Moving backwards in time, the first observable of the Universe is the Cosmic Microwave Background, signature of the recombination process occurred at z~1100. Limited by the sensitivity of current telescopes, we are now able to detect objects up to z~8. Nowadays, we are thus dealing with a lack of observational data in the range 8 Это и многое другое вы найдете в книге Reionization Signatures in Quasar Absorption Spectra (Simona Gallerani)