The Asymptotic Statistics of Random Covering Surfaces

Abstract

Let $\Gamma_{g}$ be the fundamental group of a closed connected orientable surface of genus $g\geq2$. We develop a new method for integrating over the representation space $\mathbb{X}_{g,n}=\mathrm{Hom}(\Gamma_{g},S_{n})$ where $S_{n}$ is the symmetric group of permutations of $\{1,\ldots,n\}$. Equivalently, this is the space of all vertex-labeled, $n$-sheeted covering spaces of the the closed surface of genus $g$. Given $\phi\in\mathbb{X}_{g,n}$ and $\gamma\in\Gamma_{g}$, we let $\mathsf{fix}_{\gamma}(\phi)$ be the number of fixed points of the permutation $\phi(\gamma)$. The function $\mathsf{fix}_{\gamma}$ is a special case of a natural family of functions on $\mathbb{X}_{g,n}$ called Wilson loops. Our new methodology leads to an asymptotic formula, as $n\to\infty$, for the expectation of $\mathsf{fix}_{\gamma}$ with respect to the uniform probability measure on $\mathbb{X}_{g,n}$, which is denoted by $\mathbb{E}_{g,n}[\mathsf{fix}_{\gamma}]$. We prove that if $\gamma\in\Gamma_{g}$ is not the identity, and $q$ is maximal such that $\gamma$ is a $q$th power in $\Gamma_{g}$, then \[ \mathbb{E}_{g,n}[\mathsf{fix}_{\gamma}]=d(q)+O(n^{-1}) \] as $n\to\infty$, where $d\left(q\right)$ is the number of divisors of $q$. Even the weaker corollary that $\mathbb{E}_{g,n}[\mathsf{fix}_{\gamma}]=o(n)$ as $n\to\infty$ is a new result of this paper. We also prove that if $\gamma$ is not the identity then $\mathbb{E}_{g,n}[\mathsf{fix}_{\gamma}]$ can be approximated to any order $O(n^{-M})$ by a polynomial in $n^{-1}$.