Long-Wavelength Measurements of the Cosmic Microwave Background Radiation Spectrum

The temperature of the cosmic microwave background radiation has been measured at wavelengths of 0.33, 3.0, 8.2, and 21.3 cm. These measurements represent a continuation of the work reported in 1985 by Smoot et al. The new results have a weighted average of 2.70 + or - 0.05 K and are consistent with past measurements. They limit the possible distortion of the cosmic microwave background radiation spectrum to less than 6 percent. The results of all measurements to date are consistent with a Planckian spectrum with temperature 2.74 + or - 0.02 K spanning a wavelength range of 0.l-50 cm.