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

TL;DR

This paper numerically analyzes the evolution of cosmic background and matter perturbations in viable f(R) gravity models, revealing phantom boundary crossing and anomalous growth index behavior of fluctuations.

Contribution

It demonstrates that viable f(R) models naturally exhibit phantom boundary crossing and non-monotonic, sometimes negative, growth indices of matter perturbations.

Findings

Viable f(R) models show recent crossing of the phantom boundary.

Density perturbation growth index evolves non-monotonically.

Growth index can become temporarily negative.

Abstract

Evolution of a background space-time metric and sub-horizon 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 $w_{\rm DE}=-1$. 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 non-monotonically with time and may even become negative temporarily.