The oldest known quasars (redshift = 6) display a Gunn-Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas.
Os quasares máis vellos (z > 4) mostran un efecto Gunn-Peterson e teñen zonas de absorción diante deles indicando que o medio intergaláctico nese momento era gas neutro.