  After a quantum quench, a sudden change of parameters, generic many particle
quantum systems are expected to equilibrate. A few collisions of quasiparticles
are usually sufficient to establish approximately local equilibrium. Reaching
global equilibrium is, however, much more difficult as conserved quantities
have to be transported for long distances to build up a pattern of fluctuations
characteristic for equilibrium. Here we investigate the quantum quench of the
one-dimensional bosonic Hubbard model from infinite to finite interaction
strength U using semiclassical methods for weak, and exact diagonalization for
strong quenches. Equilibrium is approached only slowly, as t^{-1/2} with
subleading corrections proportional to t^{-3/4}, consistent with predictions
from hydrodynamics. We show that these long-time tails determine the relaxation
of a wide range of physical observables.
