Dynamics and geometry of character varieties for surface groups

Abstract

The problem of classifying geometric structures on manifolds is very much related to the discussion of the automorphism groups actions on character varieties, which are spaces of equivalence classes of representations. In this chapter we survey some results on this topic, mostly focusing on representations of surface groups (both in the orientable and non-orientable cases) and free groups. An important principle in the study of the dynamics on character varieties $X=X(\pi_1(S),G)$ for surface groups $\pi_1(S)$ is the following dichotomy: when the target group $G$ is compact, $X$ has nontrivial homotopy type, and the action of the mapping class group is chaotic; whereas when the target group $G$ is non-compact, $X$ contains contractible sets on which the mapping class group acts properly. We will expand on this dichotomy in various cases. After introducing the necessary background, we will discuss representations into $\mathsf{PSL}_2(\mathbb{R})$ and $\mathsf{PGL}_2(\mathbb{R})$, discussing the number of connected components, the geometric properties (Bowditch question), the dynamics (Goldman conjecture) and some components with an `exotic' behaviour (Deroin-Tholozan representations). We will also underline how the theory for representations of fundamental groups of orientable closed hyperbolizable surfaces needs to be adapted when one considers surfaces with punctures or non-orientable surfaces. We will then discuss representations into compact groups, where we will discuss mostly ergodicity results in various settings, and some non-ergodicity results at the end. Thirdly, we will consider representations in $\mathsf{PSL}_2(\mathbb{C})$. We will discuss convex-cocompact representations, primitive-stable and Bowditch representations and their relationship. Finally, we will describe how some of the results mentioned can be generalized for representations into higher-rank Lie groups.