Causal Viscous Fluids and Non-Singular Cosmological Bounces

TL;DR

This paper explores how causal bulk-viscous fluids can enable non-singular bouncing cosmologies within various gravity theories, overcoming previous limitations and ensuring thermodynamic and causal consistency.

Contribution

It demonstrates that the Israel--Stewart formulation allows controlled null energy condition violations, enabling stable, thermodynamically consistent bouncing solutions across multiple gravity frameworks.

Findings

Analytical bounce solutions with positive entropy production.

Viscosity and relaxation time constraints for stable bounces.

Enhanced robustness of viscous bounces in extended gravity theories.

Abstract

We investigate the realization of non-singular bouncing cosmologies driven by causal bulk-viscous fluids within General Relativity, $f(R)$ gravity, and Loop Quantum Cosmology. Building on the no-go result of Eckart theory in spatially flat universes, we show that the Israel--Stewart formulation, which incorporates finite relaxation times, permits controlled violations of the null energy condition at the bounce while remaining consistent with thermodynamic and causality requirements. Analytical bounce solutions are constructed from parametrized scale factors, yielding explicit constraints on the viscosity coefficient and relaxation time that guarantee positive entropy production and stable perturbations. In extended gravity frameworks, we demonstrate that higher-curvature corrections in $f(R)$ models and quantum geometry effects in Loop Quantum Cosmology further enhance the robustness of viscous bounces. Our results establish a unified description in which bulk viscosity provides a physically consistent mechanism for singularity resolution across different theories of gravity, highlighting distinctive conditions under which smooth cosmological bounces can occur.