Critical points and supersymmetric vacua, II: Asymptotics and extremal metrics

Motivated by the vacuum selection problem of string/M theory, we study a new geometric invariant of a positive Hermitian line bundle $(L, h)\to M$ over a compact K\"ahler manifold: the expected distribution of critical points of a Gaussian random holomorphic section $s \in H^0(M, L)$ with respect to the Chern connection $\nabla_h$. It is a measure on $M$ whose total mass is the average number $\mathcal{N}^{crit}_h$ of critical points of a random holomorphic section. We are interested in the metric dependence of $\mathcal{N}^{crit}_h$, especially metrics $h$ which minimize $\mathcal{N}^{crit}_h$. We concentrate on the asymptotic minimization problem for the sequence of tensor powers $(L^N, h^N)\to M$ of the line bundle and their critical point densities $\mathcal{K}^{crit}_{N,h}(z)$. We prove that $\mathcal{K}^{crit}_{N,h}(z)$ has a complete asymptotic expansion in $N$ whose coefficients are curvature invariants of $h$. The first two terms in the expansion of $\mathcal{N}^{crit}_{N,h}$ are topological invariants of $(L, M)$. The third term is a topological invariant plus a constant $\beta_2(m)$ (depending only on the dimension $m$ of $M$) times the Calabi functional $\int_M \rho^2 dV_h$, where $\rho$ is the scalar curvature of the K\"ahler metric $\omega_h:=\frac i2 \Theta_h$. We give an integral formula for $\beta_2(m)$ and show, by a computer assisted calculation, that $\beta_2(m)>0$ for $m\leq 5$, hence that $\mathcal{N}^{crit}_{N,h}$ is asymptotically minimized by the Calabi extremal metric (when one exists). We conjecture that $\beta_2(m)>0$ in all dimensions, i.e. the Calabi extremal metric is always the asymptotic minimizer.