Geometric Rigidity via Almost-Harmonic Twisted Spinors

Abstract

We establish sharp scalar-curvature bounds and rigidity consequences of Gromov's exact-lift two-form method. Let $(M^n,g)$, $n\ge 4$ even, be a closed spin Riemannian manifold carrying a homologically $\hat{A}$-non-singular closed two-form $\omega$ whose lift to the universal cover $X$ is exact. Then $$\inf_M\scal_g\leq -\frac{4n}{n-1}λ_0(X).$$ Equality forces $g$ to be Einstein; if ${\lambda }_0(X)>0$, then $X$ is real hyperbolic, while if ${\lambda }_0(X)=0$ and ${\int }_M{\omega }^{n/2}\ne 0$, then $g$ is flat. The proof combines Gromov's twisted $L^2$-index with a conformal interpretation of the refined Kato equality and a recentering argument. The same method yields untwisted rigidity results when zero belongs to the spectrum of the Dirac operator on the universal cover, with applications to nonvanishing$\hat{A}$-genus and enlargeability.

arXiv commentImproved exposition; Comments are welcome!