After the recombination epoch neutral hydrogen was formed in the universe. Since neutral hydrogen absorbs radiation in the Lyman-alpha transition, a dark epoch begun, which was finished with the reionization of hydrogen due to radiation sources. The electromagnetic spectrum from quasars, which are at enough spatio-temporal distance (z > 6 ?) should show specific Lyman-alpha absobtion lines (inside the Ly-a forest) due to neutral hydrogen. This is called the Gunn-Peterson effect. Questions: 1. is my understanding correct? 2. is the Gunn Peterson effect only a part of the the Ly-a forest, what is the differencie or how can this effect be registered…? Thanks.