The U(1) problem: Instantons, axions, and familons

The author discusses the motivation for introducing the axion and its mass and couplings to matter. Axions may play an important role in cosmology -- they are a serious candidate to provide the nonluminous mass for which astronomical evidence is accumulating. The conversely cosmological considerations that place significant constraints on the properties of axions are examined. The framework in which axions most naturally appear is presented. It is noted that the whole superstructure of axions could be made obsolete if a good alternative approach to the problem of strong CP invariance were found. Techniques are presented for dealing with approximate Nambu-Goldstone bosons and their phenomenological (including cosmological) implications