Polynomial stability of the homology of Hurwitz spaces

Abstract

For a finite group $G$ and a conjugation-invariant subset $Q\subseteq G$, we consider the Hurwitz space ${\mathrm{Hur}}_n(Q)$ parametrising branched covers of the plane with $n$ branch points, monodromies in $G$ and local monodromies in $Q$. For $i\ge 0$ we prove that ${⨁}_n{H}_i({\mathrm{Hur}}_n(Q))$ is a finitely generated module over the ring ${⨁}_n{H}_0({\mathrm{Hur}}_n(Q))$. As a consequence, we obtain polynomial stability of homology of Hurwitz spaces: taking homology coefficients in a field, the dimension of $H_i({\mathrm{Hur}}_n(Q))$ agrees for $n$ large enough with a quasi-polynomial in $n$, whose degree is easily bounded in terms of $G$ and $Q$. Under suitable hypotheses on $G$ and $Q$, we prove classical homological stability for certain sequences of components of Hurwitz spaces. Our results generalise previous work of Ellenberg-Venkatesh-Westerland, and rely on techniques introduced by them and by Hatcher-Wahl.

24 pages, 2 figures, final version to appear in Mathematische Annalen