The tensor-to-scalar ratio in punctuated inflation

TL;DR

This paper demonstrates that punctuated inflation models can produce a tensor-to-scalar ratio exceeding unity on large scales, potentially explaining certain CMB polarization features and offering new observational signatures.

Contribution

It presents the first examples of single scalar field inflationary models with tensor-to-scalar ratio greater than one, linking large-scale power suppression to enhanced tensor modes.

Findings

Tensor-to-scalar ratio exceeds unity on large scales.

Large-scale suppression of scalar power correlates with increased tensor modes.

Enhanced B-mode polarization at low multipoles due to high r.

Abstract

Recently, we have shown that scalar spectra with lower power on large scales and certain other features naturally occur in {\it punctuated inflation}, i.e. the scenario wherein a brief period of rapid roll is sandwiched between two stages of slow roll inflation. Such spectra gain importance due to the fact that they can lead to a better fit of the observed Cosmic Microwave Background (CMB) anisotropies, when compared to the conventional, featureless, power law spectrum. In this paper, with examples from the canonical scalar field as well as the tachyonic models, we illustrate that, in punctuated inflation, a drop in the scalar power on large scales is {\it always} accompanied by a rise in the tensor power and, hence, an even more pronounced increase in the tensor-to-scalar ratio $r$ on these scales. Interestingly, we find that $r$ actually {\it exceeds well beyond unity} over a small range of scales. {\it To our knowledge, this work presents for the first time, examples of single scalar field inflationary models wherein $r\gg 1$.} This feature opens up interesting possibilities. For instance, we show that the rise in $r$ on large scales translates to a rapid increase in the angular power spectrum, $C_{\ell}^{\rm BB}$, of the B-mode polarization of the CMB at the low multipoles. We discuss the observational implications of these results.