Spectrum and Temperature

As Penzias and Wilson found, in every direction there is a low energy and very uniform radiation that fills the Universe. This is called the 3 degree background radiation, or the cosmic background radiation (CBR), or the microwave background, because the radiation is that of a blackbody with temperature slightly less than 3 degrees on the Kelvin scale (2.735 K). By Wien's law, the blackbody curve for this temperature peaks in the microwave portion of the spectrum. The following figure shows the essentially perfect blackbody spectrum obtained by the Far InfraRed Absolute Spectrometer (FIRAS) aboard NASA's Cosmic Background Explorer satellite (see the right panel for more information on COBE).

The curve shown is a theoretical blackbody curve for a temperature of 2.735 K, with the horizontal axis proportional to frequency. The agreement between theory and measurement is so spectacular that none of the data points can be distinguished from the theoretical curve on this scale! This figure represents the most perfect blackbody that has ever been observed.