  Whether, how, and to what extent solutions of Bjorken-expanding systems
become insensitive to aspects of their initial conditions is of importance for
heavy-ion collisions. Here we study 1+1D and phenomenologically relevant
boost-invariant 3+1D systems in which initial conditions approach a universal
attractor solution. In Israel-Stewart theory (IS) and kinetic theory where the
universal attractor extends to arbitrarily early times, we show that all
initial conditions approach the attractor at early times by a power-law while
their approach is exponential at late times. In these theories, the physical
mechanisms of hydrodynamization operational at late times do not drive the
approach to the attractor at early times, and the early-time attractor is
reached prior to hydrodynamization. In marked contrast, the attractor in
strongly coupled systems is realized concurrent with hydrodynamization. This
qualitative difference may offer a basis for discriminating weakly and strongly
coupled scenarios of heavy-ion collisions.
