Collisions between heavy ions at ultrarelativistic energies, used as a probe of the deconfined phase of nuclear matter, are remarkably well described by models that assume that this matter expands as a fluid of extremely low viscosity. It has however turned out to be extremely difficult to understand from the underlying Quantum Chromodynamics why such a description works so well.

In this talk, I will present some recent work that aims at studying from first principles the very first moments after such a collision, in order to attempt to bridge the gap between QCD and the observed hydrodynamical behavior.