A 2025 examine by South Korean researchers caught the eye of the astronomy neighborhood when it advised that the proof behind darkish power may very well be flawed. Darkish power makes up about 70% of the universe however no one is aware of what it’s. This makes it a frequent goal for scepticism.
Scientists had lengthy recognized that the universe was increasing. However within the late Nineteen Nineties, researchers found that this enlargement was accelerating. Astrophysicists concluded that one thing should be driving the acceleration. They named this unknown amount “darkish power”.
The 2025 re-analysis, by a staff at Yonsei College in Seoul, advised that the enlargement of the universe was not in actual fact accelerating.
Our staff on the College of Southampton has now gone through the Yonsei team’s findings. Our outcomes, printed in Month-to-month Notices of the Royal Astronomical Society (MNRAS), counsel that the outcomes have been a false alarm.
Nonetheless, the Yonsei College staff have mentioned they stand by their outcomes and {that a} comply with up examine helps their discovering that the universe is probably not accelerating.
What’s darkish power?
Within the late Nineteen Nineties, cosmology was upended by a startling discovery. Scientists knew the universe was increasing, however that they had assumed that gravity was step by step slowing down this enlargement.
Nonetheless, by observing extremely brilliant, thermonuclear explosions of white dwarf stars, two impartial groups of astronomers discovered the alternative. As a result of these explosions, often called Sort Ia supernovae, have nearly the identical intrinsic brightness, evaluating how brilliant they seem provides a exact distance.
In 1998, knowledge revealed that distant supernovae were fainter than they need to have been in a slowing universe. As a result of faintness implies distance, the observations positioned these supernovae farther away than anticipated, and implied that the cosmic enlargement was not slowing.
In reality, when the calculations have been run, it was clear that the enlargement of the universe was actually speeding up.
There was no convincing bodily clarification for this “cosmic acceleration”. Physicists labelled the mysterious, repulsive power that causes the acceleration “dark energy”.
Cosmic calibration
To know the Yonsei College group’s claims, we should perceive how astronomers use supernovae as “standard candles” to measure cosmic distances. Sort Ia supernovae are remarkably alike, however they don’t seem to be an identical.
Cosmologists appropriate for these variations. They use refined calibrations to account for relationships between the brightness of the supernovae and measurements like their color, the period of their mild curves, and the kind of galaxies they explode in.
A type of corrections is well-known however refined. It hyperlinks the brightness of a supernova to the scale of the galaxy it exploded in. Large galaxies maintain extra stars, and so extra mass. After the usual corrections, supernovae in these huge (huge) galaxies come out a couple of per cent brighter than ones in small galaxies. No person is aware of why.
Nasa / JPL-Caltech / CXC / Calar Alto O. Krause /MPIA
However greater galaxies usually have older stars which might be made up of heavier components, which might in flip affect the properties of the supernovae that a few of these stars develop into. The authors of the 2025 examine proposed a far greater evolutionary impact. They argued that the brightness of Sort Ia supernovae modifications considerably because the universe ages.
Particularly, they claimed that older white dwarf stars, that are extra frequent within the close by, present-day universe, produce brighter explosions. That may imply immediately’s supernovae are inherently a lot brighter than their distant, early-universe counterparts.
If true, the faintness we observe in distant supernovae wouldn’t be a results of their being additional away, a relationship that’s in flip pushed by darkish power, however slightly an evolutionary trait of the celebrities themselves. This declare threatened to dismantle nearly three many years of progress in astronomy.
Addressing the claims
Extraordinary claims want cautious testing. On the College of Southampton we launched into an audit of the info, utilizing observations from the Dark Energy Survey (DES), an astronomy challenge designed to constrain darkish power’s properties, alongside the identical dataset utilized by the authors of the 2025 work. In doing so, we sought to copy and take a look at their conclusions.
Our re-analysis revealed what we noticed as two issues within the Yonsei examine: a technical omission and a flawed assumption about stellar populations. As soon as corrected, the info fell again consistent with normal cosmological outcomes, together with these of DES.
CTIO/NOIRLab/NSF/AURA/T. Matsopoulos, CC BY
Crucially, the 2025 examine didn’t account for a way supernovae in huge galaxies come out a couple of per cent brighter than ones in small galaxies. When our staff reapplied this correction to the identical dataset, the correlation between a supernova’s brightness and galaxy age – introduced by the Yonsei staff of their paper – turned far weaker.
This doesn’t imply that the age of a white dwarf star has no impact on the brightness of a ensuing supernova. Certainly, most researchers within the area would doubtless settle for that the brightnesses of supernovae are extra affected by the age of the celebrities that explode than by the mass of the galaxy that it’s in.
Nonetheless, measuring a galaxy’s mass is much cheaper by way of telescope time and requires fewer technical assumptions than measurements of the ages of galaxies, that are difficult. What we confirmed is that the galaxy mass calibration is enough to the accuracy required to constrain darkish power.
Nasa, Esa, The Hubble Key Project Team, and The High-Z Supernova Search Team
Secondly, the 2025 evaluation relied on a flawed proxy, incorrectly assuming that the general age of a bunch galaxy is an identical to the particular age of the star that exploded. However galaxies are usually not uniform like this: even giant, historical galaxies comprise localised pockets of younger stars.
By overestimating the distinction in age between the celebrities that explode close by and people who explode within the distant universe, by an element of three to 5, the Yonsei College examine overestimated the quantity by which the age-brightness calibration might bias supernova distance measurements.
As soon as these omissions have been corrected, the info behaved as anticipated underneath the usual mannequin of cosmology. As such, proof for an accelerating universe stays safe.
Science friction
Whereas the 2025 examine turned out to be incorrect, the problem it posed was priceless. Difficult accepted concepts and rigorously testing our observations is prime to scientific progress.
Their claims offered a possibility for cosmology groups to return to their baseline knowledge, interrogate their core assumptions, and take a look at alternative ways of calibrating the measurements. When measuring distances, nonetheless, the age impact and the mass impact are extremely correlated, and subsequently we (the Southampton staff) consider it’s incorrect to make a full age-based adjustment to distances which have already been corrected for the mass impact.
The Southampton staff was invited, amongst others, to a workshop at Yonsei in June 2026, the place we mentioned our strategies and talked about methods to incorporate each stellar mass and stellar ages into the brightness corrections.
In a comply with up examine, just lately accepted by MNRAS, the Yonsei College staff preserve that the ages of stars have an essential impact on the supernovae they produce. They are saying that two strategies utilized in earlier research have suppressed the significance of this phenomenon.
When these results are accounted for constantly, they clarify, it results in outcomes much like these within the 2025 examine. “We subsequently proceed to seek out proof that stellar-population age is a crucial think about Sort Ia supernova standardisation,” authors Junhyuk Son and Hyejeon Cho, instructed The Dialog.
“As a result of the standard ages of supernova host populations change with cosmic time, understanding this impact is essential for precision measurements of the Universe’s enlargement historical past.”
Future measurements will depend upon understanding supernova explosions higher. It’s a job we hope the neighborhood can tackle collectively. However none of this removes the necessity for darkish power: it’s nonetheless there, and nonetheless unexplained.
In the end, we’ve got proven that current measurements of darkish power are sturdy. Moderately than debating whether or not cosmic acceleration exists, the astrophysical neighborhood can return to the thrilling job of determining what darkish power really is and the way we greatest measure it.
Over the subsequent 10 years, the Vera C. Rubin Observatory in Chile will perform a challenge often called the Legacy Survey of Space and Time, which can produce a pattern of supernovae 20 instances bigger than the DES pattern utilized in our evaluation. There is no such thing as a higher time to be investigating the mysteries of darkish power.
