A new study challenges a long-held idea that the universe’s expansion is accelerating

Sara Hashemi - Daily Correspondent

November 10, 2025 1:23 p.m.

A remnant of a Type Ia superova. NASA / CXC / U. Texas

Scientists have long held that the universe is expanding at an ever-increasing rate, driven by a mysterious but measurable force known as dark energy. Now, a new study might upend that idea, suggesting the universe’s expansion is actually slowing down—and that dark energy is diminishing, rather than stable.

These findings could have major implications for how we see the cosmos and understand its future. The work relies on observations from the Dark Energy Spectroscopic Instrument (DESI), which is using telescopic images to create a massive 3D map of the universe. Other recent results from the instrument have also implied that dark energy may not be a constant after all—and that it might be weakening.

The new work is line with those findings, says Young-Wook Lee, an astronomer at Yonsei University in South Korea who led the work, in a statement. “Our study shows that the universe has already entered a phase of decelerated expansion … and that dark energy evolves with time much more rapidly than previously thought,” he says.

The findings were published November 6 in the journal Monthly Notices of the Royal Astronomical Society.

Did you know? DESI’s stellar observations

The DESI study has already challenged prevailing assumptions about the universe. Why is the mapping project so subversive? Part of the reason lies in the massive number of observations it includes—a tidal wave of data that could contain plenty of new revelations about the universe and its eventual fate. The project has observed 15 million galaxies and quasars in its first three years, and that number is projected to hit 50 million by the time its observation ends.

To conduct their study, Lee and his team took a closer look at type Ia supernovae (pronounced “type one-A”), which occur when a white dwarf blows up in a binary system that also contains a massive star. These supernovae have long been thought to emit a constant amount of light—a light so reliable it’s known as a “standard candle” and used to measure distances in space. The light from type Ia supernovae has played a key role in suggesting the expansion of the universe is accelerating.

Yet in their new analysis of 300 galaxies, the researchers found that the brightness of these supernovae may be strongly impacted by the age of the star that blew up—implying that these reliable “candles” are more like variable lights.

Even after the team standardized the light emitted by the supernovae, those from younger star populations were fainter, on average, than those from older populations. Nor did the researchers’ results correspond to the current standard cosmological model for universe expansion. Instead, they aligned with a new model implied by the DESI observations—a model that combines data both about the supernova and about its progenitor star.

The analysis, Lee says in the statement, “shows that that the universe has already entered a decelerating phase today.”

The DESI instrument. Marilyn Sargent / Berkeley Lab

That could have major implications for the end of the universe. If dark energy continues to decrease over time, space will eventually contract. That will lead to the “Big Crunch,” a scenario in which the universe collapses back into itself.

Some scientists not involved in the work, however, contest those conclusions. Adam Riess, an astrophysicist at the Space Telescope Science Institute, tells Jonathan O’Callaghan at New Scientist that previous work from the research group, which made similar claims, has been refuted. “The same group’s new work repeats the argument with little change,” he says.

He explains that it’s difficult to measure stellar ages for type Ia supernovae at large distances, and that Lee’s team used an average stellar age for their study. “The theory behind this is weak because of a lack of certainty about how the [star] forms,” he adds.

Carlos Frenk, a cosmologist at Durham University in England, was a bit less skeptical. “It’s definitely interesting. It’s very provocative. It may well be wrong,” he says to Hannah Devlin at the Guardian. Still, “it’s not something that you can dismiss. They’ve put out a paper with tantalizing results with very profound conclusions.”

To further confirm their findings, the research team is conducting what they call an “evolution-free” test, per the statement, only studying supernovae from young galaxies. The Vera C. Rubin Observatory and other new astronomy tools will help with those observations. Will the new work further topple a cosmological model that’s been in place since the 1990s? Only time—and new observations—will tell.

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