Exact Scale-Invariant Background of Gravitational Waves from Cosmic Defects

TL;DR

Any scaling source during the radiation era, such as cosmic defects, produces an exact scale-invariant gravitational wave background, regardless of defect topology or phase transition details.

Contribution

Demonstrates the universality of scale-invariant gravitational wave spectra from scaling sources and provides numerical simulations for global O(N) theories.

Findings

Scale-invariant GW spectrum from cosmic defects confirmed

Simulation results approach large N theoretical predictions

Global cosmic strings produce significantly larger signals

Abstract

We demonstrate that any scaling source in the radiation era produces a background of gravitational waves with an exact scale-invariant power spectrum. Cosmic defects, created after a phase transition in the early Universe, are such a scaling source. We emphasise that the result is independent of the topology of the cosmic defects, the order of phase transition, and the nature of the symmetry broken, global or gauged. As an example, using large-scale numerical simulations, we calculate the scale invariant gravitational wave power spectrum generated by the dynamics of a global O(N) scalar theory. The result approaches the large N theoretical prediction as N^(-2), albeit with a large coefficient. The signal from global cosmic strings is O(100) times larger than the large N prediction.