For more than 25 years, scientists have believed that the universe is expanding faster and faster, pushed apart by a mysterious force called dark energy.
But a new study now suggests that the universe’s expansion may actually be slowing down — not speeding up — marking what could be one of the biggest shifts in cosmology in decades.
The research, published in Monthly Notices of the Royal Astronomical Society, comes from a team at Yonsei University in South Korea.
Their results challenge the long-standing belief that dark energy is constant and causes galaxies to drift apart at an ever-increasing rate.
Instead, the new findings indicate that dark energy may be weakening over time and that the universe has already entered a phase of decelerated expansion.
Lead researcher Professor Young-Wook Lee explained, “Our study shows that the universe has already started slowing down.
If this is confirmed, it would mean that dark energy evolves much more quickly than we thought — and this would completely change how we understand the universe.”
Since 1998, astronomers have based the idea of an accelerating universe on measurements of type Ia supernovae — exploding stars that serve as “standard candles” to measure cosmic distances.
Those observations showed that distant galaxies were moving away faster than expected, a discovery that earned the 2011 Nobel Prize in Physics. But the Yonsei team found that the supernovae might not be as “standard” as once believed.
By studying 300 host galaxies, they discovered that the brightness of type Ia supernovae depends on the age of the stars that created them.
Supernovae born from younger stars tend to appear slightly dimmer, while those from older stars are brighter.
When this “age bias” was corrected, the data no longer matched the widely accepted cosmological model known as ΛCDM (Lambda Cold Dark Matter), which assumes dark energy is constant.
Instead, the corrected data fit better with newer models supported by the Dark Energy Spectroscopic Instrument (DESI) project, which combines information from the cosmic microwave background — the afterglow of the Big Bang — and baryonic acoustic oscillations, or faint sound waves left over from the universe’s earliest moments. Both suggest that dark energy changes over time and is growing weaker.
When the corrected supernova data were combined with the DESI results, the evidence for an accelerating universe disappeared.
“The universe is not accelerating today,” said Professor Lee. “It has already entered a decelerating phase, just as independent measurements of the cosmic microwave background and early-universe sound waves have hinted.”
To strengthen their findings, the Yonsei team is now running additional “evolution-free” tests using only young galaxies to remove age effects completely. Early results continue to support their conclusion.
The upcoming Vera C. Rubin Observatory in Chile will play a major role in this investigation. With its massive telescope and the world’s largest digital camera, it will soon detect tens of thousands of new supernovae, allowing scientists to make far more precise measurements of how the universe expands.
If confirmed, these findings could resolve long-standing puzzles such as the “Hubble tension” — the mismatch between different methods of measuring the universe’s expansion rate — and rewrite our understanding of dark energy itself.
For now, one thing is clear: the cosmos may be telling a very different story about its future than scientists once believed.
