Inflationary predictions in scalar-tensor DBI inflation

TL;DR

This paper investigates scalar-tensor DBI inflation models, deriving analytical spectral index expressions and numerically identifying parameter regions compatible with observed cosmic microwave background data, highlighting viable scenarios with controlled boost factors.

Contribution

It provides analytical and numerical analysis of scalar-tensor DBI inflation, identifying parameter spaces that produce acceptable spectral indices and amplitude, and clarifies the impact of the coupling on the boost factor.

Findings

Regions with significant DBI effects are ruled out due to blue tilt

Viable models exhibit a red-tilted spectral index with initially suppressed boost factors

Some models show boost factors increasing later to moderate values

Abstract

The scalar-tensor Dirac-Born-Infeld (DBI) inflation scenario provides a simple mechanism to reduce the large values of the boost factor associated with single field models with DBI action, whilst still being able to drive 60 efolds of inflation. Using a slow-roll approach, we obtain an analytical expression for the spectral index of the perturbations and, moreover, determine numerically the regions of the parameter space of the model capable of giving rise to a power spectrum with amplitude and spectral index within the observed bounds. We find that regions that exhibit significant DBI effects throughout the inflationary period can be discarded by virtue of a blue-tilted spectral index, however, there are a number of viable cases --- associated with a more red-tilted spectral index --- for which the boost factor is initially suppressed by the effect of the coupling between the fields, but increases later to moderate values.