Long-lived Light Mediator to Dark Matter and Primordial Small Scale Spectrum

TL;DR

This paper investigates how a long-lived light mediator in dark matter models affects early universe perturbations, leading to enhanced small-scale dark matter structures due to a temporary matter-dominated phase.

Contribution

It introduces a model with a GeV-scale long-lived mediator that influences primordial perturbations and small-scale structure formation in the early universe.

Findings

Primordial perturbations can develop a sharp peak in the spectrum.

Smallest dark matter halos are significantly enhanced.

The model predicts observable effects on small-scale structure.

Abstract

We calculate the early universe evolution of perturbations in the dark matter energy density in the context of simple dark sector models containing a GeV scale light mediator. We consider the case that the mediator is long lived, with lifetime up to a second, and before decaying it temporarily dominates the energy density of the universe. We show that for primordial perturbations that enter the horizon around this period, the interplay between linear growth during matter domination and collisional damping can generically lead to a sharp peak in the spectrum of dark matter density perturbation. As a result, the population of the smallest DM halos gets enhanced. Possible implications of this scenario are discussed.