Gravitational Waves as a Big Bang Thermometer

TL;DR

This paper predicts the stochastic gravitational wave background from the early universe's thermal plasma, linking its properties to the maximum temperature and degrees of freedom, and explores detection prospects.

Contribution

It provides a comprehensive prediction of the Cosmic Gravitational Microwave Background for various models, including the Standard Model and extensions like MSSM and SMASH.

Findings

The gravitational wave background peaks around 80 GHz.

Predictions vary across different particle physics models.

Detection prospects depend on laboratory experiment sensitivities.

Abstract

There is a guaranteed background of stochastic gravitational waves produced in the thermal plasma in the early universe. Its energy density per logarithmic frequency interval scales with the maximum temperature $T_{\rm max}$ which the primordial plasma attained at the beginning of the standard hot big bang era. It peaks in the microwave range, at around $80\,{\rm GHz}\,[106.75/g_{*s}(T_{\rm max})]^{1/3}$, where $g_{*s}(T_{\rm max})$ is the effective number of entropy degrees of freedom in the primordial plasma at $T_{\rm max}$. We present a state-of-the-art prediction of this Cosmic Gravitational Microwave Background (CGMB) for general models, and carry out calculations for the case of the Standard Model (SM) as well as for several of its extensions. On the side of minimal extensions we consider the Neutrino Minimal SM ($\nu$MSM) and the SM - Axion - Seesaw - Higgs portal inflation model (SMASH), which provide a complete and consistent cosmological history including inflation. As an example of a non-minimal extension of the SM we consider the Minimal Supersymmetric Standard Model (MSSM). Furthermore, we discuss the current upper limits and the prospects to detect the CGMB in laboratory experiments and thus measure the maximum temperature and the effective number of degrees of freedom at the beginning of the hot big bang.