A major international analysis has challenged recent claims that the expansion of the universe may be slowing, concluding that the evidence for cosmic acceleration remains strong and that dark energy continues to play a central role in modern cosmology.
An international team of astrophysicists has reaffirmed that the universe is expanding at an accelerating rate, disputing a recent study that suggested cosmic expansion may have begun to slow.
The new research, published in Monthly Notices of the Royal Astronomical Society, argues that the established interpretation of Type Ia supernova observations remains robust. According to the researchers, the apparent challenge to cosmic acceleration arose largely from problems in how earlier data were analysed rather than from evidence that the standard cosmological picture had broken down.
Re-examining Claims of a Cosmic Slowdown
The new analysis responds directly to research from a South Korean team which proposed that the expansion of the universe could have entered a decelerating phase.
That earlier work suggested that dark energy, the still-unexplained component believed to drive the accelerated expansion of the universe, might be weakening over time.
However, researchers led by Dr. Phil Wiseman of the University of Southampton found that the measurements underlying the established picture of an accelerating universe remain reliable.
The team concluded that astronomers can continue using existing supernova-based measurements while focusing attention on the larger unresolved question: what dark energy actually is.
Type Ia Supernovae and Cosmic Expansion
Type Ia supernovae have played a crucial role in the study of the expanding universe.
These extremely luminous stellar explosions occur in systems involving white dwarf stars. Because their brightness can be calibrated, astronomers use them as cosmic distance indicators.
Observations of distant Type Ia supernovae were central to the discovery that the expansion of the universe is accelerating. That work earned Adam Riess, Brian Schmidt and Saul Perlmutter the 2011 Nobel Prize in Physics.
Both Professor Riess and Professor Schmidt participated in the latest analysis.
The recent challenge to the conventional interpretation argued that the peak brightness of Type Ia supernovae may change depending on the ages of their stellar populations. If such an effect were large enough and had not been correctly accounted for, astronomers might incorrectly interpret observations as evidence of accelerating expansion.
Problems Identified in the Earlier Analysis
The new study identified important methodological issues in the previous work.
According to the researchers, one problem involved estimating the ages of the stars that eventually produced the supernova explosions. The earlier analysis treated the age of a host galaxy as though it represented the age of the individual stellar system responsible for the supernova.
The new team argues that these quantities are not equivalent.
Researchers also found that the earlier study did not adequately account for the mass of galaxies hosting the supernovae.
Correcting for host-galaxy properties is an established part of modern supernova cosmology because differences in galactic environments can influence how observations are interpreted.
After accounting for these effects and recalibrating the supernova observations, the researchers found that the evidence supporting cosmic acceleration remained consistent.
Dark Energy Remains One of Physics' Biggest Mysteries
Although the study strengthens confidence that the universe is still accelerating, it does not resolve the fundamental mystery behind that acceleration.
Dark energy is the name given to the unknown phenomenon thought to dominate the large-scale evolution of the universe and counteract the attractive influence of gravity.
Scientists still do not know its physical nature.
The researchers noted that challenges to established cosmological measurements remain valuable because they force scientists to examine assumptions, improve calibration methods and better understand the astrophysics of supernova explosions.
The debate has therefore provided new opportunities to investigate how stellar populations, host galaxies and exploding stars may influence increasingly precise measurements of the universe.
For cosmologists, the latest analysis shifts attention back from the question of whether cosmic acceleration is real to the deeper question that has persisted since its discovery: what is causing it?
Journal reference
Phil Wiseman, Brodie Popovic, Mark Sullivan, Adam G. Riess, Dan Scolnic, Rebecca C. Chen, Tamara M. Davis, Lluรญs Galbany, Isobel M. Hook, Saurabh W. Jha, Lisa Kelsey, Yukei S. Murakami, Mickaรซl Rigault, Benjamin M. Rose, Brian Schmidt, Mat Smith, Maria Vincenzi. โStill accelerating: type Ia supernova cosmology is robust to host galaxy age evolution.โ Monthly Notices of the Royal Astronomical Society, 2026, 549(3).
DOI: https://doi.org/10.1093/mnras/stag797
Source: Royal Astronomical Society, published August 29, 2026.