Kinetic gravity braiding and natural inflation in the light of BICEP2 and PLANCK

Abstract

Based on our previous work, we constructed a phenomenological model of inflation with the higher derivative axion field in the light of recent cosmological expertiments BICEP2 and PLANCK. In order to achieve observed values for the important cosmological parameters $(n_s,r)$ we employ higher derivative kinetic term called kinetic gravity braiding (KGB) for the axion in compatible with the constant shift symmetry. Phenomenologically we choose a particular form of the braiding function $M(\phi)$ which correctly reproduces the observed value of $(n_s, r)$ based on the recent cosmological observations. Furthermore we also find axion decay constant $f$ and the scale of inflation $\Lambda$ to be naturally sub-Planckian consistent with the reheating after the end of inflation. Within the sufficient number of e-folding ${\cal N}$, we also find sub-Planckian field excursion for the axion field $\Delta \phi \simeq f$.