Abstract
We discuss the possible sources of dark matter axions in the early universe. In the standard thermal scenario, an axion string network forms at the Peccei-Quinn phase transition T∼ f a and then radiatively decays into a cosmological background of axions; to be the dark matter, these axions must have a mass m a ∼100 μ eV with specified large uncertainties. An inflationary phase with a reheat temperature below the PQ-scale T reh ≲ f a can also produce axion strings through quantum fluctuations, provided that the Hubble parameter during inflation is large H 1≳ f a; this case again implies a dark matter axion mass m a ∼100 μ eV. For a smaller Hubble parameter during inflation H 1≲ f a, ‘anthropic tuning’ allows dark matter axions to have any mass in a huge range below m a ≲1 meV.
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