An international team, including Nobel laureates Adam Ries and Brian Schmidt, found that the claim that dark energy is weakening was based on an incorrect age estimate of supernovae and the omission of a common correction for the mass of the host galaxies.
One of the most important discoveries in modern cosmology is that the universe is not just expanding—its rate of expansion is accelerating. This discovery, based on observations of Type Ia supernovae in the late 90s, led to the hypothesis of the existence of “dark energy”: a mysterious component of the universe that causes the expansion to accelerate. The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Adam Riess, and Brian Schmidt for this discovery.
But in the past year, a disturbing claim has emerged: This historic conclusion may be affected by a systematic bias in the measurement of supernovae. A team of researchers from South Korea claimed that after correcting for the age of the stellar populations in the galaxies where the supernovae occurred, the data actually indicate a universe that has begun to slow down its expansion.If the claim were true, it would require a profound change in the understanding of dark energy, and perhaps also in scenarios regarding the ultimate fate of the universe.
Now an international team of astrophysicists, including Ries and Schmidt, has published a detailed response analysis. Its conclusion is clear: the evidence that the universe is expanding at an accelerating rate remains strong, and the claim that the expansion has already slowed down does not stand up to scrutiny. (arXiv)
Standard candles — but not simple
Type Ia supernovae are used in cosmology as “standard candles”—not because they are all exactly the same, but because their light curves can be corrected and used to measure large distances in the universe. When the redshift of the light is also measured, the distance can be compared to the rate of expansion, thus reconstructing the history of the expansion of the universe.
The original discovery was surprising: distant supernovae appeared dimmer than expected in a universe whose expansion was slowing down due to gravity. The accepted interpretation was that the universe had entered a phase where expansion was accelerating a few billion years ago.
Since then, supernova measurements have become much more sophisticated. They now include corrections related to the color of the supernova, the shape of the light curve, and the properties of the galaxy in which it exploded. One important correction is the stellar mass of the host galaxy, because the environment of the supernova affects how its brightness is corrected.
What went wrong with the claim about slowdown?
The South Korean study claimed that there is a strong relationship between the age of the stellar population and the standardized brightness of the supernova. According to this argument, distant supernovae, which occurred in a younger universe, may be different in a way that makes us think that the universe is accelerating — when in fact the expansion has already begun to slow down.
The new response points to several key problems. First, the authors say, the original analysis did not include a correction for the host galaxy mass, which is a standard correction in modern supernova measurements. When this correction is added, the relationship between the galaxy age and the supernova's normalized luminosity weakens or disappears.
Second, the researchers argue that the original study confused the age of the galaxy with the age of the star or star system that led to the supernova. Within the same galaxy, stars can form at very different times. Therefore, the average age of the galaxy's stellar population is not necessarily the age of the star that exploded as a supernova. According to the new analysis, the age gap suggested in the previous study was too large, probably three to five times.
Third, if the age effect claim were true with enough force to turn the cosmological picture upside down, it should leave other clues in the data—for example, a clear shift with redshift due to the mass of the host galaxy. According to the response team, the Dark Energy Survey data do not show such a shift with significant force.
Dark energy remains an enigma
This does not mean that all the questions of cosmology have been solved. On the contrary: dark energy remains one of the great mysteries of physics. It is not clear whether it is really a cosmological constant, a property of the vacuum, some kind of dynamical field, or a sign that our theory of gravity is incomplete at the largest scales.
The tension in measuring the Hubble constant—the gap between measurements of the expansion rate in the nearby universe and values inferred from the early universe—is also still unresolved. In addition, new surveys such as DESI, Euclid, and the Vera Rubin Observatory are expected to probe the behavior of dark energy with greater precision in the coming years.
But according to the new study, at least the specific claim that Type Ia supernovae no longer support an accelerating universe does not hold up. Corrections already available in modern methods, most notably the correction for the mass of the host galaxy, reduce the claimed effect. In addition, the estimation of the age of the supernovae in the previous study was problematic.
This is how science is supposed to work.
This case illustrates the way science progresses. An unusual claim was made: perhaps the universe is no longer accelerating. It was examined by other researchers, including scientists who were involved in the original discovery, and the answer given was not “we always thought so,” but a systematic analysis of the assumptions, corrections, and data.
In the meantime, the central cosmological picture remains the same: the universe is expanding, and the available evidence still points to an accelerating expansion. What is driving this acceleration remains a mystery. But for now, at least, the big question seems to be not whether dark energy has disappeared—but what it really is.
Scientific source:
Still Accelerating: Type Ia supernova cosmology is robust to host galaxy age evolution, arXiv, 2026.
Authors: Phil Wiseman, Brodie Popovic, Mark Sullivan, Adam G. Riess, Dan Scolnic, Rebecca C. Chen, Tamara M. Davis, Lluís Galbany, Isobel M. Hook, Saurabh W. Jha, Lisa Kelsey, Yukei S. Murakami, Mickaël Rigault, Benjamin M. Rose, Brian Schmidt, Matt Smith, Maria Vincenzi. (arXiv)
The study was published in the journal Monthly Notices of the Royal Astronomical Society Of the Royal Astronomical Society.-OI: 10.1093/mnras/stag797
Short FAQ:
What is dark energy?
Dark energy is the name given to a mysterious component in the universe that appears to be causing the expansion of the universe to accelerate. Its physical nature is still unknown.
How do supernovae measure the expansion of the universe?
Type Ia supernovae serve as standard candles: after correcting their light curves, their distance can be estimated and compared to the redshift of the light they emit.
What did the South Korean study claim?
He argued that the age of stellar populations affects the brightness of supernovae in a way that could lead us to mistakenly conclude that the universe is accelerating.
What did the new study find?
He found that the claimed effect weakens or disappears when the conventional corrections are used, and that the age of the galaxy is not equivalent to the age of the star system that exploded as a supernova.
More of the topic in Hayadan:
One response
I think there are two possibilities… Either something outside the universe exerts a huge gravitational force, but then they all had to go in one direction and not like now, or there are opposing forces inside the galaxies that push the galaxies away from each other, just like magnets, only the scale is very huge. This explains both the acceleration and the growth in all directions.