The expansion of the universe is still accelerating under the influence of dark energy, and astronomers are breathing a collective sigh of relief.
A new study published in Monthly Notices of the Royal Astronomical Society has systematically refuted a 2025 claim that dark energy might be weakening — a finding that would have upended cosmology’s standard model and required a fundamental rethinking of how the universe works.
“Thankfully, we have averted this crisis, but the mystery about why the rate of expansion of the universe is still accelerating remains,” said lead author Dr. Phil Wiseman of the University of Southampton. “By proving our measurements are correct, we can get back to trying to understand what this dark energy actually is, rather than wondering if it exists at all.”
The crisis that wasn’t
The controversy began in November 2025, when a team led by Professor Young-Wook Lee at Yonsei University in South Korea published a paper in the same journal. The Yonsei team argued that Type Ia supernovae — the standard cosmological distance markers used to discover dark energy in 1998 — grow systematically brighter as the universe ages. If true, this meant astronomers had been overestimating cosmic distances for decades, and the apparent acceleration of the universe’s expansion was an illusion. In reality, they claimed, the expansion was slowing down.
“If our results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago,” Lee said at the time.
The claim sent ripples through the cosmology community. It touched the most celebrated discovery in modern astronomy: the 1998 finding by Saul Perlmutter, Adam Riess, and Brian Schmidt that the universe’s expansion is accelerating, which earned them the 2011 Nobel Prize in Physics and introduced the concept of dark energy — the mysterious force or property of space that now accounts for roughly 70 percent of the universe’s total matter-energy content.
Two errors, one correction
The new study, led by Wiseman and including both Riess and Schmidt as co-authors, identified two fundamental errors in the Yonsei analysis.
First, the South Korean team assumed that the age of a galaxy is the same as the age of the star that explodes as a Type Ia supernova. This is not correct. White dwarf stars, which produce Type Ia supernovae, can smolder for billions of years before they reach the critical mass needed to detonate, meaning a supernova explosion can occur long after its host galaxy formed.
Second, the Yonsei team failed to apply a standard cosmological correction that accounts for the mass of galaxies hosting Type Ia supernovae. This correction is routine in modern cosmology and, when properly applied, eliminates the apparent age-brightness correlation the team had reported.
When both corrections are applied, the evidence for cosmic acceleration remains fully intact.
“Extraordinary claims require especially careful testing,” said Riess, who shares the 2011 Nobel Prize for the original discovery of dark energy. “What we find is that when we calibrate these supernovae, accounting for different host environments and populations, the evidence for cosmic acceleration remains remarkably consistent.”
Co-author Professor Mark Sullivan of the University of Southampton emphasized that the challenge was scientifically productive. “Challenging accepted theories and observations is fundamental to science. This is how progress is made. Although this idea did not turn out to be correct, it has opened up new ways of thinking about how supernovae explode and how we can measure dark energy more accurately.”
What this means for cosmology
The standard cosmological model (Lambda Cold Dark Matter, or ΛCDM, for short) remains intact. Under this model, dark energy acts as a cosmological constant, driving the accelerating expansion of the universe that has now been observed across multiple independent lines of evidence: Type Ia supernovae, the cosmic microwave background, baryon acoustic oscillations, and weak gravitational lensing.
The new study does not, however, resolve all tensions in cosmology. The Dark Energy Spectroscopic Instrument, a separate international project involving more than 900 researchers from 70 institutions, released its first three years of data in March 2025 showing hints that dark energy may evolve over time, potentially weakening. That result, based on 15 million galaxies and quasars, has reached a statistical preference of 2.8 to 4.2 sigma — tantalizing but not yet the 5-sigma gold standard required for a formal discovery.
The DESI result and the Yonsei claim are methodologically unrelated. DESI’s evidence for evolving dark energy comes from baryon acoustic oscillations, not supernova age-brightness correlations. Both lines of inquiry remain active.
Brodie Popovic, a co-author of the new study at the University of Southampton, summarized the team’s assessment: “This was a good opportunity to go back and go over all of our assumptions. It turns out, yes, we do understand this stuff and we’re accounting for it in our cosmology measurement.”
The ultimate mystery — what dark energy actually is and why it causes the universe’s expansion to accelerate — remains unsolved. But the question, at least, is back on solid ground.