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Stargazing Receding Galaxies and the Range of Gravity

  1. Feb 25, 2016 #1
    Hello,

    Sorry if this is in the incorrect forum. I've come across a discussion that has made the following argument, but I'm unable reconcile the views. I've searched within and without and have come across no answer that addresses this specific claim.

    To begin, from my understanding gravity has infinite range. In a universe comprised of only two objects at any finite distance, with enough time they will collide.

    The argument is that galaxies receding from other galaxies faster than c cannot be influenced by gravity from those galaxies since gravity travels at the speed of c and would never reach them. I thought the force of gravity would only weaken, not outright cease from lack of contact. Is this an "exception" to the model we have for gravity?

    Thanks for your time.
     
  2. jcsd
  3. Feb 25, 2016 #2

    phinds

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    You are correct on all counts with the exception that 2 objects alone in the universe might not ever meet up if they had an initial velocity relative to each other (and away from each other) that is greater than their escape velocity from each other. If they are very far apart that would not have to be much velocity.

    The deal with gravity that mislead people is that gravity is a field and permeates to infinity BUT ... changes in the field propagate at the speed of light.

    All this is further complicated by the fact that recession velocity is not proper motion and objects outside of our observable universe are outside of causal contact and will remain so due to the expansion. There is actually a slight wrinkle even in that because objects not very far outside the OU are going to become inside the OU over time, but for much farther away, it holds.
     
  4. Feb 25, 2016 #3

    Jonathan Scott

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    Changes in gravity propagate at c, but the overall gravitational effect due to any collection of matter as seen from sufficient distance is hardly affected at all by whether it is intergalactic gas, a galaxy or any other form of energy-momentum. So if a signal could have got from the part of the universe where one galaxy was eventually going to appear to the other galaxy, then the other galaxy would also be affected by gravity approximately equivalent to that of the first galaxy.
     
  5. Feb 25, 2016 #4
    Thanks for the responses.

    If I'm reading correctly: would it be fairly accurate to say that changes in a gravitational field would not reach an FTL receding galaxy?
     
  6. Feb 25, 2016 #5

    Jonathan Scott

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    Changes propagate at exactly the speed of light.

    The idea of an "FTL receding galaxy" is not very meaningful here, as over cosmological distances the conventional concepts of distance and time become rather complicated. The fact that the distance to a particular galaxy is increasing faster than light does not necessarily rule out being able to send a light-speed signal which will reach it eventually. As a rough analogy, imagine that space itself is stretching (although this too can be misleading) and see that a light-speed signal will be helped on its way to some extent by the stretching process.

    If the universe is sufficiently large, there may be galaxies that are sufficiently far apart that expansion would prevent light from ever getting from one to the other. From here, if any galaxy exists beyond the "future visibility limit" then it will never be visible. However, that is a much more extreme situation than galaxies effectively receding "faster than light" due to expansion.
     
  7. Feb 25, 2016 #6

    phinds

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    and to add just one snippet of fact to what Jonathan has said, all of the galaxies out near to edge of our observable universe have recession velocities > c and those closest to the edge are receding at about 3c. We are still receiving light from all of them
     
  8. Feb 25, 2016 #7

    mathman

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    The light is very old. We would never receive signals being emitted now.
     
  9. Feb 25, 2016 #8

    phinds

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    That is not my understanding. Are you sure about that? It is my understanding that objects inside our observable universe will always be in causal contact with us, although signals will eventually become so red-shifted as to be undetectable. Undetectable, however, is not "doesn't get here".
     
  10. Feb 26, 2016 #9

    mathman

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    Frankly I'm not sure, but I would like to see a reference to a discussion of this issue.
     
  11. Feb 26, 2016 #10

    phinds

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    I don't have anything offhand, but I'm just about positive that I absorbed this from @marcus. It's certainly possible I misunderstood (wouldn't be the first time) but I'll be a bit surprised if it turns out that way. If Marcus doesn't join this thread, I'll poke around and see what I can find.
     
  12. Mar 8, 2016 #11
  13. Mar 9, 2016 #12

    Chronos

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    Gravity and light are widely believed to propogate at a velocity of c. That, of course, means any galaxy we can still see still has a gravitational influence upon us.
     
  14. Mar 9, 2016 #13
    Is it more accurate to say that any galaxy we can see had gravitational influence on us at the time it emitted the light we are seeing? My (probably incorrect) understanding is that as things begin to recede past the event horizon, we don't actually see their light vanish, but it is observed to behave similar to light coming from an object meeting the event horizon of a black hole. But then how does gravity come into play, since for example it does not redshift?
     
  15. Mar 9, 2016 #14

    Chronos

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    What you get is what you see, the gravitational effect of distant galaxies mimics. their EM fields we observe here and now. It is diluted jin the same way as light is redshifted. There is no evidence to suggest changes is either field propogate at any speed other than c.
     
  16. Mar 10, 2016 #15

    ogg

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    Part of this discussion assumes that cosmological expansion will follow our expectations (remain constant or more likely increase). Over Cosmological time/distance we have to include this stuff. If you do the math, I understand (although I've not done the math) that any two objects receding from one another by 2c will not ever be able to communicate (assuming increasing rate of expansion). (I've read 1.2c in some places, 2.2c in others...) Two objects at exactly the distance where their recessional v is c are able to communicate - think what happens if either one moves an inch closer to the other, (at exactly the time they're c apart (using recessional v as a distance metric)); suddenly they are within range. Theres also errors in about what our Observable Universe is. It is composed of (theoretically) all objects which were (or are or will ever be) in communication range. Since the OU expanded at >c, its size is larger than r =13.8 Gly
    and by quite a lot. Most of this (r = ~46 Gly) will never again communicate (interact in any way) with us. A good answer to OP's question is:"No, Gravity's range is limited to our event horizon (~14 Gly) beyond which it is either zero OR meaningless to talk about, your pick."
     
  17. Mar 11, 2016 #16
    Aha!

    https://www.physicsforums.com/threads/634757/

    I finally recalled the thread by @marcus I was looking for, it changed title. Look near the end of the first post by Marcus. It lays out just what ogg explained above. That is a really fascinating thread, I was relatively new to the forum when I came across it and was very impressed.
     
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