Two supernova studies have reopened the dark energy debate, and they do not even agree with each other
One team says cosmic acceleration may be an illusion once you account for the ages of the exploding stars. Another says acceleration is real but dark energy is fading over time. A third camp, with two Nobel laureates, says the original measurements were fine all along. It is a fight over one cosmic ruler, not a new observation, and the referee has not blown the whistle.
A new September 2026 preprint re-reduced 2,884 Type Ia supernovae and found fresh evidence that dark energy is not constant but weakening over time, echoing earlier hints from DESI and the Dark Energy Survey. It landed alongside renewed coverage of a June paper arguing something more radical: that cosmic acceleration may be an illusion once you correct supernova brightness for the ages of the stars that made them. The two challenges disagree with each other, and a rebuttal camp including two Nobel laureates says the original measurements hold up. This is a statistical dispute over existing data, not a new observation.
Dark energy is the placeholder name for the biggest mystery in physics: whatever is making the universe's expansion speed up. It was inferred in the late 1990s from distant supernovae that looked dimmer, and therefore farther away, than a steadily expanding universe could explain. That discovery won the 2011 Nobel Prize. Ever since, the standard model has treated dark energy as a cosmological constant, a fixed property of empty space making up roughly 70 per cent of the universe. In September 2026 that comfortable picture took two separate hits in the same news cycle. The trouble, for anyone hoping for a clean headline, is that the two hits point in different directions, and a well-armed third camp argues the more radical of them, the illusion claim, does not hold up.
What is the actual news?
The genuinely new item is a preprint from the University of Queensland, led by PhD candidate Ryan Camilleri with Professor Tamara Davis, submitted on 4 September 2026. It rebuilt three decades of supernova surveys into one consistent catalogue of 2,884 Type Ia supernovae and reports fresh evidence that dark energy changes over time rather than staying constant.
Its arrival coincided with a wave of coverage of a second, more provocative study: a peer-reviewed paper, published back in June 2026, arguing that the acceleration itself may be an artefact. Because that June paper resurfaced in September, the two got lumped together into a single "dark energy in doubt" story. They should not be. They make close to opposite claims.
First, what is dark energy, and how do we even measure it?
The ruler for all of this is the Type Ia supernova, the thermonuclear detonation of a white-dwarf star. After astronomers "standardise" these explosions, correcting for how quickly each one brightens and fades and for its colour, their peak brightness is nearly identical. That makes them standardisable candles: how bright one looks tells you how far away it is, and plotting distance against redshift traces how the universe has expanded. The 1998 result was that distant supernovae looked slightly too dim, which implied the expansion was accelerating, and dark energy was the explanation.
The entire current fight is about whether that ruler has a subtle flaw.
What does the new September paper say?
The Queensland team keeps acceleration and keeps dark energy, but argues dark energy is not constant. Fitting their unified 2,884-supernova catalogue together with DESI's galaxy-survey measurements and the cosmic microwave background, they find a preference for an evolving-dark-energy model over the standard constant one at about 3.3 sigma. In plainer terms, the data seem to favour a dark energy that was stronger in the past and is weakening.
"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. Davis added that the team's earlier Dark Energy Survey data "first showed hints that dark energy may be time varying, and this new compilation also sees a deviation from the standard model, although in a slightly different direction." Crucially, this paper is a preprint, submitted to a journal but not yet peer-reviewed, and it supports dark energy's existence. It sits in the same lane as DESI's results, which we covered in our look at the DESI dark-energy findings.
And the "acceleration is an illusion" paper?
This is the more radical, and more cautious-making, claim. A peer-reviewed study in Monthly Notices of the Royal Astronomical Society, led by Animesh Sah and Mohamed Rameez of the Tata Institute of Fundamental Research with Subir Sarkar of Oxford, applies a correction for the age of each supernova's parent stars to the Pantheon+ catalogue of more than 1,700 supernovae. Their argument has two prongs: that the apparent acceleration is largely directional, concentrated along the axis of our local motion through the cosmos rather than being the same in every direction, and that once the age correction is applied, the all-sky average tips towards deceleration. Since a genuine vacuum dark energy must look the same in every direction, a direction-dependent signal looks more like a local kinematic effect than a property of space.
Two important caveats. First, this paper was published in June 2026, so despite the September buzz it is not new. Second, its headline deceleration result leans on a contested stellar-age correction proposed by a separate group (Son and colleagues), and the most dramatic "tension with the standard model" figures belong to that other group, not to this analysis. It is a re-analysis of existing data, not a new observation.
So has dark energy been debunked?
No, and it is worth saying plainly. Start with the fact that the two challenger papers disagree: one says there may be no dark energy at all, the other says dark energy is real but fading. They cannot both be the headline.
Then there is the rebuttal camp, which moved fast and carries weight. A June 2026 MNRAS paper led by Phil Wiseman of the University of Southampton, with co-authors including 2011 Nobel laureates Adam Riess and Brian Schmidt, argues that supernova cosmology is robust to exactly this host-galaxy age effect: apply the standard host-mass correction, they find, and the residual age dependence vanishes, changing the dark-energy result by less than 0.01. "The previous and well accepted measurements were, in fact, fine and our current understanding of the fate of the universe remains robust," Wiseman said. Riess put it more pointedly: "Extraordinary claims require especially careful testing," and the evidence for acceleration "remains remarkably consistent." Separately, a July preprint from Ray and colleagues that reworked the same Pantheon+ data reported the universe is still accelerating, at odds with the deceleration claim. This is a contested claim in mid-argument, not an overturning.
Why does the age of stars matter so much?
Because it could quietly bend the ruler. Even after standardisation, a Type Ia supernova's brightness may still depend a little on the age of the stellar population it came from. Distant supernovae, seen in the early universe, tend to come from younger populations than nearby ones. If brightness drifts with that age and no one corrects for it, the most distant supernovae get misjudged in a way that can mimic the extra dimming attributed to acceleration. The whole dispute is whether a real residual age effect survives the corrections astronomers already apply. One side says yes and it matters enormously; the other says the existing mass correction already handles it.
When will we actually know?
With more and better supernovae, measured consistently. The referee most often named is the Vera C. Rubin Observatory, whose survey is expected to catalogue hundreds of thousands of Type Ia supernovae, far beyond the roughly 1,700 to 2,900 that both challenger papers re-analyse. Until that data arrives, this remains a fight over how to correct a few thousand existing explosions, not a new window on the cosmos. For more from the frontier, see the science section.
Three positions, one ruler
| Camp | Claim | Status |
|---|---|---|
| Sah, Rameez & Sarkar (June 2026) | Acceleration may be an illusion; directional, and deceleration once age-corrected | Peer-reviewed (MNRAS) |
| Camilleri et al. (September 2026) | Acceleration is real, but dark energy is evolving/weakening | Preprint, not yet refereed |
| Wiseman et al. / Ray et al. | The measurements are robust; acceleration holds up | Peer-reviewed rebuttal + preprint, includes 2 Nobel laureates |
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