Along with nucleosynthesis of the light elements and
the large scale structure of the universe, the cosmic
microwave background (CMB) radiation is a basic feature
of the hot Big Bang theory for the origin of the universe.
Predicted by G. Gamow, R. Alpher, and R. Herman, the CMB
is radiation emitted during the recombination epoch, when
electrons combined with atomic nuclei to make the light
elements. At recombination, the CMB radiation had a
temperature of roughly 3000 K. Today, the expansion of the
universe has cooled the temperature to just below 3 K.
A. Penzias and R. Wilson discovered the CMB in 1965. Nearly
25 years later, the launch of the
Cosmic Background Explorer (COBE) satellite yielded
unprecedented measurements of the CMB temperature
(2.725 K) and anisotropy. The anisotropy measurments
show how matter was distributed in the Universe roughly
400,000 years after the Big Bang (13 billion years ago).
The image below shows a map of the CMB acquired with the
Wilkinson Microwave Anisotropy
Probe (WMAP). Colors indicate warmer, high density (red)
and cooler, low density (blue) spots. The white bars show the
"polarization" direction of the radiation.