Explaining the Cuspy Halo Problem

[jhe1984]

Hi,

I'm reading up on CDM and related theories on Wikipedia and came across the "Cuspy Halo Problem". I followed the link, but it did not provide any further insight to help me visualize this phenomenon (or the absence of it[?]). Please help explain the "Cuspy Halo Problem" to a newcomer.

My current guess is that the problem is that CDM seems to predict a very dense, "deep", and pointed middle section (like a top) upon which the rest of the Universe is rotating, but that this prediction has not played out in observations (and may in fact have been contradicted). Is that correct?

Thanks as always!

 

[Garth]

Dark Matter is invoked to explain the gravitational dynamics of spiral galaxies and clusters, to make up cosmological density and to explain the early formation of large scale structure in the universe.

DM cannot be baryonic because there is a strict limit on the baryonic density in standard BB nuclleosynthesis and the required amount of DM exceeds this by nearly an order of magnitude. Because it is dark it is assumed to be non-interacting except gravitationally.

Unfortunately it then clumps too efficiently and should concentrate in dense regions around which ordinary matter forms galaxies, a 'cuspy halo'. However the study of galaxy dynamics requires the DM to be more evenly scattered over a region around the galaxy, in a massive dark halo, rather than at the centre. This is the 'cuspy halo' problem.

The resolution to this problem requires DM to be weakly interacting and able to repel itself so that it cannot concentrate too much. However, until we have identified the DM particle in a laboratory, measured its properties and find them to be exactly what is required to resolve the above problems we will not really know what we are talking about!

Garth

 

[SpaceTiger]

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.