Lead: New analysis led by Young-Wook Lee (Yonsei University) and colleagues argues that the cosmic expansion has already shifted from acceleration to a decelerating phase at the present epoch, a claim published in Monthly Notices of the Royal Astronomical Society on Nov. 5. The result rests on a re-evaluation of Type Ia supernova brightness that corrects for an age-related bias in progenitor stellar populations and on comparison with large-scale survey data including DESI. If confirmed, the finding challenges the standard Lambda Cold Dark Matter (LCDM) picture in which dark energy drives persistent acceleration and instead implies a time-varying dark-energy behaviour. The team says forthcoming surveys, notably observations from the Vera C. Rubin Observatory, can test the claim by providing many more well-characterized supernova hosts.
Key takeaways
- The study (published Nov. 5 in MNRAS) analyzes about 300 Type Ia supernova host galaxies and reports a 99.99% significance that a portion of the observed dimming arises from stellar-age effects, not just cosmology.
- Correcting for this age bias, the authors conclude the present-day expansion is decelerating, a direct contradiction of the standard LCDM expectation of ongoing acceleration.
- Dark energy is estimated to constitute roughly 68% of the universe’s energy budget; this work implies that its effective strength may evolve faster with time than previously modelled.
- Earlier indications from DESI (Spring 2024) suggested weakening dark energy; this study extends that possibility to claim the transition has already occurred today rather than only in the future.
- The sample size for the current analysis is ~300 host galaxies; the team projects Rubin Observatory follow-up could add >20,000 supernova host galaxies within five years to robustly confirm or refute the effect.
Background
The discovery of accelerating cosmic expansion in 1998, inferred from Type Ia supernovae acting as standard candles, led to the introduction of “dark energy” as the dominant component driving that acceleration. Two teams that reported the accelerating expansion later shared the 2011 Nobel Prize in Physics for this work. Over roughly the subsequent three decades, the Lambda Cold Dark Matter (LCDM) model became the standard cosmological framework, with dark energy parameterized as a cosmological constant that makes up about 68% of the universe’s total energy density.
However, the assumption that Type Ia supernovae are perfectly uniform has long been under scrutiny: astrophysical factors such as progenitor age, metallicity and host-galaxy environment can bias inferred distances. Large redshift surveys—including the Dark Energy Spectroscopic Instrument (DESI)—and improved supernova samples have gradually revealed small systematic effects that, when uncorrected, can mimic or modify cosmological signals. The new Yonsei-led analysis focuses specifically on an age-related luminosity bias in Type Ia supernovae and explores the cosmological consequences of correcting for that bias.
Main event
The team assembled a sample of roughly 300 Type Ia supernova host galaxies spanning a range of stellar ages and distances. They report that, after standardization procedures, supernovae from younger stellar populations remain systematically fainter than those from older populations, implying an age-dependent residual that affects distance estimates. Using an age-bias correction, the authors re-derived cosmological expansion history and found a present-day deceleration instead of continued acceleration predicted by LCDM.
The authors contrast their method with prior DESI analyses: DESI combined uncorrected supernova data with baryon acoustic oscillation (BAO) measurements and saw indications that dark energy might weaken, but still concluded the universe is accelerating now and would decelerate only in the future. By applying the age correction, Lee and collaborators find the transition has already occurred at the current epoch, producing a reversed sign in the measured cosmic acceleration parameter.
The statistical claim is strong in the paper: the reported 99.99% significance is tied to the sample and the specific modeling choices used to remove the stellar-age bias. The authors emphasize that an “evolution-free” test—using only young Type Ia supernovae from uniformly young hosts across redshift—would remove the primary astrophysical systematic and provide a cleaner cosmological measurement. They point to the Vera C. Rubin Observatory’s imminent survey capabilities as the most promising path to such a test.
Analysis & implications
If the age-bias correction and its cosmological interpretation hold up under independent scrutiny, the implications would be far-reaching: it would mean that dark energy is not a simple cosmological constant but evolves noticeably on timescales comparable to the recent cosmic epoch. That would require modifications to the dark-energy sector in cosmological models, potentially favouring dynamic scalar-field models or other exotic physics over a constant Lambda term.
A shift from acceleration to deceleration today would also affect estimates of cosmological parameters that rely on integrated distance measures, including Hubble-constant inferences, growth-of-structure calculations and projections for the ultimate fate of the universe. Some tensions in cosmology—such as the H0 discrepancy between local and early-Universe measurements—might be re-evaluated in light of a changing dark-energy behaviour, though whether this change alleviates or worsens those tensions requires detailed reanalysis.
Practically, the claim places a premium on reducing astrophysical systematics in supernova cosmology: host-galaxy properties, progenitor demographics and selection effects must be controlled at a level below the cosmological signal. Large, homogeneous samples with precise host-galaxy ages—expected from Rubin Observatory imaging and follow-up spectroscopy—are therefore critical to test the result within the next several years.
Comparison & data
| Model / Analysis | Present expansion | Primary evidence |
|---|---|---|
| LCDM (standard) | Accelerating | Type Ia supernova Hubble diagram + BAO; Lambda term |
| DESI (Spring 2024 results) | Hints of weakening dark energy; still accelerating now | Uncorrected supernova data combined with BAO |
| Yonsei et al. (Nov. 5, MNRAS) | Decelerating (current epoch) | Age-bias-corrected sample of ~300 Type Ia hosts; 99.99% significance |
This table summarizes the contrasting conclusions: the new result pivots on correcting an age-dependent residual in Type Ia standardization. The current paper’s sample is modest (~300 hosts) compared with the tens of thousands expected from upcoming Rubin observations, so statistical confirmation will rest on expanded and more uniform datasets.
Reactions & quotes
Several team members and project leads have framed the findings cautiously but with emphasis on the potential scope of change.
“Our analysis indicates the universe has already entered a decelerating phase and that dark energy evolves with time much more rapidly than previously thought.”
Young-Wook Lee, Yonsei University (lead author)
Lee’s statement outlines the team’s headline claim and the mechanism (time-evolving dark energy) they infer after applying the age-bias correction. The paper stresses that the conclusion depends strongly on the correction and on independent confirmation.
“With Rubin delivering >20,000 new supernova host galaxies within five years, we can measure host ages precisely and perform an evolution-free test.”
Chul Chung, Yonsei University (co-author)
Chung frames the near-term observational programme that would make the claim testable. Independent researchers contacted for comment in related coverage noted the importance of larger, uniformly analyzed samples and of cross-checks using BAO and other distance indicators.
Unconfirmed
- The assertion that the universe has already transitioned to deceleration is not yet independently confirmed and depends on the particular age-bias model applied to the 300-host sample.
- Whether the age correction fully accounts for other systematics (metallicity, reddening, selection effects) remains to be demonstrated with larger, homogeneous datasets.
- Claims of an immediate “paradigm shift” are premature until multiple independent surveys reproduce the effect and other cosmological probes (BAO, CMB, growth measurements) are reconciled with a changing dark energy term.
Bottom line
The Yonsei-led paper raises a provocative possibility: a time-varying dark energy that has weakened enough to put the universe into a decelerating phase today. The claim depends on a specific correction for an age-related supernova luminosity bias and on a sample of ~300 host galaxies, so it should be treated as an important but preliminary result rather than a settled revision of cosmology.
Definitive resolution will come from larger, uniform samples and cross-checks with independent probes. The Vera C. Rubin Observatory and coordinated spectroscopic follow-up offer the fastest route to a decisive test within roughly five years; until then, the cosmology community will weigh the result carefully and seek independent replication before rewriting the standard model.
Sources
- Space.com article — (news coverage)
- DESI Collaboration — (project/official instrument site)
- Vera C. Rubin Observatory / LSST — (official observatory site)
- Monthly Notices of the Royal Astronomical Society — (peer-reviewed journal)