The way it works is this: when the universe reaches a state of steady exponential expansion, you can define a radius (from wherever you are) beyond which the rest of the cosmos is forever hidden. It’s a true horizon in the sense that nothing beyond it could ever reach you. It turns out that this horizon, like a black hole’s horizon, also has an entropy associated with it, and thus a temperature. The difference is that instead of the heat going out like it does with a black hole, it goes in. The temperature is very small—something like 10-40 degrees above absolute zero—but when everything else
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