Quantum theory tells us that when absorbing light from distant light sources each element will absorb a particular wavelength corresponding to its electron configuration. When we on earth observe this light we are able to see distinct absorption patterns allowing us to identify which elements that specific beam of light encountered on its way to earth. Scientists are especially interested in the elements encountered by the beam it the star or quasar’s (a supermassive black hole found in the center of many galaxies which emits high energy light waves) atmosphere as well as the intergalactic medium. The quantity of the element encountered, known as column density can be determined as well along with temperature by observing the features of the absorption line. The doppler effect explains the reason these features occur the way they do in more detail. Because light travels at a finite speed, when we observe light from distant quasars we are essentially looking back in time and are thereby studying the evolution of gas in the cosmic history.
Since hydrogen is the most prevalent element in the universe most of the absorption is caused by light waves hitting hydrogen molecules. This type of light absorption was originally studied by Theodore Lyman, a 19th century physicist after whom the Lyman Alpha Forest (as well as the Lyman Beta Forest etc.) is named. The Lyman Alpha Forest is defined as, “the effect of seeing many such lines [primary absorption lines due to hydrogen] at different wavelengths superimposed on the quasars spectrum due to hydrogen in the intergalactic medium” (https://www.cfa.harvard.edu/~yuan-sen.ting/lyman_alpha.html) Since its original study the Lyman Alpha forest has been used to determine that the intergalactic medium is made primarily of plasma, consisting of ionized molecules. In studying the light from distant quasars astrophysicists James E. Gunn and Bruce A. Peterson calculated that in the absence of plasma all of the light from their specific quasar of study should be absorbed by stable hydrogen molecules in the intergalactic medium, this however was not the case. Because not all of the light was absorbed, they concluded that the electrons must be able to move freely signaling the presence of plasma. Today one of the biggest issues in this facet of astrophysics is the question of at what point did the intergalactic medium ionize. It will be interesting to follow this topic in the next two years as scientists find out more.