The universal cover of an affine three-manifold with holonomy of shrinkable dimension $\leq 2$

TL;DR

This paper proves that certain affine 3-manifolds with holonomy of low shrinkable dimension have universal covers diffeomorphic to br^3, advancing understanding of their topological and geometric structure.

Contribution

It establishes that affine 3-manifolds with holonomy of shrinkable dimension  2 are br^3, using Morse theory and convexity properties, and relates to the weak Markus conjecture.

Findings

Universal cover of such manifolds is br^3

2-convexity implies topological incompressibility

Application to hyperbolic 3-manifolds with cone singularities

Abstract

An affine manifold is a manifold with an affine structure, i.e. a torsion-free flat affine connection. We show that the universal cover of a closed affine 3-manifold $M$ with holonomy group of shrinkable dimension (or discompacit\'e in French) less than or equal to two is diffeomorphic to $\bR^3$. Hence, $M$ is irreducible. This follows from two results: (i) a simply connected affine 3-manifold which is 2-convex is diffeomorphic to $\bR^3$, whose proof using the Morse theory takes most of this paper; and (ii) a closed affine manifold of holonomy of shrinkable dimension less or equal to $d$ is $d$-convex. To prove (i), we show that 2-convexity is a geometric form of topological incompressibility of level sets. As a consequence, we show that the universal cover of a closed affine three-manifold with parallel volume form is diffeomorphic to $\bR^3$, a part of the weak Markus conjecture. As applications, we show that the universal cover of a hyperbolic 3-manifold with cone-type singularity of arbitrarily assigned cone-angles along a link removed with the singular locus is diffeomorphic to $\bR^3$. A fake cell has an affine structure as shown by Gromov. Such a cell must have a concave point at the boundary.