Nano-Hz gravitational wave signature from axion dark matter

TL;DR

This paper models the gravitational wave background generated by axion dark matter, revealing a detectable circular polarization signature that could explain recent NANOGrav observations.

Contribution

It introduces a nonlinear lattice simulation approach to accurately compute the gravitational wave spectrum from axion-induced gauge fields, improving upon previous linear analyses.

Findings

Predicts a gravitational wave signal detectable by SKA.

Shows the spectrum can potentially explain NANOGrav data.

Identifies the importance of nonlinear effects in gauge field dynamics.

Abstract

We calculate the accurate spectrum of the stochastic gravitational wave background from U(1) gauge fields produced by axion dark matter. The explosive production of gauge fields soon invalidates the applicability of the linear analysis and one needs nonlinear schemes. We make use of numerical lattice simulations to properly follow the nonlinear dynamics such as backreaction and rescattering which gives important contributions to the emission of gravitational waves. It turns out that the axion with the decay constant $f \sim 10^{16}$ GeV which gives the correct dark matter abundance predicts the circularly polarized gravitational wave signature detectable by SKA. We also show that the resulting gravitational wave spectrum has a potential to explain NANOGrav 12.5 year data.