How was CMB radiation first discovered?

Answer 1

Completely by accident.

Arno Penzias and Robert Wilson were scientists at Bell Laboratories who, in 1964, were seeking to find the cause of background noise in microwave signals bounced off telecommunication satellites. They found that the noise came from outer space (not the surface of the Earth), was a black-body spectrum, did not vary between day and night, and was completely isotropic. Puzzled, they strove vigorously to eliminate any possible cause (including pigeon feces in their antenna) other than microwaves from outer space.

Eighteen years earlier, George Gamow had predicted that the Big Bang would result in just that kind of radiation. However, no astronomer wanted to bother to look for it until a group at Princeton University (almost NEXT DOOR to Bell Labs!!) began, in 1964, to work out how to do so. When Penzias and Wilson saw that what the Princeton scientists were looking for, exactly matched what they had just found, they realized they had made a huge discovery. Fourteen years later they got their Nobel Prize.

Answer 2

The Cosmic Microwave Background, or CMB, is a nearly-isotropic 2.725 K signal which formed during the epoch of recombination (when the universe went from being mostly ionized to mostly neutral) approximately 300,000 years after the big bang.

It was first postulated in 1948 by George Gamow, Ralph Alpher and Robert Herman, who predicted its temperature to be approximately 5k. The same prediction was made independently by Robert Dicke in the 1960s. The first publication discussing CMB radiation was in 1964 by two Soviet physicists. That same year, the Princeton Physics Department began construction of a microwave receiver to attempt to detect the predicted signal. However, before the receiver was completed, a breakthrough was made at Bell Laboratories.

In 1965, two radio technicians at Bell Laboratories, Arno Penzias and Robert Wilson were working with a microwave receiver intended for use in satellite communications. They found the receiver was reading 3.5K too high no matter which direction it was pointed. Believing the 3.5K noise to be some sort of interference, Penzias and Wilson went to great lengths to identify and remove the source of the signal, even going so far as to chase away pigeons that had been nesting in the receiver and cleaning away the refuse they had left. But still the signal persisted. Finally, Professor Burke of MIT mentioned to the two technicians a paper he had seen written by Jim Peebels (of Princeton) describing a background microwave signal. Realizing they may have detected the signal described in the Peebels paper, Penzias and Wilson phoned Princeton and described their observations. Together with the group at Princeton, Penzias and Wilson were able to confirm that the signal they observed was indeed the signal predicted by Robert Dicke, thus confirming the discovery of the CMB.

Two papers were subsequently published side-by-side in Astrophysics Journal Letters: A theoretical paper by Dicke and a paper on the experimental results by Penzias and Wilson.