A new analysis of more than 1,700 exploded stars is reopening one of cosmology's most consequential debates: whether the universe's expansion is truly speeding up, or whether decades of supernova data have been misread.
The study, led by Subir Sarkar of Oxford's Rudolf Peierls Centre for Theoretical Physics with Animesh Sah and Mohamed Rameez of the Tata Institute of Fundamental Research in Mumbai, was published this month in Monthly Notices of the Royal Astronomical Society. It re-examines the Pantheon+ compilation, a widely used catalog of Type Ia supernovae — exploding stars bright and consistent enough to serve as cosmic distance markers, the same class of evidence behind the 2011 Nobel Prize-winning discovery of cosmic acceleration.
What the new analysis found
The team applied a correction for a factor increasingly documented in recent years: the intrinsic brightness of a Type Ia supernova appears to depend partly on the age of the star system that produced it, not just on the physics of the explosion itself. Older progenitor stars, more common in the earlier universe, may produce systematically different light curves than those observed nearby today. Once that age-dependent correction is applied, the researchers report, the signal long read as acceleration weakens — and the data instead point toward an expansion rate that is slowing down. The team also found the apparent acceleration is not uniform across the sky but concentrated in the direction of our galaxy's local motion relative to the cosmic microwave background, a directionality that is difficult to reconcile with dark energy, which by definition should push space apart uniformly in every direction.
"There is increasing evidence that the brightness of Type Ia supernovae depends on the age of the stars they come from," Sarkar said in a statement released by Oxford's physics department. "If this effect is not accounted for, it can lead to the erroneous conclusion that the expansion rate is accelerating."
The claim is far from settled. In the same journal issue, a separate paper co-authored by Maria Vincenzi, also at Oxford, reaches the opposite conclusion from an overlapping dataset, arguing the evidence for acceleration — and dark energy — holds up once other systematic effects are properly modeled. The dueling papers reflect a wider split simmering since 2024, when an unrelated galaxy-clustering survey, the Dark Energy Spectroscopic Instrument, hinted that dark energy's strength might not be constant over cosmic time, contrary to the simplest standard cosmological model.
Sarkar has long been a skeptic of the standard dark-energy picture, and his past challenges have not overturned mainstream cosmology, so most researchers are likely to treat this result as a provocative reanalysis rather than a refutation. Both camps agree the underlying supernova sample, assembled from many telescopes and surveys over two decades, has limits that make definitive answers hard to reach with current data.
That may soon change. The Vera C. Rubin Observatory in Chile, now conducting its Legacy Survey of Space and Time, is expected to observe hundreds of thousands of Type Ia supernovae over the next decade, an enormous leap from the roughly 1,550 unique events underlying Pantheon+. Sarkar's and Vincenzi's teams both say that dataset, alongside results from the newly launched Roman Space Telescope, should finally offer enough statistical power to settle whether the universe is accelerating, decelerating, or something stranger than either.
