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 (G) over cap 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 not equal 1), in agreement with the classical results of Furstenberg and Moore on the Furstenberg boundary of G(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. (C) 2021 The Authors. Published by Elsevier Masson SAS.