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About four hundred million years after the birth of our Universe the first objects were formed, which then started to ionize the surrounding gas with their strong radiation. Six hundred million years later, the all-pervasive gas was transformed from a neutral to an ionized state. This pivotal period in the history of the Universe is called the Epoch of Reionization (EoR). It holds the key to structure formation and evolution, but also represents a missing piece of the puzzle in our current knowledge of the Universe. Currently, this is changing with the completion of a new generation of radio telescopes, which are capable of directly probing the EoR. LOFAR is the first telescope of this kind, and will use an array of simple radio antennas to hunt for the radiation emitted by the neutral hydrogen during the EoR. The wavelength of this radiation is 21 cm, but on its way to us it is stretched by the Universe's expansion to the radio wavelengths of 1-2 m. However, its detection will be quite a challenge due to a number of complicating factors. For example, the desired signal is so weak that it is like a needle in a haystack, overwhelmed by the prominent foreground emission of our own Galaxy and other extragalactic radio sources. This thesis examines both the properties of the "haystack" and the way it influences the LOFAR-EoR experiment, and discusses the Cosmic Microwave Background radiation (the oldest radiation in the Universe) as an additional probe of the EoR.

Cosmological 21cm experiments: Searching for a needle in a haystack