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Gravitational waves from gauge quanta produced during inflation

Kai-Ge Zhang1,2,*, Jian-Feng He3,4,†, Chengjie Fu5,‡, and Zong-Kuan Guo3,4,6,§

  • *Contact author: zhangkaige21@mails.ucas.ac.cn
  • †Contact author: hejianfeng@itp.ac.cn
  • ‡Contact author: fucj@ahnu.edu.cn
  • §Contact author: guozk@itp.ac.cn
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Phys. Rev. D 112, 083530 – Published 16 October, 2025

DOI: https://doi.org/10.1103/p1xn-9y3c

Abstract

A fast-rolling axion can transfer its kinetic energy to a gauge field via the Chern-Simons coupling, leading to copious production of gauge quanta, which can act as a source of gravitational waves (GWs) with potentially observable amplitudes. In this work, we investigate GW production in a spectator axion model when strong backreaction is taken into account. We find that decreasing the decay constant of the axion enhances GW production. Since the initial value of the axion is larger than its quantum fluctuations, such a condition imposes a lower bound on the axion dacay constant, which sets an upper bound on the amplitude of the energy spectrum of GWs. As a result, the amplitude of the predicted GW energy spectrum is lower than 10−10 in the nHz to mHz frequency range.

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Phys. Rev. D 112, 083530– Published 16 October, 2025
  • Received 17 July 2025
  • Accepted 24 September 2025
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