Prospects for Delensing the Cosmic Microwave Background for Studying Inflation

Abstract

A detection of excess cosmic microwave background (CMB) B-mode polarization on large scales allows the possibility of measuring not only the amplitude of these fluctuations but also their scale dependence, which can be parametrized as the tensor tilt n T . Measurements of this scale dependence will be hindered by the secondary B-mode polarization anisotropy induced by gravitational lensing. Fortunately, these contaminating B modes can be estimated and removed with a sufficiently good estimate of the intervening gravitational potential and a good map of CMB E-mode polarization. We present forecasts for how well these gravitational lensing B modes can be removed, assuming that the lensing potential can be estimated either internally from CMB data or using maps of the cosmic infrared background (CIB) as a tracer. We find that CIB maps are as effective as CMB maps for delensing at the noise levels of the current generation of CMB experiments, while the CMB maps themselves will ultimately be best for delensing at polarization noise below ∆ P =1 µK-arcmin. At this sensitivity level, CMB delensing will be able to measure n T to an accuracy of 0.02 or better, which corresponds to the tensor tilt predicted by the consistency relation for single-field slow-roll models of inflation with r = 0.2. However, CIB-based delensing will not be sufficient for constraining n T in simple inflationary models.

CMB Polarization B-mode Delensing with SPTpol and Herschel

“…We note, however, f < L 150 modes contribute less to the lensing B-modes at the degree scale, where the predicted PGW signal peaks, than to the range we are testing in this work (see e.g., Figure 2 in Simard et al 2015). Thus recovering f CIB modes at < L 150 might be less critical for PGW searches.…”

“…The model used to compute this is described in Section 3.2. Depending on the angular scale, we expect 45%-65% of the CIB to be correlated with f. Almost all angular scales below < ℓ 1000 in f (with a relative weight gradually decreasing at high ℓ) are responsible for generating lensing B-modes in the range considered in this work (see Figure 2 in Simard et al 2015).…”

“…However this method is sub-optimal because the multicomponent fit is subject to the sample variance of the lensing B modes. Instead, the specific realization of lensing B modes on the sky can be characterized and removed in a process called "delensing," thus significantly improving constraints on r. While delensing has been studied at a theoretical level for many years (Kesden et al 2002;Knox & Song 2002;Seljak & Hirata 2004;Smith et al 2012;Sherwin & Schmittfull 2015;Simard et al 2015), it has only recently been performed on CMB data (Larsen et al 2016;Carron et al 2017).…”

Demonstration of Cosmic Microwave Background Delensing Using the Cosmic Infrared Background

“…This is then used to reverse the lensing in CMB maps, or, equivalently in the case of B-mode delensing, construct a linearized estimate of the lens-induced modes that is subtracted off (see, e.g., Refs. [11][12][13][14][15][16][17][18]). Though often CMBinternal delensing (i.e., using a lensing map reconstructed from the CMB itself) is considered, Ref.…”

“…Though often CMBinternal delensing (i.e., using a lensing map reconstructed from the CMB itself) is considered, Ref. [19] discuss in detail the use of the cosmic infrared background (CIB) for this purpose; see also [17]. The CIB is mostly made up of unresolved emission from dusty star-forming galaxies at high redshifts; as the CMB lensing arises from similar high redshifts, the CIB should be strongly correlated with the lensing convergence.…”