  Free cooling of a gas of inelastically colliding hard spheres represents a
central paradigm of kinetic theory of granular gases. At zero gravity the
temperature of a freely cooling homogeneous granular gas follows a power law in
time. How does gravity, which brings inhomogeneity, affect the cooling? We
combine molecular dynamics simulations, a numerical solution of hydrodynamic
equations and an analytic theory to show that a granular gas cooling under
gravity undergoes thermal collapse: it cools down to zero temperature and
condenses on the bottom of the container in a finite time.
