cosmic microwave background
The cosmic microwave background is the faint afterglow of the Big Bang — a soft, even hum of light that fills the entire sky, coming at you equally from every direction. It is the oldest light there is, released when the universe was just 380,000 years old and cooled enough for the first atoms to form. Think of it as a baby photo of the cosmos, taken when everything was still a glowing fog.
Back then this light was blazing hot, but as the universe stretched out over nearly 14 billion years it has been stretched and cooled along with it, until today it lands on us as gentle microwaves — about 2.7 degrees above absolute zero, far colder than your freezer. Astonishingly, you have already met it: roughly 1% of the static fuzz on an old untuned TV was this ancient glow, sieved out of the heavens.
It was found completely by accident. In 1965 two radio astronomers, Penzias and Wilson, kept hearing a stubborn hiss in their antenna they couldn't get rid of — they even scrubbed out pigeon droppings, suspecting the birds. The hiss turned out to be the universe itself. A common mix-up: the background isn't perfectly smooth. Its faint ripples, mapped by later satellites, are the seeds from which every galaxy and star eventually grew.
Switch an old analog TV to a dead channel: roughly 1% of that snowy static is 13.8-billion-year-old light from the Big Bang itself.
The temperature today is about 2.7 K (just 2.7 degrees above absolute zero), the same in every direction to within one part in 100,000.
Often shortened to CMB. Penzias and Wilson won the 1978 Nobel Prize for their accidental discovery, which confirmed the Big Bang over its rival, the Steady State theory. At nearby Princeton, a team led by Robert Dicke was independently building equipment to search for exactly this relic radiation; Penzias and Wilson detected it first, and the two groups published back-to-back papers in 1965.