FIMP Dark Matter in bulk viscous non-standard cosmologies

TL;DR

This paper investigates how bulk viscous non-standard cosmologies influence FIMP dark matter production, identifying new parameter regions that yield the observed relic density, differing from standard cosmology predictions.

Contribution

It extends non-standard cosmological models by incorporating bulk viscosity and interactions, exploring their effects on FIMP dark matter production and relic density.

Findings

New parameter regions for successful FIMP dark matter production.

Bulk viscosity impacts the relic density calculations.

Contrasts with predictions from standard $\\Lambda$CDM cosmology.

Abstract

In this paper, we revisit the extension of the classical non-standard cosmological model in which dissipative processes are considered through a bulk viscous term in the new field $\phi$, which interacts with the radiation component during the early universe. Specifically, we consider an interaction term of the form $\Gamma_{\phi} \rho_{\phi}$, where $\Gamma_{\phi}$ represents the decay rate of the field and $\rho_{\phi}$ denotes its energy density and a bulk viscosity described by $\xi=\xi_{0}\rho_{\phi}^{1/2}$, within the framework of Eckart's theory. This extended non-standard cosmology is employed to explore the parameter space for the production of Feebly Interacting Massive Particles (FIMPs) as Dark Matter candidates, assuming a constant thermal averaged Dark Matter production cross-section ($\langle\sigma v\rangle$), as well as a preliminary analysis of the non-constant case. In particular, for certain combinations of the model and Dark Matter parameters, namely ($T_\text{end}$,$\kappa$) and $(m_\chi,\langle\sigma v\rangle)$, where $T_\text{end}$ corresponds to the temperature at which $\phi$ decays, $\kappa$ is the ratio between the initial energy density of $\phi$ and radiation, and $m_\chi$ is the Dark Matter mass, we identify extensive new parameter regions where Dark Matter can be successfully established while reproducing the currently observed relic density, in contrast to the predictions of $\Lambda$CDM and classical non-standard cosmological scenarios.