Scalar Curvature, Volumes and the Bergman Kernel

Abstract

Motivated by the works of Gromov and LeBrun in Riemannian geometry, we study the analogous phenomena in complex geometry. We first show that both $\int_M |S_C^-(g)|^ndV_g$ and ${\rm vol}_g(M)$ (normalized by $S_C(g)\ge -1$) are bounded below by $\frac{(nπ)^n}{n!}\mathrm{CanVol}(M)$ for any Hermitian metric $g$ on a compact complex $n-$manifold $M$. Here $S_C$ denotes the Chern scalar curvature, $S_C^-=\max\{-S_C,0\}$ and ${\rm CanVol}(M)$ is the canonical volume of $M$, i.e., the volume of the canonical line bundle $K_M$. Moreover, if ${\rm vol}_g(M)=\frac{(nπ)^n}{n!}\mathrm{CanVol}(M)$ holds for some Kähler metric with $S_C\ge -1$, then it has to be the Kähler-Einstein metric of negative scalar curvature. The completely new phenomenon is that if $M$ is a compact Kähler manifold such that $K_M$ is nef, then ${\rm MinVol}_C(M)=\mathcal{I}_C(M)=\mathcal I_C^-(M)=\frac{(nπ)^n}{n!}\mathrm{CanVol}(M)$, where ${\rm MinVol}_C(M)$ is the infimum of ${\rm vol}_g(M)$ with $S_C(g)\ge -1$ and $\mathcal I_C^-(M)=\inf_g \int_M |S_C^-(g)|^ndV_g$, $\mathcal I_C(M)=\inf_g \int_M |S_C(g)|^ndV_g$. It remains unknown whether the nef condition is superfluous. The answer is positive when $M$ is obtained by blowing up a finite number of points from a projective manifold with big and nef canonical line bundle. The arguments are based on the asymptotic behaviour of the Bergman kernel of $mK_M$ as $m\rightarrow \infty$, the theory of Kähler-Ricci flow and singular Kähler-Einstein metric, as well as a very delicate gluing technique, using the Burns-Simanca metric.