The fifteen years, from 1951 to about 1965, brought a number of developments that gradually weighed in favor of the Big Bang Theory. Among these was the confirmation that radio galaxies exist. Radio galaxies are sources of radio waves. Astronomers had been aware of a far-away source of radio waves for some years but the debate over whether the source was a star or a galaxy was relevant to the Steady State or Big Bang Theory debate. The theory was that radio galaxies were assumed to be young galaxies. Under the Big Bang Theory, young galaxies should be very far away but under the Steady State Theory, young galaxies should be distributed more or less evenly throughout the universe. Therefore, if the distribution of radio galaxies could be determined, that would provide evidence for one theory or another.
By 1961 over 5,000 radio galaxies had been discovered and their distances determined, some by direct observation and some by statistical methods. The result clearly favored the Big Bang Theory: radio galaxies tended to be very far from the Milky Way. Still, there had been no knockout punch one way or the other. That changed in 1965.
Arno Penzias and Robert Wilson were working together at Bell Laboratories in 1963 in a new field called radio astronomy. They both had an interest in radio sources in the universe and convinced Bell to let them use the radio telescope during down time. In order to understand what the telescope was “seeing,” Penzias and Wilson had to filter out background noise. This noise is exactly what you hear between stations on a radio dial. It is electromagnetic radiation that comes from any number of sources: overhead power lines, a power plant, a competing and nearer radio station, for example. To study distant radio sources, Penzias and Wilson pointed their telescope at an area of the sky where there should be little interference. To their surprise, there was a persistent background noise. The two tried everything to account for this but couldn’t. Most astronomers would have ignored it because, though annoying, it wasn’t very significant.
Recall that in the 1940s Gamow, Alpher and Herman had predicted cosmic microwave background (CMB) radiation as a left-over signal from the Big Bang. Because at that time there was no way to detect CMB, their theory had languished and now, 20 years later, was all but forgotten. Certainly Penzias and Wilson weren’t aware of it at the time. Toward the end of 1964 Penzias attended a conference in Montreal, Canada, where he casually mentioned this phenomenon to Bernard Burke of Massachusetts Institute of Technology. A couple of months later Burke excitedly contacted Penzias to tell him that two theoreticians at Princeton, Robert Dicke and James Peebles, had presented a paper in which they predicted CMB (they, too, were unaware of Gamow’s, Alpher’s and Herman’s work two decades earlier).
Suddenly everything fell into place for Penzias and Wilson. The background noise had nothing to do with extraneous power sources or even “white dielectric material” (pigeon droppings) left on the horn of the telescope as they had once supposed. Instead, they had quite unwittingly and unintentionally proved the Big Bang Theory. Not only did the Big Bang Theory account for CMB but the Steady State Theory had no place for it to exist. Over the next 13 years a number of astronomers verified CMB, measured it and compared it to the predictions of the Big Bang Theory. Everything matched. In 1978 Penzias and Wilson received the Nobel Prize in physics for their discovery. This firmly cemented the Big Bang Theory as the explanation for how the universe came to be. That conclusion, however, only raised another question: what caused the Big Bang? As Carl Sagan said: “Ten or twenty billion years ago, something happened — the big bang, the event that began our universe. Why it happened is the greatest mystery we know. That it happened is reasonably clear.”