  A central question in heavy-ion collisions is how the initial
far-from-equilibrium medium evolves and thermalizes while it undergoes a rapid
longitudinal expansion. In this work we use the two-particle irreducible (2PI)
effective action for the first time to consider this question, focusing on
$\phi^4$ scalar theory truncated at three loops. We calculate the momentum
distribution of quasiparticles in the medium and show that isotropization takes
place. We furthermore consider the thermal mass of quasiparticles and the
importance of number-changing processes.
