Hawking Emission from Black Holes Evaporating toward Wormholes and the Accuracy of the WKB Approximation

Abstract

We revisit Hawking radiation from two black-hole families that can approach macroscopic wormhole configurations: the Simpson--Visser black-bounce geometry and the Casadio--Fabbri--Mazzacurati braneworld geometry. The earlier analysis of these backgrounds relied on WKB greybody factors. Here we replace that approximation by direct numerical scattering for the photon and massless Dirac channels and then recompute the emission spectra and integrated luminosities. The qualitative picture remains the same: as the wormhole endpoint is approached the black holes cool, the total flux is strongly suppressed, and the residual emission becomes increasingly fermion dominated. The quantitative picture, however, changes substantially. Close to the Schwarzschild limit the WKB estimates are reasonably accurate, but far from that limit the error can be large, and near the cold endpoint it can reach orders of magnitude. In particular, the WKB calculation can substantially overestimate the remaining luminosity precisely in the regime where the evaporation rate is most sensitive to the low-frequency tail of the greybody factors. In a fixed-parameter Simpson--Visser half-decay estimate, the direct-greybody luminosities increase the WKB lifetime coefficient by a factor of about 85. These results show that reliable evaporation rates for black holes evolving toward wormhole-like endpoints require direct numerical greybody factors rather than barrier-top WKB estimates alone.