Annihilating Cold Dark Matter

TL;DR

P plausible scenarios for avoiding the early Universe annihilation catastrophe that could result from such a large cross section of CDM particles, which seems consistent with observations.

Abstract

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 ~ 10**(-29) (m/GeV) 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.

Figures (1)

• Figure 1: The halo density at $r = 0.1 r_s$ (where the cusp problem becomes prominent) in structures of different size according to NFW nfw (solid curve). The objects are characterized by their virial velocity dispersion as indicated. Annihilation lines (dashed curves), normalized to LSB galaxies, are shown for the cases $n=0$ and 2. Above the line, annihilations are very important (at $r=0.1r_s$) and below the line they are unimportant. For $n=0$, LSB and smaller objects have their cores softened significantly, while clusters do not, consistent with observations. For $n=2$, clusters would be adversely affected.