1.1.10. The Discovery of the Cosmic Microwave Background
Gamow's prediction would be verified in 1965 with the accidental discovery of the Cosmic Microwave Background (CMB) by Penzias and Wilson of Bell Labs. Actually, a curious coincidence also led to the unknown discovery of the CMB as early as 1941. The energy density in the CMB is very similar to the energy density of ambient starlight in our own Galaxy. Observations of the excited states of the CN molecule made by McKellar (1941) indicated excitation by a photon background with a characteristic temperature of 2.3 K. But since the CN molecule could only be found where the Galaxy was, the excitation was attributed to the ambient energy density of starlight in the Galaxy as opposed to being energy external to the galaxy (i.e., the CMB). The initial measurements of Penzias and Wilson indicated that the flux density of photons at their millimeter receiver was independent of position in the sky. Failing to see any 24 hour modulation of this signal, the remaining logical conclusion was that the CMB was indeed of cosmological origin and therefore everywhere. Further observations of this CMB showed that its spectral signature was consistent with that of a blackbody, as predicted earlier by Gamow.
Today we have a precise measurement of the CMB due to the COBE (Cosmic Background Explorer) instrument launched by NASA in 1989. COBE made very sensitive measurements of the CMB and any anisotropy that might be present (this is of crucial importance in structure formation models that are discussed in chapter 5). The COBE measurements indicate that the CMB is a nearly perfect black body characterized by a temperature of 2.74 +/- 0.02 K. Prior to COBE, some balloon borne experiments indicated slight deviations from a perfect black body spectrum including the now infamous sub-mm excess in the CMB that many theorists attributed to the presence of hot dust in the early universe. However, there is nothing like good data, and the spectrum obtained by the COBE mission (see Figure 1-5) completely rules out any additional contributions to the CMB.
Figure 1-5: The COBE spectral data showing the nearly perfect blackbody fit. The plotted error bars are 400 times greater than their actual value in order that some idea of the errors can be represented. Each data point represents one of the 42 spectral bands of COBE. The x-axis is the wavenumber in cm-1 and the Y-axis is the intensity of the microwave emission. Figure is courtesy of Edward L Wright (UCLA) and is reprinted with his permission.