Cosmic Axions

The current cosmological constraints on a dark matter axion are reviewed. We describe the basic mechanisms by which axions are created in the early universe, both in the standard thermal scenario in which axion strings form and in inflation- ary models. In the thermal scenario, the dominant process for axion production is through the radiative decay of an axion string network, which implies a dark matter axion of mass m a ∼ 100 µ eV with specified large uncertainties. An inflationary phase does not affect this string bound if the reheat temperature is high T reh > ∼ f a or, conversely, for T reh < ∼ f a , if the Hubble parameter during inflation is large H 1 > f a ; in both cases, strings form and we return to the standard picture with a m a ∼ 100 µ eV dark matter axion. Inflationary models with f a > H 1 > T reh face strong CMBR constraints and require ‘anthropic misalignment’ fine-tuning in order to produce a dark matter axion; in this case, some inflation models are essentially incompatible with a detectable axion, while others can be engineered to allow a dark matter axion anywhere in a huge mass range below m a < ∼ 1meV. We endeavour to clarify the sometimes confusing and contradictory literature on axion cosmology.