Axion Cosmology

The Strong CP Problem and its resolution through the existence of an axion are briefly reviewed. The combined constraints from accelerator searches, the evolution of red giants and the duration of the SN 1987a neutrino pulse require the axion mass ma to be less than 3 · 10−3 eV. On the other hand, the requirement that the axion does not overclose the universe implies a lower bound on ma of order 10−6 eV. This lower bound can, however, be relaxed in a number of ways. If ma is near the lower bound, axions are an important contribution to the energy density of the universe in the form of (very) cold dark matter. Dark matter axions can be searched for on Earth by stimulating their conversion to microwave photons in an electromagnetic cavity permeated by a magnetic field. Using this technique, limits on the local halo density have been placed by the Axion Dark Matter eXperiment. I’ll give a status report on ADMX and its upgrade presently under construction. I’ll also report on recent results from solar axion searches and laser experiments.