A case for kinetically dominated initial conditions for inflation

TL;DR

This paper argues that initial conditions for inflation should be set during the kinetically dominated regime at the Planck epoch, especially for potentials with a plateau or hill, as most of the relevant phase space is kinetically dominated.

Contribution

It provides a detailed analysis of phase-space for inflationary potentials, showing that kinetically dominated initial conditions are natural and prevalent at the Planck epoch.

Findings

Most of the Planck surface is kinetically dominated for relevant inflaton field values.

Equipartition of energy at the Planck epoch is questionable for plateau or hill potentials.

Kinetic dominance is prevalent for small amplitude potentials consistent with current data.

Abstract

We make a case for setting initial conditions for inflation at the Planck epoch in the kinetically dominated regime. For inflationary potentials with a plateau or a hill, i.e. potentials that are bounded from above within a certain region of interest, we cannot claim complete ignorance of the energy distribution between kinetic and potential energy, and equipartition of energy at the Planck epoch becomes questionable. We analyse different classes of potentials in phase-space and quantify the fraction of the Planck surface that is kinetically dominated. For the small amplitudes of the potentials as suggested by current data, the Planck surface lies in the region of kinetic dominance for almost all values of interest of the inflaton field.