# Tests of Gravity Theories Using Supermassive Black Holes

## TL;DR

This paper investigates how supermassive black holes can be used to test theories of gravity that violate the strong equivalence principle, using observational signatures like black hole offsets in galaxy clusters.

## Contribution

It proposes a method to detect strong equivalence principle violations via black hole displacements and provides new constraints on galileon gravity parameters from existing observations.

## Key findings

- Black hole offsets can indicate violations of the strong equivalence principle.
- Current observations of M87's black hole constrain galileon gravity parameters.
- Black holes may escape galaxies within a billion years under certain conditions.

## Abstract

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.