Haagerup property and group-invariant percolation

Chiranjib Mukherjee, Konstantin Recke

Published 2023-03-30Version 1

Let $\mathcal G$ be the Cayley graph of a finitely generated group $\Gamma$. We show that $\Gamma$ has the Haagerup property if and only if for every $\alpha<1$, there is a $\Gamma$-invariant bond percolation $\mathbb P$ on $\mathcal G$ with $\mathbb E[\mathrm{deg}_{\omega}(g)]>\alpha\mathrm{deg}_{\mathcal G}(g)$ for every vertex $g$ and with the two-point function $\tau(g,h)=\mathbb P[g\leftrightarrow h]$ vanishing as $d(g,h)\to\infty$. Our result is inspired by the characterization of amenability by Benjamini, Lyons, Peres and Schramm [7]. To derive our result, we use the characterization of the Haagerup property in terms of actions on spaces with measured walls in the sense of Cherix, Martin and Valette [11]. Our proof is based on a new construction using invariant point processes on such spaces with measured walls, which leads to quantitative bounds on the two-point functions. These bounds yield in particular exponential decay of the two-point function in several examples, including co-compact Fuchsian groups and lamplighters over free groups. Moreover, our method allows us to strengthen a consequence of Kazhdan's property (T), due to Lyons and Schramm [44], to an {\em equivalence}. Namely, we show that $\Gamma$ has property (T) if and only if there exists a threshold $\alpha^*<1$ such that for every $\Gamma$-invariant bond percolation $\mathbb P$ on $\mathcal G$, $\mathbb E[\mathrm{deg}_\omega(o)]>\alpha^*\mathrm{deg}(o)$ implies that the two-point function is bounded away from zero. We extend this result to the setting of {\em relative} property (T). We then use the corresponding threshold to give a new proof of the fact, already observed by Gaboriau and Tucker-Drob [21], that there is no unique infinite cluster at the uniqueness threshold for Bernoulli bond percolation on Cayley graphs of groups admitting an infinite normal subgroup with relative property (T).