Bimetric structure formation: non-Gaussian predictions

TL;DR

This paper explores non-minimal bimetric models with a running disformal coupling, predicting tilted spectra and distinctive non-Gaussian features, extending previous minimal models with scale-invariant fluctuations.

Contribution

It introduces non-minimal bimetric models with a variable disformal coupling, deriving new predictions for spectral tilt and non-Gaussianity distortions.

Findings

Non-minimal models predict tilted spectra.

Distinctive distortions to equilateral non-Gaussianity are identified.

A consistency relation links spectral index and non-Gaussian distortion.

Abstract

The minimal bimetric theory employing a disformal transformation between matter and gravity metrics is known to produce exactly scale-invariant fluctuations. It has a purely equilateral non-Gaussian signal, with an amplitude smaller than that of DBI inflation (with opposite sign) but larger than standard inflation. We consider non-minimal bimetric models, where the coupling $B$ appearing in the disformal transformation ${\hat g}_{\mn}= g_{\mn} -B\partial_\mu\phi\partial_\nu\phi$ can run with $\phi$. For power-law $B(\phi)$ these models predict tilted spectra. For each value of the spectral index, a distinctive distortion to the equilateral property can be found. The constraint between this distortion and the spectral index can be seen as a "consistency relation" for non-minimal bimetric models.