= Event horizon inside an Oppenheimer-Snyder cloud
{title2=$\chi_{\mathcal H}(\eta)=\eta-\eta_H+\chi_0$}
Radial outgoing <null geodesics> in the interior of <Oppenheimer-Snyder collapse> obey $d\chi/d\eta=1$. Trace the ray crossing the boundary at $(\eta_H,\chi_0)$ backward: it starts at the centre at $\eta=\eta_H-\chi_0$. This ray is the <event horizon>; it can exist inside the cloud before the surface crosses $R=2M$. An <event horizon> is determined by global escape to <future null infinity>, rather than by a local stopwatch signal.
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