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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 today 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’s 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.

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The Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory in Arizona has produced the largest 3D map of our universe to date.  Marilyn Sargent/The Regents of the University of California, Lawrence Berkeley National Laboratory

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