Typical hyperbolic surfaces have an optimal spectral gap

Abstract

The first non-zero Laplace eigenvalue of a hyperbolic surface, or its spectral gap, measures how well-connected the surface is: surfaces with a large spectral gap are hard to cut in pieces, have a small diameter and fast mixing times. For large hyperbolic surfaces (of large area or large genus $g$, equivalently), we know that the spectral gap is asymptotically bounded above by $\frac 14$. The aim of these talks is to present joint work with Nalini Anantharaman, where we prove that most hyperbolic surfaces have a near-optimal spectral gap. That is to say, we prove that, for any $\epsilon > 0$, the Weil--Petersson probability for a hyperbolic surface of genus $g$ to have a spectral gap greater than $\frac 14- \epsilon$ goes to one as $g$ goes to infinity. This statement is analogous to Alon's 1986 conjecture for regular graphs, proven by Friedman in 2003. I will present our approach, which shares many similarities with Friedman's work, and introduce new tools and ideas that we have developed in order to tackle this problem.