Why do we see the cosmic microwave background as microwaves today?
Quick Answer
The early universe was hot and opaque, but as it expanded and cooled, that radiation was stretched into microwaves. It is the afterglow of the Big Bang.
The Full Story
About 380,000 years after the Big Bang, the universe cooled enough for electrons and protons to combine into neutral atoms. Light could then travel freely, and the radiation from that era has been moving through expanding space ever since. Because the universe has expanded enormously, the original visible and infrared light has been stretched into the microwave range. This cosmic microwave background, or CMB, was discovered accidentally in 1965 by Arno Penzias and Robert Wilson. It is one of the strongest pieces of evidence for the Big Bang model.
Key Facts
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