  Much recent discussion about dark matter has been centered on two seemingly
independent problems: the abundance of substructure in dark matter halos, and
the cuspiness of the halos' inner density profile. We explore possible
connections between the two problems by studying the gravitational scattering
effects due to subhalos on the phase-space distribution of dark matter
particles in the main halos. Our series of controlled numerical experiments
indicates that the number and mass density of subhalos can be high enough to
cause the collisionless dark matter particles in the inner part of a main halo
to diffuse, flattening the main halo's inner cusp within a few dynamical times.
Depending on the masses and concentration of the subhalos, the inner density
profile of the whole system (main plus sub halos) can either steepen or
flatten. Subhalo accretion can therefore introduce significant scatter in the
inner density profiles of dark matter halos, offering a possible explanation
for the range of profiles seen in both observations and cosmological
simulations.
