Bringing Isolated Dark Matter Out of Isolation: Late-time Reheating and
  Indirect Detection
  In standard cosmology, the growth of structure becomes significant following
matter-radiation equality. In non-thermal histories, where an effectively
matter-dominated phase occurs due to scalar oscillations prior to Big Bang
Nucleosynthesis, a new scale at smaller wavelengths appears in the matter power
spectrum. Density perturbations that enter the horizon during the
matter-dominated phase grow linearly with the scale factor prior to the onset
of radiation domination, which leads to enhanced inhomogeneity on small scales
if dark matter thermally and kinetically decouples during the matter-dominated
phase. The microhalos that form from these enhanced perturbations significantly
boost the self-annihilation rate for dark matter. This has important
implications for indirect detection experiments: the larger annihilation rate
will result in observable signals from dark matter candidates that are usually
deemed untestable. As a proof of principle, we consider Binos in heavy
supersymmetry with an intermediate extended Higgs sector and all other
superpartners decoupled. We find that these isolated Binos, which lie under the
neutrino floor, can account for the dark matter relic density while also
leading to observable predictions for Fermi-LAT. Current limits on the
annihilation cross section from Fermi-LAT's observations of dwarf spheroidal
galaxies may already constrain Bino dark matter up to masses $\mathcal{O}(300)$
GeV, depending on the internal structure of the microhalos. More extensive
constraints are possible with improved gamma-ray bounds and boost calculations
from $N$-body simulations.
