The growth index of matter perturbations and modified gravity
  We place tight constraints on the growth index $\gamma$ by using the recent
growth history results of 2dFGRS, SDSS-LRG, VIMOS-VLT deep Survey (VVDS) and
{\em WiggleZ} datasets. In particular, we investigate several parametrizations
of the growth index $\gamma(z)$, by comparing their cosmological evolution
using observational growth rate data at different redshifts. Utilizing a
standard likelihood analysis we find that the use of the combined growth data
provided by the 2dFGRS, SDSS-LRG, VVDS and {\em WiggleZ} galaxy surveys, puts
the most stringent constraints on the value of the growth index. As an example,
assuming a constant growth index we obtain that $\gamma=0.602\pm 0.055$ for the
concordance $\Lambda$CDM expansion model. Concerning the
Dvali-Gabadadze-Porrati gravity model, we find $\gamma=0.503\pm 0.06$ which is
lower, and almost $3\sigma$ away, from the theoretically predicted value of
$\gamma_{DGP}\simeq 11/16$. Finally, based on a time varying growth index we
also confirm that the combined growth data disfavor the DGP gravity.
