Naturally large tensor-to-scalar ratio in inflation

Abstract

Recently, BICEP2 measurements of the cosmic microwave background (CMB) $B$-mode polarization at degree angular scales has indicated the presence of tensor modes with a high tensor-to-scalar ratio of $r=0.2$ when assuming nearly scale-invariant tensor and scalar spectra, although the signal may be contaminated by dust emission as implied by the recent {\em Planck} polarization data. This result is in conflict with the {\em Planck} best-fit Lambda Cold Dark Model with $r<0.11$. Due to the fact that inflaton has to be interacting with other fields so as to convert its potential energy into radiation to reheat the Universe, the interacting inflaton may result in a suppression of the scalar spectrum at large scales. This suppression has been used to explain the observed low quadrupole in the CMB anisotropy. In this paper, we show that a combination of the tensor modes measured by BICEP2 and the large-scale suppressed scalar modes contributes to the CMB anisotropy in such a way that the resultant CMB anisotropy and polarization power spectra are consistent with both {\em Planck} and BICEP2 data. We also project our findings to cases in which $r$ may become reduced in future CMB polarization measurements.