Energy decay in three-dimensional freely cooling granular gas

The kinetic energy of a freely cooling granular gas decreases as a power law $t^{-\theta}$ at large times $t$. Two theoretical conjectures exist for the exponent $\theta$. One based on ballistic aggregation of compact spherical aggregates predicts $\theta= 2d/(d+2)$ in $d$ dimensions. The other based on Burgers equation describing anisotropic, extended clusters predicts $\theta=d/2$ when $2\le d \le 4$. We do extensive simulations in three dimensions to find that while $\theta$ is as predicted by ballistic aggregation, the cluster statistics and velocity distribution differ from it. Thus, the freely cooling granular gas fits to neither the ballistic aggregation or a Burgers equation description.