Gravitational wave background from Standard Model physics: qualitative features

Abstract

Because of physical processes ranging from microscopic particle collisions to\nmacroscopic hydrodynamic fluctuations, any plasma in thermal equilibrium emits\ngravitational waves. For the largest wavelengths the emission rate is\nproportional to the shear viscosity of the plasma. In the Standard Model at T >\n160 GeV, the shear viscosity is dominated by the most weakly interacting\nparticles, right-handed leptons, and is relatively large. We estimate the order\nof magnitude of the corresponding spectrum of gravitational waves. Even though\nat small frequencies (corresponding to the sub-Hz range relevant for planned\nobservatories such as eLISA) this background is tiny compared with that from\nnon-equilibrium sources, the total energy carried by the high-frequency part of\nthe spectrum is non-negligible if the production continues for a long time. We\nsuggest that this may constrain (weakly) the highest temperature of the\nradiation epoch. Observing the high-frequency part directly sets a very\nambitious goal for future generations of GHz-range detectors.\n