Palatini $R^2$ Quintessential Inflation

TL;DR

This paper develops a Palatini $R^2$ gravity model with a scalar field that unifies inflation and dark energy, fitting observational data without fine-tuning the cosmological constant.

Contribution

It introduces a novel Palatini $R^2$ inflationary model with a scalar field and running non-minimal coupling that explains both early and late universe acceleration.

Findings

Model fits CMB inflation data

Reproduces current dark energy density

Avoids fine-tuning of the cosmological constant

Abstract

We construct a model of quintessential inflation in Palatini $R^2$ gravity employing a scalar field with a simple exponential potential and coupled to gravity with a running non-minimal coupling. At early times, the field acts as the inflaton, while later on it becomes the current dark energy. Combining the scalar sector with an ideal fluid, we study the cosmological evolution of the model from inflation all the way to dark energy domination. We interpret the results in the Einstein frame, where a coupling emerges between the fluid and the field, feeding energy from the former to the latter during the matter-dominated era. We perform a numerical scan over the parameter space and find points that align with observations for both the inflationary CMB data and the late-time behaviour. The final dark energy density emerges from an interplay between the model parameters, without requiring the extreme fine-tuning of the cosmological constant in $\Lambda$CDM.