For those enamoured of the primaeval fireball, the relict radiation has proved a tantalising mistress. Since the famous discovery in 1965, by Penzias and Wilson, of an excess antenna temperature consistent with cosmological expectations, many observers have succeeded in measuring a flux consistent with a thermodynamic temperature of ∼3K. Until recently, however, no direct spectral measurements had been made at wavelengths shorter than the Planckian peak corresponding to radiation at this temperature. At such wavelengths atmospheric emission and absorption are overwhelming from even the highest mountain site and observations must be made from at least balloon platforms. The pioneering broadband rocket and balloon measurements covering this wavelength region produced consternation when excessively high fluxes were reported; successive flights gradually eliminated the excess, emphasising the practical difficulties of such observations. A review of this phase of the pursuit is given by Blair1. Nevertheless, it is upon direct submillimetre measurement of the spectral density Iν that confidence in the interpretation of the longer wavelength results must reside. The outcome of the first such measurement, by Queen Mary College in 1974, seemed completely to justify such confidence. Unfortunately, the subsequent observations by Berkeley, although leading to the same conclusion about the value of the thermodynamic temperature, were so discrepant in detail from those of QMC as once again to raise doubts. We have since been eagerly awaiting the results of observations by independent groups but these have been frustrated by instrumental failures. We attempt here to assess the present situation. We conclude that although present measurements indicate a flux not inconsistent with a Planckian spectrum corresponding to a temperature of ∼3K, they do not demand such an interpretation. Moreover, because we may not actually expect a Planckian curve from a fireball, very much more detailed information is needed to obtain a view of the early thermal history of the universe.
Cosmic Microwave Background Anisotropy at Degree Angular Scales and the Thermal History of the Universe