DESI maps star trails
DESI, seen here beneath star trails, aims to create a 3D map of the universe.
Dark energy, a mysterious force that drives the accelerating expansion of the universe, appears to be weakening, according to new astronomical data. If confirmed, the findings could upend our entire understanding of the cosmos.
Scientists have long assumed that dark energy—which makes up nearly 70 percent of the universe—was a constant. But last year, researchers using the Dark Energy Spectroscopic Instrument (DESI) at the Kitt Peak National Observatory in Arizona found the first hints that the force might be fluctuating over time. Their new analysis, presented this week at a meeting of the American Physical Society, bolsters those claims.
“It’s a bit more than a hint now,” says Michael Levi, the director of DESI, to Katrina Miller and Dennis Overbye at the New York Times. “It puts us in conflict with other measurements. Unless dark energy evolves—then, boy, all the ducks line up in a row.”
“We are much more certain than last year that this is definitely a thing,” adds Seshadri Nadathur, a cosmologist and member of the DESI collaboration, to Charlie Wood at Quanta Magazine.
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The research team analyzed data from the first three years of DESI’s observations, which included 15 million galaxies spanning around 11 billion years of space history. Those observations—when combined with other astrophysical data, like certain exploding stars, the afterglow of the Big Bang and the bending of light via gravitational lensing—suggest that dark energy was stronger in the past.
DESI measures the expansion of the cosmos by studying subtle patterns in the early universe known as baryon acoustic oscillations. The size of these wrinkles is directly affected by the universe’s expansion, so they can act like rulers looking into the past.
“The new findings, both from DESI and from a number of other experiments, now suggest that whatever is causing the universal expansion may be decaying—that is, decreasing in strength,” explains Arjun Dey, a project scientist for DESI, to Will Dunham at Reuters.
“This once again changes our fundamental understanding of nature, and in particular our understanding of the future of our universe,” he adds. “Will the expansion continue forever, or will the acceleration slow, stop and turn into a deceleration?”
Our understanding of how the universe is growing also informs our understanding of how it’ll end. If the universe is constantly expanding, it’ll eventually grow so large that everything will become cold, isolated and desolate. Scientists call that scenario the “big freeze.” Alternatively, dark energy could accelerate enough to tear matter and space-time apart in a “big rip.” But if dark energy weakens and the universe’s expansion slows and reverses, the cosmos will start to close in on itself instead—and eventually collapse. That’s the “big crunch.”
Not everyone is convinced by the new findings, however. They don’t yet meet the threshold for statistical significance known as “five sigma,” which physicists use to confirm discoveries—they currently sit at no higher than 4.2 sigma. According to the New York Times, that translates to a one-in-50,000 chance that the results are a fluke.
“My take-home from this analysis is that the … measurements do not yet provide decisive evidence for evolving dark energy,” says George Efstathiou, an astrophysicist at the University of Cambridge in England who was not involved in the research, to Hannah Devlin at the Guardian. “They may do as DESI accumulates more data.”
Mustapha Ishak-Boushaki, one of the DESI researchers, tells Karmela Padavic-Callaghan at New Scientist that he believes the results will reach five-sigma in the next two years as more data is captured.
“As far as theoretical models, Pandora’s box just opened. We were stuck with a cosmological constant,” he adds. “We are not stuck anymore.”
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Sara Hashemi | READ MORE
Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.