Cosmic Radiation

In a talk given at Johns Hopkins in early 1965, Princeton theoretician P. J. E. Peebles suggested that the early universe must have contained a considerable amount of radiation if the big bang model were correct. If there were little radiation, any hydrogen present in the early universe would have quickly fused to form heavier elements, and no hydrogen would be left today. This directly contradicts the observation that about 75% of the matter we see today consists of hydrogen. If, however, there were a large amount of radiation present in the early universe, the energetic photons would bust up the larger nuclei as they formed, leaving behind hydrogen. Peebles proposed that this radiation, the cosmic photons which prevented the fusion of hydrogen in the early universe still exist today but in a very altered form. There should have been little change in the number of photons, but a great change in their energy.
As the universe expanded, the wavelength of the cosmic photons should be stretched by the expansion, greatly reducing their energy. If the photons were in thermal equilibrium with very hot matter in the early universe, they should still have a thermal black body spectrum, but at a much lower temperature. He predicted that the temperature of the cosmic radiation should have dropped to around 10 kelvin. His colleagues at Princeton, P. G. Roll and D .T. Wilkinson were constructing a special antenna to detect such radiation. All of this work had been suggested by R. H. Dicke, inventor of the key microwave techniques needed to detect ten degree photons.
Peebles was not the first to suggest that there should be radiation left over from the big bang. That was first suggested in a 1948 paper by George Gamov and colleagues Ralph Alpher and Robert Herman in a model where all elements were to be created in the big bang. A more realistic model of the big bang proposed by Alpher and Herman in 1953 also led to the same prediction of cosmic radiation. In both cases, it was estimated that the thermal radiation should now have a temperature of 5 kelvin. In the early 1950s, Gamov,
Alpher and Herman were told by radio astronomers that such radiation could not be detected by equipment then available, and the effort to detect it was not pursued. Peebles was unaware of these earlier predictions.