  It is frequently supposed that quark-gluon plasma created in heavy-ion
collisions undergoes free streaming at early times. We examine this issue based
on the assumption that a universal attractor dominates the dynamics already at
the earliest stages, which offers a way to connect the initial state with the
start of the hydrodynamic expansion in an approximate but conceptually
transparent fashion. We demonstrate that the centrality dependence of the
measured particle multiplicities can be used to quantitatively constrain the
pressure anisotropy and find that it strongly depends on the model of the
initial energy deposition. As an illustration, we compare three initial state
models and show that they predict rather different early-time values of the
pressure anisotropy. This strongly suggests that assuming free streaming prior
to hydrodynamization is not necessarily compatible with a generic initial state
model and that features of the pre-hydrodynamic flow need to be matched with
the model of the initial state.
