Spectator dark matter in non-standard cosmologies
  It has been shown that the observed dark matter (DM) abundance can be
produced by amplification of quantum fluctuations of an energetically
subdominant scalar field during inflation. In this paper, we study the
robustness of this "spectator dark matter" scenario to changes in the expansion
rate of the early Universe. Compared to the standard radiation-dominated (RD)
scenario, two aspects will change: the DM energy density evolves differently as
a function of time, and also the DM isocurvature perturbation spectrum will be
different from the result in the RD case. These can impose sizeable changes to
the values of model parameters which allow the field to constitute all DM while
simultaneously satisfying all observational constraints. We study both free and
self-interacting DM in scenarios with non-standard expansion and quantify the
changes to the cases with a standard cosmological history. We also discuss
testability of the scenario through primordial DM isocurvature and
non-Gaussianity.
