Dark Energy Spectroscopic Instrument creates largest 3D map of the cosmos
DESI has already mapped out more galaxies than all previous 3D surveys combined—and it's just getting started
The Dark Energy Spectroscopic Instrument has capped off the first seven months of its survey run by creating the largest and most detailed map of the universe ever.
The instrument has smashed previous records for 3D galaxy surveys, yet it’s only about 10% of the way through its five-year mission.
Once completed, the map will yield a better understanding of dark energy, giving physicists and astronomers a better understanding of the past—and future—of the universe. Meanwhile, the technical performance and achievements of the survey are helping scientists reveal the secrets of the most powerful sources of light in the universe.
DESI scientists will present 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.
University of Michigan physicist and research professor Michael Schubnell, who was key in designing some of DESI’s instrumentation, says the project has collected more and better quality data in these first seven months than prior experiments have in the last decade.
“We are extremely happy with the performance of the instrument. This is not like a project where you work for a particular moment—in which you prepare for 10 years for this one moment or one day,” Schubnell said. “No, our experiment relies on working efficiently and reliably every night. It doesn’t stop. Even if you had a great first week, a great first six months, you have to go on, and you have to have a great five years. We’re getting fantastic data night after night, and we’re breaking our own records from now on.”
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.
“There is a lot of beauty to it,” said Berkeley Lab scientist Julien Guy. “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 NOIRLab, which the Department of Energy contracts with to operate the Mayall Telescope for the DESI survey.
U-M took on a major role in the telescope’s design and construction phase, Schubnell says. Specifically, about 20 U-M students and technicians built DESI’s 5,000 robotic “eyes,” or light-collecting fibers. Every 15 minutes, those fibers are reconfigured using a robotic positioner to point to different galaxies. Each robotic positioner consists of two small motors and a tiny computer. Schubnell developed the control electronics and software that enables communication with the robots.
“Every 15 minutes, we send commands to those 5,000 computers, and they in turn run the 10,000 monitors which move the fibers to new positions,” he said. “The precision with which this is done is amazing—we line up the fibers with galaxies to about 5-10 microns, which is a fraction of the thickness of a human hair.”
A small patch of DESI’s 5000 fiber-optic “eyes” at work. The robotic fiber positioners can be seen moving around their patrol area. The fibers themselves are back-illuminated with blue light so that their positions can be measured with the Fiber View Camera. DESI Positioners In Motion from Berkeley Lab on Vimeo. Image Credit: Claire Poppett/DESI Collaboration.
The fibers themselves are 100 microns in diameter. After the electronic eyes were developed, Schubnell spent a year at the Berkeley Lab, where scientists integrated the robotic positioners with the instrument. In 2019, he made many trips to Kitt Peak to install the instrumentation in the telescope. In late 2019, the instrument first peered out into the universe.
But 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.
Seeing dark energy’s true colors
The primary task of the survey is 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 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.
“Our science goal is to measure the imprint of waves in the primordial plasma,” Guy said. “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.
Eyes toward the future
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 10 billion years ago when galaxies were still young.
“I am thrilled to see DESI reach this milestone,” said U-M physicist Gregory Tarlé, who leads the U-M Dark Energy group and who led the DESI fiber positioner construction project. “DESI is one of the most complex instruments fielded on a telescope. It is always a bit scary to operate such an instrument, hoping that no catastrophic failures occur.
“If we can keep DESI working for the entire five-year survey as well as it has so far, we can expect great science to result. This is why scientists like Michael Schubnell, who got DESI running, keeps it running and who has a deep understanding of all the inner workings, are so indispensable.”
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, Science and Technologies Facilities Council of the United Kingdom, Gordon and Betty Moore Foundation, Heising-Simons Foundation, French Alternative Energies and Atomic Energy Commission, National Council of Science and Technology of Mexico, Ministry of Economy of Spain and by DESI member institutions.
Written by Adam Becker, Lawrence Berkeley National Laboratory