Heidelberg Joint Astronomical Colloquium

During the Epoch of Reionization (EoR) gas in the Universe ionized for the second time, after having been neutral for approximately 400 Myr during the so called "Cosmic Dark Ages". The EoR is the second of two major phase transitions of hydrogen in the Universe, the first phase being recombination at z=1100, during which hydrogen became neutral. The EoR is thought to be caused by the first astrophysical sources of radiation and its study is crucial to our understanding of the physics of these sources and how they influenced the formation of later generations of astrophysical objects. As such, the EoR is related to many fundamental questions in cosmology, galaxy formation, quasars and ultra-low metallicity stars; all leading research topics in modern astrophysics. Current constraints indicate that the EoR occurred between 6.5 < z < 12 (set by SDSS quasars and WMAP). It is widely acknowledged that the best probe of this crucial epoch is the redshifted HI 21cm emission line coming from the diffuse neutral hydrogen at these redshifts.

In my talk I will review the current understanding of this epoch and the large effort made in order to constrain it observationally. In particular, I will focus on the 21cm probe and its basic physics. I will then summarize the current observational effort to observe the 21cm radiation from the EoR and the challenges that face it. I will especially focus on the LOFAR EoR project and discuss its potential in uncovering this important event in the history of the Universe.