Antony Lewis, Alex Hall, Anthony Challinor

Lensing of the CMB is an important effect, and is usually modelled by remapping the unlensed CMB fields by a lensing deflection. However the lensing deflections also change the photon path so that the emission angle is no longer orthogonal to the background last-scattering surface. We give the first calculation of the emission-angle corrections to the standard lensing approximation from dipole (Doppler) sources for temperature and quadrupole sources for temperature and polarization. We show that while the corrections are negligible for the temperature and E-mode polarization, additional large-scale B-modes are produced with a white spectrum that dominates those from post-Born field rotation (curl lensing). On large scales about one percent of the total lensing-induced B-mode amplitude is expected to be due to this effect. However, the photon emission angle does remain orthogonal to the perturbed last-scattering surface due to time delay, and half of the large-scale emission-angle B modes cancel with B modes from time delay to give a total contribution of about half a percent. While not important for planned observations, the signal could ultimately limit the ability of delensing to reveal low amplitudes of primordial gravitational waves.We also derive the rotation of polarization due to multiple deflections between emission and observation. The rotation angle is of quadratic order in the deflection angle, and hence negligibly small: polarization typically rotates by less than an arcsecond, orders of magnitude less than a small-scale image rotates due to post-Born field rotation (which is quadratic in the shear). The field-rotation B modes dominate the other effects on small scales.

Maximilian H. Abitbol, Zeeshan Ahmed, Darcy Barron, Ritoban Basu Thakur, Amy N. Bender, Bradford A. Benson, Colin A. Bischoff, Sean A. Bryan, John E. Carlstrom, Clarence L. Chang, David T. Chuss, Ari Cukierman, Tijmen de Haan, Matt Dobbs, Tom Essinger-Hileman, Jeffrey P. Filippini, Ken Ganga, Jon E. Gudmundsson, Nils W. Halverson, Shaul Hanany, Shawn W. Henderson, Charles A. Hill, Shuay-Pwu P. Ho, Johannes Hubmayr, Kent Irwin, Oliver Jeong, Bradley R. Johnson, Sarah A. Kernasovskiy, John M. Kovac, Akito Kusaka, Adrian T. Lee, Salatino Maria, Philip Mauskopf, Jeff J. McMahon, Lorenzo Moncelsi, Andrew W. Nadolski, Johanna M. Nagy, Michael D. Niemack, Roger C. O'Brient, Stephen Padin, Stephen C. Parshley, Clement Pryke, Natalie A. Roe, Karwan Rostem, John Ruhl, Sara M. Simon, Suzanne T. Staggs, et al. (12 additional authors not shown)

CMB-S4 is a proposed experiment to map the polarization of the Cosmic Microwave Background (CMB) to nearly the cosmic variance limit for the angular scales that are accessible from the ground. The science goals and capabilities of CMB-S4 in illuminating cosmic inflation, measuring the sum of neutrino masses, searching for relativistic relics in the early universe, characterizing dark energy and dark matter, and mapping the matter distribution in the universe have been described in the CMB-S4 Science Book. This Technology Book is a companion volume to the Science Book. The ambitious science goals of the proposed "Stage-IV" CMB-S4 will require a step forward in experimental capability from the current Stage-III experiments. To guide this process, the community summarized the current state of the technology and identify R&D efforts necessary to advance it for possible use in CMB-S4. The book focused on the technical challenges in four broad areas: Telescope Design; Receiver Optics; Focal-Plane Optical Coupling; and Focal-Plane Sensor and Readout.