Searches for Astrophysical and Cosmological Axions*

The axion, a favored dark matter candidate, is expected to have a very small mass and extraordinarily weak couplings. Although it has eluded discovery or exclusion for three decades, it remains the most compelling solution to the strong-CP problem. Axions may be detected by their resonant conversion to RF photons in a microwave cavity permeated by a magnetic field. Experiments have already set significant limits on the axion's mass and photon coupling; progress in photon detection schemes at or below the standard quantum limit will soon enable definitive searches. Similarly, axions produced in the solar burning core may be detectable by their conversion to X rays in a magnetic helioscope. Significant improvements in both mass range and sensitivity of the axion helioscope will likewise be forthcoming in the next few years.