New Study Suggests Cosmic Expansion May Already Be Slowing — Implications for Dark Energy

Universe’s expansion may be slowing — and dark energy might be changing

New research from a team at Yonsei University raises a provocative possibility: the expansion of the universe, long thought to be accelerating under the influence of dark energy, may already have begun to decelerate. If confirmed, this result would challenge the standard Lambda Cold Dark Matter (LCDM) model and force a rethinking of dark energy's nature and role in cosmic history.

The team’s analysis builds on recent hints from the Dark Energy Spectroscopic Instrument (DESI) that dark energy could be weakening. The Yonsei group reports that once an apparent observational bias tied to the ages of progenitor stars is corrected, distant Type Ia supernovae — the "standard candles" used to measure cosmic expansion — appear dimmer for reasons tied to stellar populations rather than purely to cosmological expansion.

"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," said Young-Wook Lee, team leader and researcher at Yonsei University.

What the study did

Using a sample of about 300 galaxy host systems, the researchers examined Type Ia supernova brightness after applying a correction for an "age bias": younger progenitor systems can produce systematically fainter supernovae, even after standardization procedures. They report a statistical significance of 99.99% that stellar-age effects can contribute to the observed dimming, and conclude that, when this bias is accounted for, the data are inconsistent with a constant dark energy density assumed in the LCDM model.

Implications and caveats

If the team's interpretation is correct, dark energy may be evolving or weakening and the universe could already be in a decelerating phase. In extreme scenarios, a transition from expansion to contraction could point toward a "Big Crunch" in the far future. However, such dramatic conclusions remain speculative: the community will require independent confirmation, more data, and careful cross-checks using multiple cosmic probes (for example, baryon acoustic oscillations and galaxy clustering) before changing the standard cosmological picture.

Next steps

The authors say the most robust follow-up is an "evolution-free" test that uses only young Type Ia supernovae from young host galaxies across a range of distances, thereby minimizing age-related biases. The Vera C. Rubin Observatory — now starting its wide-field survey from Cerro Pachón in Chile — is expected to be crucial for this work, with plans to discover tens of thousands of new supernova host galaxies and enable precise stellar-age measurements over the next few years.

The research was published on Wednesday, Nov. 5, in the journal Monthly Notices of the Royal Astronomical Society. Until independent analyses confirm these findings, the results should be regarded as an intriguing challenge to the current cosmological paradigm rather than definitive proof of a changing cosmic fate.

Key facts:

• Study sample: ~300 galaxies with Type Ia supernovae.
• Reported statistical significance for stellar-age contribution to dimming: 99.99% (as stated by the authors).
• Main claim: After correcting for an age bias, the universe may already be in a decelerating expansion phase.
• Next test: "Evolution-free" analysis using young supernovae, aided by the Vera C. Rubin Observatory.