Gunn-Peterson test

Gunn–Peterson test

A test for the presence of neutral hydrogen and singly-ionized helium in the diffuse intergalactic medium, first proposed by J.E. Gunn and B.A. Peterson in 1965. Such hydrogen and helium are expected to cause absorption of a quasar continuum shortward of its rest-frame Lyman emission lines; the fact that this continuum is detected (in between the Lyman-alpha forest) reveals that most of the IGM is already highly ionized, at least to a redshift of 4–5.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006