The Afterglow of the Early Universe
A faint microwave glow fills the sky in every direction, carrying a snapshot of the universe when it was still extremely young.
The cosmic microwave background, or CMB, is relic radiation from the early universe. It was released when the expanding universe cooled enough for electrons and atomic nuclei to combine into neutral atoms, allowing light to travel much more freely through space.
A universe that became transparent
Before the CMB was released, the universe was filled with a hot plasma that scattered light repeatedly. Once neutral atoms formed, photons could travel long distances without being constantly deflected. Those photons continue to reach us today, although cosmic expansion has stretched their wavelengths into the microwave portion of the spectrum.
Tiny variations, enormous consequences
The CMB is remarkably uniform, but it contains tiny temperature differences across the sky. Those variations trace fluctuations in the density of the early universe. Over cosmic time, gravity amplified those small differences into the structures we see today, including galaxies and clusters.
Why the CMB matters
Measurements of the CMB help constrain fundamental cosmological parameters such as the universe's geometry, composition and expansion history. It provides an early-universe reference point against which later observations can be compared.
Looking at ancient light
The CMB is not a photograph of the universe at the instant of its beginning. It is a picture from a later stage, when the universe was hundreds of thousands of years old. Yet compared with the billions of years that followed, it is an exceptionally early observational record.
That is what makes the CMB so useful: the universe leaves behind a faint physical memory, and modern instruments can read that memory across the entire sky.