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B Question about the shape of the Universe

  1. Aug 16, 2017 #1
    If the universe was all created simultaneously in the Big Bang then why is it that all matter does not move at the same rate in relation to all surrounding matter away from the singularity causing all matter to basically form a massive increasingly expanding sphere with some significant width? If everything was roughly the same distance from the initial point then the gravity would all be on the same plane, toward all other matter aside from the early universe when the rapidly expanding distances from the mass on the opposite 'side of the sphere' would cause interference? Or would that interference and/or more localized interference be enough to permanently destroy this sphere?
  2. jcsd
  3. Aug 16, 2017 #2
    Hi and welcome!
    The big bang wasn't a singularity, it happened everywhere simultaneously and everywhere expanded away from everywhere else equally.
  4. Aug 16, 2017 #3
    Yes but at that point everywhere was at some point infinitely small and began expanding at that point
  5. Aug 16, 2017 #4
    A point is just some man-made reference of a location, that doesn't have any "size" to say it changed. If you look like Hubble did and say everything is expanding away from everything else (except gravitationally bound clusters) then you get the "point" that nowhere is special from anywhere else. It's not like our universe started at one point and expanded outward from that, only our view is limited by the time light has had to travel from what we can see.
  6. Aug 17, 2017 #5
    I'm only using the term point to refer to the very beginning of space time, everything has been accelerating away from what is now just some place in space but was at the very first second of the universe was the entire universe expanding from the singularity, I didn't realize that was even debated, I thought the big bang initiated from a singularity of nearly infinite mass and for all intensive purposes 0 volume, making this the point to which I am referring. I was watching a discussion from the world science festival and the astrophysicist who I unfortunately can not name had what he explained as the shape of the universe over time and it was a pointed bell on it's side, the point referring to this singularity. That point must still be somewhere regardless of your location or perspective
  7. Aug 17, 2017 #6


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    You're likely referring to this picture, or some variation thereof:
    The spatial location of the point at the apex of the bell is roughly where were are now. Any sufficiently distant observer is now and has always been separated from where we are or have been. Even though this separation goes to 0 in the limit as you roll back the time (that's why it's a singularity), it always stays separate.
    Another observer, some distance away, could draw an identical bell, but with them at the centre:
    And so on for infinite number of observers.

    The image of Big Bang you're stuck on is that of a point-like explosion, with debris flying apart in the shape of a spherical expanding shell. That is not the right image.

    Try this:
    Start with the current universe, and extrapolate backwards in time.

    For the sake of argument, let's assume the universe is infinite in extent - you can keep jumping to neighbouring galaxies without limit. There are galaxies distributed pretty much uniformly throughout, with no significant voids or clumps - this is what observations indicate, so you know you're on solid ground.

    Now begin rolling back the time, and contracting ALL distances by the same factor per unit of time. No matter between which two galaxies the distance is measured, it has to go down by the same factor. E.g. both a galaxy 4 billion light-years (Glyr) away from the observer, and one 2 Glyr away will have their distance to reduced by half after the same amount of time.
    From any given galaxy on which you're standing at the moment, it'll look like all other galaxies are approaching towards you, with farther galaxies approaching faster than the closer ones.

    As you keep rolling back the time, all the distances keep contracting, even though the universe stays infinite, and all the galaxies remain in the same relative positions (a galaxy at your 2 o'clock will stay there, even though it's getting closer). The further back you go, the more dense universe is, but the distribution of matter still remains just as uniform as it was when you started the process of contraction.

    At the limit of some finite point in time, all the distances approach zero - this is the singularity. It's a point in time at which the function used for describing expansion becomes singular - it stops being defined (much as f(x)=1/x is singular at x=0), results in infinities, and can no longer be used in a meaningful sense (so we don't treat it as meaningful, and stop using the BB model before we reach that point).
    It's not a point in space. In fact, if the universe is infinite, it stays infinite throughout the process of contraction, including at the singularity.

    Expansion is just that, but in reverse.
  8. Aug 18, 2017 #7
    Thank you
  9. Aug 18, 2017 #8


    Staff: Mentor

    @banderdnatch, I've never seen a layman's explanation as graphic and clear as yours. Good job.

    If you made a picture to go with it, it could be used widely, or used as an insights article.
  10. Aug 18, 2017 #9


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    Thanks. It's a bit on the short side for an Insights article, but I'll give it a thought.
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