  Non-equilibrium Green's functions provide an efficient way to describe the
evolution of the energy-momentum tensor during the early time pre-equilibrium
stage of high-energy heavy ion collisions. Besides their practical relevance
they also provide a meaningful way to address the question when and to what
extent a hydrodynamic description of the system becomes applicable. Within the
kinetic theory framework we derive a new method to calculate time dependent
non-equilibrium Green's functions describing the evolution of energy and
momentum perturbations on top of an evolving far-from-equilibrium background.
We discuss the approach towards viscous hydrodynamics along with the emergence
of various scaling phenomena for conformal systems. By comparing our results
obtained in the relaxation time approximation to previous calculations in
Yang-Mills kinetic theory, we further address the question which macroscopic
features of the energy momentum tensor are sensitive to the underlying
microscopic dynamics.
