The crisis in cosmology continues: Critical problems in a recent defense of cosmic acceleration

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. The Yonsei University team found that their analysis showed cosmic expansion may have already entered a decelerating phase, contradicting the earlier conclusion. The study found that the age-bias slope, which relates the luminosity of Type Ia supernovae to their ages, was artificially flattened when the comparison was restricted to an appropriate redshift interval. The new analysis shows that this procedure leads to a shallower slope, which the rebuttal study incorrectly reported. The study also found that the dust-extinction model used in the rebuttal analysis requires a dependence on host-galaxy mass that is opposite to the trend observed in large galaxy samples. The second problem concerns how the age-bias correction is calculated. The correction depends on the product of two quantities: the change in supernova progenitor age with redshift and the slope of the age-luminosity relation. The rebuttal focused on the possibility that supernova progenitor ages evolve less strongly with redshift than host-galaxy ages do. The new study points out that reducing the inferred age difference necessarily makes the corresponding age-luminosity slope steeper because the age difference appears in the denominator of the slope. The decrease in one quantity is therefore largely offset by the increase in the other. The Yonsei researchers argue that the rebuttal emphasized only the reduced age evolution while overlooking the accompanying increase in the age-bias slope. When both effects are treated consistently, the resulting age-bias correction—and hence the cosmological conclusion—remains largely unchanged from that of the Yonsei team's previous study. Professor Young-Wook Lee, who led the project, said, "Supernova cosmology rests on the key assumption that the luminosity calibration relation—the Phillips relation—does not change with supernova age. Our results consistently show that this assumption is not valid." The researchers also stress that their age-bias-corrected supernova results agree remarkably well with the independent DESI BAO-only result, as demonstrated in their previous study. Both point toward a universe in which dark energy evolves with time and the present expansion is no longer accelerating. To resolve the issue, the Yonsei team proposes an "evolution-free" test for future high-precision supernova cosmology. Rather than combining heterogeneous supernova populations whose average ages change with redshift, this approach would use only supernovae in young, similarly aged host galaxies across the full redshift range. The researchers report that preliminary results from such a test are consistent with their earlier findings. In the future, the much larger supernova samples expected from the Vera C. Rubin Observatory will make it possible to carry out this direct test with substantially greater precision. Such observations could provide a decisive answer to one of the most fundamental questions in modern science: what dark energy really is and whether the universe is accelerating or decelerating today.

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