Connections among three roads to cosmic acceleration: decaying vacuum, bulk viscosity, and nonlinear fluids

TL;DR

This paper explores the theoretical connections among three different approaches to explaining cosmic acceleration—decaying vacuum energy, bulk viscosity, and nonlinear fluids—showing their equivalence at the background level and providing new analytic solutions.

Contribution

It demonstrates the equivalence of these models at zeroth order and introduces new analytic solutions, including a novel Ansatz for the cosmic term.

Findings

Models are equivalent at zeroth order in cosmic evolution.

Explicit examples include power law Lambda-term, constant viscosity, and Modified Chaplygin Gas.

New analytic solutions and a novel Ansatz for the cosmic term are presented.

Abstract

We discuss the connection among three distinct classes of models often used to explain the late cosmic acceleration: decaying cosmological term, bulk viscous pressure, and nonlinear fluids. We focus on models that are equivalent at zeroth order, in the sense they lead to the same solutions for the evolution of the scale factor. More specifically, we show explicit examples where this equivalence is manifest, which include some well know models belonging to each class, such as a power law Lambda-term, a model with constant viscosity, and the Modified Chaplygin Gas. We also obtain new analytic solutions for some of these models, including a new Ansatz for the cosmic term.