In the late 1990s, two rival teams of astronomers discovered that the universe was expanding faster and faster. It was a stunning conclusion based on the dimming light of distant supernovae. This led to the idea of a mysterious force that counteracts gravity, dubbed dark energy. It was a major breakthrough that reshaped our understanding of cosmology and earned a Nobel Prize in 2011.
But now, a new study from astronomers in South Korea throws that conclusion into doubt. The researchers argue that the universe may not be speeding up at all. Instead, it could be slowing down.
"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 Young-Wook Lee, a professor of astronomy at Yonsei University in Seoul and lead author of the paper, published this month in Monthly Notices of the Royal Astronomical Society.
If confirmed, this casts our whole understanding of the universe into doubt; including how it could end.
The universe is expanding. That much is clearer. But it's less clear whether this expansion is accelerating or slowing down. At the heart of this upheaval lies the cosmic yardstick astronomers have used for decades: Type Ia supernovae.
Astronomers have long used these stellar explosions, seen across billions of light-years, to estimate distances by standardizing their brightness. These stellar explosions all reach a very similar peak intrinsic brightness, allowing astronomers to determine their distance by observing how dim they appear from Earth.
But Lee and his team argue that the light from these supernovae isn't quite as standard as once thought.
By examining 300 host galaxies with precisely measured stellar ages, the researchers found a strong correlation between the age of a star's host galaxy and the brightness of the supernova it produces. Supernovae from younger stellar populations tend to be systematically fainter, even after standardization corrections.
Our so-called "standard candles" were not so standard after all.
This introduces what the team calls a "progenitor age bias," a redshift-dependent distorsion that, when uncorrected, can make distant supernovae look dimmer -- and thus farther away -- than they truly are.
Correcting for this bias, the team found that the supernova data no longer support the idea of a universe accelerating under the grip of constant dark energy. Instead, the data align better with a time-varying model of dark energy -- one that fades over time, even allowing for deceleration in the present day.
The findings gain strength from their agreement with results from the Dark Energy Spectroscopic Instrument (DESI), an ongoing project that has already mapped over 15 million galaxies in 3D. The DESI team's baryon acoustic oscillation (BAO) data -- ripples from the early universe -- had independently suggested signs of a slowing expansion.
Researchers have long struggled to reconcile this apparent misfit. This new study explain what's going on.
When Lee's team corrected the supernova data and compared it with DESI's BAO results and data from the cosmic microwave background (CMB), they found a remarkable agreement. The trio of independent measurements -- supernovae, BAO, and CMB -- told a consistent story: the universe's expansion is no longer speeding up.
Even more striking, the combined data rule out the standard ΛCDM model (the cosmological framework that has dominated astronomy for decades) with overwhelming statistical confidence. If this study is confirmed, much of what we know about the universe is in doubt.
If the expansion of the universe has already begun to slow, the implications are profound. For one, the idea of a cosmological constant (an unchanging form of dark energy) would be invalid. The notion that we are living in an ever-accelerating cosmos, with galaxies racing apart into eternal isolation, would also fall apart.
Instead, dark energy might be a phenomenon that changes in time. We still don't know what it is, but it's weakening, and as it's weakening gravity could reassert its dominance. Think of it this way: gravity is trying to pull everything together closer and closer. Dark energy, whatever it may be, is pushing things apart. If dark energy fades in time, then over cosmic scales, gravity would start to win. Slowly but surely, everything would come closer and closer and the universe could end in something called the Big Crunch.
There are echoes here of a much older vision of the cosmos -- one that oscillates, expands and contracts, breathes in and out over eons.
But not everyone is convinced.
Adam Riess, an astrophysicist at the Space Telescope Science Institute and one of the original discoverers of dark energy, pushed back. "The same group's new work repeats the argument with little change," he told New Scientist. He pointed out the difficulty in measuring stellar ages at cosmological distances and questioned the method's assumptions.
Carlos Frenk, a cosmologist at Durham University, was more cautious. "It's definitely interesting. It's very provocative. It may well be wrong," he told The Guardian. "It's not something that you can dismiss. They've put out a paper with tantalizing results with very profound conclusions."
They also plan to use upcoming data from the Vera C. Rubin Observatory, which is expected to discover more than 20,000 new supernova host galaxies in the coming years. This massive influx of data will allow far more accurate measurements of stellar age, potentially settling the debate.
Until then, the findings challenge the two cornerstones of modern cosmology: that the universe is currently accelerating, and that dark energy is a constant force.