Multifield consequences for D-brane inflation

TL;DR

This paper investigates multifield effects in D-brane inflation, revealing consistent inflationary trajectories near inflection points, with specific predictions for observables and highlighting challenges due to isocurvature perturbations.

Contribution

The study extends the transport method to non-slow-roll regimes, providing analytic insights into multifield D-brane inflation and its observable predictions.

Findings

Inflation is always canonical near an inflection point.

Spectral index is typically blue with positive running.

Approximately 0.5% of realizations produce observable local f_NL.

Abstract

We analyse the multifield behaviour in D-brane inflation when contributions from the bulk are taken into account. For this purpose, we study a large number of realisations of the potential; we find the nature of the inflationary trajectory to be very consistent despite the complex construction. Inflation is always canonical and occurs in the vicinity of an inflection point. Extending the transport method to non-slow-roll and to calculate the running, we obtain distributions for observables. The spectral index is typically blue and the running positive, putting the model under moderate pressure from WMAP7 constraints. The local f_NL and tensor-to-scalar ratio are typically unobservably small, though we find approximately 0.5% of realisations to give observably large local f_NL. Approximating the potential as sum-separable, we are able to give fully analytic explanations for the trends in observed behaviour. Finally we find the model suffers from the persistence of isocurvature perturbations, which can be expected to cause further evolution of adiabatic perturbations after inflation. We argue this is a typical problem for models of multifield inflation involving inflection points and renders models of this type technically unpredictive without a description of reheating.