New research from Yonsei University in Seoul, Korea, casts doubt on the long-held theory that dark energy is driving away distant galaxies at an increasing rate; 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 uncover the true nature of dark energy, resolve the “Hubble tension” and understand the past and future of the universe.
The expansion of the Universe may be slowing down rather than speeding up. 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 invisible phenomenon called dark energy that acts as a kind of antigravity.
This conclusion, based on measurements of distances to distant galaxies using Type Ia supernovae, earned the 2011 Nobel Prize in Physics.
However, Yonsei University professor Lee Young-Wook and his colleagues have now provided new evidence that Type Ia supernovae, long considered the “standard candles of the Universe,” are in fact strongly influenced by the age of their progenitor stars.
“Our study shows that the Universe has already entered a slow expansion phase in the modern era and that dark energy is evolving over time much faster than previously thought,” Professor Li said.
“If these results are confirmed, it will 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 systematically appear dimmer, while supernovae from older populations appear brighter.
Based on a much larger sample of host galaxies, consisting of 300 galaxies, astronomers confirmed this effect with extremely high significance (99.999% confidence), suggesting that the dimming of distant supernovae is due not only to cosmological effects, but also to the effects of stellar astrophysics.
When this systematic bias was corrected, the supernova data no longer fit the standard ΛCDM cosmological model with a cosmological constant.
Instead, it was a much better fit to the new model endorsed by the Dark Energy Spectroscopic Instrument (DESI) project, derived from baryon acoustic oscillation (BAO) and cosmic microwave background (CMB) data.
The corrected supernova data and the BAO+CMB-only results indicate that dark energy weakens and evolves significantly over time.
More importantly, when the corrected supernova data were combined with the BAO and CMB results, the standard ΛCDM model was eliminated with overwhelming significance.
The most surprising thing is that this combined analysis shows that the Universe today is not accelerating, as previously thought, but has already entered a state of slow expansion.
“In the DESI project, key results were obtained by combining uncorrected supernova data with measurements of baryonic acoustic oscillations, leading to the conclusion that although the Universe will slow down in the future, it is still accelerating in the present,” Professor Lee said.
“In contrast, our analysis, which corrects for age bias, shows that the Universe has already entered a slowdown phase today.”
“Notably, this is consistent with what is independently predicted from BAO alone or BAO + CMB analysis, although this fact has received little attention to date.”
To further confirm their results, the researchers are now conducting an evolution-free test that uses only supernovae from young, coeval host galaxies across the entire redshift range.
The first results already confirm their main conclusion.
“Over the next five years, as the Vera K. Rubin Observatory discovers more than 20,000 new supernova host galaxies, precise age measurements will allow for a much more reliable and definitive test of supernova cosmology,” said Yonsei University professor Chul Chung.
teams paper was published today in the magazine Monthly Notices of the Royal Astronomical Society.
_____
Junhyuk Song etc.. 2025. Strong Progenitor Age Error 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
