Black hole hair in generalized scalar-tensor gravity

The most general action for a scalar field coupled to gravity that leads to second order field equations for both the metric and the scalar — Horndeski’s theory — is considered, with the extra assumption that the scalar satisfies shift symmetry. We show that in such theories the scalar field is forced to have a nontrivial configuration in black hole spacetimes, unless one carefully tunes away a linear coupling with the Gauss–Bonnet invariant. Hence, black holes for generic theories in this class will have hair. This contradicts a recent no-hair theorem, which seems to have overlooked the presence of this coupling.