Why did the early universe become transparent only after hundreds of thousands of years?
Quick Answer
Before neutral atoms formed, free electrons scattered light constantly, making the universe opaque. Once atoms formed, light could travel much more freely.
The Full Story
In the first part of cosmic history, the universe was a hot plasma of charged particles. Photons bounced off free electrons so often that light could not move far without being scattered. As expansion cooled the universe to roughly 3,000 kelvin, electrons and protons combined into neutral hydrogen and helium. This event, called recombination, made the universe transparent for the first time. The released radiation is what we now observe as the cosmic microwave background.
Key Facts
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"Why was the newborn universe foggy before it turned clear?"
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