On the decay of correlations in Sinai billiards with infinite horizon

We compute the decay of the autocorrelation function of the observable |vx| in the Sinai billiard and of the observable vx in the associated Lorentz gas with an approximation due to Baladi, Eckmann and Ruelle. We consider the standard configuration where the disks is centered inside a unit square. The asymptotic decay is found to be C(t) ∼ c(R)/t. An explicit expression is given for the prefactor c(R) as a function of the radius of the scatterer. For the small scatterer case we also present expressions for the preasymptotic regime. Our findings are supported by numerical computations.