  Quantum fields in cosmological spacetimes can experience particle production
due to their interaction with the expanding background. This effect is
particularly relevant for models of the very early Universe, when the energy
density generated through this process may back-react on the cosmological
expansion. Yet, these scenarios have not been fully explored due to the several
technical hurdles imposed by the back-reaction calculations. In this work we
review the basics of cosmological quantum particle production and demonstrate a
numerical algorithm to solve the back-reaction problem in regimes dominated by
particle production. As an illustration, we compute the effects of a massive
quantized scalar field on a cosmological bounce scenario, explicitly showing
that quantum particle production can cause the contracting phase to end in a
radiation crunch, or can delay the bounce. Finally, we discuss the relevance of
quantum particle production/annihilation to bounce and inflationary models of
the early Universe.
