Constraints on galileons from the positions of supermassive black holes
  Galileons are scalar field theories which obey the Galileon symmetry $\varphi
\to \varphi + b + c_\mu x^\mu$ and are capable of self-acceleration if they
have an inverted sign for the kinetic term. These theories violate the Strong
Equivalence Principle, such that black holes (BHs) do not couple to the
Galileon field, whereas non-relativistic objects experience a fifth force with
strength $\Delta G / G_{\rm N}$ relative to gravity. For galaxies falling down
a gradient in the Galileon field, this results in an offset between the centre
of the galaxy and its host supermassive BH. We reconstruct the local
gravitational and Galileon fields through a suite of constrained N-body
simulations (which we dub CSiBORG) and develop a Monte Carlo-based forward
model for these offsets on a galaxy-by-galaxy basis. Using the measured offset
between the optical centre and active galactic nucleus of 1916 galaxies from
the literature, propagating uncertainties in the input quantities and
marginalising over an empirical noise model describing astrophysical and
observational noise, we constrain the Galileon coupling to be $\Delta G /
G_{\rm N} < 0.16$ at $1\sigma$ confidence for Galileons with crossover scale
$r_{\rm C} \gtrsim H_0^{-1}$.
