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

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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 Rα, where α is a constant. It is found, similarly as in the case of pure Einstein gravity [Brevik and Gorbunova, Gen. Rel. Grav. 37, 2039, 2005], 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.

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