Dark energy may be even more complicated than scientists have bargained for, potentially “evolving” over time rather than remaining relentlessly constant in its acceleration of cosmic expansion. That’s according to early results from a project that is developing the largest three-dimensional map of the universe to date. The findings hint that galaxies aren’t spread across the universe as they should be if dark energy’s effects were unchanging.
Even just the possibility of this surprising twist has physicists excited. “If it’s confirmed that we’re actually seeing an evolution of dark energy, this would be a profoundly important discovery,” says Lloyd Knox, a cosmologist at the University of California, Davis, who is not involved in the new research. The seemingly academic matter has big implications for the universe, shaping whether and how it will end.
As of now, the evidence for dark energy’s evolution is intriguing yet not compelling enough for scientists to start rewriting their understanding of the universe. “But it’s a hint, and it’s very interesting, so we’re really looking forward to more data to see where that hint goes,” says Nathalie Palanque-Delabrouille, a cosmologist at Lawrence Berkeley National Laboratory and a spokesperson for the project, called the Dark Energy Spectroscopic Instrument (DESI).
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From its perch on a telescope atop Kitt Peak in Arizona, DESI seeks to revolutionize the study of dark energy, which makes up about 70 percent of the universe. Some 900 scientists are part of the project, and just like all their peers, none of them really knows what dark energy is—mostly because they can’t study it directly. Physicists can only measure dark energy by observing its effects, such as its influence on the spacing of large groups of galaxies.
That’s why DESI, at its heart, is essentially just a mission to map the distribution of matter throughout an immense swath of the observable universe. To do this, the instrument uses a technique called spectroscopy, gathering light from a galaxy, a star or some other cosmic source that is then sorted by wavelength to create a rainbowlike barcode called a spectrum. Inside that spectrum, scientists can discern the signature of the expansion of the universe—which stretches all light traveling through space. This stretching makes light appear redder, with longer wavelengths, which astronomers call redshift. They can use the phenomenon to measure relative distance, with more strongly redshifted light coming from farther out in space and thus deeper back in time.
This technique is enormously powerful for studying individual objects, but DESI can massively multitask, delivering spectra for 5,000 targets at a time. An analysis of the first year of its operations, which began in 2021, includes data on some six million objects, including galaxies, stars and quasars (the extremely luminous cores of active galaxies). “What makes DESI so unique is that it is a machine at measuring redshifts,” Palanque-Delabrouille says.
From all those spectra, the DESI team has constructed what is already the largest 3D map of the universe to date, charting 11 billion years of cosmic history—a valuable resource for even those astronomers with no interest in dark energy. For example, the DESI data can inform our understanding of tiny particles called neutrinos, as well as clusters of galaxies and the great voids that lie between them.
But for researchers focused on dark energy, DESI’s ultradetailed map reveals a subtle, telltale pattern embedded in the universe during its earliest days, when giant sound waves (now called baryon acoustic oscillations) reverberated through the primordial plasma that then suffused the cosmos. These waves left enormous bubblelike echoes in the distribution of matter upon which galaxies subsequently coalesced. Measuring the resulting ringlike galactic patterns, which are always of a characteristic size, allows cosmologists to better calibrate cosmic distances, anchoring DESI’s flood of redshift measurements and enhancing its gauge of dark energy’s activity across the universe’s history.
“DESI is a really great experiment producing stupendous data, and this first release is offering us a great taste of what’s to come,” says Adam Riess, a cosmologist at Johns Hopkins University, who shared the 2011 Nobel Prize in Physics for the discovery of dark energy but is not involved in the project.
Poring over DESI’s first year of observations, its science team opted to use an analytic technique that masked the original data to prevent any subconscious bias from altering the researchers’ work. Only a few months ago, when that masking was lifted, did they realize that the experiment’s initial results held what seemed to be a tantalizing surprise about dark energy itself rather than simple confirmation that the project was working as planned.
Going into the analysis, the scientists expected their results to align with how the universe would behave if dark energy were a constant, static phenomenon—which is indeed what the early results of DESI alone indicated. But when the cosmologists combined those results with data from other key projects investigating dark energy, they found a hint that those collective results better matched what they’d expect to see if dark energy changed, or evolved, over time. Galaxies closer to us in space and time seem, in fact, a bit too close, as if dark energy’s repulsive force had somehow begun to fade in recent cosmic epochs. “We do see an indication that the simplest model might not be the one that describes the universe,” Palanque-Delabrouille says.
Sussing out whether this hint is real rather than simply an artifact of how the data from different projects were combined will require much more information from DESI. Fortunately, the scientists already have another two years of data from the experiment to work with, and the instrument is slated to keep observing until 2026. The complete five years of data will include observations of some 40 million galaxies, Palanque-Delabrouille says.
She and her colleagues are also compiling a proposal for a second phase of the experiment. In addition, two instruments that will begin operations later this decade will be gathering similar data: the Vera C. Rubin Observatory in Chile and NASA’s Nancy Grace Roman Space Telescope. Both are designed to complement DESI in its quest to untangle dark energy, making the coming years particularly tantalizing for cosmologists.
“Finding out the nature of dark energy or what is causing the observed acceleration in the expansion of the universe is my life’s work,” says Yun Wang, a cosmologist at the California Institute of Technology, who is not involved in DESI. “So anything—any new data, any new insight on the observational side—is exciting to me.”
And the stakes are surprisingly high, she adds. “It’s huge—huge and exciting,” Wang says of the mystery of dark energy. “What dark energy is or whatever is causing the observed cosmic acceleration will ultimately determine the fate of the whole universe.”
The results are described in a set of papers posted on the preprint server arXiv.org in early April.