A new study has taken a fresh look at a late 2025 Yonsei University study claiming the universe’s expansion is stalling out and instead has found that the universe is still expanding as previously believed.
Led by researchers from the University of Southampton, the team behind the new analysis acknowledged the scientific value of the 2025 team’s work, noting that their calculations were not wrong but lacked the additional data needed to confirm the universe’s expansion rather than refute it.
The lead author of the 2025 study, Dr. Chul Chung, told The Debrief that he and his colleagues “have already submitted our response to Wiseman, Riess et al. 2026 on May 14,” adding that the paper, “Still accelerating: age bias correction in supernova cosmology is robust to host-progenitor age mapping,” has also been posted on arXiv.
“The abstract summarizes the two main issues we identify in Wiseman et al., so I think it provides a concise overview of our position,” Chung told The Debrief.
Challenging Universal Expansion
According to a statement announcing the findings, scientists first confirmed the universe’s expansion was accelerating in 2011, a discovery that won astrophysicists Professor Adam Riess and Professor Brian Schmidt the Nobel Prize in Physics. Since that discovery, cosmological models have incorporated this accelerating expansion, with experiments on dark matter and dark energy attempting to explain it.
Then last year, a separate team of astronomers announced that a new analysis of stellar phenomena indicated that the universe was no longer expanding. Adding to the controversy was the team’s claim that dark energy may not exist, citing the lack of concrete evidence for a universe undergoing accelerating expansion.
According to the Southampton-led team behind the new study, the 2025 study suggested that previous methods used to measure the universe’s expansion, such as measuring supernovae, “were fundamentally flawed.”
“This points to a time-varying dark energy and a universe that is no longer accelerating, representing a potential paradigm shift in cosmology if confirmed,” explained Professor Chul Chung, co-lead author on the study, in a 2025 email to The Debrief.
The claims sent shockwaves through the scientific community, a result that the researchers note could have dismantled the Nobel Prize-winning team’s historic findings, and “nearly three decades of astronomical progress.”
When discussing the 2025 team’s work, the lead author of the new study, Dr Phil Wiseman, said the team’s findings and the resulting debate were not necessarily incorrect. Instead, the Southampton researcher said it was primarily a result of a scientific understanding.
“The previous and well-accepted measurements were, in fact, fine and our current understanding of the fate of the universe remains robust.”
Recalculating Galaxy Masses and White Dwarf Star Ages Reestablishes Expansion
In their reanalysis, the Southampton-led team recalibrated the original data to account for variables not in the original study. According to Wiseman, these included “different host environments and populations.”
For example, the research team looked at Type 1a supernovae, which are violet, extremely bright explosions of white dwarf stars. The authors of the 2025 study claimed that this class of supernova had “different maximum brightness,” which had tricked astronomers into thinking the cosmos was accelerating, when their analysis said it was actually slowing down.
When they took a fresh look at the data, the Southampton team found that they had incorrectly estimated the age of these stars. The new study also says the 2025 team didn’t account for the mass of the galaxies hosting these cosmological phenomena, which they described as “a standard correction used in modern cosmology to prove accuracy.”
When his team factored those data points in, Wiseman said the evidence for a stalling expansion was not supported. Instead, the researcher observed that “the evidence for cosmic acceleration remains remarkably consistent.”
In their reply on ArXiv, the Yonsei University team said the new research failed to factor in two variables of their own, adding that “when these two effects are consistently combined in computing the redshift-dependent magnitude correction, the final correction, and hence the resulting cosmological impact, remain largely unchanged,” from the original 2025 calculations.
“Extraordinary Claims Require Especially Careful Testing”
When discussing the new study’s findings, Southampton Professor Mark Sullivan highlighted the benefits of scientific review, noting that “this is how progress is made.”
“Although this idea did not turn out correct, it has opened up new ways of thinking about how supernovae explode and how we can measure dark energy more accurately,” the researcher added.
Paper co-author, Dr Brodie Popovic agreed, noting that the 2025 study provided astronomers a good opportunity to “go back over all of our assumptions,” adding that “it turns out, yes, we do understand this stuff, and we’re accounting for it in our cosmology measurement.”
In a salute to the Sagan Standard established by astronomer Carl Sagan, which says “extraordinary claims require extraordinary evidence,” Reiss seemed to celebrate the new findings that reestablish his original work by quipping that “extraordinary claims require especially careful testing.”
With the accelerating expansion of the universe seemingly reestablished, at least for now, Dr Wiseman said: “we can get back to trying to understand what dark energy actually is, rather than wondering if it exists at all.”
“Thankfully we have averted this crisis, but the mystery about why the universe is still accelerating in size remains,” he added.
The study “Still Accelerating: Type Ia supernova cosmology is robust to host galaxy age evolution” was published in Monthly Notices of the Royal Astronomical Society.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.