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How was the cosmic microwave background discovered by Arno Penzias and Robert Wilson?

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In 1965, American astronomers Arno Penzias and Robert Wilson accidentally discovered the cosmic microwave background (CMB) while working at Bell Laboratories.12

The pair were using a 6-metre horn antenna, originally designed for satellite communications, to conduct radio astronomy. They detected a persistent, uniform radio signal that corresponded to an excess temperature of approximately 3 K. After meticulously eliminating all potential sources of circuit noise—and even removing pigeon droppings from the antenna—the signal remained constant regardless of the telescope's orientation.132

Penzias and Wilson consulted with Robert Dicke at Princeton University, who, along with his colleagues, had been theorizing the existence of such background radiation as a remnant of the primordial fireball. This discovery provided critical evidence for the big-bang model, confirming that the universe was once opaque and hot before cooling and expanding to its current state.1243