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Space expansion faster than c

  1. Jul 19, 2012 #1
    As I understand, every point in the the universe may be considered for that observer to be the center. Everything seems to be expanding away from that point. From our perspective on earth, the further away we observe, the faster that space is expanding, up to, and faster than the speed of light.

    If an observer was 13 billion light years away from us, would our space be expanding towards the speed of light? If so, why does it not appear to us?
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  3. Jul 19, 2012 #2


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    The nearby universe always appears to be at rest to any local observer. Only more distant regions appear to be receeding.
  4. Jul 19, 2012 #3
    Why does our local universe appear to be at rest when to a distant observer our local universe is expanding up to and past the speed of light.
  5. Jul 19, 2012 #4


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    Uh ... who is the observer that sees our local area as being at rest? (by the way, "local universe" is TERRIBLE terminology).

    If the observer is local, then the local area appears to be at rest.

    If the observer is 47Billion light years away, then the same area appears to be receding from him/her/it at about 3c.
  6. Jul 19, 2012 #5
    You're right, local universe is a horrible term. Let's use local space. What I am trying to ask is this. If an observer roughly 13 billion light years away from earth observes our solar system, galaxy or what have you, they would observe us expanding away from them at an accelerated rate. Why do we not see our local space accelerating at the rate they observe.
  7. Jul 19, 2012 #6


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    Because we are in a different frame of reference. All motion is relative and you have to define your frame of reference and what the motion is realtive TO.
  8. Jul 19, 2012 #7
    You're confusing a few concepts here. Firstly, we can talk about how quickly space is expanding with the time derivative of the scale factor. This is the same everywhere. Then, there is apparent recessional velocity. Every galaxy sees other galaxies receding, due to the metric expansion of the intermediate space. This recessional velocity is given by [itex] V = H_{0}D [/itex]. This is Hubble's law. You can see that for galaxies that are farther away, we observe a faster recessional velocity. However, that galaxy isn't actually receding. It's only apparent due to the fact that the metric of the space in between the galaxies is expanding.

    Second, space doesn't expand inside of gravitationally bound systems. So, I'm assuming you mean regions outside of our galaxy.

    So, now the answer should be clear - everyone agrees on the rate of expansion. This expansion creates the illusion that galaxies are moving away. In reality, the distance in between the galaxies is expanding, not that the galaxies themselves are moving (though, they have a very small peculiar velocity which has nothing to do with expansion and is very small).
  9. Jul 20, 2012 #8
    Is our galaxy receding near the speed of light to an observer at the edge of our observable universe?
  10. Jul 20, 2012 #9


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    No, it is receding at 3 times the speed of light.
  11. Jul 20, 2012 #10

    George Jones

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    I would say that galaxy that has "recessional speed" 3.0 times the speed of light has a speed of 0.995 times the speed of light :wink:, but this is somewhat non-standard.
  12. Jul 20, 2012 #11


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    Darksyde, you may need some background concepts otherwise you could find the information confusing.
    There's the idea of PROPER distance at some specified moment which is the distance you'd measure if you could freeze the universe right at that moment, stop the expansion process, so you could measure in some conventional way like radar without the distances changing while you were measuring. Sometimes that present value of that distance is called the "NOW distance" or "current distance" to something.

    Many of us use that concept of distance consistently or almost always. Unless otherwise specified like by saying angular-size-distance or luminosity-distance or whatever. Technically those are different. I almost always mean proper distance.

    The farthest matter we can see is the matter which gave off the ancient light called the CMB (cosmic microwave background). When we use an antenna to study that light we are seeing that matter. IT IS NOW ABOUT 46 BILLION LIGHTYEARS AWAY. Because of the expansion of distances while the light was traveling to us.

    So that effectively marks the EDGE of the currently observable universe---the farthest stuff we can see. We see it as it was when it was still hot gas and had not cooled and condensed into galaxies and stuff.


    You might want to get some hands-on experience with the standard cosmo model, so you don't just have people telling you stuff. There is a very good online calculator by a PF member named Jorrie, who posts here occasionally.

    The only problem is it tells you too much. You have to focus and just look at the numbers that matter to you and ignore the rest. You might want to try it though.

    Just go there and press the "calculate" button. He already has it set up to give you roughly the distance to the edge of the observable, and the recession speed of matter at that edge.

    It will say something like
    Proper distance now = 46000 Mly
    Proper recession speed now = 3c

    Just look at those two numbers, in those two boxes, and ignore the rest.

    Then you can put in a different number for Redshift of source now
    and press "calculate" button again.

    That is the key thing, try varying the redshift of source. He has it set to 1088 for starters because that is a good estimate of the redshift of the ancient light coming from the edge of what we can currently observe. Its waves have been stretched out by a factor of around 1100. That is why it is microwaves now, instead of the visible light and heat-glow of hot gas (which it used to be when it started out.)

    But put in something else instead of 1088. Say you heard a supernova was observed and the redshift was 2. Put a 2 into the box labeled Redshift of source, and press calculate.
    That will tell how far away the supernova is, and how rapidly the distance to it is now expanding.
    Last edited: Jul 20, 2012
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