The universe's expansion is still accelerating, researchers say

Taking a fresh look at data involving a specific type of stellar explosion, a team of researchers says it has confirmed the long-accepted notion that the universe is expanding at an accelerated rate – the very observation that led to the identification in the 1990s of an enigmatic cosmic force called dark energy.

The study's results rebut research published ‌last year that concluded that this cosmic expansion is no longer speeding up – a finding that had challenged the basic understanding of the universe.

"The universe is still accelerating," said astrophysicist Brodie Popovic of the University of Southampton in England, one of the leaders of the study published this month in the journal Monthly Notices of the Royal Astronomical Society.

"There's still a lot we don't know and are excited to learn, but we think we're on the right track," Popovic said.

The study's findings, by a team that included two Nobel Prize recipients, were guided by observations in two different datasets of a ​type of stellar explosion called a Type Ia supernova in order to calculate vast cosmic distances. These supernovas cause the destruction of an object called a white dwarf, the dense remnant of a low- to intermediate-mass star at the end of its lifecycle.

This type of supernova has proven valuable in investigating the universe's structure based on evidence that all of these explosions have roughly the same luminosity. Their observed brightness differs depending upon their distance from Earth - brighter when closer and fainter when farther – making them useful as cosmic mile markers.

By measuring the brightness of these supernovas as seen from Earth, scientists can gauge the universe's expansion rate and the variation of that rate over time. Because of the time it takes for light to travel through space, looking at distant objects in the cosmos is like looking back in time.

The Big Bang event roughly 13.8 billion years ago initiated the universe, and it has been expanding ever since. Scientists in 1998 disclosed that this expansion is accelerating, with an invisible force called dark energy as the hypothesized reason.

The universe's contents include ordinary matter – stars, planets, gas, dust and all the familiar stuff on Earth – as well as dark matter and dark energy. Ordinary matter represents an estimated 5% of the contents. Dark matter, which is known from its gravitational influences on ‌galaxies and stars, makes up an estimated 27%. Dark energy makes up an estimated 68%.

The authors of the 2025 study, which was published in the same journal as the new study, concluded that dark energy is weakening and has stopped accelerating the universe's expansion.

"Type Ia supernovae are the premier tool for measuring the expansion history of the universe, and provided the first evidence in 1998 that cosmic expansion is accelerating due to dark energy," said astrophysicist Adam Riess of Johns Hopkins University in Maryland, a co-author of the new study and a Nobel laureate in physics in 2011 for the co-discovery of the accelerating expansion ​of the universe.

Riess said that over the past decade, a Yonsei University group has argued that supernova distances should be calibrated differently by accounting for the ages of the stars that eventually explode, but no evidence for this "age effect" was found in the largest calibrated supernova samples used by the cosmology community over the last decade.

The 2025 study's authors defended their interpretation, arguing that the opposing analysis was methodologically flawed or internally inconsistent. The new study's authors, however, maintained confidence in their methods and confirmed that cosmic acceleration still holds.

The physical nature of dark energy remains unknown. Platforms such as the newly operational Vera Rubin Observatory in Chile and the forthcoming Nancy Grace Roman Space Telescope, due to be launched ​in August, may provide some insight.

"We're hoping the new data we get from Vera Rubin and the Nancy Grace Roman Space Telescope will help us narrow down what dark energy really is," Popovic said.