# On CMB B-modes and the Onset of Inflation

## TL;DR

This paper explores how early inflation dynamics could cause observable suppression in large-scale B-mode polarization of the CMB, potentially detectable by future experiments like LiteBIRD.

## Contribution

It introduces a semi-analytic method to study the impact of inflaton deceleration on primordial tensor perturbations and B-mode polarization spectrum.

## Key findings

- Suppression of large-scale B-modes may be detectable with future probes.
- The method highlights the physics behind the B-mode spectrum distortion.
- Evidence suggests reionization knowledge is crucial for detection.

## Abstract

The temperature perturbations of the cosmic microwave background radiation (CMB) appear systematically suppressed, at large angular scales, with respect to the prediction of the LambdaCDM concordance model. This behavior might be a glimpse of the inflaton deceleration from fast-roll at the start of inflation. If this were true, the spectrum of primordial tensor perturbations should be distorted in a corresponding way, with a suppression of large angular scale B-mode polarization that might be within reach for future probes like LiteBIRD. We investigate this scenario by a semi-analytic method based on a truncation of the B-mode polarization spectrum, which in our opinion has the virtue of highlighting the underlying Physics. We present some mild evidence that, with sufficient knowledge of the reionization process, the suppression of large-scale B-modes might be within reach.