Access of massive particles to photon-excluded domains

Determine whether massive particles can access the photon-excluded domains generated by the multi-peaked effective potential in horizonless symmetric black-bounce geometries.

Background

The paper studies axial gravitational perturbations and photon propagation in symmetric and asymmetric black-bounce spacetimes. For horizonless symmetric configurations with sufficiently small bounce parameter a, the effective potential develops multiple barriers, including a steep central barrier that reflects null geodesics with small impact parameters and prevents them from probing certain regions of spacetime. These photon-excluded domains produce unusual scattering behavior, nested photon rings, and brightness discontinuities in the observer’s image.

The unresolved issue is whether massive-particle trajectories are subject to the same exclusion mechanism as null geodesics, or whether their nonzero rest mass enables them to penetrate the regions inaccessible to photons. The paper explicitly leaves this question for future work.