Why is the cosmic microwave background nearly the same in every direction?
Quick Answer
The cosmic microwave background is leftover radiation from the early Universe, emitted when it first became transparent about 380,000 years after the Big Bang. Because the early Universe was hot and dense, that light is remarkably uniform across the sky.
The Full Story
The cosmic microwave background, or CMB, is the oldest light we can observe. It was released when electrons and protons combined to form neutral atoms, allowing photons to travel freely for the first time about 380,000 years after the Big Bang. The Universe at that time was extremely smooth, with tiny temperature differences of only about one part in 100,000. Those tiny variations later grew into galaxies, clusters, and the large-scale structure we see today.
Key Facts
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