Simulations of Gravitational Heating Due to Early Matter Domination

TL;DR

This paper uses cosmological N-body simulations to study how early matter domination causes microhalo formation and gravitational heating, affecting the matter power spectrum and offering observational signatures of such scenarios.

Contribution

First simulation-based analysis of halo formation and evaporation during an early matter-dominated era, linking linear power spectrum to free-streaming effects.

Findings

Free-streaming cut-off can be accurately predicted from linear power spectrum.

Microhalos can form during and after EMDE before redshift ~1000.

Gravitational heating impacts the matter power spectrum and halo formation.

Abstract

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 $\sim 1000$. 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.