The expansion of the universe may be slowing down rather than accelerating. A new study published by South Korean researchers suggests that, if confirmed, it could rewrite fundamental assumptions in cosmology.
In 1998, two teams of astronomers observing Type Ia supernovae — bright exploding stars — proposed that the universe was expanding at an accelerating rate. The most distant supernovae appeared dimmer than expected, which was interpreted as evidence that they were moving away faster than predicted. This discovery won the 2011 Nobel Prize in Physics and placed the mysterious, repulsive force known as Dark Energy at the center of cosmological theory.
However, while the nature of Dark Energy remains elusive, new research is now challenging its presumed role.
A new study led by Young-Wook Lee, an astrophysics professor at Yonsei University in South Korea, suggests the universe may already have entered a deceleration phase. The findings align with results from last year's DESI (Dark Energy Spectroscopic Instrument) project, which hinted that Dark Energy might be weakening over time.
"Dark Energy exists, but today's universe has already entered a slowdown phase," Lee said. "The fate of the universe may change — and if you can change the universe's fate, that is a major breakthrough in cosmology."
To reach this conclusion, researchers analyzed 300 galaxies and their Type Ia supernovae. The study proposes that distant supernovae appear dim not only because of their distance but also due to the age of the star systems that produced them.
Co-author Junhyuk Son said Type Ia supernovae had long been treated as nearly identical "standard candles." However, the team found that their brightness varies depending on the age of the progenitor stars: supernovae from younger stars are slightly dimmer, while those from older stars are brighter.
Son noted that the statistical confidence of this age–brightness link is 99.99 percent, arguing that this would allow more accurate use of Type Ia supernovae in measuring cosmic expansion.
If confirmed, the results would represent the most significant shift in cosmology since the 1998 discovery of accelerating expansion. They imply that the universe is no longer accelerating — and that Dark Energy is not constant but evolving over time.
If the deceleration continues, the universe could eventually stop expanding and begin to contract, potentially ending in a "Big Crunch," the opposite of the Big Bang. "Two years ago, the Big Crunch was unthinkable," Lee said. "But now we must explore whether it is genuinely possible."
The study is controversial because it challenges widely accepted theories. Nobel laureate Adam Riess argued that the research is based on "a faulty premise," noting that the idea of supernovae aging alongside the universe had already been proposed and dismissed years earlier.
Lee disputed this criticism, stating that Type Ia supernovae today are found both in younger, star-forming galaxies and in older, quiescent ellipticals. He argued that earlier dismissals of the age hypothesis relied on "deeply flawed data."
Lee expects intense debate ahead: "We have a long road before convincing the entire supernova cosmology community. Many still strongly oppose our findings."
But he says uncertainty may not last long. The new Vera C. Rubin Observatory, which began operations this year, will help settle the debate. Starting in early 2026, its 10-year Legacy Survey of Space and Time will map the sky continuously, recording asteroids, comets, exploding stars, and variable galaxies.
"Rubin will discover over 20,000 new host galaxies of supernovae with highly precise age estimates," Lee said. "Within three to five years, we'll be able to perform a more direct cosmological test without worrying about age bias."
He added: "Dark Energy keeps getting stranger. But we still lack a good theory to explain that behavior. I think we're missing something — and we may find an even more surprising answer within five years."
