About a billion years after the Big Bang, the Epoch of Reionisation saw the first light sources in the Universe slowly ionise the primordial atoms of the surrounding IGM. Learning about this distant epoch has the potential of unveiling crucial information about the formation of the first stars, galaxies, and early black holes, which sourced it. However, its observation remains elusive. Attempted measurements of the 21cm signal, coming directly from the high-redshift neutral hydrogen, are still plagued with foregrounds and systematics, whilst modelling uncertainties prevent us from extracting the reionisation signatures from large astrophysical and cosmological data sets.
In this talk, I will tell you how cosmologists are using gigantic radio interferometers in the most remote locations and state-of-the-art CMB telescopes to understand the nature and evolution of the first ever light sources. I will give an overview of what has already been done, and what is yet to achieve to get our answers. Then, I will describe my contributions to this great adventure, from modelling the evolution of the Universe and developing innovative techniques to sift through the enormous amounts of data produced, to plugging antennas in the South African desert! I will focus on the results from the Hydrogen Epoch of Reionisation Array (HERA), which has recently given the lowest upper limits on the 21cm power spectrum at redshifts z > 7, and on the re-analysis of the latest small-scale CMB observations by the South Pole Telescope. I will then argue the potential of cross correlations, especially of CMB and 21cm signal observables, to give a coherent and accurate picture of reionisation.
