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Area of Science:

  • Cosmology
  • Particle Physics

Background:

  • The QCD axion dark matter scenario relies on postinflationary Peccei-Quinn symmetry breaking.
  • Axion density is linked to the string length per unit volume (ζ/t²), with scaling (constant ζ) being the expected behavior.

Purpose of the Study:

  • To investigate claims of logarithmic time dependence in string density.
  • To compare the standard scaling model with logarithmic growth and inverse-logarithmic correction models.

Main Methods:

  • Conducted new numerical simulations of global strings.
  • Compared simulation results to different theoretical models of string network evolution.
  • Analyzed string density and mean separation (ξ) over time.

Main Results:

  • The standard scaling model (constant ζ) with ζ₀=1.19±0.20 provides the best fit to the data.
  • Apparent logarithmic corrections to ζ were identified as artifacts of initial conditions.
  • No evidence for logarithmic growth was found in residuals from the constant-ζ fit.

Conclusions:

  • The standard scaling model for cosmic strings is robust.
  • Numerical simulations can be reliably extrapolated from the Peccei-Quinn symmetry-breaking scale to the QCD scale.
  • Previous estimates of axion dark matter mass may need a ~50% upward revision.