Crescendo Beyond the Horizon: More Gravitational Waves from Domain Walls Bounded by Inflated Cosmic Strings

TL;DR

This paper explores a novel gravitational wave signal generated by a network of domain walls bounded by cosmic strings, produced during a two-step phase transition in the early universe, with potential detectability by future GW observatories.

Contribution

It introduces a new scenario where a hierarchy of symmetry-breaking scales and inflation produce a distinctive GW spectrum from string-wall networks, linking it to supersymmetric models.

Findings

The GW signal from the string-wall network can be strong and broad in frequency.

The model predicts GW signals detectable by pulsar timing arrays and space/ground-based observatories.

Detection of the GW peak can reveal the supersymmetry breaking scale and wall tension.

Abstract

Gravitational-wave (GW) signals offer a unique window into the dynamics of the early universe. GWs may be generated by the topological defects produced in the early universe, which contain information on the symmetry of UV physics. We consider the case in which a two-step phase transition produces a network of domain walls bounded by cosmic strings. Specifically, we focus on the case in which there is a hierarchy in the symmetry-breaking scales, and a period of inflation pushes the cosmic string generated in the first phase transition outside the horizon before the second phase transition. We show that the GW signal from the evolution and collapse of this string-wall network has a unique spectrum, and the resulting signal strength can be sizeable. In particular, depending on the model parameters, the resulting signal can show up in a broad range of frequencies and can be discovered by a multitude of future probes, including the pulsar timing arrays and space- and ground-based GW observatories. As an example that naturally gives rise to this scenario, we present a model with the first phase transition followed by a brief period of thermal inflation driven by the field responsible for the second stage of symmetry breaking. The model can be embedded into a supersymmetric setup, which provides a natural realization of this scenario. In this case, the successful detection of the peak of the GW spectrum probes the soft supersymmetry breaking scale and the wall tension.