RESCEU-26/10 Phantom behaviour and growth index anomalous evolution in viable f ( R ) gravity models Hayato Motohashi ∗ ( a ) , ( b ) , Alexei A. Starobinsky † ( b ) , ( c ) and Jun’ichi Yokoyama ‡ ( b ) , ( d ) ( a ) Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan ( b ) Research Center for the Early Universe (RESCEU), Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan ( c ) L. D. Landau Institute for Theoretical Physics, Moscow 119334, Russia ( d ) Institute for the Physics and Mathematics of the Universe(IPMU), The University of Tokyo, Kashiwa, Chiba, 277-8568, Japan We present numerical calculation of the evolution of a background space-time metric and sub- horizon matter density perturbations in viable f ( R ) gravity models of present dark energy and cosmic acceleration. We found that viable models generically exhibit recent crossing of the phantom boundary w$_{DE}$ = − 1. Moreover, 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. Abstract arXiv:1002.0462v2 [astro-ph.CO] 23 Feb 2010 1 ∗ Email address: motohashi@resceu.s.u-tokyo.ac.jp † Email address: alstar@landau.ac.ru ‡ Email address: yokoyama@resceu.s.u-tokyo.ac.jp