The observed growth of massive galaxy clusters

This is the third of a series of papers in which we derive simultaneous constraints on cosmolog-ical parameters and X-ray scaling relations using observations of the growth of massive, X-ray flux-selected galaxy clusters. Our data set consists of 238 clusters drawn from the ROSAT All-Sky Survey and incorporates extensive follow-up observations using the Chandra X-ray Observatory. Here we present improved constraints on departures from general relativity (GR) on cosmological scales, using the growth index, γ, to parametrize the linear growth rate of cosmic structure. Using the method of Mantz et al. (Paper I), we simultaneously and self-consistently model the growth of X-ray luminous clusters and their observable–mass scaling relations, accounting for survey biases, parameter degeneracies and the impact of systematic uncertainties. Such analysis of the survey and follow-up data is crucial, else spurious con-straints may be obtained. We combine the X-ray cluster growth data with cluster gas mass fraction, Type Ia supernova, baryon acoustic oscillation and cosmic microwave background data. We find that the combination of these data leads to a tight correlation between γ and the normalization of the matter power spectrum, σ8. Consistency with GR requires a mea