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| November 11, 2025 | Volume 21 Issue 42 |
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Colossal cosmic U-turn -- Universe's expansion is slowing, not speeding up according to new study
Researchers used type Ia supernovae, similar to SN1994d pictured in its host galaxy NGC4526, to help establish that the universe's expansion may actually have started to slow. [Credit: NASA/ESA, License type: Attribution (CC BY 4.0)/Courtesy of Royal Astronomical Society]
The universe's expansion may actually have started to slow rather than accelerating at an ever-increasing rate as previously thought, a new study suggests.
"Remarkable" findings published Nov. 6, 2025, in Monthly Notices of the Royal Astronomical Society cast doubt on the long-standing theory that a mysterious force known as "dark energy" is driving distant galaxies away increasingly faster.
Instead, they show no evidence of an accelerating universe.
If the results are confirmed, it could open an entirely new chapter in scientists' quest to uncover the true nature of dark energy, resolve the "Hubble tension," and understand the past and future of the universe.
Lead researcher Professor Young-Wook Lee, of Yonsei University in South Korea, said: "Our study shows that the universe has already entered a phase of decelerated expansion at the present epoch, and that dark energy evolves with time much more rapidly than previously thought.
"If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago."
The Hubble residual diagram before (top) and after (bottom) the age-bias correction. Corrections are applied to supernova data from the Dark Energy Survey project. After correction, the dataset no longer supports the ΛCDM model (red line) with a cosmological constant, but instead more closely fits with a time-varying dark energy model favored by a combined analysis using only baryonic acoustic oscillations and cosmic microwave background data (blue line). [Credit: Son et al., License type: Attribution (CC BY 4.0)/Courtesy of Royal Astronomical Society]
For the past three decades, astronomers have widely believed the universe is expanding at an ever-increasing rate, driven by an unseen phenomenon called dark energy that acts as a kind of anti-gravity.
This conclusion, based on distance measurements to faraway galaxies using type Ia supernovae, earned the 2011 Nobel Prize in Physics.
However, a team of astronomers at Yonsei University (Seoul, South Korea) have now put forward new evidence that type Ia supernovae, long regarded as the universe's "standard candles," are, in fact, strongly affected by the age of their progenitor stars (when they blew up) -- so these supernovae are not so "standard" after all with their brightness being a proxy for their distance.
This diagram shows how the universe appears to be in a state of decelerated expansion (red line). The dotted vertical line marks the present epoch, while the black line shows the ΛCDM prediction. The green and red lines represent the new study's model before (green) and after (red) age-bias correction, consistent with baryonic acoustic oscillations and cosmic microwave background data (blue line). [Credit: Son et al., License type: Attribution (CC BY 4.0)/Courtesy of Royal Astronomical Society]
Even after luminosity standardization, supernovae from younger stellar populations appear systematically fainter, while those from older populations appear brighter.
Based on a much larger host-galaxy sample of 300 galaxies, the new study confirmed this effect at extremely high significance (99.999% confidence), suggesting that the dimming of distant supernovae arises not only from cosmological effects but also from stellar astrophysics effects.
When this systematic bias was corrected, the supernova data no longer matched the standard ΛCDM cosmological model with a cosmological constant, researchers said.
Instead, it aligned far better with a new model favored by the Dark Energy Spectroscopic Instrument (DESI) project, derived from baryonic acoustic oscillations (BAO) -- effectively, the sound of the Big Bang -- and cosmic microwave background (CMB) data.
The corrected supernova data and the BAO+CMB-only results both indicate that dark energy weakens and evolves significantly with time.
More importantly, when the corrected supernova data were combined with BAO and CMB results, the standard ΛCDM model was ruled out with overwhelming significance, the researchers said.
Most surprising of all, this combined analysis indicates that the universe is not accelerating today unbridled as previously thought, but has already transitioned into a state of decelerated expansion.
DESI is a state-of-the-art instrument in Arizona that maps distant objects to study dark energy. [Credit: Marilyn Sargent/Berkeley Lab, License type: Attribution (CC BY 4.0)/Courtesy of Royal Astronomical Society]
Professor Lee added: "In the DESI project, the key results were obtained by combining uncorrected supernova data with baryonic acoustic oscillations measurements, leading to the conclusion that while the universe will decelerate in the future, it is still accelerating at present.
"By contrast, our analysis -- which applies the age-bias correction -- shows that the universe has already entered a decelerating phase today. Remarkably, this agrees with what is independently predicted from BAO-only or BAO+CMB analyses, though this fact has received little attention so far."
To further confirm their results, the Yonsei team is now carrying out an "evolution-free test" that uses only supernovae from young, coeval host galaxies across the full redshift range. The first results already support their main conclusion.
"Within the next five years, with the Vera C. Rubin Observatory discovering more than 20,000 new supernova host galaxies, precise age measurements will allow for a far more robust and definitive test of supernova cosmology," said research professor Chul Chung, a co-lead on the study along with PhD candidate Junhyuk Son.
The Vera C. Rubin Observatory, which sits on a mountain in the Chilean Andes, is home to the world's most powerful digital camera. It began scientific operations this year and could answer vital questions about our own solar system and the wider universe.
Front view of the fully assembled LSST Camera in its integration stand at SLAC National Laboratory when being constructed. The LSST Camera is now part of the Vera C. Rubin Observatory in northern Chile. [Credit: Photo courtesy of SLAC National Laboratory]
The LSST Camera sits atop Rubin Observatory's Simonyi Survey Telescope high in the Andes mountains of Chile. [Credit: Rubin Observatory/National Science Foundation/AURA]
After the Big Bang and the rapid expansion of the universe some 13.8 billion years ago, gravity slowed expansion down. In 1998, however, it was established that 9 billion years after the universe began, its expansion had started to speed up again, driven by a mysterious force.
Astronomers dubbed this force "dark energy," but despite it making up about 70% of the universe, it is still considered to be one of the greatest mysteries in science.
Last year, data from DESI in Tucson, AZ, suggested that the force exerted by dark energy had changed over time, and evidence for this has been growing ever since.
The hope is that with these new tools in their arsenal, astronomers will now be better equipped to find clues about what exactly dark energy is and how it influences the universe.
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The new theory is already being hotly contested. According to a recent article in New Scientist, "Adam Riess at the Space Telescope Science Institute in Maryland, one of the recipients of the 2011 Nobel prize in physics, disagrees with that claim." Riess asserts that Lee's team "used a mean stellar age derived from the host galaxy." "The theory behind this is weak because of a lack of certainty about how the [star] forms," says Riess.
We shall see how this plays out.
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Source: Royal Astronomical Society (UK)
Published November 2025
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