Cosmic microwave background - Revision history

RonzzWikiCowriterAI: Remove redundancy in the discovery narrative. AI-assisted (RonzzWikiCowriter). (via update-page on MediaWiki MCP Server)

2026-09-03T19:25:28Z

Remove redundancy in the discovery narrative. AI-assisted (RonzzWikiCowriter). (via update-page on MediaWiki MCP Server)

← Older revision		Revision as of 19:25, 3 September 2026
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	In 1964 and 1965, [[Person:Arno Penzias\|Arno Penzias]] and [[Person:Robert Woodrow Wilson\|Robert Woodrow Wilson]] were using a 20-foot horn-reflector antenna in Holmdel, New Jersey, to measure the faint radio emission of the Milky Way. The antenna had been built for the satellite-communication programme of Bell Telephone Laboratories, and the pair, both radio astronomers at the laboratory's Crawford Hill site, were characterising the sources of microwave noise that would limit such communications: every contribution to the received signal, from the atmosphere, the ground, the antenna and the receiver, had to be understood and subtracted. At a wavelength of 7.35 centimetres (a frequency of 4080 megacycles per second), they found an excess antenna temperature of about 3.5 kelvin that was isotropic, unpolarised, and present at all times of day and year, whatever part of the sky the antenna pointed at.<ref>{{#cite:Q1632}}</ref> They eliminated every source of noise they could think of, even removing a pair of pigeons that had taken up residence in the antenna throat and cleaning away the "white dielectric material" they had left behind, yet the excess remained.		In 1964 and 1965, [[Person:Arno Penzias\|Arno Penzias]] and [[Person:Robert Woodrow Wilson\|Robert Woodrow Wilson]] were using a 20-foot horn-reflector antenna in Holmdel, New Jersey, to measure the faint radio emission of the Milky Way. The antenna had been built for the satellite-communication programme of Bell Telephone Laboratories, and the pair, both radio astronomers at the laboratory's Crawford Hill site, were characterising the sources of microwave noise that would limit such communications: every contribution to the received signal, from the atmosphere, the ground, the antenna and the receiver, had to be understood and subtracted. At a wavelength of 7.35 centimetres (a frequency of 4080 megacycles per second), they found an excess antenna temperature of about 3.5 kelvin that was isotropic, unpolarised, and present at all times of day and year, whatever part of the sky the antenna pointed at.<ref>{{#cite:Q1632}}</ref> They eliminated every source of noise they could think of, even removing a pair of pigeons that had taken up residence in the antenna throat and cleaning away the "white dielectric material" they had left behind, yet the excess remained.

	The ~~two engineers were~~ not ~~looking for relic radiation~~ and did not at first understand what they had found. In early 1965 Penzias mentioned the persistent hiss to Bernard F. Burke, a radio astronomer at the Massachusetts Institute of Technology, who had heard of the prediction just made by Dicke's group at Princeton, and who put the two teams in contact. Dicke, P. James E. Peebles, Peter G. Roll and David T. Wilkinson visited Holmdel, recognised the excess as the radiation their own radiometer had been built to seek, and the two teams agreed to publish together. Penzias and Wilson reported the measurement in a short, deliberately understated paper, "A Measurement of Excess Antenna Temperature at 4080 Mc/s";<ref>{{#cite:Q1632}}</ref> Dicke's group supplied the cosmological interpretation, "Cosmic Black-Body Radiation", which appeared immediately before it in the same issue of The Astrophysical Journal.<ref>{{#cite:Q1643}}</ref> The radiation became known as the cosmic microwave background.		The pair had not set out to test any cosmological prediction and did not at first understand what they had found. In early 1965 Penzias mentioned the persistent hiss to Bernard F. Burke, a radio astronomer at the Massachusetts Institute of Technology, who had heard of the prediction just made by Dicke's group at Princeton, and who put the two teams in contact. Dicke, P. James E. Peebles, Peter G. Roll and David T. Wilkinson visited Holmdel, recognised the excess as the radiation their own radiometer had been built to seek, and the two teams agreed to publish together. Penzias and Wilson reported the measurement in a short, deliberately understated paper, "A Measurement of Excess Antenna Temperature at 4080 Mc/s";<ref>{{#cite:Q1632}}</ref> Dicke's group supplied the cosmological interpretation, "Cosmic Black-Body Radiation", which appeared immediately before it in the same issue of The Astrophysical Journal.<ref>{{#cite:Q1643}}</ref> The radiation became known as the cosmic microwave background.

	<blockquote>		<blockquote>

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2026-09-03T19:24:25Z

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Rongzhou: /* The anisotropy of the CMB */

2026-09-03T19:19:44Z

The anisotropy of the CMB

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 == References ==

Rongzhou: /* The anisotropy of the CMB */

2026-09-03T19:19:04Z

The anisotropy of the CMB

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 == The anisotropy of the CMB ==
-=== The dipole ===
+=== The Doppler shift dipole ===
 [[File:Planck CMB map.jpg|thumb|upright=1.6|All-sky map of the cosmic microwave background measured by ESA's Planck satellite. The colour differences are temperature fluctuations of about one part in 100,000. Credit: ESA and the Planck Collaboration (CC BY 4.0).]]
-The largest apparent departure from uniformity of the CMB is a dipole: the sky is very slightly hotter in one direction and colder in the opposite one, by about one part in a thousand, a temperature difference of a few millikelvin. The dipole does not originate in the early universe but in our own motion: the Solar System is moving at a few hundred kilometres per second relative to the rest frame defined by the CMB, and the resulting Doppler shift makes the radiation slightly hotter in the direction of motion and colder behind us.<ref>{{#cite:Q1635}}</ref> Removing this dipole is the first step in analysing the CMB, and it leaves a pattern that is uniform to one part in 100,000.
+The largest apparent departure from uniformity of the CMB is a dipole: observing from earth, the CMB is slightly hotter in one direction by a few millikelvin.
-=== Fluctuations of one part in 100,000 ===
+The dipole is a result of our own motion: the Solar System is moving at a few hundred kilometres per second relative to the rest frame defined by the CMB, and the resulting Doppler shift makes the radiation slightly hotter in our direction of motion.<ref>{{#cite:Q1635}}</ref>
-After the dipole is subtracted, the CMB temperature is almost perfectly constant across the sky: the remaining fluctuations, first detected by COBE in 1992, are at the level of about 30 microkelvin, one part in 100,000 of the mean.<ref>{{#cite:Q1635}}</ref> These fluctuations are the imprint of tiny density variations present in the universe at recombination, the "seeds" from which galaxies, clusters and the large-scale structure of the cosmos later grew by gravitational instability: slightly overdense regions had slightly hotter radiation and eventually collapsed into the structures seen today.
+=== Fluctuations of 10 ppm ===
-The pattern of the fluctuations is nearly scale-invariant, meaning that patches of very different angular size have comparable amplitude, as predicted by the theory of cosmic inflation. The statistics of the pattern are described by expanding the temperature map into spherical harmonics, each labelled by a multipole ℓ that roughly corresponds to an angular scale of 180°/ℓ.
+After adjusting for Doppler effects, the CMB temperature is almost perfectly constant across the sky: the remaining fluctuations, first detected by COBE in 1992, are at the level of about 30 microkelvin, which is about 10 ppm of the mean.<ref>{{#cite:Q1635}}</ref> These fluctuations are imprints of tiny density variations present in the universe at recombination, and the origin from which galaxies, clusters and the large-scale structure of the cosmos later grew by gravitational instability: slightly denser regions had slightly hotter radiation and collapsed into the cosmic structures seen today over billions of years.
 == The anisotropy of the CMB ==

Rongzhou: /* The angular power spectrum */

2026-09-03T19:18:17Z

The angular power spectrum

← Older revision		Revision as of 19:18, 3 September 2026
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	The pattern of the fluctuations is nearly scale-invariant, meaning that patches of very different angular size have comparable amplitude, as predicted by the theory of cosmic inflation. The statistics of the pattern are described by expanding the temperature map into spherical harmonics, each labelled by a multipole ℓ that roughly corresponds to an angular scale of 180°/ℓ.		The pattern of the fluctuations is nearly scale-invariant, meaning that patches of very different angular size have comparable amplitude, as predicted by the theory of cosmic inflation. The statistics of the pattern are described by expanding the temperature map into spherical harmonics, each labelled by a multipole ℓ that roughly corresponds to an angular scale of 180°/ℓ.

			== The anisotropy of the CMB ==

			=== The Doppler shift dipole ===

			[[File:Planck CMB map.jpg\|thumb\|upright=1.6\|All-sky map of the cosmic microwave background measured by ESA's Planck satellite. The colour differences are temperature fluctuations of about one part in 100,000. Credit: ESA and the Planck Collaboration (CC BY 4.0).]]

			The largest apparent departure from uniformity of the CMB is a dipole: observing from earth, the CMB is slightly hotter in one direction by a few millikelvin.

			The dipole is a result of our own motion: the Solar System is moving at a few hundred kilometres per second relative to the rest frame defined by the CMB, and the resulting Doppler shift makes the radiation slightly hotter in our direction of motion.<ref>{{#cite:Q1635}}</ref>

			=== Fluctuations of 10 ppm ===

			After adjusting for Doppler effects, the CMB temperature is almost perfectly constant across the sky: the remaining fluctuations, first detected by COBE in 1992, are at the level of about 30 microkelvin, which is about 10 ppm of the mean.<ref>{{#cite:Q1635}}</ref> These fluctuations are imprints of tiny density variations present in the universe at recombination, and the origin from which galaxies, clusters and the large-scale structure of the cosmos later grew by gravitational instability: slightly denser regions had slightly hotter radiation and collapsed into the cosmic structures seen today over billions of years.

			=== Polarisation ===

			The CMB is faintly polarised at the level of a few microkelvins, because the last scattering of the photons was not isotropic: the quadrupolar pattern of the incoming radiation around each scattering electron imprinted a preferred direction on the scattered light.

	=== The angular power spectrum ===		=== The angular power spectrum ===
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	[[File:CMB angular power spectrum (WMAP).svg\|thumb\|upright=1.5\|The angular power spectrum of the CMB temperature anisotropies from three years of WMAP data (points) together with measurements from ground-based and balloon-borne experiments (coloured points); the solid curve is the best-fitting cosmological model. Credit: NASA/WMAP Science Team (public domain).]]		[[File:CMB angular power spectrum (WMAP).svg\|thumb\|upright=1.5\|The angular power spectrum of the CMB temperature anisotropies from three years of WMAP data (points) together with measurements from ground-based and balloon-borne experiments (coloured points); the solid curve is the best-fitting cosmological model. Credit: NASA/WMAP Science Team (public domain).]]

	The statistical ~~content~~ of the fluctuations is ~~usually summarised by the angular power spectrum,~~ the variance of the spherical-harmonic coefficients C<sub>ℓ</sub> as a function of multipole ℓ, conventionally plotted in the dimensionless form		The angular power spectrum provides a statistical description of the fluctuations in CMB. It is the variance of the spherical-harmonic coefficients C<sub>ℓ</sub> as a function of multipole ℓ, conventionally plotted in the dimensionless form

	{{#content:Q1639}}		{{#content:Q1639}}

	The power spectrum of the CMB is not flat: it shows a series of peaks and troughs, the acoustic peaks, which ~~are among~~ the ~~most important measurements in cosmology. They arise because, before recombination, the photon–baryon fluid supported sound waves:~~ gravity ~~compressed overdense regions while~~ radiation pressure pushed back, setting up oscillations that froze into the temperature pattern when the radiation decoupled. Modes that had completed a whole number of half-oscillations by decoupling appear as peaks in the power spectrum, and modes caught at other phases appear as troughs.		The power spectrum of the CMB is not flat: it shows a series of peaks and troughs, known as the '''acoustic peaks''', which arose from oscillations caused by the counteracting effects between gravity and radiation pressure during recombination.

	~~Each feature~~ of the power spectrum ~~carries specific information~~:		Different features of the power spectrum provides diverse insights on the universe:<ref>{{#cite:Q1232}}</ref> <!-- verify citation covers every feature! -->

	* the position of the first peak, at ℓ ≈ 200, is set by the angular size of the sound horizon and measures the total density of the universe, pinning down its geometry;~~<ref>{{#cite:Q1232}}</ref>~~		* the position of the first peak, at ℓ ≈ 200, is set by the angular size of the sound horizon and measures the total density of the universe, pinning down its geometry;
	* the odd–even alternation in the heights of successive peaks~~, the pattern of compressional versus rarefactional modes, measures~~ the baryon density~~, since baryons load the compressional modes~~;		* the odd–even alternation in the heights of successive peaks meausres the baryon density;
	* the overall damping of the peaks at high ℓ, caused by photons diffusing out of small regions during recombination (Silk damping), measures the matter density and the thickness of the last scattering surface;		* the overall damping of the peaks at high ℓ, caused by photons diffusing out of small regions during recombination, an effect known as Silk damping, measures the matter density and the thickness of the last scattering surface;
	* the plateau of power at low ℓ, the Sachs–Wolfe effect, reflects the gravitational potential fluctuations and measures the amplitude and tilt of the primordial fluctuation spectrum;		* the plateau of power at low ℓ, the Sachs–Wolfe effect, reflects the gravitational potential fluctuations and measures the amplitude and tilt of the primordial fluctuation spectrum;
	* gravitational lensing of the CMB by intervening matter slightly smooths the peaks and adds power at small scales, providing a ~~direct~~ measurement of the growth of ~~structure, sensitive to dark matter~~.		* gravitational lensing of the CMB by intervening matter slightly smooths the peaks and adds power at small scales, providing a measurement for the growth of cosmic structures.

	Fitting the full spectrum together with the polarisation measurements and the lensing signal yields the parameters of the standard model of cosmology, ~~and it is in this way that the CMB constrains~~ the geometry, contents and age of the universe ~~quoted above~~.<ref>{{#cite:Q1232}}</ref>		Fitting the full spectrum together with the polarisation measurements and the lensing signal yields the parameters of the [[standard model of cosmology]], hence constraining the geometry, contents and age of the universe.<ref>{{#cite:Q1232}}</ref>

	<uml>		<uml> <!-- use a<sub>b</sub> instead of a_b -->
	@startuml		@startuml
	!theme bluegray		!theme bluegray

Rongzhou: /* Origin and physical properties */

2026-09-03T18:47:20Z

Origin and physical properties

← Older revision		Revision as of 18:47, 3 September 2026
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	=== The surface of last scattering ===		=== The surface of last scattering ===

	In the hot early universe, matter was ionised: free protons and electrons formed a plasma that scattered ~~light~~ almost continuously~~, so that radiation and matter were~~ tightly coupled and ~~the universe was opaque~~. ~~As the~~ universe expanded~~, it cooled, until, a~~ few hundred thousand years after the Big Bang, ~~its~~ temperature had fallen to about 3000 kelvin and the free electrons combined with protons to form neutral hydrogen~~, in the~~ epoch known as recombination. With the charged particles gone, ~~the~~ radiation ~~was suddenly~~ free to travel~~: the universe became transparent~~, ~~and~~ the ~~photons that had been~~ scattered ~~for the last time began to stream freely~~. Those photons ~~are~~ the cosmic microwave background we observe today.		In the hot early universe, matter was ionised: free protons and electrons formed a plasma that scattered photons almost continuously. Radiation was tightly coupled with matter and cannot travel freely.

			The universe cooled down as it expanded: A few hundred thousand years after the Big Bang, the temperature of the universe had fallen to about 3000 kelvin and the free electrons combined with protons to form neutral hydrogen. This epoch is known as recombination.

			With the charged particles gone, radiation became free to travel, in whatever direction the cosmic plasma last scattered them into, just before recombination. Those photons form the basis of the cosmic microwave background we observe today.

	The region from which these photons reach us is called the surface of last scattering. It is not a physical surface but a spherical shell centred on the observer, at a redshift of about 1090, corresponding to a time when the universe was about 380,000 years old and roughly one-thousandth of its present size.<ref>{{#cite:Q1232}}</ref> Since decoupling, the expansion of the universe has stretched the wavelength of each photon by a factor of (1 + ''z''), so that the temperature of the radiation scales with redshift as		The region from which these photons reach us is called the surface of last scattering. It is not a physical surface but a spherical shell centred on the observer, at a redshift of about 1090, corresponding to a time when the universe was about 380,000 years old and roughly one-thousandth of its present size.<ref>{{#cite:Q1232}}</ref> Since decoupling, the expansion of the universe has stretched the wavelength of each photon by a factor of (1 + ''z''), so that the temperature of the radiation scales with redshift as
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	{{#content:Q1638}}		{{#content:Q1638}}

	For ''T'' ≈ 2.7 K this gives a peak wavelength of roughly a millimetre, squarely in the microwave band~~, which is why the radiation is observed with microwave instruments~~.		For ''T'' ≈ 2.7 K this gives a peak wavelength of roughly a millimetre, squarely in the microwave band.

	~~That the~~ CMB ~~is such~~ a ~~precise~~ blackbody is ~~powerful evidence for~~ the hot Big Bang theory. Only radiation that was once in thermal equilibrium with matter in an extremely hot, dense phase can plausibly fill the sky with a blackbody spectrum to this accuracy; steady-state cosmologies, in which the radiation would have to be produced by astrophysical sources, have no natural way of generating it.<ref>{{#cite:Q1634}}</ref>		CMB as a nearly a perfect blackbody is strongly consistent with the expectations of the hot Big Bang theory. Only radiation that was once in thermal equilibrium with matter in an extremely hot, dense phase can plausibly fill the sky with a blackbody spectrum to this accuracy; steady-state cosmologies, in which the radiation would have to be produced by astrophysical sources, have no natural way of generating it.<ref>{{#cite:Q1634}}</ref>

	=== A geometric probe of the universe ===		=== A geometric probe of the universe ===
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	{{#content:Q1560}}		{{#content:Q1560}}

	where ''H'' is the Hubble parameter and ''G'' the gravitational constant. The ratio of any component's density to the critical density is its density parameter, and the total density parameter of the universe is usually written ''Ω''<sub>total</sub>. The angular scale of the acoustic peaks of the CMB (described below) depends on the total density: in a positively curved universe, features would appear larger than in a flat one, and in a negatively curved universe they would appear smaller. The Planck measurements of the peaks imply a total density parameter of 1 to within a few tenths of a percent, and a curvature density parameter ''Ω''<sub>K</sub> = 0.0007 ± 0.0019: the universe is spatially flat ~~to very high precision.<ref>{{#cite:Q1232}}</ref>~~		where ''H'' is the Hubble parameter and ''G'' the gravitational constant. The ratio of any component's density to the critical density is its density parameter, and the total density parameter of the universe is usually written ''Ω''<sub>total</sub>. The angular scale of the acoustic peaks of the CMB (described below) depends on the total density: in a positively curved universe, features would appear larger than in a flat one, and in a negatively curved universe they would appear smaller. The Planck measurements of the peaks imply a total density parameter of 1 to within a few tenths of a percent, and a curvature density parameter ''Ω''<sub>K</sub> = 0.0007 ± 0.0019, suggesting that the universe is spatially flat.<ref>{{#cite:Q1232}}</ref>

	Flatness alone does not fix the composition, but the detailed shape of the CMB pattern does. Planck finds that ordinary baryonic matter contributes about 5% of the total density, dark matter about 27%, and dark energy about 68%, with the dark energy behaving like a cosmological constant.<ref>{{#cite:Q1232}}</ref> ~~These are the values quoted in the articles on [[dark energy]] and [[dark matter]], which the CMB constrains jointly with the other cosmological probes.~~

	== The anisotropy of the CMB ==		== The anisotropy of the CMB ==

Rongzhou: /* Significance */

2026-09-03T16:11:29Z

Significance

← Older revision		Revision as of 16:11, 3 September 2026
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	A primordial B-mode signal would be the signature of gravitational waves generated during cosmic inflation, stretched to cosmological scales by the rapid expansion. Its detection would directly probe the energy scale of inflation. So far, searches combining Planck data with ground-based and balloon-borne experiments such as BICEP/Keck have found no convincing primordial B-mode signal, and instead measure the E-mode pattern precisely and the lensing-induced B-modes, continuing to tighten the constraints on the standard model.<ref>{{#cite:Q1232}}</ref>		A primordial B-mode signal would be the signature of gravitational waves generated during cosmic inflation, stretched to cosmological scales by the rapid expansion. Its detection would directly probe the energy scale of inflation. So far, searches combining Planck data with ground-based and balloon-borne experiments such as BICEP/Keck have found no convincing primordial B-mode signal, and instead measure the E-mode pattern precisely and the lensing-induced B-modes, continuing to tighten the constraints on the standard model.<ref>{{#cite:Q1232}}</ref>

	~~== Significance ==~~

	The cosmic microwave background underpins the modern cosmological model in several distinct ways. Its near-perfect blackbody spectrum is the direct observational evidence that the universe went through a hot, dense, radiation-dominated phase, and it rules out cosmologies without such a phase.<ref>{{#cite:Q1634}}</ref> Its temperature fluctuations provide the initial conditions for the growth of all cosmic structure, from galaxies to the [[Cosmic web\|cosmic web]], and confirm that those fluctuations were nearly scale-invariant, as inflation predicts.<ref>{{#cite:Q1232}}</ref> Its acoustic peaks provide a "standard ruler" of known physical length, the sound horizon at recombination, whose apparent size at different distances anchors the baryon acoustic oscillation measurements used to trace the expansion history of the universe and to probe [[dark energy]].<ref>{{#cite:Q1232}}</ref> And its measurement of the geometry and contents of the universe, together with the independent supernova and large-scale-structure evidence, establishes the cosmic energy budget: a spatially flat universe whose energy is dominated by dark energy and dark matter, with ordinary matter a minor ingredient.<ref>{{#cite:Q1232}}</ref>

	For these reasons the CMB is, as of September 2026, one of the most active frontiers of observational cosmology. Current and planned experiments continue to map its polarisation at ever higher sensitivity: ground-based instruments such as the Simons Observatory and CMB-S4, and the planned JAXA-led LiteBIRD satellite mission, target above all the search for the primordial B-modes that would reveal the physics of inflation.

	== References ==		== References ==

Rongzhou: /* History */

2026-09-03T16:09:47Z

History

← Older revision		Revision as of 16:09, 3 September 2026
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	=== Prediction ===		=== Prediction ===

	~~The idea that~~ the ~~universe began in a hot~~, ~~dense state was developed in the 1940s by~~ [[Person:George Gamow\|George Gamow]] and his collaborators~~, who realised~~ that a universe ~~that was once~~ enormously hot should still be filled with the radiation left over from that era~~, cooled by the expansion to a temperature of a few kelvin~~. ~~In 1948,~~ [[Person:Ralph Asher Alpher\|Ralph Asher Alpher]] and [[Person:Robert Herman\|Robert Herman]] estimated the temperature of this relic radiation ~~and obtained a value of~~ a few ~~kelvin, remarkably close to the value measured seventeen years later~~.<ref>{{#cite:Q1633}}</ref> The prediction attracted little attention at the time, and the radiation was not searched for ~~observationally~~.		In the 1940s, [[Person:George Gamow\|George Gamow]] and his collaborators demonstrated that if the universe began in an enormously hot and dense state as suggested by the [[Big Bang theory]], it should still be filled with the radiation left over from that era. [[Person:Ralph Asher Alpher\|Ralph Asher Alpher]] and [[Person:Robert Herman\|Robert Herman]] further estimated that taking into account the cooling provided by an expanding universe, the temperature of this relic radiation should be only a few kelvins.<ref>{{#cite:Q1633}}</ref> The prediction attracted little attention at the time, and the observation of the radiation was not actively searched for.

	The idea was independently revived in the early 1960s. [[Person:Robert H. Dicke\|Robert H. Dicke]] and his group at Princeton University reasoned that the universe must be filled with a relic radiation of a few kelvin, and began building a small radiometer to look for it at a wavelength of about 3 centimetres. The detection, however, came first, and from an unexpected quarter.		=== Accidental discovery (1964–1965) ===

	~~=== Accidental discovery (1964–1965) ===~~		In the early 1960s, a group of physicists led by [[Person:Robert H. Dicke\|Robert H. Dicke]] at Princeton University revisisted the idea. They began building a small radiometer to look for CMB at a wavelength of about 3 centimetres. Their progress, however, was superceded by a couple of telecommunication engineers that were not looking for CMB at all.

	[[File:Holmdel Horn Antenna.jpg\|thumb\|The Holmdel horn antenna at Bell Telephone Laboratories in New Jersey, with which Penzias and Wilson discovered the cosmic microwave background in 1965. Credit: Erik Dunham (CC BY-SA 3.0).]]		[[File:Holmdel Horn Antenna.jpg\|thumb\|The Holmdel horn antenna at Bell Telephone Laboratories in New Jersey, with which Penzias and Wilson discovered the cosmic microwave background in 1965. Credit: Erik Dunham (CC BY-SA 3.0).]]

	In 1964 and 1965, [[Person:Arno Penzias\|Arno Penzias]] and [[Person:Robert Woodrow Wilson\|Robert Woodrow Wilson]] were using a 20-foot horn-reflector antenna in Holmdel, New Jersey, built for satellite communications, to measure the faint radio emission of the Milky Way. At a wavelength of 7.35 centimetres (a frequency of 4080 megacycles per second), they found an excess antenna temperature of about 3.5 kelvin that was isotropic, unpolarised, and present at all times of day and year, whatever part of the sky the antenna pointed at.<ref>{{#cite:Q1632}}</ref> They eliminated every source of noise they could think of, even removing a pair of pigeons that had taken up residence in the antenna throat and cleaning away the "white dielectric material" they had left behind, yet the excess remained.		In 1964 and 1965, [[Person:Arno Penzias\|Arno Penzias]] and [[Person:Robert Woodrow Wilson\|Robert Woodrow Wilson]] were using a 20-foot horn-reflector antenna in Holmdel, New Jersey, built for satellite communications, to measure the faint radio emission of the Milky Way. At a wavelength of 7.35 centimetres (a frequency of 4080 megacycles per second), they found an excess antenna temperature of about 3.5 kelvin that was isotropic, unpolarised, and present at all times of day and year, whatever part of the sky the antenna pointed at.<ref>{{#cite:Q1632}}</ref> They eliminated every source of noise they could think of, even removing a pair of pigeons that had taken up residence in the antenna throat and cleaning away the "white dielectric material" they had left behind, yet the excess remained.

			<!-- better precision: what were the Bell Lab engineers trying to do? How did those people who weren't even looking for CMB know to publish in an astrophysical journal?-->

	Penzias and Wilson reported the measurement in a short paper, "A Measurement of Excess Antenna Temperature at 4080 Mc/s", published in 1965 in The Astrophysical Journal.<ref>{{#cite:Q1632}}</ref> Almost simultaneously, and in the same issue of the journal, Dicke's group published the companion paper interpreting the excess as the relic radiation of the Big Bang, which they had been about to search for. The two groups had been in contact: when Dicke heard of the Bell Labs result, he is said to have told his colleagues, "Well, boys, we've been scooped." The radiation became known as the cosmic microwave background.		Penzias and Wilson reported the measurement in a short paper, "A Measurement of Excess Antenna Temperature at 4080 Mc/s", published in 1965 in The Astrophysical Journal.<ref>{{#cite:Q1632}}</ref> Almost simultaneously, and in the same issue of the journal, Dicke's group published the companion paper interpreting the excess as the relic radiation of the Big Bang, which they had been about to search for. The two groups had been in contact: when Dicke heard of the Bell Labs result, he is said to have told his colleagues, "Well, boys, we've been scooped." The radiation became known as the cosmic microwave background.
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	=== COBE: the spectrum and the fluctuations ===		=== COBE: the spectrum and the fluctuations ===

	~~The next decisive step came with~~ NASA's Cosmic Background Explorer (COBE) satellite~~, launched in November 1989, which~~ carried three instruments ~~designed~~ to study the radiation. The Far-Infrared Absolute Spectrophotometer (FIRAS), ~~led by [[Person:John C. Mather\|John C. Mather]], compared the CMB with an internal blackbody calibrator and showed that its spectrum is that of a near~~-~~perfect blackbody, with any deviations from a perfect blackbody limited to about 50 parts per million of the peak brightness.<ref>{{#cite:Q1634}}</ref~~> The Differential Microwave Radiometer (DMR)~~, led by [[Person:George F. Smoot\|George F. Smoot]], searched for spatial variations in the temperature; in April 1992 the team announced the detection of the long~~-~~sought temperature fluctuations, at the level of about one part in 100,000.<ref~~>~~{{#cite:Q1635}}~~<~~/ref~~>		In 1989, NASA launched its Cosmic Background Explorer (COBE) satellite. It carried three instruments dedicated to study the radiation of CMB:

			* The Far-Infrared Absolute Spectrophotometer (FIRAS), <!-- What does it do ? -->
			* The Differential Microwave Radiometer (DMR) <!-- What does it do ? -->
			* <!-- ?? -->

	The COBE results transformed the study of the CMB: the measurement of a perfect blackbody spectrum confirmed that the radiation is a relic of a hot, dense, thermalised early universe, while the detection of fluctuations showed that the early universe was not perfectly smooth, providing the seeds from which galaxies and clusters later grew. [[Person:John C. Mather\|John C. Mather]] and [[Person:George F. Smoot\|George F. Smoot]] shared the 2006 Nobel Prize in Physics for these measurements.		<!-- how many ? --> significant conclusions were obtained through the analysis of data collected by COBE. A team led by [[Person:John C. Mather\|John C. Mather]], compared <!-- what --> with an internal blackbody calibrator and showed that its spectrum corresponds to near-perfect blackbody, with any deviations from a perfect blackbody limited to about 50ppm of the peak brightness.<ref>{{#cite:Q1634}}</ref> Another team, led by [[Person:George F. Smoot\|George F. Smoot]], discovered spatial temperature fluctuations at the level of about 10 ppm.<ref>{{#cite:Q1635}}</ref>

			The COBE results transformed the study of the CMB: the measurement of a near-perfect blackbody spectrum confirmed that the radiation is a relic of a hot, dense, thermalised early universe, while the detection of fluctuations showed that the early universe was not perfectly smooth, providing the seeds from which galaxies and clusters later grew.

			<blockquote>
			[[Person:John C. Mather\|John C. Mather]] and [[Person:George F. Smoot\|George F. Smoot]] shared the 2006 Nobel Prize in Physics for these measurements.
			</blockquote>

	=== WMAP and Planck ===		=== WMAP and Planck ===
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	[[File:COBE WMAP Planck comparison (PIA16874).jpg\|thumb\|upright=1.6\|The evolution of CMB maps: COBE (left, 1989), WMAP (centre, 2001) and Planck (right, 2009). Each generation resolved the temperature pattern with greater sensitivity. Credit: NASA/JPL-Caltech/ESA (public domain).]]		[[File:COBE WMAP Planck comparison (PIA16874).jpg\|thumb\|upright=1.6\|The evolution of CMB maps: COBE (left, 1989), WMAP (centre, 2001) and Planck (right, 2009). Each generation resolved the temperature pattern with greater sensitivity. Credit: NASA/JPL-Caltech/ESA (public domain).]]

	Two subsequent space missions ~~mapped the CMB with far higher sensitivity and resolution.~~ NASA's Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, and ~~its successor,~~ ESA's Planck satellite, launched in 2009, ~~measured~~ the temperature and polarisation of the CMB across the whole sky~~. The final full~~-~~mission Planck~~ results, ~~published in 2020, determined the parameters of the standard cosmological model~~ to ~~percent-level~~ precision~~, measuring~~ the age of the universe ~~at 13.787 billion years, the Hubble constant at 67.4 km s<sup>−1</sup> Mpc<sup>−1</sup>,~~ and ~~the tilt of the primordial fluctuation spectrum at about 0.965, all consistent with a spatially flat universe dominated by~~ dark energy ~~and dark matter~~.~~<ref>{{#cite:Q1232}}</ref>~~		Two subsequent space missions, NASA's Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, and ESA's Planck satellite, launched in 2009, mapped the temperature and polarisation of the CMB across the whole sky <!-- is "sky" the precise term? --> with far higher sensitivity and resolution compared to the COBE results, providing precious data to determine with high precision the age of the universe and to estimate [[dark energy\|the cosmological constant]].

	<uml>		<uml>
	@startuml		@startuml
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	rectangle "1948\nAlpher and Herman predict a\nrelic radiation of a few kelvin" as Y1948		rectangle "1948\nAlpher and Herman predict a\nrelic radiation of a few kelvin" as Y1948
	rectangle "1965\nPenzias and Wilson detect\nan isotropic 3.5 K excess" as Y1965		rectangle "1965\nPenzias and Wilson detect\nan isotropic 3.5 K excess" as Y1965
	rectangle "1978\nNobel Prize in Physics to\nPenzias and Wilson" as Y1978		rectangle "1978\nNobel Prize in Physics\nawarded to\nPenzias and Wilson" as Y1978
	rectangle "1989–1992\nCOBE: blackbody spectrum and\ntemperature fluctuations" as YCOBE		rectangle "1989–1992\nCOBE: blackbody spectrum and\ntemperature fluctuations" as YCOBE
	rectangle "2000\nBOOMERanG resolves the\nfirst acoustic peak" as Y2000		rectangle "2000\nBOOMERanG resolves the\nfirst acoustic peak" as Y2000
	rectangle "2001–2010\nWMAP maps temperature and\npolarisation" as YWMAP		rectangle "2001–2010\nWMAP maps temperature and\npolarisation" as YWMAP
	rectangle "2009–2018\nPlanck: ~~final full-mission~~\~~ncosmological~~ parameters" as YPL		rectangle "2009–2018\nPlanck: higher precision mapping\n to determine\n* cosmological parameters\n* age of universe" as YPL

	Y1948 --> Y1965 ~~: prediction confirmed by accident~~		Y1948 --> Y1965
	Y1965 --> Y1978 ~~: discovery recognised~~		Y1965 --> Y1978
	Y1978 --> YCOBE ~~: from detection to precision~~		Y1978 --> YCOBE
	YCOBE --> Y2000		YCOBE --> Y2000
	Y2000 --> YWMAP		Y2000 --> YWMAP
	YWMAP --> YPL		YWMAP --> YPL

	~~note bottom of YPL~~
	~~Each mission sharpens the measurement~~
	~~of the same ancient radiation, tightening~~
	~~the parameters of the ΛCDM model.~~
	~~end note~~

	@enduml		@enduml
	</uml>		</uml>

Rongzhou at 15:36, 3 September 2026

2026-09-03T15:36:22Z

← Older revision		Revision as of 15:36, 3 September 2026
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	'''The cosmic microwave background''' ('''CMB''') is ~~the~~ faint electromagnetic radiation that ~~fills~~ the universe, a relic of the hot, dense, nearly uniform state in which the [[~~Universe\|~~universe]] began about 13.8 billion years ago.<ref>{{#cite:Q1232}}</ref> ~~It is the oldest light that can be observed: it was released~~ when the universe had cooled enough to become transparent, a few hundred thousand years after the [[Big Bang theory\|Big Bang]], ~~and its photons have travelled across~~ the ~~universe almost undisturbed ever since. Because the expansion of the universe has stretched these photons by a factor of about 1100, the radiation is~~ observed today ~~not as visible light but as microwaves, forming a near-perfect blackbody at a temperature of about 2.7 kelvin~~.		'''The cosmic microwave background''' ('''CMB''') is a faint electromagnetic radiation that permeates the entire universe. It is a relic of the hot, dense, and nearly uniform state in which the [[universe]] began about 13.8 billion years ago.<ref>{{#cite:Q1232}}</ref> Released when the universe had cooled enough to become transparent, a few hundred thousand years after the [[Big Bang theory\|Big Bang]], it is the oldest light that can be observed today.

	~~The CMB was predicted in~~ the ~~1940s as~~ a ~~consequence~~ of the ~~hot Big Bang theory and discovered by accident in 1965 by [[Person:Arno Penzias\|Arno Penzias]] and [[Person:Robert Woodrow Wilson\|Robert Woodrow Wilson]]~~, ~~two radio astronomers~~ at ~~Bell Telephone Laboratories, who were awarded the 1978 Nobel Prize in Physics for the detection~~.<~~ref>{{#cite:Q1632}}</ref~~>		Because the expansion of the universe has stretched the photons of the cosmic microwave background by a factor of about 1100, the radiation is observed today not as visible light but as microwaves, forming a near-perfect blackbody at a temperature of about 2.7 kelvin. <!-- missing citation -->

	Because ~~its~~ spectrum and ~~its~~ spatial pattern can be measured with exquisite precision, ~~the CMB~~ has become the single most informative observable in cosmology. ~~Its~~ temperature is almost perfectly uniform across ~~the sky~~, ~~varying~~ by ~~only~~ about ~~one part in 100,000, and that faint pattern of hot and cold spots~~ encodes the geometry, composition, age and history of the universe~~: it~~ is the strongest evidence that the universe is spatially flat, and it anchors the measurement of the abundance of ordinary matter, [[dark matter]] and [[dark energy]].<ref>{{#cite:Q1232}}</ref>		The existence of CMB was predicted in the 1940s as a necessary consequence of the Big Bang. The first observation, by accident, was in 1965 by [[Person:Arno Penzias\|Arno Penzias]] and [[Person:Robert Woodrow Wilson\|Robert Woodrow Wilson]], two radio astronomers at Bell Telephone Laboratories, while they were trying to minimise noise for their ultra-sensitive satelite communication ground receiver. They were awarded the 1978 Nobel Prize in Physics for the detection.<ref>{{#cite:Q1632}}</ref>

			Because the spectrum and spatial pattern of CMB can be measured with exquisite precision, it has become the single most informative observable in cosmology. Although its temperature is almost perfectly uniform across space, tiny variations by about 10 ppm encodes the geometry, composition, age and history of the universe. It is the strongest evidence that the universe is spatially flat, and it anchors the measurement of the abundance of ordinary matter, [[dark matter]] and [[dark energy]].<ref>{{#cite:Q1232}}</ref>

	== History ==		== History ==

RonzzWikiCowriterAI: Refine FIRAS precision wording and current-experiments sentence. AI-assisted (RonzzWikiCowriter). (via update-page on MediaWiki MCP Server)

2026-09-03T13:47:36Z

Refine FIRAS precision wording and current-experiments sentence. AI-assisted (RonzzWikiCowriter). (via update-page on MediaWiki MCP Server)

← Older revision		Revision as of 13:47, 3 September 2026
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	=== COBE: the spectrum and the fluctuations ===		=== COBE: the spectrum and the fluctuations ===

	The next decisive step came with NASA's Cosmic Background Explorer (COBE) satellite, launched in November 1989, which carried three instruments designed to study the radiation. The Far-Infrared Absolute Spectrophotometer (FIRAS), led by [[Person:John C. Mather\|John C. Mather]], compared the CMB with an internal blackbody calibrator and showed that its spectrum is that of a near-perfect blackbody, with no deviations ~~detectable at the level of~~ about ~~one part in 10,000~~ of the peak brightness.<ref>{{#cite:Q1634}}</ref> The Differential Microwave Radiometer (DMR), led by [[Person:George F. Smoot\|George F. Smoot]], searched for spatial variations in the temperature; in April 1992 the team announced the detection of the long-sought temperature fluctuations, at the level of about one part in 100,000.<ref>{{#cite:Q1635}}</ref>		The next decisive step came with NASA's Cosmic Background Explorer (COBE) satellite, launched in November 1989, which carried three instruments designed to study the radiation. The Far-Infrared Absolute Spectrophotometer (FIRAS), led by [[Person:John C. Mather\|John C. Mather]], compared the CMB with an internal blackbody calibrator and showed that its spectrum is that of a near-perfect blackbody, with any deviations from a perfect blackbody limited to about 50 parts per million of the peak brightness.<ref>{{#cite:Q1634}}</ref> The Differential Microwave Radiometer (DMR), led by [[Person:George F. Smoot\|George F. Smoot]], searched for spatial variations in the temperature; in April 1992 the team announced the detection of the long-sought temperature fluctuations, at the level of about one part in 100,000.<ref>{{#cite:Q1635}}</ref>

	The COBE results transformed the study of the CMB: the measurement of a perfect blackbody spectrum confirmed that the radiation is a relic of a hot, dense, thermalised early universe, while the detection of fluctuations showed that the early universe was not perfectly smooth, providing the seeds from which galaxies and clusters later grew. [[Person:John C. Mather\|John C. Mather]] and [[Person:George F. Smoot\|George F. Smoot]] shared the 2006 Nobel Prize in Physics for these measurements.		The COBE results transformed the study of the CMB: the measurement of a perfect blackbody spectrum confirmed that the radiation is a relic of a hot, dense, thermalised early universe, while the detection of fluctuations showed that the early universe was not perfectly smooth, providing the seeds from which galaxies and clusters later grew. [[Person:John C. Mather\|John C. Mather]] and [[Person:George F. Smoot\|George F. Smoot]] shared the 2006 Nobel Prize in Physics for these measurements.
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	The cosmic microwave background underpins the modern cosmological model in several distinct ways. Its near-perfect blackbody spectrum is the direct observational evidence that the universe went through a hot, dense, radiation-dominated phase, and it rules out cosmologies without such a phase.<ref>{{#cite:Q1634}}</ref> Its temperature fluctuations provide the initial conditions for the growth of all cosmic structure, from galaxies to the [[Cosmic web\|cosmic web]], and confirm that those fluctuations were nearly scale-invariant, as inflation predicts.<ref>{{#cite:Q1232}}</ref> Its acoustic peaks provide a "standard ruler" of known physical length, the sound horizon at recombination, whose apparent size at different distances anchors the baryon acoustic oscillation measurements used to trace the expansion history of the universe and to probe [[dark energy]].<ref>{{#cite:Q1232}}</ref> And its measurement of the geometry and contents of the universe, together with the independent supernova and large-scale-structure evidence, establishes the cosmic energy budget: a spatially flat universe whose energy is dominated by dark energy and dark matter, with ordinary matter a minor ingredient.<ref>{{#cite:Q1232}}</ref>		The cosmic microwave background underpins the modern cosmological model in several distinct ways. Its near-perfect blackbody spectrum is the direct observational evidence that the universe went through a hot, dense, radiation-dominated phase, and it rules out cosmologies without such a phase.<ref>{{#cite:Q1634}}</ref> Its temperature fluctuations provide the initial conditions for the growth of all cosmic structure, from galaxies to the [[Cosmic web\|cosmic web]], and confirm that those fluctuations were nearly scale-invariant, as inflation predicts.<ref>{{#cite:Q1232}}</ref> Its acoustic peaks provide a "standard ruler" of known physical length, the sound horizon at recombination, whose apparent size at different distances anchors the baryon acoustic oscillation measurements used to trace the expansion history of the universe and to probe [[dark energy]].<ref>{{#cite:Q1232}}</ref> And its measurement of the geometry and contents of the universe, together with the independent supernova and large-scale-structure evidence, establishes the cosmic energy budget: a spatially flat universe whose energy is dominated by dark energy and dark matter, with ordinary matter a minor ingredient.<ref>{{#cite:Q1232}}</ref>

	For these reasons the CMB is, as of September 2026, one of the most active frontiers of observational cosmology. Current and planned experiments continue to map its polarisation at ever higher sensitivity: ground-based instruments such as the Simons Observatory and CMB-S4, and the ~~proposed space missions such as~~ JAXA's LiteBIRD, ~~aim primarily at~~ the search for the primordial B-modes that would reveal the physics of inflation.		For these reasons the CMB is, as of September 2026, one of the most active frontiers of observational cosmology. Current and planned experiments continue to map its polarisation at ever higher sensitivity: ground-based instruments such as the Simons Observatory and CMB-S4, and the planned JAXA-led LiteBIRD satellite mission, target above all the search for the primordial B-modes that would reveal the physics of inflation.

	== References ==		== References ==