says Victoria Fawcett, an astronomy graduate student at Durham University in the UK. Quasars are excellent probes of the early universe because of their sheer power; DESI’s data will go back in time 11 billion years.
Fawcett and her colleagues are using DESI data to understand the evolution of quasars themselves. It is thought that quasars start out surrounded by an envelope of dust, which reddens the light they give off, like the sun through haze. As they age, they drive off this dust and become bluer. But it has been hard to test this theory, because of the paucity of data on red quasars. DESI is changing that, finding more quasars than any prior survey, with an estimated 2.4 million quasars expected in the final survey data.
Star trails over the Nicholas U. Mayall 4-meter Telescope on Kitt Peak National Observatory near Tucson, Arizona. (Credit: KPNO/NOIRLab/NSF/AURA/P. Marenfeld)
“DESI is really great because it’s picking up much fainter and much redder objects,” says Fawcett. That, she adds, allows scientists to test ideas about quasar evolution that just couldn’t be tested before. And this isn’t just limited to quasars. “We’re finding quite a lot of exotic systems,” Fawcett says, “including large samples of rare objects that we just haven’t been able to study in detail before.”
There’s more to come for DESI. The survey 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, DESI is expected to have over 35 million galaxies in its catalog, enabling an enormous variety of cosmology and astrophysics research.
“All this data is just there, and it’s just waiting to be analyzed,” says Pucha. “And then we will find so much amazing stuff about galaxies. For me, that’s exciting.”
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 by the DESI member institutions.
The DESI collaboration is honored to be permitted to conduct scientific research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.
Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
After the Big Bang, the universe, glowing brightly, was opaque and so hot that atoms could not form. Eventually cooling down to about minus 454 degrees Fahrenheit (-270 degrees Celsius), much of the energy from the Big Bang took the form of light. This afterglow, known as the cosmic microwave background, can now be seen with telescopes at microwave frequencies invisible to human eyes. It has tiny fluctuations in temperature that provide information about the early universe.
Now scientists might have an explanation for the existence of an especially cold region in the afterglow, known as the CMB Cold Spot. Its origin has been a mystery so far but might be attributed to the largest absence of galaxies ever discovered.
Scientists used data collected by the Dark Energy Survey to confirm the existence of one of the largest supervoids known to humanity, the Eridanus supervoid, as reported in a paper published in December 2021. This once-hypothesized but now-confirmed void in the cosmic web might be a possible cause for the anomaly in the CMB.
Observations for the Dark Energy Survey were carried out, using the Blanco Telescope in the Andes mountains of Chile. Scientists used its data to create a map of dark matter in the region of sky that contains the Eridanus supervoid and CMB Cold Spot. Photo: Reidar Hahn, Fermilab
The Eridanus supervoid
The cosmic web is made of clusters and superclusters of galaxies. They are pulled together by the attractive force of gravity and accelerated away from each other by the repulsive force of a mysterious, not-yet-understood phenomenon called dark energy.
Between these clusters of galaxies are voids: vast regions of space that contain fewer galaxies, and thus less ordinary matter, and less dark matter than exists within the galaxy clusters.
Among the largest structures known to humanity, the supervoid in the constellation Eridanus is a massive, elongated, cigar-shaped void in the cosmic web that’s 1.8 billion lightyears wide and has been observed to contain about 30% less matter than the surrounding galactic region. Its center is located 2 billion lightyears from Earth, making it the dominant underdensity of matter in our galactic neighborhood.
Mapping dark matter
To make this discovery, scientists used Dark Energy Survey data to create a map of dark matter in the same direction as the CMB Cold Spot, by observing the effect of gravitational lensing. It’s a phenomenon that occurs when the paths of light are warped by the gravitational influence of dark matter.
The Cold Spot resides in the constellation Eridanus in the southern galactic hemisphere. The inset shows the microwave temperature map of this patch of sky, as mapped by the European Space Agency Planck satellite. The main figure depicts the map of the dark matter distribution created by the Dark Energy Survey team. Image: Gergö Kránicz and András Kovács
“This map of dark matter is the largest ever such map that’s been created,” said Niall Jeffrey, the scientist who worked on the construction of a dark matter map. “We have been able to map out dark matter over a quarter of the Southern Hemisphere.”
Scientists previously counted the number of galaxies visible in the location of the CMB Cold Spot and found an underdensity of galaxies in that region. The new map shows there is a matching underdensity of invisible dark matter.
“It is enough of a new element in the long history of the CMB Cold Spot problem that after this, people will at least be sure that there is a supervoid.” – András Kovács
Using voids to understand dark energy
The imager of the Dark Energy Camera can record tiny amounts of light from distant galaxies and supernova. Photo: Reidar Hahn, Fermilab
The Dark Energy Survey is an international effort to understand the effect dark energy has on the acceleration of the universe. It involves 300 scientists from 25 institutions in seven countries.
The Dark Energy Survey documents hundreds of millions of galaxies, supernovae and patterns within the cosmic web, using a 570-Megapixel digital camera, called the DECam, high in the Chilean Andes. This camera’s construction and integration of components was led by the U.S. Department of Energy’s Fermi National Accelerator Laboratory.
“We were thinking many years ago, a decade and a half at least, how would voids affect the present acceleration of the universe,” said Juan Garcia-Bellido, a cosmologist from IFT-Madrid and co-author of the paper.
At the largest scales of the universe, there is a tug-of-war between the gravitational forces and the expansion of the universe from dark energy, making some of the voids between galactic clusters deeper.
“Photons or particles of light enter into a void at a time before the void starts deepening and leave after the void has become deeper,” said Garcia-Bellido. “This process means that there is a net energy loss in that journey; that’s called the Integrated Sachs-Wolfe effect. When photons fall into a potential well, they gain energy, and when they come out of a potential well, they lose energy. This is the gravitational redshift effect.”
Open questions
Although the new result confirms that the Eridanus supervoid is gigantic, it still is not sufficient to explain the discrepancy between the predictions of the current standard cosmological model used to predict the behavior of dark energy—known as the Lambda Cold Dark Matter model—and the observed change in temperature in the Cold Spot that can be attributed to the supervoid’s effect on photons from the CMB.
“Having the coincidence of these two individually rare structures in the cosmic web and in the CMB is basically not enough to prove causality with the scientific standard,” said András Kovács, the lead researcher on this project.
“It is enough of a new element in the long history of the CMB Cold Spot problem that after this, people will at least be sure that there is a supervoid, which is a good thing because some people have debated that,” said Kovács.
In short, there are two ways to think about this problem: Either the Lambda-CDM model is correct, and the CMB Cold Spot is an extreme anomaly that coincidentally has a massive supervoid in front of it, or the Lambda-CDM model is incorrect, and the Integrated Sachs-Wolfe effect is stronger in supervoids than expected.
The latter would indicate a greater influence of dark energy on the universe and possibly faster cosmic expansion. Interestingly, this possibility is backed up by evidence from other, more distant supervoids. Moreover, the Dark Energy Survey team observed that the lensing signal from the Eridanus supervoid is slightly weaker than expected.
“The trouble is that typical alternative models cannot explain this discrepancy either, so if true, it might mean that we do not understand something very deep about dark energy,” said Kovács.
The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Studies and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science, Technology and Innovation, the German Research Foundation and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration.
Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.