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.
Highlights
The current acceleration of the universe is often attributed to dark energy, an unknown fluid with effective equation of state (EoS) parameter w close to −1
In Ref. [8], it was shown that the coupling of zero-pressure dark matter with phantom dark energy could avoid a big rip singularity, and the universe might evolve to asymptotic de Sitter space
Little rip models provide an evolution for the universe intermediate between asymptotic de Sitter expansion and models with a big rip singularity
Summary
The current acceleration of the universe is often attributed to dark energy, an unknown fluid with effective equation of state (EoS) parameter w close to −1. Phantom dark energy models can lead to a singularity in which the scale factor and density become infinite at a finite time; such a singularity is called a big rip [3, 4], or Type I singularity [5]. W is less than −1, so that the dark energy density increases with time, but w approaches −1 asymptotically and sufficiently rapidly that a singularity is avoided This proposed non-singular cosmology was called a “little rip” because it leads to a dissolution of bound structures at some point in the future (similar to the effect of a big rip singularity).
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