Dark Matter Relics and the Expansion Rate in Scalar-Tensor Theories
  We study the impact of a modified expansion rate on the dark matter relic
abundance in a class of scalar-tensor theories. The scalar-tensor theories we
consider are motivated from string theory constructions, which have conformal
as well as disformally coupled matter to the scalar. We investigate the effects
of such a conformal coupling to the dark matter relic abundance for a wide
range of initial conditions, masses and cross-sections. We find that exploiting
all possible initial conditions, the annihilation cross-section required to
satisfy the dark matter content can differ from the thermal average
cross-section in the standard case. We also study the expansion rate in the
disformal case and find that physically relevant solutions require a nontrivial
relation between the conformal and disformal functions. We study the effects of
the disformal coupling in an explicit example where the disformal function is
quadratic.
