What is the cosmic microwave background?
Quick Answer
The cosmic microwave background is faint radiation left over from the early universe, about 380,000 years after the Big Bang. It is now observed as microwaves filling all of space.
The Full Story
The cosmic microwave background, or CMB, is the oldest light we can detect. It was released when the universe cooled enough for atoms to form and light to travel freely, about 380,000 years after the Big Bang. Penzias and Wilson discovered it accidentally in 1965, and the finding became one of the strongest pieces of evidence for the Big Bang model. Space missions such as COBE, WMAP, and Planck measured tiny temperature variations in the CMB that helped map the early universe. Those fluctuations show how galaxies and large-scale structures later formed.
Key Facts
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"The oldest light in the universe is still around us."
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