A Spectral Perspective on Neumann-Zagier

We provide a new topological interpretation of the symplectic properties of gluing equations for triangulations of hyperbolic 3-manifolds, first discovered by Neumann and Zagier. We also extend the symplectic properties to more general gluings of PGL(2,C) flat connections on the boundaries of 3-manifolds with topological ideal triangulations, proving that gluing is a K_2 symplectic reduction of PGL(2,C) moduli spaces. Recently, such symplectic properties have been central in constructing quantum PGL(2,C) invariants of 3-manifolds. Our methods adapt the spectral network construction of Gaiotto-Moore-Neitzke to relate framed flat PGL(2,C) connections on the boundary C of a 3-manifold to flat GL(1,C) connections on a double branched cover S -> C of the boundary. Then moduli spaces of both PGL(2,C) connections on C and GL(1,C) connections on S gain coordinates labelled by the first homology of S, and inherit symplectic properties from the intersection form on homology.