Topological Multi-Natural Inflation

Abstract

We investigate topological inflation in minimal multi-natural inflation, in which the axion potential consists of two cosine terms. Focusing on the high-scale regime, we perform a systematic parameter scan for representative ratios of the two periods and identify regions consistent with current constraints on the scalar spectral index and tensor-to-scalar ratio, including those from the Atacama Cosmology Telescope. The interplay of the two harmonics allows departures from the quartic hilltop limit and accommodates the higher spectral index favored by recent cosmological data combinations. We characterize the hilltop and near-inflection-point branches and analyze the associated vacuum structure and domain-wall properties. We also discuss a symmetry-breaking origin of the required domain walls when the inflaton is the angular component of a complex scalar field. We identify regions in which the observational constraints and the adopted criteria for topological inflation are simultaneously satisfied, including regions with a sub-Planckian axion decay constant. Our results show that the observational tension encountered in the minimal quartic double-well model need not extend to topological inflation in a minimal multi-natural axion potential.