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Authors: Steven Furlanetto, Chris L. Carilli, Jordan Mirocha, James Aguirre, Yacine Ali-Haimoud, Marcelo Alvarez, Adam Beardsley, George Becker, Judd D. Bowman, Patrick Breysse, Volker Bromm, Philip Bull, Jack Burns, Isabella P. Carucci, Tzu-Ching Chang, Xuelei Chen, Hsin Chiang, Joanne Cohn, Frederick Davies, David DeBoer, Joshua Dillon, Olivier Doré, Cora Dvorkin, Anastasia Fialkov, Nick Gnedin, Bryna Hazelton, Daniel Jacobs, Kirit Karkare, Saul Kohn, Leon Koopmans, Ely Kovetz, Paul La Plante, Adam Lidz, Adrian Liu, Yin-Zhe Ma, Yi Mao, Kiyoshi Masui, Matthew McQuinn, Andrei Mesinger, Julian Munoz, Steven Murray, Aaron Parsons, Jonathan Pober, Benjamin Saliwanchik, Jonathan Sievers, Eric Switzer, Nithyanandan Thyagarajan, Hy Trac, Eli Visbal, Matias Zaldarriaga

Abstract: The epoch of reionization, when photons from early galaxies ionized the intergalactic medium about a billion years after the Big Bang, is the last major phase transition in the Universe's history. Measuring the characteristics of the transition is important for understanding early galaxies and the cosmic web and for modeling dwarf galaxies in the later Universe. But such measurements require probes of the intergalactic medium itself. Here we describe how the 21-cm line of neutral hydrogen provides a powerful probe of the reionization process and therefore important constraints on both the galaxies and intergalactic absorbers at that time. While existing experiments will make precise statistical measurements over the next decade, we argue that improved 21-cm analysis techniques - allowing imaging of the neutral gas itself - as well as improved theoretical models, are crucial for testing our understanding of this important era.