Dark energy is the name given to whatever causes the universe's expansion to accelerate. It behaves like a repulsive force spread uniformly through space and does not interact with ordinary matter or light. Despite comprising roughly 70% of the universe's total energy, it has never been directly detected — its existence is inferred from the expansion rate of the universe.

The universe is still accelerating — two Nobel laureates had to prove it again

Q: How do we know the universe is accelerating?

By comparing how fast distant Type Ia supernovae are receding against nearby ones. These stellar explosions have consistent brightness, letting astronomers measure distances precisely. Distant supernovae appear farther away than a decelerating universe would predict — meaning expansion has sped up over time. Multiple independent methods confirm this result.

Dark energy is the strangest thing we have ever confirmed exists. It fills all of space, pushes galaxies apart faster and faster, and nobody can say what it is. What astronomers have known, with compounding confidence for nearly thirty years, is that it must be there — because the universe is accelerating. When a team from Yonsei University in South Korea challenged that conclusion in a peer-reviewed paper, the scientists who first proved it went back to the data.

They have now published their response in the same journal that ran the original challenge. The universe is still accelerating. The Nobel Prize is still justified. The mystery, though, is as deep as it has ever been.

Dark energy is so central to modern cosmology that it is built into the standard model of the universe — the equations that describe everything from the Big Bang to the fate of every galaxy. If the universe were decelerating, that model would need fundamental revision. The Yonsei study made precisely that claim: the universe had already entered a phase of slowing expansion, and dark energy was weakening more rapidly than any model predicted. The claim was extraordinary, and so was the response.

A cosmic thermometer

The instrument at the center of this debate is a type of stellar explosion called a Type Ia supernova. These events occur when a white dwarf star in a binary system draws too much mass from its companion and ignites in a runaway thermonuclear detonation — a flash of light that briefly outshines an entire galaxy. Remarkably, these explosions are consistent enough in brightness to serve as what astronomers call a standard candle: if you know how bright an object truly is, its apparent dimness tells you exactly how far away it is.

Three physicists — Adam Riess, Brian Schmidt, and Saul Perlmutter — used this method to measure how fast distant galaxies were receding. What they found overturned cosmology: galaxies were not simply moving away; they were moving away faster than they used to. The universe’s expansion was accelerating, driven by some invisible force they called dark energy. The discovery earned all three the Nobel Prize in Physics.

The Yonsei team used the same instrument — more than three hundred Type Ia supernovae — to reach the opposite conclusion. Their analysis argued that once you corrected for “age bias” (the idea that different stellar populations produce slightly different-looking supernovae), the accelerating expansion signature vanishes. If correct, dark energy would not be a fixed feature of the cosmos but a fading one, already in retreat.

Where the challenge broke down

Dr. Phil Wiseman of the University of Southampton led the rebuttal team, working alongside the two Nobel laureates. Their analysis identified two specific errors embedded in the Yonsei methodology.

The first was an age conflation. The Korean study used the age of the host galaxy as a proxy for the age of the individual supernova within it — a substitution that does not hold. Galaxies contain stars of vastly different ages, and the supernova that detonates today may have originated in a stellar system billions of years older or younger than the galaxy’s average. Using galaxy age as a stand-in for supernova age systematically distorts the analysis.

The second error was the omission of a host galaxy mass correction — a standard step in modern supernova cosmology. The brightness of a Type Ia supernova varies subtly depending on the mass of the galaxy it occurs in, and this correction has been applied as routine practice for years. Its absence introduced a systematic bias into the Korean team’s distance measurements, precisely the measurements that underpin any claim about expansion history.

“The previous and well-accepted measurements were, in fact, fine,” Wiseman stated, “and our current understanding of the fate of the universe remains robust.”

Professor Sullivan, a co-author, noted that questioning established ideas remains essential to scientific progress — and that this challenge, even though incorrect, prompted the community to examine supernova astrophysics and cosmological methodology with renewed precision. A scientific debate that ended with the original answer unchanged still moved the field.

What the confirmation does not settle

The rebuttal confirms cosmic acceleration. It does not explain what produces it.

Dark energy constitutes roughly 70% of the total energy in the universe, yet it produces no light, no detectable particles, and no direct signal beyond the gravitational imprint it leaves on the universe’s growth rate. Its existence is inferred entirely from what it does — and what it does is push. Every method that has measured cosmic expansion, from Type Ia supernovae to the cosmic microwave background to baryon acoustic oscillations to gravitational wave sirens, returns the same result: the universe is not just expanding, it is expanding faster than it used to.

What none of those methods has settled is whether dark energy is fixed — a cosmological constant Einstein introduced and then called his greatest blunder — or whether it is dynamic, an evolving field that might behave differently in the future than it does today. This distinction matters because the universe’s fate depends on it. A fixed dark energy produces a universe that expands forever at an accelerating rate, growing colder and emptier over trillions of years. An evolving dark energy introduces the possibility of much stranger outcomes.

Early results from the DESI survey in 2025 hinted at possible evolution in dark energy, though the statistical significance was not high enough to be conclusive. Wiseman’s team is explicit about the limits of their finding: what they have rebutted is the specific claim that acceleration has already stopped. Whether dark energy itself is changing over time remains an open and active question.

Common questions about dark energy

What is dark energy?

Dark energy is the name given to whatever is causing the universe’s expansion to accelerate. It behaves like a repulsive force spread uniformly through space and does not interact with ordinary matter or light — making it invisible by every instrument we have built. Despite comprising roughly 70% of the universe’s total energy, it has never been directly detected. Its existence is inferred entirely from the expansion rate of the universe as measured through supernovae, galaxy surveys, and the cosmic microwave background.

How do we know the universe is accelerating?

By comparing how fast distant supernovae are receding against how fast nearby ones are. In a decelerating universe, very distant explosions would appear to be receding faster than expected relative to their brightness. Instead, they appear dimmer — meaning they are farther away than deceleration would predict, which means the expansion has sped up over time. This signature has been confirmed across multiple independent observational methods, each using different physics.

Could dark energy be changing over time?

Possibly. The simplest model treats dark energy as a cosmological constant — fixed, uniform, and permanent. But early results from the DESI survey and some supernova compilations have hinted at possible deviations. The question is genuinely open. This rebuttal confirms that acceleration has not stopped; it does not resolve whether the strength of dark energy is constant.

Does this resolve the Hubble tension?

No. The Hubble tension — the persistent disagreement between different methods of measuring the universe’s current expansion rate — is a separate and unresolved problem. This rebuttal addresses only whether the universe is still accelerating, not the precise rate at which it is doing so.

The Nancy Grace Roman Space Telescope, which launched days before this rebuttal was published, is expected to deliver the largest supernova survey in history over its five-year mission. Combined with Euclid’s weak-lensing data and DESI’s galaxy maps, Roman’s observations should substantially narrow the range of possible dark energy models — and either confirm the cosmological constant or force a revision nobody has yet predicted.

Reference: Wiseman et al., “Still accelerating: type Ia supernova cosmology is robust to host galaxy age evolution,” Monthly Notices of the Royal Astronomical Society, 2026; 549(3). DOI: 10.1093/mnras/stag797

Tags: astronomy, dark energy, Adam Riess, Brian Schmidt, cosmic acceleration, Nobel Prize