Horizon Problem

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The universe appears to look the same in every direction, although the two opposite sides of the universe are far enough apart that light starting at one side traveling toward the other side could not have made it across in the time after the Big Bang. Normally when things look the same, it is because there is a causal connection between them, but that can't be the case here. This is what physicists call the "horizon problem." (Unit: 4)