A null hypersurface has a degenerate induced metric. Its normal covector raises to a null vector tangent to the hypersurface, whose integral curves are its null generators.
A trapped surface is a closed spacelike codimension-two surface whose two future-directed orthogonal null congruences both have negative expansion.
An anti-trapped surface is the time reverse of a trapped surface: both future-directed orthogonal null expansions are positive.
Articles by others on the same topic
A null hypersurface is a concept from the field of differential geometry and general relativity, relating to the geometry of spacetime. In general, a hypersurface is a submanifold of one dimension less than its ambient manifold. For example, in a four-dimensional spacetime (which typically includes three spatial dimensions and one time dimension), a hypersurface is a three-dimensional surface. A **null hypersurface** specifically refers to a hypersurface where the normal vector at each point is a null vector.