Fermi Gamma-ray Space Telescope

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

Sten Delos
(Adrienne Erickcek, Tim Linden)


Fermi-LAT observations provide a unique probe of the Universe’s first second because an early matter-dominated era (EMDE) prior to the onset of Big Bang nucleosynthesis leads to a distinctive gamma-ray signature. An EMDE, which is an unavoidable consequence of several early-Universe models, enhances the growth of small-scale density fluctuations. As a result, most of the dark matter is 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 and thus the evolution of the early universe and the origins of dark matter. We also discuss implications for the expected emission from the Galactic center if most of the dark matter is contained in EMDE-seeded microhalos.