The monopole spectrum of the Cosmic Microwave Background Radiation using the original data set by the FIRAS team, available at http://lambda.gsfc.nasa.gov/product/cobe/firas_monopole_get.cfm. The vertical axis "MJy/sr" corresponds to 10⁶ jansky per steradian, where a jansky is 10^-26 Watts per square-meter per Hertz. The horizontal axis ("1/cm") corresponds to the reciprocal of the microwave wavelength (in cm), which is proportional to the microwave frequency. The error bars are too small to be displayed by a computer screen, but vastly exaggerated error bars were included to show the measured data points. Made with GNUPlot.
NASA's comment on the original picture: "Cosmic Microwave Background (CMB) spectrum plotted in waves per centimeter vs. intensity. The solid curve shows the expected intensity from a single temperature blackbody spectrum, as predicted by the hot Big Bang theory. A blackbody is a hypothetical body that absorbs all electromagnetic radiation falling on it and reflects none whatsoever. The FIRAS data were taken at 34 positions equally spaced along this curve. The FIRAS data match the curve so exactly, with error uncertainties less than the width of the blackbody curve, that it is impossible to distinguish the data from the theoretical curve. These precise CMB measurements show that 99.97% of the radiant energy of the Universe was released within the first year after the Big Bang itself. All theories that attempt to explain the origin of large scale structure seen in the Universe today must now conform to the constraints imposed by these measurements. The results show that the radiation matches the predictions of the hot Big Bang theory to an extraordinary degree. See Mather et al. 1994, Astrophysical Journal, 420, 439, "Measurement of the Cosmic Microwave Background Spectrum by the COBE FIRAS Instrument,"Wright et al. 1994, Astrophysical Journal, 420, 450,"Interpretation of the COBE FIRAS CMBR Spectrum," and Fixsen et al. 1996, Astrophysical Journal, 473, 576,"The Cosmic Microwave Background Spectrum from the Full COBE FIRAS Data Sets" for details."