A Faint Noise Reveals the Afterglow of the Early Universe

In 1964, physicists Arno Allan Penzias and Robert Woodrow Wilson were calibrating a large horn antenna at a Bell Telephone Laboratories facility situated on Crawford Hill in Holmdel, New Jersey. The six-meter instrument had originally been constructed to capture faint radio signals reflected from Echo balloon satellites. To prepare the receiver for delicate astronomical observations, the two researchers methodically worked to eliminate all identifiable sources of external and internal interference. They accounted for commercial radio broadcasts and radar transmissions, while chilling the receiving equipment with liquid helium to minus 269 degrees Celsius, just four kelvins above absolute zero, to minimize thermal disruption from the apparatus itself.

Despite these precautions, their measurements revealed an unexpected, isotropic hiss that remained constant regardless of the direction in which the antenna pointed. Observed at a wavelength of 7.35 centimeters, the antenna's noise temperature came to about 7.5 kelvins, well above the 3.3 kelvins expected from the atmosphere and the antenna itself. It persisted unvaried through day and night across the entire sky. Suspecting practical flaws within the structure, Penzias and Wilson thoroughly inspected the instrument, evicted a pair of nesting pigeons, and cleaned away the droppings that had collected inside the horn. Even after these physical corrections, the uniform noise continued unabated, indicating that it originated beyond both the Solar System and the Milky Way.