sented an explanation of its anomalous evolution in terms of time dependence of G$_{eff}$ . Since γ has characteristic time and wavenumber dependence, future detailed observations may yield useful information on the validity of f ( R ) gravity through this quantity, although current constraints have been obtained assuming that it is constant both in time and in wavenumber[4, 39]. Another related observational test of this model is supplied by the large-scale structure of the Universe which should be different from that in the ΛCDM model. In particular, voids are expected to be more pronounced since the effective gravi- tational constant is bigger inside them compared to large matter overdensities where it is practically equal to that measured in laboratory. HM and JY are grateful to T. Narikawa and K. Yamamoto for useful communications. We thank T. Kobayashi for poiting out a typo in Ref. [15] and in the first version of this manuscript. AS acknowledges RESCEU hospitality as a visiting professor. He was also partially supported by the grant RFBR 08-02-00923 and by the Scientific Programme “As- tronomy” of the Russian Academy of Sciences. This work was supported in part by JSPS Grant-in-Aid for Scientific Research No. 19340054(JY), JSPS Core-to-Core program “In- ternational Research Network on Dark Energy”, and Global COE Program “the Physical Sciences Frontier”, MEXT, Japan. Acknowledgments [1] V. Sahni and A. A. Starobinsky, Int. J. Mod. Phys. D 9 , 373 (2000) [arXiv:astro-ph/9904398]; P. J. E. Peebles and B. Ratra, Rev. Mod. Phys. 75 , 559 (2003) [arXiv:astro-ph/0207347]; T. Padmanabhan, Phys. Rept. 380 , 235 (2003) [arXiv:hep-th/0212290]; V. Sahni, Lect. Notes Phys. 653 , 141 (2004) [arXiv:astro-ph/0403324]; E. J. Copeland, M. Sami and S. Tsujikawa, Int. J. Mod. Phys. D 15 , 1753 (2006) [arXiv:hep-th/0603057]; V. Sahni and A. A. Starobinsky, Int. J. Mod. Phys. D 15 , 2105 (2006) [arXiv:astro-ph/0610026]. [2] E. Komatsu et al. , arXiv:1001.4538 [astro-ph.CO]. [3] A. Shafieloo, V. Sahni and A. A. Starobinsky, Phys. Rev. D 80 , 101301 (R) (2009) [arXiv:0903.5141 [astro-ph.CO]]. 17