  A Bose gas in a double-well potential, exhibiting a true Bose-Einstein
condensate (BEC) amplitude and initially performing Josephson oscillations, is
a prototype of an isolated, non-equilibrium many-body system. We investigate
the quasiparticle (QP) creation and thermalization dynamics of this system by
solving the time-dependent Keldysh-Bogoliubov equations. We find avalanche-like
QP creation due to a parametric resonance between BEC and QP oscillations,
followed by slow, exponential relaxation to a thermal state at an elevated
temperature, controlled by the initial excitation energy of the oscillating BEC
above its ground state. The crossover between the two regimes occurs because of
an effective decoupling of the QP and BEC oscillations. This dynamics is
analogous to elementary particle creation in models of the early universe. The
thermalization in our set-up occurs because the BEC acts as a grand canonical
reservoir for the quasiparticle system.
