Modified Gravity Need Not Apply

In Dark Matter by Brian Koberlein

In the battle to describe the motion of galaxies, two models fight head to head. In one corner is dark matter, a strange form of matter that surrounds galaxies, and can be seen only through its gravitational tug on light. In the other corner is Modified Newtonian Dynamics (MoND), which proposes a new approach to gravitational interactions, just as Einstein’s model superseded Newton’s

Dark matter has long been the favorite contender, since it explains a range of observed phenomena such as the large scale clustering of galaxies as well as the motion of gas and dust within spiral galaxies.  But MoND also has a few things going for it. Overall, it’s predictions agree with dwarf galaxies better than dark matter, and MoND also matches the rotation of spiral galaxies. As long as direct detection of dark matter continues to elude us, modified gravity models will be eager challengers.

Recently MoND regained some interested after a study showed that the rotation of galaxies seemed to correlate with the distribution of visible matter in those galaxies. This is exactly what one would expect from MoND models, and seemed to go against the predictions of dark matter. But new computer simulations show that dark matter can account for these correlations after all.

The team developed a computer model that accounted not just for stars and dark matter, but also the interstellar gas. In modeling the formation and evolution of galaxies, they found that a correlation forms between the distribution of matter and their rotation. They also found the relation between mass, size and luminosity of galaxies in their model matched that of observed galaxies.

So dark matter can account for the rotational dynamics of a galaxy just as MoND models can. That doesn’t rule out MoND, but it does mean that MoND isn’t necessary in this case.

Paper: Aaron D. Ludlow, et al. Mass-Discrepancy Acceleration Relation: A Natural Outcome of Galaxy Formation
in Cold Dark Matter Halos. PRL 118, 161103 DOI: 10.1103/PhysRevLett.118.161103 (2017)