Universal Flow in the First Stage of Relativistic Heavy Ion Collisions
  In the first moments of a relativistic heavy ion collision explosive collective flow begins to grow before the matter has yet equilibrated. Here it is found that as long as the stress-energy tensor is traceless, early flow is independent of whether the matter is composed of fields or particles, equilibrated or not, or whether the stress-energy tensor is isotropic. This eliminates much of the uncertainty in modeling early stages of a collision. 
Joshua Vredevoogd and Scott Pratt