Astrophysical Probes of the Vainshtein Mechanism: Stars and Galaxies
  Ghost-free theories beyond the Horndeski class exhibit a partial breaking of
the Vainshtein mechanism inside non-relativistic sources of finite extent. We
exploit this breaking to identify new and novel astrophysical probes of these
theories. Non-relativistic objects feel a gravitational force that is weaker
than that predicted by general relativity. The new equation of hydrostatic
equilibrium equation is derived and solved to predict the modified behaviour of
stars. It is found that main-sequence stars are dimmer and cooler than their
general relativity counterparts but the red giant phase is largely
indistinguishable. The rotation curves and lensing potential of Milky Way-like
galaxies are calculated. The circular velocities are smaller than predicted by
general relativity at fixed radius and the lensing mass is larger than the
dynamical mass. We discuss potential astrophysical probes of these theories and
identify strong lensing as a particularly promising candidate.
