The oldest light in the universe
The cosmic microwave background (CMB) is radiation released when the universe became transparent at 380,000 years after the Big Bang. At this recombination era, the temperature had fallen to 3,000 Kelvin, cool enough for electrons to bind with nuclei. Before this moment, photons scattered endlessly off free electrons in an opaque plasma. After, neutral atoms formed, space cleared, and those ancient photons flew freely across the expanding universe.
Today, the CMB has cooled to 2.7 Kelvin due to the expansion of space. This radiation fills every direction of the sky at nearly equal brightness, making it the most isotropic measurement in astronomy. Discovery of the CMB in 1964 was accidental; Bell Labs engineers were trying to eliminate noise from their antenna when they found a faint, uniform glow that would not go away.
Temperature fluctuations and structure
Though seemingly smooth, the CMB contains tiny temperature variations of one part in 100,000. Hotter regions correspond to slightly denser patches of primordial gas; these seeds grew via gravity into the galaxies, stars, and galaxy clusters we see today. Maps of the CMB therefore show the universe in its infancy, before structure became pronounced. The pattern of these fluctuations reveals the universe's geometry, composition, and age with unprecedented precision.
A fossil record of the early universe
The CMB encodes answers to fundamental questions. Its scale (wavelength spectrum) reveals whether space is flat, curved, or warped. Its polarization exposes the imprint of gravitational waves from the first fraction of a second. Its temperature gradient reveals motion of the Solar System through the universe. The CMB is the most scrutinized electromagnetic signal in history, and improved measurements have consistently revealed surprises about dark matter, dark energy, and cosmic geometry.