Models for Little Rip Dark Energy

TL;DR

This paper explores specific dark energy models leading to a little rip cosmology, where the universe's expansion accelerates indefinitely without a finite-time singularity, and analyzes conditions distinguishing these models from de Sitter expansion.

Contribution

It provides detailed conditions and models for little rip cosmology, including effects of dark matter coupling and scalar field models, expanding understanding of late-time universe evolution.

Findings

Little rip models have infinite lifetime despite local instability.

Coupling with dark matter can convert little rip into de Sitter expansion.

Conditions on scalar fields determine if models exhibit little rip behavior.

Abstract

We examine in more detail specific models which yield a little rip cosmology, i.e., a universe in which the dark energy density increases without bound but the universe never reaches a finite-time singularity. We derive the conditions for the little rip in terms of the inertial force in the expanding universe and present two representative models to illustrate in more detail the difference between little rip models and those which are asymptotically de Sitter. We derive conditions on the equation of state parameter of the dark energy to distinguish between the two types of models. We show that coupling between dark matter and dark energy with a little rip equation of state can alter the evolution, changing the little rip into an asymptotic de Sitter expansion. We give conditions on minimally-coupled phantom scalar field models and on scalar-tensor models that indicate whether or not they correspond to a little rip expansion. We show that, counterintuitively, despite local instability, a little-rip cosmology has an infinite lifetime.