Constraints on modified gravity from the observed X-ray luminosity function of galaxy clusters

TL;DR

This study uses galaxy cluster X-ray luminosity functions combined with other cosmological data to test for deviations from General Relativity, finding no significant evidence for modified gravity.

Contribution

It provides the tightest constraints to date on the growth index gamma, testing modified gravity theories against observational data.

Findings

Gamma consistent with GR prediction (~0.55) within uncertainties

No evidence for deviations from General Relativity on cosmological scales

Combined data sets improve constraints on growth index and cosmological parameters

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, Omega_m(z)^gamma, for which GR predicts a growth index gamma~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: Omega_m, w, and Omega_k, 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 gamma=0.51+0.16-0.15 and Omega_m=0.27+-0.02 (68.3 per cent confidence limits), for a flat cosmological constant (LCDM) background model. Allowing w to be a free parameter, we find gamma=0.44+0.17-0.15. Relaxing the flatness prior in the LCDM model, we obtain gamma=0.51+0.19-0.16. When in addition to the XLF data we use the CMB data to constrain gamma through the ISW effect, we obtain a combined constraint of gamma=0.45+0.14-0.12 for the flat LCDM model. Our analysis provides the tightest constraints to date on the growth index. We find no evidence for departures from General Relativity on cosmological scales.