Lipschitz rigidity for scalar curvature

TL;DR

This paper proves that under certain low-regularity conditions, a 1-Lipschitz map of non-zero degree from a spin manifold with scalar curvature bounded below by a specific constant to a sphere must be an isometry, extending previous rigidity results.

Contribution

It generalizes Lipschitz rigidity results to metrics of low regularity and connects harmonic spinors with quasiregular maps, answering a question of Gromov.

Findings

Any 1-Lipschitz map of non-zero degree under these conditions is an isometry.

Spectral properties of Dirac operators can be used for low-regularity metrics.

The existence of harmonic spinors implies quasiregularity of the map.

Abstract

Let $M$ be a closed smooth connected spin manifold of even dimension $n$, let $g$ be a Riemannian metric of regularity $W^{1,p}$, $p > n$, on $M$ whose distributional scalar curvature in the sense of Lee-LeFloch is bounded below by $n(n-1)$, and let $f \colon (M,g) \to \mathbb{S}^n$ be a $1$-Lipschitz continuous (not necessarily smooth) map of non-zero degree to the unit $n$-sphere. Then $f$ is a metric isometry. This generalizes a result of Llarull (1998) and answers in the affirmative a question of Gromov (2019) in his "Four lectures". Our proof is based on spectral properties of Dirac operators for low regularity Riemannian metrics and twisted with Lipschitz bundles. We argue that the existence of a non-zero harmonic spinor field forces $f$ to be quasiregular in the sense of Reshetnyak, and in this way connect the powerful theory for quasiregular maps to the Atiyah-Singer index theorem.