So; what is the Cosmic Microwave Background (CMB) radiation, and why is it there?  As early as 1941, Andrew McKellar, studying interstellar absorption lines, observed an average black-body equivalent temperature of 2.3 K.  In 1948, theoretical studies predicted the background temperature, although the estimates of its value back then were somewhat high based, as it was, on the Universe being only three billion years old.  Essentially, it is a form of electromagnetic radiation that fills the universe.  If you tune your TV between channels, you see a ghostly white fog.  Some of it is due to the pervasive CMB.  Using radio telescopes, there is a faint, near homogenous background of radiation that is strongest in microwaves. It was discovered, serendipitously, in 1964 by Arno Penzias and Robert Wilson while working for Bell Labs in New Jersey.  In 1978, they received the Nobel Prize for their work.

CMB fits well within the Big Bang model for the birth of the Universe.  Recall from the discussion of the Early Universe that at about 380,000 years, the Universe becomes transparent following recombination.  The photons that existed at that time no longer reacted with atomic nuclei, which had ceased to be ionized and had combined with electrons to form neutral atoms.  As the Universe expanded, the photons lost energy.  As photons always travel at light speed, when they loose energy, their wavelength increases.  Today, after some 13 billion years, the CMB has a black-body equivalent temperature of about 2.725 K, which corresponds to microwaves.  The CMB is almost uniform in any direction, but does contain small irregularities, known as anisotropies, which vary with the size of the region examined.  To date, the Big Bang has been the model that most closely explains not just the existence of the CMB, but the anisotropies as well.  The Cosmic Background Explorer (COBE), launched in 1989, was the first probe to look at the CMB.  The Wilkinson Microwave Anisotropy Probe (WMAP), launched in June 2001, was designed to produce much more detailed and accurate maps of the CMB at a resolution of 0.2 degrees.  Separately, the Sloan Digital Sky Survey (SDSS) mapped 230 million celestial objects, and measured the spectra of 930,000 galaxies, 120,000 quasars, and 225,000 stars.  Together with the WMAP results, particularly the most recent seven year results published in January 2010, the data confirmed that:
Astronomy & Cosmology


Large Scale Structure of the Universe

Cosmic Microwave Background

the Universe is 13.75 0.13 Billion years old.
The current expansion rate of the universe, the Hubble constant is 70.5 1.3 km/second/Mpc.
The geometry of the Universe is Euclidean to within 2%; that is, it is essentially flat. Check image to the right for more information about the possible geometries of the Universe. 
It corroborated the standard "cold dark matter" model of the universe, suggesting that the constituents of the Universe are:
     ordinary matter comprises 4.6% 0.1%,
     cold dark matter comprises 23.3% 1.3%,
     dark energy comprises 72.1% 1.5%, causing the expansion rate of the universe to speed up.
Less than 1% of the current contents of the universe is in the form of neutrinos. 
There could be one or even two more species of neutrino that have not been observed. 
The next phase of CMB analysis is the European Space Agency's (ESA) Planck spacecraft, which launched on the 14th of May, 2009.  Amongst the many improvements, it has three times the resolution and ten times the sensitivity of WMAP.  Initial data has proved to be excellent, and there is a lot of information, and some preliminary pictures, on the ESA Planck website
Various Possible Geometries of the Universe