Axion strings are superconducting

TL;DR

This paper investigates the superconducting properties of QCD axion strings and their cosmological implications, including effects on string stability, relic abundance, and the early universe dynamics, especially in the presence of primordial magnetic fields.

Contribution

It introduces the concept of superconducting axion strings, analyzes their stability and evolution, and explores how primordial magnetic fields influence their dynamics and the resulting axion relic abundance.

Findings

Vortons from axion strings can overclose the universe without current leakage.

Primordial magnetic fields induce large currents, slowing strings and increasing their density.

The QCD axion relic abundance and parameter space are significantly affected.

Abstract

We explore the cosmological consequences of the superconductivity of QCD axion strings. Axion strings can support a sizeable chiral electric current and charge, which alters their early universe dynamics. Shrinking axion string loops can become effectively stable remnants called vortons, supported by the electromagnetic force of the string current. Generically, vortons produced by axion strings overclose the universe, unless there are efficient current leakage processes. Furthermore, if a primordial magnetic field (PMF) exists in the early universe, a large current is induced on axion strings, creating a significant drag force from interactions with the surrounding plasma. As a result, the strings are slowed down, which leads to an orders of magnitude enhancement in the number of strings per Hubble volume. Finally, we study the implications for the QCD axion relic abundance. The QCD axion window is shifted by orders of magnitude in some parts of our parameter space.