Random walks, WPD actions, and the Cremona group

Abstract

We study random walks on groups of isometries of non-proper delta-hyperbolic spaces under the assumption that at least one element in the group satisfies Bestvina-Fujiwara's WPD condition. We show that in this case typical elements are WPD, and the Poisson boundary coincides with the Gromov boundary. Moreover, we show that the random walk satisfies a form of asymptotic acylindricality, and we use this to show that the normal closure of random elements yields almost surely infinitely many different normal subgroups. Moreover, the probability that the normal closure is free tends to 1 if and only if the maximal normal subgroup coincides with the center of the group. We apply such techniques to the Cremona group, thus obtaining that the dynamical degree of random Cremona transformations grows exponentially fast, producing many different normal subgroups, and identifying the Poisson boundary. We also give a new identification of the Poisson boundary of Out(F_n). Our methods give bounds on the rates of convergence for these results.