# Null hypersurfaces and trapping horizons

## TL;DR

This paper investigates the geometric properties of null hypersurfaces and trapped submanifolds in spacetimes with constant sectional curvature, establishing conditions under which trapping horizons are non-expanding.

## Contribution

It provides new results on the existence and nature of trapped submanifolds and horizons in null hypersurfaces within constant curvature spacetimes, linking curvature, energy conditions, and horizon properties.

## Key findings

- Null non-expanding horizons cannot exist in non-positive curvature spacetimes.
- In positive curvature spacetimes satisfying Einstein's equations, null trapping horizons are non-expanding.
- Cross-sections of marginally outer trapped tubes have constant sectional curvature equal to the ambient space.

## Abstract

The purpose of the present work is to study (marginally) trapped submanifolds lying in a null hypersurface. Let $(M,g,N)\to\Bm(c)$ be a null hypersurface of a space-time with constant sectional curvature $c$, endowed with a Screen Integrable and Conformal rigging $N$. The (Marginally) Trapped Submanifolds we are interested with are particular leaves of the screen distribution according to the sign of their expansions. We prove that if $c$ is non-positive, then $\Bm$ cannot contain a null non-expanding horizon. In the case $c$ is positive, we show that if $\Bm$ satisfies Einstein's equation and dominant energy condition holds, then any null trapping horizon of $\Bm$ is a null non-expanding horizon. More generally we prove that in a spacetime $\Bm(c)$ with constant sectional curvature $c$, cross-sections of a marginally outer trapped tube are Riemann manifold with the same constant sectional curvature $c$.