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Thread: Are areas of the universe receding from each other at faster than light speeds?

  1. #1 Are areas of the universe receding from each other at faster than light speeds? 
    Forum Radioactive Isotope
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    I think I have understood this to be the case and that the reason is that space ( or spacetime?) is undergoing expansion.

    If so then it must follow that there are areas of the universe that are receding from our area at FTL (is that a used abbreviation?) speeds?

    If this is true then is that a trivial observation or does it say anything about the nature of these areas?

    Can we say they these areas are cut off from us in the same way as the centres of black holes are (still?) thought to be?

    Can we be sure that the laws of physics there are the same as they are here if there is no longer any way for them to be connected to our area by light?


    By the way in the gravity theories that posit gravitons (unless I am misremembering and gravity is only defined as a geometrical property) are gravitons supposed to propagate at the speed of light?


    Last edited by geordief; April 13th, 2014 at 05:54 AM.
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    Moderator Moderator Markus Hanke's Avatar
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    If so then it must follow that there are areas of the universe that are receding from our area at FTL (is that a used abbreviation?) speeds?
    Yes, very distant objects ( ~ billions of light years ) recede from us with an apparent velocity greater than c.

    If this is true then is that a trivial observation or does it say anything about the nature of these areas?
    It doesn't say anything about the local regions where those objects are located, because the apparent recession is due to the metric expansion of the space between us and the object, and not due to local motion of the object itself.

    Can we say they these areas are cut off from us in the same way as the centres of black holes are (still?) thought to be?
    Not really, because we can see them, so their light arrives here where we are. What we can't do though is signal back, because our signals will never arrive there.

    Can we be sure that the laws of physics there are the same as they are here if there is no longer any way for them to be connected to our area by light?
    Of course there can never be complete certainty in science, but the very fact that ordinary light is being detected here ( combined with the fact that we don't observe anything out of the ordinary in those objects ) would indicate that the same laws of physics apply there.


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    Quote Originally Posted by Markus Hanke View Post

    Not really, because we can see them, so their light arrives here where we are. What we can't do though is signal back, because our signals will never arrive there.
    So the light has traveled from that area to ours at a speed that appears to be faster than c but is really just c?

    What is the reason it cannot make the return journey in the same way?

    It is not because the light left the other area at a time when the 2 areas were not mutually receding (or is it apparently receding ?)at faster then light speeds is it?
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    Quagma SpeedFreek's Avatar
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    Quote Originally Posted by geordief View Post
    So the light has traveled from that area to ours at a speed that appears to be faster than c but is really just c?
    Not really.

    The universe is 13.7 billion years old. No light has crossed a distance of more than 13.7 billion light-years in order to reach us, so no light has apparently moved faster than light.

    Those galaxies were closer to us in the past, when their light was emitted, but the subsequent expansion has put them more than 13.7 billion light-years away. If everything started off in the same place, then anything that has achieved a distance of more than 13.7 billion light-years in 13.7 billion years has apparently receded faster than light.

    As an example, the galaxies that we see at the "Hubble distance" (where objects apparently recede at the speed of light), emitted the light we see when they were only 5.7 billion light-years away, a little over 9 billion years ago. Their light took ~9 billion years to reach us, crossing a distance that was originally only 5.7 billion light-years. The reason that light took longer than 5.7 billion years to reach us is the expansion of the universe, putting more distance in front (and behind!) that light as it made its journey towards us. That expansion has "moved" those galaxies away to 13.7 billion light-years away during that lights 9 billion year journey. so those galaxies are now at a distance which means they would have had to recede at the speed of light for the whole history of the universe in order to get there, but their light moved towards us at the speed of light, through a universe that was increasing in size the whole time.

    Quote Originally Posted by geordief View Post
    What is the reason it cannot make the return journey in the same way?
    The acceleration of the expansion, due to dark energy. The expansion of the universe was decelerating for the first 7-8 billion years, but then the decelerating expansion turned into accelerating expansion, which throws up a cosmic event horizon (which is more distant than the Hubble distance").

    In a universe that is expanding at a decelerating rate, or even a constant rate, any light can travel any distance given enough time, apparent recession at the speed of light or above is not a limit on observation, it just means the light takes more time to cross that distance.

    But in a universe that is undergoing accelerating expansion, there is a limit to the distance that light can travel, as there comes a point when the accelerating expansion puts more distance between the light and its target than that light can cross even given infinite time!

    The basic concept of how light can cross distances where objects are apparently receding from each other faster than light is given in the link below. Have a look at it, and see if it helps:

    Ant on a rubber rope - Wikipedia, the free encyclopedia
    Last edited by SpeedFreek; April 13th, 2014 at 06:27 AM.
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by geordief View Post
    So the light has traveled from that area to ours at a speed that appears to be faster than c but is really just c?
    As SpeedFreek said.
    Imagine a cake with raisins in it - put it into the oven, and the dough will start to expand. The distance between the raisins increases, even though they remain perfectly stationary with respect to the dough; it is the dough itself which expands. Same in space - most objects are ( roughly ) stationary, but because space between them expands, their distances from one another increase, and you get an apparent recession effect. The objects themselves, however, never move.
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    thanks.That is something to be going over.
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