$3+1$ nonlinear evolution of Ricci-coupled scalar-Gauss-Bonnet gravity

Scalar-Gauss-Bonnet (sGB) gravity with an additional coupling between the scalar field and the Ricci scalar exhibits very interesting properties related to black hole stability, evasion of binary pulsar constraints, and general relativity as a late-time cosmology attractor. Furthermore, it was demonstrated that a spherically symmetric collapse is well posed for a wide range of parameters. In the present paper we examine further the well-posedness through $3+1$ evolution of static and rotating black holes. We show that the evolution is indeed hyperbolic if the weak coupling condition is not severely violated. The loss of hyperbolicity is caused by the gravitational sector of the physical modes, thus it is not an artifact of the gauge choice. We further seek to compare the Ricci-coupled sGB theory against the standard sGB gravity with additional terms in the Gauss-Bonnet coupling. We find strong similarities in terms of well-posedness, but we also point out important differences in the stationary solutions. As a by-product, we show strong indications that stationary near-extremal scalarized black holes exist within the Ricci-coupled sGB theory, where the scalar field is sourced by the spacetime curvature rather than the black hole spin.