Although dark matter is a central element of the standard cosmological model, it is not without its problems. Persistent mysteries persist about it, including the fact that scientists have found no direct evidence of its presence in particle form.
Despite extensive research, we have yet to detect dark matter particles. Some astronomers therefore favor an alternative, such as modified Newtonian dynamics (MoND) or the modified gravity model. And a new study of galactic rotation seems to confirm them.
The idea for the MoND was inspired by galactic rotation. Most of a galaxy’s visible matter is clustered in the center. One might therefore expect stars near the center to have faster orbital speeds than stars further away, like the planets in our solar system. What we observe is that the stars in a galaxy all spin at roughly the same speed. The spin curve is essentially flat rather than falling. 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.
Rotation curve of the typical spiral galaxy M 33 (yellow and blue dots with error bars) and that predicted from the distribution of visible matter (white line). The divergence between the two curves is explained by the addition of a halo of dark matter surrounding the galaxy. Credit: Wikipedia
At interstellar distances, the gravitational attraction between stars is essentially Newtonian. So, rather than modifying general relativity, Milgrom proposed modifying Newton’s universal law of gravity. He argued that instead of the force of attraction being a pure inverse square relationship, gravity has a small residual force, regardless of distance. This residue is only about 10 trillionths of a G, but it is enough to explain galactic rotation curves.
Of course, just adding a small term to Newton’s gravity means changing Einstein’s equations as well. This is why the MoND has been generalized in various ways, such as AQUAL, which means “a quadradic Lagrangian”. Both AQUAL and the standard LCDM model can explain the observed galactic rotation curves, but there are some subtle differences.
This is where a recent study comes in. A difference between AQUAL and LCDM is the rotational speeds of stars in inner orbit compared to stars in outer orbit. For LCDM, both should be governed by the distribution of matter, so the curve should be smooth. AQUAL foresees a small bend in the curve due to the dynamics of the theory. It is too small to be measured in a single galaxy, but statistically there should be a small shift between the inner and outer velocity distributions.
Measured lag between internal and external stellar motions. Credit: Kyu-Hyun Chae
The author of this article therefore examined the high-resolution velocity curves of 152 galaxies as observed in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. He found a mismatch in agreement with AQUAL. The data seems to support modified gravity compared to standard dark matter cosmology.
The result is exciting, but it doesn’t conclusively reverse dark matter. The AQUAL model has its own problems, such as its disagreement with the observation of gravitational lenses of galaxies. But it’s a victory for the outsider theory, causing some astronomers to shout “Vive la MoND!” “.