Ultraviolet completion of the inflationary paradigm

TL;DR

This paper introduces a new nonlocal quantum gravity model that completes $f(R)$ theories, including Starobinsky inflation, ensuring compatibility with observations and resolving previous issues with renormalizability and stability.

Contribution

It proposes a nonlocal, super-renormalizable or finite quantum completion of $f(R)$ theories that preserves classical inflationary solutions and improves theoretical consistency.

Findings

The model maintains classical background solutions and perturbation equations.

It overcomes previous incompatibilities between renormalizability and stability.

The theory is at least super-renormalizable, possibly finite, ensuring a consistent quantum gravity framework.

Abstract

After an exhaustive introduction highlighting the strengths and weaknesses of the non-local models proposed so far as ultraviolet completions of the Starobinsky theory, we propose a new nonlocal completion of a general $f(R)$ theory (in the Einstein's frame) suitable for driving inflation in the early universe consistently with observations. The nonlocal theory shares with $f(R)$ the same background solutions and the same equations of motion for perturbations at linear and nonlinear level. Therefore, the classical cosmological observables are not affected by the nonlocal operators needed for the quantum completion. Our construction applies to any local action written in the Einstein's frame, but we will provide the details only for two explicit examples: the Starobinsky model and a general $f(R)$ theory. The new model overcomes the incompatibility of renormalizability and stability present in the previous proposals. Since the nonlocal theory is at least super-renormalizable, but can also be finite depending on the details of the model, this work shows the consistency of the inflationary paradigm with a well-defined quantum theory of gravity at high energy. We could rephrase the latter statement saying that the success of the $f(R)$ theories in cosmology can be traced back to the existence of an ultraviolet completion that preserves all the classical features. The inflationary paradigm survives, or it is actually insensitive to quantum gravity, because it is an exact solution of quantum gravity, up to perturbative corrections.