Cosmic background radiation is

electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

that fills all space. The origin of this radiation depends on the region of the

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

that is observed. One component is the

cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...

. This component is redshifted

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

s that have freely streamed from an epoch when the Universe became transparent for the first time to radiation. Its discovery and detailed observations of its properties are considered one of the major confirmations of the

Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...

. Background radiation is largely homogeneous and isotropic. A slight detectable

anisotropy Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ve ...

is present which correlates to

galaxy filaments In cosmology, galaxy filaments are the largest known structures in the universe, consisting of walls of galactic superclusters. These massive, thread-like formations can commonly reach 50 to 80 megaparsecs ()—with the largest found to date b ...

and

voids Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, ...

. The discovery (by chance in 1965) of the cosmic background radiation suggests that the early universe was dominated by a radiation field, a field of extremely high temperature and pressure. There is background radiation observed across all wavelength regimes, peaking in

microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...

, but also notable in

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

and

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

regimes. Fluctuations in cosmic background radiation across regimes create parameters for the amount of baryonic matter in the universe. See cosmic infrared background and X-ray background. See also

cosmic neutrino background The cosmic neutrino background is a proposed background particle radiation composed of neutrinos. They are sometimes known as relic neutrinos or sometimes abbreviated CNB or CB, where the symbol is the Greek letter '' nu'', standard particle p ...

and

extragalactic background light The diffuse extragalactic background light (EBL) is all the accumulated radiation in the universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers almost all wavelengths of the electrom ...

. The Sunyaev–Zel'dovich effect shows the phenomena of radiant cosmic background radiation interacting with "

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

" clouds distorting the spectrum of the radiation.

Timeline of significant events

1896:

Charles Édouard Guillaume Charles Édouard Guillaume (; 15 February 1861 – 13 June 1938) was a Swiss physicist who received the Nobel Prize in Physics in 1920 "for the service he had rendered to precision measurements in physics by his discovery of anomalies in nickel ...

estimates the "radiation of the stars" to be 5.6 K. 1926: Sir

Arthur Eddington Sir Arthur Stanley Eddington, (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lu ...

estimates the non-thermal radiation of

starlight Starlight is the light emitted by stars. It typically refers to visible electromagnetic radiation from stars other than the Sun, observable from Earth at night, although a component of starlight is observable from Earth during daytime. Sunlig ...

in the galaxy has an effective temperature of 3.2 K

1930s: Erich Regener calculates that the non-thermal spectrum of cosmic rays in the galaxy has an effective temperature of 2.8 K. 1931: The term ''microwave'' first appears in print: "When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon." ''Telegraph & Telephone Journal'' XVII. 179/1" 1938:

Walther Nernst Walther Hermann Nernst (; 25 June 1864 – 18 November 1941) was a German physical chemist known for his work in thermodynamics, physical chemistry, electrochemistry, and solid-state physics. His formulation of the Nernst heat theorem helped ...

re-estimates the cosmic ray temperature as 0.75 K. 1946: The term "

" is first used in print in an astronomical context in an article "Microwave Radiation from the Sun and Moon" by

Robert Dicke Robert Henry Dicke (; May 6, 1916 – March 4, 1997) was an American astronomer and physicist who made important contributions to the fields of astrophysics, atomic physics, cosmology and gravity. He was the Albert Einstein Professor in Scien ...

and Robert Beringer. 1946:

predicts a microwave background radiation temperature of 20 K (ref: Helge Kragh) 1946: Robert Dicke predicts a microwave background radiation temperature of "less than 20 K" but later revised to 45 K (ref: Stephen G. Brush). 1946:

George Gamow George Gamow (sometimes Gammoff; born Georgiy Antonovich Gamov; ; 4 March 1904 – 19 August 1968) was a Soviet and American polymath, theoretical physicist and cosmologist. He was an early advocate and developer of Georges Lemaître's Big Ba ...

estimates a temperature of 50 K. 1948: Ralph Alpher and Robert Herman re-estimate Gamow's estimate at 5 K. 1949: Ralph Alpher and Robert Herman re-re-estimate Gamow's estimate at 28 K. 1960s: Robert Dicke re-estimates a MBR (microwave background radiation) temperature of 40 K (ref: Helge Kragh). 1965:

Arno Penzias Arno Allan Penzias (; April 26, 1933 – January 22, 2024) was an American physicist and radio astronomer. Along with Robert Woodrow Wilson, he discovered the cosmic microwave background radiation, for which he shared the Nobel Prize in Physi ...

and

Robert Woodrow Wilson Robert Woodrow Wilson (born January 10, 1936) is an American astronomer who, along with Arno Allan Penzias, discovered cosmic microwave background radiation (CMB) in 1964. The pair won the 1978 Nobel Prize in Physics for its discovery. While ...

measure the temperature to be approximately 3 K. Robert Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson interpret this radiation as a signature of the Big Bang.

References

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External links

The Diffuse X-ray and Gamma-ray Background & Deep Fields
Observational astronomy Physical cosmology Concepts in astronomy Electromagnetic radiation