2 Axion Cosmology

The cosmological properties of axions are reviewed .W e discuss the axions produced by thermal processes in the early universe, the evolution of the av- erage axion field between the Peccei-Quinn and QCD phase-transitions, the domain- wall problem and its possible resolutions, the population of cold axions produced by vacuum realignment, string decay and domain wall decay, and, finally, axion miniclusters and axion isocurvature perturbations. For background information on the strong CP problem and on laboratory and astrophysical axion constraints, the reader is referred to Chaps. 1 and 3. In this chapter, we will be concerned only with the cosmological properties of axions. We merely mention that different authors may use different definitions of the axion decay constant fa. We define normalization by the action density for QCD plus an axion LQCD+a = − 1 G b G bμν + 1 ∂μ a∂ μ a