Crossing of the w=-1 Barrier in Two-Fluid Viscous Modified Gravity

TL;DR

This paper investigates how a two-fluid viscous modified gravity model allows the dark energy equation of state parameter to cross the w=-1 barrier without scalar fields, highlighting the role of bulk viscosity in late universe singularities.

Contribution

It introduces a two-fluid viscous modified gravity framework that explains crossing the w=-1 barrier without scalar fields, extending previous single-fluid models.

Findings

Fluid can transition from quintessence to phantom regime due to viscosity.

Two-fluid model is necessary as viscosities evolve differently over time.

Scalar fields are not required for crossing the phantom barrier.

Abstract

Singularities in the dark energy late universe are discussed, under the assumption that the Lagrangian contains the Einstein term R plus a modified gravity term of the form R^\alpha, where \alpha is a constant. It is found, similarly as in the case of pure Einstein gravity [I. Brevik and O. Gorbunova, Gen. Rel. Grav. 37 (2005), 2039], that the fluid can pass from the quintessence region (w>-1) into the phantom region (w<-1) as a consequence of a bulk viscosity varying with time. It becomes necessary now, however, to allow for a two-fluid model, since the viscosities for the two components vary differently with time. No scalar fields are needed for the description of the passage through the phantom barrier.