  Renormalization-group methods provide a viable approach for investigating the
emergent collective behavior of classical and quantum statistical systems in
both equilibrium and nonequilibrium conditions. Within this approach we
investigate here the dynamics of an isolated quantum system represented by a
scalar $\phi^4$ theory after a global quench of the potential close to a
dynamical critical point. We demonstrate that, within a pre-thermal regime, the
time dependence of the relevant correlations is characterized by a short-time
universal exponent, which we calculate at the lowest order in a dimensional
expansion.
