The EoR, which starts about 400 million years after the Big-Bang, represents a major phase transition for hydrogen. Due to the formation of the first astrophysical sources, hydrogen in this epoch transforms from fully neutral to fully ionized. The EoR could be traced in space and time using relic radio emission that will be observed by the LOFAR radio telescope starting from the end of this year.
The EoR is determined by how and when the Universe started forming astrophysical objects and how the ionizing radiation from these objects permeates and fills the intergalactic medium. The EoR is related to many fundamental questions in cosmology, properties of the first (mini-)quasars, formation of very metal-poor stars and a slew of other important research topics in astrophysics. Hence uncovering it will have far reaching implications on the study of structure formation in the early Universe.
Currently, there are only few observational constraints on the epoch of reionization. The CMB temperature and polarization data obtained by the WMAP satellite allow measurement of the total Thomson scattering of the primordial CMB photons off intervening free electrons produced by the epoch of reionization along the line of sight. They show that the CMB intensity has only been damped by ~ 9%, indicating that the Universe was mostly neutral for 400 million years and then ionized. However, the Thomson scattering measurement is an integral constraint telling us little about the sources of reionization, its duration or how it propagated to fill the whole Universe.
Another constraint comes from specific features in the spectra of distant quasars, known as the Lyman forest. These features, which are due to neutral hydrogen, indicate two important facts about reionization. First, hydrogen in the recent Universe is highly ionized, only 1 part in 10000 being neutral. Second, the neutral fraction of hydrogen in the distant Universe suddenly increases at redshift 6.5, i.e., about 900 million years after the Big Bang, demarcating the end of the reionization process. Despite these data providing strong constraints on the ionization state of the Universe at redshifts below 6.5, they say very little about the reionization process itself. Another couple of constraints come also from the Lyman forest systems, IGM temperature and the number of ionizing photons per baryon, suggesting the bulk of the reionization process occurs at late redshifts z ≈ [6-9].
A whole slew of possible constraints currently discussed in the literature are either very controversial, very weak or, as is often the case, both. Most are very interesting and exciting, but can be investigated reliably only with a new generation of instruments such as the James Webb Space Telescope, replacing the Hubble Space Telescope in the next decade.
The imminent availability of observations of redshifted 21 cm radiation from the Universe's dark ages and the EoR will be one of the most exciting developments in the study of cosmology and galaxy and structure formation in recent years. Currently, there are a number of instruments that are designed to measure this radiation. In this contribution I have argued that despite the many difficulties that face such measurements they will provide a major breakthrough in our understanding of this crucial epoch. In particular current radio telescopes, such as LOFAR, will be able to provide us with the global history of the EoR progression, the fluctuations power spectrum during the EoR, etc., up to z ≈ 11. These measurements will usher the study of the high redshift Universe into a new era which will bridge, at least in part, the large gap that currently exists in observation between the very high redshift Universe (z ≈ 1100) as probed by the CMB and the low redshift Universe (z 6).
Although the current generation of telescopes have a great promise they will also have limitations. For example they have neither the resolution, the sensitivity nor the frequency coverage to address many fundamental issues, like the nature of the first sources. Crucially, they will not provide a lot of information about the dark ages which is only accessible through very low frequencies in the range of 40-120 (z ≈ 35-11). Fortunately, in the future SKA can improve dramatically on the current instruments in terms of sensitivity, redshift coverage and resolution.
The next decade will be extremely exciting for studying the high redshift Universe, especially as these radio telescopes gradually come online, starting with LOFAR, GMRT and MWA. They promise to resolve many of the puzzles we have today pertaining to the formation and evolution of the first objects cosmology, and the physical processes in the high redshift intergalactic medium.
Acknowledgements I would like to thank Geraint Harker, Stephen Rafter and Rajat M. Thomas for careful reading of the manuscript. Many of the results shown here have been obtained in collaboration with the members of the LOFAR EoR project whose contribution I would like to acknowledge. I would like also thank the editors of this book for giving me the opportunity to write this chapter.