The epoch of reionization occurred when some form of energy, possibly radiation from the first generation of stars that formed after the Big Bang or quasars, heated surrounding gas and ionized it. This changed the universe from being relatively transparent to opaque since radiation scattered off the ionized gas. This article is part of a special issue devoted to the IAU General Assembly, helping experts in this field learn about current research.

http://www.raa-journal.org/raa/index.php/raa/article/view/1198