Simulations of gravitational heating due to early matter domination

In cosmologies with an early matter -dominated era (EMDE) prior to Big Bang nucleosynthesis, the boosted growth of small-scale matter perturbations during the EMDE leads to microhalo formation long before halos would otherwise begin to form. For a range of models, halos can even form during the EMDE itself. These halos would dissipate at the end of the EMDE, releasing their gravitationally heated dark matter and thereby imprinting a free -streaming cut-off on the matter power spectrum. We conduct the first cosmological N -body simulations of the formation and evaporation of halos during and after an EMDE. We show that in these scenarios, the free -streaming cut-off after the EMDE can be predicted accurately from the linear matter power spectrum. Although the free streaming can erase much of the EMDE-driven boost to density perturbations, we use our findings to show that the (re -)formation of halos after the EMDE nevertheless proceeds before redshift similar to 1000. Early -forming microhalos are a key observational signature of an EMDE, and our prescription for the impact of gravitational heating will allow studies of the observational status and prospects of EMDE scenarios to cover a much wider range of parameters.