  We consider the evolution of the gravitational wave spectrum for super-Hubble
modes in interaction with a relativistic fluid, which is regarded as an
effective description of fluctuations in a light scalar minimally coupled
field, during the earliest epoch of the radiation dominated era after the end
of inflation. We obtain the initial conditions for gravitons and fluid from
quantum fluctuations at the end of inflation, and assume instantaneous
reheating. We model the fluid by using relativistic causal hydrodynamics. There
are two dimensionful parameters, the relaxation time $\tau$ and temperature. In
particular we study the interaction between gravitational waves and the non
trivial tensor (spin 2) part of the fluid energy-momentum tensor. Our main
result is that the new dimensionful parameter $\tau$ introduces a new relevant
scale which distinguishes two kinds of super-Hubble modes. For modes with
$H^{-1}<\lambda<\tau$ the fluid-graviton interaction increases the amplitude of
the primordial gravitational wave spectrum at the electroweak transition by a
factor of about $1.3$ with respect to the usual scale invariant spectrum.
