In section 3. Cosmology
Cosmic microwave background constraints in light of priors over reionization histories
Understanding how the gas between galaxies was transformed from the neutral state left over from the recombination epoch into the ionized state that we observe today is one of the most urgent open questions in physical cosmology. Such process occurred within one billion years of the Big Bang, and is known as cosmic reionization. It can be investigated with several different techniques. Among the most widely used is the study of the imprint left by the scattering of cosmic microwave background photons of free electrons along their path from the last scattering surface to us. Such an integral signal has been measured with exquisite precision by the Planck satellite, and interpreted as indicating an early start to reionization. However, that interpretation is not unique as it depends on the detailed reionization history which is currently not known precisely. To overcome this problem the authors use a non-parametric reconstruction approach to set tight limits on the extent of early reionization. This result might be in tension with the recent EDGES detection of a 21 cm line reionization signal, if the latter is to be confirmed.