New theory tries to explain why the universe is expanding at an ever-increasing rate

Astronomers know the universe is expanding, and that its rate of expansion is continually accelerating. Most explanations rely on “dark energy” to explain this phenomenon, but a new study takes a different path.

An international team of researchers from the Center for Applied Space Technology and Microgravity (ZARM) at the University of Bremen and the Transylvanian University of Brașov in Romania has concluded that the universe’s expansion may be explained – at least in part – without invoking dark energy.

This intriguing theory changes the geometry of spacetime itself and shows that acceleration can appear even when spacetime contains no extra energy component.

The approach keeps the ingredients simple and rewrites the rules for measuring distance and time.

By deriving the large‑scale equations within this new setup, the authors show how the universe’s accelerated expansion can arise without inserting a cosmological constant into the equations where, in their opinion, it doesn’t belong.

Understanding the Standard Model

Einstein’s general relativity links matter and geometry: mass and energy curve spacetime, and that curvature guides motion.

Assuming the universe looks the same on large scales leads to the Friedmann equations, which track how the universe’s scale changes with time.

To match the observed acceleration with those equations, cosmologists usually add a constant energy density, often called dark energy. It fits the data well, but it extends the model by adding a separate ingredient.

How Finsler geometry differs

The study replaces the usual Riemannian geometry with Finsler geometry. In the standard case, measured intervals do not depend on direction or on the state of motion.

In a Finsler spacetime, the metric can depend on both. This direction‑ and velocity‑dependent structure sits inside the definition of spacetime rather than outside it.

Working in that framework, the authors derive gravitational field equations and then obtain a cosmological analog of the Friedmann equations, the Finsler–Friedmann equation.

It plays the same role as in standard cosmology but reflects the new metric’s dependence on motion and direction.

Matter, geometry, and universe expansion

The treatment of matter changes, too. In general relativity, gases and other many‑particle systems are summarized by an energy-momentum tensor built from the second moment of a one‑particle distribution function.

That summary discards higher moments that describe asymmetries and detailed structure.

Here, geometry and matter live on the same phase space (positions and velocities). Because the metric depends on direction and speed, higher moments naturally contribute to gravity.

The kinetic description stays intact instead of being reduced to a single, second‑moment summary. Matter and geometry impact the universe’s expansion by being matched at the same level of detail.

Acceleration without dark energy

Solving the Finsler-Friedmann equation for a homogeneous and isotropic universe (it appears the same when looking in any direction) yields a vacuum solution with exponential growth of the scale factor.

In the standard picture, that behavior requires a cosmological constant. In this construction, the acceleration arises from the geometry itself.

This is the central result: by allowing direction‑ and velocity‑dependent measurements, the expansion accelerates even when the average energy density carries no dedicated dark‑energy term.

Causality and local physics

Changing geometry raises questions about cause and effect. The causal structure appears in the shape of light cones, which determine which events can influence others.

The analysis shows only mild deformations of light cones relative to the standard case. For observers and particles moving slowly with respect to the cosmic rest frame, the differences remain small.

That behavior helps preserve tested predictions of general relativity at ordinary speeds and scales. This framework offers a way to adjust the large‑scale expansion while keeping well‑known local physics intact.

Measuring universe expansion

The path forward relies on hard data. Supernova measurements compare brightness and distance to map the expansion history.

Galaxy surveys measure a preferred scale set by baryon acoustic oscillations (a regular pattern in galaxy spacing).

The cosmic microwave background records early‑universe geometry and contents. The growth of structure tracks how small density ripples became galaxies and clusters.

The Standard Model performs well across these tests. A Finsler‑based model, as proposed in this study, must meet the same benchmarks.

It needs concrete predictions for distance-redshift relations, the BAO scale, the microwave background pattern, and the time dependence of structure growth.

Gravitational lensing provides another powerful check because it traces how light follows spacetime geometry near mass.

Many questions left unanswered

Any alternative theory must stay consistent with precision tests in the solar system and with measurements from binary pulsars.

It should also fit what we know about the early universe, including a very rapid expansion phase and its later transition to standard behavior.

Stability and the behavior of small disturbances matter as well; the evolution of ripples influences structure, lensing, and tiny directional differences.

These questions set clear targets for theory and observation. They determine whether the geometry‑driven mechanism matches the sky as well as or better than the current model.

Universe, expansion rate, and next steps

This work shows two levers in cosmology: the inventory of what fills the universe and the rules that measure it.

The usual path explains acceleration by adding a constant energy density. This study keeps the inventory lean and modifies the metric so that direction and motion influence measured intervals.

Within that framework, the Universe’s accelerated expansion appears without the need for mysterious and elusive dark energy.

“This is an exciting indication that we may be able to explain the accelerated expansion of the universe without dark energy, based on a generalized spacetime geometry,” concludes Christian Pfeifer, ZARM physicist and member of the research team.

“The new geometry opens up completely new possibilities for better understanding the laws of nature in the cosmos.”

If predictions align with supernovae, galaxy clustering, the microwave background, lensing, and structure growth, then geometry alone could account for the observed acceleration.

If not, the comparison will still sharpen our understanding of how measurement rules shape cosmic dynamics. Either way, the idea is testable.

The full study was published in the Journal of Cosmology and Astroparticle Physics.

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