Adapted from a release by Bill Schulz, Lawrence Berkeley National Laboratory.

A five-year quest to map the universe and unravel the mysteries of dark energy began officially on May 17, 2021, at Kitt Peak National Observatory near Tucson, Arizona. The Dark Energy Spectroscopic Instrument (DESI) will capture and study the light from more than 30 million galaxies and other distant objects, allowing scientists to construct a 3-D map of the universe with unprecedented detail.

DESI is an international science collaboration that includes physicists from the University of Utah and is managed by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) with primary funding from DOE’s Office of Science.

Spectra collected by DESI are the components of light corresponding to the colors of the rainbow. As the universe expands, galaxies move away from each other and their light is shifted to longer, redder wavelengths. The more distant the galaxy, the greater its “redshift.” The project will collect and model the data for tens of millions of spectra, which requires training the automated software using a combination of deep observations and visual inspection. Angela Berti, a postdoctoral researcher at the U, is heavily involved in the effort.

“I’ve been doing visual inspection—measuring some redshifts by hand the old-fashioned way—to assess the algorithms we’ll use to measure millions more redshifts automatically. Understanding which spectral features our modeling techniques don’t handle well helps us improve that modeling overall,” said Berti. “You’re never going to have an analysis that gets every single redshift perfectly. But you can optimize and figure out how to reduce your error as much as possible.”

Berti is also working on tools that will identify how the many different types of galaxies congregate together in space.

Allyson Brodzeller, a doctoral student at the U, has been developing new techniques to model data for quasars, bright, distant objects powered by black holes billions of times more massive than the sun. She is using techniques in machine learning to group quasars together by similar features.

“This allows us to classify the objects by their properties in the data and to identify quasar spectra that don’t perfectly fit into the groups,” Brodzeller said. “Establishing these classifications will allow us to get better estimates of their redshift and properties.”

The detailed distribution of galaxies in the map is expected to yield new insights on the influence and nature of dark energy.

“Dark energy is one of the key science drivers for DESI,” said project co-spokesperson Kyle Dawson, a professor of physics and astronomy at U. “The goal is not so much to find out how much there is—we know that about 70% of the energy in the universe today is dark energy—but to study its properties.”

“The universe is expanding at a rate determined by its total energy contents,” Dawson explained. “As the DESI instrument looks out in space and time, we can literally take snapshots today, yesterday, 1 billion years ago, 2 billion years ago as far back in time as possible. We can then figure out the energy content in these snapshots—and see how it is evolving.”

The formal start of DESI’s five-year survey follows a four-month trial run of its custom instrumentation that captured more than four million galaxy spectra—more than the combined output of all previous spectroscopic surveys.

The DESI instrument resides at the retrofitted Mayall Telescope at the National Science Foundation’s Kitt Peak National Observatory. The instrument includes new optics that increase the field of view of the telescope and includes 5,000 robotically controlled optical fibers to gather spectroscopic data from an equal number of objects in the telescope’s field of view.

“We’re not using the biggest telescopes,” said Berkeley Lab’s David Schlegel, who is a DESI project scientist.  “It’s that the instruments are better and very highly multiplexed, meaning that we can capture the light from many different objects at once.”

“DESI is the most ambitious of a new generation of instruments aimed at better understanding the cosmos—in particular, its dark energy component,” said project co-spokesperson Nathalie Palanque-Delabrouille, a cosmologist at France’s Alternative Energies and Atomic Energy Commission. She said the scientific program—including her own interest in quasars—will allow researchers to address with precision two primary questions: What is dark energy? And, to which degree does gravity follow the laws of general relativity (which form the basis of our understanding of the cosmos)?

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 authors are honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

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 energy.gov/science.