New supernova studies cast doubt on dark energy theory

Two new analyses published this month are raising fresh questions about one of modern cosmology's foundational ideas: that a mysterious force called dark energy is driving the universe's expansion to accelerate. The studies arrive amid a broader period of uncertainty about the standard cosmological model, which failed to win majority support in the largest-ever survey of physicists worldwide.

A Universe That May Be Slowing Down

A study published in Monthly Notices of the Royal Astronomical Society by researchers at the Tata Institute of Fundamental Research in Mumbai and the University of Oxford reanalyzed the Pantheon+ dataset of more than 1,700 Type Ia supernovae — the same class of stellar explosions central to the Nobel Prize-winning discovery of cosmic acceleration in the late 1990s. After applying corrections for the ages of the stars that produce these explosions, the team found that the data no longer favored an accelerating universe. Instead, their results pointed toward a cosmos whose expansion is slowing down.sciencedaily+2

"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. "If this effect is not accounted for, it can lead to the erroneous conclusion that the expansion rate is accelerating."sciencedaily

The researchers also found that the apparent acceleration varies depending on direction — a result difficult to reconcile with dark energy, which should act uniformly across all of space.arxiv+1

A Separate Study Finds Dark Energy May Be Changing

In a separate effort released last week, a team led by University of Queensland Ph.D. candidate Ryan Camilleri compiled 2,884 Type Ia supernovae into the most comprehensive dataset of its kind, combining historic observations with data from the Dark Energy Survey. Rather than confirming that dark energy is fixed and unchanging, as the standard ΛCDM model assumes, the analysis found further evidence that dark energy may vary over time.sci+2

"Instead of confirming the standard model of cosmology which assumes dark energy is fixed and unchanging, we have more evidence that dark energy may change over time," Camilleri said. The finding echoes results from the Dark Energy Spectroscopic Instrument, which independently detected hints of time-varying dark energy.china.org.cn+1

Cosmologists Remain Divided

Not all researchers accept these conclusions. A paper published in the same journal issue as Sarkar's study, co-authored by Professor Maria Vincenzi at Oxford, argues that the evidence for cosmic acceleration remains robust after accounting for the effects Sarkar's team identified. A separate August analysis also pushed back, concluding that the universe continues to expand as current models predict.physics.ox+3

The debate unfolds against a backdrop of wider uncertainty. The largest-ever survey of physicists, led by Perimeter Institute's Niayesh Afshordi and published earlier this year, found that the ΛCDM model failed to secure majority support among respondents. Only 51% agreed the early universe underwent inflation, and views on dark matter were scattered across multiple competing explanations.scitechdaily+1

Both sides now look to the Rubin Observatory's Legacy Survey of Space and Time, which will measure hundreds of thousands of supernovae and could help settle whether the universe is truly accelerating — or whether one of cosmology's biggest discoveries needs to be reconsidered.sciencedaily