The cusp properties of High Harmonic Loops

TL;DR

This paper statistically analyzes high harmonic cosmic string loops to better understand cusp properties, revealing that cusp numbers can be larger than previously assumed, which impacts gravitational wave signal predictions.

Contribution

It provides a detailed statistical study of cusp sharpness and frequency on high harmonic loops, refining assumptions used in gravitational wave predictions from cosmic strings.

Findings

Sharpness parameters are generally close to unity, with some exceptions.

Cusp number per period scales with the number of harmonics and can be significantly larger.

Potential for larger gravitational wave signals than previously estimated.

Abstract

In determining the gravitational signal of cusps from a network of cosmic strings loops, a number of key parameters have to be assumed. These include the typical number of cusps per period of string oscillation and the typical values of the sharpness parameters of left and right moving waves on the string, evaluated at the cusp event. Both of these are important, as the power stored in the gravitational waves emitted from the loops of string is proportional to the number of cusps per period, and inversely proportional to the product of the sharpness parameters associated with the left and right moving modes on the string. In suitable units both of these quantities are usually thought to be of order unity. In order to try and place these parameters on a more robust footing, we analyse in detail a large number of randomly chosen loops of string that can have high harmonics associated with them, such as one might expect to form by chopping off an infinite string in the early universe. This allows us to analyse tens of thousands of loops and obtain detailed statistics on these crucial parameters. While we find in general the sharpness parameters are indeed close to unity, as assumed in previous work (with occasional exceptions where they can become $O(10^{-2})$), the cusp number per period scales directly with the number of harmonics on the loop and can be significantly larger than unity. This opens up the possibility of larger signals than would have otherwise been expected, potentially leading to tighter bounds on the dimensionless cosmic string tension $G\mu$.