Testing General Relativity on Cosmic Scales with the Observed Abundance of Massive Clusters

Using observations of the growth of massive, X-ray flux-selected galaxy clusters, we obtain tight constraints on both the growth and expansion histories of the Universe. Our data set consists of 238 cluster detections from the ROSAT All-Sky Survey (RASS), and incorporates follow-up observations of 94 of those clusters from the Chandra X-ray Observatory and/or ROSAT. Using a new statistical framework that self-consistently produces simultaneous constraints on cosmology and X-ray scaling relations we account for survey biases, parameter degeneracies and the impact of systematic uncertainties to obtain improved constraints on departures from General Relativity (GR) on cosmological scales. We parametrize the linear growth rate of cosmic structure with a power law of the mean matter density to the growth index, gamma. Combining the X-ray cluster growth data with cluster gas-mass fraction (f(gas)), Type Ia supernovae (SNIa), baryon acoustic oscillations (BAO), and cosmic microwave background (CMB) data, we find a tight correlation between gamma and the normalization of the matter power spectrum, sigma(8). Allowing gamma and the dark energy equation of state parameter, w, to take any constant values, we find no evidence for departures from the standard paradigm, GR+ACDM (cosmological constant plus cold dark matter). Our results highlight the power of X-ray cluster studies to constrain cosmology.