Topological Defects and nano-Hz Gravitational Waves in Aligned Axion
  Models
  We study the formation and evolution of topological defects in an aligned
axion model with multiple Peccei-Quinn scalars, where the QCD axion is realized
by a certain combination of the axions with decay constants much smaller than
the conventional Peccei-Quinn breaking scale. When the underlying U(1)
symmetries are spontaneously broken, the aligned structure in the axion field
space exhibits itself as a complicated string-wall network in the real space.
We find that the string-wall network likely survives until the QCD phase
transition if the number of the Peccei-Quinn scalars is greater than two. The
string-wall system collapses during the QCD phase transition, producing a
significant amount of gravitational waves in the nano-Hz range at present. The
typical decay constant is constrained to be below O(100) TeV by the pulsar
timing observations, and the constraint will be improved by a factor of 2 in
the future SKA observations.
