Astronomers at the Center of Applied Space Technology and Microgravity (ZARM) in Germany have published a new study arguing that dark energy may not be necessary. It’s far from a definitive nail in the coffin of the “energy of space,” but it does offer the tantalizing possibility that cosmology might be just a little easier to fathom than conventionally believed.
Though widely supported by scientists, the idea of dark energy has nonetheless always been a bit of an unsatisfying aspect of modern cosmology. It was created to explain the Hubble-born knowledge that the universe is expanding at an accelerating rate, in defiance of the predictions of Einstein’s gravitational theory, which predicts that it should be expanding ever more slowly.
Dark energy was initially (and, to a certain extent, still is) defined as an unknown phenomenon that has whatever properties are needed to explain this unexplainable fact. So, if the observation required that space had an intrinsic property pushing it to expand, then dark energy was defined as an intrinsic property of space that pushes it to expand. The magnitude of the effect? Well, that would be whatever it needs to be to offset gravity and create the observed rate of expansion.
NASA gravity well diagram
Credit: NASA
There have been other studies narrowing down (“constraining”) the possibility space for dark energy since then, but they haven’t fundamentally changed the conversation; the most concrete evidence that there must be a dark energy remains the original insight that universal expansion is speeding up.
As you can likely tell, this is not a traditionally sound scientific approach, but simply the best one that science has available right now. Because of the rather post hoc nature of dark energy’s definition, many scientists have naturally rejected it and assumed that more experimental theories would eventually crop up to replace it.
The most commonly cited alternative theory is gravity itself, though with critical modifications from Einstein’s original formulation that help explain recent observations, such as the expansion of the universe. These so-called “modified gravity” models aim to explain what dark energy does without positing that dark energy exists at all.
One such theory of modified gravity is called Finsler gravity, which basically maps Einstein’s relativity onto a model of the universe using a more complex geometry than Einstein assumed. Put simply, Einstein assumed an isotropic universe, or one that looks and behaves the same in all directions.
chandra isotropic universe survey
This survey from the Chandra X-Ray Observatory looked to see whether the universe was isotropic, and it failed to confirm that assumption. Credit: Chandra X-Ray Observatory
Finsler gravity instead maps the universe as anisotropic, meaning that directionality can exist, and the universe could expand at different speeds in different directions. According to the study, if you reformulate the laws of cosmology under the Finsler gravity assumption, you obtain a model of the universe that generally exhibits the observed behavior. No dark energy required.
This hinges on Finsler gravity’s ability to model the gravitational behavior of gases more accurately, with big implications for understanding the evolution of a universe that was entirely gaseous at the beginning.
By no means does this study disprove the existence of dark energy, but it does at least suggest that there could be other, more classically satisfying explanations.