FIMP Dark Matter in bulk viscous non-standard cosmologies

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 $ϕ$, which interacts with the radiation component during the early universe. Specifically, we consider an interaction term of the form $Γ_ϕ ρ_ϕ$, where $Γ_ϕ$ represents the decay rate of the field and $ρ_ϕ$ denotes its energy density and a bulk viscosity described by $ξ=ξ_{0}ρ_ϕ^{1/2}$, 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 ($\langleσv\rangle$), as well as a preliminary analysis of the non-constant case. In particular, for certain combinations of the model and Dark Matter parameters, namely ($T_\text{end}$,$κ$) and $(m_χ,\langleσv\rangle)$, where $T_\text{end}$ corresponds to the temperature at which $ϕ$ decays, $κ$ is the ratio between the initial energy density of $ϕ$ and radiation, and $m_χ$ 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 $Λ$CDM and classical non-standard cosmological scenarios.