Axion Dark Matter from Cosmic String Network

TL;DR

This paper uses lattice simulations to estimate axion dark matter produced by cosmic strings, confirming logarithmic growth behaviors and analyzing how initial conditions and string properties influence axion abundance.

Contribution

It introduces a new string identification method and provides a detailed analysis of factors affecting axion dark matter production from cosmic strings.

Findings

Confirmed logarithmic growth in string number and axion spectrum index

Demonstrated insensitivity of scaling behaviors to initial conditions

Derived a lower bound on the axion mass

Abstract

We perform the lattice simulation to estimate the axion dark matter abundance radiated from the global cosmic strings in the post-inflationary scenario. The independent numerical confirmation on the recently observed logarithmic growth in both the number of strings per Hubble patch and the spectral index of the power law scaling for the axion spectrum is reported. These logarithmic scalings are checked against two different prescriptions for generating initial random field configurations, namely fat-string type and thermal phase transition. We discuss a possible strong correlation between the axion spectrum and the string evolutions with different initial conditions to support the insensitivity of scaling behaviors against different initial data and we provide a qualitative understanding of it. The impact of various combinations of the power law of the axion spectrum, nonlinearities around the QCD scale, and average inter-string distances on the axion abundance is discussed. Additionally, we introduce a new novel string identification method, based on the tetrahedralization of the space, which guarantees the connectedness of the strings and provides a convenient way of assigning the core location. Finally we derive the lower bound on the axion mass.