  The study of how fast thermalization in heavy ion collisions occurs has been
one of the central topics in the heavy ion community. In the weak coupling
picture this thermalization occurs from "the bottom up": high energy partons,
formed early in the collision, radiate low energy gluons which then proceed to
equilibrate among themselves, forming a thermal bath that brings the high
energy sector to equilibrium. In this scheme we apply a model on parton energy
loss to discuss the effects of medium expansion on the thermalization problem
and estimate the average transverse momentum diffusivity for thermalization in
a Bjorken expanding medium.
