Phantom boundary crossing and anomalous growth index of fluctuations in viable f(R) models of cosmic acceleration

Abstract

The evolution of a background space-time metric and subhorizon matter density perturbations in the Universe is numerically analyzed in viable f(R) models of present dark energy and cosmic acceleration. It is found that viable models generically exhibit recent crossing of the phantom boundary ωDE = - 1. Furthermore, it is shown that, as a consequence of the anomalous growth of density perturbations during the end of the matter-dominated stage, their growth index evolves nonmonotonically with time and may even become negative temporarily.