THE observed spectrum of the microwave background radiation1 and the observations of interstellar absorption lines2 point to a thermal origin, while the extreme isotropy of this radiation, not easily consistent with superposition of emission from discrete sources34, suggests that the microwave background is primordial. If so, the recent observations at sub-millimetre wavelengths (Shivanandan et al.5, Houck and Harwit6 and Muehlner and Weiss7) must have a different origin. Processes before or during the recombination epoch cannot distort a primordial blackbody spectrum to the extent indicated by the sub-millimetre observations8,9. If the observed isotropy6 of the sub-millimetre flux (I/I 0.5 within a beamwidth 0.008 steradians11) is an indication of its extragalactic origin, it must have been produced at an epoch later than the recombination epoch and probably after or during the formation of galaxies, but, in this case, it may be due to superposition of radiation from discrete sources. Another possibility is that the sub-millimetre flux is due to some optically thick resonant line in the galaxy10, or to several lines within a narrow band of frequency. Finally, a recent observation from Mauna Kea shows a background emission feature near 870 m, consistent with ref. 7 and possibly attributable to atmospheric N2O (I. Nolt, private communication). This observation was, however, made from a low altitude ( 5 km), where the infrared N2O column density is high and not well determined (M. Werner, private communication). These observations need verification.
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