Is the expansion of the universe slowing down?

It is widely accepted that our universe is expanding at an ever-increasing rate. But could we be wrong? This is what a group of South Korean scientists says, but other scientists are very concerned about this work.

Our universe has been expanding since the big bang 13.8 billion years ago. Several lines of evidence, including observations of distant dying stars called Type Ia supernovae, suggest that this expansion is accelerating. One of the main explanations for this acceleration is a mysterious force called dark energy, the discovery of which won the 2011 Nobel Prize in Physics.

Young-Wook Lee, of Yonsei University in South Korea, and colleagues now say this could be false. Type Ia supernovae occur when the remnant core of a star like our sun, known as a white dwarf, explodes in a binary system. Astronomers use these “standard candles” as reliable measures of distance across the cosmos, because they are thought to be uniformly luminous.

But Lee and his team say that brightness varies greatly with the age of stars, based on their analysis of 300 host galaxies. They say that distant supernovae can appear fainter than expected and this is usually attributed to the accelerated expansion of the universe, but, once this “age bias” is taken into account, the accelerated expansion disappears.

Instead, Lee says their findings suggest that the expansion of the universe began to slow 1.5 billion years ago, and could even reverse in the future, a scenario called the “big crunch” in which the universe could end with a reverse big bang. Previously, he said, “a major crisis was out of the question. But now it’s a possibility.”

Adam Riess of the Space Telescope Science Institute in Maryland, one of the 2011 Nobel Prize winners in physics, disagrees with this assertion, pointing to previous work by the group in 2020 that had been refuted. “New work from the same group takes up this argument with few changes,” he says, pointing out that it is very difficult to measure star ages for Type Ia supernovae at large distances. He says Lee’s team used an average stellar age derived from the host galaxy. “The theory behind this is weak due to a lack of certainty about how the [star] shapes,” Riess explains.

There are known issues with how age affects the brightness of Type Ia supernovae in the universe, says Mark Sullivan of the University of Southampton, UK, but these are already accounted for in measurements of dark energy. “I’m very skeptical that this will lead to a decelerating universe,” he says.

Upcoming observations with the Vera C. Rubin Observatory in Chile are expected to significantly increase the number of known Type Ia supernovae in the universe, from the thousands cataloged today to tens of thousands. This will allow us to “map the expansion history” of the universe much further back in time, Sullivan says, potentially ruling out the claims of Lee’s team.

The exact nature of dark energy, however, remains mysterious. Earlier this year, results from the Dark Energy Spectroscopic Instrument Survey indicated that dark energy may not be a constant force, but could vary over time. While this doesn’t mean the universe is decelerating right now, it could suggest that the rate of expansion has changed over the history of the universe.

“The needle is much more indicative that dark energy is a kind of dynamic thing, not a cosmological constant,” says Ed Macaulay of Queen Mary University of London. “I think that’s a really interesting question.”