Is the expansion of the universe slowing down?

It is widely believed that our Universe is expanding at an ever-accelerating rate. But could we be wrong? That's what a group of South Korean scientists claimed in a new paper, but other scientists have expressed serious concerns about the work.

Our Universe has been expanding since the Big Bang, which occurred 13.8 billion years ago. Some evidence, including observations of distant dying stars called Type 1a supernovae, suggests that this expansion is accelerating. One of the main theories to explain this acceleration is a mysterious force called dark energywhose discovery earned him the 2011 Nobel Prize in Physics.

Young-Wook Lee from Yonsei University in South Korea and colleagues now say that may be wrong. Type 1a 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 so-called “standard spark plugs” as reliable measurements of distances in the Universe, since they are believed to be equally bright.

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

Instead, Lee said, their results suggest that the expansion of the universe began to slow down 1.5 billion years ago and may even reverse in the future. This is a scenario that astronomers call the “big crunch,” in which the universe could end reverse Big Bang. Previously, he said, “there was no question of a big crisis. But now it is possible.”

Adam Reiss at the Space Telescope Science Institute in the US, one of the 2011 Nobel Prize winners in physics, disagrees with this claim, pointing to the group's earlier work in 2020 that was refuted. “New work from the same group repeats this argument with minor changes,” he says, noting that measuring the stellar ages of Type 1a supernovae at large distances is very difficult. He says Lee's team used average stellar ages derived from the host galaxy. “The theory behind this is weak due to a lack of certainty about how [star] shapes,” says Reiss.

There are known problems with how age affects the brightness of Type 1a supernovae in the Universe, he says. Mark Sullivan at the University of Southampton, but they are already included in dark energy measurements. “I'm very skeptical that this will slow down the universe,” he says.

Upcoming observations from Vera K. Rubin Observatory In Chile, the number of known Type 1a supernovae in the universe is expected to increase significantly from the thousands cataloged today to the 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 Lee's team's claims.

However, the exact nature of dark energy remains mysterious. Earlier this year, results from a study conducted by the Dark Energy Spectroscopic Instrument (DESI) showed that dark energy may not be a constant force, but may change over time. While this does not mean that the Universe is slowing down now, it may mean that the rate of expansion has changed over the history of the Universe.

“The arrow indicates much more that dark energy is some kind of dynamical thing rather than a cosmological constant,” says Ed Macaulay at Queen Mary University of London. “I think that's a really interesting question.”