Researchers from the Tata Institute of Fundamental Research in Mumbai and the University of Oxford have published findings that question a central concept in modern cosmology: the idea that the universe is expanding at an accelerating rate due to dark energy. Their research, appearing in Monthly Notices of the Royal Astronomical Society, suggests the observational evidence supporting cosmic acceleration may not be as robust as commonly accepted.
The team, led by Professor Subir Sarkar of Oxford’s Rudolf Peierls Centre for Theoretical Physics, conducted a comprehensive reexamination of the Pantheon+ dataset, which comprises observations of more than 1,700 Type Ia supernovae. Type Ia supernovae have been instrumental in cosmology for over 25 years as tools for measuring universal expansion. The original discovery that expansion appears to accelerate was honored with the 2011 Nobel Prize in Physics.
The researchers applied a newly proposed correction accounting for the ages of stars that produce Type Ia supernovae explosions. When this correction was incorporated into their analysis, the data no longer indicated uniform acceleration across the universe. Instead, the findings suggest cosmic expansion may actually be decelerating overall. Additionally, the team investigated directional dependence, discovering that apparent acceleration appears concentrated along the direction of local motion as indicated by the cosmic microwave background, an effect inconsistent with dark energy predictions.
Despite these findings, the scientific community remains divided on the matter. A concurrent paper by Professor Maria Vincenzi, also from Oxford, published in the same journal issue, maintains that available observations continue to support ongoing cosmic acceleration. Vincenzi’s team points to extensive expertise in supernova astrophysics and expressed confidence in the cosmological framework developed over recent decades.
Resolution may come through future observations. The Rubin Observatory’s Legacy Survey of Space and Time is expected to measure hundreds of thousands of supernovae, providing a substantially larger sample that could definitively test whether the universe is accelerating, whether any such effect varies directionally, and the role dark energy plays in cosmic expansion.
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