Impact of the small-scale structure on the Stochastic Background of Gravitational Waves from cosmic strings

TL;DR

This paper investigates how small-scale structures on cosmic strings influence the stochastic gravitational wave background, revealing significant effects at high frequencies and classifying expected spectra into four distinct types.

Contribution

It introduces a framework to analyze the impact of small-scale loop populations on the gravitational wave background from cosmic strings, considering different models and uncertainties.

Findings

Extra small loop population affects high-frequency gravitational wave signals.

Four classes of gravitational wave spectra are identified based on models.

Uncertainties in Polchinski-Rocha exponents lead to diverse spectrum predictions.

Abstract

Numerical simulations and analytical models suggest that infinite cosmic strings produce cosmic string loops of all sizes with a given power-law. Precise estimations of the power-law exponent are still matter of debate while numerical simulations do not incorporate all the radiation and back-reaction effects expected to affect the network at small scales. Previously it has been shown, using a Boltzmann approach, that depending on the steepness of the loop production function and the gravitational back-reaction scale, a so-called Extra Population of Small Loops (EPSL) can be generated in the loop number density. We propose a framework to study the influence of this extra population of small loops on the Stochastic Background of Gravitational Waves (SBGW). We show that this extra population can have a significant signature at frequencies higher than $H_0(\Gamma G\mu)^{-1}$ where $\Gamma$ is of order $50$ and $H_0$ is the Hubble constant. We propose a complete classification of the gravitational wave power spectra expected from cosmic strings into four classes, including the model of Blanco-Pillado, Olum and Shlaer and the model of Lorenz, Ringeval and Sakellariadou. Finally we show that given the uncertainties on the Polchinski-Rocha exponents, two hybrid classes of gravitational wave power spectrum can be considered giving very different predictions for the SBGW.