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 lifetimes proportional to their initial rest-frame length, l init . We study a population with l init up to 6000 inverse mass units and measure the proportionality constant to be 0.14 AE 0.04, independently of the initial lengths. We propose a new method to construct oscillating nonself intersecting loops from initially stationary strings and show that by contrast these loops have lifetimes scaling approximately as l 2 init , in line with previous works on artificially created string configurations. We show that the oscillating strings have a mean-square velocity ofv 2 ≃ 0.500 AE 0.004, consistent with the Nambu-Goto value of 1=2, while the network loops havev 2 ≃ 0.40 AE 0.04. We argue that whatever the mechanism behind the network loop decay is, it is nonlinear, 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 parametrizing the uncertainty as the fraction f 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 NG < 0.1 at 95% confidence level. If the recently reported NANOgrav signal is due to cosmic strings, f NG must be greater than 10 −3 in order not to violate bounds from the cosmic microwave background.

Stochastic gravitational-wave background from metastable cosmic strings

“…The main differences concern the number density of small loops and the treatment of gravitational backreaction, which can smooth out string singularities. Moreover, the entire picture of NG string loops decaying by gravitational radiation has been challenged by fieldtheoretic simulations suggesting a much faster decay of the network whose origin, however, remains mysterious [4,14]. In this paper, we follow the approach in Ref.…”

The Global 21 cm Signal of a Network of Cosmic String Wakes

“…Hence if the SGWB reported by NANOGrav is due to Abelian-Higgs cosmic strings, a fraction of those loops are Nambu-Goto-like and survive to radiate gravitationally. Hindmarsh et al (2021) find that this fraction needs to be between 10 −3 and 0.1.…”

Internal Excitations of Global Vortices

“…However, the results obtained from simulations of string networks seem to describe a different behaviour [32]. It has also been suggested that internal degrees of freedom found in these models [33] can play a significant role in that case [32,34] 1 . Note, however, that this effect was not observed in [39] where these internal modes do not seem to be clearly excited.…”