Cosmic strings are stable topological defect solutions of field theories which may have formed in symmetry breaking phase transitions in the early Universe.
The detection or non-detection of these relics could constrain the physics of very high energies.
In an expanding and decelerating universe, a cosmic string network relaxes towards an attractor configuration exhibiting universal properties — known as a scaling solution — and it subsequently remains self-similar with the Hubble radius.
Hence if cosmic strings were formed in phase transitions early in the history of the universe, scaling implies that they should be present all over the sky with a surface density growing with redshift z.
Strings induce anisotropies in the Cosmic Microwave Background (CMB) and they have been searched for in the Planck data.
However, CMB photons come from the highest observable redshift set by their last scattering surface.
For gravitons, z is only bounded by our understanding of the Friedmann-Lemai^tre model, or more probably by the redshift at which cosmic inflation ended.
For this reason, the stochastic gravitational wave background (SGWB) is an observable particularly sensitive to cosmic strings and could provide the opportunity for a first detection.