Yonsei University Astronomers Challenge 27-Year-Old Theory of Universe's Accelerating Expansion
- MM24 News Desk
- 11 hours ago
- 3 min read
A team of astronomers from Yonsei University in South Korea has published groundbreaking findings suggesting the universe's expansion is now slowing down, directly challenging the long-held Nobel Prize-winning theory of an ever-accelerating cosmos driven by dark energy. Led by Professor Young-Wook Lee, the study, published in Monthly Notices of the Royal Astronomical Society, corrects a systematic bias in cosmic "standard candles" and could upend our fundamental understanding of the universe's fate.
For nearly three decades, the cornerstone of modern cosmology has been the certain knowledge that the universe is not just expanding, but that its expansion is speeding up at an ever-increasing rate. This discovery of an accelerating universe, propelled by a mysterious repulsive force dubbed dark energy, earned the 2011 Nobel Prize in Physics. But what if a subtle flaw in our cosmic measuring sticks has led us astray for the past 27 years?
According to a new study from Yonsei University, that is precisely what has happened. The research, led by Professor Young-Wook Lee from the Center for Galaxy Evolution Research, presents a compelling case that the universe has already transitioned into a phase of decelerated expansion.
"Our study shows that the universe has already entered a phase of decelerated expansion at the present epoch," stated Professor Lee, a finding that, if confirmed, would fundamentally rewrite the story of our cosmos.
The crux of this paradigm shift lies in a critical re-examination of type Ia supernovae. These exploding stars have long been considered cosmology's "standard candles"—reliable markers for measuring vast cosmic distances because their peak brightness is thought to be consistent. It was by observing these supernovae that scientists originally concluded the expansion was accelerating. However, the Yonsei University team discovered a crucial oversight, reported Monthly Notices of the Royal Astronomical Society.
Their work, based on a much larger sample of 300 host galaxies, revealed that the age of the stars that produce these supernovae creates a significant bias. Even after standard corrections, supernovae from younger stellar populations appear systematically fainter, while those from older populations appear brighter. When the team applied a correction for this "age-bias," the evidence for an accelerating universe vanished.
So, what does the data show now? The corrected supernova measurements no longer fit the standard Lambda Cold Dark Matter (ΛCDM) model, which relies on a constant form of dark energy. Instead, they align strikingly well with a new model emerging from the Dark Energy Spectroscopic Instrument (DESI) project, which uses Baryonic Acoustic Oscillations (BAO)—fossilized sound waves from the Big Bang—and Cosmic Microwave Background data. This combined analysis points toward a dark energy that is not constant but is weakening over time.
The implications are staggering. "If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago," Professor Lee explained. This isn't just a minor adjustment; it's a potential overhaul of our understanding of the cosmos's engine. The research suggests that the mysterious force making up about 70 percent of the universe is fading, allowing gravity to regain the upper hand and slow the expansion.
This new conclusion directly contrasts with the recent interpretation from the DESI collaboration, which also found evolving dark energy but concluded the universe is still accelerating today, with deceleration set for the future. The key difference lies in the supernova correction. "By contrast, our analysis... shows that the universe has already entered a decelerating phase today," Lee clarified.
To solidify their revolutionary claim, the team is now conducting an "evolution-free test" using only supernovae from similarly young galaxies. The future looks even brighter for resolving this cosmic mystery.
The Vera C. Rubin Observatory in Chile, which began operations this year, will soon supercharge this research. With its powerful digital camera, it is expected to discover over 20,000 new supernova host galaxies, providing the data needed for a definitive test within the next five years. This could finally unravel the true nature of dark energy, one of the greatest enigmas in all of science.