  We propose a new scenario characterizing the transition of the quark-gluon
plasma (QGP) produced in heavy-ion collisions from a highly non-equilibrium
state at early times toward a fluid described by hydrodynamics at late times.
We develop an analogy to the evolution of a quantum mechanical system that is
governed by the instantaneous ground states. In the simplest case, these slow
modes are "pre-hydrodynamic" in the sense that they are initially distinct
from, but evolve continuously into, hydrodynamic modes. For a class of
collision integrals, the pre-hydrodynamic mode represents the angular
distribution (in momentum space) of those gluons that carry most of the energy.
We illustrate this scenario using a kinetic description of weakly-coupled
Bjorken expanding plasma. Rapid longitudinal expansion drives a reduction in
the degrees of freedom at early times. In the relaxation time approximation for
the collision integral, we show quantitatively that the full kinetic theory
evolution is dominated by the pre-hydrodynamic mode. We elaborate on the
criterion for the dominance of pre-hydrodynamic slow modes and speculate that
adiabatic hydrodynamization may describe the pre-equilibrium behavior of the
QGP produced in heavy-ion collisions.
