  The tensor modes reentering the Hubble radius when the plasma is dominated by
a stiff fluid lead to a spectral energy density whose blue slope depends on the
total post-inflationary sound speed. This result gets however corrected by a
secondary (gauge-dependent) term coming from the curvature inhomogeneities that
reenter all along the same stage of expansion. In comparison with the
first-order result, the secondary contribution is shown to be always suppressed
inside the sound horizon and its effect on the total spectral energy density of
the relic gravitons is therefore negligible for all phenomenological purposes.
It is also suggested that the effective anisotropic stress of the curvature
inhomogeneities can be obtained from the functional derivative of the
second-order action of curvature inhomogeneities with respect to the background
metric.
