This week I will be giving a talk on a new paper that showed up on astro-ph positing that flat galactic rotation curves can be explained by string theory (!). Galactic rotation curves – that is, graphs of the speed at which stars and gas rotate about the centre of the galaxy – are found to remain flat or even rise far outside where the visible galaxy ends, and we can’t account for that using only Newtonian dynamics assuming the visible matter is the only matter there; they should eventually fall off gradually rather than maintaining very high speeds. So astronomers postulate the existence of dark matter to add extra acceleration to the matter orbiting the galaxy centre, to make up for the apparent difference. We’ve never directly detected the components of dark matter.* Other theories exist to explain this, such as an ad hoc modification of the Newtonian force law (modified Newtonian dynamics, or MOND) that is not theoretically motivated.
The very intriguing string theory argument, meanwhile, seems to be that in the Nappi-Witten model of IIB string theory, where the string theory equations can be solved exactly in the case of a plane polarized gravitational field background, it can be shown that the gauge potential couples to the worldsheet of the strings via the gravimagnetic field (a gravimagnetic field is a field produced by a moving mass in general relativity, in the same way that a moving electric charge produces a magnetic field in electromagnetism). Therefore strings interacting under this field will follow Landau orbits, much like a charged particle moving in a magnetic field.
So this means (and here is the great logical leap of the paper) that stars themselves will follow the same trajectories (they don’t provide a description for how this happens) – and this adds a term linear in the circular speed to the force equation which compensates for the drop in speed far from the galaxy centre! C’est un miracle! And it looks like it sort of works in real galaxies, too. Except sometimes. Probably when the galaxy doesn’t benefit from string background rotation, since it needs a componenet of the gravimagnetic field perpendicular to the galactic plane to act. But maybe there is somethnig to this idea.
Well, it’s fun to speculate…
*We have never directly detected the components of dark matter experimentally but there are a couple of results that corroborate it: first, we know that the dynamics of satellite galaxies suggest a density drop off of r^(-3) far from the centre, which is predicted by cosmological simulations of structure formation. Second, the ‘Bullet Cluster‘ result, in which dark matter was detected in a colliding galaxy cluster, is the first direct detection of a dark matter halo.