NANOGrav spectral index $\gamma=3$ from melting domain walls

TL;DR

This paper proposes that melting domain walls in the early Universe emit gravitational waves with a spectral index matching NANOGrav's recent data, linking high-energy physics, dark matter, and gravitational wave observations.

Contribution

It introduces a high-energy physics scenario involving a feebly coupled scalar field that produces melting domain walls, explaining the NANOGrav gravitational wave spectral shape and suggesting ultra-light dark matter.

Findings

Gravitational wave spectrum with spectral index $oldsymbol{}$ matches NANOGrav data.

Model predicts ultra-light dark matter below $10^{-11}-10^{-12}$ eV.

Parameter space aligns with gravitational wave observations and superradiance effects.

Abstract

We discuss cosmic domain walls described by a tension red-shifting with the expansion of the Universe. These melting domain walls emit gravitational waves with the low-frequency spectral shape $\Omega_{gw}\propto f^{2}$ corresponding to the spectral index $\gamma=3$ favoured by the recent NANOGrav 15 yrs data. We discuss a concrete high-energy physics scenario leading to such a melting domain wall network in the early Universe. This scenario involves a feebly coupled scalar field, which can serve as a promising dark matter candidate. We identify parameters of the model matching the gravitational wave characteristics observed in the NANOGrav data. The dark matter mass is pushed to the ultra-light range below $10^{-11}-10^{-12}\,\text{eV}$ which is accessible through planned observations thanks to the effects of superradiance of rotating black holes.