Modified natural inflation: A small single field model with a large tensor to scalar ratio

TL;DR

This paper proposes a modified natural inflation model incorporating higher derivative kinetic terms to achieve a large tensor-to-scalar ratio while maintaining sub-Planckian field values, aligning with recent cosmological observations.

Contribution

It introduces a kinetic gravity braiding modification to natural inflation, enabling large tensor modes with sub-Planckian decay constants and analyzing preheating challenges.

Findings

Achieves $n_s \,\approx\, 0.96$ and $r > 0.1$ in the model.

Maintains sub-Planckian axion decay constant $f$ and inflation scale $\\Lambda$.

Preheating via Chern-Simons coupling is unlikely in this framework.

Abstract

In this paper we explored in detail a phenomenological model of modified single field natural inflation in light of recent cosmological experiments, BICEP2. Our main goal is to construct an inflationary model which not only predicts the important cosmological quantities such as $(n_s, r)$ compatible with experimental observation, but also is consistent with the low energy effective theory framework. Therefore, all the fundamental scale apart from $M_p$ and quantities of our interest should be within the sub-Planckian region. In order to achieve our goal we modify the usual single field natural inflationary model by a specific form of higher derivative kinetic term called kinetic gravity braiding (KGB). One of our guiding principles to construct such a model is the constant shift symmetry of the axion. We have chosen the form of the KGB term in such a way that it predicts the required value of $n_s\simeq 0.96$ and a large tensor to scalar ratio $r> 0.1$. Importantly for a wide range of parameter space our model has sub-Planckian axion decay constant $f$ and the scale of inflation $\Lambda$. However, the reheating after the end of inflation limits the value of $f$ so that we are unable to get $f$ to be significantly lower than $M_p$. Furthermore, we find sub-Planckian field excursion for the axion field $\Delta \phi \simeq f$ for the sufficient number of e-folding ${\cal N} \gtrsim 50 $. We also discussed in detail about the natural preheating mechanism in our model based on the recently proposed Chern-Simon coupling. We found this gravity mediated preheating is very difficult to achieve in our model. With our general analytic argument, we also would like to emphasize that Chern-Simons mediated preheating is very unlikely to happen in any slow roll inflationary model.