The hiss that was always there.

In 1964 Arno Penzias and Robert Wilson had the use of an ultra-quiet radio ear: a horn-shaped antenna about six metres across, on a hill in Holmdel, New Jersey. Bell Labs had built it to catch the faint signals of the first communication satellites. Penzias and Wilson wanted to turn it on the sky.

First they had to account for every whisper of noise the antenna made by itself. One whisper would not go away: a low, steady hiss, the same wherever they pointed, from one season to the next.

They checked their receiver against a reference cooled in liquid helium. They found a pair of pigeons nesting in the horn, removed them, and cleaned out the horn. The hiss stayed. By the spring of 1965 they had ruled out every cause they could think of, in the antenna and in the air, and the hiss was still there, wherever they pointed.

Not far away, at Princeton University, Robert Dicke and his colleagues were building an instrument to look for exactly such a hiss: the light left over from the universe’s hot beginning, cooled by its long expansion to a faint glow of microwaves. A phone call brought the two groups together, and in 1965 they reported side by side, the measurement and what it meant. Penzias and Wilson shared the Nobel Prize in Physics in 1978.

That light, the cosmic microwave background, fills the universe. Measured far more precisely since, it is about 2.7 degrees above absolute zero, and every sugar cube’s worth of space, the room you are in included, holds hundreds of particles of that light. It was there before anyone built a radio, and before there was anyone to build one. It reached the horn in Holmdel the way it reaches you now.

Hints of it had turned up before, in earlier measurements, and nobody had recognised them for what they were. The universe was not hiding it. It was simply what was there, waiting to be seen.