One point of evidence in support of dark matter is the way in which the speed of stars, gas and dust in a galaxy varies with their distance from the center of the galaxy, known as the galactic rotation curve. Most of the visible matter of a galaxy is concentrated near the center of a galaxy, so we would expect that more central stars should move much faster than stars on the outer rim. Thus the rotation curve should decrease with distance. However most galaxies have a fairly flat rotation curve, meaning outer stars move about as fast as inner stars. This and other evidence as led us to develop the theory of dark matter. But new research on galactic rotation curves has found an odd correlation, and it could mean that dark matter is wrong after all.
The orbital speed of stars in a galaxy depends upon the gravitational pull of a galaxies mass. The more strongly a star is pulled toward galactic center, the greater its radial acceleration, and the faster it needs to move to overcome that pull. This is similar to the planets in our solar system. Mercury is close to the Sun, and therefore has a large radial acceleration due to the Sun’s gravity. Distant Pluto has only a small radial acceleration. So Mercury zips around the Sun at 48 km/s, while Pluto chugs along at less than 5 km/s. According to the dark matter model, a galaxy’s mass isn’t concentrated in its center. Most of the visible matter is, but a galaxy is surrounded by a halo of dark matter. Most of a galaxy’s mass is dark matter, and most of it is in the halo.
There’s a lot of other evidence to support dark matter, but there have also been alternative models such as Modified Newtonian Dynamics (MoND). In this model, the radial acceleration of a star deviates very slightly from that predicted by Newtonian physics and general relativity. The difference is too small to notice on the scale of our solar system, but on a galactic scale that difference adds up, producing a galactic rotation curve just as we observe. MoND and related theories can accurately describe rotation curves, but they fail to describe other effects such as large scale galactic clustering and the mass distributions of colliding galaxies, so dark matter is the dominant model in astrophysics. But new research on the radial acceleration of stars could bring MoND back into favor.
The correlation between radial and gravitational accelerations is pretty strong. Credit: McGaugh, et al.
The researchers looked at the observed rotation curves for 153 galaxies, and calculated the radial acceleration at various distances in each galaxy. They then compared these results to the gravitational acceleration as predicted by the distribution of visible matter within a galaxy (technically the distribution of baryonic mass). They found a strong correlation between the two. When the gravitational acceleration was stronger, so was the radial acceleration, and when one was weaker, so was the other. What’s interesting is that this relation holds up in a range of galaxies. It didn’t matter whether most of the visible matter was clustered in the center or not, the relation still held. It’s also a purely empirical correlation, so there is no strong theoretical component to make it work.
So what gives? The researchers propose three possibilities. The first is that the correlation could be due to the dynamics of galaxy formation. It’s not clear how this would occur, but there are aspects of galactic evolution we don’t fully understand. The second is that the distribution of dark matter and baryonic matter within a galaxy are correlated. This would require some new kind of dark matter physics that makes dark matter clump in the same way that regular matter does. The third and perhaps most intriguing idea is that it really is due to some kind of modified dynamics.
The correlation between radial acceleration and mass distribution is strong enough that it points to baryonic matter being the source of the acceleration. A relation such as this is exactly the kind of thing MoND models would predict, and it contradicts current dark matter models. If this result is replicated, dark matter will have some explaining to do if it wants to remain the dominant theory. But the challenge for MoND is also just beginning. To succeed in the end it will have to account for things like large scale clustering, where it currently fails miserably.
The game between dark matter and MoND just got a bit more interesting.
Paper: Stacy McGaugh, et al. The Radial Acceleration Relation in Rotationally Supported Galaxies. arXiv:1609.05917 [astro-ph.GA] (2016)