Moduli spaces of flat tori with prescribed holonomy

77 pages, 15 figuresWe generalise to the genus one case several results of Thurston concerning moduli spaces of flat Euclidean structures with conical singularities on the two dimensional sphere. More precisely, we study the moduli space of flat tori with $n$ cone points and a prescribed holonomy $\rho$. In his paper `Flat Surfaces' Veech has established that under some assumptions on the cone angles, such a moduli space ${\mathcal{F}}_{[\rho]}\subset \mathscr M_{1,n}$ carries a natural geometric structure modeled on the complex hyperbolic space ${\mathbb C}{\mathbb{H} }^{n-1}$ which is not metrically complete. Using surgeries for flat surfaces, we prove that the metric completion $\overline{{\mathcal{F}}_{[\rho]}}$ is obtained by adjoining to $ {\mathcal{F}}_{[\rho]} $ certain strata that are themselves moduli spaces of flat surfaces of genus 0 or 1, obtained as degenerations of the flat tori whose moduli space is $ {\mathcal{F}}_{[\rho]}$. We show that the ${\mathbb C}{\mathbb{H} }^{n-1}$-structure of $ {\mathcal{F}}_{[\rho]}$ extends to a complex hyperbolic cone-manifold structure of finite volume on $ \overline{{\mathcal{F}}_{[\rho]}}$ and we compute the cone angles associated to the different strata of codimension 1. Finally, we address the question of whether or not the holonomy of Veech's ${\mathbb C}{\mathbb{H} }^{n-1}$-structure on $ \mathcal F_\rho$ has a discrete image in $ {\rm Aut}({\mathbb C}{\mathbb{H} }^{n-1})=\mathrm{PU}(1,n-1)$. We outline a general strategy to find moduli spaces $\mathcal F_{[\rho]}$ whose ${\mathbb C}{\mathbb{H} }^{n-1}$-holonomy gives rise to lattices in $\mathrm{PU}(1,n-1)$ and eventually we give a finite list of $\mathcal F_{[\rho]}$'s whose holonomy is an arithmetic lattice