Fine tuning and the ratio of tensor to scalar density fluctuations from cosmological inflation

TL;DR

This paper explores how natural inflationary models constrain the ratio of tensor to scalar fluctuations, finding that naturalness does not necessarily imply a detectable level of gravitational waves in the CMB.

Contribution

It derives constraints on the tensor-to-scalar ratio in natural inflation models, highlighting its role as a discriminator among different inflationary scenarios.

Findings

Natural inflation models do not require a large tensor-to-scalar ratio.

The tensor-to-scalar ratio is a sensitive indicator to distinguish inflationary models.

Naturalness constraints do not necessarily lead to detectable B-mode polarization.

Abstract

The form of the inflationary potential is severely restricted if one requires that it be natural in the technical sense, i.e. terms of unrelated origin are not required to be correlated. We determine the constraints on observables that are implied in such natural inflationary models, in particular on $r$, the ratio of tensor to scalar perturbations. We find that the naturalness constraint does not require $r$ to be lare enough to be detectable by the forthcoming searches for B-mode polarisation in CMB maps. We show also that the value of $r$ is a sensitive discriminator between inflationary models.