Strong ergodicity, property (T), and orbit equivalence rigidity for translation actions

Abstract: We study equivalence relations that arise from translation actions $\Gamma\curvearrowright G$ which are associated to dense embeddings $\Gamma<G$ of countable groups into second countable locally compact groups. Assuming that $G$ is simply connected and the action $\Gamma\curvearrowright G$ is strongly ergodic, we prove that $\Gamma\curvearrowright G$ is orbit equivalent to another such translation action $\Lambda\curvearrowright H$ if and only if there exists an isomorphism $\delta:G\rightarrow H$ such that $\delta(\Gamma)=\Lambda$. If $G$ is moreover a real algebraic group, then we establish analogous rigidity results for the translation actions of $\Gamma$ on homogeneous spaces of the form $G/\Sigma$, where $\Sigma<G$ is either a discrete or an algebraic subgroup. We also prove that if $G$ is simply connected and the action $\Gamma\curvearrowright G$ has property (T), then any cocycle $w:\Gamma\times G\rightarrow\Lambda$ with values into a countable group $\Lambda$ is cohomologous to a homomorphism $\delta:\Gamma\rightarrow\Lambda$. As a consequence, we deduce that the action $\Gamma\curvearrowright G$ is orbit equivalent superrigid: any free nonsingular action $\Lambda\curvearrowright Y$ which is orbit equivalent to $\Gamma\curvearrowright G$, is necessarily conjugate to an induction of $\Gamma\curvearrowright G$.