For more than two decades, one of the greatest mysteries in modern physics has been a simple but troubling question: why is the universe expanding faster and faster?
Until now, scientists have relied on a hypothetical and invisible force called dark energy to explain this cosmic acceleration. But a new study suggests that this mysterious ingredient may not be necessary at all.
Instead, researchers propose that gravity itself — when described in a more general way — could naturally cause the universe to speed up.
A Long-Standing Cosmic Puzzle
Our current understanding of the universe is based on Einstein’s general theory of relativity and the standard model of particle physics. Using these equations, scientists can describe how space expands over time.
However, when astronomers compare these equations with real observations — such as distant supernovae and the cosmic microwave background — something doesn’t match. The expansion of the universe is accelerating, not slowing down.
To fix this, cosmologists added an extra component to the equations: dark energy, a hypothetical form of energy that fills space and pushes everything apart. Today, dark energy is thought to make up nearly 70% of the universe.
The problem? No one knows what dark energy actually is.
Why Dark Energy Has Always Been Unsatisfying
Dark energy was never derived naturally from Einstein’s theory. It was added as a mathematical “patch” to make the equations fit the data.
This has always made physicists uncomfortable.
“If your theory needs an unknown ingredient just to work, maybe the theory itself needs to be rethought,” many researchers have argued.
A New Idea: Extended Gravity
Now, a team from the University of Bremen (ZARM) in Germany and the Transylvanian University of Brașov in Romania suggests a different solution.
Their research, published in the Journal of Cosmology and Astroparticle Physics, is based on a modified version of gravity known as Finsler gravity.
Unlike standard general relativity, which uses a simpler geometric description of spacetime, Finsler gravity allows spacetime to have a more complex and flexible structure.
This turns out to make a big difference on cosmic scales.
Acceleration Without Dark Energy
When the researchers applied this extended gravity theory to the equations that describe the expanding universe, they made a surprising discovery:
The universe can accelerate all by itself — even without dark energy.
In this model, the geometry of spacetime alone is enough to produce an accelerating expansion. No mysterious extra energy is required.
In other words, space itself may be responsible for the cosmic speed-up.
“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,” says physicist Christian Pfeifer from ZARM.
A Radical New Perspective
If this approach turns out to be correct, it could fundamentally change how we understand the universe.
Instead of searching for an unknown cosmic substance, scientists might need to rethink the nature of space, time, and gravity itself.
However, researchers stress that this is not yet the final answer. The new theory still needs to be tested against many different astronomical observations.
What This Means for Cosmology
For now, dark energy remains the standard explanation. But this new work shows that there may be another path forward — one that doesn’t rely on invisible components, but on a deeper understanding of spacetime geometry.
If confirmed, it could mark one of the biggest shifts in theoretical physics since Einstein.
Journal Reference:
- Christian Pfeifer, Nicoleta Voicu, Annamária Friedl-Szász, Elena Popovici-Popescu. From kinetic gases to an exponentially expanding universe — the Finsler-Friedmann equation. Journal of Cosmology and Astroparticle Physics, 2025; 2025 (10): 050 DOI: 10.1088/1475-7516/2025/10/050
