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Evolution of a background space-time metric and sub-horizon matter densityperturbations in the Universe is numerically analyzed in viable $f(R)$ modelsof present dark energy and cosmic acceleration. It is found that viable modelsgenerically exhibit recent crossing of the phantom boundary $w_{\rm DE}=-1$.Furthermore, it is shown that, as a consequence of the anomalous growth ofdensity perturbations during the end of the matter-dominated stage, theirgrowth index evolves non-monotonically with time and may even become negativetemporarily.

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