How Thermal Inflation can save Minimal Hybrid Inflation in Supergravity

TL;DR

This paper shows that adding a brief period of thermal inflation after primordial inflation can make minimal hybrid inflation in supergravity consistent with observational data, especially the spectral index.

Contribution

It introduces a novel approach of using thermal inflation to salvage minimal hybrid inflation in supergravity without complex Kahler modifications.

Findings

Spectral index within 1-sigma Planck bounds for large VEV

Tensor-to-scalar ratio potentially observable soon

Thermal inflation can be applied to other models

Abstract

Minimal hybrid inflation in supergravity has been ruled out by the 2015 Planck observations because the spectral index of the produced curvature perturbation falls outside observational bounds. To resurrect the model, a number of modifications have been put forward but many of them spoil the accidental cancellation that resolves the eta-problem and require complicated Kahler constructions to counterbalance the lost cancellation. In contrast, in this paper the model is rendered viable by supplementing the scenario with a brief period of thermal inflation, which follows the reheating of primordial inflation. The scalar field responsible for thermal inflation requires a large non-zero vacuum expectation value (VEV) and a flat potential. We investigate the VEV of such a flaton field and its subsequent effect on the inflationary observables. We find that, for large VEV, minimal hybrid inflation in supergravity produces a spectral index within the 1-sigma Planck bound and a tensor-to-scalar ratio which may be observable in the near future. The mechanism is applicable to other inflationary models.