  We report a new area law in General Relativity. A future holographic screen
is a hypersurface foliated by marginally trapped surfaces. We show that their
area increases monotonically along the foliation. Future holographic screens
can easily be found in collapsing stars and near a big crunch. Past holographic
screens exist in any expanding universe and obey a similar theorem, yielding
the first rigorous area law in big bang cosmology. Unlike event horizons, these
objects can be identified at finite time and without reference to an asymptotic
boundary. The Bousso bound is not used, but it naturally suggests a
thermodynamic interpretation of our result.
