At present, the universe has a very low density (only a few atoms per cubic meter) and is there transparent. Since it is expanding, it is clear that it was denser and hotter in the past. Much earlier it was in fact completely opaque. The main source of opacity is due to free electrons which interact with photons through Thomson scattering. We know that the transition from the opaque epoch to the transparent one corresponds to the epoch where most of free electron were captured by atomic nuclei (hydrogen and helium) as the unverse cooled down because of expansion. The epoch is referred to as recombination, although the universe had always been ionized earlier. This epoch occured quite early in the history of the universe: its temperature was around 3000K, the corresponding redshift being around 1100 and its “age” (or more precisely the time that has elapsed since nucleosynthesis) was only a few 100,000 years.

After recombination, photons can travel without being scattered by free electrons. For this reason, recombination is also called the ``last scattering epoch’’. A large number of astrophysical processes have produced photons since then, but at present, these primordial photons still represent by far the largest contribution, both in photon number and in energy, to radiation. Today, these photons are detected under the form of an almost perfect black body at a temperaure around T=2.726K and form the Cosmic Microwave Background, or CMB.

Since CMB photons travelled freely since recombination, the region from which the photons we observe were emitted is a sphere centered on us and is called the last scattering surface. Note that this sphere has no physical reality: another observer would define another last scattering surface centered on him- or herself.

The main reason why CMB is useful for cosmology is that its epoch of emission is very old: no other astrophysical object is known to be that old and by definition it is the oldest electromagnetic image we can see.

The strategy to extract science from CMB bears a lot of similarities with heliosismology: if one looks toward the sun, one sees only its surface as space between Earth and the Sun is transparent and its interior is opaque. It is however possible to get a lot of information about the interior of the Sun only by looking at its surface. The reason is that the Sun is not a static or stationary object, but is constantly vibrating. Some of these vibrations can be observed (through Doppler effect) at its surface. These vibration modes do not only travel at the surface of the Sun but also travel more or less deeply inside the star. By studying all these vibration modes it is therefore possible to extract the sound speed profile inside the Sun as deeply as modes can travel. One can then reconstruct the radial profile of most of the relevant physical parameters. The situation is very similar with CMB: the last scattering surface plays the role of the surface of the Sun, and one observes tiny temperature fluctuations on the last scattering surface. These temperature fluctuations are interpreted as density waves which already existed at the last scattering epoch. Analyzing carefully these fluctuations allows to obtain some informations about the last scattering epoch and earlier. The analogy with heliosismology is however not perfect. In particular there are three crucial differences between the Sun and the CMB:
 The apparent frequency of the density waves we observe is extremely large, around several 10,000,000 years. Therefore we have a static rather than dynamical view of the last scattering surface. We therefore have far less information as compared as a long timescale observation of the Sun.
 Although the universe is transparent, we see the whole universe between the last scattering surface and us. Foreground removal is therefore a preliminary step before obtaining a cleaned CMB map and is a complicated procedure. CMB maps we can extract are therefore noisy.
 Although we know that some process has generated these density waves in the early universe, such process probably occured very early in the history of the universe (i.e., much earlier than nucleosynthesis) and is not well known at present. The interpretation of CMB fluctuation both relies on known physics (propagation of density waves at recombination) as well as hypothesis on physical processes which are far from our reach experimentally.

Although the parameters that can be extracted from the study of CMB fluctuations are usually called under the common name of cosmological parameters, they usually belong to one of these two categories:
 The astrophysical parameters, which are those who influence the propagation of density wave at recombination; the matter content of the universe (baryon density, dark matter density, etc) belongs to this category,
 The primordial parameters, which have to do with the (still unknown) physical processes that generated density waves in the early universe. Since these processes are not well known, there is a lot of freedom in the way one parametrizes them, and one has to keep in mind that the extraction of the astrophysical parameters is in general very dependent on the choice of this parametrization.

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