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B Big Bang theory and the known universe

  1. Jun 1, 2017 #1
    I just want to verify from physicists whether what I have read in this article is true: http://www.astro.ucla.edu/~wright/infpoint.html
    It says:
    The Universe was not concentrated into a point at the time of the Big Bang. But the observable Universe was concentrated into a point
     
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  3. Jun 1, 2017 #2

    Drakkith

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    I'm under the impression that the observable universe was concentrated into a very small, but nonzero volume at the time of the big bang. However I'm certainly not an expert on this and would also like to see a good answer.
     
  4. Jun 1, 2017 #3

    Bandersnatch

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    Yes, it is. You can generally take everything from Ned Wright's website to be correct.

    It was a point in the limt of the singularity, just as every other finite volume. The distinction made in the linked article is between finite and infinite volume - the latter remains infinite even if you shrink all finite distances to a point.
     
  5. Jun 1, 2017 #4
    Thank you!
     
  6. Jun 1, 2017 #5

    Orodruin

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    A limit that most physicists would agree is not physical, but rather a result of extrapolating the theory to a regime where it is no longer valid. For the intended readership, that may not be relevant though.
     
  7. Jun 2, 2017 #6

    vanhees71

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    I'd also say that the unavoidable singularities (like black holes, big bangs/crunches) of standard GR are manifestations of our ignorance rather than really describing nature. Usually one argues that for very small space-time volumes classical gravity shouldn't be valid anymore and that you need some quantum theory of gravity, but we still don't have a fully satisfactory theory, and before we don't have it, we cannot check, whether this really solves the problem of singularities in GR.
     
  8. Jun 2, 2017 #7

    russ_watters

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    Maybe, but I think it is important to keep that tentative until evidence uncovers the flaws rather than assuming the way the universe has to work; like light is a wave so it has to have a medium.
     
  9. Jun 16, 2017 #8
    What do you think of this statement about the Big Bang: "... In reality, the expansion did not start in a single point but started in a huge number of points that appeared in a spherical shape. When scientists give up the idea of creation starting in a singularity and look for a spherical beginning, they will get their mathematical equations to work."
     
  10. Jun 16, 2017 #9

    Bandersnatch

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    I think it would be very hard to give up on an idea one doesn't hold, or make work something that is not faulty. Or trivially easy, depending on how you read that.

    As is often the case, before one can begin to think outside the box, first one needs to learn where the box is.
     
  11. Jun 16, 2017 #10

    phinds

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    I think you don't understand the meaning of "singularity" because if you did you would have ignored this nonsense instead of asking about it. "Singularity" in this context means "the place where the math model gives nonphysical results and we don't know WHAT is/was happening". It does NOT mean "a point in space" as that writer mistakenly believes.

    Also only pop-sci misrepresentation contend that the universe started at a point in space. The big bang happened everywhere at once. This is a bit hard to get your head around when you first encounter it.
     
  12. Jun 16, 2017 #11

    Drakkith

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    It's wrong. Modeling the universe as starting in a spherical shape solves absolutely nothing and introduces unjustified constraints.
     
  13. Jun 18, 2017 #12
    I read a very interesting article about the big bang:

    https://insidetheperimeter.ca/things-didnt-go-so-smoothly-at-the-big-bang/

    The Big Bang referred as the initial singularity. Mathematicians use the term "singularity" to indicate that the GR equations are failing when the temperature and energy density become infinity . It doesn't have any physical meaning. You need a more accurate theory to describe the law of physics when "singularities" appear in the equations.
     
  14. Jun 18, 2017 #13

    Ssnow

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    The use of the word ''observable'' is misleading... there is implicitly an observer that is no part of the Universe? ...
     
  15. Jun 18, 2017 #14

    Bandersnatch

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    No, but there are observers who see different patches of the universe.
     
  16. Jun 18, 2017 #15

    Ssnow

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    Ok this makes sense,
    thanks
     
  17. Jun 18, 2017 #16

    phinds

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    No, it's not the word "observable" in that sentence that is misleading (as has already been explained), it's the word "point". While it is true that the observable portion of the universe started off as a very small volume, it did not start off as a point. I haven't done the math but I've seen various statements ranging from the size of an atom to the size of a golf ball (but I think the golf ball statement is likely way too big)
     
    Last edited: Jun 18, 2017
  18. Jun 18, 2017 #17

    Chronos

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    The initial size of the observable universe depends on the number of e-folds it experienced during inflation [an e-fold roughly doubles the size of the observable universe]. The consensus choice is 50-60 e-folds, as discussed here; How long before the end of inflation were observable perturbations produced?,
    https://arxiv.org/abs/astro-ph/0305263, . Of course, this assumes inflation theory and standard cosmological models are correct. Obviously, the observable universe could never been size zero under this scenario, otherwise it would have remained zero. By the same token, this does not preclude a universe of infinite size, since a universe of infinite size could have undergone any number of e-folds and still remain infinite.
     
  19. Jun 19, 2017 #18

    russ_watters

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    Actually, I think it is the word "observable" that is at issue and by discarding it you are missing it:

    The "obervable universe" would have to start at a single point, as any light cone does, because that's how it is defined. The "observable universe" is the expanding sphere of our light cone that started at whatever point we choose to define as the start, regardless of whether the *actual* universe started at a single point.

    Why? Because the "start" has to be t=0, which means that light hasn't had a chance to propagate yet to some finite size of the sphere.
     
  20. Jun 19, 2017 #19

    russ_watters

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    There is implicitly an observer that was present at the chosen point, for the event which we define as the "start" at t=0. By definition, it must be a point.
     
  21. Jun 19, 2017 #20

    Ssnow

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    I think here the word ''point'' is not the same of the primitive concept in geometry.
     
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