FIMP Dark Matter in Bulk Viscous Non-Standard Cosmologies

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 Γϕρϕ, where Γϕ represents the decay rate of the field and ρϕ denotes its energy density and a bulk viscosity described by ξ=ξ0ρϕ1/2, 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 (⟨σv⟩), as well as a preliminary analysis of the non-constant case. In particular, for certain combinations of the model and Dark Matter parameters, namely (Tend,κ) and (mχ,⟨σv⟩), where Tend corresponds to the temperature at which ϕ decays, κ is the ratio between the initial energy density of ϕ and radiation, and mχ 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 ΛCDM and classical non-standard cosmological scenarios.