The CMB (Cosmic Microwave backround)
The CMB is composed of a bath of photons which were emitted following
the big bang when the universe was still extremely hot and dense - so hot
and dense that all the matter and radiation was bound together in a photon-baryon
fluid. At this time, the universe was too hot to form stars, galaxies,
or any other structure as we know it today. Although this fluid was incredibly
uniform, there were tiny (one part in 100,000) inhomogeneities in
its density. As the universe expanded and cooled enough to form neutral
atoms, the photons decoupled from the baryons. The baryons then proceeded
to gravitationally collapse into potential wells formed by the denser regions
initiating the formation of the first stars and galaxies while the photons
were left to travel essentially inimpeded ever since, cooling with the
expansion of the universe. Observed with microwave detectors, these photons
(CMB photons) are effectively a shapshot of the very early universe.
Denser and less dense regions in the primordial mass distribution are reflected
as hotter and colder regions observed with our detectors. The way
in which these inhomogeneities (anisotropy in the CMB) are distributed
on the sky gives us insight into how the universe came to be what we observe
today. By comparing these measurements with a set of structure formation
models, we can estimate cosmological parameters such as the geometry of
the universe, its composition, and its rate of expansion.
For an exellent discussion of CMB theory pitched to many different levels, check out Wayne Hu's web site. For a brief review of recent CMB experiments see the a conference proceedings (astro-ph/0112052) that I wrote up last fall.