Gunn–Peterson trough

In astronomical spectroscopy, the Gunn–Peterson trough is a feature of the spectra of bright high redshift sources, particularly quasars and gamma-ray burst afterglows, due to the presence of neutral hydrogen in the Intergalactic medium (IGM).

The trough is characterized by suppression of electromagnetic emission from the source at wavelengths less than that of the Lyman-alpha line at the redshift of the emitted light.

It was not until 2001, with the discovery of a quasar with a redshift z = 6.28 by Robert Becker and others[3] using data from the Sloan Digital Sky Survey, that a Gunn–Peterson trough was finally observed.

The article also included quasars at redshifts of z = 5.82 and z = 5.99, and, while each of these exhibited absorption at wavelengths on the blue side of the Lyman-alpha transition, there were numerous spikes in flux as well.

The flux of the quasar at z = 6.28, however, was effectively zero beyond the Lyman-alpha limit, meaning that the neutral hydrogen fraction in the IGM must have been larger than ~10−3.