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Detecting the 21 cm emission from this time, all the way through to the end of reionization, has been proposed as a powerful way of studying early structure formation. [9] This period of the Universe's history corresponds to redshifts of z ≈ 30 {\displaystyle z\approx 30} to z ≈ 6 − 12 {\displaystyle z\approx 6-12} , implying a frequency ...
A hydrogen atom with proton and electron spins aligned (top) undergoes a flip of the electron spin, resulting in emission of a photon with a 21 cm wavelength (bottom) The hydrogen line, 21 centimeter line, or H I line [a] is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms.
This is accomplished by looking at the 21-cm line emission produced by hot diffuse neutral hydrogen from distant galaxy clusters and from the intracluster medium. [1] This neutral hydrogen traces out the large scale structures in the universe, and so can be used to map out the large scale Baryon Acoustic Oscillation (BAO) structure of the universe.
For example, the 2 → 1 line is called "Lyman-alpha" (Ly-α), while the 7 → 3 line is called "Paschen-delta" (Pa-δ). Energy level diagram of electrons in hydrogen atom. There are emission lines from hydrogen that fall outside of these series, such as the 21 cm line.
The waterhole, or water hole, is an especially quiet band of the electromagnetic spectrum between 1420 and 1662 megahertz, corresponding to wavelengths of 18–21 centimeters. It is a popular observing frequency used by radio telescopes in radio astronomy. [1]
Hyperfine structure gives the 21 cm line observed in H I regions in interstellar medium. Carl Sagan and Frank Drake considered the hyperfine transition of hydrogen to be a sufficiently universal phenomenon so as to be used as a base unit of time and length on the Pioneer plaque and later Voyager Golden Record .
The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of 1.420 405 751 768 GHz, [1] which corresponds to the 21 cm line in the hydrogen spectrum. Hydrogen masers are very complex devices and sell for as much as US$235,000. [2] There are two types to be distinguished: active and passive.
The presence of other molecules close to the molecule involved affects both line width and line position. It is the dominant process for liquids and solids. An extreme example of this effect is the influence of hydrogen bonding on the spectra of protic liquids. Observed spectral line shape and line width are also affected by instrumental factors.