Bimetric structure formation: non-Gaussian predictions

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 ${\hat g}_{\mn}= g_{\mn} -B\partial_μϕ\partial_νϕ$ can run with $ϕ$. For power-law $B(ϕ)$ 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.

17 pages, 2 figures. References and minor comments added in revised version