Production of gravitational waves by inflationary transitions in aligned natural inflation

Abstract The original axion natural inflation model predicts a tensor-to-scalar ratio exceeding experimental limits. Conversely, in aligned axion inflation, inflation can proceed along trajectories emerging from near a saddle point of the two-field potential and ending through an instability in the orthogonal direction. Such solutions satisfy present observational limits and will be tested by future CMB experiments. Previous studies have suggested the possibility of two distinct inflationary stages separated by a transition characterized by rapid oscillations of the fields. In this work, we demonstrate that the existence of these two stages is a generic feature of the model. We explore a possible phenomenological signature of the transition when a U(1) gauge field is coupled to the axions, namely, the production of gravitational waves (GWs) sourced by gauge quanta generated during the transition. This mechanism produces a feature similar to those seen in spectator axion models or axion inflation with appropriate potentials, i.e. a strongly scale-dependent power spectrum. The scale at which the GW spectrum is produced is determined by the duration of the second inflationary phase. Consequently, the spectrum may peak at different frequencies, potentially detectable by future GW experiments.