Indian-Led Study Questions Whether Dark Energy Is Real

A new analysis led by researchers at the Tata Institute of Fundamental Research (TIFR) in Mumbai, together with Professor Subir Sarkar of the University of Oxford, is challenging a central idea in modern cosmology: that the universe is expanding at an accelerating rate because of "dark energy." The findings, published as a letter in Monthly Notices of the Royal Astronomical Society, argue the evidence for cosmic acceleration may not be as strong as widely assumed. A separate paper in the same journal issue, co-authored by Professor Maria Vincenzi of Oxford, reaches the opposite conclusion, so the debate remains unsettled.

Sarkar worked with Animesh Sah and Mohamed Rameez of TIFR to revisit the Pantheon+ dataset, which contains observations of more than 1,700 Type Ia supernovae. For more than 25 years, astronomers have used these exploding stars to track how the universe has expanded over time; measurements of them were central to the 2011 Nobel Prize-winning discovery that the expansion appears to be speeding up.

The team applied a recently proposed correction related to the ages of the stars that go on to produce Type Ia supernova explosions. "There is increasing evidence that the brightness of Type Ia supernovae depends on the age of the stars they come from," said Sarkar. "If this effect is not accounted for, it can lead to the erroneous conclusion that the expansion rate is accelerating." Once the correction was applied, the researchers found the observations no longer favored uniform acceleration, and instead suggested cosmic expansion may be slowing.

The team also tested whether the apparent acceleration looks the same in every direction, as the standard cosmological model assumes. They found it is directed mainly along the direction the solar system is moving locally, marked by a hotspot in the cosmic microwave background, and fades with distance. Sarkar said this directional pattern rejects dark energy regardless of whether the stellar-age correction is applied, and that applying the correction turns the remaining isotropic component into a deceleration.

Vincenzi's paper, using the same broader evidence, argues cosmic acceleration is still supported, and that the finding "allows the research community to focus on one of the biggest unanswered questions in physics: the nature of dark energy itself." Both interpretations face a bigger test soon: the Rubin Observatory's Legacy Survey of Space and Time is expected to measure hundreds of thousands of supernovae.