The crisis in cosmology continues: Critical problems in a recent defense of cosmic acceleration

Doubts about the standard model of cosmology are deepening after a new study identified fundamental problems in a recent analysis that defended the conventional view that the universe is currently undergoing accelerated expansion driven by dark energy.

Last November, researchers at Yonsei University in South Korea reported that, once a systematic bias related to the ages of Type Ia supernovae is taken into account, the universe may no longer be accelerating as previously thought. Instead, their analysis suggested that cosmic expansion has already entered a decelerating phase and that dark energy may not be a cosmological constant but rather a rapidly evolving component that is weakening with time.

The result attracted considerable attention because it closely agreed with the prediction from an independent cosmological probe: the baryon acoustic oscillation (BAO)-only measurements from the Dark Energy Spectroscopic Instrument (DESI).

In June, an international team led by researchers at the University of Southampton and including two Nobel laureates published a rebuttal challenging the Yonsei result. Their analysis found a much shallower relationship between supernova luminosity and age than that reported by the Yonsei team. They also argued that the ages of supernova progenitors may evolve much less with redshift than the ages of their host galaxies.

On this basis, the Southampton–led team concluded that the required age-bias correction would be small and that the expansion of the universe is still accelerating. The emerging crisis in cosmology therefore appeared to have been averted.

A disputed slope in the data

But a new study by the Yonsei researchers, published in Monthly Notices of the Royal Astronomical Society, finds that both central arguments advanced in the rebuttal are affected by critical analytical problems that undermine its defense of cosmic acceleration.

The crisis in cosmology, the researchers conclude, continues.

The first problem concerns the measurement of the “age-bias slope”—the relationship between the standardized luminosities of Type Ia supernovae and their ages.

To measure this relationship reliably, supernovae must be compared within a sufficiently narrow redshift interval. Otherwise, the evolution of the supernova population across the redshift range becomes entangled with the age-luminosity correlation being measured.

The rebuttal study instead combined supernovae over an unusually broad redshift interval. The new analysis demonstrates that this procedure artificially flattens the measured age-bias slope. As progressively higher-redshift supernovae are added to the sample, the recovered slope becomes systematically shallower. The same trend is also recovered in mock data constructed to reproduce the relevant observational conditions.

“The shallow age-bias slope reported in the rebuttal is therefore the result of a clear methodological problem in the analysis,” said lead author Dr. Chul Chung of Yonsei University. “When the comparison is restricted to an appropriate redshift interval, the strong age dependence reappears.”

The Yonsei team also found that the dust-extinction model underlying the rebuttal analysis requires a dependence on host-galaxy mass that is exactly opposite to the trend observed in large galaxy samples. The researchers therefore argue that this part of the model lacks observational support.

Why the correction still holds

The second problem concerns how the age-bias correction is calculated.

The correction depends on the product of two quantities: the change in supernova progenitor age with redshift and the slope of the age-luminosity relation. The rebuttal focused on the possibility that supernova progenitor ages evolve less strongly with redshift than host-galaxy ages do.

However, the new study points out that reducing the inferred age difference necessarily makes the corresponding age-luminosity slope steeper because the age difference appears in the denominator of the slope. The decrease in one quantity is therefore largely offset by the increase in the other.

According to the Yonsei researchers, the rebuttal emphasized only the reduced age evolution while overlooking the accompanying increase in the age-bias slope. When both effects are treated consistently, the resulting age-bias correction—and hence the cosmological conclusion—remains largely unchanged from that of the Yonsei team’s previous study.

Professor Young-Wook Lee, who led the project, said, “Supernova cosmology rests on the key assumption that the luminosity calibration relation—the Phillips relation—does not change with supernova age. Our results consistently show that this assumption is not valid.”

“To defend an extraordinary claim such as the accelerated expansion of the universe, this fundamental assumption must first be demonstrated to be correct,” Lee added. “Instead, the observational evidence against it is becoming stronger. Under these circumstances, we must seriously question whether the evidence still supports an accelerating universe and a cosmological constant.”

A cleaner test ahead

The researchers also stress that their age-bias-corrected supernova results agree remarkably well with the independent DESI BAO-only result, as demonstrated in their previous study. Both point toward a universe in which dark energy evolves with time and the present expansion is no longer accelerating.

To resolve the issue, the Yonsei team proposes an “evolution-free” test for future high-precision supernova cosmology. Rather than combining heterogeneous supernova populations whose average ages change with redshift, this approach would use only supernovae in young, similarly aged host galaxies across the full redshift range.

The researchers report that preliminary results from such a test are consistent with their earlier findings. In the future, the much larger supernova samples expected from the Vera C. Rubin Observatory will make it possible to carry out this direct test with substantially greater precision.

Such observations could provide a decisive answer to one of the most fundamental questions in modern science: what dark energy really is and whether the universe is accelerating or decelerating today.

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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