Handlebodies, Outer space, and tropical geometry

TL;DR

This paper explores the tropicalization of moduli spaces of Riemann surfaces and handlebodies, connecting complex geometry, tropical geometry, and geometric group theory through new constructions and generalizations.

Contribution

It introduces the notion of stable complex handlebodies and shows how certain complex moduli spaces relate to their tropical counterparts, extending known relationships to punctured cases.

Findings

CV_{g,n}^* as tropicalization of complex handlebody moduli space

Construction of a simply connected partial compactification with normal crossings

Extension of relationships between geometric group theory objects to punctured surfaces

Abstract

The moduli space of graphs $M_{g,n}^{\mathrm{trop}}$ is a polyhedral object that mimics the behavior of the moduli spaces $M_{g,n}$, $\overline{M}_{g,n}$ of (stable) Riemann surfaces; this relationship has been made precise in several different ways, which collectively identify $M_{g,n}^{\mathrm{trop}}$ as the "tropicalization" of $M_{g,n}$. We describe how this relationship lifts to some objects that live over $M_{g,n}$ (like Teichm\"uller space) and that live over $M_{g,n}^{\mathrm{trop}}$ (like the Culler-Vogtmann space $CV_{g,n}^*$). We introduce the notion of a stable complex handlebody, and show that $CV_{g,n}^*$ can be viewed as the tropicalization of a certain complex manifold $hT(V_{g,n})$ that parametrizes complex handlebodies. An important ingredient is our construction of a partial compactification $\overline{hT}(V_{g,n})\supset hT(V_{g,n})$, which we prove is a simply connected complex manifold with simple normal crossings boundary. When $n=0$, $hT(V_{g,n})$ coincides with the moduli space of Schottky groups, $\overline{hT}(V_{g,n})$ coincides with Gerritzen-Herrlich's extended Schottky space, and $CV_{g,0}^*$ is the simplicial completion of the original Outer space. The resulting picture fits together many familiar objects from geometric group theory and surface topology, including Harvey's curve complex, mapping class groups of surfaces and handlebodies, and augmented Teichm\"uller space. Many of the relationships between the objects that we see in this picture already exist in the literature, but we add some new ones, and generalize several existing relationships to include a number $n>0$ of punctures/leaves.