# Cuspy Halo Problems...

by jhe1984
Tags: cuspy, halo
 Emeritus Sci Advisor PF Gold P: 2,977 Both simulations and observations indicate the existence of extended dark matter halos, the problem has to do with the inner slope of the density profile. If we describe the inner density profile of a galaxy as a power law: $$\rho\propto r^{\alpha}$$ we find that simulations predict $-1.5 \lesssim \alpha \lesssim -1$, while observations indicate $-1 \lesssim \alpha \lesssim 0$. Both the observations and simulations have potentially significant sources of error, so we're still ironing out the details, but there does appear to be a discrepancy. There have been several suggested resolutions to this problem, one of which Garth already mentioned -- self-interacting dark matter. Although popular, this is far from being the universally accepted solution. Some of us, myself included, suspect that the problem is simply that the simulations aren't good enough yet. It's an issue not only of numerical resolution (which is steadily improving with time), but also of our models of the baryonic component of the galaxy (ordinary matter). Finally, some folks believe that this is evidence for even more exotic physics, such as the extra dimensions predicted by string theory. Stay tuned, this is a very active area of research.