Is Dark Energy Getting Weaker? New Evidence Strengthens the Case.
Introduction
Last spring, a team of nearly 1,000 cosmologists announced that dark energy — the enigmatic agent propelling the universe to swell in size at an ever-increasing rate — might be slackening. The bombshell result, based on the team’s observations of the motions of millions of galaxies combined with other data, was tentative and preliminary. Today, the scientists report that they have analyzed more than twice as much data as before and that it points more strongly to the same conclusion: Dark energy is losing steam.
“We are much more certain than last year that this is definitely a thing,” said Seshadri Nadathur, a member of the Dark Energy Spectroscopic Instrument (DESI) collaboration, the group behind the new result.
Their finding, presented today at the Global Physics Summit in Anaheim, California, aligns with that of a second group of cosmologists, the 400-strong Dark Energy Survey (DES). Having also observed a huge swath of the cosmos, DES reported evidence of varying dark energy in a paper earlier this month and in a talk today at the Anaheim meeting.
“It’s interesting that things are pushing in this direction and that multiple experiments are seeing some tension” with the idea that dark energy is constant, said Michael Troxel, a member of the DES team based at Duke University.
If the evidence of evolving dark energy holds up as more data accrues — and this is not guaranteed — it would upend cosmologists’ understanding of our ultimate destiny. Dark energy that has a constant density and pressure would doom our cosmos to expand for all time until unbridgeable gulfs separate every particle from all the others, snuffing out all activity. But dark energy that evolves makes alternative futures possible. “It challenges the fate of the universe,” said Mustapha Ishak-Boushaki, a cosmologist at the University of Texas at Dallas and DESI team member. “It’s game-changing.”
Evolving or weakening dark energy would also rewrite our picture of present-day reality. The most straightforward idea is that dark energy is the energy of the vacuum of space itself, which should be an unchanging feature of quantum physics. Evolving dark energy would herald the presence of something extra, some previously undetected ingredient in the fundamental recipe of the cosmos. The missing part could be as simple as a new type of particle, or it could reveal a subtle failure of Einstein’s theory of gravity. It might even lead researchers down a path that ends at a new fundamental theory of physics.
“It sounds like it will be a paradigm shift, something that will change our understanding and the way we are putting all the pieces together,” Ishak-Boushaki said.
Mapping the Cosmos
Astrophysicists first detected the influence of dark energy in the late 1990s. Two teams observed dozens of faraway supernovas and found that the most distant ones had traveled even farther from our Milky Way galaxy than had been expected. Something appeared to be speeding up the expansion of the universe.
Theoretical physicists knew exactly what that something should be: the energy of space itself. In his theory of gravity, Einstein noted a mathematical slot for a “cosmological constant” — energy that has a constant density and pressure everywhere, causing repulsion. As for the source of this energy, physicists knew that quantum fields, the entities responsible for particles like electrons and photons, contribute an energetic sizzle to otherwise-empty space. This energy would be too mild to matter over a few meters, but on a cosmic scale it should add up, sweeping galaxies away from one another faster and faster as more space (and therefore more of this vacuum energy) accumulates. The discovery that the universe’s expansion is indeed accelerating vindicated physicists’ understanding of both quantum fields and gravitation, even as it raised new questions.
The Dark Energy Spectroscopic Instrument (the black cylinder in the photo) is mounted on the Mayall Telescope atop Kitt Peak in Arizona.
Marilyn Sargent, Multimedia Prod
But cosmology has come a long way in recent decades. DESI and DES are both mapping millions of celestial objects, which gives them the resolution needed to determine whether dark energy is really a cosmological constant or subtly changing.
DESI has especially crisp vision. The telescope, which sits atop Kitt Peak in Arizona, is equipped with thousands of swiveling robotic eyes. Since May 2021, these eyes have flitted back and forth night after night, pointing their fiber optic cables at galaxy after galaxy and collecting their light. In its first year of operation, the telescope observed six million galaxies, precisely pinpointing the speeds at which they are moving away from Earth.
Many of the galaxies are so distant that it has taken their light billions of years to reach us. Altogether, their light illuminates the last 11 billion years or so of cosmic history. The DESI cosmologists focused on identifying the way that galaxies subtly cluster into roughly spherical shells of a particular size, remnants of ripples that traveled through the universe when it was much younger and denser. They used these galaxy shells to reconstruct the universe’s expansion in freeze-frame detail.
Last April, the DESI scientists shared the results of their first year of observations. The data yielded signs that dark energy may have been weakening over the last few billion years. Its density did not look constant.
DESI researchers were thrilled, but cautious. They spoke with precision, describing their findings as “hints” rather than “evidence” and carefully adding caveats. Team members emphasized past experiences of seeing anomalies in physics melt away with additional data.
There was “this big question mark: How robust are these new hints?” said Kim Berghaus, a theoretical physicist at the California Institute of Technology who is not part of the DESI team. “At first a lot of people thought that they would go away.”
DESI’s new map includes 15 million galaxies spanning 32 billion light-years of space. (The universe is less than 14 billion years old, but cosmic expansion has stretched it out.)
DESI Collaboration and KPNO
NOIRLab/NSF/AURA/R. Proctor
Then last fall, the collaboration released a more detailed analysis that considered more subtle patterns in the locations of galaxies, beyond the more obvious spherical shells. The hints of changing dark energy persisted. “Everyone breathed a sigh of relief,” said Dillon Brout, a cosmologist at Boston University who is part of both the DESI and DES teams.
The group’s new result is based on three years of stargazing by the robotic eyes on Kitt Peak. With the analysis of this fresh data, the vibe has shifted from relief to elation.
Millions More Galaxies
Where DESI’s first year of data contained six million galaxies, the three-year data set spans nearly 15 million. The team again identified the spherical shells of galaxies and again reconstructed the last 10 billion years of cosmic expansion — this time, with even greater resolution.
For months, they honed their computer code on mock data and a scrambled version of the real data, looking for bugs and verifying that their analysis worked as it should. On the evening of Dec. 10, 2024, representatives of the collaboration met in Cancún, Mexico, and spent three hours debating whether they felt satisfied that they had looked at the analysis from every possible angle.
When they decided they had, a handful of researchers retired to their hotel rooms to unscramble the data and produce the final plots. Among them was Nadathur, a cosmologist at the University of Portsmouth in the U.K. He reveled in the privilege of being one of the first humans on Earth to glean the secrets of the largest and most detailed galaxy map made to date. Two days later, he took the stage before around 200 of his DESI colleagues, with hundreds more team members watching remotely, and shared the results. “It was the best experience of my professional career,” he said.
In isolation, DESI’s 15 million galaxies could match either an evolving dark energy model or the standard theory of cosmology, known as the Lambda-CDM model, which assumes a cosmological constant. (Lambda is the Greek symbol used for Einstein’s cosmological constant, and CDM stands for cold dark matter.) But when DESI researchers also factored in preexisting data on the locations of thousands of supernovas in nearby galaxies, and conditions in the universe’s early days as revealed by remnants of ancient light (called the cosmic microwave background), the combined data sets departed starkly from Lambda-CDM and pointed to an evolution in dark energy.
DESI has some 5,000 robotic eyes that swivel toward distant galaxies, collecting their light one by one.
DESI Collaboration
DESI reported last spring that the combined data sets diverged from what the Lambda-CDM model would predict by as much as 3.9 “sigma,” a measure of statistical significance. Now that figure has ticked up to 4.2 sigma. Assuming that the researchers have not made a mistake, this number means that the odds are only about 1 in 30,000 that Lambda-CDM is the correct model of the cosmos. That’s roughly as likely as flipping a coin 15 times and getting 15 heads in a row.
Further strengthening the results, the tension with Lambda-CDM persists (though at lower levels) even if one deletes the cosmic microwave background data or the supernova data. This indicates that the problem isn’t with any one data set.
“They have to all conspire to be very wrong,” Ishak-Boushaki said, “which I think is very, extremely unlikely.”
The DES team reached a similar conclusion. Over five years, their telescope in the Chilean Andes snapped high-resolution photographs of 12% of the sky, creating the most extensive catalog of supernovas to date and locating the same spherical shells traced out by many millions of galaxies (albeit with less precision than DESI). Combining that data with state-of-the-art observations of the cosmic microwave background, they found a tension with Lambda-CDM of 3.2 sigma that disappears when dark energy is presumed to change. “This paints a nice consistent picture,” Troxel said.
The DES team cautions, however, that statistical analysis in cosmology is a tricky business. Normally, you can do an experiment over and over until you’re convinced your theory is right or wrong. But cosmologists get only one universe to observe. Since that’s the case, tensions between data and theory can arise from assumptions made about the theory itself, as opposed to unexpected patterns in the data. “Like it or not, there is some subjectivity to it,” said Chihway Chang, a cosmologist at the University of Chicago and member of the DES team.
Another challenge in interpreting the new results is that, at least in DESI’s cosmic history, dark energy seems to have gradually intensified for billions of years before starting to slack off about six billion years ago. The intensifying behavior isn’t unthinkable, but theorists find it quite unnatural, calling it “phantom” dark energy.
They’ve already come up with explanations for why the phantom isn’t really there. Perhaps the data from that earlier period isn’t painting an accurate picture. Dark energy’s effect on the universe in its early years was mild because there was so much less space back then; the signal from that era is weaker. Berghaus feels cautiously optimistic that DESI is onto something real, but her hope hinges on dark energy turning out to weaken consistently throughout time. “If it does go into the phantom crossing,” she said, she “would lean more toward a systematic error.”
The Dark Energy Survey spent five years mapping the southern sky using a camera mounted on the Víctor M. Blanco Telescope (center) in Chile.
Reidar Hahn/Fermilab
Varying dark energy would vastly expand the range of possibilities for where the universe is headed. The expansion might grind to a halt, and gravity could bring everything crashing together. Or dark energy could go phantom again, and the expanding universe could even pick up more speed. Everything depends on the details of what’s generating this energy.
“Now a Pandora’s box is open for theories,” Ishak-Boushaki said.
A Clash of Fundamentals
The accumulating evidence for varying dark energy has inspired polar reactions among theoretical physicists.
For Cumrun Vafa, a physicist at Harvard University, the notion of a universe where the total energy in space slowly dissipates is entirely natural. He specializes in string theory, the speculative framework that attempts to explain all matter and forces in terms of vibrating strings of energy. In attempting to build universes out of strings, he says, you can’t construct one where the cubic meters of space maintain a positive energy forever. Eventually, the energy must fall, either abruptly, or slowly over time. “The theory kind of demands that it change,” he said. “The only question is how quickly.” He feels especially excited about the DESI data because it aligns with a slow decline that appears in many models of stringy universes.
Raphael Bousso, a theoretical physicist at the University of California, Berkeley, feels that the claimed variation in dark energy is so improbable it verges on impossible. Just from the behavior of particles we know to exist, the vacuum must have some constant energy, he pointed out; it isn’t optional. So DESI would have to be picking up on the subtle influence of some additional, undiscovered field that acts as a gradually thinning repulsive fluid — one layered on top of, and barely distinguishable from, the constant vacuum energy. He deems it far more likely that some subtle misstep or misfortune has led the DESI team to mismeasure a cosmological constant.
“It walks like a duck. It quacks like a duck. Is it a unicorn in a duck suit?” Bousso said. “For three sigma, even four, I’m not going to lose any sleep.”
While Bousso sleeps soundly, the unblinking robotic eyes on Kitt Peak will continue to stare out across the universe and into the past, catching photons for a third (and final) cosmic map. This one will include 50 million galaxies and could take shape by late 2026 or early 2027. Later this year, the DES collaboration plans to publish observations of how galaxies and matter have clustered over the eons, a process that reflects the tug-of-war between gravity’s pull and dark energy’s push. Their results should further clarify whether dark energy is running out of steam.
Researchers resoundingly agree that it’s a thrilling time to be a cosmologist. Berghaus sees the dark energy anomaly as the latest in a handful of compelling cosmological mysteries. With a new wave of next-generation telescopes coming online over the next decade, it feels to her like it’s morning in cosmology. “We’re having all these tensions now under Lambda-CDM that are popping up,” she said. “So from that perspective, I don’t think it’s the end of the story.”
