Borel and volume classes for dense representations of discrete groups

Abstract

We show that the bounded Borel class of any dense representation $ρ: G\to \PSL_n\bC$ is non-zero in degree three bounded cohomology and has maximal semi-norm, for any discrete group $G$. When $n=2$, the Borel class is equal to the $3$-dimensional hyperbolic volume class. Using tools from the theory of Kleinian groups, we show that the volume class of a dense representation $ρ: G\to \PSL_2\bC$ is uniformly separated in semi-norm from any other representation $ρ': G\to \G$ for which there is a subgroup $H\le G$ on which $ρ$ is still dense but $ρ'$ is discrete or indiscrete but stabilizes a point, line, or plane in $\bH^3\cup \partial \bH^3$. We exhibit a family of dense representations of a non-abelian free group on two letters and a family of discontinuous dense representations of $\PSL_2\bR$, whose volume classes are linearly independent and satisfy some additional properties; the cardinality of these families is that of the continuum. We explain how the strategy employed may be used to produce non-trivial volume classes in higher dimensions, contingent on the existence of a family of hyperbolic manifolds with certain topological and geometric properties.