Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 311 / 1 / d / Solution

Using $v=t+r_{\star }$ puts the black string metric into regular ingoing form,Its inverse metric givesFor $r<r_{+}$, $f<0$, so $\nabla r$ is timelike. It is future-directed by continuity from the future horizon, where it equals the future generator ${\partial }_v$. If $X$ is any future-directed causal tangent, thenThus $r$ decreases strictly along every future-directed causal curve in the interior. No such curve can cross back through $r=r_{+}$ or reach future null infinity. The region $r<r_{+}$ is consequently outside the causal past of future null infinity and is part of the black hole region.