Cosmic background radiation

Article By:

Wright, Edward L. Department of Astronomy, University of California, Los Angeles, California.

Last reviewed:September 2018

DOI:https://doi.org/10.1036/1097-8542.163750

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• Cosmic background radiation, published January 2014:Download PDF Get Adobe Acrobat Reader

The electromagnetic radiation leftover from the origin of the universe in the big bang. Cosmic background radiation is the oldest light in the universe, emitted when the 13.8-billion-year-old universe was about 380,000 years old. The most studied cosmic background radiation peaks in the microwave portion of the spectrum and is thus known as cosmic microwave background. This light provides unique cosmological information regarding the age, composition, earliest conditions, and subsequent evolution of the universe (Fig. 1). The cosmic microwave background was emitted before any astronomical objects such as stars or galaxies existed, and is now observed to be like the thermal emission from a black object (blackbody) with a temperature of 2.725 K (Kelvin above absolute zero, or −454.8°F) observed all over the sky. The energy density of this radiation is larger than that of any other cosmic radiation field. See also: Absolute zero; Astronomy; Blackbody; Electromagnetic radiation; Galaxy; Heat radiation; Radiation; Star; Temperature; Universe

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