Future evolution and finite-time singularities inF(R)gravity unifying inflation and cosmic acceleration

Abstract

We study the future evolution of quintessence/phantom-dominated epoch in modified $F(R)$ gravity which unifies the early-time inflation with late-time acceleration and which is consistent with observational tests. Using the reconstruction technique it is demonstrated that there are models where any known (big rip, II, III, or IV type) singularity may classically occur. From another side, in Einstein frame (scalar-tensor description) only IV type singularity occurs. Near the singularity the classical description breaks up, and it is demonstrated that quantum effects act against the singularity and may prevent its appearance. The realistic $F(R)$ gravity which is future singularity free is proposed. We point out that additional modification of any $F(R)$ gravity by the terms relevant at the early universe is possible, in such a way that future singularity does not occur even classically.