  A variety of supergravity and string models involve hidden sectors where the
hidden sectors may couple feebly with the visible sectors via a variety of
portals. While the coupling of the hidden sector to the visible sector is
feeble its coupling to the inflaton is largely unknown. It could couple feebly
or with the same strength as the visible sector which would result in either a
cold or a hot hidden sector at the end of reheating. These two possibilities
could lead to significantly different outcomes for observables. We investigate
the thermal evolution of the two sectors in a cosmologically consistent hidden
sector dark matter model where the hidden sector and the visible sector are
thermally coupled. Within this framework we analyze several phenomena to
illustrate their dependence on the initial conditions. These include the
allowed parameter space of models, dark matter relic density, proton-dark
matter cross section, effective massless neutrino species at BBN time,
self-interacting dark matter cross-section, where self-interaction occurs via
exchange of dark photon, and Sommerfeld enhancement. Finally fits to the
velocity dependence of dark matter cross sections from galaxy scales to the
scale of galaxy clusters is given. The analysis indicates significant effects
of the initial conditions on the observables listed above. The analysis is
carried out within the framework where dark matter is constituted of dark
fermions and the mediation between the visible and the hidden sector occurs via
the exchange of dark photons. The techniques discussed here may have
applications for a wider class of hidden sector models using different
mediations between the visible and the hidden sectors to explore the impact of
Big Bang initial conditions on observable physics.
