Remnant heat from the "big bang," first detected in 1965, is in the form of microwave radiation because the universe has cooled considerably since its birth. But the universe acts as a perfect emitter and absorber of radiation, and for this reason it is called a blackbody as all objects are which emit and absorb perfectly.

Our Sun is almost a blackbody, so we'll use it as an example of what to expect from the much cooler universe. First, the Sun, at a temperature of 5,800 K, emits electromagnetic waves predominantly in the form of heat. The universe, however, has cooled to 2.7 K so the electromagnetic waves from its beginning are now microwaves (called the Cosmic Microwave Background, or CMB). Another blackbody property of the Sun is its emission of light of all colors (or wavelengths, as seen through a prism) as well as heat (infrared) and ultraviolet radiation. This range of wavelengths is easily detected because the Sun is comparatively hot, but the CMB also creates a range of microwave wavelengths (equivalent to colors) consistent with a blackbody of much lower temperature.

The FIRAS instrument on the COBE (Cosmic Background Explorer) satellite measured this range of wavelengths and found them to be exactly as expected for a blackbody, as shown below.



South Carolina State University, 08/26/2012