The origins of the CMBR lie in an early hot phase of the expansion of the Universe, where the details of its generation are erased by the close coupling of radiation and matter. Later energy releases, interactions with matter at different temperatures, and other effects can modify the spectrum and brightness distribution of the CMBR. Cosmological data on the gross properties of the Universe are contained in the integrated properties of the CMBR, such as the spectrum and the large-scale brightness structure. Detailed information about the properties and formation of present-day objects, such as clusters of galaxies, is encoded in the small-scale structures in the brightness.
A critical stage in the development of the CMBR occurs when the expansion of the Universe causes the temperature to drop to about 3000 K. At earlier times (higher redshifts), matter and radiation were in good thermal contact because of the abundance of free electrons. But at this stage the number of free electrons drops rapidly as matter becomes neutral, and the radiation and matter become thermally decoupled, so that the temperatures of the photon and matter fluids evolve almost independently. We can distinguish three events that occur at almost the same time: the non-relativistic and relativistic (photon plus neutrino) mass densities are equal at redshift
most electrons have become bound to ions at the redshift of
recombination,
and the interaction length of photons and electrons exceeds the scale
of the Universe at the redshift of decoupling
(approximate forms taken from
Kolb & Turner 1990).
In these relations,
One of the important changes that occurred during this period, because
of the change in the interactions of photons and electrons, was that
the length scale on which gravitational collapse can occur dropped
dramatically, so that fluctuations in the mass density that were
stabilized by the radiation field before recombination became unstable
after recombination, and were able to collapse (slowly - the
expansion of the Universe causes the collapse of gravitationally bound
objects to be power-law rather than exponential in time:
Landau & Lifshitz 1962;
see descriptions in
Kolb & Turner 1990).
Matter
over-densities and under-densities present at recombination, and which
later became the large-scale objects that we see in the present-day
Universe, such as clusters of galaxies,
caused fluctuations in the intensity of the radiation field through
their gravitational perturbations (the Sachs-Wolfe effect;
Sachs & Wolfe 1967),
through thermodynamic fluctuations in the density of
radiation coupled to the matter, and through Doppler shifts due to
motions of the surface of last scattering. Recent reviews of the
introduction of primordial structure in the CMBR by objects near
recombination are given by
Bond (1995) and
White et al. (1994).
0
is the present-day mass density of the Universe,
and B is the
present-day baryon density, both in units
of the critical density, 
crit (equation 2). The
redshifts of recombination and decoupling are similar, and neither
phenomenon is sharply-defined, so that there was
a moderately broad redshift range from 1500 to 1000
(about 1.6 x 105 (0
h1002)-1/2
years after the Big Bang) when the Universe was becoming
neutral, matter-dominated, and transparent to radiation. At some time
about then, most of the photons that are now in the
cosmic background radiation were scattered by electrons for the last
time, and we often refer to a sphere of last scattering or
redshift of last scattering at this epoch.