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. 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." 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.
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