Axions as Hot and Cold Dark Matter
  The presence of a hot dark matter component has been hinted at 3 sigma by a
combination of the results from different cosmological observations. We examine
a possibility that pseudo Nambu-Goldstone bosons account for both hot and cold
dark matter components. We show that the QCD axions can do the job for the
axion decay constant f_a < O(10^10) GeV, if they are produced by the saxion
decay and the domain wall annihilation. We also investigate the cases of
thermal QCD axions, pseudo Nambu-Goldstone bosons coupled to the standard model
sector through the Higgs portal, and axions produced by modulus decay.
