The dark energy spectroscopic instrument (DESI) has completed the first seven months of its exploration run by shattering all previous records for three-dimensional galaxy studies and creating the largest and most detailed map of the universe ever. Yet it is only about 10% of the way through its five-year mission. When done, the phenomenally detailed 3D map will provide a better understanding of dark energy, thereby giving physicists and astronomers a better understanding of the past – and future – of the universe. Meanwhile, the study’s impressive technical performance and literally cosmic results so far are helping scientists uncover the secrets behind the most powerful light sources in the universe.
DESI is an international scientific collaboration managed by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) with primary funding for construction and operation from the DOE’s Office of Science.
DESI researchers will present the instrument’s performance and some early astrophysical results this week at a Berkeley Lab-hosted webinar called CosmoPalooza, which will also include updates from other leading cosmological experiments.
“There’s a lot of beauty in it,” said Berkeley Lab researcher Julien Guy, one of the speakers. “In the distribution of the galaxies on the 3D map, there are huge clusters, filaments and cavities. They are the largest structures in the universe. But in them you will find an imprint of the very early universe and the history of its expansion since then.”
DESI has come a long way in reaching this point. The instrument was originally proposed more than ten years ago, and construction of the instrument began in 2015. It was installed by the Nicholas U. Mayall 4-meter telescope at the Kitt Peak National Observatory near Tucson, Arizona. The Kitt Peak National Observatory is a program of the National Science Foundations (NSF) NOIRLab that the Department of Energy is contracting to operate the Mayall telescope for the DESI study. The instrument saw the first light at the end of 2019. Then, during its validation phase, the coronavirus pandemic hit and shut down the telescope for several months, though some work continued at a distance. In December 2020, DESI turned its eyes to the sky again and tested its hardware and software, and by May 2021, it was ready to begin its scientific study.
However, work on the DESI itself did not end when the study started. “There is constant work going on to make this instrument work,” said physicist Klaus Honscheid of Ohio State University, co-instrument researcher on the project, who will deliver the first paper from the CosmoPalooza DESI session. Honscheid and his team ensure that the instrument runs smoothly and automatically, ideally without input during a night’s observation. “The feedback I get from the night watchmen is that the guards are boring, which I take as a compliment,” he said.
But the monotonous productivity requires incredibly detailed control over each of the 5,000 groundbreaking robots that place optical fibers on the DESI instrument, ensuring that their positions are accurate to within 10 microns. “Ten microns is small,” Honscheid said. “It’s less than the thickness of a human hair. And you have to place each robot to collect the light from galaxies billions of light-years away. Every time I think of this system, I wonder how we could handle it at all? DESI’s Success as an instrument is something to be very proud of. “
To see the true colors of dark energy
The level of accuracy is needed to perform the primary task of the study: to collect 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 light has been redshifted – stretched toward the red end of the spectrum by the expansion of the universe in the billions of years it traveled before reaching Earth. It is these redshifts that let DESI see the depths 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 map clusters and superheaps of galaxies. These structures carry echoes of their original formation as they were only ripples in the infant’s cosmos. By teasing these echoes, physicists can use DESI’s data to determine the universe’s expansion history.
“Our scientific goal is to measure the imprint of waves in the original plasmasaid Guy. “It’s amazing that we can actually detect the effect of these waves billions of years later, and so quickly in our study.”
Understanding the history of expansion is crucial, with nothing less than the fate of the entire universe at stake. Today, about 70% of the universe’s content is dark energy, a mysterious form of energy that drives the universe’s expansion ever faster. As the universe expands, more dark energy emerges, accelerating the expansion more, in a cycle that drives the fraction of dark energy in the universe ever upward. Dark energy will ultimately determine the fate of the universe: will it expand forever? Will it collapse about itself again, about one Big bang in reverse? Or will it tear 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 history of expansion with the history of growth, cosmologists can check whether Einstein’s general theory of relativity holds over these enormous 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 study. Meanwhile, DESI is already making a breakthrough in our understanding of the distant past, more than 10 billion years ago, when galaxies were still young.
“It’s pretty amazing,” said Ragadeepika Pucha, a graduate student in astronomy at the University of Arizona who works at DESI. “DESI will tell us more about the physics behind galaxy formation and evolution.”
Pucha and her colleagues use DESI data to understand the behavior of black holes with intermediate masses in small galaxies. Huge black holes are thought to inhabit the nuclei of almost all large galaxies, just like our own The Milky Way. But whether small galaxies always contain their own (smaller) black holes in their nuclei is still unknown. Black holes themselves can be almost impossible to find – but if they attract enough material, they will be easier to spot. When gas, dust and other material fall into black hole heated (to temperatures warmer than the star of a star) on its way in, an active galactic core (AGN) is formed. In large galaxies, AGNs are among the brightest objects in the known universe. But in smaller galaxies, AGNs can be much weaker and harder to distinguish from newborn stars. The spectra taken by DESI can help solve this problem – and its wide range across the sky will provide more information about the nuclei of small galaxies than ever before. These nuclei, in turn, will give scientists clues as to how bright AGNs were formed in the very early universe.
Quasars – a particularly bright array of galaxies – are among the brightest and most distant objects known. “I like to think of them as lampposts looking back in time in the history of the universe,” said Victoria Fawcett, an astronomy student at Durham University in the United Kingdom. Quasars are excellent probes in the early universe because of their sheer power; DESI’s data will go back 11 billion years.
Fawcett and her colleagues use DESI data to understand the evolution of quasars themselves. It is believed that quasars start surrounded by a mantle of dust, which blushes the light they emit, just like the sun through haze. As they get older, they drive this dust away and become more blue. However, it has been difficult to test this theory due to lack of data on red quasars. DESI is changing that by finding more quasars than any previous survey, with an estimated 2.4 million quasars expected in the final survey data.
“DESI is really amazing because it picks up much weaker and much redder objects,” Fawcett said. That, she adds, allows scientists to test ideas about quasar evolution that just could not be tested before. And this is not just limited to quasars. “We find quite a few exotic systems, including large specimens of rare objects, that we just have not been able to study in detail before,” Fawcett said.
There is more on the way to DESI. The study has already cataloged over 7.5 million galaxies and adds 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 a huge range of research in cosmology and astrophysics.
“All this data is just there and they’m just waiting to be analyzed,” Pucha said. “And then we will find so many amazing things about galaxies. For me, it’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 US National Science Foundation, Science and Technologies Facilities Council in the United Kingdom, Gordon and Betty Moore Foundation, Heising-Simons Foundation, French Alternative Energies and Atomic Energy Commission (CEA), National Council for Science and Technology in Mexico, the Spanish Ministry of Economy and by the DESI member institutions.
The DESI collaboration is honored to be allowed to conduct scientific research on Iolkam Du’ag (Kitt Peak), a mountain of particular importance to the Tohono O’odham Nation.