Speaker
Description
The growing sensitivity of gravitational-wave detectors enables increasingly precise tests of black hole (BH) ringdown spectroscopy. Yet BH quasinormal modes (QNMs) are spectrally unstable: small near horizon modifications can produce a drastically different QNM spectrum, while causality requires the prompt ringdown to remain BH-like until the reflected signal returns. Quantum BHs with potentially large interior reflection provide a natural setting for this tension. The relation between their time-domain waveform and different QNM spectra, although repeatedly discussed, still lacks a consistent treatment. In this work, we systematically examine the time-domain Green function for quantum BHs, considering sources located outside and inside the light-ring potential barrier. By decomposing the Green function into causally distinct components and choosing the corresponding inverse-Laplace contours consistently, we clarify how the response is built from different sets of QNMs. We find that the quantum BH QNM reconstruction provides a faithful description once the curved-spacetime region is probed, but its practical efficiency depends strongly on the evolutionary stage. Before interior reflection becomes relevant, we prove that this basis is formally equivalent to the BH QNM and tail expansions, although its convergence properties depend on the source location. At late times, the long-lived modes always provide an efficient basis. These results are confirmed by time-domain simulations and provide a unified causal picture of BH spectroscopy and quantum BH seismology.