Disformally coupled inflation

TL;DR

This paper explores a novel disformal coupling between two scalar fields during inflation, revealing that such interactions can support viable inflation with distinctive predictions like low tensor-to-scalar ratio.

Contribution

It introduces a new disformal coupling framework in inflationary models, analyzing its effects on dynamics, perturbations, and observational predictions.

Findings

Disformal coupling can inhibit one field from inflating while allowing the other to drive inflation.

The model predicts a low tensor-to-scalar ratio consistent with current observations.

Perturbation spectra are derived for exponential coupling models, showing viable inflationary scenarios.

Abstract

A disformal coupling between two scalar fields is considered in the context of cosmological inflation. The coupling introduces novel derivative interactions mixing the kinetic terms of the fields but without introducing superluminal or unstable propagation of the two scalar fluctuation modes. Though the typical effect of the disformal coupling is to inhibit one of the fields from inflating the universe, the energy density of the other field can drive viable near Sitter -inflation in the presence of nontrivial disformal dynamics, in particular when one assumes exponential instead of power-law form for the couplings. The linear perturbation equations are written for the two-field system, its canonical degrees of freedom are quantised, their spectra are derived and the inflationary predictions are reported for numerically solved exponential models. A generic prediction is low tensor-to-scalar ratio.