NANOGrav spectral index <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi><mml:mo>=</mml:mo><mml:mn>3</mml:mn></mml:math> from melting domain walls
Eugeny Babichev, Dmitry Gorbunov, Sabir Ramazanov, Rome Samanta, Alexander Vikman
Abstract
We discuss cosmic domain walls described by a tension redshifting with the expansion of the Universe. These melting domain walls emit gravitational waves with the low-frequency spectral shape ${\mathrm{\ensuremath{\Omega}}}_{gw}\ensuremath{\propto}{f}^{2}$ corresponding to the spectral index $\ensuremath{\gamma}=3$ favored 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 ultralight range below ${10}^{\ensuremath{-}11}\ensuremath{-}{10}^{\ensuremath{-}12}\text{ }\text{ }\mathrm{eV}$ which is accessible through planned observations thanks to the effects of superradiance of rotating black holes.