Entropy Fluctuations in Brane Inflation Models

TL;DR

The paper investigates entropy (isocurvature) fluctuations arising at the end of brane inflation in warped throats, focusing on the tachyon as the waterfall field. Using a two-field framework with an inflaton-like field $\psi$ and a tachyon field $T$, it demonstrates that tachyonic growth of the entropy mode $\delta s$ can seed a rapidly growing curvature perturbation $\\mathscr{R}$ on cosmological scales, potentially dominating the primordial perturbation. In the absence of back-reaction, the induced curvature fluctuations can be of comparable magnitude to the primordial ones, suggesting a curvaton-like contribution to the observed spectrum; however, back-reaction effects can shut off the resonance in realistic models, though toy scenarios show possible parameter regions where entropy effects remain important. Overall, the work highlights that end-of-inflation entropy dynamics can place significant constraints on brane-inflation models and may alter the inferred cosmological perturbation budget via entropy-to-curvature transfer.

Abstract

We study the development of entropy fluctuations in brane inflation in a warped throat, including the brane-antibrane tachyon as the waterfall field. We find that there is a period at the end of inflation during which the entropy mode associated with the tachyon field increases exponentially. In turn, the induced entropy seeds a contribution to the curvature fluctuation on cosmological scales which grows rapidly and could exceed the primordial curvature perturbation. We identify parameter values for which in the absence of back-reaction the induced curvature fluctuations are larger than the primordial adiabatic ones. In the specific model we study, however, back-reaction limits the growth of the entropy fluctuations. We discuss situations in which back-reaction effects are less constraining. The lesson of our investigation is that the study of the development of entropy fluctuations at the end of the period of inflation can lead to constraints on models of brane inflation and suggests that the curvaton mechanism may contribute significantly to the spectrum of cosmological perturbations.