String melting in a photon bath

TL;DR

This paper calculates the decay rate of metastable cosmic strings interacting with a photon thermal bath, revealing temperature-dependent effects on bubble nucleation and string melting, with implications for topological stability.

Contribution

It introduces a novel instanton approach for non-thermal scalar fields coupled to a photon bath, analyzing metastability and decay in out-of-equilibrium conditions.

Findings

Decay rate depends on temperature via photon bath effects

String core melts during tunneling process

Bubble expansion occurs at the speed of light

Abstract

We compute the decay rate of a metastable cosmic string in contact with a thermal bath by finding the instanton solution. The new feature is that this decay rate is found in the context of non thermal scalar fields in contact with a thermal bath of photons. In general, to make topologically unstable strings stable, one can couple them to such a bath. The resulting plasma effect creates metastable configurations which can decay from the false vacuum to the true vacuum. In our specific set-up, the instanton computation is realized for the case of two out-of-equilibrium complex scalar fields : one is charged and coupled to the photon field, and the other is neutral. New effects coming from the thermal bath of photons make the radius of the nucleated bubble and most of the relevant physical quantities temperature-dependent. However, the temperature appears in a different way than in the purely thermal case, where all scalar fields are in thermal equilibrium. As a result of the tunneling, the core of the initial string melts while bubbles of true vacuum expand at the speed of light.