Cosmic Microwave Background
The cosmic microwave background (CMB) is a faint, extremely uniform bath of microwave radiation arriving from every direction in the sky, with a spectrum very close to that of a blackbody at about 2.725 K. In standard cosmology it is interpreted as relic radiation released when the expanding universe first became transparent to light, roughly 380,000 years after the Big Bang.
It was found accidentally in 1964–65 by Arno Penzias and Robert Wilson at Bell Telephone Laboratories in Holmdel, New Jersey, who could not eliminate a persistent excess antenna temperature of a few kelvin from their horn reflector at 7.35 cm. Their brief 1965 announcement in the Astrophysical Journal appeared alongside a companion paper by Robert Dicke, Jim Peebles, Peter Roll and David Wilkinson interpreting the signal cosmologically. Penzias and Wilson shared the 1978 Nobel Prize in Physics. The COBE satellite's FIRAS instrument showed in 1990 that the spectrum matches a blackbody to remarkable precision, and COBE's DMR instrument detected in 1992 the temperature anisotropies — variations of order one part in 100,000 — whose statistics were later mapped in detail by WMAP and by Planck. Superimposed on this is a much larger dipole, at the millikelvin level, attributed to the motion of the solar system relative to the radiation field.
Within the standard model the CMB is the single most quantitatively powerful cosmological dataset: the angular power spectrum of its fluctuations is what fixes most of the parameters of the concordance model, including the Hubble constant and the density attributed to dark matter and dark energy.
A note on prediction
The CMB is usually presented as a successful prediction of the hot Big Bang, and Ralph Alpher, Robert Herman and George Gamow did estimate a present-day relic temperature of a few kelvin in the late 1940s. It is equally true, and less often mentioned, that several earlier workers — including Arthur Eddington, Erich Regener and Walther Nernst — obtained temperatures of a few kelvin for the interstellar radiation field on quite different, non-cosmological grounds. Critics of the standard model make much of this; defenders reply that only the hot Big Bang predicts a blackbody of that temperature filling all space. Both points are fair, and the disagreement is about which prediction counts.
On this wiki
The CMB is the observation that most sharply divides the researchers catalogued here, because a non-expanding universe still has to explain it. Related material is indexed under Category:Big Bang and Category:Cosmology.
The CMB as thermalised starlight or plasma emission. The most developed alternative here is Lyndon Ashmore's "new tired light", in which photons crossing intergalactic space recoil off free electrons, losing a little energy — producing the redshift — and re-emitting that energy at microwave wavelengths. His Recoil Interaction Between Photons and The Electrons In The Plasma Of Intergalactic Space Leading To The Hubble Constant And CMB derives both the Hubble constant and the background from the same mechanism, and the laboratory side is argued in Intrinsic Plasma Redshifts Now Reproduced in the Laboratory: A Discussion in Terms of New Tired Light. A related programme is Ari Brynjolfsson's plasma redshift cosmology. Plasma cosmologists such as Eric J Lerner argue instead that the background is starlight thermalised and isotropised by filamentary plasma structures.
The CMB read as an aether or motion signal. Because the dipole defines a frame in which the radiation is isotropic, several authors here treat it as a recovered preferred frame. Hartwig Wolfgang Thim proposes an oscillator experiment on exactly this basis in How Much Lower is the Frequency of a Solid State Oscillator When it is Moving Relative to the CMB (Cosmic Microwave Background), and Glen W Deen connects the dipole to Dayton Miller's aether-drift results in D. C. Miller's Ether Wind Velocity Predicts Rotation of the CMBR Anistropy Vector of ca. 27 Arc Minutes per Year and D. C. Miller's 1933 Cosmic Ether Model. See Aether.
Direct critiques. Paul Marmet argues against the standard interpretation in Big Bang Cosmology Meets an Astronomical Death, and William C Mitchell in Bye Bye Big Bang, Hello Reality and No Neutrinos, No Big Bang. Alexander A Scarborough offers a non-cosmological origin in The Cosmic Microwave Background: A New Perspective on the 27 K Radiation, and Joseph J Smulsky discusses observational tests in Cosmic Microwave Background Radiation and New Space Projects.
The honest summary of the disagreement is this: the blackbody quality of the spectrum is the hardest single fact for any local or scattering origin to reproduce, and the alternatives above are attempts to meet it rather than evasions of it.