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The Lyman-alpha forest is a series of absorption lines in electromagnetic radiation (EMR) from distant galaxies and quasars consisting of Lyman alpha (Ly-α), the first spectral line of the Lyman series (121.6 nm), redshifted at various degrees. As continuum emission from the distant astronomical object passes through intervening intergalactic clouds (i.e., in the intergalactic medium (IGM)), neutral hydrogen (such as in an HI region) absorbs some of the light at the lyman-alpha wavelength, producing absorption lines. We observe the absorption lines at the redshifts of the intervening clouds that formed them, allowing us to detect these clouds, along with information on their size (from the line depth), and their location (the same celestial coordinates as the further light source, and the redshift of the absorption line). Such a forest can be discerned in a high spectral resolution spectrum of a single point source such as a quasar, yielding information regarding the material along the line of sight. The information inherent in such a forest is useful for studying the large scale structure of the universe. The Baryon Oscillation Spectroscopic Survey (BOSS) uses this technique to find evidence of baryon acoustic oscillations.
The forest of absorption lines at the redshift of the quasar provide clues to the quasar's characteristics and history, through a phenomena termed the proximity effect. EMR from quasars tends to ionize the surrounding IGM, making it more transparent at the Lyman-α wavelength, resulting in a redshift-range immediately in front of the quasar in which there is less Lyman-α absorption. The affected region is termed the proximity zone. The effect has the potential of showing how long the quasar has (had) been shining, giving some indication of the age of the quasar at that time, and how long the supermassive black hole (SMBH) has been there. It also has the potential of showing earlier variation of the brightness of the quasar as a function of time.
Lyman-beta forests are also observed, and have value in that it can be observed from the ground at somewhat smaller redshifts, adding to the data that can be collected by ground telescopes.