The Lyman-alpha forest is a series of absorption lines in electromagnetic radiation (EMR) from distant galaxies and quasars consisting of Lyman alpha, the first spectral line of the Lyman series (121.6 nm), redshifted to various degrees. As light passes through intervening intergalactic clouds (i.e., in the intergalactic medium (IGM)) neutral hydrogen (such as in an HI region), radiation is absorbed, and by recognizing and measuring the shifted Lyman-alpha lines, characteristics of the intervening clouds can be determined including location and size. This 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.
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. This phenomena has been termed the proximity effect (and the affected region, the proximity zone) which 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 has been there. It also has the potential of showing earlier variation of the brightness of the quasar as a function of time.