CMBPol Mission Concept Study: Gravitational Lensing

TL;DR

The paper evaluates the scientific benefits and technical considerations of measuring gravitational lensing in CMB polarization, highlighting its role in constraining cosmological parameters and improving primordial gravitational wave detection.

Contribution

It provides a comprehensive analysis of the science case for CMB lensing measurements, including parameter constraints, contamination reduction, and instrumental design trade-offs.

Findings

CMB lensing constrains neutrino mass and structure growth.

High-resolution measurements enable lens reconstruction to separate lensing and primordial signals.

Contamination from non-CMB foregrounds and systematics must be carefully controlled.

Abstract

Gravitational lensing of the cosmic microwave background by large-scale structure in the late universe is both a source of cosmological information and a potential contaminant of primordial gravity waves. Because lensing imprints growth of structure in the late universe on the CMB, measurements of CMB lensing will constrain parameters to which the CMB would not otherwise be sensitive, such as neutrino mass. If the instrumental noise is sufficiently small (<~ 5 uK-arcmin), the gravitational lensing contribution to the large-scale B-mode will be the limiting source of contamination when constraining a stochastic background of gravity waves in the early universe, one of the most exciting prospects for future CMB polarization experiments. High-sensitivity measurements of small-scale B-modes can reduce this contamination through a lens reconstruction technique that separates the lensing and primordial contributions to the B-mode on large scales. A fundamental design decision for a future CMB polarization experiment such as CMBpol is whether to have coarse angular resolution so that only the large-scale B-mode (and the large-scale E-mode from reionization) is measured, or high resolution to additionally measure CMB lensing. The purpose of this white paper is to evaluate the science case for CMB lensing in polarization: constraints on cosmological parameters, increased sensitivity to the gravity wave B-mode via lens reconstruction, expected level of contamination from non-CMB foregrounds, and required control of beam systematics.