  Thermal fluctuations in the early universe plasma and in very hot
astrophysical objects are an unavoidable source of gravitational waves (GW).
Differently from previous studies on the subject, we approach this problem
using methods based on field theory at finite temperature. Such an approach
allows to probe the infrared region of the spectrum where dissipative effects
are dominant. Incidentally, this region is the most interesting from the point
of view of the detectability perspectives. We find significant deviations from
a Planck spectrum.
