Although dark matter is an essential part of the standard cosmological model, it is not without its problems. There are still nagging mysteries about these objects, not least of which is the fact that scientists haven’t found direct evidence for the particles.
Despite our many searches, we have not yet discovered dark matter particles. So some astronomers prefer an alternative, such as the modified Newtonian dynamics (MoND) or modified gravitational model. A new study of galactic rotation seems to back them up.
The idea of MoND is inspired by the rotation of the galaxy. Most of the visible matter in the galaxy collects in the middle, so you’d expect that stars closer to the center would have faster orbital velocities than stars farther away, similar to planets in our solar system. What we notice is that the stars in the whole galaxy rotate at roughly the same speed. The rotation curve is basically flat rather than declining. The dark matter solution is that galaxies are surrounded by a halo of invisible matter, but in 1983 Mordehay Milgrom argued that our gravitational model must be wrong.
In interstellar distances, the gravitational attraction between stars is mainly Newtonian. So instead of modifying general relativity, Milgrom suggested modifying Newton’s universal law of gravitation. He argued that instead of the force of attraction as a pure inverse square relationship, gravity has a small residual pull regardless of distance. This remnant is only about 10 trillionths of a G, but it is enough to explain the galactic rotation curves.
Of course, just adding a small term to Newtonian gravity means you also have to tweak Einstein’s equations as well. So MoND has been generalized in various ways, such as AQUAL, which stands for “quadruple Lagrangian”. Both the AQUAL model and the standard LCDM can explain the observed galactic rotation curves, but there are some subtle differences.
This is where a recent study comes in. One difference between AQUAL and LCDM is the rotation speed of inner orbit stars versus outer orbit stars. For LCDM, both must be subject to texture distribution, so the curve must be smooth. AQUAL predicts a small gap in the curve due to the dynamics of the theory. It’s too small to measure in a single galaxy, but statistically there should be little shift between the inward and outward velocity distributions.
So the author of this paper considered high-resolution velocity curves for 152 galaxies as observed in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. He found a shift in agreement with AQUAL. The data appears to support modified gravity over standard dark matter cosmology.
The result is exciting, but it doesn’t conclusively flip dark matter. The AQUAL model has its own problems, such as its inconsistency with gravitational lensing observed by galaxies. But it’s a victory for the underdog theory, which has some astronomers exclaiming “Vive le MoND!”
Research published on arXiv Prepress server.
Kyu-Hyun Chae, Discerning Dark Matter, Modified Gravity, and Modified Inertia with the Inner and Outer Parts of the Galactic Rotation Curves, arXiv (2022). DOI: 10.48550/arxiv.2207.11069
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