Where are the trapped surfaces?

TL;DR

This paper investigates the boundaries of trapped surfaces in spacetime, demonstrating that in certain solutions like Vaidya, these boundaries lie inside the event horizon and presenting examples of trapped surfaces extending outside the apparent horizon.

Contribution

It introduces a detailed analysis of trapped surface boundaries in specific spacetime models and constructs examples of trapped surfaces extending beyond the apparent horizon.

Findings

Boundaries of trapped surfaces can lie strictly inside the event horizon.

Existence of closed trapped surfaces extending outside the apparent horizon.

Analysis applied to simple examples and the Vaidya solution.

Abstract

We discuss the boundary of the spacetime region through each point of which a trapped surface passes, first in some simple soluble examples, and then in the self-similar Vaidya solution. For the latter the boundary must lie strictly inside the event horizon. We present a class of closed trapped surfaces extending strictly outside the apparent horizon.