For 10 points, what is this electromagnetic radiation that is usually named for its vacuum wavelength?
Red shift alters this entity in the Alcock-Paczynski Effect. When absorbed by the interstellar medium, it can be used to calculate the ISM's kinetic temperature. Since a supply of neutral matter is required for it to exist, detection of this entity is virtually the only source of astronomical data for the era in between recombination and reionization. It was predicted by van de Hulst in the 1940's, but was actually discovered by Ewen and Purcell, who used radio astronomy to probe the arms of the Milky Way. It forms one bound(*) of what SETI researchers often refer to as "the water hole." Corresponding to a frequency of about 1420 megahertz, this radiation results from an atomic transition involving an approximately 6 micro-electronvolt energy change between two hyperfine levels of its namesake element, hydrogen. For 10 points, what is this electromagnetic radiation that is usually named for its vacuum wavelength?
Red shift alters this entity in the Alcock-Paczynski Effect.
21 cm line [prompt on the hydrogen line before mention]