Breaking a dark degeneracy: The gamma-ray signature of early matter
  domination
  The Universe's early thermal history is poorly constrained, and it is
possible that it underwent a period of early matter domination driven by a
heavy particle or an oscillating scalar field that decayed into radiation
before the onset of Big Bang nucleosynthesis. The entropy sourced by this
particle's decay reduces the cross section required for thermal-relic dark
matter to achieve the observed abundance. This degeneracy between dark matter
properties and the thermal history vastly widens the field of viable dark
matter candidates, undermining efforts to constrain dark matter's identity.
Fortunately, an early matter-dominated era also amplifies density fluctuations
at small scales and leads to early microhalo formation, boosting the dark
matter annihilation rate and bringing smaller cross sections into the view of
existing indirect-detection probes. Employing several recently developed models
of microhalo formation and evolution, we develop a procedure to derive
indirect-detection constraints on dark matter annihilation in cosmologies with
early matter domination. This procedure properly accounts for the unique
morphology of microhalo-dominated signals. While constraints depend on dark
matter's free-streaming scale, the microhalos make it possible to obtain upper
bounds as small as $\langle\sigma v\rangle \lesssim 10^{-32}$ cm$^3$s$^{-1}$
using Fermi-LAT observations of the isotropic gamma-ray background and the
Draco dwarf galaxy.
