Nonperturbative gedanken experiments in Einstein-dilaton-Gauss-Bonnet gravity: nonlinear transitions and tests of the cosmic censorship beyond General Relativity

Abstract: As the only gravity theory with quadratic curvature terms and second-order field equations, Einstein-dilaton-Gauss-Bonnet gravity is a natural testbed to probe the high-curvature regime beyond General Relativity in a fully nonperturbative way. Due to nonperturbative effects of the dilatonic coupling, black holes in this theory have a minimum mass which separates a stable branch from an unstable one. The minimum mass solution is a double point in the phase diagram of the theory, wherein the critical black hole and a wormhole solution coexist. We perform extensive nonlinear simulations of the spherical collapse onto black holes with scalar hair in this theory, especially focusing on the region near the minimum mass. We study the nonlinear transition from the unstable to the stable branch and assess the nonlinear stability of the latter. Furthermore, motivated by modeling the mass loss induced by Hawking radiation near the minimum mass at the classical level, we study the collapse of a phantom field onto the black hole. When the black-hole mass decreases past the critical value, the apparent horizon shrinks significantly, eventually unveiling a high-curvature elliptic region. We argue that evaporation in this theory is bound to either violate the weak cosmic censorship or produce horizonless remnants. Addressing the end-state might require a different evolution scheme.