Primordial fluctuations and non-Gaussianities from multifield DBI Galileon inflation

TL;DR

This paper explores a multifield DBI Galileon inflation model, revealing conditions for stable quasi-exponential expansion, and demonstrates the potential for large orthogonal non-Gaussianities in primordial fluctuations, providing new observational signatures.

Contribution

It introduces a novel inflationary model combining DBI and Galileon terms, analyzes its stability, and predicts distinctive non-Gaussian features in the primordial bispectrum.

Findings

Stable quasi-exponential expansion possible with induced gravity.

Large orthogonal non-Gaussianities can be generated.

Bispectrum shape and sign diagnose induced gravity strength.

Abstract

We study a cosmological scenario in which the DBI action governing the motion of a D3-brane in a higher-dimensional spacetime is supplemented with an induced gravity term. The latter reduces to the quartic Galileon Lagrangian when the motion of the brane is non-relativistic and we show that it tends to violate the null energy condition and to render cosmological fluctuations ghosts. There nonetheless exists an interesting parameter space in which a stable phase of quasi-exponential expansion can be achieved while the induced gravity leaves non trivial imprints. We derive the exact second-order action governing the dynamics of linear perturbations and we show that it can be simply understood through a bimetric perspective. In the relativistic regime, we also calculate the dominant contribution to the primordial bispectrum and demonstrate that large non-Gaussianities of orthogonal shape can be generated, for the first time in a concrete model. More generally, we find that the sign and the shape of the bispectrum offer powerful diagnostics of the precise strength of the induced gravity.