New measurements of galaxy rotation lean toward modified gravity as dark matter explanation

Although dark matter is a central part of the standard cosmological model, it is not without problems. There are still lingering mysteries about the stuff, not the least of which is the fact that scientists haven’t found any direct evidence of particles.

Despite many searches, we have yet to detect dark matter particles. So some astronomers favor an alternative, such as modified Newtonian dynamics (MoND) or modified gravity model. And a new study of galactic rotation seems to back them up.

The idea of ​​MoND was inspired by galactic rotation. Most of the visible matter in a galaxy is clustered in the center, so stars closer to the center would be expected to have faster orbital velocities than stars further away, such as the planets in our solar system. What we observe is that the stars in a galaxy rotate at roughly the same speed. The rotation curve is essentially flat rather than down. The dark matter solution is that galaxies are surrounded by a halo of invisible matter, but in 1983 Mordehai Milgrom argued that our gravitational model must be wrong.

At interstellar distances, the gravitational pull between stars is essentially Newtonian. So instead of modifying general relativity, Milgrom proposed to modify Newton’s universal law of gravity. He argued that instead of the attractive force being a pure inverse square relationship, gravity has a small attractive force regardless of distance. This remnant is only about 10 trillion G, but it is enough to explain the galactic rotation curves.

Of course, just adding a small term to Newton’s gravity means you have to modify Einstein’s equations as well. Thus, MoND has been generalized in various ways, such as AQUAL, which stands for “a quadratic Lagrangian”. Both AQUAL and the standard LCDM model can explain the observed Galactic rotation curves, but there are some subtle differences.

That’s where a recent study comes into play. A difference between AQUAL and LCDM is in the rotation velocities of the stars in the inner orbit and those in the outer orbit. By LCDM, both should be governed by the matter distribution, so the curve should be smooth. AQUAL predicts a small twist in the curve due to the dynamics of the theory. It is too small to measure in a single galaxy, but statistically, there should be a small shift between the inner and outer velocity distributions.

Thus, the author of this paper analyzed the high-resolution velocity curves of 152 galaxies as observed in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. He found a change according to AQUAL. The data appear to support modified gravity over standard dark matter cosmology.

The result is exciting, but not conclusively overturned dark matter. The AQUAL model has its own problems, such as its disagreement with the gravitational lensing observed by galaxies. But it’s a win for the underdog theory, prompting some astronomers to cheer “Vive le MoND!”

The research is published in archiv preprint server.