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arXiv · 2307.04582

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

E. Babichev·D. Gorbunov·S. Ramazanov·R. Samanta·A. Vikman
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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.

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E. Babichev, D. Gorbunov, S. Ramazanov, R. Samanta, A. Vikman. 2023-07-10. NANOGrav spectral index $\gamma=3$ from melting domain walls. https://doi.org/10.1103/physrevd.108.123529

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