Doubling of Asymptotically Flat Half-spaces and the Riemannian Penrose Inequality

Building on previous works of Bray, of Miao, and of Almaraz, Barbosa, and de Lima, we develop a doubling procedure for asymptotically flat half-spaces (M, g) with horizon boundary Sigma subset of M and mass m is an element of R. If 3 <= dim(M) <= 7, (M, g) has non-negative scalar curvature, and the boundary partial derivative M is mean-convex, we obtain the Riemannian Penrose-type inequality m >= (1/2)(n/n-1) (|Sigma|/omega(n-1))(n-2/n-1) as a corollary. Moreover, in the case where partial derivative M is not totally geodesic, we show how to construct local perturbations of (M, g) that increase the scalar curvature. As a consequence, we show that equality holds in the above inequality if and only if the exterior region of (M, g) is isometric to a Schwarzschild half-space. Previously, these results were only known in the case where dim(M)=3 and Sigma is a connected free boundary hypersurface.