A study published in the Monthly Notices of the Royal Astronomical Society suggests that the expansion rate of the universe may be slowing down. The research, led by Professor Young-Wook Lee's team at Yonsei University in South Korea, provides new insights into the current theoretical model of accelerating cosmic expansion through a re-analysis of supernova data.
The research team found that the brightness of Type Ia supernovae is affected by the age of their progenitor stars, resulting in systematic deviations. Supernovae in younger stellar populations appear dimmer, while those in older populations appear brighter. Analysis based on a sample of 300 host galaxies confirmed this effect with 99.999% confidence. Professor Young-Wook Lee stated: "Our study indicates that the universe may have entered a phase of decelerating expansion in the current epoch, with dark energy evolving faster than expected."
When this age-related bias is corrected, the supernova data no longer fully conform to the standard ΛCDM cosmological model. The corrected data show better agreement with baryon acoustic oscillation and cosmic microwave background radiation data obtained from the Dark Energy Spectroscopic Instrument project. The comprehensive analysis suggests that dark energy may be weakening over time, and the state of cosmic expansion may have shifted from acceleration to deceleration.
The research team is further validating this finding through the "no-evolution test," with preliminary results supporting the main conclusions. With the Vera C. Rubin Observatory in Chile expected to discover more than 20,000 new supernova host galaxies over the next five years, cosmic expansion research will receive richer data support. The observatory began scientific observations in 2025 and will provide new observational evidence for studies of cosmic evolution.
This study provides a new perspective for understanding the nature of dark energy and cosmic evolution. Continued observations will help scientists further explore the dynamic changes in cosmic expansion.
