Spectrum of the Cosmic Microwave Background: Results and Prospects

Abstract

The spectrum of the cosmic microwave background carries information on energetic processes in the universe from redshift z ~10⁶ to the present. COBE measurements limit deviations from a blackbody spectrum to less than 0.03% of the CMB peak flux from 0.5-5 mm wavelength, corresponding to energy release AE/E < 2 x 10^-4 for redshifts 10³ < z < 10⁶. A series of balloon-borne and ground-based measurements show no significant evidence for spectral distortions at longer wavelengths. Is this the end of the story? Can new observations provide significant new understanding of physical processes in the early universe? I review mechanisms which can distort the CMB spectrum and discuss observing strategies to complement the COBE results.