New study says universe’s expansion is still accelerating

WASHINGTON: A new study based on fresh analysis of Type Ia supernova data has concluded that the universe is continuing to expand at an accelerating pace, reaffirming the view that led scientists in the 1990s to propose the existence of dark energy.

The research, published this month in the journal Monthly Notices of the Royal Astronomical Society, pushes back against a study published last year in the same journal that had argued the expansion was no longer speeding up. The new work was carried out by a team that included two Nobel Prize winners and drew on observations from two separate datasets involving Type Ia supernovas.

Astrophysicist Brodie Popovic of the University of Southampton, one of the study’s leaders, said the findings support the long-standing understanding of cosmic expansion.

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

Type Ia supernovas are stellar explosions that destroy a white dwarf, the dense remnant left behind by a low- to intermediate-mass star at the end of its life cycle. Scientists use them to estimate vast cosmic distances because they are understood to have broadly similar luminosity. Their apparent brightness changes depending on how far they are from Earth, allowing researchers to use them as distance markers and to measure how fast the universe is expanding and how that rate has changed over time.

Because light takes time to travel across space, observing very distant objects also allows scientists to look back into earlier stages of the universe. The universe began about 13.8 billion years ago with the Big Bang and has been expanding since then. In 1998, scientists announced that this expansion was accelerating, with dark energy proposed as the cause.

Ordinary matter — including stars, planets, gas, dust and familiar material on Earth — accounts for about 5% of the universe’s contents. Dark matter, identified through its gravitational effects on galaxies and stars, makes up an estimated 27%, while dark energy accounts for about 68%.

Adam Riess, an astrophysicist at Johns Hopkins University and a co-author of the new study, said Type Ia supernovae have long been central to measuring the universe’s expansion history. Riess, who shared the 2011 Nobel Prize in physics for the co-discovery of accelerating cosmic expansion, said:

"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,"

Riess said a group at Yonsei University had argued over the past decade that supernova distances should be calibrated differently by taking into account the ages of the stars that later explode, and that this so-called age effect could significantly change the evidence for acceleration. He said:

"In our study, we found no evidence for the claimed 'age effect' in the largest calibrated supernova samples used by the cosmology community over the last decade,"

The earlier study’s authors had concluded that dark energy was weakening and had stopped driving accelerated expansion. But Young-Wook Lee of Yonsei University in Seoul, one of the leaders of that 2025 study, defended his team’s conclusions and said the main arguments in the new paper have serious shortcomings. Lee said they have "serious methodological flaws or lead to conclusions that are internally inconsistent by their own logic."