New research from Yonsei University in Seoul, Korea, casts doubt on the long-held theory that dark energy is chasing distant galaxies ever faster; instead, it shows no evidence of an accelerating Universe. If the results are confirmed, it could open a whole new chapter in scientists’ quest to discover the true nature of dark energy, resolve the “Hubble tension” and understand the past and future of the Universe.
The expansion of the Universe could slow down instead of speeding up. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
Over the past three decades, astronomers have widely believed that the Universe is expanding at an ever-increasing rate, driven by an invisible phenomenon called dark energy that acts as a kind of anti-gravity.
This conclusion, based on distance measurements to distant galaxies using Type Ia supernovae, earned the 2011 Nobel Prize in Physics.
However, Professor Young-Wook Lee of Yonsei University and colleagues have presented new evidence that Type Ia supernovae, long considered the “standard candles” of the Universe, are in fact strongly affected by the age of their progenitor stars.
“Our study shows that the Universe has already entered a phase of slowed expansion in the current era and that dark energy is changing over time much faster than previously thought,” Professor Lee said.
“If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago.”
Even after luminosity normalization, supernovae from younger stellar populations appear systematically fainter, while those from older populations appear brighter.
Based on a much larger host galaxy sample of 300 galaxies, astronomers confirmed this effect with extremely high significance (99.999% confidence), suggesting that the attenuation of distant supernovae results not only from cosmological effects but also from stellar astrophysical effects.
When this systematic bias was corrected, the supernova data no longer fit the standard cosmological model ΛCDM with a cosmological constant.
Instead, it aligned much better with a new model favored by the Dark Energy Spectroscopic Instrument (DESI) project, derived from Baryonian Acoustic Oscillations (BAO) and Cosmic Microwave Background (CMB) data.
The supernova-corrected data and BAO+CMB-only results indicate that dark energy weakens and changes significantly with time.
More importantly, when the supernova corrected data were combined with the BAO and CMB results, the standard ΛCDM model was excluded with overwhelming importance.
Most surprisingly, this combined analysis indicates that the Universe is not accelerating today as previously thought, but has already shifted to a state of slowed expansion.
“In the DESI project, key results were obtained by combining uncorrected supernova data with measurements of baryonic acoustic oscillations, which led to the conclusion that although the Universe will decelerate in the future, it is still accelerating at present,” said Professor Lee.
“On the other hand, our analysis – which applies age bias correction – shows that the Universe has already entered a deceleration phase today.”
“Remarkably, this is consistent with what is independently predicted from BAO only or BAO+CMB analyses, although this fact has received little attention so far. »
To further confirm their results, the researchers are currently performing a no-evolution test, which uses only supernovae from contemporary young host galaxies across the entire redshift range.
The first results already support their main conclusion.
“Over the next five years, with the discovery of more than 20,000 new supernova host galaxies by the Vera C. Rubin Observatory, precise age measurements will enable much more robust and definitive testing of supernova cosmology,” said Professor Chul Chung of Yonsei University.
The team’s paper was published today in the Monthly Notices of the Royal Astronomical Society.
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Junhyuk’s son and others. 2025. Strong progenitor age bias in supernova cosmology – II. Alignment with DESI BAO and signs of a non-accelerating Universe. MNRAS 544 (1): 975-987; doi: 10.1093/mnras/staf1685
