Effects of friction on cosmic strings

TL;DR

This paper investigates how friction affects the evolution of cosmic strings, their perturbations, and the potential formation of black holes, concluding that black hole formation via this process is negligible within observational limits.

Contribution

It provides a detailed analysis of frictional effects on cosmic string perturbations, loop dynamics, and black hole formation, including new insights into the suppression of perturbations and loop behavior.

Findings

Friction exponentially suppresses perturbations crossing the horizon before time t_*

Small loops tend to be circular at collapse, affecting black hole formation

Black hole formation from loops is below observational limits, offering no bounds on string tension μ

Abstract

We study the evolution of cosmic strings taking into account the frictional force due to the surrounding radiation. We consider small perturbations on straight strings, oscillation of circular loops and small perturbations on circular loops. For straight strings, friction exponentially suppresses perturbations whose co-moving scale crosses the horizon before cosmological time $t_*\sim \mu^{-2}$ (in Planck units), where $\mu$ is the string tension. Loops with size much smaller than $t_*$ will be approximately circular at the time when they start the relativistic collapse. We investigate the possibility that such loops will form black holes. We find that the number of black holes which are formed through this process is well bellow present observational limits, so this does not give any lower or upper bounds on $\mu$. We also consider the case of straight strings attached to walls and circular holes that can spontaneously nucleate on metastable domain walls.