The Mysterious ‘Dark’ Energy That Permeates the Universe Is Slowly Eroding

Physicists call the dark energy that drives the universe “the cosmological constant.” Now the largest map of the cosmos to date hints that this mysterious energy has been changing over billions of years.

Physicists have deduced subtle hints that the mysterious “dark” energy that drives the universe to expand faster and faster may be slightly weakening with time. It’s a finding that has the potential to shake the foundations of physics.

“If true, it would be the first real clue we have gotten about the nature of dark energy in 25 years,” said Adam Riess, an astrophysicist at Johns Hopkins University who won the Nobel Prize for co-discovering dark energy in 1998.

The new observations come from the Dark Energy Spectroscopic Instrument (DESI) team, which unveiled a map of the cosmos of unprecedented scope, along with a bonanza of measurements derived from the map. To many researchers, the highlight is a plot showing that three different combinations of observations all insinuate that the influence of dark energy may have eroded over the eons.

“It’s possible we’re seeing hints of dark energy evolving,” said Dillon Brout of Boston University, a member of the DESI team.

Researchers inside and outside of the collaboration all stress that the evidence is not strong enough to claim a discovery. The observations favor the erosion of dark energy with the sort of middling statistical significance that could easily vanish with additional data. But researchers also note that three distinct sets of observations all point in the same intriguing direction, one that’s at odds with the standard picture of dark energy as the intrinsic energy of the vacuum of space—the quantity that Albert Einstein dubbed the “cosmological constant” due to its unvarying nature.

“It’s exciting,” said Sesh Nadathur, a cosmologist at the University of Portsmouth who worked on the DESI analysis. “If dark energy is not a cosmological constant, that’s going to be a huge discovery.”

Rise of the Cosmological Constant

In 1998, Riess’ group, along with another team of astronomers led by Saul Perlmutter, used the light of dozens of distant, dying stars called supernovas to illuminate the structure of the cosmos. They discovered that the expansion of the universe is growing faster as it ages.

According to Einstein’s general theory of relativity, any matter or energy can drive cosmic expansion. But as space expands, all the familiar kinds of matter and energy become less dense as they spread out in a roomier universe. As their densities fall, the expansion of the universe should slow down, not speed up.

One substance that does not become diluted with the expansion of space, however, is space itself. If the vacuum has an energy of its own, then as more vacuum (and therefore more energy) is created, the expansion will speed up, just as Riess’ and Perlmutter’s teams observed. Their discovery of the accelerating expansion of the universe revealed the presence of a tiny amount of energy associated with the vacuum of space—dark energy.

Conveniently, Einstein had considered such a possibility while developing general relativity. To stop the dilution of matter from collapsing the universe, he imagined that all of space might be infused with a fixed amount of extra energy, represented by the symbol Λ, called lambda, and referred to as the cosmological constant. Einstein’s intuition turned out to be off, as the universe isn’t balanced in the way he imagined. But after the 1998 discovery that space seems to be pushing everything outward, his cosmological constant returned and took its place at the heart of the current standard model of cosmology, a set of intertwined ingredients named the “Lambda CDM model.”

“It’s simple. It’s one number. It has some story you can attach to it. That’s why it’s believed to be constant,” said Licia Verde, a theoretical cosmologist and member of the DESI collaboration.

Now a new generation of cosmologists wielding a new generation of telescopes may be picking up the first whispers of a richer story.

Mapping the Heavens

One of those telescopes sits on Kitt Peak in Arizona. The DESI team has outfitted the telescope’s four-meter mirror with 5,000 robotic fibers that automatically swivel toward their celestial targets. The automation enables lightning-fast data collection as compared with the previous flagship galaxy survey, the Sloan Digital Sky Survey (SDSS), which relied on similar fibers that had to be plugged in to patterned metal plates by hand. On one recent record-setting night, DESI was able to record the locations of nearly 200,000 galaxies.

From May 2021 to June 2022, the robotic fibers slurped up photons arriving at Earth from different eras of cosmic history. The DESI researchers have since transformed that data into the most detailed cosmic map ever made. It features the precise locations of about 6 million galaxies as they existed between roughly 2 and 12 billion years ago (out of the universe’s 13.8 billion–year history). “DESI is a really great experiment producing stupendous data,” said Riess.

Earth is at the center of this thin slice of DESI’s 3D map of the universe. Magnification reveals the distribution

of galaxies and voids.

Courtesy of Claire Lamman/DESI collaboration; custom colormap package by cmastro

The secret to DESI’s precision mapping is its ability to collect spectra of galaxies—data-rich plots recording the intensity of each hue of light. A spectrum reveals how quickly a galaxy is moving away from us and therefore which era of cosmic history we’re seeing it in (the faster a galaxy is receding, the older it is). That lets you situate the galaxies relative to each other, but to calibrate the map with the correct distances relative to Earth—essential information for a full reconstruction of cosmic history—you need something else.

For the DESI collaboration, that something was a patchwork of frozen density ripples left behind from the early universe. For the first couple hundred thousand years after the Big Bang, the cosmos was a hot, thick soup of mostly matter and light. Gravity pulled the matter inward while light pushed it outward, and the struggle set off density ripples spreading outward from a smattering of initial dense spots in the soup. After the universe cooled and atoms formed, it became transparent. Light streamed outward, leaving the ripples—called baryonic acoustic oscillations (BAOs)—frozen in place.

The end result was a series of overlapping spheres with slightly denser shells measuring roughly a billion light-years across—the distance BAOs had time to travel before freezing. Those dense shells went on to form slightly more galaxies than other locations did, and when DESI researchers map millions of galaxies, they can detect traces of these spheres. Closer spheres appear bigger than distant ones, but since DESI researchers know the spheres are all the same size, they can tell how far away from Earth the galaxies really are and resize the map accordingly.

To avoid unconsciously influencing their results, the researchers conducted a “blind” analysis, working with measurements that had been randomly shuffled around to obscure any physical patterns. Then the collaboration met in Hawaii last December to unscramble the results and see what sort of map the Kitt Peak robotic fibers had observed.

Nadathur, who was watching live over Zoom from his home in the United Kingdom, felt a thrill when the map was revealed, because it seemed a bit strange. “If you had enough experience with BAO data, you could see that something was going to be needed that was a bit different from the standard model,” Nadathur said. “I knew that Lambda CDM wasn’t quite the whole picture.”

Over the following week, as the researchers combed through the new data set, analyzing it and blending it with other large cosmological data sets, they discovered the source of the oddness and exchanged a flurry of Slack messages.

“One of my colleagues posted a plot showing this dark energy constraint and didn’t write any words. Just the plot and an exploding head emoji,” Nadathur said.

Data for Days

DESI aims to pin down how the universe has expanded over time by observing different types of galaxies as they appeared during seven epochs of cosmological history. They then see how well these seven snapshots line up with the evolution predicted by Lambda CDM. They also consider how well other theories do—such as theories that allow dark energy to vary between snapshots.

With the first year of DESI data alone, Lambda CDM fits the snapshots nearly as well as a variable dark matter model. It’s only when the collaboration combines the DESI map with other snapshots—light known as the cosmic microwave background and a series of three recent supernova maps—that the two theories start to drift apart.

They found that the results varied from the prediction of Lambda CDM by 2.5, 3.5, or 3.9 “sigmas,” depending on which of the three supernova catalogs they included. Imagine flipping a coin 100 times. The prediction for a fair coin is 50 heads and 50 tails. If you get 60 heads, that’s two sigma away from the mean; the odds of it happening by chance (as opposed to the coin being rigged) are 1 in 20. If you get 75 heads—which has a 1-in-2,000,000 chance of happening randomly—that’s a five-sigma result, the gold standard for claiming a discovery in physics. The sigma values DESI obtained fall somewhere in between; they could be rare statistical fluctuations or real evidence that dark energy is changing.

The DESI instrument is equipped with thousands of robotic

fibers to dramatically speed up data collection.

Courtesy of Marilyn Sargent/The Regents of the University

of California, Lawrence Berkeley National Laboratory

While researchers find these numbers tantalizing, they also warn against reading too much into the higher values. The universe is a lot more complicated than a coin, and the statistical significances depend on subtle assumptions in the data analysis.

A stronger reason for enthusiasm is the fact that all three supernova catalogs—which span somewhat independent populations of supernovas—hint that dark energy is varying in the same way: Its power is waning, or as cosmologists say, “thawing.” “When we swap out all of these complementary data sets, they all tend to converge on this slightly negative number,” Brout said. If the discrepancy were random, the data sets would be more likely to point in different directions.

Joshua Frieman, a cosmologist at the University of Chicago and a member of the DESI collaboration who didn’t work on the data analysis, said he would be glad to see Lambda CDM fall. As a theorist, he proposed theories of thawing dark energy in the 1990s, and he more recently cofounded the Dark Energy Survey—a project that searched for deviations from the standard model from 2013 to 2019 and created one of the three supernova catalogs DESI used. But he also remembers being burnt by disappearing cosmological anomalies in the past. “My reaction to this is to be intrigued,” but “until the errors get smaller, I’m not going to write my [Nobel] acceptance speech,” Frieman joked.

“Statistically speaking, it could disappear,” Brout said of the discrepancy with the Lambda CDM model. “We are now going all out to find out if it will.”

After wrapping up their third year of observations earlier this week, the DESI researchers expect that their next map will contain nearly twice as many galaxies as the map unveiled today. And now that they have more experience doing the BAO analysis, they plan to get the updated three-year map out quickly. Next comes a five-year map of 40 million galaxies.

Beyond DESI, a slew of new instruments are coming online in the coming years, including the 8.4-meter Vera Rubin Observatory in Chile, NASA’s Nancy Grace Roman Space Telescope, and the European Space Agency’s Euclid mission.

“Our data in cosmology has made enormous leaps over the last 25 years, and it’s about to make bigger leaps,” Frieman said.

As they amass new observations, researchers may continue to find that dark energy appears as constant as it has for a generation. Or, if the trend continues in the direction suggested by DESI’s results, it could change everything.

New Physics

If dark energy is weakening, it can’t be a cosmological constant. Instead, it may be the same sort of field that many cosmologists think sparked a moment of exponential expansion during the universe’s birth. This kind of “scalar field” could fill space with an amount of energy that looks constant at first—like the cosmological constant—but eventually starts to slip over time.

“The idea that dark energy is varying is very natural,” said Paul Steinhardt, a cosmologist at Princeton University. Otherwise, he continued, “it would be the only form of energy we know which is absolutely constant in space and time.”

But that variability would bring about a profound paradigm shift: We would not be living in a vacuum, which is defined as the lowest-energy state of the universe. Instead, we would inhabit an energized state that’s slowly sliding toward a true vacuum. “We’re used to thinking that we’re living in the vacuum,” Steinhardt said, “but no one promised you that.”

The fate of the cosmos would depend on how quickly the number previously known as the cosmological constant declines, and how far it might go. If it reaches zero, cosmic acceleration would stop. If it dips far enough below zero, the expansion of space would turn to a slow contraction—the sort of reversal required for cyclic theories of cosmology, such as those developed by Steinhardt.

String theorists share a similar outlook. With their proposal that everything boils down to the vibration of strings, they can weave together universes with different numbers of dimensions and all manner of exotic particles and forces. But they can’t easily construct a universe that permanently maintains a stable positive energy, as our universe has seemed to. Instead, in string theory, the energy must either gently fall over the course of billions of years or violently drop to zero or a negative value. “Essentially, all string theorists believe that it’s one or the other. We do not know which one,” said Cumrun Vafa of Harvard University.

Observational evidence for a gradual decline of dark energy would be a boon for the gentle-fall scenario. “That would be amazing. It would be the most important discovery since the discovery of dark energy itself,” Vafa said.

But for now, any such speculations are rooted in the DESI analysis in only the loosest of ways. Cosmologists will have to observe many millions more galaxies before seriously entertaining thoughts of revolution.

“If this holds up, it could light the way to a new, potentially deeper understanding of the universe,” Riess said. “The next few years should be very revealing.”

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