For a quarter-century, the cosmos seemed settled on one astonishing point: the Universe is not just expanding, it is accelerating. A new analysis now challenges that bedrock idea, arguing the expansion has already begun to slow.
If the finding holds, it could reshape everything from how we measure cosmic distances to what we think dark energy really is—and where our Universe is headed.
From runaway expansion to a provocative rethink
Since 1998, observations of Type Ia supernovae—stellar explosions bright enough to be seen across billions of light-years—have served as the Universe’s distance markers. Those measurements revealed that faraway galaxies were receding faster than expected, a result that earned the 2011 Nobel Prize in Physics and vaulted “dark energy” into cosmology’s lexicon.
The standard model, known as Lambda-CDM, assumes dark energy is a cosmological constant: steady, featureless, and dominant enough to drive an ever-faster expansion. But a team led by researchers at Yonsei University in Seoul argues that a subtle bias in supernova data has skewed that conclusion.
The new claim: a slowing Universe, not a speeding one
In a study published in Monthly Notices of the Royal Astronomical Society, the team reports that Type Ia supernovae are not perfectly standardized after all. Their peak brightness appears to depend on the age of the stars that produced them: explosions from younger stellar populations look systematically fainter, while those from older populations look brighter, even after luminosity corrections.
The researchers used ages of 300 supernova host galaxies to quantify this effect at extremely high significance, and then corrected the supernova data accordingly. Once that correction was applied, the supernova-based evidence for present-day acceleration vanished.
“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,” said Professor Young-Wook Lee of Yonsei University.
Crucially, the corrected supernova results align better with a newer cosmological picture favored by independent measurements from the Dark Energy Spectroscopic Instrument (DESI). Those measurements, rooted in baryon acoustic oscillations (BAO) and the cosmic microwave background (CMB), suggest dark energy may be changing with time rather than remaining constant.
What changed in the data—and why it matters
Type Ia supernovae have long been treated as “standard candles”: once their light curves are standardized, their intrinsic brightness is assumed to be uniform. That assumption lets astronomers turn apparent brightness into distance, building a ladder that reaches across the observable Universe. The Yonsei team says the ladder’s rungs have an overlooked tilt—linked to progenitor age—that becomes more important as one looks deeper into space and further back in time.
Correcting for that tilt does two things. First, it reduces the inferred dimming of distant supernovae that had been attributed to accelerating expansion. Second, it changes how supernova distances mesh with BAO and CMB constraints, breaking the tight embrace between supernova data and the classic Lambda-CDM model.
“By contrast, our analysis — which applies the age-bias correction — shows that the Universe has already entered a decelerating phase today,” Professor Lee said. “Remarkably, this agrees with what is independently predicted from BAO-only or BAO+CMB analyses, though this fact has received little attention so far.”
DESI, BAO, and an evolving dark energy
BAO act as a cosmic “standard ruler,” an imprint of primordial sound waves frozen into the distribution of galaxies. DESI’s early results, combined with CMB data, have already hinted that dark energy might not be a fixed cosmological constant. The Yonsei reanalysis dovetails with that possibility, finding a better fit when dark energy’s influence weakens with time.
In earlier combined analyses, DESI and uncorrected supernova data pointed to a Universe still accelerating today but likely to decelerate in the future. The new work argues that, after the age-bias correction, the transition to deceleration has already occurred.
What this would—and would not—change
Deceleration does not mean contraction. The new claim is that the expansion rate is slowing relative to gravity’s pull, not that galaxies are now rushing back together. The Universe would still be getting larger; it just would not be speeding up in doing so.
If dark energy is evolving rather than constant, the implications are profound. Theoretical models of dynamical dark energy—where the parameter describing cosmic pressure, w, changes over time—would take center stage. The timeline of the distant future would also be rewritten, with possibilities ranging from a gentle coasting expansion to more exotic fates, depending on how dark energy’s strength evolves.
Closer to home, an evolving-dark-energy picture might help ease the “Hubble tension,” the persistent disagreement between expansion rates inferred from the early Universe (CMB) and those measured from the local Universe (supernovae and other probes). A systematic bias in supernova distances would ripple through that debate.
Caution, debate, and the road to confirmation
Extraordinary claims in cosmology demand extraordinary cross-checks. Supernova standardization has been refined for decades, and numerous teams have hunted for hidden systematics. The Yonsei analysis will now face intense scrutiny: independent reanalyses of the same supernova compilations, fresh measurements of host-galaxy ages, and tests that restrict samples to uniform stellar populations.
“Within the next five years, with the Vera C. Rubin Observatory discovering more than 20,000 new supernova host galaxies, precise age measurements will allow for a far more robust and definitive test of supernova cosmology,” said Professor Chul Chung of Yonsei University.
Beyond supernovae, other cosmological yardsticks will weigh in. Weak gravitational lensing tracks the clumping of matter; redshift-space distortions chart the growth of cosmic structure; time-delay measurements from lensed quasars provide expansion-rate checks untethered to supernova physics. If multiple independent probes converge on a decelerating present-day Universe, Lambda-CDM would face its most serious test yet.
How we’ll know
The authors say they are conducting an “evolution-free” test that uses only supernovae from young, coeval host galaxies across the full redshift range—minimizing the very age effects they identified. Early results, they report, support the main conclusion, though comprehensive details will need to be vetted by the broader community.
Meanwhile, DESI will release progressively larger galaxy samples, the European Space Agency’s Euclid mission is beginning precision measurements of cosmic geometry, and NASA’s Nancy Grace Roman Space Telescope will add deep infrared supernova surveys. Together with the Rubin Observatory’s avalanche of discoveries, these projects will either cement an evolving-dark-energy paradigm—or reaffirm the accelerating Universe with tighter error bars.
The bigger picture
Cosmology advances by sharpening its tools and questioning its assumptions. Type Ia supernovae transformed our view of the Universe, but they are complicated astrophysical events born in diverse environments. If the brightness of these explosions depends on the age of their progenitors more than we thought, our cosmic yardstick needs a careful recalibration.
Whether the Universe is already decelerating or not, the message is the same: when our standard candles flicker with hidden histories, the cosmos nudges us to look again. That is not a setback. It is how science turns uncertainty into understanding—and why the next few years of sky surveys may redraw the most fundamental curve in the Universe: how fast it grows with time.