Stochastic gravitational waves from long cosmic strings

TL;DR

This paper calculates a lower bound on the stochastic gravitational wave background from long cosmic strings across cosmic history, using numerical simulations and analytical methods, highlighting the dominant contribution from matter era long strings.

Contribution

It provides a robust lower bound on the gravitational wave signal from cosmic strings by excluding loop contributions and employing Nambu-Goto simulations across all cosmic eras.

Findings

The strain power spectrum peaks on Hubble scales.

High frequency oscillations are observed at large wavenumbers.

Most high frequency power originates from long strings in the matter era.

Abstract

We compute the expected strain power spectrum and energy density parameter of the stochastic gravitational wave background (SGWB) created by a network of long cosmic strings evolving during the whole cosmic history. As opposed to other studies, the contribution of cosmic string loops is discarded and our result provides a robust lower bound of the expected signal that is applicable to most string models. Our approach uses Nambu-Goto numerical simulations, running during the radiation, transition and matter eras, in which we compute the two-point unequal-time anisotropic stress correlators. These ones act as source terms in the linearised equations of motion for the tensor modes, that we solve using an exact Green's function integrator. Today, we find that the rescaled strain power spectrum $(k/\mathcal{H}_0)^2 \mathcal{P}_h$ peaks on Hubble scales and exhibits, at large wavenumbers, high frequency oscillations around a plateau of amplitude $100 (GU)^2$. Most of the high frequency power is generated by the long strings present in the matter era, the radiation era contribution being smaller.