In the presence of nonminimal gravitational couplings, matter field perturbations on a static black hole spacetime may develop unphysical poles in their linearized equations. Physical solutions confined in the domain between the event horizon and a pole satisfy a boundary value problem, although with boundary conditions which are different from standard quasinormal modes. We refer to them as “trapped quasinormal modes.” Focusing on a Schwarzschild black hole in Einstein-Proca theory, we find that trapped quasinormal modes accurately capture the behavior of perturbations under time evolution. In particular, axial-vector modes are unstable, with a growth rate that increases with multipole number. More interestingly, we uncover a new instability that affects monopole perturbations. These results confirm the existence of a novel destabilization mechanism of black holes by nonminimally coupled vector fields, with potential implications to well-studied models of modified gravity and cosmology based on vector particles.

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• Alternative gravity theories
• Dark energy
• Classical black holes