More history in abbreviated form is in this CMB time line .
The universe was once very hot and dense, the photons and baryons would have formed a plasma. As the universe expanded and cooled there came a point when the radiation (photons) decoupled from the matter. The radiation cooled and is now at 2.73 Kelvin. The fact that the spectrum (see figure) of the radiation is almost exactly that of a black body implies that it could not have had its origin through any prosaic means. This has led to the death of the steady state theory.
This diagram, centered on the observer (you), shows a representation of the universe where the angle represents the angle of view and the distance (radius) from the center measures both distance and time since light travels at a finite speed. The scale is non-linear and maked in terms of redshift z which is the fractional amoumt by which emitted light is stretched by the expansion of the Universe in the travel time from its emission and reaching the observer. The Universe changes scale by a factor (1 + z) in this time and the light wavelength changes by the same factor.
The temperature of the cosmic background radiation changes down by the same factor (1 + z). At early epochs (for z>1000), this temperature was high enough that most of the universe- was ionized, and therefore opaque ("optically thick"). The surface z=1000 is sometimes called the cosmic photosphere, in comparison with the photosphere (apparent surface) of the Sun. It is the surface from which the cosmic background photons last scattered before coming to us.
The light coming from this cosmic photosphere (surface of last scattering) can be used to make an image of the early Universe. One can then learn about the Universe when it was a 1000 times smaller than the present. In 1967 Sachs and Wolfe determined that any observed temperature variations were directly related to variations in the density variations at that time.
However at the time, cosmologists, having very litle data, fell back on the Cosmological Principle: which states that the universe, on the average, looks the same from any point. It is motivated by the Copernican argument that the Earth is not in a central, preferred position. If the universe is locally locally isotropic, as viewed from any point, it is also uniform. So the cosmological principle states that the universe is approximately isotropic and homogeneous, as viewed by any observer at local rest. (See U2 Anisotropy Experiment for the effect of observer motion.) The CMB should then appear to be approximately isotropic.
Further investigations, including more recent ones by the COBE satellite (Smoot et. al.), confirmed the virtual isotropy of the CMB to better than one part in ten-thousand.
A map of the sky at microwave frequencies, showing that the CMB is almost completely the same in all directions.
Given this qualification checked in limited regions by small angular scale observations, any attempt to interpret the origin of the CMB as due to present astrophysical phenomena (i.e. stars, radio galaxies, etc.) is discredited. Therefore, the only satisfactory explanation for the existence of the CMB lies in the physics of the early Universe.
While the CMB is predicted to be very smooth, the lack of features cannot be perfect. At some level one expects to see irregularities, or anisotropies, in the temperature of the radiation. These temperature fluctuations are the imprints of processes and features of the early universe. The COBE DMR instrument first detected these imprints and made them public in 1992.
Usually the features of the Universe and the CMB are interpreted in the context of a cosmological model - The Big Bang - which is derived from general cosmological principles and observations. Some of the features of the Big Bang model are discussed in the Science Goals subsection on the Origin of Large Scale Structure.
Pictograph of Signal Flow Concept
for attaining cosmological model tests and parameters from CMB observations.
Words on signal flow Software concept
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