Ultra slow-roll inflation demystified

Konstantinos Dimopoulos

doi:10.1016/j.physletb.2017.10.066

Abstract

Ultra-slow-roll (USR) inflation is a new mode of inflation which corresponds to the occasions when the inflaton field must traverse an extremely flat part of the scalar potential, when the usual slow-roll (SR) fails. We investigate USR and obtain an estimate for how long it lasts, given the initial kinetic density of the inflaton. We also find that, if the initial kinetic density is small enough, USR can be avoided and the usual SR treatment is valid. This has important implications for inflection-point inflation.

Highlights

Cosmic inflation is an organic component of the concordance model of cosmology
We have showed that this is a temporary phase of inflation, not an attractor, and obtained an estimate of how many efolds it lasts, depending on the initial kinetic density of the inflaton field
We have discussed how the field can depart from the usual slow-roll (SR) when crossing an extremely flat patch in the scalar potential

Summary

Introduction

Cosmic inflation is an organic component of the concordance model of cosmology. It is a period of exponential expansion in the early Universe, which determines the initial conditions for the subsequent Hot Big Bang cosmology. Inflation is modelled through the inflationary paradigm, which suggests that the Universe undergoes inflation when dominated by the potential density of a scalar field (inflaton). It was recently realised that SR may end when the potential becomes steep and curved, as is for the end of inflation, and when it suddenly becomes extremely flat, too flat for the regular SR assumptions to apply In this case, the system engages in so-called ultra slow-roll (USR) inflation. It is fair to say that the community seems little aware of USR, so the hope is that our treatment may be revealing of USR’s nature This is a acute problem in models of inflection-point inflation, where a region of USR exists around the inflection point.

Ultra-slow roll inflation

Ultra-slow-roll in linear inflation and beyond

Ultra-slow-roll in inflection-point inflation

Quantum diffusion

Perturbations

Conclusions