One of the least explored phases of the Universe’s evolution is its very first billion years. During this period, the Universe underwent the fundamental transition from one with no sources of light, to the one lit by the incredible variety of stars and galaxies which we observe at later times. Understanding how the first billion years unfolded hence addresses the fundamental question of how all stars and galaxies in the Universe were born.
Paradoxically, our best shot at directly detecting light emitted from this period is not to observe the radiation emitted from the first sources of light themselves, but rather the light emitted by the gas surrounding them, the InterGalactic Medium (IGM).
One of the main components of the IGM during this phase was neutral hydrogen (HI), which emits a characteristic emission at the 21cm wavelength. The radiation emitted by the first stars and galaxies however destroyed – or ionised – the HI, leading to a progressive fading of the 21cm emission on cosmological scales, until no neutral hydrogen was left.
By studying how the HI was progressively destroyed by the light emitted by surrounding sources, we can reconstruct how these first stars and galaxies were born and evolved through time.
Due to the importance of the HI ionisation process in our observation of this phase, this period is commonly referred to as the Epoch of Reionisation (EoR).
Ground-breaking technological advances in cosmological 21cm observations, which for the first time will allow us to directly observe the large-scale evolution of EoR, will soon be within reach, thanks to the development of instruments such as HERA and SKA.