Multi-field DBI inflation: introducing bulk forms and revisiting the gravitational wave constraints

TL;DR

This paper analyzes multi-field DBI inflation models with bulk form fields, showing their effects cancel out for scalar perturbations, and revisits gravitational wave constraints, finding multi-field models remain compatible with observations.

Contribution

It introduces the effects of bulk form fields in multi-field DBI inflation and demonstrates their cancellation in perturbations, updating gravitational wave constraint analysis.

Findings

Bulk form fields do not affect scalar perturbations or non-Gaussianities.

Multi-field DBI inflation remains compatible with gravitational wave data.

Vector perturbations may be amplified under certain conditions.

Abstract

We study multi-field Dirac-Born-Infeld (DBI) inflation models, taking into account the NS-NS and R-R bulk fields present in generic flux compactifications. We compute the second-order action, which governs the behaviour of linear cosmological perturbations, as well as the third-order action, which can be used to calculate non-Gaussianities in these models. Remarkably, for scalar-type perturbations, we show that the contributions due to the various form fields exactly cancel in both the second- and third-order actions. Primordial perturbations and their non-Gaussianities are therefore unaffected by the presence of form fields and our previous results are unmodified. We also study vector-type perturbations associated with the U(1) gauge field confined on the D3-brane, and discuss whether their quantum fluctuations can be amplified. Finally, we revisit the gravitational wave constraints on DBI inflation and show that an ultra-violet DBI multi-field scenario is still compatible with data, in contrast with the single field case, provided there is a transfer from entropy into adiabatic perturbations.