Embedding DBI inflation in scalar-tensor theory

TL;DR

This paper investigates a scalar-tensor inflation model with a DBI matter component, analyzing how the coupling affects inflation dynamics and primordial perturbations, revealing increased efolds and altered power spectrum characteristics.

Contribution

It introduces a novel scalar-tensor inflation framework with a DBI matter field and explores its impact on background evolution and perturbation spectra.

Findings

Coupling drives the scalar field to its effective potential minimum.

The DBI field influences the number of efolds and the boost factor.

Primordial power spectrum depends on the modified DBI field perturbations.

Abstract

The Dirac-Born-Infeld (DBI) action has been widely studied as an interesting example of a model of k-inflation in which the sound speed of the cosmological perturbations differs from unity. In this article we consider a scalar-tensor theory in which the matter component is a field with a DBI action. Transforming to the Einstein frame, we explore the effect of the resulting coupling on the background dynamics of the fields and the first-order perturbations. We find that the coupling forces the scalar field into the minimum of its effective potential. While the additional scalar field contributes significantly to the energy density during inflation, the dynamics are determined by the DBI field, which has the interesting effect of increasing the number of efolds of inflation and decreasing the boost factor of the DBI field. Focusing on this case, we show, with the benefit of numerical examples, that the power spectrum of the primordial perturbations is determined by the behaviour of the perturbations of the modified DBI field.