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Asked by anon-214572 to Helen, Farah, Dave, Cheryl, Bastian, Alun on 11 Jun 2019.
Question: ive heard the universe is expanding faster then the speed of light is this true
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Dave Underhill answered on 11 Jun 2019:
Yes it is. It is all down to the frame of reference used. In special relativity (E=mc2) nothing can go faster than the speed of light, now that may seem like a contradiction but bear with me 🙂 In special relativity distance is measured within its own local frame of reference (where spacetime is flat and unchanging) however, this does not apply where spacetime curves (essentially when we insert gravity). Here we meet the difference between special and general relativity – general relativity does allow for things to move faster than the speed of light and special relativity is a special case within general relativity. Look at the earth, as we stand on it it looks flat, and it is in our local frame of reference, but if we fly in a plane we can see that it isn’t flat, at this larger frame of reference we can see the curvature. So where 4-dimensional spacetime can be considered flat, essentially when gravity is acting uniformly on that spacetime, special relativity holds and nothing can move faster than the speed of light. However, when we look at universal scales the frame is not flat and unchanging, gravity causes distortions in both space and time and thus it is possible on the larger scale of general relativity for things to move faster than the speed of light – or at least that’s how I understand it 🙂
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Comments
Harrison commented on :
Just to add a bit to Dave’s explanation. The key point is that two objects that are near each other cannot have a speed between them that is greater than the speed of light. This is true in special and general relativity. (By the way, near might still be hundreds of millions of miles apart!) But, in general relativity, spacetime, which is a merging of time and space and is therefore a 4-dimensional thing, can be warped in all kinds of strange ways by every form of energy. It is therefore possible to warp spacetime in such a way that if you were able to compare the motions of two objects that are extremely far away from each other you would find a relative speed between them that exceeds the speed of light. Indeed, there are galaxies that are so far away from us that the distance between them and us is growing at greater than the speed of light. Consequently, the light from these galaxies can’t keep up and we shall never see these galaxies. So, we live in a universe in which there is a horizon beyond which we cannot see. Actually, it is a bit more sobering than that. We now know that the expansion of space is accelerating. Therefore, by the time you finish reading my comment, there will be galaxies that have just slipped beyond our cosmic horizon. Indeed, day by day, if the acceleration continues, there is less and less of the universe that we can see. If the acceleration continues, in the very distant future the Milky Way will be alone in a vast empty void of space.
Dave commented on :
Thanks, Harrison, explained much better than I could 🙂