A long neck principle for Riemannian spin manifolds with positive scalar curvature

Abstract: We develop index theory on compact Riemannian spin manifolds with boundary in the case when the topological information is encoded by bundles which are supported away from the boundary. As a first application, we establish a "long neck principle" for a compact Riemannian spin $n$-manifold with boundary $X$, stating that if $\textrm{scal}(X)\geq n(n-1)$ and there is a nonzero degree map into the sphere $f\colon X\to S^n$ which is strictly area decreasing, then the distance between the support of $\textrm{d} f$ and the boundary of $X$ is at most $\pi/n$. This answers, in the spin setting and for strictly area decreasing maps, a question recently asked by Gromov. As a second application, we consider a Riemannian manifold $X$ obtained by removing $k$ pairwise disjoint embedded $n$-balls from a closed spin $n$-manifold $Y$. We show that if $\textrm{scal}(X)>\sigma>0$ and $Y$ satisfies a certain condition expressed in terms of higher index theory, then the radius of a geodesic collar neighborhood of $\partial X$ is at most $\pi \sqrt{(n-1)/(n\sigma)}$. Finally, we consider the case of a Riemannian $n$-manifold $V$ diffeomorphic to $N\times [-1,1]$, with $N$ a closed spin manifold with nonvanishing Rosenberg index. In this case, we show that if $\textrm{scal}(V)\geq\sigma>0$, then the distance between the boundary components of $V$ is at most $2\pi \sqrt{(n-1)/(n\sigma)}$. This last constant is sharp by an argument due to Gromov.