Particle Creation in Bouncing Cosmologies
  We investigate scalar particle creation in a set of bouncing models where the
bounce occurs due to quantum cosmological effects described by the
Wheeler-DeWitt equation. The scalar field can be either conformally or
minimally coupled to gravity, and it can be massive or massless, without self
interaction. The analysis is made for models containing a single radiation
fluid, and for the more realistic case of models containing the usual observed
radiation and dust fluids, which can fit most of the observed features of our
Universe, including an almost scale invariant power spectrum of scalar
cosmological perturbations. In the conformal coupling case, the particle
production is negligible. In the minimal coupling case, for massive particles,
the results point to the same physical conclusion within observational
constraints: particle production is most important at the bounce energy scale,
and it is not sensitive neither to its mass nor whether there is dust in the
background model. The only caveat is the case where the particle mass is larger
than the bounce energy scale. On the other hand, the energy density of produced
massive particles depend on their masses and the energy scale of the bounce.
For very large masses and deep bounces, this energy density may overcome that
of the background. In the case of massless particles, the energy density of
produced particles can become comparable to the background energy density only
for bounces occurring at energy scales comparable to the Planck scale or above,
which lies beyond the scope of this paper: we expect that the simple
Wheeler-DeWitt approach we are using should be valid only at scales some few
orders of magnitude below the Planck energy. Nevertheless, in the case in which
dust is present, there is an infrared divergence, which becomes important only
for scales much larger than today's Hubble radius.
