Observing Invisible Axions with Gravitational Waves

TL;DR

This paper predicts that ultralight axion-like particles can produce a detectable gravitational wave background if the Peccei-Quinn symmetry is restored after inflation, with specific spectral features and broad frequency coverage.

Contribution

It combines effective field theory and numerical simulations to show how axion strings generate a distinctive gravitational wave spectrum with observable implications.

Findings

The gravitational wave spectrum has logarithmic deviations from scale invariance.

A single ultralight axion-like particle with certain parameters produces an observable signal.

The spectrum spans a wide frequency range, accessible to multiple experiments.

Abstract

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 $10^{14}~{\rm GeV}$ and any mass between $10^{-18}~{\rm eV}$ and $10^{-28}~{\rm eV}$ 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.