Noncommutative Poisson boundaries and Furstenberg-Hamana boundaries of Drinfeld doubles
We clarify the relation between noncommutative Poisson boundaries and Furstenberg-Hamana boundaries of quantum groups. Specifically, given a compact quantum group $G$, we show that in many cases where the Poisson boundary of the dual discrete quantum group $\hat G$ has been computed, the underlying topological boundary either coincides with the Furstenberg-Hamana boundary of the Drinfeld double $D(G)$ of $G$ or is a quotient of it. This includes the $q$-deformations of compact Lie groups, free orthogonal and free unitary quantum groups, quantum automorphism groups of finite dimensional C$^*$-algebras. In particular, the boundary of $D(G_q)$ for the $q$-deformation of a compact connected semisimple Lie group $G$ is $G_q/T$ (for $q\ne1$), in agreement with the classical results of Furstenberg and Moore on the Furstenberg boundary of $G_{\mathbb C}$. We show also that the construction of the Furstenberg-Hamana boundary of $D(G)$ respects monoidal equivalence and, in fact, can be carried out entirely at the level of the representation category of $G$. This leads to a notion of the Furstenberg-Hamana boundary of a rigid C$^*$-tensor category.