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 (Formula presented.), which interacts with the radiation component during the early universe. Specifically, we consider an interaction term of the form (Formula presented.), where (Formula presented.) represents the decay rate of the field and (Formula presented.) denotes its energy density and a bulk viscosity described by (Formula presented.), 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 ((Formula presented.)), as well as a preliminary analysis of the non-constant case. In particular, for certain combinations of the model and Dark Matter parameters, namely ((Formula presented.), (Formula presented.)) and (Formula presented.), where (Formula presented.) corresponds to the temperature at which (Formula presented.) decays, (Formula presented.) is the ratio between the initial energy density of (Formula presented.) and radiation, and (Formula presented.) 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 (Formula presented.) CDM and classical non-standard cosmological scenarios. © 2025 by the authors.