After five years of mapping the sky in 3D — an area that stretches from Earth’s front porch to about 11 billion light-years away — researchers at the Dark Energy Spectroscopic Instrument (DESI) project are pausing for a moment of appreciation from their peers.
In January, DESI — which includes researchers from 70 institutions across the world, including Yale — will receive the American Astronomical Society’s 2026 Lancelot M. Berkeley Prize for meritorious work in astronomy. The prize honors not only DESI’s efforts at creating the largest 3D map of the universe, but also its goal of learning more about dark energy.
Dark energy is an unseen, theorized energy that physicists say may account for three-quarters of the mass-energy content in the universe. Dark energy does not emit light or any other radiation that we can observe, and any information about its nature comes from indirect methods, such as surveying the distribution of far-off galaxies in three dimensions.
This is where DESI comes in. Using the 4-meter Mayall Telescope at Kitt Peak Observatory in Arizona, DESI captures light from 5,000 galaxies simultaneously and has mapped the locations of more than 30 million galaxies and quasars across one-third of the sky.
The U.S. Department of Energy’s Lawrence Berkeley National Laboratory manages DESI, which includes 750 researchers.
“DESI has worked wonderfully well so far,” said Charles Baltay, the Eugene Higgins Professor Emeritus of Physics and Astronomy in Yale’s Faculty of Arts and Sciences and a co-founding researcher at DESI. “We’re even slightly ahead of schedule.”
Charles Baltay
Others who have been part of the DESI team include Yale’s Nikhil Padmanabhan, associate professor of physics and of astronomy in FAS; David Rabinowitz, who recently retired as a senior research scientist in physics; and Xinyi Chen, ’24 Ph.D., who is now at Ohio State University, have also been part of the DESI team. Baltay and Rabinowitz are also members of Yale’s Wright Lab.
In an interview, Baltay places DESI in context with the groundbreaking science of earlier eras and discusses Yale’s contributions to the experiment. The interview has been edited and condensed.
What scientific question is at the heart of DESI?
Charles Baltay: The most important science question there is — what is the universe made of?
The universe has much, much more matter than what we’re aware of. Everything we know — people, planets, particles, every bit of knowledge, every book in the Library of Congress — amounts to maybe a lousy 4% of the matter in the universe. The other 96% we know nothing about, and that’s how it stands even now.
What could be more exciting than finding out what the universe is made of?
How does a 3D map of galaxies help with that?
Baltay: As a part of the mapping process, DESI looks for the imprinted signs of intense heat [called baryon acoustic oscillations] that filled the universe for the first 400,000 years after the Big Bang. Our oscillation measurements can be used to study the distribution of galaxies and the way they cluster, and test it against Lambda CDM, the leading physics model of the universe.
The Dark Energy Spectroscopic Instrument has made the largest 3D map of the universe to date. Fly through millions of galaxies mapped using coordinate data from DESI. (Credit: DESI collaboration and Fiske Planetarium, CU Boulder)
What has DESI found so far?
Baltay: It has added to the idea that we need new physics to explain our observations.
More than 20 years ago, my friend Saul Perlmutter [Baltay’s longtime collaborator, and a 2011 Nobel laureate] helped to show that the expansion of the universe isn’t slowing down, as our present understanding of physics tells us it should. No — the expansion of the universe is accelerating.
This implies dark energy must be some form of repulsive energy, pushing the universe apart. And as we analyze all five years of DESI’s data, we are waiting to see if it suggests that dark energy may not be constant with time. There is an indication from the first year of data that it may be changing as time goes along, and not Einstein’s cosmological constant.
How consequential would that discovery be, historically?
Baltay: It’s a huge fork in the road that would direct research for the next few decades.
In a sense, it’s similar to 100 years ago, when we thought physics was established, even boring. And then the discovery of atoms came along and atoms didn’t do what physics at the time said they would do, leading to the discovery of quantum mechanics and the theory of relativity.
How has Yale contributed to DESI’s work?
Baltay: We designed, built, and installed DESI’s Fiberview Camera, which is essential to the whole experiment.
The function of this camera is to align the 5,000 optical fibers on the DESI focal plane with the target galaxies, with the precision required to take the desired spectra. The spectrum measurements are used to determine distance for objects millions of light-years away to obtain a three-dimensional distribution of 30 million galaxies.
It took three to four years to perfect, but without it the experiment doesn’t work.
The DESI Fiberview camera system
How personally fulfilling is it to see DESI’s success and what it portends for a better understanding of dark energy?
Baltay: You work for years, hoping you’ll help discover something exciting or significant — and this could be a landmark experiment.
Long ago, Aristotle looked at the sky and said the universe was static. The stars didn’t move. And then, much later, Einstein realized that general relativity is inconsistent with this. He came up with the idea of a cosmological constant, a repulsive gravity.
But then in 1929, Hubbell came along with his discovery of the expanding universe, and the implication that expansion would slow down over time. That idea prevailed until 1999, when Perlmutter returned us to the cosmological constant idea.
And now we have DESI. If the full DESI data set confirms the indication that dark energy is changing with time, it will show that dark energy is different from Einstein’s cosmological constant. It’s an exciting time.
