Scientists might be mistaken about dark energy, but they’re correct about gravity, according to a new study. Dark energy, the enigmatic force driving the accelerated expansion of the universe, is generally believed to maintain a constant density throughout cosmic history. However, dark energy might actually be diminishing, as researchers from the Dark Energy Spectroscopic Instrument (DESI) collaboration suggest in a series of papers posted on November 19 to the project’s website and arXiv.org. This finding reinforces an earlier report from April by the same team that reached a similar conclusion. Simultaneously, the new analysis—a more comprehensive examination of the same data used in the earlier report—confirms that the DESI data align with general relativity, Albert Einstein’s theory of gravity, with no support for alternative, “modified gravity” theories.
DESI creates a 3-D map of galaxies across the cosmos. The project’s previous analysis concentrated solely on one type of information derived from this map: baryon acoustic oscillations, sound waves from the early universe that left imprints on the cosmos still visible today. The new analysis incorporates data on how galaxies and other structures evolve over cosmic history. “This is the first time we are sensitive to how structure grows with time,” says cosmologist Dragan Huterer of the University of Michigan in Ann Arbor. “This is significant because the growth of structure is well known to be very sensitive to dark energy and modified gravity.”
In both analyses, the researchers detected signs of a variation in dark energy’s equation of state, the relationship between its pressure and density over time. “We are pointing at the same conclusion, and this is … completely reassuring,” says cosmologist Pauline Zarrouk of CNRS and the Laboratoire de Physique Nucléaire et de Hautes Énergies in Paris. Because the two analyses are based on the same data, “if we were not seeing the same [conclusion], that would really be an issue.” (In both cases, the team combined DESI’s data with other cosmological data, including data on the cosmic microwave background, the oldest light in the universe.)
With the first result, DESI researchers were taking a bold stance, says physicist Daniel Scolnic of Duke University. “They’re not backing away from that. A lot of times when there’s some big result in cosmology, it feels like a month later … it’s gone.” But with DESI, “their neck’s still out. I really respect that and appreciate that.” If dark energy is confirmed to vary, it would significantly impact cosmology, challenging the standard cosmological model. That model has been highly successful in describing the cosmos but includes poorly understood components, like dark energy and the likewise unidentified source of mass called dark matter.
In an effort to provide a more satisfying explanation of the cosmos, some scientists are modifying general relativity, which describes gravity as a result of mass warping spacetime. Modified gravity theories could potentially eliminate the need for dark matter or dark energy. But the structure formation observed by DESI was consistent with that predicted by general relativity. And there’s no evidence for modified gravity, although the theories are not fully ruled out. In the new study, a puzzle remains about the masses of neutrinos, lightweight subatomic particles abundant in the cosmos. Like DESI’s first analysis, the new findings indicate that the sum of the masses of the three types of neutrinos is smaller than expected, at least by some accounts. That could suggest that cosmologists have misunderstood something about the nature of the cosmos or about neutrinos themselves.
In 2025, the DESI collaboration plans to release results based on the project’s first three years of data. That will be a true test of how robust the results are, including whether dark energy indeed changes over time. Scolnic envisions the standard cosmological model as a bonfire. While scientists had been enjoying sitting around the warm glow, with the DESI results, sparks have begun to fly. “This is when you tell everyone, ‘Let’s just take a step back from the bonfire, just to be safe. … We’re not throwing water on the whole thing, but definitely just one step back.’”
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