Double-graviton production from Standard Model plasma

Abstract

The thermal plasma filling the early universe generated a stochastic gravitational wave background that peaks in the microwave frequency range today. If the graviton production rate is expressed as a series in a fine-structure constant, $\alpha$, and the temperature over the Planck mass, $T_{}^2/m_{\mathrm{pl}}^2$, then the lowest-order contributions come from single ($\sim \alpha T_{}^2/m_{\mathrm{pl}}^2$) and double ($\sim T_{}^4/m_{\mathrm{pl}}^4$) graviton production via $2\to 2$ scatterings. We show that in the Standard Model, single-graviton production dominates if the maximal temperature is smaller than $4\times 10_{}^{18}$ GeV. This justifies previous calculations which relied solely on single-graviton production. We mention Beyond the Standard Model scenarios in which the single and double-graviton contributions could be of comparable magnitudes. Finally, we elaborate on what these results imply for the range of applicability of General Relativity as an effective theory.

16 pages. v2: clarifications added

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• Stochastic gravitational wave from graviton bremsstrahlung in inflaton decay into massive spin 3/2 particles