Accretion onto Oscillating Cosmic String Loops

TL;DR

This paper investigates how the finite size of oscillating cosmic string loops affects the accretion process in the early universe, revealing differences from point-mass models and analyzing the impact of loop motion.

Contribution

It introduces a Zel'dovich approximation-based method to study accretion onto extended loops, highlighting size effects and comparing with point-mass results.

Findings

Finite loop size suppresses early accretion compared to point masses.

Turnaround surface shape is altered by loop size during early accretion.

Accreted mass growth rate scales as (1+z)^{-3/2} at high redshift.

Abstract

Cosmic string loops are non-linear density fluctuations which form in the early universe and could play an important role in explaining many phenomena which are in tension with the standard $\Lambda$CDM model. Hence, the details of the accretion process onto cosmic string loops should be studied in detail. Most previous works view loops as point masses and ignore the impact of a finite loop size. In this work, we utilize the Zel'dovich approximation to calculate the non-linear mass sourced by a static extended loop with a time-averaged density profile derived from the trajectory of the loop oscillation, and compare the result with what is obtained for a point-mass source. We find that the finite size of a loop mainly affects the evolution of turnaround shells during the early stages of accretion, converging to the point mass result after a critical redshift, $z^{(II)/(III)}_{c}$. For $z>z^{(II)/(III)}_{c}$, the total accreted mass surrounding a loop is suppressed relative to the point mass case and has a growth rate proportional to $(1+z)^{-3/2}$. As an immediate extension, we also qualitatively analyse the accretion onto moving point masses and onto moving extended loops. In addition to the reduction in the nonlinear mass, the loop finite size also changes the shape of the turnaround surface at early stages of accretion.