Observing Invisible Axions with Gravitational Waves

Understand

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.