Mirror QCD phase transition as the origin of the nanohertz Stochastic Gravitational-Wave Background

TL;DR

This paper proposes that a first-order phase transition in the mirror QCD epoch could produce a gravitational-wave background matching PTA observations and potentially resolve cosmological tensions, linking dark matter, gravitational waves, and cosmology.

Contribution

It introduces a novel connection between mirror QCD phase transition and the nHz gravitational-wave background, also addressing cosmological tensions within a unified framework.

Findings

SGWB signal from mirror QCD phase transition explains PTA data.

Parameter space can resolve H0 and S8 tensions.

Next-generation CMB experiments can test the model.

Abstract

Several Pulsar Timing Array (PTA) collaborations have recently provided strong evidence for a nHz Stochastic Gravitational-Wave Background (SGWB). Here 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 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 of these three phenomena for $0.2 < \Delta N_{\rm eff} < 0.5$, where the mirror dark matter component constitutes about $30\%$ of the total dark matter abundance. Next generation CMB experiment such as CMB-S4 is able to test this parameter region.