The Doppler shift of this phenomenon is commonly used to plot galaxy rotation curves.
Temperature deviations leading to this phenomenon are strongly dependent on the decoupling induced by the Wouthuysen-Field coupling. This phenomenon is the most common structural probe of the cold neutral and warm neutral components of the ISM. When this phenomenon takes on a two-horned structure, it is indicative of a rotating gaseous disk. The width of this phenomenon was correlated with absolute magnitude by Tully and Fisher to derive their namesake relation. This phenomenon is used by experiments like PAPER and LOFAR to probe the era between recombination and re-ionization, a time called the (*) "dark ages". The Doppler shift of this phenomenon is commonly used to plot galaxy rotation curves. Originally predicted by van de Hulst and discovered by Ewen and Purcell, this phenomenon has a frequency of about 1420 megahertz and occurs due to a split in the 2S-sub-one-half ground state due to hyperfine interactions. For 10 points, name this radiation emitted by neutral hydrogen atoms, usually named for its wavelength.
Temperature deviations leading to this phenomenon are strongly dependent on the decoupling induced by the Wouthuysen-Field coupling.
21 cm line [accept hydrogen line before mention; accept Lyman-alpha line before the word “ISM”]