The growth index of matter perturbations and modified gravity

TL;DR

This paper constrains the growth index of matter perturbations using recent galaxy survey data, testing modified gravity models and finding that observational data disfavor the DGP gravity model.

Contribution

It provides the most stringent observational constraints on the growth index and tests the viability of modified gravity models using multiple galaxy surveys.

Findings

The growth index for DM is .602 b1 0.055.

The DGP gravity model has a growth index .503 b1 0.06, nearly 3 standard deviations away from the theoretical prediction.

Observational data disfavor the DGP gravity model based on growth rate measurements.

Abstract

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.