The QCD Axion and Gravitational Waves in light of NANOGrav results

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration has recently reported strong evidence for a stochastic process affecting the 12.5 yr dataset of pulsar timing residuals. We show that the signal can be interpreted in terms of a stochastic gravitational wave background emitted from a network of axionic strings in the early Universe. The spontaneous breaking of the Peccei-Quinn symmetry originate the axionic string network and the QCD axion, the dark matter particle in the model. We explore a non-standard cosmological model driven by an exotic scalar field $\phi$ which evolves under the influence of a self-interacting potential; the axion field starts t…