Axion dark matter: strings and their cores
  Axions constitute a well-motivated dark matter candidate, and if PQ symmetry
breaking occurred after inflation, it should be possible to make a clean
prediction for the relation between the axion mass and the axion dark matter
density. We show that axion (or other global) string networks in 3D have a
network density that depends logarithmically on the string separation-to-core
ratio. This logarithm would be about 10 times larger in axion cosmology than
what we can achieve in numerical simulations. We simulate axion production in
the early Universe, finding that, for the separation-to-core ratios we can
achieve, the changing density of the network has little impact on the axion
production efficiency.
