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.