Universe Expansion Now Enters Phase of Slowing According to New Research

by Sophie Jenkins
London, UK (SPX) Nov 06, 2025

A new study led by scientists from Yonsei University suggests the universe has begun to slow its expansion, contradicting the widely held belief that this rate has been accelerating. Their research raises substantial doubts about the standard model that identifies dark energy as the cause for ongoing acceleration.

The investigation, published in Monthly Notices of the Royal Astronomical Society, examined type Ia supernovae as cosmic distance markers. The team found that the luminosity of these supernovae is significantly affected by the age of their progenitor stars, even after conventional standardisation methods were applied. Supernovae originating from younger stellar populations appeared systematically fainter, while those from older populations appeared brighter.

Analyzing a sample covering 300 host galaxies, the authors confirmed this so-called age-bias effect with high statistical confidence. When adjusting for this bias, distant supernova data diverged from predictions of the standard ?CDM cosmological model, which incorporates a cosmological constant.

Instead, the recalibrated results agreed with more recent models that describe dark energy as a force which evolves and weakens over time. The findings also showed that upon combining the corrected supernovae data with measurements of baryonic acoustic oscillations and cosmic microwave background, the evidence for a currently accelerating universe was rejected with overwhelming significance.

Lead author Professor Young-Wook Lee noted, "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."

The Yonsei team is conducting further evolution-free tests using only supernovae from young, coeval galaxies across multiple redshifts. Initial results are consistent with their main conclusion. Anticipated surveys using the Vera C. Rubin Observatory are expected to provide larger samples and allow even more robust testing of supernova cosmology in the next five years.

The study argues that while previous analyses, such as those from the Dark Energy Spectroscopic Instrument (DESI) project, suggested the universe would decelerate in the future but remains in a phase of acceleration, the newly corrected data show deceleration has already started. These results align closely with predictions using only baryonic acoustic oscillations and cosmic microwave background data.

Research Report:Strong Progenitor Age-bias in Supernova Cosmology. II. Alignment with DESI BAO and Signs of a Non-Accelerating Universe