Cosmic and local mass density of ‘‘invisible’’ axions

If the Universe is axion dominated it may be possible to detect the axions which comprise the bulk of the mass density of the Universe. The feasibility of the proposed experiments depends crucially upon knowing the axion mass (or equivalently, the Peccei-Quinn symmetry-breaking scale) and the local mass density of axions. In an axion-dominated universe our galactic halo should be comprised primarily of axions. We calculate the local halo density to be at least 510^-25 g cm^-3, and at most a factor of 2 larger. Unfortunately, it is not possible to pin down the axion mass, even to within an order of magnitude. In an axion-dominated universe we place an upper limit to the axion mass of about 10^-4 eV. We give precise formulas for the axion mass in an axion-dominated universe, and clearly point out all the uncertainties involved in determining the precise value of the mass.