Crossing of the w=-1 barrier in viscous modified gravity

TL;DR

This paper explores how bulk viscosity in a modified gravity model with scalar curvature power alpha can cause the universe to cross the w=-1 barrier, leading to a Big Rip singularity, extending previous Einstein gravity results.

Contribution

It demonstrates the conditions under which bulk viscosity induces crossing of the w=-1 barrier in a generalized gravity framework, broadening understanding of cosmic singularities.

Findings

Bulk viscosity can drive the universe into the phantom region.

Crossing of the w=-1 barrier depends on the power of scalar expansion in viscosity.

Specific examples illustrate the transition to Big Rip in modified gravity.

Abstract

We consider a modified form of gravity in which the action contains a power alpha of the scalar curvature. It is shown how the presence of a bulk viscosity in a spatially flat universe may drive the cosmic fluid into the phantom region (w<-1) and thus into a Big Rip singularity, even if it lies in the quintessence region (w>-1) in the non-viscous case. The condition for this to occur is that the bulk viscosity contains the power (2 alpha-1) of the scalar expansion. Two specific examples are discussed in detail. The present paper is a generalization of the recent investigation dealing with barrier crossing in Einstein's gravity: I. Brevik and O. Gorbunova, Gen. Relativ. Grav. 37 (2005) 2039.