Cosmological stretching of perturbations on a cosmic string

TL;DR

This paper studies how cosmological expansion influences small-scale structures on cosmic strings, revealing that short modes maintain their amplitude after horizon entry, affecting their relative strength.

Contribution

It demonstrates that cosmological expansion causes the amplitude-to-wavelength ratio of short string modes to increase after horizon entry, a novel insight into string perturbation evolution.

Findings

Short modes' physical amplitude remains unchanged after horizon entry.

The amplitude-to-wavelength ratio increases due to cosmological stretching.

Longer modes influence the evolution of smaller-scale structures.

Abstract

We investigate the effects of cosmological expansion on the spectrum of small-scale structure on a cosmic string. We simulate the evolution of a string with two modes that differ in wavelength by one order of magnitude. Once the short mode is inside the horizon, we find that its physical amplitude remains unchanged, in spite of the fact that its comoving wavelength decreases as the longer mode enters the horizon. Thus the ratio of amplitude to wavelength for the short mode becomes larger than it would be in the absence of the long mode.