Circle-valued angle structures and obstruction theory

Abstract

We study spaces of circle-valued angle structures, introduced by Feng Luo, on ideal triangulations of 3-manifolds. We prove that the connected components of these spaces are enumerated by certain cohomology groups of the 3-manifold with $\mathbb{Z}_2$-coefficients. Our main theorem shows that this establishes a geometrically natural bijection between the connected components of the spaces of circle-valued angle structures and the obstruction classes to lifting boundary-parabolic $PSL(2,\mathbb{C})$-representations of the fundamental group of the 3-manifold to boundary-unipotent representations into $SL(2,\mathbb{C})$. In particular, these connected components have topological and algebraic significance independent of the ideal triangulations chosen to construct them. The motivation and main application of this study is to understand the domain of the state-integral defining the meromorphic 3D-index of Garoufalidis and Kashaev, necessary in order to classify the boundary-parabolic representations contributing to its asymptotics.