This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Doubts about the standard model of cosmology are deepening after a new study identified fundamental problems in a recent analysis that defended the conventional view that the universe is currently undergoing accelerated expansion driven by dark energy. Last November, researchers at Yonsei University in South Korea reported that, once a systematic bias related to the ages of Type Ia supernovae is taken into account, the universe may no longer be accelerating as previously thought.
Instead, their analysis suggested that cosmic expansion has already entered a decelerating phase and that dark energy may not be a cosmological constant but rather a rapidly evolving component that is weakening with time. The result attracted considerable attention because it closely agreed with the prediction from an independent cosmological probe: the baryon acoustic oscillation (BAO)-only measurements from the Dark Energy Spectroscopic Instrument (DESI). In June, an international team led by researchers at the University of Southampton and including two Nobel laureates published a rebuttal challenging the Yonsei result.
Their analysis found a much shallower relationship between supernova luminosity and age than that reported by the Yonsei team. They also argued that the ages of supernova progenitors may evolve much less with redshift than the ages of their host galaxies. On this basis, the Southampton–led team concluded that the required age-bias correction would be small and that the expansion of the universe is still accelerating.
The emerging crisis in cosmology therefore appeared to have been averted. But a new study by the Yonsei researchers, published in Monthly Notices of the Royal Astronomical Society, finds that both central arguments advanced in the rebuttal are affected by critical analytical problems that undermine its defense of cosmic acceleration. The crisis in cosmology, the researchers conclude, continues.
The first problem concerns the measurement of the "age-bias slope"—the relationship between the standardized luminosities of Type Ia supernovae and their ages. To measure this relationship reliably, supernovae must be compared within a sufficiently narrow redshift interval. Otherwise, the evolution of the supernova population across the redshift range becomes entangled with the age-luminosity correlation being measured.
The rebuttal study instead combined supernovae over an unusually broad redshift interval. The new analysis demonstrates that this procedure artificially flattens the measured age-bias slope. As progressively higher-redshift supernovae are added to the sample, the recovered slope becomes systematically shallower.
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