The Universe may have already started slowing down

The findings, published November 6 in Monthly Notices of the Royal Astronomical Society, question the long-accepted belief that a mysterious force known as “dark energy” is pushing galaxies apart at an ever-increasing rate. Instead, researchers found no convincing evidence that the universe is still accelerating.

If confirmed, the results could reshape scientists’ understanding of dark energy, help resolve the long-standing “Hubble tension,” and transform theories about the universe’s past and future.

Evidence for a Cosmic Slowdown

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.”

For nearly three decades, astronomers have believed that the universe’s expansion was accelerating due to dark energy, a mysterious force acting as a kind of “anti-gravity.” This conclusion was originally based on measurements of distant type Ia supernovae, a discovery that earned the 2011 Nobel Prize in Physics.

Rethinking the Universe’s “Standard Candles”

The new research from Yonsei University challenges that foundation. Type Ia supernovae, long considered reliable “standard candles” for measuring cosmic distances, appear to be influenced by the age of the stars that create them.

Even after standardizing their brightness, the team found that supernovae originating from younger stars tend to look fainter, while those from older stars appear brighter. Analyzing data from 300 host galaxies, the researchers confirmed this age effect with an extraordinary level of confidence (99.999%).

This means that part of the dimming once attributed to cosmic acceleration could actually result from stellar population differences rather than universal expansion.

A New Model Emerges

When the team corrected for this age-related bias, the supernova data no longer fit the standard ΛCDM model, which assumes a constant form of dark energy. Instead, it matched more closely with a newer model supported by the Dark Energy Spectroscopic Instrument (DESI) project.

This alternative model draws on baryonic acoustic oscillations (BAO) — essentially ancient sound waves from the Big Bang — and data from the cosmic microwave background (CMB). Both sources suggest that dark energy is not constant but instead weakens and changes over time.

When researchers combined the corrected supernova data with BAO and CMB results, the evidence became overwhelming: the universe does not appear to be accelerating anymore, but has entered a phase of decelerated expansion.

A Universe Already Slowing

Professor Lee explained, “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.”

Testing the Findings

To strengthen their conclusions, the Yonsei team is performing what they call an “evolution-free test.” This approach examines only supernovae from young, coeval galaxies — those with stars of similar ages — across the entire redshift range. Early results already support the main finding.

“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 author of the study, along with PhD candidate Junhyuk Son.

The Vera C. Rubin Observatory and the Future of Cosmology

Located high in the Chilean Andes, the Vera C. Rubin Observatory houses the world’s most powerful digital camera. Having begun scientific operations this year, it is expected to revolutionize our understanding of both the solar system and the broader universe.

After the Big Bang, roughly 13.8 billion years ago, the universe expanded rapidly before gravity slowed it down. Then, around nine billion years after its birth, scientists discovered that expansion had begun speeding up again. This was attributed to dark energy, which is believed to make up about 70 percent of the universe.

Dark Energy’s Mystery Deepens

Despite decades of study, dark energy remains one of science’s most puzzling enigmas. Last year, data from DESI in Tucson, Arizona hinted that the influence of dark energy might have changed over time, an idea now gaining traction with the Yonsei team’s new results.

With advanced instruments like DESI and the Vera C. Rubin Observatory, astronomers hope to finally uncover what dark energy really is — and how it shapes the fate of the universe.