New research indicates that the universe’s expansion may be slowing down rather than accelerating, challenging the prevailing theories in modern cosmology. The study, published on November 6, 2025, in the Monthly Notices of the Royal Astronomical Society, suggests that dark energy, the mysterious force believed to drive galaxies apart, may be weakening. If validated, these findings could significantly alter our understanding of the cosmos and address the long-standing “Hubble tension.”
A team led by Professor Young-Wook Lee from Yonsei University in South Korea conducted a comprehensive analysis that contradicts nearly three decades of assumptions about cosmic expansion. Traditionally, astronomers have believed that dark energy is responsible for an accelerating universe, a conclusion drawn from observations of distant type Ia supernovae that earned the 2011 Nobel Prize in Physics.
However, this new research indicates that there is no substantial evidence supporting the idea that the universe is currently experiencing acceleration. Professor Lee stated, “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.”
Rethinking Cosmic Measurements
The research scrutinizes the reliability of type Ia supernovae as cosmic distance indicators—previously considered “standard candles.” The team found that the brightness of these supernovae is influenced by the age of the stars that produce them. They analyzed data from 300 host galaxies and discovered a remarkable correlation: supernovae from younger stars appeared fainter, while those from older stars were brighter. This age effect was confirmed with a confidence level of 99.999%.
The implications of these findings are significant. What was once attributed to cosmic acceleration may have been misattributed dimming resulting from variations in stellar populations. When the researchers adjusted for this age-related bias, the data no longer conformed to the standard ΛCDM model, which assumes a constant dark energy form. It instead aligned with a newer model supported by the Dark Energy Spectroscopic Instrument (DESI) project. This alternative perspective suggests that dark energy is not static but changes over time.
Combining the corrected supernova data with results from baryonic acoustic oscillations (BAO) and the cosmic microwave background (CMB) provided compelling evidence that the universe has entered a phase of decelerated expansion.
Future Research and Observations
Professor Lee emphasized the importance of their findings, stating that while previous analyses suggested ongoing acceleration, their method indicates that the universe has already begun to slow down. This conclusion is also consistent with predictions derived from BAO analyses, a fact that has not garnered significant attention until now.
To further validate their conclusions, the Yonsei team is conducting what they refer to as an “evolution-free test.” This method focuses on supernovae from young, coeval galaxies—those with similar star ages—across the entire redshift range. Early results are already bolstering their main finding.
The Vera C. Rubin Observatory, located in the Chilean Andes, is expected to play a pivotal role in this ongoing research. Equipped with the world’s most advanced digital camera, it commenced scientific operations this year and is anticipated to enhance our understanding of both the solar system and the broader universe. With plans to discover over 20,000 new supernova host galaxies, precise age measurements will soon provide more robust tests of supernova cosmology.
The mystery of dark energy remains one of the most perplexing challenges in contemporary science. Previous data from DESI suggested that dark energy’s influence might have altered over time, a notion that gains traction with the Yonsei team’s findings. With advanced tools like DESI and the Vera C. Rubin Observatory, astronomers are poised to unravel the true nature of dark energy and its role in shaping the universe’s destiny.
