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Far away galaxies moving faster than light?

  1. Apr 28, 2012 #1
    Good evening. I just watched a documentary called The Universe-Light Speed. In it, astrophysicist Alexei Filippenko said that galaxies which are very far are moving away from us faster than the speed of light. Also, cosmologist Alan Guth said that during inflation the universe expanded faster than the speed of light. What did they mean exactly? I know that neither matter or energy can break the limit imposed by the speed of light, so I am a little confused.

    Thank you in advance.
     
  2. jcsd
  3. Apr 28, 2012 #2
    Carlos, nothing can move through spacetime faster than c. But, the expansion of the universe is the growth of space itself, which is not limited. So, essentially, relativity places a limit on local frames of reference, but there is no limit on global frames on an expanding universe. (e.g. receding galaxies in an expanding universe)

    Also, remember that it isn't the galaxies themselves that are receding. They appear to be receding because new space is being created in between galaxies.
     
  4. Apr 28, 2012 #3
    Oh ok, then it is not accurate to say that the galaxies are moving away from us faster than light, but rather it's spacetime in between us and those galaxies that is moving (expanding) faster than light. But, if we measure the time dilation of such a galaxy from the reference frame of Earth, how will we perceive it? How do we perceive the characteristics of an object which appears to move faster than the speed of light?
     
  5. Apr 29, 2012 #4

    Chalnoth

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    Well, if we try to measure the speed directly, such as through the redshift, we don't get a speed faster than light. The "speed" that is talked about is the inferred speed due to the change in distance over time. This speed is faster than light.

    But the time dilation can be read directly off of the redshift of the object.
     
  6. Apr 29, 2012 #5
    I think that you should be much more careful with that kind of interpretation. Galaxies are separating due to the inertia, that is due to the fact that they were doing so in the past. As a result space between them is expanding.
     
  7. Apr 29, 2012 #6

    Chalnoth

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    Both ways of looking at this situation are equivalent.
     
  8. Apr 29, 2012 #7
    Hi.

    Not only between the galaxies, at anywhere, for an example between my head and foot, do space expand?

    Regards.
     
  9. Apr 29, 2012 #8

    Chalnoth

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    No. The expansion of space is an average effect on large distances. The local matter density is more than enough to overcome the expansion.
     
  10. Apr 29, 2012 #9
    Well, in most aspects I agree, but then again you can't say that galaxies themselves are not receding as a physical interpretation.
     
  11. Apr 29, 2012 #10

    Chalnoth

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    Sure you can. Why couldn't you? Perhaps the simplest way of looking at the expansion is that galaxies are mostly stationary with respect to a space-time that is expanding.
     
  12. Apr 29, 2012 #11
    Hi.

    If the expansion of space is an average effect on large distances, would not it contradict expansion of wavelength of each photon from the stars which is very short of 10^-6 m or so. It suggests that expansion occurs homogeneously in such micro scale.

    Regards.
     
  13. Apr 29, 2012 #12

    Chalnoth

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    No, that's an incorrect understanding of the situation. Perhaps a better way to understand it is that different parts of the universe expand at different rates, so that when a photon of any wavelength passes through an expanding region, it expands. It is the parts of the universe with very little matter, such as the vast space between galaxies, that expand.
     
  14. Apr 29, 2012 #13
    I have to go, will respond later. Hope we can continue this discussion.
     
  15. Apr 29, 2012 #14
    Hi. OK. As for the vast space between galaxies that expand,

    the expansion is very homogeneous in micro scale as homogeneous expansion of visible light wavelength show it. Is it OK?

    Regards.
     
  16. Apr 29, 2012 #15

    Chalnoth

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    No, the expansion is only homogeneous at very large scales. At smaller scales, different parts of the universe expand at different rates (with no expansion at all in gravitationally-bound areas, such as within galaxy clusters or galaxies). But the expansion impacts all objects within the expanding region identically. The wavelength of a photon passing through an expanding region just does not matter.
     
  17. Apr 29, 2012 #16
    Hi.

    I still feel wonder that during traveling in expanding space the top and the tail of the wave train of one photon, say 1 meter length or so is expanded as pulled rubber. Expansion is very homogeneous so wave length is exactly proportional everywhere in this 1 meter. All the colleague photons are expanded perfectly in the same manner, not like that some are red light but others remain violet.

    Regards.
     
  18. Apr 29, 2012 #17

    Chalnoth

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    No, expansion isn't about stretching anything. Photons aren't "pulled" by the expansion.

    A better way to think about it is that because of the curved space-time they traverse, when they arrive at their destination it is as if they arrive with a relative speed difference compared to where they arrived from.
     
  19. Apr 30, 2012 #18
    NO spacetime is neither created nor destroyed. Whoever said that spacetime is being created between two galaxes to accomodate for the expansion is wrong. It is called expansion for a reason -- we are literally expanding, or stretching if you will, the fabric of spacetime as the universe grows.

    The spacetime can be stretched out to all infinities as well as compacted to the singularity of planck length in all directions.
     
  20. Apr 30, 2012 #19

    Chalnoth

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    I don't understand your objection here. There is no conservation of space-time.
     
  21. May 6, 2012 #20
    Before we begin viewing such galaxies as somehow unusually exotc due to their near-faster light redshift, please keep in mind that any denizen of those far-flung galaxies looking our way would see us receding from them at near-faster speed of light as well. In fact, to them we would seem on the brink of going over the universal horizon of visibility ready to dissapear forever from their detection just as we see them.
     
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