COBE Satellite Finds No Hint of Excess in The Cosmic Microwave Spectrum

For several years cosmologists have been kept busy puzzling over reports of a large excess in the cosmic microwave background spectrum at wavelengths shorter than 1 mm. Now they've been given a brief respite. At the January meeting of the American Astronomical Society in Washington, DC, John Mather of NASA's Goddard Space Flight Center presented the first spectral results from COBE—the Cosmic Background Explorer—which had been launched into Earth orbit just eight weeks earlier. The moment Mather placed the COBE spectrum (reproduced on this page) on the overhead projector, the packed lecture hall burst into sustained applause. The contrast with all the earlier, fragmentary data was astonishing. The spectral measurements, from 1 cm down to 0.5 mm, fit perfectly to a Planck blackbody radiation curve for a temperature of $\text{2.735±0.06\hspace{0.17em}}\mathrm{K}.$ At the 1% level one could see no deviation from an ideal blackbody spectrum. And it's not just a question of fitting a shape. With only one free parameter—temperature—the normalization also has to come out right. For a given temperature the absolute brightness of a blackbody spectrum is specified.