Electromagnetic Spectrum
The electromagnetic spectrum is the full range of electromagnetic radiation, from the longest radio waves to the shortest gamma rays. All electromagnetic radiation is the same physical phenomenon – oscillating electric and magnetic fields propagating through space at the speed of light – differing only in wavelength and frequency.
The spectrum, from longest to shortest wavelength:
| Band | Wavelength | Cosmological relevance |
|---|---|---|
| Radio | > 1 mm | Radio galaxies, pulsars, 21-cm hydrogen line |
| Microwave | 1 mm - 1 m | Cosmic microwave background |
| Infrared | 700 nm - 1 mm | High-redshift galaxies, dust-obscured regions |
| Visible | 400 - 700 nm | Optical galaxy surveys, supernovae |
| Ultraviolet | 10 - 400 nm | Hot stars, active galactic nuclei |
| X-ray | 0.01 - 10 nm | Galaxy cluster gas, black hole accretion |
| Gamma ray | < 0.01 nm | Gamma-ray bursts, extreme particle acceleration |
Wavelength and frequency are related by . Shorter wavelength means higher frequency means higher energy per photon (, where is Planck’s constant).
Cosmology uses the entire spectrum. The cosmic microwave background peaks in microwaves. Galaxy surveys operate in visible and infrared. Redshift moves light along the spectrum: a galaxy whose light was emitted in the ultraviolet may be observed in the infrared if it is at high enough redshift. X-ray telescopes reveal the hot gas in galaxy clusters. Radio telescopes map neutral hydrogen. No single band gives a complete picture; cosmology depends on observations across the entire electromagnetic spectrum.
Last reviewed .