How did Arno Penzias and Robert Wilson detect the cosmic microwave background?

In 1965, American astronomers Arno Penzias and Robert W. Wilson were working at Bell Laboratories using a 6-metre horn antenna originally designed to detect signals from high-altitude balloons for satellite communications.12

While adapting the antenna for radio astronomy, they detected a persistent, uniform signal that corresponded to an excess temperature of approximately 3 K. To ensure the signal was not an error, they meticulously eliminated all potential sources of circuit noise, including removing pigeons that had been roosting in the horn. Despite these efforts, the signal remained constant regardless of the direction in which the telescope was pointed.31

They eventually consulted with physicist Robert Dicke at Princeton University, who recognized the signal as the cosmic microwave background—the residual radiation from the big-bang model of the universe's origin. This discovery provided critical evidence for the big bang theory and earned Penzias and Wilson the 1978 Nobel Prize for Physics.312