Scaling Density of Axion Strings

Mark Hindmarsh, Joanes Lizarraga, Asier Lopez-Eiguren, and Jon Urrestilla
Phys. Rev. Lett. 124, 021301 – Published 16 January 2020
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Abstract

In the QCD axion dark matter scenario with postinflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time t, by convention written ζ/t2. The expectation has been that the dimensionless parameter ζ tends to a constant ζ0, a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations ζ shows a logarithmic increase with time, while theoretical modeling suggests an inverse logarithmic correction. Either case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings, we compare the standard scaling (constant-ζ) model to the logarithmic growth and inverse-logarithmic correction models. In the standard scaling model, by fitting to linear growth in the mean string separation ξ=t/ζ, we find ζ0=1.19±0.20. We conclude that the apparent corrections to ζ are artifacts of the initial conditions, rather than a property of the scaling network. The residuals from the constant-ζ (linear ξ) fit also show no evidence for logarithmic growth, restoring confidence that numerical simulations can be simply extrapolated from the Peccei-Quinn symmetry-breaking scale to the QCD scale. Reanalysis of previous work on the axion number density suggests that recent estimates of the axion dark matter mass in the postinflationary symmetry-breaking scenario we study should be increased by about 50%.

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  • Received 7 October 2019

DOI:https://doi.org/10.1103/PhysRevLett.124.021301

© 2020 American Physical Society

Physics Subject Headings (PhySH)

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Mark Hindmarsh1,2,*, Joanes Lizarraga3,†, Asier Lopez-Eiguren1,‡, and Jon Urrestilla3,§

  • 1Department of Physics and Helsinki Institute of Physics, University of Helsinki, PL 64, FI-00014, Finland
  • 2Department of Physics and Astronomy, University of Sussex, Falmer, Brighton BN1 9QH, United Kingdom
  • 3Department of Theoretical Physics, University of the Basque Country UPV/EHU, 48080 Bilbao, Spain

  • *mark.hindmarsh@helsinki.fi
  • joanes.lizarraga@ehu.eus
  • asier.lopezeiguren@helsinki.fi
  • §jon.urrestilla@ehu.eus

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Issue

Vol. 124, Iss. 2 — 17 January 2020

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