Dissipative hidden sector dark matter
  A simple way of explaining dark matter without modifying known Standard Model
physics is to require the existence of a hidden (dark) sector, which interacts
with the visible one predominantly via gravity. We consider a hidden sector
containing two stable particles charged under an unbroken $U(1)^{'}$ gauge
symmetry, hence featuring dissipative interactions. The massless gauge field
associated with this symmetry, the dark photon, can interact via kinetic mixing
with the ordinary photon. In fact, such an interaction of strength $\epsilon
\sim 10 ^{-9}$ appears to be necessary in order to explain galactic structure.
We calculate the effect of this new physics on Big Bang Nucleosynthesis and its
contribution to the relativistic energy density at Hydrogen recombination. We
then examine the process of dark recombination, during which neutral dark
states are formed, which is important for large-scale structure formation.
Galactic structure is considered next, focussing on spiral and irregular
galaxies. For these galaxies we modelled the dark matter halo (at the current
epoch) as a dissipative plasma of dark matter particles, where the energy lost
due to dissipation is compensated by the energy produced from ordinary
supernovae (the core-collapse energy is transferred to the hidden sector via
kinetic mixing induced processes in the supernova core). We find that such a
dynamical halo model can reproduce several observed features of disk galaxies,
including the cored density profile and the Tully-Fisher relation. We also
discuss how elliptical and dwarf spheroidal galaxies could fit into this
picture. Finally, these analyses are combined to set bounds on the parameter
space of our model, which can serve as a guideline for future experimental
searches.
