In a quiet room in Seoul, a group of astronomers watch numbers scroll across a screen. It's the new map of the universe, traced by thousands of supernovae. Inside, they say, lies evidence that theexpansion of the universe It's no longer accelerating. Galaxies we thought were in eternal flight appear to be slowing down, as if the cosmos had taken its foot off the accelerator. After 27 years, the theory ofdark energy may no longer be the last word on our cosmic history.
A hypothesis that overturns the Nobel Prize
It was 1998 when the universe suddenly accelerated (or so it seemed). Two groups of researchers, observing distant supernovae, they concluded that a mysterious force was pushing everything further and further apart. They called it dark energyThe idea earned a Nobel Prize and almost unanimous consensus: the cosmos was expanding ever faster, and no one knew why.
Today, the team led by Young-Wook Lee of Yonsei university proposes a historic reversal. No more acceleration, but an ongoing slowdown. According to the study published in the Monthly Notices of the Royal Astronomical Society, the expansion rate is already slowing. In effect, the universe would have stopped racing and started breathing.
Supernovae and old deceptions
It's all about the Type Ia supernovae, the "standard candles" that allowed us to measure cosmic distances. But perhaps they weren't so standard. The Korean team demonstrated that their brightness varies based on the age of the stars from which they were born: the younger ones appear fainter, the older ones brighter. A detail that could change the entire interpretation of the data.
Once this "age bias" has been corrected, the standard cosmological model, the famous ΛCDM, no longer holds up. The recalibrated data aligns better with a decelerating universe. In other words, it's not gravity that's losing strength, but dark energy that's fading.
The crucial fact: the new analyses, combined with the project results Desi and the observations of the cosmic microwave background, rule out with a very high probability (99.999%) that the universe is still accelerating. Goodbye expansion. In its place, a slow but statistically significant slowdown.
A cosmos that changes its mind
For cosmologists, this is a silent revolution. If confirmed, it would mean that the universe has passed its "peak acceleration" and is moving toward a more stable phase, perhaps destined to reverse course in the next billions of years. No longer an infinite expansion, but a more complex cycle, where matter and energy once again take on greater weight.
Side effect: a more humble cosmology
The idea of a slowing cosmos challenges even our imagination. For decades, we've imagined a universe in flight, dominated by an invisible force. Now we discover that that force can diminish, perhaps even vanish. It's not the end of the world, but the end of a certainty. And in astronomy, certainties always last too short.
Perhaps the simplest lesson is this: the universe, too, has its cycles. It grows, accelerates, and slows down. Like us, it has phases of excitement and calm. And if the cosmos slows down, we might too: at least for a moment, just to look at it better.
