  The evolution of the Universe between inflation and the onset of big bang
nucleosynthesis is difficult to probe and largely unconstrained. This ignorance
profoundly limits our understanding of dark matter: we cannot calculate its
thermal relic abundance without knowing when the Universe became radiation
dominated. Fortunately, small-scale density perturbations provide a probe of
the early Universe that could break this degeneracy. If dark matter is a
thermal relic, density perturbations that enter the horizon during an early
matter-dominated era grow linearly with the scale factor prior to reheating.
The resulting abundance of substructure boosts the annihilation rate by several
orders of magnitude, which can compensate for the smaller annihilation cross
sections that are required to generate the observed dark matter density in
these scenarios. In particular, thermal relics with masses less than a TeV that
thermally and kinetically decouple prior to reheating may already be ruled out
by Fermi-LAT observations of dwarf spheroidal galaxies. Although these
constraints are subject to uncertainties regarding the internal structure of
the microhalos that form from the enhanced perturbations, they open up the
possibility of using gamma-ray observations to learn about the reheating of the
Universe.
