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Physics
Special and General Relativity
Understanding the Cosmic Microwave Background
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[QUOTE="Ibix, post: 5448479, member: 365269"] Following the Big Bang, the universe was filled with hot plasma, all of it glowing and all of it absorbing light. At some point it cooled enough that it started to form atoms and light had a chance to propagate. It's the glow emitted then that we are seeing. It came from every point in the universe, so one year after atoms started to form you would have been seeing the glow from one light year away; ten years after you would have been seeing the glow from atoms ten light years away. Now we are seeing light emitted a very long way away, 13.9bn years ago. It doesn't look really hot because the universe is expanding and that has red-shifted the glow to around 3K. Think of Earth as a rock sticking out of a pond. Throw a stone into the pond and it makes ripples which spread out, wash over the rock and carry on. This is analogous to a supernova - a short (on cosmological timescales) flash at a single point in space. If you aren't looking when the flash washes past Earth, you've missed it. Now build a grid of squares an inch by an inch with a stone at each junction. Hold it over the pond and add a mechanism that makes it drop all of its stones at once. There will be ripples everywhere when they all drop. Stones near the "Earth" will make ripples that will wash quickly past and be gone. But there are ripples from the next stone right behind, and more ripples behind those ones. There will always be ripples because ripples started from everywhere. There will always be CMB because it came from everywhere. Does that help? [/QUOTE]
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Understanding the Cosmic Microwave Background
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