Lyman-alpha forest

[3] Subsequently, the spectra of many other high-redshift quasars were observed to have the same system of narrow absorption lines.

[5] For a neutral hydrogen atom, spectral lines are formed when an electron transitions between energy levels.

Since neutral hydrogen clouds in the intergalactic medium are at different degrees of redshift (due to their varying distance from Earth), their absorption lines are observed at a range of wavelengths.

Each individual cloud leaves its fingerprint as an absorption line at a different position in the observed spectrum.

The Lyman-alpha forest is an important probe of the intergalactic medium and can be used to determine the frequency and density of clouds containing neutral hydrogen, as well as their temperature.

A computer simulation of a possible Lyman-alpha forest configuration at z = 3
A quasar spectrum with Lyman absorbers being continuously redshifted due to cosmic expansion forming a "forest" of lines.