Speaker
Description
Layering is an ubiquitous feature of astrophysical objects. Motivated by the fact that physical black holes retain such structures in the time definition all outside probes are selected and monitored, we investigate linear perturbations and gravitational-wave echoes of a compact object composed of two concentric thin shells. Compared with the single-shell configuration, the double shell structure introduces an extra peak in the effective potential and partitions the spacetime into multiple resonant cavities. As the mass distribution between the two shells varies, the echo waveform and its spectrum undergo systematic evolution. In particular, we identify a direct correspondence between cavity lengths and the migration of resonance frequencies, and uncover a nontrivial branch permutation of spectral peaks arising from the coupling among multiple cavities. These results reveal generic wave-propagation signatures in the layered compact objects and suggest that gravitational-wave echoes may encode information about their internal structure.