Annihilating Cold Dark Matter

Structure formation with cold dark matter (CDM) predicts halos with a central density cusp, which are observationally disfavored. If CDM particles have an annihilation cross section $\sigma v\sim {10}^{-29\mathrm{}}\left(m/\mathrm{GeV}\right){\mathrm{cm}}^2$, then annihilations will soften the cusps. We discuss plausible scenarios for avoiding the early Universe annihilation catastrophe that could result from such a large cross section. The predicted scaling of core density with halo mass depends upon the velocity dependence of $\sigma v$, and $s$-wave annihilation leads to a core density nearly independent of halo mass, which seems consistent with observations.