Quasinormal modes of deformed black holes: parity and isospectrality breaking

While most gravitational wave tests of General Relativity (GR) are designed to be theory-agnostic, theory-specific approaches can deliver more stringent and physically meaningful constraints. In the case of ringdown-based tests of GR, such theory-specific approaches require computing the quasinormal mode (QNM) spectrum in particular beyond-GR theories. In this talk, we discuss using the modified Teukolsky formalism to compute the QNM shifts of parametrically deformed black holes, focusing on the Johannsen-Psaltis metric. We find that the beyond-GR effects break the isospectrality between even and odd parity perturbations but do not mix definite-parity modes. Inspired by this example, we give a condition that a beyond-GR metric must satisfy to admit definite-parity QNMs. This work advances understanding of parity and isospectrality breaking in beyond-GR QNMs, offering a path to interpret potential ringdown deviations from GR.