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

Recent research from Yonsei University in South Korea has reignited doubts about the standard model of cosmology, which posits that the universe is currently accelerating in its expansion due to dark energy. A study published in Monthly Notices of the Royal Astronomical Society highlights fundamental issues with a previous analysis defending this view.

The Yonsei researchers discovered that a systematic bias related to the ages of Type Ia supernovae significantly impacts the measurement of cosmic expansion. By correcting for this bias, they found that cosmic expansion may be entering a decelerating phase instead of continuing to accelerate. This challenges the prevailing cosmological model and suggests that dark energy could be a component that evolves over time rather than a constant.

The new study refutes a recent rebuttal by an international team from the University of Southampton, including two Nobel laureates. The rebuttal argued that the Yonsei findings were affected by a shallow relationship between supernova luminosity and age caused by wide redshift intervals. However, the Yonsei team demonstrated that this criticism misrepresented their methodology, as their analysis showed a deeper age-dependence when applying appropriate redshift limits.

Moreover, the Yonsei researchers identified a second critical issue: the way the age-bias correction is calculated. They showed that the correction relies on the interplay between the change in supernova progenitor ages with redshift and the slope of the age-luminosity relation. The rebuttal overlooked the fact that reducing the inferred age difference largely increases the corresponding age-luminosity slope, maintaining the overall effect of the correction.

Professor Young-Wook Lee, lead researcher, emphasized that these findings conflict with the fundamental assumption that the luminosity calibration relation—the Phillips relation—remains constant with supernova age. This assumption is crucial for defending an accelerating universe model. The researchers propose an "evolution-free" test for future supernova cosmology studies, focusing on supernovae in young, similarly aged host galaxies to avoid systematic biases.

Their results align well with independent observations from the Dark Energy Spectroscopic Instrument (DESI), further supporting the possibility of an evolving universe with time-dependent dark energy.