A new analysis led by researchers at the Tata Institute of Fundamental Research, Mumbai, together with Professor Subir Sarkar of the University of Oxford, is challenging one of the central ideas in modern cosmology: that the universe is expanding at an accelerating rate because of “dark energy” associated with the quantum vacuum.
Their letter, published in Monthly Notices of the Royal Astronomical Society, argues that the evidence for cosmic acceleration may not be as strong as widely assumed. The conclusion is disputed. In the same issue of the journal, a separate paper co-authored by Professor Maria Vincenzi, also at the University of Oxford, concludes that observations continue to support an accelerating universe.
Reexamining More Than 1,700 Supernovae
Professor Sarkar, from Oxford’s Rudolf Peierls Centre for Theoretical Physics, worked with Animesh Sah and Mohamed Rameez of the Tata Institute of Fundamental Research in India to revisit one of cosmology’s most important collections of observational data. Their analysis focused on Pantheon+, a dataset containing observations of more than 1,700 Type Ia supernovae.
For more than 25 years, astronomers have relied on these exploding stars to track how the universe has expanded over time. Measurements of Type Ia supernovae were central to the discovery that the expansion of the universe appears to be speeding up, a breakthrough recognized with the 2011 Nobel Prize in Physics.
The researchers examined the Pantheon+ observations while including a recently proposed correction related to the ages of the stars that eventually produce Type Ia supernova explosions. They also investigated whether the apparent acceleration looks the same in every direction, as the standard cosmological model assumes.
“There is increasing evidence that the brightness of Type Ia supernovae depends on the age of the stars they come from,” said Professor Subir Sarkar of Oxford’s Rudolf Peierls Centre for Theoretical Physics, a co-author of the study. “If this effect is not accounted for, it can lead to the erroneous conclusion that the expansion rate is accelerating.”
A Universe That May Be Slowing Down
Once the researchers applied the stellar age correction, they found that the observations no longer favored a universe undergoing uniform acceleration. Their analysis instead indicates that, overall, cosmic expansion may be slowing rather than speeding up.
The team also examined whether the apparent acceleration could be anisotropic. In other words, they asked whether the effect varies depending on the direction in which it is measured. Such a result would conflict with the standard cosmological assumption that the universe behaves similarly in every direction on large scales.
Sarkar and his colleagues argue that if the apparent acceleration is directional, dark energy could not be responsible for it. In their interpretation, an effect produced by the quantum vacuum should not vary from one direction to another.
“We found that the inferred acceleration is directed mainly along the direction that we are moving locally, as indicated by the hotspot in the cosmic microwave background, and dies away with distance,” explains Professor Sarkar. “This is unaffected by the correction to the supernova brightness – so rejects dark energy independently of whether the correction is applied or not. The correction turns the isotropic component into a deceleration – which again rules out dark energy.”
Cosmologists Remain Divided
The findings challenge the prevailing view that the universe continues to expand at an accelerating rate, the conclusion associated with the Nobel Prize-winning discovery announced more than two decades ago.
However, other researchers examining the same broader question have reached a different conclusion. A paper published in the same journal issue and co-authored by Professor Maria Vincenzi of the University of Oxford argues that the available evidence still supports ongoing cosmic acceleration.
She comments: “The lead authors of our study are world experts in understanding how the environments of Type Ia supernovae affect cosmological measurements with more than a decade of experience in both supernova astrophysics and galaxy evolution. Our recent findings provide further confidence in the cosmological framework that has emerged over the past three decades and allow the research community to focus on one of the biggest unanswered questions in physics: the nature of dark energy itself.”
Rubin Observatory Could Help Settle the Debate
Both interpretations will soon face a much larger observational test. Data from the Rubin Observatory’s Legacy Survey of Space and Time (LSST) are expected to provide measurements of hundreds of thousands of supernovae.
That enormous new sample should give cosmologists a powerful opportunity to test whether the universe is truly accelerating, whether the apparent effect varies with direction, and what role, if any, dark energy plays in shaping the expansion of the cosmos.