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The death of dark energy is a false alarm – the…

Science · August 16, 2026 · The Conversation · 7 min

A recent paper claimed that dark energy might not exist. But new research suggests this conclusion was premature.

<a href="https://theconversation.com/profiles/phil-wiseman-1513455">Phil Wiseman</a>, <a href="https://theconversation.com/institutions/university-of-southampton-1093">University of Southampton</a> and <a href="https://theconversation.com/profiles/mark-sullivan-164795">Mark Sullivan</a>, <a href="https://theconversation.com/institutions/university-of-southampton-1093">University of Southampton</a>

A 2025 study by South Korean researchers caught the attention of the astronomy community when it suggested that the evidence behind dark energy could be wrong. Dark energy makes up about 70% of the universe but nobody knows what it is. This makes it a frequent target for scepticism.

Scientists had long known that the universe was expanding. But in the late 1990s, researchers discovered that this expansion was accelerating. Astrophysicists concluded that something must be driving the acceleration. They named this unknown quantity “dark energy”.

<a href="https://academic.oup.com/mnras/article/544/1/975/8281988">The 2025 re-analysis</a>, by a team at Yonsei University in Seoul, suggested that the expansion of the universe was not in fact accelerating.

Our team at the University of Southampton has now <a href="https://academic.oup.com/mnras/article/549/3/stag797/8703725">gone through the Yonsei team’s findings</a>. Our results, published in Monthly Notices of the Royal Astronomical Society (MNRAS), suggest that the results were a false alarm.

However, the Yonsei University team have said they stand by their results and that a follow up study supports their finding that the universe may not be accelerating.

<h2>What is dark energy?</h2>

In the late 1990s, cosmology was upended by a startling discovery. Scientists knew the universe was expanding, but they had assumed that gravity was gradually slowing down this expansion.

However, by observing incredibly bright, thermonuclear explosions of white dwarf stars, two independent teams of astronomers found the opposite. Because these explosions, known as Type Ia supernovae, have almost the same intrinsic brightness, comparing how bright they appear gives a precise distance.

In 1998, data revealed that distant <a href="https://iopscience.iop.org/article/10.1086/300499">supernovae were fainter</a> than they should have been in a slowing universe. Because faintness implies distance, the observations placed these supernovae farther away than expected, and implied that the cosmic expansion <a href="https://iopscience.iop.org/article/10.1086/307221">was not slowing</a>.

In fact, when the calculations were run, it was clear that the expansion of the universe was <a href="https://arxiv.org/pdf/astro-ph/0303428">actually speeding up</a>.

There was no convincing physical explanation for this “cosmic acceleration”. Physicists labelled the mysterious, repulsive force that causes the acceleration <a href="https://science.nasa.gov/dark-energy/">“dark energy”</a>.

<h2>Cosmic calibration</h2>

To understand the Yonsei University group’s claims, we must understand how astronomers use supernovae <a href="https://www.nobelprize.org/uploads/2019/05/popular-physicsprize2011.pdf">as “standard candles”</a> to measure cosmic distances. Type Ia supernovae are remarkably alike, but they are not identical.

Cosmologists correct for these differences. They use subtle calibrations to account for relationships between the brightness of the supernovae and measurements like their colour, the duration of their light curves, and the type of galaxies they explode in.

One of those corrections is well known but subtle. It links the brightness of a supernova to the size of the galaxy it exploded in. Big galaxies hold more stars, and so more mass. After the standard corrections, supernovae in those big (massive) galaxies come out a few per cent brighter than ones in small galaxies. Nobody knows why.

<figure class="align-center "> <img alt="" src="https://images.theconversation.com/files/749498/original/file-20260722-57-45t8pt.jpg?ixlib=rb-4.1.1&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/749498/original/file-20260722-57-45t8pt.jpg?ixlib=rb-4.1.1&q=45&auto=format&w=600&h=413&fit=crop&dpr=1 600w, https://images.theconversation.com/files/749498/original/file-20260722-57-45t8pt.jpg?ixlib=rb-4.1.1&q=30&auto=format&w=600&h=413&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/749498/original/file-20260722-57-45t8pt.jpg?ixlib=rb-4.1.1&q=15&auto=format&w=600&h=413&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/749498/original/file-20260722-57-45t8pt.jpg?ixlib=rb-4.1.1&q=45&auto=format&w=754&h=519&fit=crop&dpr=1 754w, https://images.theconversation.com/files/749498/original/file-20260722-57-45t8pt.jpg?ixlib=rb-4.1.1&q=30&auto=format&w=754&h=519&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/749498/original/file-20260722-57-45t8pt.jpg?ixlib=rb-4.1.1&q=15&auto=format&w=754&h=519&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"> <figcaption> SN 1572 is the remnant of a Type Ia supernova. <a class="source" href="https://www.spitzer.caltech.edu/image/sig08-016-vivid-view-of-tychos-supernova-remnant">Nasa / JPL-Caltech / CXC / Calar Alto O. Krause /MPIA</a> </figcaption> </figure>

But bigger galaxies typically have older stars that are made up of heavier elements, which could in turn influence the properties of the supernovae that some of those stars become. The authors of the 2025 study proposed a far bigger evolutionary effect. They argued that the brightness of Type Ia supernovae changes significantly as the universe ages.

Specifically, they claimed that older white dwarf stars, which are more common in the nearby, present-day universe, produce brighter explosions. That would mean today’s supernovae are inherently much brighter than their distant, early-universe counterparts.

If true, the faintness we observe in distant supernovae would not be a result of their being further away, a relationship that is in turn driven by dark energy, but rather an evolutionary trait of the stars themselves. This claim threatened to dismantle almost three decades of progress in astronomy.

<h2>Addressing the claims</h2>

Extraordinary claims need careful testing. At the University of Southampton we embarked on an audit of the data, using observations from the <a href="https://www.darkenergysurvey.org/">Dark Energy Survey (DES)</a>, an astronomy project designed to constrain dark energy’s properties, alongside the same dataset used by the authors of the 2025 work. In doing so, we sought to replicate and test their conclusions.

Our re-analysis revealed what we saw as two problems in the Yonsei study: a technical omission and a flawed assumption about stellar populations. Once corrected, the data fell back in line with standard cosmological results, including those of DES.

<figure class="align-center "> <img alt="Victor M Blanco telescope in Chile." src="https://images.theconversation.com/files/749505/original/file-20260722-57-y6oh9g.jpg?ixlib=rb-4.1.1&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/749505/original/file-20260722-57-y6oh9g.jpg?ixlib=rb-4.1.1&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/749505/original/file-20260722-57-y6oh9g.jpg?ixlib=rb-4.1.1&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/749505/original/file-20260722-57-y6oh9g.jpg?ixlib=rb-4.1.1&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/749505/original/file-20260722-57-y6oh9g.jpg?ixlib=rb-4.1.1&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/749505/original/file-20260722-57-y6oh9g.jpg?ixlib=rb-4.1.1&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/749505/original/file-20260722-57-y6oh9g.jpg?ixlib=rb-4.1.1&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"> <figcaption> The Dark Energy Survey gathered data using a wide-field camera mounted on the Victor M Blanco telescope in Chile. <a class="source" href="https://noirlab.edu/public/images/iotw2331a/">CTIO/NOIRLab/NSF/AURA/T. Matsopoulos</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a> </figcaption> </figure>

Crucially, the 2025 study did not account for how supernovae in big galaxies come out a few per cent brighter than ones in small galaxies. When our team reapplied this correction to the same dataset, the correlation between a supernova’s brightness and galaxy age – presented by the Yonsei team in their paper – became far weaker.

This doesn’t mean that the age of a white dwarf star has no effect on the brightness of a resulting supernova. Indeed, most researchers in the field would likely accept that the brightnesses of supernovae are more affected by the age of the stars that explode than by the mass of the galaxy that it is in.

However, measuring a galaxy’s mass is far less expensive in terms of telescope time and requires fewer technical assumptions than measurements of the ages of galaxies, which are challenging. What we showed is that the galaxy mass calibration is adequate to the accuracy required to constrain dark energy.

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