F(R) cosmology in presence of a phantom fluid and its scalar-tensor counterpart:towards a unified precision model of the universe evolution

The behavior of cosmological evolution is studied in the case when a phantom fluid, that contributes to the universe accelerated expansion, is introduced in an F (R) model. At the early stages of the universe history, the dark fluid is seen to give rise to a deceleration of its expansion. For t close to present time it works as an additional contribution to the effective cosmological constant and, later, it produces the transition to a phantom era, which could actually be taking place right now in some regions of the cosmos, and might have observable consequences. For t close to the Rip time, the universe becomes completely dominated by the dark fluid, whose equation of state is phantom-like at that time. Our model, which is able to reproduce the dark energy period quite precisely, may still be modified in such a way that the epoch dominated by an effective cosmological constant, produced by the F (R) term and by the dark fluid contribution, becomes significantly shorter, what happens when a matter term is included. The dark fluid with phantom behavior gives rise to a super-accelerated phase, as compared with the case where just the viable F (R) term contributes. It is also seen explicitly that an F (R) theory can be constructed from a phantom model in a scalar-tensor theory, in which the scalar field does not behave as phantom (in the latter case the action for F (R) would be complex). Promising F (R) models which can cross the phantom divide in a convenient way are constructed explicitly.