Even Dark Matter Can’t Catch a Break From Gravity, Study Suggests

There’s still a lot we don’t know about dark matter, or the “missing†mass supposedly constituting around 85% of the universe. Physicists have good reason to believe it’s out there. New research confirms at least one long-standing hunch about dark matter—that it’s influenced by gravity and can be detected through its gravitational effects.

A Nature Communications paper published November 3 presents a thorough mathematical analysis of galactic redshifts, or shifts in the wavelengths of a galaxy as it moves away from us. The datasets were a combination of the first three years of measurements from the Dark Energy Survey, as well as 22 data points from various spectroscopic surveys. The researchers assessed how well the cosmological data conformed to gravitational force and Euler’s equations, a set of tools used by astrophysicists to account for the effects of general relativity.

They found strong evidence to support the idea that dark matter, just like anything else in the universe, behaves according to the rules of gravity. At the same time, they did spot a small likelihood of a mysterious fifth force, which, if real, could drastically change the search for dark matter.

Dips in spacetime

For the study, the researchers turned to gravitational wells—sizeable “dips†in the fabric of spacetime caused by the gravitational force of massive celestial bodies. They tracked how the presence of gravitational waves caused shifts in the velocity of galaxies, represented by the redshift data.

“If dark matter is not subject to a fifth force, then galaxies—which are mostly made of dark matter—will fall into these wells like ordinary matter, governed solely by gravity,†explained Camille Bonvin, study co-author and a physicist at the University of Geneva in Switzerland, in a release.

If dark matter is exempt from gravity, however, the galaxies would interact with gravitational waves in different ways, according to the paper. The team’s calculations demonstrated that galactic motions indeed obey general relativity and Euler’s equations—meaning, by extension, it is highly likely that the same applies to dark matter.

The fifth force

Another part of the study was to consider the possibility of an unknown “fifth†force at play with dark matter. So far, physicists have confirmed four fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—that govern how all matter behaves. But could dark matter be different? Could it operate in accordance with other rules?

In short, the researchers’ conclusion was a weak “maybe.†The conclusions “do not yet rule out the presence of an unknown force,†admitted Nastassia Grimm, study lead author and an astrophysicist at the University of Portsmouth in the United Kingdom, in the release. If there were such a force, however, its strength could not exceed 7% the strength of gravity, she added, or it would have already appeared in observations.

Still, in the slim chance that the fifth force exists, it would “not only break Euler’s equation, but it would also leave an impact on the evolution of the density fluctuations and the gravitational potentials,†according to the paper. Basically, a fifth force would warrant a complete reassessment of the way astrophysicists study galaxies.

Whether that would be a nightmare or the start of a fantastic journey, the researchers can’t be sure yet. That said, the researchers anticipate they’ll soon have a chance to probe this further, using the next round of data from the Vera C. Rubin Observatory and the Dark Energy Survey.