It’s the thermal afterglow of the universe’s hot, dense infancy, stretched to microwave wavelengths by cosmic expansion. Its tiny temperature fluctuations reveal density differences that seeded galaxies. Discovered accidentally by Penzias and Wilson in 1965, it confirmed Big Bang predictions. Satellites like COBE, WMAP, and Planck mapped it in exquisite detail. These maps provide a blueprint of the early universe. It’s literally the universe’s baby picture.
It matters because the CMB confirms the Big Bang and informs cosmological parameters.
It also allows precise tests of theories like inflation and dark matter distribution.
The cosmic microwave background is a 13.8-billion-year-old snapshot of the early universe.
National Aeronautics and Space Administration [nasa.gov]