Domain wall interpretation of the PTA signal confronting black hole overproduction

TL;DR

This paper explores how domain wall networks could explain the PTA-detected gravitational wave background, while also predicting the formation of primordial black holes and a secondary high-frequency gravitational wave signal.

Contribution

It provides a Bayesian analysis linking domain wall dynamics to PTA signals and predicts associated primordial black hole production and secondary gravitational waves.

Findings

DWs can produce the observed PTA SGWB without overproducing PBHs.

Primordial black holes formed can be of solar mass and merge to generate detectable high-frequency GWs.

Secondary kilohertz GW signals could be observed by current or future interferometers.

Abstract

Recently, Pulsar Timing Array (PTA) collaborations have detected a stochastic gravitational wave background (SGWB) at nano-Hz frequencies, with Domain Wall networks (DWs) proposed as potential sources. To be cosmologically viable, they must annihilate before dominating the universe energy budget, thus generating a SGWB. While sub-horizon DWs shrink and decay rapidly, causality requires DWs with super-horizon size to continue growing until they reach the Hubble horizon. Those entering the latest can be heavier than a Hubble patch and collapse into Primordial Black Holes (PBHs). We conduct a Bayesian analysis of the PTA signal, interpreting it as an outcome of SGWB from DW networks, with a prior ensuring no PBH overproduction. Our findings indicate that DWs result in the production of solar-mass PBHs. The binary mergers occurring within these PBHs generate a second SGWB in the kilo-Hz domain which could be observable in on-going or planned Earth-based interferometers.