  Preheating and other particle production phenomena in the early Universe can
give rise to high- energy out-of-equilibrium fermions with an anisotropic
stress. We develop a formalism to calculate the spectrum of gravitational waves
due to fermions, and apply it to a variety of scenarios after inflation. We pay
particular attention to regularization issues. We show that fermion production
sources a stochastic background of gravitational waves with a significant
amplitude, but we find that typical frequencies of this new background are not
within the presently accessible direct detec- tion range. However,
small-coupling scenarios might still produce a signal observable by planned
detectors, and thus open a new window into the physics of the very early
Universe.
