Band width estimates via the Dirac operator

Abstract

Let $M$ be a closed connected spin manifold such that its spinor Dirac operator has non-vanishing (Rosenberg) index. We prove that for any Riemannian metric on $V=M \times [-1,1]$ with scalar curvature bounded below by $\sigma \gt 0$, the distance between the boundary components of $V$ is at most $C_n / \sqrt{\sigma}$, where $C_n = \sqrt{(n-1)/n} \cdot C$ with $C \lt 8(1+\sqrt{2})$ being a universal constant. This verifies a conjecture of Gromov for such manifolds. In particular, our result applies to all high-dimensional closed simply connected manifolds $M$ which do not admit a metric of positive scalar curvature. We also establish a quadratic decay estimate for the scalar curvature of complete metrics on manifolds, such as $M \times \mathbb{R}^2$, which contain $M$ as a codimension two submanifold in a suitable way. Furthermore, we introduce the “$\mathcal{KO}$-width” of a closed manifold and deduce that infinite $\mathcal{KO}$-width is an obstruction to positive scalar curvature.