Phenomenology of dynamical trapped regions

Abstract

Regular black holes (RBHs) and horizonless black hole mimickers (BHMs) are often studied as stationary alternatives to classical black holes, although their interiors may be unstable or undergoing relaxation. We investigate the phenomenological consequences of this evolution by numerically evolving linear scalar perturbations on prescribed, time-dependent Hayward-like geometries. We consider BHM--RBH--BHM histories in which an initially horizonless object temporarily develops a trapped region before returning to a horizonless configuration, through either a single bounce or a sequence of repeated bounces. Using horizon-penetrating Painlevé--Gullstrand coordinates, we follow the perturbations through the formation and disappearance of trapping horizons and compute the resulting waveforms and scalar-field energy. For histories with identical initial and final geometries, we find that changing the duration of the intermediate regular black hole phase produces differences in the amplitude and phase of the late-time signal, including its echoes. Longer trapped phases also yield greater energy amplification, consistent with the blueshift of outgoing modes near the inner trapping horizon. These results show that the late-time response of a dynamically relaxing BHM need not be determined by its final configuration alone: it can retain a memory of the dynamical history of its interior.