Linked from

The 18 pages that link to 21-centimeter line, each with the reason it gives.

• Cosmic microwave backgroundCompared with: It can probe later cosmic epochs beyond the background’s last-scattering surface.

• Interstellar mediumRelated: Its emission maps atomic hydrogen across the Milky Way, including regions opaque to visible light.

• Radio astronomyRelated: Mapping this line traces hydrogen across galaxies and the distant universe.

• HydrogenRelated: This signal lets astronomers map vast clouds of hydrogen in the Milky Way and beyond.

• Spectral lineBroader topic: Its line maps neutral hydrogen across the Milky Way and other galaxies.

• Hydrogen atomBroader topic: This line maps hydrogen gas across galaxies and the early universe.

• Hyperfine structureBroader topic: This transition traces neutral hydrogen across interstellar and intergalactic space.

• Hydrogen maserRelated: The same transition is central to the maser and became a landmark in radio astronomy.

• Neutral hydrogenBroader topic: This is the characteristic signal produced by neutral hydrogen’s spin transition.

• Voyager Golden RecordRelated: The cover’s timing diagram uses this transition as a universal unit.

• Pioneer plaqueRelated: Its wavelength and frequency provide the plaque’s proposed universal units of length and time.

• Galactic planeRelated: Its emission maps hydrogen across the Milky Way’s disk, including regions hidden by dust.

• Zone of AvoidanceRelated: Radio surveys using this emission can map gas and galaxies through the obscuring plane.

• Dark galaxyRelated: It can reveal gas in systems too faint to detect through starlight.

• High-velocity cloudRelated: Its Doppler shift reveals the line-of-sight velocities used to identify high-velocity clouds.

• Interstellar cloudRelated: Its signal maps atomic hydrogen in clouds that may be inconspicuous in visible light.

• Observational cosmologyRelated: Its signals may trace hydrogen and structure during the universe’s early, poorly observed eras.

• Astronomical radio sourceBroader topic: It lets radio telescopes trace neutral hydrogen across interstellar and intergalactic space.