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.Comment: 16 pages latex, no figure