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B The malleability of space

  1. Aug 30, 2017 #1
    Does the universe’s ongoing expansion mean that some finite amount of space-time is stretching, or that there is simply more and more of the same (space-time) being produced? Given the obvious difference between the dense primordial universe and the present, I’m guessing the former, which, if so, might this not subtly impact things like gravity (i.e., its conduction of virtual particle-waves) ?
     
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  3. Aug 30, 2017 #2

    phinds

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    "stretching" of space is purely a pop-sci fantasy. Space is just geometry, not "stuff" that can stretch or bend. Things get farther apart but nothing is being produced. Google "metric expansion"
     
  4. Aug 30, 2017 #3

    Grinkle

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    B thread, please go easy on me.

    As space expands, if there are not more Higgs particles created to maintain a consistent Higgs field strength, whatever strength means in that context, wouldn't the Higgs field decay as time goes on?

    I have no way to defend the implication that a fixed number of Higgs particles would somehow spread apart from each other as space expands if no new ones are created.
     
  5. Aug 30, 2017 #4

    PeterDonis

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    Neither. When we say the universe is "expanding", we are saying something about its 4-dimensional geometry. That geometry does not "change"; it just is.
     
  6. Aug 30, 2017 #5

    PeterDonis

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    You don't need to create more Higgs particles to maintain a constant Higgs field strength.

    No. The Higgs field (more precisely, the vacuum expectation value of that field) stays constant because it's in its lowest energy state. It can't decay because there is no state with lower energy for it to decay into.
     
  7. Aug 30, 2017 #6

    Chronos

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    To quote Einstein, empty space has no ponderable properties. Lacking ponderable properties means to speak of it being stretched, torn, broken or otherwise defeormed in any way is virtually meaningless. The geometry of spcetime is an imaginary grid we overlay the vacuum with to compare the geometrical relationships between adjoining regions of spacetime. Geometry does have ponderable properties, which makes it useful for visualization purposes, but, should not be confused with spacetime, which does not..
     
  8. Aug 31, 2017 #7
    Wikiing up on a vacuum's ZPE (which seems to have "ponderable properties") I'm having trouble with the assertion here that "Space is just geometry," which was, to me, already putting the cart before the horse. Mathematics makes for exact and helpful metaphors that can be applied to, but <>, space.
     
  9. Aug 31, 2017 #8

    Grinkle

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    The point I am confused on is whether current models and experimental evidence predict there is any such thing as empty space. Minimally, space contains the Higgs field, and maybe minimally it also contains other things that distinguish it from empty that a theory of quantum gravity could describe.
     
  10. Aug 31, 2017 #9

    PeterDonis

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    Current models don't "predict" that there is empty space; empty space (more precisely, spacetime) is an axiom on which the models are built. Some of the theories of quantum gravity that are being worked on do not have this axiom; they attempt to build "empty space" (spacetime) out of other more fundamental entities. But those theories are still speculative at this point.

    Experimental evidence doesn't "predict" that there is empty space; it just tells us there is, by commonplace observation. That's why our current models can adopt this as an axiom without an issue.
     
  11. Aug 31, 2017 #10

    PeterDonis

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    Why?
     
  12. Aug 31, 2017 #11
    I just meant that geometry/math can be used to describe/model certain aspects of space, but not all. That space is probably more than geometry. E.g., if Feynman's and Wheeler's calculations suggesting "the zero-point radiation of the vacuum to be an order of magnitude greater than nuclear energy." Or if Paul Davies' "Many physicists believe that "the vacuum holds the key to a full understanding of nature" is true, then space is more than geometry.
     
  13. Aug 31, 2017 #12

    PeterDonis

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    No, those things are not saying that space is more than geometry; they are saying that "vacuum" as a concept is not the same as "space" as a concept. (Or spacetime, if we are using relativity, as we should.)
     
  14. Aug 31, 2017 #13
    Then getting back to my question, it would seem the universe's expansion would involve either creating ever more of this vacuum or changing the quality/nature of it, and I'm still thinking the latter.
     
  15. Aug 31, 2017 #14

    PeterDonis

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    No, because the "vacuum" is not something that gets "created" or "stretches" as the universe "expands". The universe is a 4-dimensional geometry, and the "vacuum" is a 4-dimensional solution to a 4-dimensional field equation on that 4-dimensional geometry. It doesn't "change"; it just has particular values at each point of the 4-dimensional geometry. The solution describing the vacuum for our universe already contains all the information about the whole 4-dimensional geometry; nothing has to be "created" in one part of that geometry vs. another.
     
  16. Aug 31, 2017 #15

    Grinkle

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    Ah - thanks.
     
  17. Sep 1, 2017 #16
    According to SM, every point in space contains all fields. There no "holes" in them. Mathematically speaking, "every point of spacetime is assigned a value" and that's what field is. (The value can be just a real number, a complex number, a matrix, a tensor etc).

    The important point is a field can easily have zero value in a large area of space. If you have a cubic kilometer of space where photon field is everywhere zero, it's just a place which has no photons moving trough it. But it's not a cubic kilometer "without photon field".
     
  18. Sep 1, 2017 #17

    Grinkle

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    No issue with this.

    Thinking a bit more, I'm probably just wanting someone to explain to me what the nature of dark energy is. :smile:
     
  19. Sep 1, 2017 #18

    PeterDonis

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    We don't know at present. We just know that, from the standpoint of our cosmological models, dark energy looks like a small positive cosmological constant. But we don't know why the cosmological constant has the small positive value that it has.
     
  20. Sep 1, 2017 #19
    Seems widely theorized to uniformly fill otherwise empty space. If so, would expansion be introducing more of this energy or diluting the concentration of some fixed amount? In either case, expansion would seem to be a lot more than a geometric phenom.
     
  21. Sep 1, 2017 #20

    PeterDonis

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    It uniformly fills spacetime. (That's true whether spacetime is "otherwise empty" or not. The dark energy--small positive cosmological constant--is everywhere, not just in "otherwise empty" regions.) That's what "small positive cosmological constant" means.

    Neither of these is correct. I've already explained the correct viewpoint earlier in this thread.
     
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