A Yonsei University paper in Monthly Notices of the Royal Astronomical Society argues that the brightness of some Type Ia supernovae depends on the age of the progenitor star, based on DESI observations. Correcting for that effect, the authors find much of the evidence for accelerated expansion is reduced and report a model with time-varying dark energy and possible deceleration. The claim is controversial: Nobel laureate Adam Riess and others urge caution, noting current data are insufficient. The Yonsei team plans further studies to test their results.
New Study Challenges Cosmic Acceleration — Could a 'Big Crunch' Be Possible?
A Yonsei University paper in Monthly Notices of the Royal Astronomical Society argues that the brightness of some Type Ia supernovae depends on the age of the progenitor star, based on DESI observations. Correcting for that effect, the authors find much of the evidence for accelerated expansion is reduced and report a model with time-varying dark energy and possible deceleration. The claim is controversial: Nobel laureate Adam Riess and others urge caution, noting current data are insufficient. The Yonsei team plans further studies to test their results.
11:21 PM, 11/11/2025Science
New Study Questions Whether the Universe Is Still Accelerating
AUSTIN — A provocative new paper from researchers at Yonsei University suggests the long-held view that the universe's expansion is accelerating may need revision. Published this month in Monthly Notices of the Royal Astronomical Society, the study uses observations from the Dark Energy Spectroscopic Instrument (DESI) to revisit how Type Ia supernovae are used to measure cosmic expansion.
What the authors claim
The team reports that the measured brightness of some Type Ia supernovae appears to depend on the age of the progenitor star. Because these supernovae have been treated as "standard candles"—objects of nearly uniform intrinsic brightness used to infer distances—any systematic variation could affect conclusions about the universe's expansion rate.
"Younger (stars) will produce fainter supernovae compared to older (stars). And the supernovae in the distant universe are younger compared to the local count," Professor Young‑Wook Lee, lead author of the paper, told the Associated Press.
After applying an age-based correction to the supernova brightnesses, the authors say much of the dimming previously attributed to accelerated expansion is reduced. They report results that are inconsistent with a simple cosmological constant driving perpetual acceleration and instead favor a decelerating expansion with time-varying dark energy. If that trend continued, they warn, it could open the door to scenarios such as a "Big Crunch," in which expansion reverses and the universe collapses.
Community response and next steps
The claim has drawn skepticism from some experts. Dr. Adam Riess of Johns Hopkins University, a co-recipient of the 2011 Nobel Prize for work confirming cosmic acceleration, told New Scientist that similar proposals have been made before but remain tentative because current datasets do not yet provide detailed information about progenitor stars and the supernovae they produce.
Other researchers emphasize that DESI is a powerful new resource and that further, independent analyses—using larger samples and complementary probes such as baryon acoustic oscillations and the cosmic microwave background—are needed to test whether the age effect fully explains the supernova dimming.
The Yonsei team says it will continue follow-up studies to refine its methods and expand the dataset. Until independent confirmations appear, the wider cosmology community is likely to treat these results as interesting but not yet definitive.
Bottom line: The new paper raises an important methodological question about Type Ia supernovae and cosmic expansion. It highlights the need for additional data and cross-checks before overturning the prevailing model of dark-energy-driven acceleration.