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Spectrum of radiation from global strings and the relic axion density

Richard A. Battye*, Lukasz P. Bunio†, Steven J. Cotterill‡, and Pranav B. Gangrekalve Manoj§

  • Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom

  • *Contact author: Richard.Battye@manchester.ac.uk
  • †Contact author: lukasz.bunio@postgrad.manchester.ac.uk
  • ‡Contact author: steven.cotterill@manchester.ac.uk
  • §Contact author: pbg@sfu.ca
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Phys. Rev. D 113, 123540 – Published 22 June, 2026

DOI: https://doi.org/10.1103/jd13-bjjx

Abstract

We discuss key aspects of the nature of radiation from global strings and its impact on the relic axion density. Using a simple model we demonstrate the dependence on the spectrum of radiation emitted by strings. We then study the radiation emitted by perturbed straight strings paying particular attention to the difference between the overall phase of the field and the small perturbations about the string solution which are the axions. We find that a significant correction is required to be sure that one is analyzing the axions and not the self-field of the string. Typically this requires one to excise a sizeable region around the string—something which is not usually done in the case of numerical field theory simulations of string networks. We have measured the spectrum of radiation from these strings and find that it is compatible with an exponential, as predicted by the Nambu-like Kalb-Ramond action, and in particular is not a “hard” spectrum often found in string network simulations. We conclude by attempting to assess the uncertainties on relic density and find that this leads to a wide range of possible axion masses when compared to the measured density from the cosmic microwave background, albeit that they are typically higher than what is predicted by the initial misalignment mechanism. If the primary mode of decay is via a “soft spectrum” from loops produced close to the backreaction scale we find that ma≈160  μeV and a detection frequency f≈38  GHz. If axions are primarily emitted directly by the string network, and we use emission spectra reported in typical field theory simulations, then ma≈4  μeV and f≈1  GHz, however this increases to ma≈125  μeV and f≈30  GHz using our spectra for the case of an oscillating string. In all scenarios there are significant remaining uncertainties that we delineate.

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Phys. Rev. D 113, 123540– Published 22 June, 2026
  • Received 10 February 2026
  • Accepted 15 May 2026
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