= Apparent horizon
{wiki}
An <apparent horizon> is the outer boundary of the future-trapped region on a chosen spatial slice, under the usual regularity conditions. Its location depends on the slice; it is not the globally defined <event horizon>. At a regular boundary the outward <null expansion> vanishes while the inward <null expansion> is negative. Its area cannot be substituted into every version of the <Penrose inequality> without the appropriate area and initial-data hypotheses.
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