Black hole boundaries

TL;DR

This paper reviews traditional and alternative, quasilocal definitions of black hole boundaries, emphasizing their mathematical properties and relevance in various areas of relativity and quantum gravity.

Contribution

It provides a comprehensive overview of classical and recent quasilocal horizon concepts, highlighting their differences from traditional event horizons.

Findings

Traditional black holes are defined by causal structure.

Alternative horizons are associated with zero outward null expansion.

Recent work explores quasilocal horizons in various contexts.

Abstract

Classical black holes and event horizons are highly non-local objects, defined in relation to the causal past of future null infinity. Alternative, quasilocal characterizations of black holes are often used in mathematical, quantum, and numerical relativity. These include apparent, killing, trapping, isolated, dynamical, and slowly evolving horizons. All of these are closely associated with two-surfaces of zero outward null expansion. This paper reviews the traditional definition of black holes and provides an overview of some of the more recent work on alternative horizons.