Rigidity of Kleinian groups via self-joinings: measure theoretic criterion

Abstract: Let $n, m\ge 2$. Let $\Gamma<\text{SO}^\circ(n+1,1)$ be a Zariski dense convex cocompact subgroup. Let $\rho : \Gamma \to \text{SO}^\circ(m+1,1)$ be a Zariski dense convex cocompact faithful representation and $f:\Lambda\to \mathbb{S}^{m}$ the $\rho$-boundary map on the limit set of $\Gamma$. Let $$\Lambda_f:= \bigcup \left{ C \cap \Lambda : \begin{matrix} C \subset \mathbb{S}^n \text{ is a circle such that} \ f(C \cap \Lambda) \text{ is contained in a proper sphere } \text{of } \mathbb{S}^m \end{matrix} \right}.$$ When $\Lambda$ is doubly stable (e.g., $\Omega=\mathbb{S}^n-\Lambda$ is disconnected), we prove the following dichotomy $$\text{either}\quad \Lambda_f= \Lambda \quad \text{ or } \quad \cal H^{\delta}(\Lambda_f) =0,$$ where ${\cal H}^\delta$ is the Hausdorff measure of dimension $\delta=\dim_H \Lambda$. Moreover, in the former case, we have $n=m$ and $\rho$ is a conjugation by a M\"obius transformation on $\mathbb{S}^n$. Our proof uses ergodic theory for directional diagonal flows and conformal measure theory of discrete subgroups of higher rank semisimple Lie groups, applied to the self-joining subgroup $\Gamma_\rho=(\operatorname{id} \times \rho)(\Gamma) < \text{SO}^\circ(n+1,1)\times \text{SO}^\circ(m+1,1)$.