| Abstract
| - We use measurements of the growth of cosmic structure, as inferred from the observed evolution of the X-ray luminosity function (XLF) of galaxy clusters, to constrain departures from general relativity (GR) on cosmological scales. We employ the popular growth rate parameterization, Ωm(z)γ, for which GR predicts a growth index γ∼ 0.55. We use observations of the cosmic microwave background (CMB), type Ia supernovae (SNIa) and X-ray cluster gas mass fractions (fgas), to simultaneously constrain the expansion history and energy content of the Universe, as described by the background model parameters: Ωm, w and Ωk, i.e. the mean matter density, the dark energy equation of state parameter and the mean curvature, respectively. Using conservative allowances for systematic uncertainties, in particular for the evolution of the mass-luminosity scaling relation in the XLF analysis, we find γ= 0.51+0.16−0.15 and Ωm= 0.27 ± 0.02 (68.3 per cent confidence limits), for a flat cosmological constant, cold dark matter (ΛCDM) background model. Allowing w to be a free parameter, we find γ= 0.44+0.17−0.15. Relaxing the flatness prior in the ΛCDM model, we obtain γ= 0.51+0.19−0.16. When in addition to the XLF data we use the CMB data to constrain γ through the ISW effect, we obtain a combined constraint of γ= 0.45+0.14−0.12 for the flat ΛCDM model. Our analysis provides the tightest constraints to date on the growth index. We find no evidence for departures from GR on cosmological scales.
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