Nano-Hz gravitational wave signature from axion dark matter

Understand

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.