RESCEU-5/10 arXiv:1002.1141v2 [astro-ph.CO] 2 Mar 2010 Phantom boundary crossing and anomalous growth index of fluctuations in viable f ( R ) models of cosmic acceleration Hayato Motohashi 1 , $^{2}$, Alexei A. Starobinsky 2 , $^{3}$, and Jun’ichi Yokoyama 2 , 4 1 Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan 2 Research Center for the Early Universe (RESCEU), Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan 3 L. D. Landau Institute for Theoretical Physics, Moscow 119334, Russia 4 Institute for the Physics and Mathematics of the Universe(IPMU), The University of Tokyo, Kashiwa, Chiba, 277-8568, Japan 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$_{DE}$ = − 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. 1 Abstract