The debate over whether the universe is really accelerating has flared up again. A team at Yonsei University, publishing in Monthly Notices of the Royal Astronomical Society, defends its controversial claim that a hidden age bias in Type Ia supernovae could mean cosmic expansion has already begun to decelerate, with dark energy weakening over time. They argue a recent defense of acceleration — co-authored by two Nobel laureates — rests on flawed analysis of the "age-bias slope." This remains a genuinely open, contested question that the Vera C. Rubin Observatory may soon help resolve.
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![Pinterest](https://pinterest.com/pin/create/button/?url=https://skycr.org/2026/08/26/cosmic-acceleration-debate-supernova-age-bias/&media=https://skycr.org/wp-content/uploads/2026/08/cosmology.webp&description=The debate over whether the universe is accelerating has reignited. A team at Yonsei University, publishing in Monthly Notices of the Royal Astronomical Society, defends its claim that a hidden age bias in Type Ia supernovae undermines cosmic acceleration, disputing a recent rebuttal co-authored by two Nobel laureates. The fight centers on the "age-bias slope" and whether the Phillips relation changes with supernova age. Rubin Observatory data may soon settle it.){kind=link}