Stochastic background of gravitational waves from fermions

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 oef this new background are not within the presently accessible direct detection 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.

Figure 1

Spectra of GWs at the time of production, in the scenario I with $E_I\sim {10}^{16}\mathrm{GeV}$ and ${\omega }_0\sim {10}^{14}\mathrm{GeV}$, for $q={10}^2$ (dashed line, peaked at $k=8q^{1/4}{\omega }_0$) and $q={10}^6$ (solid line, peaked at $k=6q^{1/4}{\omega }_0$). The spectra in the scenario II for the same parameters look qualitatively the same.