The very first luminous objects which formed in the Universe had a
profound effect on the surrounding environment. Until that time, the
Universe was composed almost entirely of neutral gas in the form of
neutral hydrogen atoms. The first light in the Universe ionised the surrounding gas and this is known as the Epoch of Reionisation. It marks the end of the Cosmic Dark Ages.
It is possible to probe the Epoch of Reionisation and trace the history of the formation of the first luminous objects by sensitive measurements of the spectral signature of neutral hydrogen. Neutral hydrogen emits energy at radio wavelengths, and neutral hydrogen at earlier times in the Universe is redshifted to longer wavelengths. These wavelengths are the primary observing range of the Square Kilometre Array.
The SKA will detect neutral hydrogen and the onset of reionisation, and it will trace the reionsation history of the Universe up to the present time.