A Rigorous Derivation of Haff's Law for a Periodic Two-Disk Fluid

We derive Haff's cooling law for a periodic fluid consisting of two hard disks per unit cell by reducing it to a point particle moving inside a Sinai billiard with finite horizon with an inelastic collision rule. Indeed, our results also apply to general dispersing billiards with piece-wise smooth boundary with finite horizon and no cusps.