Observable dark radiation from cosmologically safe QCD axion
  We propose a QCD axion model that avoids the cosmological domain wall
problem, introducing a global SU(3)_f family symmetry to which we embed the
unwanted PQ discrete symmetry. The spontaneous breaking of SU(3)_f and PQ
symmetry predicts eight NG bosons as well as axion, all of which contribute to
dark radiation in the Universe. The derivation from the standard model
prediction of dark radiation can be observed by future observations of CMB
fluctuations. Our model also predicts a sizable exotic kaon decay rate, which
is marginally consistent with the present collider data and would be tested by
future collider experiments.
