Marginally outer trapped surface
= Marginally outer trapped surface
{title2=$\theta_+=0$}
A closed spacelike two-surface is marginally outer trapped when its future outgoing <null expansion> is zero. For the future black-hole type, the ingoing <null expansion> is negative. An <apparent horizon> on a chosen spatial slice is ordinarily its outermost such surface under suitable regularity assumptions. The notion depends on the slice and is not the global definition of an <event horizon>.