Trapped Surface

A trapped surface is a region of spacetime so strongly curved by gravity that even light emitted outwards is dragged inwards. Normally, if you switch on a light, the wavefront expands in every direction. Inside a trapped surface, gravity is so intense that both the inward and the outward-going light actually converge toward a smaller area. Once a trapped surface forms, collapse to a singularity becomes unavoidable.

Why it mattered to Hawking

The trapped surface is the crucial idea behind the most important result of Hawking's early career. It was introduced by Roger Penrose in 1965, and its power was that it let physicists prove things about black holes without tracking the messy details of a collapsing star. Penrose showed that once a trapped surface exists, general relativity guarantees a singularity must follow.

Hawking took this concept and applied it, in reverse, to the entire universe, reasoning that the expanding cosmos run backwards behaves like a collapse and must therefore also harbour a singularity at its origin. The trapped surface is thus the technical seed of the Penrose-Hawking singularity theorems, and a perfect illustration of how a single sharp idea can unlock an entire field.