Axions and dark matter caustics

The strong CP problem and its resolution through the existence of an axion are briefly reviewed. If the axion mass ma similar to 10(-5) eV, axions contribute significantly to the cosmological energy density in the form of cold dark matter. Dark matter axions can be detected by resonant conversion to microwave photons in a cavity permeated by a static magnetic field and tuned to the axion mass. The infall of cold dark matter, whether it be axions or neutralinos, onto an isolated galaxy such as our own produces caustic rings in the halo dark matter distribution. A recent study of 32 extended and well-measured galactic rotation curves shows evidence of such caustics.