I admit that I am not a physicist, but this really surprises me. Is this actually true?
Yes - nothing can travel "through" space faster than the speed of light. However if the space itself is expanding then two objects which are at rest (relative to their local environment) can move away from each other at speeds greatly in excess of the speed of light. To conceptualise this - imagine a balloon with dots drawn all over it. Blow up the balloon. Now none of the dots have moved relative to the balloon itself, yet they are now further apart from each other. Read Lawrence Krauss's book "The Physics of Star Trek" - he explains this beautifully (exploitation of this was the justification for how the warp drive worked) - MMGB
In the [observable universe]? (that sphere around us of radius n light years, where n is the age of the universe in years) I do not think it is possible for two objects to recedes faster than the speed of light. Due to the initial inflation the universe is much larger than the observable universe and perhaps the example here is the relative motions of galaxies that are farther apart than n light years and so are not mutually observable -- however such objects can never be seen from each other and in generaal it is impossible for either to have any influence on the other ever (I think). --Eob
Eob - you're absolutely correct. Take Galaxy A (at the edge of our sphere of light perception, and receding at just below c, relative to us) and Galaxy B (likewise at the edge of our sphere of light perception, but diametrically opposed to Galaxy A, with the Earth at the centre of the imaginary line connecting all three). The light from Galaxy A and B is travelling at c and hence we can just perceive each. But relative to each other, they are receding at the '''linear addition of their recession velocities". They would not ever be able to perceive each other. Hence the "observable universe" of Galaxy A would have the earth at the extreme fringe of it (or "where the Earth was going to be when it formed" relative to their timeline) and nothing beyond it. Galaxy B would not exist, nor would it have ever existed, in their reference frame. Likewise, there is probably a Galaxy C beyond Galaxy B that we cannot perceive, and never will.
The definition of our "observable universe" is "all the stuff that is moving away from us at less than the speed of light, hence we can still see it. Some versions of the inflationary hypothesis speculate that we can only see 1 millionth to a billionth of the real cosmos. Not that it matters, we cannot (by definition) ever verify it one way or the other (barring the discovery of wormholes, and let's NOT go there). - MMGB
No, objects in the observable universe can be receding faster than the speed of light. It works because what we see lies in the distant past - just because the objects are too far away for light to reach us now, doesn't mean they were when the light was emitted.
I am not saying that it is wrong that galaxies can move apart faster than the speed of light--but it is completely alien to my (admittedly limited) understanding of relativistic physics. Egern
I'm not sure of the which is right, but perhaps this issue is being conflated with the apparent superluminal velocity of distant galaxies? [1] -- DrBob
Nah, it's quite definite that some galaxies are receding at true superluminal velocities. This works because limits on relative velocities - you're quite right that local rest doesn't come into it - only apply to objects starting at the same point in space and time. The possible paths of an object there make up a cone, called the point's future, and contain no relatively superluminal paths. In flat static spacetime all cones are oriented the same way, so the same applies, but with the expansion of the universe the cones are relatively tilted. Remember relativity is a local theory. -- Josh Grosse
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For the record, the stars older than the universe thing was more or less resolved a while ago. The linked page doesn't appear that well informed, I'm sure we can do better.