Tests of Gravity Theories Using Supermassive Black Holes
  Scalar-tensor theories of gravity generally violate the strong equivalence
principle, namely compact objects have a suppressed coupling to the scalar
force, causing them to fall slower. A black hole is the extreme example where
such a coupling vanishes, i.e. black hole has no scalar hair. Following earlier
work, we explore observational scenarios for detecting strong equivalence
principle violation, focusing on galileon gravity as an example. For galaxies
in-falling towards galaxy clusters, the supermassive black hole can be offset
from the galaxy center away from the direction of the cluster. Hence, well
resolved images of galaxies around nearby clusters can be used to identify the
displaced black hole via the star cluster bound to it. We show that this signal
is accessible with imaging surveys, both ongoing ones such as the Dark Energy
Survey, and future ground and space based surveys. Already, the observation of
the central black hole in M~87 places new constraints on the galileon
parameters, which we present here. $\mathcal{O}(1)$ matter couplings are
disfavored for a large region of the parameter space. We also find a novel
phenomenon whereby the black hole can escape the galaxy completely in less than
one billion years.
