  In ultra-relativistic heavy ion collisions, the matter formed shortly after
the collision is a dense, out of equilibrium, system of gluons characterized by
a semi-hard momentum scale $Q_{\rm s}$. Simple power counting arguments
indicate that this system is over-occupied: the gluon occupation number is
parametrically large when compared to a system in thermal equilibrium with the
same energy density. On short time scales, soft elastic scatterings tend to
drive the system towards the formation of a Bose--Einstein condensate that
contains a large fraction of the gluons while contributing little to the energy
density. The lifetime and existence of this condensate depends on whether
inelastic processes, that occur on the same time scale as the elastic ones,
preferably increase or decrease the number of gluons. During this overpopulated
stage, and all the way to thermalization, the system behaves as a strongly
interacting fluid, even though the elementary coupling constant is small. We
argue that while complete isotropization may never be reached, the system may
yet evolve for a long time with a fixed anisotropy between average longitudinal
and transverse momenta.
