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 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 co-founded 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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