Double-graviton production from Standard Model plasma

J. Ghiglieri, M. Laine, J. Schütte-Engel, E. Speranza.

16 Jan 2024.

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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, $T^2_{ } / m_{\rm pl}^2$ , then the lowest-order contributions come from single ( $\sim \alpha T^2_{ }/m_{\rm pl}^2$ ) and double ( $\sim T^4_{ }/m_{\rm pl}^4$ ) 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.