Mirror QCD phase transition as the origin of the nanohertz Stochastic Gravitational-Wave Background detected by the Pulsar Timing Arrays

Abstract

Recent independent announcements by several collaborations have shown strong evidence of a Stochastic Gravitational-Wave Background (SGWB) detected through Pulsar Timing Arrays (PTAs). In this study, we investigate the implications of a first-order phase transition occurring within the early universe's dark quantum chromodynamics (dQCD) epoch, specifically within the framework of the mirror twin Higgs dark sector model. Our analysis indicates a distinguishable SGWB signal originating from this phase transition, which can explain the measurements obtained by PTAs. Remarkably, a significant portion of the parameter space within the mirror twin Higgs model that accounts for the SGWB signal also effectively resolves the existing tensions in both the $H_0$ and $S_8$ measurements in Cosmology. This intriguing correlation suggests a possible common origin for these three phenomena. Furthermore, the parameter region, $0.2 < \Delta N_{\rm eff} < 0.5$, where the mirror dark matter component constitutes less than $30\%$ of the total dark matter abundance, can accommodate all current cosmological observations and PTA measurements.Comment: 6 pages, 3 figure