The afterglow of the Big Bang was discovered by accident in 1965

In 1965, Arno Penzias and Robert Wilson at Bell Labs in New Jersey were trying to track down a faint, persistent hiss in their microwave antenna. They checked the electronics, hunted for interference, and even evicted a pair of pigeons whose nest they suspected. The noise stubbornly remained, coming from every direction in the sky and never varying with the time of day or season.

Just 30 miles away, a Princeton team led by Robert Dicke and Jim Peebles had calculated that a hot, dense early universe should have left behind a faint bath of radiation, and they were building a detector to find exactly that signal. When Penzias phoned to describe his inexplicable noise, Dicke reportedly hung up and told his colleagues, “Boys, we’ve been scooped.”

What the two engineers had stumbled onto was the cosmic microwave background — relic light from about 380,000 years after the Big Bang, when the cooling universe first let light travel freely. It is a near-perfect blackbody at 2.725 K, and its faint temperature ripples, just 1 part in 100,000, mark the density seeds that grew into every galaxy.

The accidental discovery confirmed the Big Bang picture in one stroke and earned Penzias and Wilson the 1978 Nobel Prize in Physics.

It also settled a fierce debate. The rival Steady State theory had no reason to expect such a glow, but a hot origin did: light emitted at roughly 3,000 K and stretched by billions of years of cosmic expansion would arrive today, redshifted, exactly as microwaves.