  Extending previous results [Phys. Rev.D {\bf 56}, 6351 (1997)], we estimate
the back reaction of cosmological gravitons in the expansion dynamics, during
the matter age. Tensor perturbations with scales larger than the Hubble radius
are created due to the inequivalence of vacua at different times.
  During noninflationary phases these perturbations become effective
gravitational waves as they enter the Hubble radius, adding new contributions
to the energy density of the subhorizon waves. During the radiation epoch the
creation of these effective gravitons may lead to a departure from the standard
behaviour $a(t)\propto t^{1/2}$, with possible consequences to several cosmic
processes, such as primordial nucleosynthesis. We examine the implications of
this phenomenom during the matter dominated era, assuming an initial
inflationary period. The dynamical equation obeyed by the scale factor is
derived and numerically solved for different values of the relevant parameters
involved.

Astrophysics