Cosmological implications of a Dark Matter self-interaction energy
  density
  We investigate cosmological constraints on an energy density contribution of
elastic dark matter self-interactions characterized by the mass of the exchange
particle and coupling constant. Because of the expansion behaviour in a
Robertson-Walker metric we investigate self-interacting dark matter that is
warm in the case of thermal relics. The scaling behaviour of dark matter
self-interaction energy density shows that it can be the dominant contribution
(only) in the very early universe. Thus its impact on primordial
nucleosynthesis is used to restrict the interaction strength, which we find to
be at least as strong as the strong interaction. Furthermore we explore dark
matter decoupling in a self-interaction dominated universe, which is done for
the self-interacting warm dark matter as well as for collisionless cold dark
matter in a two component scenario. We find that strong dark matter
self-interactions do not contradict super-weak inelastic interactions between
self-interacting dark matter and baryonic matter and that the natural scale of
collisionless cold dark matter decoupling exceeds the weak scale and depends
linearly on the particle mass. Finally structure formation analysis reveals a
linear growing solution during self-interaction domination; however, only
non-cosmological scales are enhanced.
