The Cosmology of Atomic Dark Matter
  While, to ensure successful cosmology, dark matter (DM) must kinematically
decouple from the standard model plasma very early in the history of the
Universe, it can remain coupled to a bath of "dark radiation" until a
relatively late epoch. One minimal theory that realizes such a scenario is the
Atomic Dark Matter model, in which two fermions oppositely charged under a new
U(1) dark force are initially coupled to a thermal bath of "dark photons" but
eventually recombine into neutral atom-like bound states and begin forming
gravitationally-bound structures. As dark atoms have (dark) atom-sized
geometric cross sections, this model also provides an example of
self-interacting DM with a velocity-dependent cross section. Delayed kinetic
decoupling in this scenario predicts novel DM properties on small scales but
retains the success of cold DM on larger scales. We calculate the atomic
physics necessary to capture the thermal history of this dark sector and show
significant improvements over the standard atomic hydrogen calculation are
needed. We solve the Boltzmann equations that govern the evolution of
cosmological fluctuations in this model and find in detail the impact of the
atomic DM scenario on the matter power spectrum and the cosmic microwave
background (CMB). This scenario imprints a new length scale, the
Dark-Acoustic-Oscillation (DAO) scale, on the matter density field. This DAO
scale shapes the small-scale matter power spectrum and determines the minimal
DM halo mass at late times which may be many orders of magnitude larger than in
a typical WIMP scenario. This model necessarily includes an extra dark
radiation component, which may be favoured by current CMB experiments, and we
quantify CMB signatures that distinguish an atomic DM scenario from a standard
$\Lambda$CDM model containing extra free-streaming particles. [Abridged]
