Class field theory as a dynamical system

This is the text from a talk at the Arbeitstagung 2011, which can serve as an introduction to arxiv:1009.0736 and arXiv:1007.0907. I first discuss how a global field is determined by a certain dynamical system, and how this relates to abelian L-series determining those fields. I then discuss an analog in Riemannian geometry, and how it leads to a metric in the space of closed Riemannian manifolds.