Boundary representations of hyperbolic groups

Abstract

Let $\Gamma$ be a Gromov hyperbolic group, endowed with an arbitrary left-invariant hyperbolic metric, quasi-isometric to a word metric. The action of $\Gamma$ on its boundary $\partial\Gamma$ endowed with the Patterson-Sullivan measure $\mu$, after an appropriate normalization, gives rise to a faithful unitary representation of $\Gamma$ on $L^2(\partial\Gamma,\mu)$. We show that these representations are irreducible, and give criteria for their unitary equivalence in terms of the metrics on $\Gamma$. Special cases include quasi-regular representations on the Poisson boundary.