Cosmic Reionization and the First Nonlinear Structures in the Universe

TL;DR

This paper discusses the early stages of cosmic reionization, highlighting its significance as a milestone in universe evolution and its role in understanding the formation of the first structures.

Contribution

It provides an overview of expectations, open questions, and historical context regarding the ionization of hydrogen and helium in the early universe.

Findings

Reionization marks a key milestone in cosmic evolution.

Open questions remain about the timing and processes of reionization.

Reionization serves as a probe for early structure formation.

Abstract

In this introductory chapter, we outline expectations for when and how the hydrogen and helium atoms in the universe turned from neutral to ionized, focusing on the earliest, least well understood stages, and emphasizing the most important open questions. We include a historical summary, and highlight the role of reionization as one of the few milestones in the evolution of the universe since the Big Bang, and its status as a unique probe of the beginning stages of structure formation.