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The formation of the first galaxies marks a major transition in the evolution of structure in the Universe. These same galaxies with their zero metallicity Population III stars, second generation Population II stars, and black hole driven sources (e.g., mini-quasars, x-ray binaries, etc.) transformed the intergalactic medium from neutral to ionized. This process, known as the Epoch of Reionization (EoR), is the central topic discussed in this chapter.

As mentioned in chapter # 1 [by A. Loeb in this book], about 400,000 years after the Big Bang, the Universe's density decreased enough so that the temperature fell below 3000 K, allowing ions and electrons to (re)combine into neutral hydrogen and helium – the fraction of heavier elements wasf negligible. Immediately afterwards, photons decoupled from baryons and the Universe became transparent, leaving a relic signature known as the cosmic microwave background (CMB) radiation. This event ushered the Universe into a period of darkness, known as the dark ages.

The dark ages ended about 400 million years later, when the first galaxies formed and start emitting ionizing radiation. Initially during the EoR, the intergalactic medium (IGM) is neutral except in regions surrounding the first objects. However, as this reionization progresses, an evolving patchwork of neutral (H I) and ionized hydrogen (H II) regions unfolds. After a sufficient number of UV-radiation emitting objects formed, the temperature and the ionized fraction of the gas in the Universe increase rapidly until eventually the ionized regions permeate to fill the whole Universe [9, 115, 30, 44, 41, 67, 135].

The current constraints strongly suggest that the EoR roughly occurs within the redshift range of z ~ [6-15]. Figure 1 shows a space-redshift slice of a simulation of the progression of reionization with time and how it appears in 21 cm brightness temperature, which is proportional to the density of neutral hydrogen (see section 4). At high redshifts most of the gas is neutral, hence, the signal is mostly sensitive to cosmological density fluctuations, whereas at lower redshifts ionization bubbles start to appear until they fill the whole Universe [9].

Figure 1

Figure 1. This figure shows a slice through redshift of the 21 cm radiation in which the reionization process progresses through the volume of a cosmological simulation with radiative transfer [206].

The EoR is a watershed epoch in the history of the Universe. Prior to it, the formation and evolution of structure was dominated by dark matter alone, while baryonic matter played a marginal role. The EoR marks the transition to an era in which the role of cosmic gas in the formation and evolution of structure became prominent and, on small scales, even dominant.

The details of the reionization scenario I have laid out are yet to be clarified. For example, it is not known what controls the formation of the first objects and how much ionizing radiation they produce, or how the ionization bubbles expand into the intergalactic medium and what they ionize first, high-density or low-density regions?. The answer to these questions and many others that arise in the context of studying the EoR needs knowledge of fundamental issues in cosmology, galaxy formation, quasars and the physics of very metal poor stars; all including foremost research in topics in modern astrophysics. Substantial theoretical and observational efforts are currently dedicated to understanding the physical processes that trigger this epoch and govern its evolution, and ramifications on subsequent structure formation (c.f., [9, 30, 44, 41, 67]). However, despite the pivotal role played by the EoR in cosmic history, observational support for the proposed scenarios is very scarce, and when available, is indirect and model dependent.

In principle, there are many different ways to observationally probe the EoR. In this contribution, I mainly focus on the redshifted 21cm emission line from neutral hydrogen at high redshifts. This is one of the most promising techniques for studying the dark ages and the EoR. To date, there are a number of telescopes dedicated to measure this faint radiation. In the short term, these consist of: The Low Frequency Array (LOFAR), the Murchison Widefield Array (MWA), Precision Array to Probe Epoch of Reionization (PAPER) and Giant Metrewave Radio Telescope (GMRT), while, on a somewhat longer time scales the Square Kilometer Array (SKA). One of the most challenging tasks in studying the EoR is to extract and identify the cosmological signal from the data and interpret it correctly. This is because the detectable signal in the frequency range relevant to the EoR is composed of a number of components – the cosmological EoR signal, extragalactic and Galactic foreground, ionospheric distortions, instrumental response and noise – each with its own physical origin and statistical properties.

Figure 2 shows a sketch of the likely evolution of reionization from the neutral hydrogen point of view. The figure emphasizes the other non-cosmological effects that are seen with the 21 cm experiments, e.g., foreground, ionosphere and instrumental effects. The radio antennas seen at the bottom are LOFAR's Low Band Antennas.

Figure 2

Figure 2. This figure shows a sketch of the likely development of the EoR. About 500,000 years after the Big Bang (z ~ 1000) hydrogen recombined and remained neutral for a few hundred million years during the dark ages. At a redshift, z ~ 15, the first stars, galaxies and quasars began to form, heating and reionizing the hydrogen gas. The neutral IGM can be observed with LOFAR up to z ≈ 11.5 through its redshifted 21cm spin-flip transition. However, many atmospheric, galactic and extra-galactic emission contaminate the 21 cm signal.

In this chapter I discuss various observational and theoretical aspects of the Epoch of Reionization. In section 2 the current observational scene is reviewed, specifically focusing on the CMB data and the Lyman alpha forest spectra. In sections 3 and 4, we discuss, respectively, the physics of the reionization process and the 21 cm line transition and how it could be used to probe reionization. The redshifted 21 cm experiments their potentials and the challenges are discussed in section 5. Extraction and quantification of the information stored in the redshifted 21 cm data using various statistics is discussed in section 6. This chapter concludes with a brief summary (section 7).

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