Gravitational wave background from Standard Model physics: Qualitative features

Understand

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.