New research from Yonsei University in Seoul, Korea, casts doubt on the long-standing theory that dark energy is driving distant galaxies away increasingly faster; instead, it shows no evidence of an accelerating Universe. If the results are confirmed it could open an entirely new chapter in scientists’ quest to uncover the true nature of dark energy, resolve the ‘Hubble tension,’ and understand the past and future of the Universe.
Universe’s expansion may be slowing, not accelerating. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
For the past three decades, astronomers have widely believed that the Universe is expanding at an ever-increasing rate, driven by an unseen phenomenon called dark energy that acts as a kind of anti-gravity.
This conclusion, based on distance measurements to faraway galaxies using Type Ia supernovae, earned the 2011 Nobel Prize in Physics.
However, Yonsei University’s Professor Young-Wook Lee and colleagues have now put forward new evidence that Type Ia supernovae, long regarded as the Universe’s ‘standard candles,’ are in fact strongly affected by the age of their progenitor stars.
“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,” 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 standardization, 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, the astronomers confirmed this effect at extremely high significance (99.999% confidence), suggesting that the dimming of distant supernovae arises not only from cosmological effects but also from stellar astrophysics effects.
When this systematic bias was corrected, the supernova data no longer matched the Standard ΛCDM cosmological model with a cosmological constant.
Instead, it aligned far better with a new model favored by the Dark Energy Spectroscopic Instrument (DESI) project, derived from Baryon Acoustic Oscillations (BAO) and Cosmic Microwave Background (CMB) data.
The corrected supernova data and the BAO+CMB-only results both indicate that dark energy weakens and evolves significantly with time.
More importantly, when the corrected supernova data were combined with BAO and CMB results, the Standard ΛCDM model was ruled out with overwhelming significance.
Most surprising of all, this combined analysis indicates that the Universe is not accelerating today as previously thought, but has already transitioned into a state of decelerated expansion.
“In the DESI project, the key results were obtained by combining uncorrected supernova data with baryonic acoustic oscillations measurements, leading to the conclusion that while the Universe will decelerate in the future, it is still accelerating at present,” Professor Lee said.
“By contrast, our analysis — which applies the age-bias correction — shows that the Universe has already entered a decelerating phase today.”
“Remarkably, this agrees with what is independently predicted from BAO-only or BAO+CMB analyses, though this fact has received little attention so far.”
To further confirm their results, the researchers are now carrying out an evolution-free test, which uses only supernovae from young, coeval host galaxies across the full redshift range.
The first results already support their main conclusion.
“Within the next five years, with the Vera C. Rubin Observatory discovering more than 20,000 new supernova host galaxies, precise age measurements will allow for a far more robust and definitive test of supernova cosmology,” said Yonsei University’s Professor Chul Chung.
The team’s paper was published today in the Monthly Notices of the Royal Astronomical Society.
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Junhyuk Son et al. 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