Cosmic microwave background
The faint glow of microwaves filling the whole sky, left over from the hot early universe, at under 3 degrees above absolute zero.
Everything glows according to its temperature. This is blackbody radiation: a warm object gives off a characteristic spread of wavelengths, peaking at shorter wavelengths the hotter it is. A stove ring glows red, the Sun's surface white-yellow, and your own body gives off invisible infrared.
The cosmic microwave background is the coldest and largest example: a nearly uniform glow of microwaves arriving from every direction in the sky. Its spectrum matches a blackbody at 2.725 kelvin, less than three degrees above absolute zero. It is light released about 380,000 years after the Big Bang, when the universe had cooled enough for atoms to form and light could at last travel freely. The expansion of the universe has since stretched that once-hot light into microwaves.
It was found by accident. In 1964 Arno Penzias and Robert Wilson, working with a sensitive horn antenna at Bell Labs in New Jersey, could not get rid of a faint, steady noise that came from all directions, whatever they tried. It turned out to be the afterglow of the early universe, and it earned them the 1978 Nobel Prize in Physics.
Its almost perfect smoothness, and the faint ripples within it, are among the strongest evidence for the Big Bang.
Coming up in future stories
- They Blamed the Pigeons for the Big Bang 1964–65 · Holmdel, New Jersey