Observing invisible axions with gravitational waves

Journal article

If the Peccei-Quinn symmetry associated to an axion has ever been restored after inflation, axion strings inevitably produce a contribution to the stochastic gravitational wave background. Combining effective field theory analysis with numerical simulations, we show that the resulting gravitational wave spectrum has logarithmic deviations from a scale invariant form with an amplitude that is significantly enhanced at low frequencies. As a result, a single ultralight axion-like particle with a decay constant larger than 10¹⁴ GeV and any mass between 10^-18 eV and 10^-28 eV leads to an observable gravitational wave spectrum and is compatible with constraints on the post-inflationary scenario from dark matter overproduction, isocurvature and dark radiation. Since the spectrum extends over a wide range of frequencies, the resulting signal could be detected by multiple experiments. We describe straightforward ways in which the Peccei-Quinn symmetry can be restored after inflation for such decay constants. We also comment on the recent possible NANOgrav signal in light of our results.

Authors: Gorghetto, M; Hardy, E; Nicolaescu, H

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: Journal of Cosmology and Astroparticle Physics
• Volume: 2021
• Issue: 06
• Article number: 034
• Publication date: 2021-06-17
• Acceptance date: 2021-04-19
• DOI: 10.1088/1475-7516/2021/06/034
• EISSN: 1475-7516
• Language: English
• Keywords: gravitational waves / theory; cosmology of theories beyond the SM; Cosmic strings; monopoles; domain walls; axions