Redshift distortions as a probe of gravity

Redshift distortion measurements from galaxy surveys include sensitivity to the gravitational growth index distinguishing other theories from Einstein gravity. This gravitational sensitivity is substantially free from uncertainty in the effective equation of state of the cosmic expansion history. We estimate the future sensitivity and complementarity of this technique exploring the growth history of the universe. We also illustrate the bias in the traditional application to matter density determination using f = Ω m ( a ) 0.6 , and how to avoid it.