Loop decay in Abelian-Higgs string networks

Abstract

We study the decay of cosmic string loops in the Abelian-Higgs model. We confirm earlier results that loops formed by intersections of infinite strings formed from random-field initial conditions disappear quickly, with lifetime proportional to their initial rest-frame length ${\ell }_{\text{init}}$. We study a population with ${\ell }_{\text{init}}$ up to $6000$ inverse mass units, and measure the proportionality constant to be $0.14\pm 0.04$, independently of the initial lengths. We propose a new method to construct oscillating non-self intersecting loops from initially stationary strings, and show that by contrast these loops have lifetimes scaling approximately as ${\ell }_{\text{init}}^2$, in line with previous works on artificially created string configurations. We show that the oscillating strings have mean-square velocity ${\bar{v}}^2\simeq 0.500\pm 0.004$, consistent with the Nambu-Goto value of $1/2$, while the network loops have ${\bar{v}}^2\simeq 0.40\pm 0.04$. We argue that whatever the mechanism behind the network loop decay is, it is non-linear, can only be suppressed by careful tuning of initial conditions, and is much stronger than gravitational radiation. An implication is that one cannot use the Nambu-Goto model to derive robust constraints on the tension of field theory strings. We advocate parametrising the uncertainty as the fraction $f_{\text{NG}}$ of Nambu-Goto-like loops surviving to radiate gravitationally. None of the 31 large network loops created survived longer than 0.25 of their initial length, so one can estimate that $f_{\text{NG}}<0.1$ at $95$% confidence level. If the recently reported NANOgrav signal is due to cosmic strings, $f_{\text{NG}}$ must be greater than $10^{-3}$ in order not to violate bounds from the Cosmic Microwave Background.

arXiv comment16 pages, 20 figures. Movies can be found as ancillary files

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