On CMB B-modes and the Onset of Inflation

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.