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superluminal expansion [Q&A]


Atul of Canara College asks, is it true that the Universe is expanding at a speed greater than light speed? And if so, does that not conflict with Relativity?

Yes, but no. And no.

Yes, the Universe is expanding, but there is no single speed that characterizes that expansion. However, two objects separated by a suitably large distance do recede from each other at speeds that exceed light speed.

But this does not contradict the basic result of Relativity, which says that no object or signal may go from one point to another faster than light. The expansion of the Universe is a change in the scale of space-time, and does not represent the physical translation of an object. There is no information that is being transferred at superluminal velocities.

For a simple example of how anybody can produce this kind of superluminal speeds, take a flashlight, point it in front of you, and turn it on. Then sweep it up to point overhead, and turn it off. Now consider the wavefront that emanates from the flashlight, and in particular, the extreme edges of the wavefront. The speed at which these edges diverge from each other is ~1.41 times the speed of light, by simple Pythagorean construction. But no information is being transmitted at this speed between the two edge points, so there is no violation of Relativity. Analogously, the metric of space-time can expand at any speed without coming into conflict with Relativity.