Rapid roll Inflation with Conformal Coupling

TL;DR

This paper explores a novel inflationary model with a conformally coupled scalar field that rapidly rolls, offering an alternative to slow-roll inflation, and discusses its implications for string-theoretic warped inflation scenarios.

Contribution

It introduces a rapid roll inflation model with conformal coupling, demonstrating its viability and implications for string-inspired inflationary frameworks.

Findings

Existence of an attractor inflationary solution with rapid rolling

Warped inflation can achieve 37 efolds with low energy scales

Conformal coupling prevents scalar fields from generating cosmological fluctuations

Abstract

Usual inflation is realized with a slow rolling scalar field minimally coupled to gravity. In contrast, we consider dynamics of a scalar with a flat effective potential, conformally coupled to gravity. Surprisingly, it contains an attractor inflationary solution with the rapidly rolling inflaton field. We discuss models with the conformal inflaton with a flat potential (including hybrid inflation). There is no generation of cosmological fluctuations from the conformally coupled inflaton. We consider realizations of modulated (inhomogeneous reheating) or curvaton cosmological fluctuations in these models. We also implement these unusual features for the popular string-theoretic warped inflationary scenario, based on the interacting D3-anti D3 branes. The original warped brane inflation suffers a large inflaton mass due to conformal coupling to 4-dimensional gravity. Instead of considering this as a problem and trying to cure it with extra engineering, we show that warped inflation with the conformally coupled, rapidly rolling inflaton is yet possible with N=37 efoldings, which requires low energy scales 1-100 TeV of inflation. Coincidentally, the same warping numerology can be responsible for the hierarchy. It is shown that the scalars associated with angular isometries of the warped geometry of compact manifold (e.g. S^3 of KS geometry) have solutions identical to conformally coupled modes and also cannot be responsible for cosmological fluctuations. We discuss other possibilities.