Chapter 6: The Prediction of Cosmic Background Radiation

Around 1950, Gamow proposed that many of the heavy elements currently found in the Universe were synthesized by thermonuclear reactions when the Universe was very young. To enable such reactions to proceed fast enough to offset the rapid decrease of density caused by the expansion of the early Universe, the matter in the Big Bang must have started at very high temperatures, and associated with it would be a thermal radiation field whose energy density greatly exceeded that of matter.

As the Universe expanded, both matter and radiation would cool, become too low for further nuclear reactions, and eventually the matter and radiation will follow separate thermal histories. Past this epoch the radiation is no longer in thermodynamic equilibrium with matter but it would continue to cool as each photon is redshifted by the cosmological expansion. Under these conditions it can be shown that the radiation continues to maintain a thermal (i.e. black body) spectral distribution characterized by lower and lower radiation temperatures as the Universe continues to expand.

According to early estimates of Gamow, by now the radiation would be no more than tens of degrees above absolute zero.

Dicke and Peebles worked to improve the Big Bang nucleosynthesis calculations when it became clear that stellar nucleosynthesis could probably not explain the present cosmic abundance of He. In particular they concluded that the best available data led to a prediction of a few Kelvin of the present radiation temperature. They set out to build an appropriate antenna to detect this radiation.