  A numerical molecular dynamics experiment measuring the two-time correlation
function of the transversal velocity field in the homogeneous cooling state of
a granular gas is reported. By measuring the decay rate and the amplitude of
the correlations, the accuracy of the Landau-Langevin equation of fluctuating
hydrodynamics is checked. The results indicate that although a Langevin
approach can be valid, the fluctuation-dissipation relation must be modified,
since the viscosity parameter appearing in it differs from the usual
hydrodynamic shear viscosity.
