  In conformally flat background geometries the long wavelength gravitons can
be described in the fluid approximation and they induce scalar fluctuations
both during inflation and in the subsequent radiation-dominated epoch. While
this effect is minute and suppressed for a de Sitter stage of expansion, the
fluctuations of the energy-momentum pseudo-tensor of the graviton fluid lead to
curvature perturbations that increase with time all along the post-inflationary
evolution. An explicit calculation of these effects is presented for a standard
thermal history and it is shown that the growth of the curvature perturbations
caused by the long wavelength modes is approximately compensated by the slope
of the power spectra of the energy density, pressure and anisotropic stress of
the relic gravitons.
