New measurements of the rotation of galaxies point to altered gravity as an explanation for dark matter
Although dark matter is a central part of the standard cosmological model, it is not without problems. There are still nagging mysteries about the stuff, not least the fact that scientists haven’t found any direct particle evidence of it.
Despite numerous searches, we have not yet been able to detect it dark matter particles. So some astronomers prefer an alternative, such as modified Newtonian dynamics (MoND) or modified gravity model. And a new study of galactic rotation seems to support them.
The idea of MoND is inspired by galactic rotation. Most of the visible matter in a galaxy is clustered in the center, so you would expect stars closer to the center to have higher orbital velocities than stars further away, similar to our solar system’s planets. What we observe is that stars in a galaxy all rotate at approximately the same speed. The rotational curve is essentially flat instead of tapering off. The solution to dark matter is that galaxies are surrounded by a halo of invisible matter, but in 1983 Mordehai Milgrom argued that our gravity model must be wrong.
At interstellar distances, the attraction between stars is essentially Newtonian. So instead of adjusting general relativity, Milgrom suggested changing Newton’s universal law of gravity. He argued that instead of the attraction being a pure inverse square relationship, gravity has a small lasting pull regardless of distance. This remnant is only about 10 trillionths of a G, but it’s enough to explain galactic rotational curves.
Of course, adding a little term to Newton’s gravity means adjusting Einstein’s equations as well. So MoND has been generalized in various ways, such as AQUAL, which stands for “a quadradic Lagrangian”. Both AQUAL and the standard LCDM model can explain observed galactic rotation curves, but there are some subtle differences.
This is where a recent study comes into play. One difference between AQUAL and LCDM is in the rotational speeds of stars in the inner orbit versus stars in the outer orbit. For LCDM, both must be governed by the distribution of matter, so the curve must be smooth. AQUAL predicts a small kink 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 a small shift between the inner and outer velocity distributions.
So the author of this paper looked at high-resolution velocity curves of 152 galaxies as observed in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. He found a shift in consultation with AQUAL. The data appears to support modified gravity over standard dark matter cosmology.
The result is exciting, but it doesn’t tip over definitively dark matter. The AQUAL model has its own issues, such as disagreement with observed gravitational lensing galaxies. But it’s a win for the underdog theory, leading some astronomers to shout “Vive le MoND!”
The research has been published on the arXiv preprint server.
Kyu-Hyun Chae, Distinguishing Dark Matter, Modified Gravity, and Modified Inertia with the Inner and Outer Parts of Galactic Rotation Curves, arXiv (2022). DOI: 10.48550/arxiv.2207.11069
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