PLANCK-SCALE CORRECTIONS TO AXION MODELS

It has been argued that quantum gravitational effects will violate all nonlocal symmetries. Peccei-Quinn symmetries must therefore be an "accidental" or automatic consequence of local gauge symmetry. Moreover, higher-dimensional operators suppressed by powers of M(Pl) are expected to explicitly violate the Peccei-Quinn symmetry. Unless these operators are of dimension d greater-than-or-equal-to 10, axion models do not solve the strong CP problem in a natural fashion. A small gravitationally induced contribution to the axion mass has little if any effect on the density of relic axions. If d = 10, 11, or 12 these operators can solve the axion domain-wall problem, and we describe a simple class of Kim-Shifman-Vainshtein-Zakharov axion models where this occurs. We also study the astrophysics and cosmology of "heavy axions" in models where 5 less-than-or-equal-to d less-than-or-equal-to 10.