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Horizon problem
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===Causal information propagation=== In accepted relativistic physical theories, no information can travel faster than the [[speed of light]]. In this context, "information" means "any sort of physical interaction". For instance, heat will naturally flow from a hotter area to a cooler one, and in physics terms, this is one example of information exchange. Given the example above, the two galaxies in question cannot have shared any sort of information; they are not in [[Causality (physics)|causal contact]]. In the absence of common initial conditions, one would expect, then, that their physical properties would be different, and more generally, that the universe as a whole would have varying properties in causally disconnected regions.
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