Testing gravity using galaxy clusters: new constraints on beyond Horndeski theories

Abstract

The Beyond Horndeski class of alternative gravity theories allow for\nSelf-accelerating de-Sitter cosmologies with no need for a cosmological\nconstant. This makes them viable alternatives to $\\Lambda$CDM and so testing\ntheir small-scale predictions against General Relativity is of paramount\nimportance. These theories generically predict deviations in both the Newtonian\nforce law and the gravitational lensing of light inside extended objects.\nTherefore, by simultaneously fitting the X-ray and lensing profiles of galaxy\nclusters new constraints can be obtained. In this work, we apply this\nmethodology to the stacked profiles of 58 high-redshift ($ 0.1<z<1.2$) clusters\nusing X-ray surface brightness profiles from the XMM Cluster Survey and weak\nlensing profiles from CFHTLenS. By performing a multi-parameter Markov chain\nMonte Carlo analysis, we are able to place new constraints on the parameters\ngoverning deviations from Newton's law $\\Upsilon_{1}=-0.11^{+0.93}_{-0.67}$ and\nlight bending $\\Upsilon_{2}=-0.22^{+1.22}_{-1.19}$. Both constraints are\nconsistent with General Relativity, for which $\\Upsilon_{1}=\\Upsilon_{2}=0$. We\npresent here the first observational constraints on $\\Upsilon_{2}$, as well as\nthe first extragalactic measurement of both parameters.\n