  In a matter-filled spacetime, perhaps with positive cosmological constant, a
stable marginally outer trapped 2-sphere must satisfy a certain area
inequality. Namely, as discussed in the paper, its area must be bounded above
by $4\pi/c$, where $c > 0$ is a lower bound on a natural energy-momentum term.
We then consider the rigidity that results for stable, or weakly outermost,
marginally outer trapped 2-spheres that achieve this upper bound on the area.
In particular, we prove a splitting result for 3-dimensional initial data sets
analogous to a result of Bray, Brendle and Neves [10] concerning area
minimizing 2-spheres in Riemannian 3-manifolds with positive scalar curvature.
We further show that these initial data sets locally embed as spacelike
hypersurfaces into the Nariai spacetime. Connections to the Vaidya spacetime
and dynamical horizons are also discussed.
