The gamma-ray signature of an early matter-dominated era

The thermal history of the Universe between inflation and Big Bang nucleosynthesis is unknown, but an early matter-dominated era (EMDE), driven by either an oscillating scalar field or a massive particle, is an unavoidable consequence of several early-Universe models. An EMDE enhances the growth of small-scale density fluctuations, causing most of the dark matter to be bound in sub-earth-mass microhalos at high redshift, long before the formation of galaxies. Dark matter annihilation within these unresolved microhalos generates a gamma-ray signal that broadly tracks the dark matter content, yielding a similar signature to that of decaying dark matter. However, in dense regions such as galactic centers, tidal stripping and other disruptive processes suppress the microhalo gamma-ray signal, resulting in a distinctive emission profile. We use N-body simulations to study microhalos subjected to tidal forces in order to determine the EMDE emission profile for dwarf spheroidal galaxies. We then analyze the gamma-ray emission from these galaxies to constrain the dark matter annihilation cross-section and microhalo properties, thereby probing the evolution of the early universe and the origins of dark matter.