The Lyman Alpha Series is a method used by physicists and cosmologists to detect the presence of hydrogen, usually in the form of vast clouds, in the universe and Quasars, or quasi-stellar radio sources. These facts aid cosmological research by giving one an idea about where the quasars are located, how quickly they are receding, the boundaries of the universe and more. First, though, one must be familiar with the properties of hydrogen.
Hydrogen is the lightest and most abundant element in the universe. It is composed of one proton and one electron. As explained in the article on atoms, electrons have energy levels starting from n=1, the lowest. When energy is applied to them, they basically “jump up” to another level, n>1, depending on the amount of energy added. When light at a wavelength of 1216 Å (Angstroms) hits a hydrogen atom, the electron becomes excited, absorbing this energy and jumping to the next energy level.

Scientists use this fact to determine the size and location of hydrogen clouds. If light is plotted on a graph, and there is a “dip” on the graph that corresponds to a wavelength of 1216 Å, then this means that hydrogen is present somewhere between the Earth and the Quasar. We can derive the location of the cloud by determining the how much the light has been stretched. This stretching occurs because the universe is expanding. Scientists take the wavelength of light received on Earth and extrapolate backwards until they obtain a value of 1216 Å. More extrapolation can be done in order to determine the location of the Quasar that has emitted the light. As said before, when light at a wavelength of 1216 Å strikes a hydrogen atom, the electron absorbs the energy and jumps up to a higher energy level.

The electron, however very quickly moves back down to n=1 and emits a photon. In other words, when the electron is boosted to a higher energy level, it does not stay there; it comes back down quickly. Thus, the extra photons that are emitted from these clouds create a bulge in the graph. The combination of absorption and emission lines on a graph are what scientists call the Lyman Alpha Forest.