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

Understand

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.