NATURAL INFLATION WITH PSEUDO NAMBU-GOLDSTONE BOSONS

We show that a pseudo Nambu-Goldstone boson, with a potential of the form V(phi) = LAMBDA-4[1 +/- cos(phi/f)], can naturally give rise to an epoch of inflation in the early Universe. Successful inflation can be achieved if f approximately m(pl) and LAMBDA approximately m(GUT). Such mass scales arise in particle-physics models with a gauge group that becomes strongly interacting at a scale approximately LAMBDA, e.g., as can happen in superstring theories. The density fluctuation spectrum is non-scale-invariant, with extra power on large length scales.