  We provide a detailed description of the nonequilibrium time evolution of an
interacting homogeneous Bose-Einstein condensate. We use a nonperturbative
in-medium quantum field theory approach as a microscopic model for the Bose
gas. The real-time dynamics of the condensate is encoded in a set of
self-consistent equations which corresponds to an infinite sum of ladder
Feynman diagrams. The crucial role played by the interaction between
fluctuations for the instability generation is thoroughly described.
