Evolution of dissipative and non-dissipative universes in holographic cosmological models with a power-law term

Abstract

Density perturbations related to structure formations are expected to be different in dissipative and non-dissipative universes, even if the background evolution of the two universes is the same. To clarify the difference between the two universes, first-order density perturbations are studied, using two types of holographic cosmological models. The first type is a "$\Lambda(t)$ model" similar to a time-varying $\Lambda(t)$ cosmology for the non-dissipative universe. The second type is a "BV model" similar to a bulk viscous cosmology for the dissipative universe. To systematically examine the two different universes, a power-law term proportional to $H^{\alpha}$ is applied to the $\Lambda(t)$ and BV (bulk-viscous-cosmology-like) models, assuming a flat Friedmann--Robertson--Walker model for the late universe. Here, $H$ is the Hubble parameter and $\alpha$ is a free parameter whose value is a real number. The $\Lambda(t)$-$H^{\alpha}$ and BV-$H^{\alpha}$ models are used to examine first-order density perturbations for matter, in which the background evolution of the two models is equivalent. In addition, thermodynamic constraints on the two models are discussed, with a focus on the maximization of entropy on the horizon of the universe, extending previous analyses [Phys. Rev. D 100, 123545 (2019) (arXiv:1911.08306); 102, 063512 (2020) (arXiv:2006.09650)]. Consequently, the $\Lambda(t)$-$H^{\alpha}$ model for small $|\alpha|$ values is found to be consistent with observations and satisfies the thermodynamic constraints, compared with the BV-$H^{\alpha}$ model. The results show that the non-dissipative universe described by the $\Lambda(t)$-$H^{\alpha}$ model similar to lambda cold dark matter models is likely favored.