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 nonrelativistic sources of finite extent. We exploit this breaking to identify new and novel astrophysical probes of these theories. Nonrelativistic objects feel a gravitational force that is weaker than that predicted by general relativity. The new equation of hydrostatic equilibrium is derived and solved to predict the modified behavior 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 smaller than the dynamical mass. We discuss potential astrophysical probes of these theories and identify strong lensing as a particularly promising candidate.