# High Energy Theory Seminar: "Equivalence Principle in Scalar-tensor Theories"

We study the question of whether the equivalence principle holds for extended objects moving on cosmological backgrounds in modified gravity theories. We do so within the framework of effective field theory of dark energy focusing in particular on the subclass corresponding to the Horndeski theories. These are the most general second order scalar-tensor theories with non-linear derivative self-interactions allowing the Vainshtein screening mechanism to operate on small scales. We show how the approximate shift symmetry of the action, together with the special derivative structure of the scalar self-interactions insures that the equivalence principle is obeyed for screened extended objects with negligible self-gravitational energy. We comment on the possibility of extending the argument to the case of black holes.