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Thread: If the universe was growing faster than the speed of light, could there be matter at the edge of the universe?

  1. #1 If the universe was growing faster than the speed of light, could there be matter at the edge of the universe? 
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    Since scientists say:
    1) Matter and/or energy, cant travel faster than 299 792 458 m / s relative to anything else.
    2) The universe expands faster than the speed of light.

    So is it technically right to say that there is no energy at the edges of the universe? (since energy can't take enough time to travel there as the max speed it can go is 299 792 458 m / s)


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  3. #2  
    Brassica oleracea Strange's Avatar
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    Firstly, expansion is not a speed. The relative speed of things depends on their distance away. If it isn't obvious why I can draw a picture.

    Secondly, as a result objects far enough away are moving away from us with a velocity greater than the speed of light.

    The "can't move faster than light" restriction comes from special relativity and is therefore a purely local restriction. It doesn't apply to the expansion of space its self.


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  4. #3  
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    Quote Originally Posted by Strange View Post
    Firstly, expansion is not a speed.
    then what is it? http://en.wikipedia.org/wiki/Faster-...rsal_expansion
    The relative speed of things depends on their distance away.
    not direction? distance? i dont get how distance affect speed of things
    If it isn't obvious why I can draw a picture.
    yes, please, it would help me a lot

    Secondly, as a result objects far enough away are moving away from us with a velocity greater than the speed of light.
    really? Travelling faster than light.? - Yahoo! Answersthis question i posed on yahoo answers 3 months ago, the answer i got was: "If you see A move to the left with velocity Va and B move to the right with velocity Vb, then B does not see A move away with velocity Va + Vb (the classical case) but with velocity

    (Va + Vb)/ ( 1 + Va Vb /c^2 )

    If you take Va = 0.7 c and Vb = 0.8 c, then the speed that B sees A receding with is

    (0.7c + 0.8c)/(1+0.7*0.8) = 0.96 c

    This relativistic addition of velocities will always result in a speed less than c.
    When you have two photons flying in opposite directions, one will see the velocity of the other not as 2c, but as

    (c+c)/(1+c*c/c^2) = c"
    It is really different than what you are saying here, that objects can travel > c, (it matters if it is far away enough)
    but what he says is that no object can travel > c, (no matter what)

    The "can't move faster than light" restriction comes from special relativity and is therefore a purely local restriction. It doesn't apply to the expansion of space its self.
    Why not? please explain whats "local restriction" sorry if my knowledge in this "local/non-local" part is rather weaker than other parts of my knowledge
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  5. #4  
    Brassica oleracea Strange's Avatar
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    I don't have much time right now, so just a couple of quick points:

    Maybe it helps to think of it in terms of scaling; as the universe expands (linearly) distances get scaled by the same amount. So, if we say galaxy B is at distance 1 and C is at distance 2; then after a time when all distances have increased by 10%, galaxy B will be at 1.1 and C will be at 2.2 - i.e. C will have "moved" twice as far (0.2) as B (0.1). This is just a direct consequence of linear scaling/expansion.

    I have drawn the ASCII art explanation so many times, I should really remember to save it ... Here is a very quick version:

    A series of points in space, an equal distance apart:
    T: A . B . C . D . . .

    After some time, the space between each of them (that's important) has increased by the same amount:
    T+1: A . . B . . C . . D . . .
    T+2: A . . . B . . . C . . . D . . .

    So, at time T+1 the distance between A and B has increased by 1 dot. The relative velocity is therefore 1 dot per unit time (I am deliberately not attempting to provide realistic values for distance or time).

    But the distance between A and C has increased by 2 dots. So the relative velocity is therefore 2 dots per unit time.

    And the same is true for any pair of points you choose. There will be some point (Z, for example) which will at some time, be moving faster than the speed of light - from the point of view of A. However, from the point of view of Y, Z will only be moving away at 1 dot per unit time.

    So, the velocity is not a "real" velocity. It is due to a change in scale or metric; i.e. what we measure rather than distant galaxies flying through space.

    Your addition of velocities bit is absolutely correct: for situations where special relativity (SR) can be applied. But is only a subset of general relativity (GR). SR doesn't work on the scale of the whole universe. For that we need GR (which not only describes the expansion but predicted it before it was observed). There is no simple "can't go faster than light rule" in GR. (in fact there is nothing simple in GR!)

    Hope that helps a bit...
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    So you are saying, galaxies can travel faster than the speed of light, relative to earth, is it? im not quite sure what you are saying by
    linear scaling
    and this forumla: (c+c)/(1+c*c/c^2) = c" cant be used in that scale?
    then what scale can this forumla:
    (c+c)/(1+c*c/c^2) = c" be used?
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    i don't think you can use that formular in that case, because it's about the distance of 2 objects increasing between them without a change in velocity of the objects.

    you can use this formular to find the velocity of a 2nd object moving relative to another in a distance of you; factoring in time delation. i.e.

    a train traveling at 0.7c and someone running inside the train at 0.4c, in the direction the train is moving, wouldn't add to 1.1c because it clocks slower. so using that formula will show you the speed of the running person to be 0.86c.

    and a light beam traveling inside the train, in the direction the train is moving, to be 1c.
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    So you are saying, galaxies can travel faster than the speed of light, relative to earth
    They're not moving through space. The space between us and those galaxies is expanding. The more space there is, the more (faster) the expansion. The space between us and those galaxies is expanding at such a rate that light will never be able to cross the ever expanding distance. That's what receeding faster than light means, in this case.
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    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    but if you would have send a beam of light to a mirror on a galaxy, that expands away from us, before and after the expansion; you could work out by how much that galaxy would have had to accelerate to = that distance.
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    There is no 'before and after the expansion'. It's happening now. And has been expanding faster for the last 7 billion years.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    that's not how i meant it. i meant, if you could reflect a beam of light to an expanding galaxy now, and again, at a later time, you could find out by how much that galaxy would have had to accelerate to travel that distance.
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  12. #11  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by curious mind View Post
    that's not how i meant it. i meant, if you could reflect a beam of light to an expanding galaxy now, and again, at a later time, you could find out by how much that galaxy would have had to accelerate to travel that distance.
    But, apart from the practical difficulties (impossibilities), it would take billions of years. We can directly measure their (apparent) velocity using red-shift.
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    Quote Originally Posted by AlexG View Post
    So you are saying, galaxies can travel faster than the speed of light, relative to earth
    They're not moving through space. The space between us and those galaxies is expanding. The more space there is, the more (faster) the expansion. The space between us and those galaxies is expanding at such a rate that light will never be able to cross the ever expanding distance. That's what receeding faster than light means, in this case.
    yeah, thanks this is a nice explanation, probably the best here, i understand much more now, although i still dont know much :X
    well, so are space only increasing between galaxies? what about space at the edge of the universe? isn't there an edge of the universe?
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    Quote Originally Posted by ryanawe123 View Post
    well, so are space only increasing between galaxies? what about space at the edge of the universe? isn't there an edge of the universe?
    Not even between galaxies, but only between clusters of galaxies. You can think of things at smaller scales being held together by their gravity. Expansion only occurs on very large scales where gravity is weak enough.

    And, in our current models, there is no edge. Either the universe is infinite or is is "finite but unbounded" - think of the surface of the earth, as an analogy: it is of finite area but it has no edge.
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    oh my my my, this is gunna make me rethink about the universe, i always thought there was an end.

    so, if we keep travelling in a direction (i know we wont reach the end, i know we can never, etc.) but we would get back to the same spot?

    i said it because if we keep travelling in a direction on earth, we would come back to the same spot
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  16. #15  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by ryanawe123 View Post
    so, if we keep travelling in a direction (i know we wont reach the end, i know we can never, etc.) but we would get back to the same spot?
    Probably. Unless it is infinite.
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  17. #16  
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by ryanawe123 View Post
    so, if we keep travelling in a direction (i know we wont reach the end, i know we can never, etc.) but we would get back to the same spot?
    Probably. Unless it is infinite.
    so we dont know?
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  18. #17  
    Brassica oleracea Strange's Avatar
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    No we don't know. We know it is either very, very big or infinite.

    Actually, if the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies could be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. However, I think this has been ruled out by various observations (but this is way beyond my area of expertise).
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    like....... not observing duplicate patterns of stars?
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