In astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars.
These absorption lines result from intergalactic gas through which the galaxy or quasar's light has traveled. Since the absorption and emission of light follow the laws of quantum mechanics, only photons with specific energies can be absorbed. This causes each individual absorption line. The forest is created by the fact that photons that come to us from distant light sources show Hubble redshift that depends on the distance between us and the source of light.
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. Searching for lines from other elements like helium, carbon and silicon (matching in redshift), the abundance of heavier elements in the clouds can also be studied. A cloud with a high column density of neutral hydrogen will show typical damping wings around the line and is referred to as a damped Lyman-alpha system.
For quasars at higher redshift the number of lines in the forest is higher, until at a redshift of about 6, there is so much neutral hydrogen in the intergalactic medium that the forest turns into a Gunn-Peterson trough. This shows the end of the reionization of the universe.
Read more about Lyman-alpha Forest: Cosmology
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