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I Question about "the big bang happened everywhere at once"

  1. Jan 14, 2017 #1
    Is it fair to think about the statement "the big bang happened everywhere at once." as meaning the singularity that spawned the "big bang" was very large by cosmic scales, even infinitely large? (I am aware that the word "singularity" refers to a place where the math breaks down and not a point in space. I also am aware that "before" the big bang there was no time or space and that giving the "singularity" a spacial measurement is strange at best - but I want to pose the question nonetheless.)
    Another way to ask this question is if the singularity was infinite in size and the big bang was in any way comparable to a pre inflation era inflation of the singularity.
     
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  3. Jan 14, 2017 #2

    phinds

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    You are asking a question about the characteristics of a thing described by the term "singularity" which you admit to understanding just means "the place where the math breaks down". It's exactly like asking "what do we know about this thing that we don't know anything about?".
     
  4. Jan 14, 2017 #3

    PeterDonis

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    There is no such thing. More precisely, even though an "initial singularity" appears in a particular set of highly idealized models, nobody actually believes that part of the models.

    The proper use of the term "Big Bang" is to refer to the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have reliable evidence. (In inflation models, this state occurs at the end of inflation.) "The Big Bang happened everywhere at once" is just a way of emphasizing that this state was a state of the entire universe, not an isolated piece of it.
     
  5. Jan 14, 2017 #4

    Chronos

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    One possibility is space is the universal gravitational field. It therefore lacks any context before gravity broke free from the single unified force during the Planck epoch. One must also be careful with the notion of time. It is not really useful without space, so terms like before and prior are merely an awkward attempt to preserve some sense of causal continuity in the natal universe.
     
  6. Jan 14, 2017 #5

    PeterDonis

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    What is this a reference to?
     
  7. Jan 14, 2017 #6
    The singularity, as you said yourself, is not 'a thing'.
    It means conditions that cannot be described mathematically with our current best theories.
    It is pointless to speculate about the evolution of an object whose state is undefinable to begin with.
     
  8. Jan 14, 2017 #7

    Chronos

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    The essay by Norton [http://www.pitt.edu/~jdnorton/papers/Einstein_reality_space.pdf] captures the essence for the argument of space as a consequence of the gravitational field. Page 181, in particular, captures the Einstein perspective and unequivocally establishes this assertion. Einstein is attributed with having responded to an interview question with - "People before me believed that if all the matter in the universe were removed, only space and time would exist. My theory proves that space and time would disappear along with matter."
     
    Last edited: Jan 14, 2017
  9. Jan 14, 2017 #8
    interesting.
     
  10. Jan 14, 2017 #9

    PeterDonis

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    This looks like a philosophy paper, not a physics paper. (Which doesn't mean it's not interesting, just that it's not about physics.) I was looking for something that gave testable consequences for "space as a consequence of the gravitational field". There is no way to test whether space (more precisely spacetime) would continue to exist in the complete absence of matter, so asserting that it would "disappear" (or that it wouldn't) is not a statement of physics, it's a statement of philosophy.

    I agree that it describes Einstein's viewpoint well, but I don't see that viewpoint as being "unequivocally established" by anything in this paper, on page 181 or anywhere else.

    I don't think Einstein was correct in this assertion, since flat Minkowski spacetime, with zero stress-energy everywhere, is a valid solution of his field equation.
     
  11. Jan 14, 2017 #10

    PeterDonis

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    To try and get this subthread back on topic, there are models in which "spacetime" is not really meaningful at the "beginning" of the universe. For example, Hawking's "no boundary" model, in which, heuristically, there is an "earliest time" at which Lorentzian spacetime (one timelike and three spacelike dimensions) "emerges" from Euclidean "spacetime" (four dimensions all the same--not really describable as "spacelike" or "timelike", just all the same), and the Euclidean spacetime is, heuristically, a hemisphere, so that there is no "initial singularity" and everything is nice and smooth. I don't think this model is considered as a likely contender by most cosmologists, though.
     
  12. Jan 14, 2017 #11

    PeterDonis

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    Also, the models that have gravity unified with the other interactions until after the Planck epoch are based on gravity as a spin-2 quantum field on a flat background spacetime, so I don't think they support the idea that spacetime "lacks any context" before gravity broke free.
     
  13. Jan 14, 2017 #12
    A nice and smooth sphere of what though?
    If not a nice smooth sphere of something, what else could it be?
     
  14. Jan 14, 2017 #13

    PeterDonis

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    I don't think there's a simple answer to this question. The best answer I can give would be "whatever kind of quantum stuff you get when spacetime is Euclidean instead of Lorentzian".
     
  15. Jan 14, 2017 #14
    Phase changes, but why?, I guess it doesn't matter
     
  16. Jan 15, 2017 #15

    Chronos

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    I would call Norton more of an historical recapitulation. My intent was to raise the fact Einstein considered traditional concepts of space and time as no more than gravitational field artifacts. I consider that worthy of attention as a plausible model.
     
    Last edited: Jan 15, 2017
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