Change is in the air. New data strengthen a hint that dark energy, long thought to be constant force in the universe, might change over time.
Dark energy explains the observation that the universe’s expansion rate is accelerating. But its origins are unknown. It’s typically expected to have constant density across the billions of years of the universe’s history. So when researchers from the Dark Energy Spectroscopic Instrument, or DESI, reported in 2024 that dark energy might vary over time based on their first year of data, it shook cosmology to its core.
Many scientists expected that the standard picture would prevail with additional data from DESI. But that hasn’t happened. Instead, with three years of DESI data, the preference for a changing, or dynamical dark energy has grown.
“I am shocked,” says cosmologist Eleonora Di Valentino of the University of Sheffield in England, who wasn’t involved with the research. “It means that really there is the possibility of new physics, and that’s very exciting.”
DESI has mapped out the locations of more than 14 million galaxies and quasars, the extremely bright, active cores of distant galaxies. The researchers measured a phenomenon called baryon acoustic oscillations, ring-shaped patterns imprinted on the cosmos in the early universe. The researchers combined their data with other datasets, including catalogs of exploding stars called supernovas and observations of ancient light called the cosmic microwave background.
Together, the data match dynamical dark energy better than the standard picture, by a statistical measure as large as 4.2 sigma, depending on which data are used, the researchers report in a paper published on DESI’s website March 19 and in a talk at the American Physical Society’s Global Physics Summit in Anaheim, Calif. That approaches the benchmark commonly required for a discovery, five sigma.
But the standard cosmological model with constant dark energy, called lambda CDM, is not ruled out. “That lambda CDM, for all of its shortcomings, really works pretty well,” says cosmologist Michael Turner of the University of Chicago, who was not involved with the research. DESI found that constant lambda CDM can explain the data, but a model with dynamical dark energy fit the data better.
The researchers performed a variety of cross-checks, including leaving out the cosmic microwave background or supernova data. Dynamical dark energy still won out.
With the first result, DESI scientists still questioned whether they’d missed some subtle effect that would account for the dark energy surprise. “We were all worried that there was something that had stayed under the rug and not been discovered,” says DESI physicist Nathalie Palanque-Delabrouille of Lawrence Berkeley National Laboratory in California. “But now… we’re much more confident that we’ve explored all possible options, and this result … is really what the data is telling us. So this is very exciting.”
Other experiments will soon weigh in on dark energy. The European Space Agency’s Euclid space telescope, launched in 2023, released early data on March 19, observing 26 million galaxies. “In order to be able to see whether it’s a simple cosmological constant or it’s something more complex, we need Euclid; we need to look at all the cosmic history,” says Euclid astrophysicist Xavier Dupac of ESA. In the future, Euclid scientists will look at how galaxies cluster together in different eras of the universe’s history, using that information to tease out how the cosmos expanded.
The fate of the universe may be at stake. In scientists’ standard picture, the cosmos expands indefinitely. But the extremely distant future may look different in a universe with changing dark energy. “The universe may stop expanding and then recollapse in a ‘Big Crunch,’” says DESI physicist Mustapha Ishak-Boushaki of the University of Texas at Dallas, “something that in the last 25 years we thought was out of consideration.”
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