Is the Universe Still Accelerating? The Yonsei–Southampton Supernova Dispute Explained

The Yonsei–Southampton dispute concerns a fundamental question in cosmology: is the expansion of the Universe still accelerating, or is some of the apparent acceleration caused by a systematic bias in measurements of Type Ia supernovae?

The answer has not been settled. The Yonsei team argues that the ages of the stellar populations behind these explosions affect their brightness even after standard corrections. The Southampton-led team—including Nobel laureates Adam Riess and Brian Schmidt—argues that this effect has been substantially overestimated and that cosmic acceleration remains the best explanation of the data. 813

What Yonsei claimed in 2025

Type Ia supernovae are used as “standard candles.” Astronomers compare their observed brightness with their expected intrinsic brightness to estimate cosmic distances. If their luminosity changes systematically with the age of the surrounding stellar population, however, that comparison could distort the inferred history of cosmic expansion.

In a study published in November 2025, the Yonsei team reported a strong relationship between the standardized brightness of Type Ia supernovae and the age of the stars in their host galaxies. It calculated that changes in host-galaxy ages with redshift create a significant systematic shift in the distances inferred from supernovae. After applying an age-related correction, the team found no convincing evidence of present-day acceleration and proposed that the Universe may have entered a phase of decelerating expansion.

If confirmed, the result would have major implications for the standard ΛCDM model. In that framework, the cosmological constant Λ—the simplest form of dark energy—is associated with the accelerating expansion of the Universe. A recent transition to deceleration would require either evolving or weakening dark energy, or a major revision of how supernovae and gravity are interpreted.

Southampton’s response

The Southampton-led team challenged two central elements of the Yonsei analysis.

1. A galaxy’s age is not necessarily the progenitor’s age

Yonsei uses the age of a host galaxy’s stellar population as an indicator of the age of the star—or white-dwarf system—that ultimately exploded. Southampton argues that these are different quantities. A galaxy contains stars of many ages, so its average stellar age does not directly determine the age of the specific progenitor system involved in a supernova. 18

Under this interpretation, the way host-galaxy ages change with redshift does not automatically translate into an equivalent change in the ages of supernova progenitors.

2. The host-galaxy mass correction

Southampton also argued that Yonsei’s analysis did not include the conventional correction for host-galaxy mass, often called the “mass step.” This adjustment is used in modern supernova analyses because the brightness of these explosions is linked to environmental properties that also correlate with the age of stellar populations.

When the Southampton team applied the correction to the same data, it found a much weaker relationship between host-galaxy age and the Hubble residual—the difference between a supernova’s measured brightness and the brightness expected after standardization. Its conclusion was that the age correction was not large enough to erase the evidence for cosmic acceleration.

Yonsei’s counter-rebuttal

Yonsei argues that the Southampton analysis does not resolve the issue. In its newer analysis, the team says Southampton combined supernovae across an excessively broad redshift range, approximately z = 0.04 to z = 0.42.

Across that interval, the mean age of host galaxies changes by roughly 3 billion years. Yonsei argues that grouping objects with such different ages before performing the regression can make the age–brightness relationship appear artificially flatter than it really is. 2

The team also says Southampton treated two linked quantities as too independent:

• the slope of the relationship between age and brightness, and
• the extent to which progenitor ages change with redshift.

Even if progenitor-age evolution is smaller than the evolution in the average age of the host galaxies, Yonsei argues, a steeper age–luminosity slope could still produce a substantial overall correction. The team also questions whether the treatment of galaxy mass and dust in the rebuttal analysis may have artificially suppressed the age signal. 35

What the disagreement is really about

This is not simply a contest between “acceleration” and “deceleration.” At its core, it is a disagreement over whether Type Ia supernova data have been corrected adequately for differences in stellar populations and galactic environments.

Southampton’s position is that:

• the age of a host galaxy cannot be used directly as the age of the progenitor,
• the host-galaxy mass correction absorbs much of the apparent age dependence, and
• the remaining effect is too small to overturn the standard cosmological interpretation.

Yonsei’s position is that:

• using host-galaxy age as a proxy is justified by its relationship to the underlying stellar populations,
• analyzing supernovae across a broad redshift range underestimates the age–brightness slope, and
• the age correction can remain significant even if progenitor-age evolution is smaller than the evolution in host-galaxy ages. 2

Until larger, more homogeneous samples are analyzed independently, neither side has turned the dispute into a definitive overthrow of the established cosmological picture.

What DESI adds to the debate

Results from the Dark Energy Spectroscopic Instrument, or DESI, provide important context but do not settle the supernova disagreement on their own. DESI’s measurements of baryon acoustic oscillations (BAO) map the relationship between cosmic distance and redshift using a method different from that of supernovae.

Some combinations of DESI data with measurements of the cosmic microwave background and supernova samples have shown a preference for models in which dark energy changes over time. In one DESI–CMB combination, the preference over ΛCDM was reported at the 3.1-sigma level. When different supernova samples are added, the statistical preference changes depending on the sample used.

That is not independent proof that the Universe is currently slowing down. BAO measurements constrain the history of cosmic distances, while the interpretation in terms of evolving dark energy depends on the model and on which datasets are combined. More recent analyses describe the preference for dynamical dark energy as modest or inconclusive, and some find no statistically significant advantage over ΛCDM. 17

How could the dispute be resolved?

One particularly clean test would compare supernovae in host galaxies with as similar stellar ages as possible at different redshifts. That would reduce the need for a large, theoretically imposed age correction.

Other priorities include:

• larger supernova samples selected in a uniform way,
• spectroscopic measurements of host galaxies,
• a clearer distinction between the age of a galaxy’s overall stellar population and the age of the actual progenitor system,
• independent checks of mass, dust and local-environment effects, and
• new observations from major facilities such as the Vera C. Rubin Observatory and NASA’s Nancy Grace Roman Space Telescope.

The bottom line

The most cautious summary is that cosmic acceleration remains the established interpretation, while the new dispute concerns how reliably Type Ia supernovae can be corrected for age and environment. Southampton argues that Yonsei’s corrections are too large; Yonsei responds that the rebuttal underestimated the age slope and failed to account properly for the mathematical connection between the relevant quantities. 18

At the same time, DESI results leave open the possibility that dark energy is not perfectly constant. That possibility is still a long way from proving that the Universe has already shifted from accelerating expansion to deceleration. The outcome will depend on better supernova measurements and independent cosmological tests.