Supernova explosion in the center of the galaxy “Elements of this image furnished by NASA” (Credit: muratart on Shutterstock)

After accounting for star age at time of explosion, researchers say our universe may be slowing down.

In A Nutshell

• Younger supernovae appear dimmer than older ones, but astronomers haven’t been accounting for this age difference when measuring cosmic expansion

• After correcting for stellar age bias, the data flips: instead of showing an accelerating universe, measurements now suggest the cosmos may already be slowing down

• Three independent measurement techniques now align after the correction, telling the same story about a decelerating universe rather than conflicting results

• If confirmed, dark energy isn’t a constant force pushing the universe apart forever; it may be weakening, changing predictions about the cosmos’s ultimate fate

For 27 years, astronomers have asserted the universe is speeding up. According to the prevailing theory, “dark energy” has been pushing everything apart faster and faster. Now, a study is suggesting we’ve been misreading the evidence all along.

How could this have happened? We’ve been comparing apples to oranges, or more precisely, young exploding stars to old ones, without accounting for the difference.

Researchers at Yonsei University in South Korea discovered that the age of a star matters when it explodes. Stars that detonated billions of years ago came from younger stellar populations than those exploding today, and younger supernovae appear dimmer. Astronomers have been interpreting that dimness as proof of cosmic acceleration when it might just be an age effect.

After correcting for this bias, the data tells a different story. The universe might not be accelerating at all. In fact, it may already be slowing down.

The Cosmic Speed Trap

Since 1998, astronomers have used exploding stars called Type Ia supernovae as cosmic measuring sticks. When two teams noticed distant explosions looked dimmer than expected, they concluded something must be stretching space itself, making the light travel farther. They called it dark energy, and the discovery won a Nobel Prize in 2011.

But that conclusion assumed that a supernova’s brightness doesn’t depend on the age of the star that exploded. New measurements show that assumption is wrong.

Lead author Junhyuk Son and his colleagues measured the ages of galaxies where supernovae occurred. A clear pattern emerged. Supernovae from younger galaxies consistently appear dimmer than those from older galaxies.

The standard correction astronomers use, which is based ongalaxy size, doesn’t fix this problem. Galaxy size changes slowly over time, but galaxy age changes dramatically. They’ve been correcting for the wrong thing.

The Vera C. Rubin Observatory began scientific operations this year and could answer vital questions about our own solar system and the wider universe. (Credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA — Attribution (CC BY 4.0))

“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,” lead researcher Young-Wook Lee explains in a statement. “If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago.”

What the Corrected Data Shows

When the team applied their age correction and combined supernova measurements with two other independent ways of measuring cosmic expansion (baryon acoustic oscillations from the early universe and the cosmic microwave background), the results shifted dramatically.

Before the correction, combining all three datasets gave a deceleration parameter of -0.27 ± 0.06 (negative means accelerating). After the correction, that number flipped to +0.178 ± 0.061 (positive means decelerating). That’s a huge swing.

Even more striking, the corrected supernova data now agrees much better with what those other two measurement methods were already suggesting when analyzed without supernovae. It’s as if the corrected data from three completely independent techniques are finally telling the same story.

The tension with thestandard model of cosmology (where dark energy is a constant) jumped to more than 9-sigma after the correction. That’s far beyond what scientists normally consider statistically significant.

Double-Checking the Work

To verify their findings, the researchers tried something clever. They selected only supernovae from consistently young galaxies at all distances, with no age mixing whatsoever. If the age bias is real, this “apples-to-apples” comparison should avoid the problem entirely.

Published in Monthly Notices of the Royal Astronomical Society, those results also favor a slowing universe. That’s independent confirmation the age effect is real.

DESI is a state-of-the-art instrument which maps distant objects to study dark energy. (Credit: Marilyn Sargent/Berkeley Lab — Attribution (CC BY 4.0))

How This Could Ease the Hubble Constant Mismatch

If these findings hold up, they don’t eliminate dark energy. But they fundamentally change what dark energy is doing. Instead of being Einstein’s “cosmological constant” that pushes forever, dark energy might be something that strengthened in the past but is weakening now. The universe’s long-term fate looks different—less “Big Rip” tearing everything apart, more gentle slowdown.

The results might also help solve another puzzle: the “Hubble tension,” where different measurement methods give different values for how fast the universe expands. If the supernova samples used for calibration have systematically younger stellar populations than comparison samples, age bias could account for some of that mismatch.

The systematic error Son and colleagues identified is too big to ignore, even if other complications remain to be sorted out. Whether the universe is truly decelerating today is still an open question, but the cosmic speedometer definitely needs recalibration

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Paper Summary

Methodology

Researchers measured stellar population ages in supernova host galaxies using spectroscopy and found supernovae from younger galaxies appear 0.030 magnitudes dimmer per billion years of age difference. They calculated how progenitor age evolves with cosmic time using star formation history and applied this as a correction to supernova distance measurements. They analyzed both constant and time-varying dark energy models using supernova data (Pantheon+ and DES-SN5YR), baryon acoustic oscillations (DESI), and cosmic microwave background measurements (Planck and ACT). A separate test used only supernovae from young galaxies to avoid age bias without corrections.

Results

Before correction, combining BAO, CMB, and DES supernova data suggested continued cosmic acceleration with a deceleration parameter q₀ = -0.27 ± 0.06. After age-bias correction, the deceleration parameter became q₀ = +0.178 ± 0.061, suggesting current deceleration. The corrected supernova distances aligned closely with predictions from baryon acoustic oscillations and cosmic microwave background data analyzed independently. Tension with the standard cosmological constant model increased to more than 9-sigma after correction. The evolution-free test using only young-host supernovae confirmed these results.

Limitations

The study applied mean corrections based on predicted age evolution rather than individual measurements for all host galaxies. High-redshift galaxy ages remain sparsely measured. The age-bias slope comes from samples at z < 0.45 and assumes this relationship holds at all distances. How the age correction interacts with other systematic effects needs further study. The evolution-free test uses smaller sample sizes with larger uncertainties but provides independent confirmation.

Funding and Disclosures

The National Research Foundation of Korea supported this work through the Center for Galaxy Evolution Research. The study used publicly available data. No competing interests were declared.

Publication Information

Son, J., Lee, Y.-W., Chung, C., Park, S., & Cho, H. (2025). “Strong progenitor age bias in supernova cosmology – II. Alignment with DESI BAO and signs of a non-accelerating universe,” published November 6, 2025 in Monthly Notices of the Royal Astronomical Society, 544(1), 975-987.

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