Solutions to the strong-CP problem in a world with gravity☆

We examine various solutions of the strong-CP problem to determine their sensitivity to possible violations of global symmetries by Plauck scale physics. While some solutions remain viable even in the face of such effects. Violations of the Peccei-Quinn (PQ) symmetry by non-renormalizable operators of dimension less than 10 will generally shift the value of {bar {theta}} to values inconsistent with the experimental bound {bar {theta}} {approx_lt} 10{sup {minus}}9. We show that it is possible to construct axion models where gauge symmetries protect PQ symmetry to the requisite level.