The Epoch of Reionization (EoR) is a term used to describe the period during which the gas in the Universe went from being almost completely neutral to a state in which it became almost completely ionized. This watershed event - which has occurred when the Universe was a few hundred million years old (about a twentieth of its current age) and the first radiating objects formed - is intimately linked to many fundamental questions in cosmology and structure formation and evolution.
Despite its pivotal role, the EoR is one of the least understood epochs in the Universe's evolution. A large amount of theoretical effort, guided by very limited observational evidence, is currently dedicated to understanding the physical processes that trigger this epoch, govern its evolution, and what ramifications it had on subsequent structure formation. In the near future, the LOFAR telescope, which has the EoR as one of its key projects, is set to measure the neutral gas fraction in the Universe as a function of redshift and angular position through the hydrogen hyperfine spin-flip 21 cm line. The 21 cm line is, probably, the only observable tracers of the gas during the EoR. It allows detailed mapping of the EoR as it progresses in time and space (see Figure 1).
Figure 1. This Figure shows a sketch of the likely development of the EoR. About 0.4 million years after the Big Bang (z~1100) hydrogen recombined and remained neutral for about 400 million years until the first radiation emitting sources emerged, an era known as the “Universe’s dark ages”. At z~10, the first stars, galaxies and quasars began to form and heat and ionize the intergalactic medium. The neutral IGM can be observed with LOFAR through its redshifted 21cm spin-flip transition to redshift 11.5 (when the Universe was 400 million years old). However, many instrumental, ionospheric, Galactic and extra-galactic contaminants corrupt the 21 cm signal. (Courtesy of V. Jelić).