  Coarsening of an isolated far-from-equilibrium quantum system is a
paradigmatic many-body phenomenon, relevant from subnuclear to cosmological
lengthscales, and predicted to feature universal dynamic scaling. Here, we
observe universal scaling in the coarsening of a homogeneous two-dimensional
Bose gas, with exponents that match analytical predictions. For different
initial states, we reveal universal scaling in the experimentally accessible
finite-time dynamics by elucidating and accounting for the
initial-state-dependent prescaling effects. The methods we introduce establish
direct comparison between cold-atom experiments and non-equilibrium field
theory, and are applicable to any study of universality far from equilibrium.
