What's the directional argument?

The researchers went further. They tested whether the apparent acceleration looks the same in every direction — a core assumption of the standard cosmological model.

Their analysis suggests it does not. The inferred acceleration lines up mostly with our own galaxy's motion relative to the cosmic microwave background, the faint afterglow of the Big Bang, and fades at greater distances.

"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," Sarkar said. "The correction turns the isotropic component into a deceleration, which again rules out dark energy."

Because dark energy, if real, should push the universe outward equally in every direction, a directional effect would be hard to square with that interpretation.

Why is this controversial?

The paper appears alongside a counter-study, also in Monthly Notices of the Royal Astronomical Society and co-authored by Professor Maria Vincenzi of Oxford. Using overlapping supernova data, that group concluded the evidence still supports an accelerating cosmos.

"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," Vincenzi said. "Our recent findings provide further confidence in the cosmological framework that has emerged over the past three decades."

Both teams examined how stellar age affects supernova brightness. They disagree on whether the correction is large enough to overturn acceleration.

Can upcoming observatories settle the debate?

Yes — at least in principle. The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will record hundreds of thousands of new supernovae within the next decade. That sample will be large enough to test:

• Whether the universe is truly accelerating
• Whether the effect varies with direction
• What role, if any, stellar age plays in supernova brightness

Until those data arrive, the dispute remains live. The 2011 Nobel Prize for the discovery of acceleration will not be undone by a single reanalysis. But the new result means cosmologists cannot ignore the question of whether their primary evidence has been misread.

What would it mean if the universe is decelerating?

A slowing universe would remove the most direct observational motivation for dark energy — a placeholder name for whatever is supposedly driving the apparent acceleration. It would not, however, eliminate the broader mystery of what the universe is mostly made of. Ordinary matter accounts for roughly 5% of cosmic contents. Dark matter, inferred from galaxy rotation and gravitational lensing, accounts for another ~25%. Whatever the verdict on acceleration, the rest of the cosmic inventory remains unexplained.

For now, two peer-reviewed studies sit side by side in the same journal, drawing opposite conclusions from overlapping data. The next round of evidence will come not from laptops, but from telescopes in Chile.