Axion dark matter from topological defects
  The cosmological scenario where the Peccei-Quinn symmetry is broken after
inflation is investigated. In this scenario, topological defects such as
strings and domain walls produce a large number of axions, which contribute to
the cold dark matter of the universe. The previous estimations of the cold dark
matter abundance are updated and refined based on the field-theoretic
simulations with improved grid sizes. The possible uncertainties originated in
the numerical calculations are also discussed. It is found that axions can be
responsible for the cold dark matter in the mass range $m_a=(0.8-1.3)\times
10^{-4}\mathrm{eV}$ for the models with the domain wall number $N_{\rm DW}=1$,
and $m_a\approx\mathcal{O}(10^{-4}-10^{-2})\mathrm{eV}$ with a mild tuning of
parameters for the models with $N_{\rm DW}>1$. Such higher mass ranges can be
probed in future experimental studies.
