The saga that threatened to rewrite the fate of the cosmos seems to have reached a resting point. An international team led by the University of Southampton thoroughly reviewed the data that in November 2025 had suggested that the expansion of the universe was slowing, and concluded that the cosmic acceleration – driven by the elusive dark energy – remains firm, as predicted by standard models of cosmology.

For nearly thirty years, the astronomical community took an extraordinary fact for granted: the universe is not only expanding, but it is doing so faster and faster. The finding, made in the late 1990s by observing Type Ia supernovae, earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics, and gave rise to the concept of dark energy: an unknown force that pushes the cosmos outward and that, according to current estimates, makes up about 70% of the total content of the universe.

That consensus was shaken in November 2025, when a team from Yonsei University in Seoul, led by Professor Young-Wook Lee, published a study in the journal Monthly Notices of the Royal Astronomical Society (MNRAS) that argued exactly the opposite: that the expansion had already entered a phase of slowdown. The news swept through the scientific world like a shockwave, because if confirmed, it would force a rewriting of the predicted future for the universe.

Lee's team analyzed a sample of more than 300 host galaxies for Type Ia supernovae—astronomy's so-called "standard candles," explosions of white dwarfs that always reach a similar intrinsic brightness and are therefore used to measure cosmic distances. Their central argument was that the brightness of these explosions depends not only on distance, but also on the age of the progenitor star: supernovae born from young stars would be slightly dimmer than those from old stars.

By applying this stellar age correction to the data, the South Korean researchers claimed to obtain a result that coincided with the independent measurements of the DESI (Dark Energy Spectroscopic Instrument) project, based on acoustic oscillations of baryons. According to their interpretation, both lines of evidence pointed in the same direction: dark energy would be weakening over time, and the universe, far from accelerating, would have already begun to slow down. The team claimed to have a statistical confidence of 99.99% in the age-brightness relationship they proposed.

Backing up DESI data

What gave strength to Yonsei's study was its apparent alignment with previous results from the DESI project itself, which in 2024 had already hinted that dark energy might not be an immutable constant, but a quantity that evolves over time. That coincidence between two independent methods—supernovae and acoustic oscillations—was interpreted by many as a serious indication that the standard cosmological model, known as ΛCDM, needed a thorough overhaul.

Faced with such a claim, the response of the scientific community was to put the data under the magnifying glass. The Southampton team, led by researcher Phil Wiseman and Professor Mark Sullivan, undertook an independent audit using observations from the Dark Energy Survey (DES), a project specifically designed to narrow down the properties of dark energy, in addition to the same dataset used by the South Korean team.

The result, published on June 10, 2026 in MNRAS under the title "Still accelerating: type Ia supernova cosmology is robust to host galaxy age evolution", identified two specific problems in the original analysis. The first was a technical omission: the 2025 study had not applied the standard correction for mass of the host galaxy, a well-established adjustment that shows that supernovae occurring in large galaxies are, on average, a few percentage points brighter than those in small galaxies.

The second problem was conceptual: Lee's team had assumed that the age of a host galaxy was equivalent to the age of the individual star that ended up exploding as a supernova, a simplification that, according to Southampton, introduces a systematic bias into the results. By reinstating the galactic mass correction and correcting for that assumption, the correlation between brightness and stellar age that the South Korean team had presented as evidence weakened substantially, and the data realigned with the standard cosmological model.

•     Omission of galactic mass correction: a standard setting in modern cosmology that the 2025 study did not incorporate, and which alone explains much of the signal that had been attributed to stellar age.

•     Erroneous assumption about stellar age: the average age of a galaxy was equated with the point age of the star that exploded, when both magnitudes are not interchangeable.

•     Correcting for both points, the correlation between brightness and age of the stellar population—which was at the heart of the South Korean argument—loses statistical force significantly.

Dark energy is a hypothetical component that would act as a kind of antigravity, pushing space to expand faster and faster. No one knows for sure what it is. It is estimated that it represents around 70% of the total content of the universe, well ahead of dark matter (about 25%) and ordinary matter – planets, stars, galaxies – which barely accounts for the remaining 5%. Scientists know that the universe slowed down for a good part of its history due to the effect of gravity, and that about 9,000 million years ago that trend was reversed: something began to gain momentum against gravity and the expansion began to accelerate. That "something" is, to this day, one of the greatest open mysteries in physics.

Far from closing the debate with the stroke of a pen, Professor Mark Sullivan himself, co-author of the Southampton study, underlined the value of the process: "Questioning accepted theories and observations is fundamental to science. This is how you progress. Although this idea did not turn out to be correct, it opened up new ways of thinking about how supernovae explode and how we can measure dark energy more accurately," he said.

Yonsei's team, for its part, did not give up. In June 2026, it participated in a joint workshop with researchers from Southampton, and in a follow-up study – already accepted in MMNAS – it argued that the age of stellar populations is still a relevant factor in the standardization of Type Ia supernovae, and that certain methods would have underestimated its importance. The academic discussion, in other words, is still open, although the majority consensus has once again leaned towards the standard model.

The South Korean team itself agrees with its critics on one point: the next major astronomical surveys will have the final word. The Vera C. Rubin Observatory, which has just begun full scientific operations, plans to discover more than 20,000 new supernova host galaxies in the coming years, with much more precise age measurements than those available so far. Added to that will be data from the European Space Agency's Euclid Space Telescope and NASA's future Nancy Grace Roman telescope, both specifically designed to test the nature of dark energy with unprecedented precision.

Until that data is available, the position of standard cosmology emerges stronger from this round, but the underlying enigma—what dark energy really is and why it dominates the fate of the universe—remains as open as the day it was discovered.

❓ Was it confirmed that the universe continues to expand faster and faster?

Yes. The University of Southampton study, published in June 2026 in the Monthly Notices of the Royal Astronomical Society, concluded that the accelerating expansion of the universe is still in place and that the data are consistent with the standard cosmological model.

❓ What did the study say that cast doubt on cosmic acceleration?

Research from Yonsei University (Seoul), published in November 2025, argued that when correcting the brightness of supernovae for the age of the progenitor star, the expansion of the universe showed signs of slowing down, which would imply a weakening of dark energy.

❓ What mistakes did the Southampton team find?

He found that the original study did not apply the standard correction for mass of the host galaxy and that it incorrectly assumed that the age of a galaxy is equivalent to the age of the star that exploded as a supernova.

❓ What is dark energy?

It is an unknown component that represents about 70% of the universe and acts as a repulsive force, accelerating the expansion of the cosmos for about 9,000 million years. Its exact nature remains one of the great mysteries of physics.

❓ Is the scientific debate on this topic closed?

Not quite. Yonsei's team presented a follow-up study defending their original hypothesis, and both groups agree that observatories such as the Vera C. Rubin, Euclid and the Roman telescope will provide definitive data in the coming years to settle the discussion.

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