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How can the Universe expand faster than the speed of light during inflation?

Some sources indicate that the big bang caused an expansion which traveled faster than the speed of light. How can this be?

You ask a good question, one whose answer lies in the subtle difference between expansion that is faster than the speed of light and the propagation of information that is faster than the speed of light. The latter is forbidden by fundamental physical laws, but the former is allowed; that is, as long as you are not transmitting any information (like a light pulse), you can make something happen at a speed that is faster than that of light. The expansion of the Universe is a "growth" of the spacetime itself; this spacetime may move faster than the speed of light relative to some other location, as long as the two locations can't communicate with each other (or, in terms of light rays, these two parts of the Universe can't see each other). According to the theory of inflation, the Universe grew by a factor of 10 to the sixtieth power in less than 10 to the negative thirty seconds, so the "edges" of the Universe were expanding away from each other faster than the speed of light; however, as long as those edges can't see each other (which is what we always assume), there is no physical law that forbids it.

November 2002, Kristine Spekkens (more by Kristine Spekkens) (Like this Answer)

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