Multi-component Dark Matter and Small Scale Structure Formation

TL;DR

This paper investigates how mixed non-thermal dark matter components, specifically WIMPs and axions, evolve during an early matter-dominated epoch, affecting small-scale structure formation and the potential for axion miniclusters.

Contribution

It provides a detailed analysis of the evolution of WIMP and axion perturbations during EMDEs, exploring conditions for axion minicluster formation within observational constraints.

Findings

EMDEs with low reheat temperatures favor axion minicluster formation.

WIMP relic density is constrained by moduli decay, affecting dark matter composition.

Enhanced perturbations could lead to early small-scale structure formation.

Abstract

We consider the evolution of non-thermal dark matter perturbations in models which contain both Weakly Interacting Massive Particles (WIMPs) and axions. Using constraints from existing observations we examine the percentage of WIMPs and axions that may comprise the cosmological dark matter budget in models with an Early Matter Dominated Epoch (EMDE) -- where entropy production is important. After carefully tracking the thermal evolution of the various species by solving the Boltzmann equations, we consider the enhancement of perturbations that may have led to early structure formation for axions and WIMPs. We investigate the impact of enhanced perturbations on the parameter space of both species, after imposing existing constraints from indirect detection experiments. Given these constraints we establish the feasibility of axions to form miniclusters in the early universe in EMDEs for a given percentage of allowed WIMPs. We find that EMDEs with low reheat temperatures near the BBN limit are preferred for axion minicluster formation. When the EMDE is caused by string moduli, the WIMP contribution to the relic density is set by the moduli branching to dark matter at the level of $\lesssim \mathcal{O}(10^{-4})$.