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 $\Lambda$CDM 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 Newton's law $\Upsilon_{1}=-0.11^{+0.93}_{-0.67}$ and
light bending $\Upsilon_{2}=-0.22^{+1.22}_{-1.19}$. Both constraints are
consistent with General Relativity, for which $\Upsilon_{1}=\Upsilon_{2}=0$. We
present here the first observational constraints on $\Upsilon_{2}$, as well as
the first extragalactic measurement of both parameters.
