Gravitational waves from melting cosmic strings

TL;DR

This paper investigates gravitational wave emissions from melting cosmic strings in a scale-free model, revealing a distinctive non-flat spectrum with a peak frequency potentially detectable by future observatories.

Contribution

It introduces a scale-free model of cosmic string formation where string tension decreases over time, and estimates the unique gravitational wave spectrum emitted by such melting strings.

Findings

The gravitational wave spectrum from melting strings is non-flat with a characteristic falloff.

The peak frequency of the spectrum is model-dependent and within future detector sensitivity.

The dynamics of melting strings are equivalent to constant tension strings in Minkowski space when neglecting backreaction.

Abstract

Appearance of cosmic strings in the early Universe is a common manifestation of new physics typically linked to some high energy scale. In this paper, we discuss a different situation, where a model underlying cosmic string formation is approximately scale free. String tension is naturally related to the square of the temperature of the hot primordial plasma in such a setting, and hence decreases with (cosmic) time. With gravitational backreaction neglected, the dynamics of these melting strings in an expanding Universe is equivalent to the dynamics of constant tension strings in a Minkowski spacetime. We provide an estimate for the emission of gravitational waves from string loops. Contrary to the standard case, the resulting spectrum is markedly non-flat and has a characteristic falloff at frequencies below the peak one. The peak frequency is defined by the underlying model and lies in the range accessible by the future detectors for very weak couplings involved.