This article covers a provocative study by Young-Wook Lee and colleagues that challenges the consensus that cosmic expansion is accelerating. The authors argue that accounting for the ages of type Ia supernova progenitors removes the apparent acceleration and suggests deceleration began ~1.5 billion years ago. Critics, including Nobel laureate Adam Riess, warn that progenitor ages are hard to determine, while others call the results provocative but not easily dismissed. Planned observations from the Vera C. Rubin Observatory—expected to find > 20,000 new supernova hosts within five years—should provide decisive tests.
New Study Challenges Cosmic Acceleration — Could the Universe Be Slowing Down?
This article covers a provocative study by Young-Wook Lee and colleagues that challenges the consensus that cosmic expansion is accelerating. The authors argue that accounting for the ages of type Ia supernova progenitors removes the apparent acceleration and suggests deceleration began ~1.5 billion years ago. Critics, including Nobel laureate Adam Riess, warn that progenitor ages are hard to determine, while others call the results provocative but not easily dismissed. Planned observations from the Vera C. Rubin Observatory—expected to find > 20,000 new supernova hosts within five years—should provide decisive tests.
02:54, 10.11.2025Science
New Paper Questions the Evidence for an Accelerating Universe
A contentious new study published in Monthly Notices of the Royal Astronomical Society and led by Yonsei University astronomer Young-Wook Lee argues that the widely reported acceleration of the universe's expansion may be an artifact of how we measure exploding stars. By accounting for the ages of type Ia supernova progenitors, the team finds the apparent cosmic acceleration disappears and suggests the expansion has been decelerating for roughly 1.5 billion years.
Why this matters
Type Ia supernovae have long been treated as "standard candles"—objects of nearly uniform intrinsic brightness—allowing astronomers to estimate distances using the inverse-square law. Observations that distant type Ia supernovae appear dimmer than expected were interpreted as evidence that cosmic expansion is speeding up, leading to the introduction of a mysterious component called dark energy.
Lee and colleagues contend that intrinsic brightness varies with the age of the progenitor systems. Using a sample drawn from 300 galaxies, they modelled how progenitor age affects supernova brightness and concluded that stellar astrophysics effects, rather than purely cosmological distance, can produce the observed dimming.
"Our study shows that the universe has already entered a phase of decelerated expansion at the present epoch and that dark energy evolves with time much more rapidly than previously thought," Lee said in a statement.
Implications and reactions
If Lee's interpretation holds up under independent scrutiny, it would force a major revision to current cosmological models and our understanding of dark energy — even opening the theoretical possibility of a future contraction or 'big crunch.' But the claim is controversial and far from settled.
Adam Riess (Space Telescope Science Institute), a co-recipient of the 2011 Nobel Prize for work that established the accelerating expansion, cautioned that progenitor ages are difficult to determine: "The theory behind this is weak because of a lack of certainty about how the [star] forms."
Other experts describe the work as provocative but worthy of careful consideration. University of Durham cosmologist Carlos Frenk called the results "tantalising" and said they "cannot simply be dismissed."
How the question could be resolved
Testing this hypothesis requires many more well-characterised supernovae with reliable host-galaxy age measurements. Co-lead author Chul Chung noted that the Vera C. Rubin Observatory in Chile will soon transform the dataset: over the next five years, it is expected to discover more than 20,000 new supernova host galaxies, enabling much tighter tests of how progenitor age correlates with brightness.
For now, the study is an important reminder that cosmological conclusions depend critically on astrophysical assumptions. Independent analyses, larger samples, and complementary distance probes (for example, baryon acoustic oscillations and the cosmic microwave background) will be needed before the community revises the standard picture.
Reference: Young-Wook Lee et al., Monthly Notices of the Royal Astronomical Society.