Apparent horizon as a membrane

TL;DR

This paper models the near-horizon geometry of physical black holes as a membrane, deriving explicit expressions for physical quantities and connecting near-horizon physics with observable signatures.

Contribution

It introduces a membrane description of the apparent horizon in dynamical black holes, providing explicit formulas and insights into near-horizon geometry and surface gravity.

Findings

Derived closed-form expressions for redshift, acceleration, and extrinsic curvature.

Established a membrane stress tensor using junction conditions.

Connected near-horizon geometry with observable signatures and Rindler space.

Abstract

The requirement that a trapped spacetime domain forms in finite time for distant observers is logically possible and sometimes unavoidable, but its consequences are not yet fully understood. In spherical symmetry, the characterization of the near-horizon geometry of these physical black holes is complete and shows marked differences from their eternal counterparts. Whether these differences lead to observable signatures remains unclear. We construct an approximate near-horizon metric that encapsulates them and is suitable for modeling. The timelike apparent horizon of physical black holes provides a natural surface for a consistent membrane description: we obtain closed-form expressions for the redshift, proper acceleration, and extrinsic curvature, and assign a two-dimensional viscous-fluid stress tensor via junction conditions. These results also provide an additional perspective on the relation between Rindler and near-horizon geometries. Among dynamical generalizations of surface gravity, only a subset applies to these models. We complete their analysis and recover the intuitive definition of surface gravity -- the acceleration in the frame of a near-horizon observer, redshifted to infinity -- directly from the membrane acceleration.