Yonsei team rebuts critique, says universe expansion may be slowing

September 14, 2026 at 12:07 AM2 min

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Astrophysicists from Yonsei University in South Korea published a rebuttal in Monthly Notices of the Royal Astronomical Society, arguing that a June 2026 critique of their work contains methodological errors. They maintain that after correcting for the age of Type Ia supernovae, the data suggest the universe's expansion may have shifted from acceleration to deceleration. The team says their corrected results align with independent baryon acoustic oscillation data from the DESI instrument.

Key Facts

The Yonsei University team published their rebuttal in Monthly Notices of the Royal Astronomical Society.
The June 2026 critique came from an international group at the University of Southampton.
The Yonsei team argues that the critique used too broad a redshift range of 0.04 < z < 0.42, over which the average age of host galaxies changes by about 3 billion years.
The corrected supernova results align with independent baryon acoustic oscillation data from the DESI instrument.
The authors acknowledge that their work does not provide irrefutable proof that cosmic acceleration has stopped.

The Rebuttal

The Yonsei University team, led by Dr. Chul Chung, published a response to the June 2026 critique in Monthly Notices of the Royal Astronomical Society. They argue that the Southampton group's analysis combined supernovae from too broad a redshift range, 0.04 < z < 0.42, where the average age of host galaxies changes by about 3 billion years. Dr. Chung stated that the shallow age-bias slope reported in the critique results from a clear methodological problem, and that restricting the comparison to an appropriate redshift interval restores the strong age dependence. The team also addressed the conversion of host galaxy age to progenitor age, showing that the same effect that makes the age evolution less pronounced also steepens the luminosity-age relation. When both factors are considered together, the final correction for cosmological calculations remains nearly the same as in the original Korean study.

DESI Alignment

The corrected supernova results match independent baryon acoustic oscillation data from the Dark Energy Spectroscopic Instrument (DESI). The Yonsei team says this alignment further points to an evolving nature of dark energy, challenging the assumption of a cosmological constant in the Lambda-CDM model. The authors acknowledge that their work does not provide irrefutable proof that cosmic acceleration has stopped. They argue, however, that it revives the debate over whether systematic errors in Type Ia supernova interpretation drive one of cosmology's most important measurements.