Gravitational wave background from Standard Model physics: Qualitative features

TL;DR

This paper estimates the gravitational wave background produced by Standard Model plasma at high temperatures, highlighting its potential as a probe of early universe conditions and the challenges for future high-frequency detectors.

Contribution

It provides a qualitative estimate of the gravitational wave spectrum from Standard Model physics at high temperatures, emphasizing the role of shear viscosity and high-frequency signals.

Findings

High-frequency gravitational wave background is non-negligible if production lasts long.

The background is tiny at sub-Hz frequencies but significant at GHz-range frequencies.

Observations could constrain the maximum temperature of the early universe.

Abstract

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.