Self-interacting dark matter

A new type of dark matter is considered. As with cold dark matter, when the dark matter is nonrelativistic, it does not couple to the electron-photon plasma, so its entropy per comoving volume is fixed. The new feature is that there is a cosmological era where number-changing reactions keep the dark matter in chemical equilibrium so that the chemical potential vanishes. This has several interesting consequences: during this era the dark matter cannibalizes its rest mass to keep warm, its temperature dropping only logarithmically with scale. We have done a general study of such self-interacting dark matter to identify the interesting ranges for its mass, coupling, and entropy. There are two consequences of this scheme which are particularly noteworthy. The unusual evolution of energy density and density perturbations allows for the possibility of decreasing the predicted anisotropies in the cosmic microwave background by a factor of 2 or more. This dark matter allows a completely new scheme for processing density perturbations. The theory introduces a new cosmological mass scale: the Jeans mass when the number-changing processes decouple. Perturbations on this supercluster scale are the first to go nonlinear.