CMB-S4: Forecasting Constraints on Primordial Gravitational Waves
Authors: Abazajian Kevork , Addison Graeme E. , Adshead Peter , Ahmed Zeeshan , Akerib Daniel , Ali Aamir , Allen Steven W. , Alonso David , Alvarez Marcelo , Amin Mustafa A. , Anderson Adam , Arnold Kam S. , Ashton Peter , Baccigalupi Carlo , Bard Debbie , Barkats Denis , Barron Darcy , Barry Peter S. , Bartlett James G. , Basu Thakur Ritoban , Battaglia Nicholas , Bean Rachel , Bebek Chris , Bender Amy N. , Benson Bradford A. , Bianchini Federico , Bischoff Colin A. , Bleem Lindsey , Bock James J. , Bocquet Sebastian , Boddy Kimberly K. , Richard Bond J. , Borrill Julian , Bouchet François R. , Brinckmann Thejs , Brown Michael L. , Bryan Sean , Buza Victor , Byrum Karen , Hervias Caimapo Carlos , Calabrese Erminia , Calafut Victoria , Caldwell Robert , Carlstrom John E. , Carron Julien , Cecil Thomas , Challinor Anthony , Chang Clarence L. , Chinone Yuji , Sherry Cho Hsiao-Mei , Cooray Asantha , Coulton Will , Crawford Thomas M. , Crites Abigail , Cukierman Ari , Cyr-Racine Francis-Yan , de Haan Tijmen , Delabrouille Jacques , Devlin Mark , Di Valentino Eleonora , Dierickx Marion , Dobbs Matt , Duff Shannon , Dvorkin Cora , Eimer Joseph , Elleflot Tucker , Errard Josquin , Essinger-Hileman Thomas , Fabbian Giulio , Feng Chang , Ferraro Simone , Filippini Jeffrey P. , Flauger Raphael , Flaugher Brenna , Fraisse Aurelien A. , Frolov Andrei , Galitzki Nicholas , Gallardo Patricio A. , Galli Silvia , Ganga Ken , Gerbino Martina , Gluscevic Vera , Goeckner-Wald Neil , Green Daniel , Grin Daniel , Grohs Evan , Gualtieri Riccardo , Gudmundsson Jon E. , Gullett Ian , Gupta Nikhel , Habib Salman , Halpern Mark , Halverson Nils W. , Hanany Shaul , Harrington Kathleen , Hasegawa Masaya , Hasselfield Matthew , Hazumi Masashi , Heitmann Katrin , Henderson Shawn , Hensley Brandon , Hill Charles , Colin Hill J. , Hložek Renée , Patty Ho Shuay-Pwu , Hoang Thuong , Holder Gil , Holzapfel William , Hood John , Hubmayr Johannes , Huffenberger Kevin M. , Hui Howard , Irwin Kent , Jeong Oliver , Johnson Bradley R. , Jones William C. , Hwan Kang Jae , Karkare Kirit S. , Katayama Nobuhiko , Keskitalo Reijo , Kisner Theodore , Knox Lloyd , Koopman Brian J. , Kosowsky Arthur , Kovac John , Kovetz Ely D. , Kuhlmann Steve , Kuo Chao-lin , Kusaka Akito , Lähteenmäki Anne , Lawrence Charles R. , Lee Adrian T. , Lewis Antony , Li Dale , Linder Eric , Loverde Marilena , Lowitz Amy , Lubin Phil , Madhavacheril Mathew S. , Mantz Adam , Marques Gabriela , Matsuda Frederick , Mauskopf Philip , McCarrick Heather , McMahon Jeffrey , Daniel Meerburg P. , Melin Jean-Baptiste , Menanteau Felipe , Meyers Joel , Millea Marius , Mohr Joseph , Moncelsi Lorenzo , Monzani Maria , Mroczkowski Tony , Mukherjee Suvodip , Nagy Johanna , Namikawa Toshiya , Nati Federico , Natoli Tyler , Newburgh Laura , Niemack Michael D. , Nishino Haruki , Nord Brian , Novosad Valentine , O’Brient Roger , Padin Stephen , Palladino Steven , Partridge Bruce , Petravick Don , Pierpaoli Elena , Pogosian Levon , Prabhu Karthik , Pryke Clement , Puglisi Giuseppe , Racine Benjamin , Rahlin Alexandra , Sathyanarayana Rao Mayuri , Raveri Marco , Reichardt Christian L. , Remazeilles Mathieu , Rocha Graca , Roe Natalie A. , Roy Anirban , Ruhl John E. , Salatino Maria , Saliwanchik Benjamin , Schaan Emmanuel , Schillaci Alessandro , Schmitt Benjamin , Schmittfull Marcel M. , Scott Douglas , Sehgal Neelima , Shandera Sarah , Sherwin Blake D. , Shirokoff Erik , Simon Sara M. , Slosar Anže , Spergel David , St. Germaine Tyler , Staggs Suzanne T. , Stark Antony , Starkman Glenn D. , Stompor Radek , Stoughton Chris , Suzuki Aritoki , Tajima Osamu , Teply Grant P. , Thompson Keith , Thorne Ben , Timbie Peter , Tomasi Maurizio , Tristram Matthieu , Tucker Gregory , Umiltà Caterina , van Engelen Alexander , Vavagiakis Eve M. , Vieira Joaquin D. , Vieregg Abigail G. , Wagoner Kasey , Wallisch Benjamin , Wang Gensheng , Watson Scott , Westbrook Ben , Whitehorn Nathan , Wollack Edward J. , Kimmy Wu W. L. , Xu Zhilei , Eric Yang H. Y. , Yasini Siavash , Yefremenko Volodymyr G. , Won Yoon Ki , Young Edward , Yu Cyndia , Zonca Andrea ,   The CMB-S4 Collaboration
Published: February 2022

Abstract
CMB-S4—the next-generation ground-based cosmic microwave background (CMB) experiment—is set to significantly advance the sensitivity of CMB measurements and enhance our understanding of the origin and evolution of the universe. Among the science cases pursued with CMB-S4, the quest for detecting primordial gravitational waves is a central driver of the experimental design. This work details the development of a forecasting framework that includes a power-spectrum-based semianalytic projection tool, targeted explicitly toward optimizing constraints on the tensor-to-scalar ratio, r, in the presence of Galactic foregrounds and gravitational lensing of the CMB. This framework is unique in its direct use of information from the achieved performance of current Stage 2–3 CMB experiments to robustly forecast the science reach of upcoming CMB-polarization endeavors. The methodology allows for rapid iteration over experimental configurations and offers a flexible way to optimize the design of future experiments, given a desired scientific goal. To form a closed-loop process, we couple this semianalytic tool with map-based validation studies, which allow for the injection of additional complexity and verification of our forecasts with several independent analysis methods. We document multiple rounds of forecasts for CMB-S4 using this process and the resulting establishment of the current reference design of the primordial gravitational-wave component of the Stage-4 experiment, optimized to achieve our science goals of detecting primordial gravitational waves for r > 0.003 at greater than 5σ, or in the absence of a detection, of reaching an upper limit of r < 0.001 at 95% CL.