Gravitational wave background from Standard Model physics: Qualitative
  features
  Because of physical processes ranging from microscopic particle collisions to
macroscopic hydrodynamic fluctuations, any plasma in thermal equilibrium emits
gravitational waves. For the largest wavelengths the emission rate is
proportional to the shear viscosity of the plasma. In the Standard Model at T >
160 GeV, the shear viscosity is dominated by the most weakly interacting
particles, right-handed leptons, and is relatively large. We estimate the order
of magnitude of the corresponding spectrum of gravitational waves. Even though
at small frequencies (corresponding to the sub-Hz range relevant for planned
observatories such as eLISA) this background is tiny compared with that from
non-equilibrium sources, the total energy carried by the high-frequency part of
the spectrum is non-negligible if the production continues for a long time. We
suggest that this may constrain (weakly) the highest temperature of the
radiation epoch. Observing the high-frequency part directly sets a very
ambitious goal for future generations of GHz-range detectors.
