The cosmic microwave background, or CMB, is the oldest light in the universe. It was released about 380,000 years after the Big Bang, when the universe cooled enough for electrons and protons to combine into neutral hydrogen. Before that, light could not travel freely. After, it could. That light has been traveling ever since, stretched by the expansion of the universe into microwave wavelengths.

The CMB is remarkably uniform. Its temperature is 2.725 Kelvin, with tiny fluctuations of a few parts per hundred thousand. Those fluctuations are the seeds of everything we see today: galaxies, clusters, and superclusters. They mark regions that were slightly denser than average, where gravity eventually pulled matter together.

The CMB was discovered by accident in 1964. Arno Penzias and Robert Wilson were trying to reduce noise in a radio antenna and found a persistent signal coming from all directions. They had found the afterglow of the Big Bang. The discovery earned them the Nobel Prize.

Satellites like COBE, WMAP, and Planck have mapped the CMB in exquisite detail. Their data pin down the age, composition, and geometry of the universe. It is the single most important observation in modern cosmology.