The expansion of the universe is now slowing down, not accelerating, new research suggests.

A new study from Yonsei University in South Korea suggests that the expansion of the universe may have begun to slow down rather than accelerating at an ever-increasing rate as previously thought.

The remarkable findings published in the Monthly Notices of the Royal Astronomical Society call into question the long-held theory that a mysterious force known as dark energy is pushing distant galaxies away at ever-increasing speeds . Instead, they show no evidence of an accelerating universe.

If the results are confirmed, it could open a whole new chapter in scientists' quest to discover 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 states: "Our study shows that the universe has already entered a phase of decelerating expansion in the present epoch and that dark energy evolves over time much more rapidly than previously thought. If these results are confirmed, it would represent a major paradigm shift in cosmology since the discovery of dark energy 27 years ago."

For the past three decades, astronomers have widely believed that the universe is expanding at an ever-increasing rate, driven by an invisible phenomenon called dark energy that acts as a kind of antigravity. This conclusion, based on distance measurements to distant galaxies using Type Ia supernovae, earned him the 2011 Nobel Prize in Physics.

However, a team of astronomers from Yonsei University has presented new evidence that Type Ia supernovae, long considered the "standard candles" of the universe, are actually strongly affected by the age of their parent stars.

Even after standardization of luminosity, supernovae from younger stellar populations appear systematically fainter, while those from older populations appear brighter.

Based on a much larger sample of 300 host galaxies, the new study confirmed this effect with extremely high significance (99.999% confidence), suggesting that the obscuring of distant supernovae arises not only from cosmological effects but also from stellar astrophysical effects.

When this systematic bias was corrected, the supernova data no longer matched the standard ?CDM cosmological model with a cosmological constant, the researchers said.

In contrast, it aligned much better with a new model favored by the Dark Energy Spectroscopic Instrument (DESI) project, derived from baryonic acoustic oscillations (BAOs)—effectively the sound of the Big Bang—and data from the cosmic microwave background (CMB). Corrected supernova data and results obtained solely with BAO+CMB indicate that dark energy weakens and evolves significantly over time.

More importantly, when the corrected supernova data were combined with the BAO and CMB results, the standard ?CDM model was ruled out with overwhelming significance, the researchers said.

The most surprising thing of all is that this combined analysis indicates that the universe is not accelerating today as previously thought, but has already entered a state of decelerating expansion.

Professor Lee adds: "In the DESI project, the key results were obtained by combining uncorrected supernova data with baryonic acoustic oscillation measurements, leading to the conclusion that while the universe will slow down in the future, it is still accelerating today. In contrast, our analysis—which applies age bias correction—shows that the universe has already entered a slowing phase. Remarkably, this agrees with what is independently predicted by BAO-only or BAO+CMB analyses, although this fact has received little attention until now."

To further confirm their results, Yonsei's team is conducting an "evolution-free test," which uses only supernovae from young, contemporaneous host galaxies across the entire redshift range. The initial results already support their main conclusion.

"In the next five years, with the Vera C. Rubin Observatory's discovery of more than 20,000 new supernova host galaxies, precise age measurements will allow for a much more robust and definitive test of supernova cosmology," notes research professor Chul Chung, co-director of the study along with PhD candidate Junhyuk Son.

The Vera C. Rubin Observatory, located in the Chilean Andes, houses the world's most powerful digital camera. It began scientific operations this year and could answer crucial questions about our solar system and the universe in general.

After the Big Bang and the rapid expansion of the universe some 13.8 billion years ago, gravity slowed its expansion. But in 1998 it was discovered that, nine billion years after the beginning of the universe, its expansion had begun to accelerate again, driven by a mysterious force.

Astronomers called it dark energy, but despite making up approximately 70 percent of the universe, it is still considered one of the greatest mysteries of science.

Last year, data from DESI in Tucson, Arizona, suggested that the strength of dark energy had changed over time, evidence that has continued to grow since then. It is hoped that, with these new tools at their disposal, astronomers will be better equipped to find clues about exactly what dark energy is and how it influences the universe.