Speaker
Description
We study gravitational quasinormal modes (QNMs) for two classes of compact object geometries, using the spectrum as a direct probe of spacetime structure and physical parameters. We focus on gravitational QNMs as they encode the intrinsic spectrum of the metric and characterize its stability independently of any interaction with external fields.
In the first part, we consider the Plebański-Demiański family of metrics, the most general Petrov Type-D electrovacuum solution of general relativity, which encompasses as special subcases the Kerr, Schwarzschild, Reissner-Nordström, and C-metric spacetimes, among others. We compute the QNM spectrum and analyze its dependence on the full set of physical parameters, including mass, spin, electric charge, magnetic charge, NUT parameter, acceleration, and twist. We then perform a stability analysis, looking for parameter combinations that may trigger instabilities, and investigate the conditions under which isospectrality is preserved, identifying the minimal subset of parameters that must be set to zero in order to recover it.
In the second part, we turn to black hole mimickers, solutions of general relativity that violate one or more hypotheses of the Penrose singularity theorem and therefore do not necessarily contain a singularity. These objects may or may not possess an event horizon, yet many feature a photon sphere and can produce a shadow observationally indistinguishable from that of a true black hole in Event Horizon Telescope (EHT) data. We compute the QNM spectrum of selected mimicker metrics, study their stability, and search for the characteristic echoes in the late-time ringdown, a distinctive spectral signature of these objects. Finally, we discuss how the QNM spectrum, combined with EHT shadow data, may allow us to constrain the parameters of mimicker metrics, providing a discriminator that shadow observations alone cannot supply.