Predicting the CMB: The hazards of being first

Carlstrom, Crawford, and Knox reply: Victor Alpher indeed points out an important part of the story, one that we included in an earlier version of our article , but in our efforts to drastically shorten the manuscript it was relegated to a single reference. The earlier draft read, “That Arno Penzias and Robert Wilson’s signal was so quickly and correctly interpreted is a consequence of some of the earliest connections between particle physics and cosmology. In the 1940s Ralph Alpher and George Gamow were considering the hot, dense, early universe as a possible site for nucleosynthesis. They found that in order to produce the amount of helium observed in the local universe, there had to be about 10¹⁰ thermal photons for every nucleon and correctly predicted that this background of photons would persist to the present day as a thermal bath at a few kelvin. When Robert Dicke and his collaborators at Princeton University—who also predicted the existence of the cosmic microwave background—learned of the measurement at nearby Bell Labs, they immediately understood the significance.”