The Cosmic Microwave Background

The cosmic microwave background, or CMB, is a faint glow of microwave radiation that fills the entire sky in every direction. It is the cooled-down afterglow of the hot, dense early universe, light released about 380,000 years after the Big Bang, when the cosmos had expanded and cooled enough to become transparent for the first time. In effect, it is a photograph of the universe in its infancy.

Discovered by accident in 1965, the CMB is one of the most important findings in the history of cosmology. Its existence confirmed that the universe really did begin in a hot, dense state, settling a major scientific debate, and its temperature, almost perfectly uniform across the whole sky, fits the Big Bang model with extraordinary precision.

Why it mattered to Hawking

The tiny variations in the CMB, regions very slightly hotter or cooler than average, are the seeds from which galaxies later grew. Hawking did important early work on exactly this: how minute quantum fluctuations in the very early universe could have been stretched by its rapid expansion into the ripples we now see imprinted on the CMB, and from there into the large-scale structure of the cosmos. The map of the early universe, in other words, carries traces of the quantum physics he spent his life studying.