A novel investigation originating from Yonsei University in Seoul, Korea, is challenging the prevailing cosmological model, which posits that dark energy is propelling distant galaxies apart at an ever-increasing velocity. The findings suggest, contrary to widespread belief, that there is no empirical substantiation for an accelerating cosmic expansion. Should these outcomes withstand scrutiny and verification, they could herald a new epoch in humanity’s pursuit of understanding the fundamental nature of dark energy, potentially resolving the persistent ‘Hubble tension,’ and providing unprecedented insights into the Universe’s historical trajectory and its ultimate destiny.
Universe’s expansion may be slowing, not accelerating. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
For the preceding three decades, the astronomical community has largely adhered to the conviction that the cosmos is undergoing expansion at a continually escalating rate. This phenomenon has been attributed to an enigmatic force known as dark energy, theorized to function as a form of repulsive gravitational force.
This widely accepted conclusion, derived from distance estimations to remote galaxies utilizing Type Ia supernovae, was recognized with the Nobel Prize in Physics in 2011.
However, Professor Young-Wook Lee from Yonsei University, along with his research associates, has now presented novel evidence indicating that Type Ia supernovae, long considered reliable cosmic distance markers or ‘standard candles,’ are in reality profoundly influenced by the age of their progenitor stellar populations.
“Our investigation indicates that the Universe has already entered a phase of decelerated expansion at the current epoch, and that dark energy undergoes temporal evolution at a considerably swifter pace than was previously surmised,” stated Professor Lee.
“If these findings are substantiated, it would signify a monumental paradigm shift in cosmology, comparable in impact to the initial discovery of dark energy some 27 years prior.”
Even subsequent to luminosity standardization procedures, supernovae originating from younger stellar assemblages appear systematically less luminous, whereas those stemming from more mature populations manifest as brighter.
Leveraging a significantly larger dataset encompassing 300 host galaxies, the researchers confirmed this effect with an exceptionally high degree of statistical confidence (99.999%), suggesting that the diminished brightness observed in distant supernovae is a consequence not solely of cosmic factors but also of astrophysical processes related to stellar evolution.
Upon accounting for this systematic deviation, the supernova observational data ceased to be congruent with the standard ΛCDM cosmological model, which incorporates a cosmological constant.
Instead, the data exhibited a far superior alignment with a nascent model favored by the Dark Energy Spectroscopic Instrument (DESI) project, a model derived from Baryon Acoustic Oscillations (BAO) and Cosmic Microwave Background (CMB) measurements.
Both the recalibrated supernova data and the results obtained exclusively from BAO and CMB analyses point towards a weakening of dark energy and its significant evolution over cosmic timescales.
Crucially, when the adjusted supernova data were integrated with information from BAO and CMB observations, the standard ΛCDM model was unequivocally refuted with overwhelming statistical certainty.
Perhaps the most startling revelation from this amalgamated analysis is the indication that the Universe is not presently accelerating, contrary to prior assumptions, but has rather transitioned into a state characterized by decelerated expansion.
“Within the DESI project, the pivotal findings were achieved by merging unadjusted supernova data with baryonic acoustic oscillation measurements, leading to the conclusion that while the Universe is projected to decelerate in the future, it continues to accelerate at the present moment,” Professor Lee elaborated.
“In contrast, our analysis—which incorporates the correction for age-related bias—demonstrates that the Universe has already embarked upon a decelerating trajectory today.”
“Remarkably, this outcome aligns with independent predictions derived from BAO-only or BAO+CMB analyses, a concordance that has thus far garnered limited attention.”
To provide further corroboration for their findings, the research team is currently conducting an ‘evolution-free’ test. This methodology exclusively utilizes supernovae sourced from young, coeval host galaxies across the entire redshift spectrum.
Preliminary outcomes from this test already lend strong support to their central conclusion.
“Within the upcoming five years, with the Vera C. Rubin Observatory expected to identify upwards of 20,000 novel supernova host galaxies, precise age determinations will facilitate a considerably more robust and definitive evaluation of supernova cosmology,” commented Professor Chul Chung of Yonsei University.
The research team’s scholarly publication has been officially released 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
