According to the standard cosmological model, the Universe has been in a phase of accelerated expansion for several billion years—an expansion expected to continue forever, leading to a colder, increasingly inhospitable cosmos where even distant galaxies would eventually fade from view. Not so, suggests a team of South Korean researchers, who argue instead that the Universe has already shifted from acceleration to deceleration, calling into question the idea of eternal expansion.
When Hubert Reeves completed his PhD in nuclear astrophysics in the late 1950s, the prevailing cosmological model (supported by most cosmologists and championed by Fred Hoyle along with Hermann Bondi and Thomas Gold in 1948) was the now-defunct steady state model. It proposed an infinite Universe expanding at a constant rate for all eternity. In this view, the dilution of matter—seen as a gas of galaxies—was offset by the continuous creation of new matter, a gas of light atoms that condensed into stars and galaxies, eventually forming heavier elements through stellar nucleosynthesis.
But everything changed in 1965 with the discovery of the cosmic microwave background, which delivered the final blow to the already-weakened steady state model (undermined earlier by the discovery of quasars) and confirmed the Big Bang theory proposed by Lemaître, Gamow and Alpher. The model that replaced it predicted a Universe whose expansion rate had been slowing down since the Big Bang. That is, until a new surprise arrived in the late 1990s when two teams of astronomers—using Type Ia supernovae—independently discovered that around 7 billion years ago, deceleration gave way to accelerated expansion.
To explain this reversal, physicists brought back into Einstein’s gravitational equations a new physical constant—interpreted as a form of exotic energy producing a paradoxically repulsive gravitational effect: Einstein’s famous cosmological constant, first introduced in 1917 in the earliest relativistic model of the Universe.
In recent years, however, evidence has been mounting that this constant may not actually be constant, and instead might vary over time—an idea compatible with some theories of new physics. These theories attempt to explain the so-called dark energy, the mysterious component behaving like a time-dependent energy field.
Presentation by Françoise Combes of her 2016-2017 course: “Dark Energy and Models of the Universe.” The cosmologist and astrophysicist explains the problem of dark energy, its possible solutions, and the observation programs planned to solve the enigma of its nature. © Collège de France
Supernovae, fossil radiation and BAO: tools for probing dark energy
Type Ia supernovae are explosions of white dwarfs—stellar remnants with the mass of the Sun packed into the volume of Earth. They are extremely bright, visible billions of light years away, and their luminosity varies very little, making them reliable standard candles for astronomers.
By measuring their brightness, researchers can estimate the distance of the galaxies in which they occurred, sometimes billions of years in the past. Combined with measurements of their spectral shift, scientists can determine the value and possible evolution of the Universe’s expansion speed.
This also produces a measurement known as the Hubble–Lemaître constant, which links a distant galaxy’s spectral shift to its distance from the Milky Way, offering clues about the value and behavior of the cosmological constant.
Independent estimates come from studying the cosmic microwave background—the oldest observable light in the Universe, emitted about 380,000 years after the Big Bang—and from the study of large-scale structures made of galaxies and clusters. These structures still bear the imprint of sound waves in the hot plasma of the early Universe, known as baryon acoustic oscillations (BAO).
In recent years, a growing disagreement has emerged between the Hubble–Lemaître constant derived from the cosmic microwave background and the one derived from supernovae. One way to reconcile this tension is to allow dark energy to evolve over time. A new paper published in Monthly Notices of the Royal Astronomical Society, with a freely accessible version on arXiv, has just added fresh weight to this idea.
In a press release from the Royal Astronomical Society, lead author Young-Wook Lee of Yonsei University in South Korea states: “Our study shows that the Universe has already entered a phase of slowed expansion in the present epoch and that dark energy evolves much faster than previously thought. If confirmed, this would represent a major paradigm shift in cosmology since the discovery of dark energy 27 years ago.”
“Does the dark universe hold new surprises for us?” A talk by French cosmologist and physicist Nathalie Palanque-Delabrouille, member of the French Academy of Sciences and of the Lawrence Berkeley National Laboratory (U.S. Department of Energy’s Office of Science). This lecture, organized on November 14, 2024, by the Paris-South local chapter of the French Physical Society, remains relevant for understanding the 2025 results concerning DESI. © French Physical Society
Supernovae evolving like galaxies
But how can the Yonsei University team propose such a hypothesis?
White dwarfs were chosen to probe the distant Universe because they were believed to explode with a constant luminosity, acting as standard candles. However, independent studies now suggest this assumption is flawed. Just as galaxies evolve chemically, white dwarfs do not form identically across cosmic time. More precisely, their intrinsic brightness at the moment of the thermonuclear explosion depends on the age of their progenitor stars. Supernovae from younger stellar populations appear systematically dimmer, while those from older populations appear brighter.
This introduces a bias: without accounting for stellar age, the true luminosity of Type Ia supernovae—and therefore all conclusions drawn from them—cannot be accurately determined.
The researchers explain that even before correcting for this bias, combining cosmic microwave background data with the latest BAO measurements and recent supernova data suggested that the observable Universe’s expansion would slow down in the future.
But now, after including the supernova age-related bias in the calculations, the result becomes much more dramatic: deceleration would already be underway, with a statistical significance exceeding 9 sigma—well above the 5-sigma threshold usually considered sufficient to claim a discovery. The correlation between galaxy age, progenitor age and supernova luminosity already meets this level of confidence.
Still, caution is essential. More precise supernova data from the Euclid satellite and the Vera C. Rubin Observatory will be needed in the coming years.
If the evolution of dark energy is confirmed, the next challenge will be identifying the underlying physics and the Universe’s ultimate fate. One possibility is a future Big Crunch in tens of billions of years, followed by a new Big Bang.
Many quantum gravity theories—also theories of unification—introduce new quantum fields behaving as scalar fields, reminiscent of the one associated with the Brout–Englert–Higgs boson. It is possible to introduce a time-varying scalar field called a quintessence field, a nod to the fifth element of Aristotelian philosophy. Its energy density, which would change over time, would play the role of dark energy.
Laurent Sacco
Journalist
Born in Vichy in 1969, I grew up during the Apollo era, inspired by space exploration, nuclear energy, and major scientific discoveries. Early on, I developed a passion for quantum physics, relativity, and epistemology, influenced by thinkers like Russell, Popper, and Teilhard de Chardin, as well as scientists such as Paul Davies and Haroun Tazieff.
I studied particle physics at Blaise-Pascal University in Clermont-Ferrand, with a parallel interest in geosciences and paleontology, where I later worked on fossil reconstructions. Curious and multidisciplinary, I joined Futura to write about quantum theory, black holes, cosmology, and astrophysics, while continuing to explore topics like exobiology, volcanology, mathematics, and energy issues.
I’ve interviewed renowned scientists such as Françoise Combes, Abhay Ashtekar, and Aurélien Barrau, and completed advanced courses in astrophysics at the Paris and Côte d’Azur Observatories. Since 2024, I’ve served on the scientific committee of the Cosmos prize. I also remain deeply connected to the Russian and Ukrainian scientific traditions, which shaped my early academic learning.