Metastable Strings and Gravitational Waves Enable Stochastic Background Origin in One-Scale Models

The recent detection of a persistent, low-frequency gravitational-wave background by Pulsar Timing Array experiments suggests the existence of cosmic strings, and new research explores how these structures might originate from relatively simple dark sector models. James Ingoldby, Valentin V. Khoze, and Jessica Turner, all from Durham University, investigate metastable cosmic strings formed through a single-stage symmetry breaking process driven by a single Higgs field. Their work demonstrates that these strings, while initially stable, decay through the creation of monopole-antimonopole pairs, and crucially, the team delineates the specific parameter space where this decay rate reproduces the observed gravitational-wave signal. This achievement provides a minimal and predictive framework for understanding the origin of the observed background, avoiding the need for more complex models involving multiple Higgs fields or symmetry breaking stages.

Investigations explore various potential sources, refine data analysis techniques, and collaborate to interpret the signals. Studies delve into conventional astrophysical sources, including supermassive black hole binaries and primordial black holes, as well as hypothetical sources like cosmic strings and scalar induced gravitational waves generated during inflation. Beyond conventional astrophysics, research explores physics beyond the Standard Model and early universe scenarios, including first-order phase transitions, cosmic inflation, and Grand Unified Theories.

Investigations also explore supersymmetry, axions, and breaking of B-L symmetry as potential sources, scrutinizing theoretical models like string theory and metastable strings. Researchers are examining the dynamics of these defects, including their interactions and decay. A key theme uniting these investigations is the prevalence of topological defects, such as cosmic strings, domain walls, and monopoles, predicted by many beyond-the-Standard-Model theories. The detection of gravitational waves provides a new window into the very early universe, allowing scientists to probe physics at extremely high energies. This work explores a specific model featuring an electroweak-like dark sector with a single-stage symmetry breaking, demonstrating how such a configuration can naturally generate these strings and predict their decay rate. Researchers calculated the classical string tension and established a dimensionless parameter to describe this connection, mapping theoretical parameters to observable quantities. Specifically, the calculations confirm the reliability of a simplified approach throughout the relevant parameter space, without requiring complex extensions to the model. Scientists constructed solutions for these strings, embedded within a model featuring a single symmetry-breaking scale, and calculated their decay rate. By mapping the resulting parameters onto the ranges indicated by PTA analyses, they identified overlapping regions where these classically stable strings reproduce the observed gravitational wave signal. This work establishes a connection between theoretical models of dark sectors and observational data from gravitational wave astronomy.

The team verified the reliability of a simplified approach throughout the relevant parameter space, ensuring the accuracy of their calculations. They showed that the physics of these defects is governed by a single scale, contrasting with more complex scenarios. The research provides a robust framework for exploring the connection between dark sector physics and the observed gravitational wave background, paving the way for future studies with more complex models and refined observational data.