Tests of Gravity Theories Using Supermassive Black Holes

Scalar-tensor theories of gravity generally violate the strongequivalence principle, namely that compact objects have a suppressedcoupling to the scalar force, which causes them to fall slower. A blackhole is the extreme example where such a coupling vanishes, i.e., blackholes have no scalar hair. We explore observational scenarios fordetecting strong equivalence principle violation, focusing on galileongravity as an example. For galaxies infalling toward galaxy clusters,the supermassive black hole can be offset from the galaxy center awayfrom the direction of the cluster. Well-resolved images of galaxiesaround nearby clusters can therefore be used to identify the displacedblack hole via the star cluster bound to it. We show that this signal isaccessible with imaging surveys, both ongoing ones such as the DarkEnergy Survey and future ground- and space-based surveys. Already, theobservation of the central black hole in M 87 places new constraints onthe galileon parameters, which we present here. { O }(1) mattercouplings are disfavored for a large region of the parameter space. Wealso find a novel phenomenon whereby the black hole can escape thegalaxy completely in less than one billion years.