Kinetic Gravity Braiding and axion inflation

TL;DR

This paper introduces a new inflationary model using higher derivative kinetic gravity braiding of an axion field, allowing for sub-Planckian decay constants and consistent with observed CMB spectra.

Contribution

The authors develop a novel inflationary framework with higher derivative kinetic gravity braiding, enabling sub-Planckian axion decay constants and addressing limitations of natural inflation.

Findings

Model achieves sub-Planckian axion decay constants.

Induces an almost scale-invariant power spectrum.

Operates within a wide parameter regime.

Abstract

We constructed a new class of inflationary model with the higher derivative axion field which obeys constant shift symmetry. In the usual axion (natural) inflation, the axion decay constant is predicted to be in the super-Planckian regime which is believed to be incompatible with an effective field theory framework. With a novel mechanism originating from a higher derivative kinetic gravity braiding (KGB) of an axion field we found that there exist a huge parameter regime in our model where axion decay constant could be naturally sub-Planckian. Thanks to the KGB which effectively reduces the Planck constant. This effectively reduced Planck scale provides us the mechanism of further lowering down the speed of an axion field rolling down its potential without introducing super-Planckian axion decay constant. We also find that with that wide range of parameter values, our model induces almost scale invariant power spectrum as observed in CMB experiments.