Entropic cosmology in a dissipative universe

TL;DR

This paper explores how dissipative processes in a cosmological fluid influence universe evolution and structure formation, proposing a model that interpolates between non-dissipative and fully dissipative scenarios, with implications for observed matter clustering.

Contribution

It introduces a phenomenological entropic-force model incorporating dissipation effects and derives a matter density contrast evolution equation, highlighting the impact of dissipation on structure growth.

Findings

Dissipation rate affects density perturbations even with identical background evolution.

Low dissipation rates (less than 0.1) align well with observed growth rates.

Higher dissipation rates cause deviations from observed structure formation patterns.

Abstract

The bulk viscosity of cosmological fluid and the creation of cold dark matter both result in the generation of irreversible entropy (related to dissipative processes) in a homogeneous and isotropic universe. To consider such effects, the general cosmological equations are reformulated, focusing on a spatially flat matter-dominated universe. A phenomenological entropic-force model is examined that includes constant terms as a function of the dissipation rate ranging from $\tilde{\mu} =0$, corresponding to a nondissipative $\Lambda$CDM (lambda cold dark matter) model, to $\tilde{\mu} =1$, corresponding to a fully-dissipative CCDM (creation of cold dark matter) model. A time evolution equation is derived for the matter density contrast, in order to characterize density perturbations in the present entropic-force model. It is found that the dissipation rate affects the density perturbations even if the background evolution of the late universe is equivalent to that of a fine-tuned pure $\Lambda$CDM model. With increasing dissipation rate $\tilde{\mu}$, the calculated growth rate for the clustering gradually deviates from observations, especially at low redshifts. However, the growth rate for low $\tilde{\mu}$ (less than 0.1) is found to agree well with measurements. A low-dissipation model predicts a smaller growth rate than does the pure $\Lambda$CDM model (for which $\tilde{\mu} =0$). More detailed data are needed to distinguish the low-dissipation model from the pure $\Lambda$CDM one.