Metastable cosmic strings are broken at the start

TL;DR

This paper demonstrates that metastable cosmic strings tend to break early due to thermal effects or monopole attachment, significantly impacting their longevity and observational signatures, especially in light of recent pulsar timing data.

Contribution

It reveals that early-time breaking of metastable cosmic strings is more common than previously thought, revising conditions for their survival and implications for cosmological observations.

Findings

Early breaking of strings via thermal effects or monopoles is prevalent.

Survival of strings to NANOGrav epoch requires higher monopole mass-to-tension ratio.

Quantum tunnelling occurs mainly at high-tension points, increasing breaking rates.

Abstract

We show that metastable cosmic strings break at early times, either via finite-temperature effects or by attaching to pre-existing monopoles during network percolation. The resulting segments can be initially super-horizon in size and thus persist for a significant amount of time. If the strings do not re-percolate, the network's eventual destruction is typically due to this early-time breaking rather than late-time quantum tunnelling. Survival of strings to epochs probed by NANOGrav requires $m_M^2/\mu\gtrsim 10^3$, where $m_M$ and $\mu$ are the monopole mass and the string tension respectively, over an order of magnitude larger than previous estimates. We also revisit quantum-tunnelling induced breaking. Results from numerical simulations suggest that this occurs mainly at rare high-tension points on the strings, yielding a rate much larger than is usually assumed. We briefly discuss the related scenario of flux tubes in a dark QCD-like hidden sector with dark-quark masses above the confinement scale.