Relic gravitons and viscous cosmologies

TL;DR

This paper explores viscous cosmological models that maintain positive entropy and could affect relic graviton spectra, addressing previous issues with negative bulk viscosity during cosmic evolution.

Contribution

It introduces viscous cosmological models with positive bulk viscosity throughout their evolution, consistent with the second law of thermodynamics, and examines their impact on relic graviton spectra.

Findings

Models with positive bulk viscosity during all evolution

Compatibility with the second law of thermodynamics

Potential influence on relic graviton energy spectra

Abstract

Previously it was shown that there exists a class of viscous cosmological models which violate the dominant energy condition for a limited amount of time after which they are smoothly connected to the ordinary radiation era (which preserves the dominant energy conditions). This violation of the dominant energy condition at an early cosmological epoch may influence the slopes of energy spectra of relic gravitons that might be of experimental relevance. However, the bulk viscosity coefficient of these cosmologies became negative during the ordinary radiation era, and then the entropy of the sources driving the geometry decreases with time. We show that in the presence of viscous sources with a linear barotropic equation of state $p=\gamma \rho$ we get viscous cosmological models with positive bulk viscous stress during all their evolution, and hence the matter entropy increases with the expansion time. In other words, in the framework of viscous cosmologies, there exist isotropic models compatible with the standard second law of thermodynamics which also may influence the slopes of energy spectra of relic gravitons.