A new theory of gravity could explain cosmic acceleration without dark energy
The accelerating expansion of the universe is usually explained by an invisible force known as dark energy. But a new study suggests this mysterious ingredient may not be necessary after all. Using an extended version of Einstein’s gravity, researchers found that cosmic acceleration can arise naturally from a more general geometry of spacetime. The result hints at a radical new way to understand why the universe keeps speeding up.
Why the universe is expanding faster and faster remains one of the biggest open questions in physics. Current theories cannot fully explain this accelerating growth. Today's standard picture of the universe is built on Einstein's general theory of relativity and the standard model of particle physics. Within this framework, scientists usually assume the presence of a mysterious force known as "dark energy" to account for the speeding expansion of space. Despite its widespread use in cosmology, the true nature and origin of dark energy are still unknown.
Now, researchers from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen, working with colleagues at the Transylvanian University of Brașov in Romania, suggest a different way of looking at the problem. Their work indicates that the universe's expansion might be explained -- at least partially -- without invoking dark energy at all.
Why Dark Energy Was Added in the First Place
Cosmologists use Einstein's general theory of relativity along with the Friedmann equations to describe how the universe changes over time. However, when these equations are applied to real astronomical observations, they fall short. To match what telescopes see, scientists must manually add an extra "dark energy term" to the equations. This addition is not derived naturally from the theory itself, which has long made it an unsatisfying solution.
A New Approach Using Extended Gravity
Because of this limitation, the team at ZARM and their Romanian collaborators explored an alternative idea. Their results, published in the Journal of Cosmology and Astroparticle Physics, rely on an extension of general relativity (GR) known as Finsler gravity. This approach has been developeded over recent years and uses a broader description of spacetime geometry.
Unlike the standard formulation of GRT, Finsler gravity can describe the gravitational behavior of gases more precisely. This difference turns out to be crucial when modeling the large-scale behavior of the universe.
Accelerated Expansion Without Dark Energy
When the researchers applied Finsler gravity to the Friedmann equations, they uncovered a striking result. The modified equations, known as the Finsler-Friedmann equations, naturally predict an accelerating universe even in empty space. No extra assumptions are required, and no additional "dark energy" term needs to be added by hand.
"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," says Christian Pfeifer, ZARM physicist and member of the research team. "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."