On the Lattice of Boundaries and the Entropy Spectrum of Hyperbolic Groups

Abstract

Let $\Gamma$ be a non-elementary hyperbolic group and $\mu$ be a probability on $\Gamma$. We study the $\mu$-proximal, stationary actions, also known as boundary actions, of $\Gamma$. In particular, we are interested in the spectrum of Furstenberg entropies of $(\Gamma,\mu)$-boundaries, and the lattice-theoretic and topological structure of the set $\mathcal{BL}(\Gamma,\mu)$ of boundaries. We prove that all hyperbolic groups have infinitely many distinct boundaries, which attain an infinite set of distinct entropies. Additionally, for simple random walks on non-abelian free groups $F_d$, we establish that there are infinitely many boundaries whose entropy is greater than $\frac{1}{2}-\epsilon$ times the entropy of Poisson boundary, when the rank $d$ is large. General results of independent interest about the order-theoretic and continuity properties of Furstenberg entropy for countable groups are attained along the way. This includes the result that under mild assumptions, the spectrum of boundary entropies $\mathcal{H}_{\text{bound}}(\Gamma,\mu)$ is closed.