The effect of multiple features on the power spectrum in two-field inflation

Abstract

We extend our previous workon the enhancement of the curvature spectrum during inflation to the two-field case.We identify the slow-roll parameter η as the quantitythat can trigger the rapid growth of perturbations.Its two components, η ∥ along the background trajectoryand η ⊥ perpendicular to it, remain small during most of the evolution,apart from short intervals during which they take large, positive or negative, values.The typical reason for the appearance of strong features in η ∥ issharp steps or inflection points in the inflaton potential,while η ⊥ grows large during sharp turns infield space. We focus on the additive effect of several featuresleading to the resonant growth of the curvature spectrum.Three or four features in the evolution of η are sufficient in order to induce an enhancement of the power spectrum by six or seven orders of magnitude, which canlead to the significant production of primordial black holes and stochasticgravitational waves.A big part of our study focuses on understanding the evolutionof the perturbations and the resulting spectra through analytic means.The presence of multiple features inthe background evolution points to a more complex inflationary paradigm,which is also more natural in the multi-field case.The critical examination of this possibility is withinthe reach of experiment.