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

TL;DR

This paper explores F(R) cosmology with a phantom fluid, demonstrating its impact on universe acceleration, transitions, and scalar-tensor equivalence, aiming for a unified model of cosmic evolution.

Contribution

It introduces a model combining F(R) gravity and phantom fluids, explicitly constructing F(R) models that can cross the phantom divide and analyzing their cosmological implications.

Findings

The model reproduces dark energy period accurately.

Phantom fluid induces super-acceleration and late-time dominance.

Scalar-tensor correspondence avoids complex actions.

Abstract

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.