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Gravities effect on the boundaries of the Universe

  1. Nov 20, 2009 #1
    Some theories of the expanding universe say that eventually the universe's expansion could be slowed and eventually pulled back in on itself. My question is why can we assume the edge of the universe is effected by gravity? Does this barrier necessarily have to be made up of a physical substance?

    Also this might be a weird question but what do yo think would happen if I flew up next to this expanding barrier and tried to put my hand through it? Would I lose my hand or possible my hand wouldn't penetrate it.
     
  2. jcsd
  3. Nov 21, 2009 #2

    Chalnoth

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    What edge?
     
  4. Nov 21, 2009 #3

    Chronos

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    The mainstream consensus is the universe has no 'edge'. Interestingly enough, we are at the temporal 'edge' of the universe - the most ancient part of the cosmos observable to us. Everything else we observe appears as it was in our past.
     
  5. Nov 21, 2009 #4
    Hmmmmmmm....... This doesn't seem to make sense to me, but is interesting. So the space is infinte but the universe isn't. The universe just keeps expanding out into empty space? Or does this theory not really imply anything in particular?
     
  6. Nov 21, 2009 #5

    Chalnoth

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    Our universe isn't expanding "into" anything at all. New space is being created between galaxies, that is all.
     
  7. Nov 22, 2009 #6

    Chronos

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    Current evidence suggests the observable universe is finite in every respect.
     
  8. Nov 22, 2009 #7

    It would be more fair to say that any observer, where ever it is, is on temporal "edge" of universe, not that we have some special position.
     
  9. Nov 22, 2009 #8

    marcus

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    When astronomers use the term observable universe they mean something quite special (the part we have received light from, or some other type of radiation, and can therefore see) which is different from the universe as a whole.
    One of the first thing one learns in astro is to make this distinction.

    Using standard techniques we can infer that the universe is much larger than the observable portion, and further, using General Relativity and a basic working assumption of uniformity used in cosmology we can deduce some things about the universe outside the immediately observable portion.

    Certainly I know of no evidence that the universe is finite!
    Conversely it is obvious that the observable universe is finite, one hardly needs detailed evidence.
    It's confusing when people gloss over the distinction.
     
    Last edited: Nov 22, 2009
  10. Nov 22, 2009 #9

    jambaugh

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    No. The universe has no edge in the same way a balloon has no edge. But the expanding balloon is an analogy. The balloon does expand into higher dimensional space. There is no reason to assume the universe is doing the same. The universe "is all of space" it is not an object expanding in a larger space (necessarily). We may model cosmology by embedding a 4-dimensional universe in a large space (in effect using the balloon analogy as a model) but the model is not the theory and we needn't take seriously the non-operational components such as extra dimensions.

    The reason we may use such a model is because it is much easier to deal with mathematically and conceptually. But don't confuse the tools with the machine they are being used to construct.
     
  11. Nov 22, 2009 #10
    I have much doubts about "new space is being created" statement being correct. Time is expanding as well, but we do not say "new time is being created", do we?
     
  12. Nov 22, 2009 #11

    Chalnoth

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    In what way is time expanding?
     
  13. Nov 22, 2009 #12
    Well it is not "stretching" like space, but you must agree that now there is more time, that we have experienced, then 5 minutes ago. I am not questioning fact that there is more and more space between galaxies, I am saying that "space is being created" is not the best description of what is happening, especially if we have no clue what "creation of space" really mean, and what would be a mechanism for such thing.
     
  14. Nov 22, 2009 #13

    Chalnoth

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    Er, that's the passage of time, not expansion of it. In GR terms, there is no more now than there was previously, we've just moved along our geodesics a bit further.
     
  15. Nov 22, 2009 #14
    Ok, forget time. How exactly space is being created?
     
  16. Nov 22, 2009 #15

    Chalnoth

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    Through gravity, which describes the geometry of space-time. There is simply more space after some amount of expansion than there was before.
     
  17. Nov 22, 2009 #16
    "There is simply more space" is much better. As far as semantics is questioned, you should agree that "space is being created" is not the correct statement.
     
  18. Nov 22, 2009 #17

    Chalnoth

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    Why not?
     
  19. Nov 22, 2009 #18
    Because it means that some creation process is at work.
     
  20. Nov 22, 2009 #19
    Because the space where no matter exist is that space that increase in time and because that space is quantum vacuum with a energy density greater than zero we can say "something is created".
    This quantum vacuum is very different by "nothing" notion. IT IS something.
     
  21. Nov 22, 2009 #20
    Of course it is something. That is the issue here. Again what you can tell me about "space creation" other then there is more and more space?
    I just opened faucet, and guess what, there is more and more water in my sink. Can I claim that water is "being created" in my sink?
     
  22. Nov 22, 2009 #21
    Is not the same thing. You can see that the water is coming from your faucet and you can deduce that it is created somewhere else.

    It is not the case with cosmological vacuum = space. No one can say where are coming from this new quantum vacuum (QV), at least until now. So, a logical assumption can be that is "just created". Maybe a more logical one is assumption that something else is transformed in QV but this don't exclude first assumption.
     
  23. Nov 22, 2009 #22

    So because you can not deduce where the vacuum is coming from, you are claiming that it is being created? Very scientific indeed. Why should we worry about anything then. It's all just being created.
     
  24. Nov 22, 2009 #23

    jambaugh

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    Space is not a substance like water or even energy. It is at best a relationship between objects. Now again we may model space and space-time via analogy as a hyper-surface embedding in larger dimensions and within that model the increasing measure comes from "elsewhere" within the enveloping space of the model. But again that is an analogue making it easier to express dynamic curvature and geometry and not to be confused with observable reality.

    Space or Volume is not a conserved quantity in the same sense as energy or momentum or lepton number and one doesn't need to explain an increase or decrease in space in terms of a creation process or influx from elsewhere.
     
  25. Nov 22, 2009 #24

    Chalnoth

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    Well, first, energy is just a property of matter, not a substance in and of itself. But we can say that space-time is every bit as physical as the electromagnetic field: after all, it can carry energy and momentum.

    But yes, it is true that space isn't a conserved quantity, so we needn't balk at the generation of new space. Though I must mention that energy is also not necessarily a conserved quantity: it depends upon what you're talking about.
     
  26. Nov 22, 2009 #25

    It doesn't make any difference if space isn't conserved quantity. When you say "space is being created", you have no argument to put it behind other than - it can be so. That is not always the best argument.
     
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