The QCD Axion and Gravitational Waves in light of NANOGrav results

TL;DR

This paper proposes that the gravitational wave background detected by NANOGrav can be explained by axionic strings from the early universe, with a non-standard cosmology affecting the axion mass and GW spectrum, aligning with observations.

Contribution

It introduces a novel interpretation of NANOGrav signals as arising from axionic strings in a non-standard cosmological model, expanding the understanding of dark matter and gravitational waves.

Findings

The NANOGrav signal can be explained by axionic strings in a modified cosmology.

The axion mass is smaller in non-standard cosmology, increasing GW spectrum.

Parameter space aligns with observations within 95 ext{ and }68 ext{ extbackslash } limits.

Abstract

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 to oscillate during the modified cosmology, and provides the dark matter observed. For an equation of state $w_\phi < 1/3$, the QCD axion mass is smaller than expected in the standard cosmology and the GW spectrum from axionic strings is larger. We assess the parameter space of the model which is consistent with the NANOGrav-$12.5\,$yr detection, which can be explained within 95\% limit by a QCD axion field evolving in a dust-like scenario, as well as within 68\% limit in a cosmology with $w_\phi < 0$.