Constraints on cosmic strings from ultracompact minihalos

TL;DR

This paper investigates how cosmic string loops can seed ultracompact minihalos (UCMHs) and uses observational data to constrain the cosmic string tension, highlighting the importance of loop velocity assumptions.

Contribution

It provides a detailed analysis of dark matter accretion onto cosmic string loops and revises previous limits on string tension by considering realistic loop velocities.

Findings

Limit on cosmic string tension: Gμ ≤ 5×10⁻⁸

UCMH abundance constraints depend on loop velocities

Realistic velocity assumptions weaken previous bounds

Abstract

Cosmic strings are expected to form loops. These can act as seeds for accretion of dark matter, leading to the formation of ultracompact minihalos (UCMHs). We perform a detailed study of the accretion of dark matter onto cosmic string loops and compute the resulting mass distribution of UCMHs. We then apply observational limits on the present-day abundance of UCMHs to derive corresponding limits on the cosmic string tension $G\mu$. The bounds are strongly dependent upon the assumed distribution of loop velocities and their impacts on UCMH formation. Under the assumption that a loop can move up to a thousand times its own radius and still form a UCMH, we find a limit of $G\mu\le 5\times10^{-8}$. We show, in opposition to previous results, that strong limits on the cosmic string tension are not obtainable from UCMHs when more stringent (and realistic) requirements are placed on loop velocities.