Although dark matter is a central part of the standard cosmological model, it is not without problems. There continue to be nagging mysteries about the stuff, not the least of which is the fact that scientists haven’t found any direct particle evidence for it. Despite many searches, we have yet to discover dark matter particles. So some astronomers favor an alternative, such as Modified Newtonian Dynamics (MoND) or the Modified Gravitational Model. And a new study of galactic rotation seems to back them up.
The idea for MoND was inspired by galactic rotation. Most of the visible material in a galaxy is clustered in the center, so you’d expect stars closer to the center to have higher orbital speeds than stars farther away, like the planets in our solar system. What we observe is that stars in a galaxy all rotate at approximately the same speed. The rotation curve is essentially flat rather than falling off. The solution for dark matter is that galaxies are surrounded by a halo of invisible matter, but in 1983 Mordehai Milgrom argued that our model of gravity must be wrong.
At interstellar distances, the gravitational attraction between stars is essentially Newtonian. So instead of modifying general relativity, Milgrom proposed modifying Newton’s universal law of gravitation. He argued that instead of the force of attraction being a pure inverse square relationship, gravity has a small residual force independent of distance. This remainder is only about ten trillionths of a gee, but it is enough to explain galactic rotation curves.
Of course, just adding a small term to Newton’s gravity means you have to modify 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 in. One difference between AQUAL and LCDM is the rotation rates of inner-orbit stars vs. stars in outer orbit. For LCDM, both should be governed by the distribution of matter, so the curve should be smooth. AQUAL predicts a slight bend 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. So the author of this paper looked at high-resolution velocity curves for 152 galaxies as observed in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. He found a shift consistent with AQUAL. The data appear to support modified gravity over standard dark matter cosmology.
The result is exciting, but it does not finally overturn dark matter. The AQUAL model has its own problems, such as its disagreement with observed gravitational lensing of galaxies. But it’s a victory for the underdog theory, prompting some astronomers to cheer, “Vive le MoND!”
This article was originally published on The universe today by Brian Koberlein. Read the original article here.