Physicists from the Center of Applied Space Technology and Microgravity at the University of Bremen and the Transylvanian University of Brașov have unveiled a new theoretical framework that could rewrite how we understand the accelerating expansion of the Universe — and potentially render the mysterious dark energy obsolete. They’ve proposed that the acceleration may be a fundamental feature of spacetime geometry itself, rather than the product of an unknown cosmic force.
This artist’s impression shows the evolution of the Universe beginning with the Big Bang on the left followed by the appearance of the Cosmic Microwave Background. The formation of the first stars ends the cosmic dark ages, followed by the formation of galaxies. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
For more than a quarter-century, scientists have grappled with a startling observation: quite contrary to expectations, the expansion of the Universe is not slowing under gravity’s pull but speeding up.
In the 1990s, astronomers inferred this acceleration by observing distant Type Ia supernovae, a discovery that led to the widespread postulation of dark energy — a pervasive, unseen component thought to drive cosmic acceleration.
Yet despite its central role in the Standard Model, dark energy’s physical nature remains elusive.
In their new work, Dr. Christian Pfeifer and his colleagues argue that the expansion of the Universe could be explained, at least in part, by modifying the underlying geometric framework used to describe gravity.
At the heart of modern cosmology lies Einstein’s general theory of relativity, which describes how matter and energy shape spacetime.
The evolution of the Universe itself is modeled using the Friedmann equations, derived from Einstein’s theory.
The team’s new solution builds on an extension of Einstein’s theory known as Finsler gravity.
Developed over recent years, the framework generalizes the geometry of spacetime itself, allowing for a more detailed description of how matter — especially gases — behaves under gravity.
Unlike general relativity, which relies on a specific geometric structure, Finsler gravity permits a richer and more flexible spacetime geometry.
Using this approach, the study authors recalculated the equations that govern the Universe’s expansion.
When expressed within the Finsler framework, the modified Friedmann equations naturally predict an accelerated expansion of the Universe — even in a vacuum, and without introducing any additional dark energy component.
In other words, the acceleration emerges from the geometry of spacetime itself.
“This is an exciting indication that we may be able to explain the accelerated expansion of the Universe, at least in parts, without dark energy, on the basis of a generalized spacetime geometry,” Dr. Pfeifer said.
The idea does not claim to eliminate dark energy entirely, nor does it immediately overturn the Standard Model.
Instead, it suggests that at least some of the effects attributed to dark energy might arise from a deeper and more nuanced description of gravity.
“This new geometric point of view on the dark energy problem opens up new possibilities for better understanding the laws of nature in the cosmos,” Dr. Pfeifer said.
The team’s paper was published in the Journal of Cosmology and Astroparticle Physics.
_____
Christian Pfeifer et al. 2025. From kinetic gases to an exponentially expanding Universe — the Finsler-Friedmann equation. JCAP 10: 050; doi: 10.1088/1475-7516/2025/10/050
