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

The Beyond Horndeski class of alternative gravity theories allow for Self-accelerating de-Sitter cosmologies with no need for a cosmological constant. This makes them viable alternatives to LambdaCDM and so testing their small-scale predictions against General Relativity is of paramount importance. These theories generically predict deviations in both the Newtonian force law and the gravitational lensing of light inside extended objects. Therefore, by simultaneously fitting the X-ray and lensing profiles of galaxy clusters new constraints can be obtained. In this work, we apply this methodology to the stacked profiles of 58 high-redshift (0.1< z <1.2) clusters using X-ray surface brightness profiles from the XMM Cluster Survey and weak lensing profiles from CFHTLenS. By performing a multi-parameter Markov chain Monte Carlo analysis, we are able to place new constraints on the parameters governing deviations from Newtons law ϒ₁ = -0.11^+0.93_-0.67 and light bending Υ₂ = -0.22^+1.22_-1.19. Both constraints are consistent with General Relativity, for which Υ₁ = Υ₂ = 0. We present here the first observational constraints on ϒ₂, as well as the first extragalactic measurement of both parameters.