Like many of science's greatest discoveries, the one that earned Arno Penzias his Nobel Prize was an event of pure serendipity. While tuning a small, yet very powerful and highly sensitive horn antenna for conducting radio astronomy experiments, Arno Penzias and Robert Wilson noted a constant low level noise disrupting their reception. Despite their efforts, Penzias and Wilson could not find any evidence of malfunction in their equipment. Further, the static persisted regardless of the direction the antenna was pointing. As they continued their investigation, Penzias and Wilson came to realize that they had stumbled onto the most conclusive evidence to date supporting the Big Bang Theory.
Penzias and Wilson's discovery was the watershed providing the critical evidence for a theory first developed by George Henri Lemaitre and Edwin Hubble in the 20s and the 30s. Basing his study upon well-established mathematical principles, Lemaitre proved that Einstein's theory of general relativity was incorrect in asserting that the universe was static and that a better model could be constructed based upon the theory of an expanding universe. Observational data was provided by Hubble, who in 1929 announced that galaxies could be measured moving away from our own. Lemaitre thus speculated that the universe must have been created by the explosion of some original atom, in a "big bang."
Working in 1965, Penzias and Wilson were not looking for evidence of the Big Bang. However, by this time astronomers had begun to speculate about the conditions at the beginning of the universe. An explosion of such size and temperature to bring the universe into being must have left some mark. As Penzias and Wilson continued to pursue their disruptive "static," they came to realize that they had discovered the remnants of this first cataclysm.