Average diffusion rate of Ehrenfest Wind-tree billiards

Abstract

One of the versions of the wind-tree model of Boltzmann gas, suggested by Paul and Tatiana Ehrenfest more than a century ago, can be seen as a billiard in the plane endowed with $\mathbb{Z}\oplus\mathbb{Z}$-periodic rectangular obstacles. In the breakthrough paper by V. Delecroix, P. Hubert and S. Lelievre the authors proved, that the diffusion rate of trajectories in such a billiard is equal to $\frac{2}{3}$, that is the \textit{maximal} distance from the origin achieved by a point of a typical trajectory on a segment of time $[0,t]$ grows roughly as $t^\frac{2}{3}$ for large $t$. Here $\frac{2}{3}$ is the Lyapunov exponent of the associated renormalizing dynamical system. This pioneering result does not tell, however, whether trajectories spend most of the time close or far from the initial point. In the current paper, we prove that the \textit{average} distance from the origin grows with the same rate $t^\frac{2}{3}$. In plain terms, it means that trajectories mostly stay as far as possible from the initial point (though, it is known that the wind-tree billiard is recurrent, so trajectories occasionally pass close to the initial point). More generally, fundamental rigidity results by A.Eskin and M.Mirzakhani completed by certain genericity results by J.Chaika and A.Eskin imply that the diffusion rate of almost all flat geodesic rays on any $\mathbb{Z}^d$-cover of a closed translation surface $S$ is given by certain Lyapunov exponent of the Kontsevich--Zorich cocycle on the $\text{SL}_2(\mathbb{R})$-orbit closure of $S$. In this paper we prove that in this most general setting, the \textit{average} and \textit{maximum} diffusion rates coincide.