Jordan Ellenberg (Wisconsin)

Random matrices, random permutations, conjectures of arithmetic distribution over function fields, topology of Hurwitz spaces

A Hurwitz space HG,n is an algebraic variety parametrizing branched covers of the projective line with some fixed finite Galois group G. We will prove that, under some hypotheses on G, the rational ith homology of the Hurwitz spaces stabilizes when the number of branch points is sufficiently large compared to i.

This purely topological theorem has some interesting number-theoretic consequences. It implies, for instance, a weak form of the Cohen- Lenstra conjectures over rational function fields, and some quantitative inverse Galois results over function fields. For instance, we show that the average size of the p-part of the class number of a hyperelliptic genus-g curve over Fq is bounded independently of g, when q is large enough relative to p; the key point here is q can be held fixed while g grows.

I will try to give a general overview of the dictionary between conjectures about topology of moduli spaces, on the one hand, and arithmetic distribution conjectures (Cohen-Lenstra, Bhargava, Malle, inverse Galois....) on the other. In particular, I will explain how vanishing statements in cohomology imply that a natural Frobenius action behaves like a random matrix, a random permutation, or a random element of some other more exotic monodromy group.

(joint work with Akshay Venkatesh and Craig Westerland.)