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 , 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 .