Cosmic string Y-junctions: a comparison between field theoretic and Nambu-Goto dynamics

TL;DR

This paper compares the dynamics of cosmic string Y-junction formation using field theoretic simulations and the Nambu-Goto approximation, finding moderate predictive accuracy and close late-time behavior.

Contribution

It provides a detailed comparison between field theoretic and Nambu-Goto models for cosmic string Y-junction formation, highlighting their agreement and limitations.

Findings

Nambu-Goto predicts junction formation reasonably well.

Late-time dynamics closely match Nambu-Goto results.

Minimal radiative emission observed from Y-junctions.

Abstract

We explore the formation of cosmic string Y-junctions when strings of two different types collide, which has recently become important since string theory can yield cosmic strings of distinct types. Using a model containing two types of local U(1) string and stable composites, we simulate the collision of two straight strings and investigate whether the dynamics matches that previously obtained using the Nambu-Goto action, which is not strictly valid close to the junction. We find that the Nambu-Goto action performs only moderately well at predicting when the collision results in the formation of a pair of Y-junctions (with a composite string connecting them). However, we find that when they do form, the late time dynamics matches those of the Nambu-Goto approximation very closely. We also see little radiative emission from the Y-junction system, which suggests that radiative decay due to bridge formation does not appear to be a means via which a cosmological network of such string would rapidly lose energy.