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, the initial growth of the flow is independent of whether the matter is composed of fields or particles, equilibrated or not, or whether the stress-energy tensor is isotropic. By comparing several models, it appears that this equivalence extends for times of the order of 1 fm/c. This is sufficiently long to allow one to initialize the flow in hydrodynamic calculations independently of the early physics, and reduces the uncertainty involved in modeling the collision.