This work presents a phenomenological boundary-layer model describing possible near-horizon transport effects in regions of strong curvature, formulated on a classical general-relativity background. All deviations from the standard prediction are encoded in a small set of dimensionless parameters representing impedance contrast, reflectivity, absorption, and shear of a conserved temporal current, while the Einstein equations in the exterior spacetime remain unchanged. The model predicts two observable signatures. In gravitational-wave ringdowns, partial reflection near the horizon produces a sequence of delayed echoes whose spacing is determined by the effective cavity length. In primordial black-hole evaporation, the same boundary parameters modify the emission conditions, producing mild spectral hardening and a slow drift of the polarization angle in high-energy radiation. Because both effects depend on the same scaling combination of parameters, gravitational-wave and gamma-ray observations provide a direct consistency test of the effective description. The formulation is strictly four-dimensional, reduces smoothly to the general-relativity limit when the boundary layer vanishes, and is intended as a minimal testable model that can be applied directly to existing LIGO, Virgo, KAGRA, and high-energy data.
Roy Herbert (Sat,) studied this question.