Understanding the Epoch of Cosmic Reionization ― Challenges and Progress

The aim of this volume is to summarize the current status and future outlook of the reionization field, on both the theoretical and observational fronts. It brings together leading experts in many sub-disciplines, highlighting the measurements that are likely to drive the growing understanding of reionization and the cosmic dawn, and lays out a roadmap to interpreting the wealth of upcoming observations. What is the best use of limited observational resources? How to develop theoretical tools tailored for each observation? Ultimately, what will be learned about the epoch of reionization and the Universe's galactic ancestors?
The birth of the first stars and galaxies, and their impact on the diffuse matter permeating the early Universe, is one of the final frontiers in cosmology. Recently, measurements of the fluctuations in the cosmic microwave background (CMB), sourced only a few hundred thousand years after the Big Bang, provided robust insight into the overall physical content of our Universe. On the other end of the timeline, groundbreaking telescopes, like the Hubble Space Telescope, provide us a picture of the complexities of the galaxy-rich universe in which we now live, billions of years later. However, we know almost nothing about the astrophysics of the first billion years.
During this relatively brief epoch, a tiny fraction of matter condensed inside the first galaxies, forming the first stars. The light from these objects and subsequent generations of galaxies began spreading throughout the vast, web-like intergalactic medium (IGM). This culminated in the final major phase change of our universe, cosmological reionization, when ionizing radiation stripped electrons from almost every atom in our universe. Reionization lifted the cosmic fog, allowing visible light to finally spread throughout space. The universe was no longer a cold, dark place. This mysterious epoch or reionization corresponds to the transition between the relative simplicity of the early universe, and the complexity of the present-day one, 14 billion years later. It is fundamental in understanding cosmic origins, and its impact on structure formation resonates even to this day. Until recently, there was very little observational insight into the epoch of reionization. Subsequent observational and theoretical advancements have begun to paint a picture of a complicated, extended, inhomogeneous process, whose details remain elusive. At its core, the process of cosmological reionization involves understanding how stars and clumps of gas impact each other and eventually the entire Universe. The challenges associated with such an enormous range of relevant scales, coupled with our relatively poor understanding of the dominant astrophysics, have thus far impeded efforts to form a solid theoretical framework for reionization. As such, the interpretation of even the sparse reionization data currently available remains controversial. The ability to interpret this wealth of data is also increasing thanks to more sophisticated analytical and numerical approaches. Investigations have become subtler, discarding the "one size fits all" approach to simulations in favor of focused studies with specialized tools placing astrophysics on the cusp of a dramatic increase in knowledge of this uncharted cosmological frontier.

Andrei Mesinger is a Junior Professor at Scuola Normale Superiore of Pisa who received his PhD at Columbia. Dr. Mesinger was a Hubble Postdoctoral Fellow at the Princeton University Department of Astrophysical Sciences and a Postdoctoral Fellow at the Physics Departments of Yale University and UCLA. His research interests are first light, reionization, high-redshift sources, and modeling techniques.