Bounds on the Horndeski Gauge-Gravity Coupling
  The Horndeski gauge-gravity coupling is the leading non-minimal interaction
between gravity and gauge bosons, and it preserves all the symmetries and the
number of physical degrees of freedom of the standard model of particle physics
and general relativity. In this paper we study the effects of the non-minimal
interaction in astronomy and cosmology, and obtain upper bounds on the
associated dimensionless coupling constant $\lambda$. From the modification of
equations of motion of gauge bosons applied to compact astronomical objects, we
find upper bounds $|\lambda| \lesssim 10^{88}$, $|\lambda| \lesssim 10^{75}$
and $|\lambda| \lesssim 10^{84}$ from a black hole shadow, neutron stars and
white dwarfs, respectively. The bound $|\lambda| \lesssim 10^{75}$ that is
deduced from neutron stars is the strongest and provides twenty orders of
magnitude improvement of the previously known best bound on this parameter. On
the other hand, the effects of this term on modification of the gravitational
Poisson equation lead to a weaker bound $|\lambda| \lesssim 10^{98}$. From the
propagation of gravitational waves we also find $|\lambda| \lesssim 10^{119}$,
which is even weaker.
