Freezing-In Gravitational Waves

TL;DR

This paper explores the production of stochastic gravitational wave backgrounds in the early universe, emphasizing the role of graviton pair production and quantum gravity effects, and computes the spectrum within a scalar field model.

Contribution

It introduces the concept of gravitational wave freeze-in production, including pair production processes and quantum gravity effects, expanding on previous single graviton production analyses.

Findings

Graviton pair production can significantly enhance the GW background under certain conditions.

Quantum gravity effects are smaller than leading contributions by a factor of $(T_{max}/m_p)^2$.

The CGMB spectrum is explicitly computed within a scalar quartic interaction model.

Abstract

The thermal plasma in the early universe produced a stochastic gravitational wave (GW) background, which peaks today in the microwave regime and was dubbed the cosmic gravitational microwave background (CGMB). In previous works only single graviton production processes that contribute to the CGMB have been considered. Here we also investigate graviton pair production processes and show that these can lead to a significant contribution if the ratio between the maximum temperature and the Planck mass, $T_{\rm max}/m_{\rm p}$, divided by the internal coupling in the heat bath is large enough. As the dark matter freeze-in production mechanism is conceptually very similar to the GW production mechanism from the primordial thermal plasma, we refer to the latter as ``GW freeze-in production''. We show that quantum gravity effects appear in single graviton production and are smaller by a factor $(T_{\rm max}/m_{\rm p})^2$ than the leading order contribution. In our work we explicitly compute the CGMB spectrum within a scalar model with quartic interaction.