Gravitational Lensing

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.In CMB polarization, gravitational lensing is the largest guaranteed source of B‐mode (or curl‐like) polarization. Future CMB polarization experiments with sufficient sensitivity to measure B‐modes on small angular scales (l∼1000) can measure lensing with better sensitivity, and on different scales, than could be achieved by measuring CMB temperature alone. If the instrumental noise is sufficiently small (≲5 μK‐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.