Gravitational Waves as a Big Bang Thermometer

Understand

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.