Natural inflation at the grand unification scale

Natural inflation driven by pseudo-Nambu-Goldstone bosons (PNGB) has a problem that the nearly scale invariant spectrum of density perturbations is attained only when the symmetry breaking scale is of the order of Planck scale. We show here that if one couples the PNGB to a thermal bath as in warm inflation models, the amplitude and spectral index which agrees with the Wilkinson Microwave Anisotropy Probe (WMAP) data is obtained with the symmetry breaking in the grand unified theory (GUT) scale. We give a GUT model of PNGB arising out of spontaneously broken lepton number at the GUT scale which gives rise to heavy Majorana masses for the right-handed neutrinos which is needed in seesaw models. This model also generates a lepton asymmetry because of the derivative coupling of the PNGB to the lepton current. A characteristic feature of this model is the prediction of large non-Gaussianity which may be observed in the forthcoming PLANCK experiment.