Domain wall interpretation of the PTA signal confronting black hole overproduction

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). By applying percolation theory, we pioneer an estimation of the PBH abundance originating from DW networks. We conduct a Bayesian analysis of the PTA signal, interpreting it as an outcome of SGWB from DW networks, accounting for PBH overproduction as a prior. We included contributions from supermassive black hole binaries along with their astrophysical priors. Our findings indicate that DWs, as the proposed source of the PTA signal, result in the production of PBHs about ten times heavier than the sun. 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 if the correlation length of the DW network is greater than approximately 60$\%$ than the cosmic horizon, $L \gtrsim 0.6 t$.Comment: Major improvements of the PBH formation modeling, of the depth of the Bayesian analysis and of the SMBH binary prior (5 pages, 4 figures + appendix, 21 pages in total