Reheating in R2 Palatini inflationary models

Abstract

We consider R 2 inflation in the Palatini gravity assuming the existence of scalar fields, coupled to gravity in the most general manner. These theories, in the Einstein frame, and for one scalar field h, share common features with K -inflation models. We apply this formalism for the study of popular inflationary models, whose potentials are monomials, V ∼ h n , with n a positive even integer. We also study the Higgs model non-minimally coupled to gravity. Although these have been recently studied, in the framework of the Palatini approach, we show that the scalar power spectrum severely constrains these models. Although we do not propose a particular reheating mechanism, we show that the quadratic ∼ h 2 and the Higgs model can survive these constraints with a maximum reheating temperature as large as ∼ 10 15 GeV , when reheating is instantaneous. However, this can be only attained at the cost of a delicate fine-tuning of couplings. Deviations from this fine-tuned values can still yield predictions compatible with the cosmological data, for couplings that lie in very tight range, giving lower reheating temperatures.

Tracing the high energy theory of gravity: an introduction to Palatini inflation

“…Refs. [32,33,38,42,49,50] for the inclusion of an R 2 term and Refs. [36,41] for non-vanishing torsion and kinetic terms for the metric in the Palatini case, which introduce multiple extra terms to the action.…”

Initial conditions for plateau inflation: a case study

“…[2,[50][51][52][53][54][55][56][57][58][59][60][61]) and α = 0 to the scenario studied in Refs. [32,33,38,42,46,47], while α = 0 = ξ represents the minimally coupled case where only the form of V (φ) is important.…”

Constant-roll in the Palatini-R2 models

“…The choice of the parameters 6 α = 10 7 and λ = 10 −13 are such that they reproduce the observable value of the scalar power spectrum, as was also noted in [44,46,49]. If we allow for the case of varying α, while keeping the other two parameters β and λ at constant values, we obtain the following table.…”