Boundary regularity and stability for spaces with Ricci bounded below

This paper studies the structure and stability of boundaries in noncollapsed RCD(K,N) spaces, that is, metric-measure spaces (X, d, H^N) with Ricci curvature bounded below. Our main structural result is that the boundary ∂X is homeomorphic to a manifold away from a set of codimension 2, and is N- 1 rectifiable. Along the way, we show effective measure bounds on the boundary and its tubular neighborhoods. These results are new even for Gromov–Hausdorff limits (MiN,dgi,pi)→(X,d,p) of smooth manifolds with boundary, and require new techniques beyond those needed to prove the analogous statements for the regular set, in particular when it comes to the manifold structure of the boundary ∂X. The key local result is an ε-regularity theorem, which tells us that if a ball B₂(p) ⊂ X is sufficiently close to a half space B2(0)⊂R+N in the Gromov–Hausdorff sense, then B₁(p) is biHölder to an open set of R+N. In particular, ∂X is itself homeomorphic to B₁(0 ^N-1) near B₁(p). Further, the boundary ∂X is N- 1 rectifiable and the boundary measure [InlineEquation not available: see fulltext.] is Ahlfors regular on B₁(p) with volume close to the Euclidean volume. Our second collection of results involve the stability of the boundary with respect to noncollapsed mGH convergence X_i→ X. Specifically, we show a boundary volume convergence which tells us that the N- 1 Hausdorff measures on the boundaries converge [InlineEquation not available: see fulltext.] to the limit Hausdorff measure on ∂X. We will see that a consequence of this is that if the X_i are boundary free then so is X.