In an expanding universe, the density of the universe continually falls. Today the density of the universe is so low that radiation can propagate across ten billion parsecs without interacting with matter; but in the distant past, the density was high enough that the radiation interacted continually and strongly with matter. This interaction characterized the early universe from until about one hundred thousand years after its beginning. The radiation at this time behaved like radiation trapped within the interior of a star: the radiation stayed in thermal equilibrium with the matter, and so was characterized by a black body spectrum.
The interesting time for observations is the transition of the universe from this high density regime to the current low-density regime. The reason this regime is interesting is that we can to see the radiation from this time. This radiation reaches us from the most distant observable parts of universe, the parts that have the highest redshifts. The radiation that escaped these parts as the universe went transparent 100,000 years ago is just now arriving at Earth. In effect, when we look out into the universe, we see out until we reach this photosphere that encloses us like water surrounding an air bubble.
Why does the universe go from opaque to transparent at early times? The reason is that the radiative processes are very efficient at exchanging energy between radiation and matter at early times, but are inefficient at latter times. The radiative processes are the same as found at the centers of stars: radiation scatters with free electrons ( Compton scattering), and the emission and absorption of radiation as free electrons pass by ions ( bremsstrahlung emission and absorption). These processes are proportional to the square of the density, and are therefore proportional to the scale of the universe to the sixth power. This means that when the scale of the universe doubles in size, the rate of interaction between the matter and the radiation decreases by a factor of sixty-four. So while the radiation can maintain thermal equilibrium with matter at early times, eventually the density drops to the point that the universe doubles in time before the radiation can thermalize with matter. At this time, the characteristics of this radiation are set for all later times. The radiation is black body radiation, with a temperature and energy density characterized by a single parameter—the temperature of the radiation.
With time, the photosphere releasing this radiation receded away from us. The relic radiation created where we now live is 5 billion parsecs away from us. What we see today is the relic radiation created by a photosphere that was 5 billion parsecs from us when it emitted its radiation. Because of the photosphere's motion away from us, this radiation is heavily redshifted. Within the photosphere, the radiation has a temperature of around 5,000° Kelvin; today it appears to us as black body radiation with a temperature of only 2.73° K. Because this relic radiation is redshifted into the microwave band, it is referred to as the microwave background.
The temperature we see today is related to the temperature at the time of its creation by Tc = ( 1 + z ) To, where z is the redshift. Because the universe becomes transparent when the matter has a temperature of roughly 3000°K, the relic radiation we see comes from a region with a redshift of z = 1000.
One of the more unusual aspects of the microwave background is that it sets a preferred rest frame within our local universe. An object is at rest relative to the microwave background if the temperature is the same in all directions. If we move relative to this rest frame, the microwave background in our direction of motion is blue-shifted, so that its temperature appears higher than 2.73°K. Each point in our universe has a unique rest frame, one that is different from every from the rest frame in the universe.
This ability to define a unique rest frame at every point in the universe is somewhat startling, because one of the basic assumptions of physics is that there is no preferred reference frame, meaning that the physics we do here on Earth is identical to the physics done by a traveler moving at a high rate of speed. Of course the basic point of this assumption still holds; after all, special relativity still holds. While the microwave background defines a unique rest frame, the local physics does not.
The microwave background is an important feature of the big bang theory of the universe, and its detection proved to be the most persuasive piece of evidence for the theory.
Current research on the microwave background focuses on deviations of the microwave background from pure isotropy. The reason is that today we see a very clumpy universe, filled with galaxies and clusters of galaxies. For these to have formed, pockets of high density must have existed in the early universe. These fluctuations appear as fluctuations in the microwave background., and by studying these fluctuations, we can learn the initial conditions that lead to the galaxies.