The bispectrum of matter perturbations from cosmic strings

TL;DR

This paper calculates the matter bispectrum induced by cosmic strings using two methods, confirming their agreement, and compares the results to inflationary perturbations, finding limited potential for constraints on cosmic strings.

Contribution

First calculation of the matter bispectrum from cosmic strings using two independent methods, with analysis of compensation factors and comparison to existing models.

Findings

The two methods agree qualitatively and quantitatively.

Compensation factors are essential in both models.

Cosmic string bispectrum is unlikely to constrain string populations effectively.

Abstract

We present the first calculation of the bispectrum of the matter perturbations induced by cosmic strings. The calculation is performed in two different ways: the first uses the unequal time correlators (UETCs) of the string network - computed using a Gaussian model previously employed for cosmic string power spectra. The second approach uses the wake model, where string density perturbations are concentrated in sheet-like structures whose surface density grows with time. The qualitative and quantitative agreement of the two gives confidence to the results. An essential ingredient in the UETC approach is the inclusion of compensation factors in the integration with the Green's function of the matter and radiation fluids, and we show that these compensation factors must be included in the wake model also. We also present a comparison of the UETCs computed in the Gaussian model, and those computed in the unconnected segment model (USM) used by the standard cosmic string perturbation package CMBACT. We compare numerical estimates for the bispectrum of cosmic strings to those produced by perturbations from an inflationary era, and discover that, despite the intrinsically non-Gaussian nature of string-induced perturbations, the matter bispectrum is unlikely to produce competitive constraints on a population of cosmic strings.