Einstein’s gravity persists despite a dark energy puzzle

Scientists may be wrong about dark energy. But they’re right about gravity, a new study suggests.

Dark energy, the mysterious phenomenon causing the accelerating expansion of the cosmos, is widely thought to have had a constant density throughout the history of the universe. But dark energy may be dwindling, researchers from the Dark Energy Spectroscopic Instrument, or DESI, collaboration report Nov. 19 in a batch of papers posted on the project’s website and arXiv.org.

The finding reaffirms an April report by the same team that reached a similar conclusion (SN: 4/4/24). At the same time, the new analysis – a more complete look at the same data used in the previous report – confirms that the DESI data agree with general relativity, Albert Einstein’s theory of gravity, with no evidence for alternative theories, of ” modified gravity”.

DESI makes a 3-D map of galaxies throughout the cosmos. Previous analysis of the project focused on just one type of information gleaned from that map: baryon acoustic oscillations, sound waves in the early universe that left traces in the cosmos that are visible today (SN: 3/4/19).

The new analysis adds information about how galaxies and other structures evolve over cosmic history. “This is the first time we are sensitive to how the structure grows over time,” says cosmologist Dragan Huterer of the University of Michigan in Ann Arbor. “This is significant because structure growth is known to be very sensitive to dark energy and modified gravity.”

In both analyses, the researchers found signs of a change in dark energy’s equation of state, the relationship between pressure and its density, over time. “We are pointing to the same conclusion, and this is … completely reassuring,” says cosmologist Pauline Zarrouk of the CNRS and the Laboratoire de Physique Nucléaire et de Hautes Énergies in Paris. Because both analyzes are based on the same data, “if we wouldn’t see the same thing [conclusion]that would really be a problem.” (In both cases, the team combined the DESI 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 sticking their necks out, says physicist Daniel Scolnic of Duke University. “They are not backing down from this. A lot of times when there’s a big result in cosmology, it feels like a month later … it’s gone.” But with DESI, “their neck is still out. I really respect him and appreciate him.”

If dark energy is confirmed to change, it would send a jolt through cosmology, overturning the accepted theory of scientists, the standard cosmological model. This theory has been extremely successful in describing the cosmos, but it includes poorly understood components such as dark energy and an unidentified source of mass called dark matter (SN: 26.8.24).

In an effort for a more satisfactory explanation of the cosmos, some scientists are changing general relativity, which describes gravity as the result of a massive distortion of space-time. Modified theories of gravity may eliminate the need for dark matter or dark energy (SN: 7/5/24). But the structure formation that DESI observed was consistent with that predicted by general relativity. And there is no evidence for modified gravity, although theories are not completely ruled out.

In the new study, there is a puzzle about the masses of neutrinos, the light subatomic particles that abound in the cosmos. Like the first DESI analysis, the new findings show that the sum of the masses of the three neutrino types is smaller than expected, at least by some accounts (SN: 20.9.24). This may imply 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 data from the first three years of the project. This will be a real test of how robust the results are, including whether dark energy really changes over time.

Scolnic envisions the standard cosmological model as a fire. While scientists have been enjoying sitting around the warm glow, with the DESI results, sparks are starting to fly. “That’s when you say to everybody, ‘Let’s take a step back from the fire, just to be safe. … We’re not throwing water on the whole thing, but definitely just a step back.’”