Cosmological fluctuations from infrared cascading during inflation

Abstract

We propose a qualitatively new mechanism for generating cosmological fluctuations from inflation. The nonequilibrium excitation of interacting scalar fields often evolves into infrared (IR) and ultraviolet cascading, resulting in an intermediate scaling regime. We observe elements of this phenomenon in a simple model with inflaton $\ensuremath{\phi}$ and isoinflaton $\ensuremath{\chi}$ fields interacting during inflation via the coupling ${g}^{2}(\ensuremath{\phi}\ensuremath{-}{\ensuremath{\phi}}_{0}{)}^{2}{\ensuremath{\chi}}^{2}$. Isoinflaton particles are created during inflation when they become instantaneously massless at $\ensuremath{\phi}={\ensuremath{\phi}}_{0}$, with occupation numbers not exceeding unity. Previous studies have focused on the momentary slowing down of the condensate $\ensuremath{\phi}(t)$ by back-reaction effects. Here, we point out that very quickly the produced $\ensuremath{\chi}$ particles become heavy and their multiple rescatterings off the homogeneous condensate $\ensuremath{\phi}(t)$ generates Bremsstrahlung radiation of light inflaton IR fluctuations with high occupation numbers. The subsequent evolution of these IR fluctuations is qualitatively similar to that of the usual inflationary fluctuations, but their initial amplitude is different. The IR cascading generates a bump-shaped contribution to the cosmological curvature fluctuations, which can even dominate over the usual fluctuations for ${g}^{2}>0.06$. The IR cascading curvature fluctuations are significantly non-Gaussian, and the strength and location of the bump are model dependent, through ${g}^{2}$ and ${\ensuremath{\phi}}_{0}$. The effect from IR cascading fluctuations is significantly larger than that from the momentary slowing down of $\ensuremath{\phi}(t)$. With a sequence of such bursts of particle production, the superposition of the bumps can lead to a new broadband non-Gaussian component of cosmological fluctuations added to the usual fluctuations. Such a sequence of particle creation events can, but need not, lead to trapped inflation.