  We consider a quantum quench in which two initially independent condensates
are suddenly coupled, and study the subsequent "rephasing" dynamics. For weak
couplings, the time-evolution of physical observables is predicted to follow
universal scaling laws, connecting the short-time dynamics to the long-time
non-perturbative regime. We first present a two-mode model valid in two and
three dimensions and then move to one dimension, where the problem is described
by a gapped Sine-Gordon theory. Combining analytical and numerical methods, we
compute universal time-dependent expectation values, allowing a quantitative
comparison with future experiments.
