Dark energy, the invisible something that is supposed to make up about 70% of the universe, may not be needed to explain why space keeps speeding up.
An international team based at the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen and the Transylvanian University of Brașov has shown that an extended version of Einstein’s gravity can produce an accelerating universe using geometry alone.
If that holds up, one of the biggest ingredients in modern cosmology suddenly looks less like a mysterious substance and more like a bookkeeping trick.
Why dark energy was invented in the first place
Back in 1998, astronomers discovered that distant supernovae were dimmer than expected, which suggested the universe was not just expanding but doing so faster and faster.
To make the equations match the data, cosmologists added a new term that behaves like a constant energy density filling space. That term became known as dark energy and today it is thought to dominate the cosmic energy budget.
There was always a catch. Dark energy did not emerge naturally from physics. It was something put into the equations by hand so the numbers would fit. Imagine balancing your monthly budget by adding a mystery income line labeled “other” and never really explaining where the money comes from.
So physicists kept asking a simple question. What if the equations themselves are too limited?
Enter Finsler gravity
The new work looks at gravity through a more general type of geometry called Finsler geometry. In Einstein’s general relativity, spacetime is modeled with Riemannian geometry, which depends on position but not on the direction or speed of a moving object.
Finsler geometry adds that extra dependence, which turns out to matter when you try to describe gases and particles that move in many different ways on very large scales.
This richer framework lets the team connect gravity directly to the full behavior of a “kinetic gas” using what physicists call the one-particle-distribution function. In standard cosmology, only one part of that function feeds into gravity, through the usual energy momentum tensor. In the Finsler picture, all the moments of that distribution can influence the gravitational field.
In everyday terms, it is like moving from looking only at the average speed in highway traffic to considering the whole spread of speeds, from slow trucks to speeding motorcycles, when you decide how to design the road.
The Finsler Friedmann equation and an accelerating vacuum
Working in a universe that is homogeneous and isotropic on large scales, the researchers derived what they call the Finsler Friedmann equation. This equation determines both how the cosmic scale factor evolves in time and how the causal structure of spacetime looks, all within the Finsler gravity framework.
The surprising part is what happens in a vacuum. Even with no matter and no cosmological constant, the equation naturally admits a solution where the universe expands exponentially with time.
In other words, accelerated expansion appears as a built-in feature of the geometry rather than something forced by adding a separate dark energy term.
The causal structure in this solution is only mildly different from the standard Friedmann Lemaître Robertson Walker cosmology, and for slowly-moving objects the two are nearly indistinguishable.
Christian Pfeifer, physicist at ZARM and co-author of the study, calls the result “an exciting indication that we may be able to explain the accelerated expansion of the universe” without treating dark energy as a separate ingredient. He adds that this geometric point of view could open new possibilities for understanding the laws of nature at cosmic scales.
What changes and what does not
For the most part, this is a theoretical advance. No telescope has suddenly seen dark energy vanish. Instead, the work shows that a generalized spacetime geometry can reproduce one of the main jobs usually assigned to dark energy, at least in idealized models.
Future studies will have to test whether Finsler gravity can match the full set of observations, from the cosmic microwave background to the growth of large-scale structure.
At the end of the day, that is what will decide whether this framework becomes a serious rival to the standard picture or simply a useful thought experiment about how flexible gravity might be.
For people on Earth, none of this will make tonight’s sky look different and it will not shrink the electric bill. Yet it quietly reshapes the background story of the universe that sets the stage for every galaxy, star, planet, and ecosystem.
If accelerated expansion can come from the shape of spacetime itself, then our cosmic environment may be telling us that geometry, not a mysterious extra fluid, drives the largest-scale evolution of nature.
The study was published in the Journal of Cosmology and Astroparticle Physics.
