Constraining primordial tensor features with the anisotropies of the Cosmic Microwave Background

TL;DR

This paper investigates the possibility of scale-dependent features in primordial gravitational wave spectra using CMB data, finding no current evidence but highlighting future experimental prospects for detection.

Contribution

It introduces a method to search for scale-dependent tensor features in CMB data and forecasts the improved constraints from upcoming experiments.

Findings

No evidence for tensor spectrum bumps in current data.

Future experiments like LiteBIRD and CMB-S4 will enhance detection capabilities.

Delensing improves constraints at smaller angular scales.

Abstract

It is commonly assumed that the stochastic background of gravitational waves on cosmological scales follows an almost scale-independent power spectrum, as generically predicted by the inflationary paradigm. However, it is not inconceivable that the spectrum could have strongly scale-dependent features, generated, e.g., via transient dynamics of spectator axion-gauge fields during inflation. Using the temperature and polarisation maps from the \textit{Planck} and BICEP/Keck datasets, we search for such features, taking the example of a log-normal bump in the primordial tensor spectrum at CMB scales. We do not find any evidence for the existence of bump-like tensor features at present, but demonstrate that future CMB experiments such as LiteBIRD and CMB-S4 will greatly improve our prospects of determining the amplitude, location and width of such a bump. We also highlight the role of delensing in constraining these features at angular scales $\ell\gtrsim 100$.