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Flat universe and big bang theory

  1. May 2, 2013 #1
    Hello everybody! This is my first post!

    I was wondering about the fact that we have measure our universe to be flat very accurately. This means that it is also infinite and if it is infinite now it was infinite from the beginning!

    This means that at bb the time collapses but not the space right?

    With this in mind I am trying to figure out how the expansion took place...

    How it is possible to have a hot dense universe at the beginning which is also infinite?

    We say that every galaxy is moving away from the others so if we move backwards in time this motion leads us to a single point. How is that compatible with an infinite universe from its beginning?

    Thanks in advance for your answers.
  2. jcsd
  3. May 2, 2013 #2
    The flat geometry does not mean an infinite universe. The question of whether the universe is finite or infinite is still up for debate.

    Also the hot dense state is of an unknown size and origin.
    The singularity descriptive used in the BB scenario is a descriptive of when the maths breaks down.

    welcome to the forum.
  4. May 2, 2013 #3


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    That's a bit of a misleading way to state it because it implies that the universe IS flat, which is NOT an established fact. The universe has been measured to be within some small percentage of being flat and personally I'd find it to be one hell of a coincidence if it is that close and NOT flat, but none-the-less, it is not yet proven to be flat.

    Well, you got the second part of that right. As Mordred pointed out, flat does NOT mean infinite.

    I have not idea what you mean by this.

    Why would it NOT be possible. Hot and dense does not preclude infinite.

    Nonsense ... I know it seems that way at first, but if it is infinite now it was infinite at the beginning (and if not, then not)

    See above
  5. May 2, 2013 #4
    Thank you for your answers,

    I thought that after Planck mission results we could conclude that it is flat. I apologise for that...

    I read in many articles that if our universe is flat or open is infinite and if it is close is finite. I had no idea that flat and finite is a possibility! What consequences could this have for the future of our universe if it is true?

    I ment that the space was instantaneously infinite (it was not "growing") but the time has a zero value at that moment... Isn't that correct?

    I don't have clear in my mind if by infinite space we also mean that infinite material was created during the big bang... If space was infinite, big bang was taking place everywhere in space right? I imagine that the material would have infinitely space to spread so you we can't have a dense universe.

    I agree that is nonsense, but if you ask someone what proofs we have about BB and how the whole idea was created it is one of the first things that you get as an answer for some reason...
  6. May 2, 2013 #5


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    I think what you are missing here is the concept of bounded/unbounded. A universe can be flat and finite but unbounded. That means that if you go in the right direction for long enough you'll get back where you started. In the real universe, because of expansion, you actually would NOT get there because the universe is expanding faster than you can travel, but you get the idea ...

    Ah ... yes, that's a reasonable way of looking at it. I think we all have trouble with the singularity. I know I do.

    Infinite and dense are not mutually exclusive. I think probably what' misleading you is this: Infinite space with a fixed amount of matter can get LOTS bigger, so the density goes down; that is what is believed to have happened (if the start was infinite). Google "Hilbert's Hotel"

    Well, I wouldn't call it "some reason" I would call it the very specific reason that pretty much ALL popularizations make that moronic statement. As has been discussed here on this forum several times, even reputable physicists (to say nothing of the media whores like Michio Kaku) say it even when they know better. It seems to be a prerequisite of TV shows in particular that you leave truth and complexity at the door and make dumb-downed statements that you know are not true.
  7. May 2, 2013 #6
    The planck data showed that it is flat or extremely close to flat. For example the universe could be so large that the entire observable portion could look for all intensive purposes as being flat.
    flat and finite is possible as there are geometric shapes that can be flat, finite and no edge. Mobeous strip, torus and Klein bottle. I merely point these shapes out as an example.
    However I am not stating the universe is one of those shapes, the first reason of sheer size is the more likely scenario.
    The part of the BB instantaneous infinite is misleading. We do not know what occurs prior to 10-43 seconds. This is the point where all the maths breaks down or singularity in this usage.

    Time is a tricky subject in cosmology to say the least, For example some models describe time as continuous. In particular the large variety of multi-verse theorem.
    so time isn't accurately considered as starting from the moment of our universes creation.

    I just noticed Phinds post but I'll leave this post as additional information.
    Last edited: May 2, 2013
  8. May 2, 2013 #7
  9. May 2, 2013 #8
    Your answers are very helpful!!!

    I understand what you said... My problem is that I think that at the very beginning, I have an infine space to fill it with a finite amount of matter evenly...

    If we measure our universe to be infinite now, can't we conclude that it began infinite, despite what happens before the planck time? Is there a possibility the universe from finite become an infinite one?

    Thanks, I will read them as soon as possible
  10. May 2, 2013 #9


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    If the universe is infinite now, then it was always infinite. But we have no knowledge of the size of the universe, so we can make no such conclusion.
  11. May 2, 2013 #10
    There is no reason to suspect a finite amount of energy or matter either. If you have an infinite universe its quite reasonable to assume an infinite amount of energy/matter.

    we can only measure what is in our observable portion of the universe, and how energy and matter behaved in the observable portion.

    edit: to expand on this,

    All our mathematics, studies and descriptions of the quantity of energy, mass, curvature and shape are derived from the observable portion of our universe. To attempt to describe beyond that leads to conjecture. The observable portion is always finite so mass energy values are finite values.
    Last edited: May 2, 2013
  12. May 2, 2013 #11


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    vattoman, another thing to keep in mind is this --- if you have an infinitely large universe filled with matter the density of lead, then you have an infinite amount of matter. If you have an infinitely large universe filled with matter that is the density of deep space (damned near nothing, but not zero) then you have an infinite amount of matter.

    I think a lot of your problem with all of this is that you have not yet come to terms with various ramification of "infinite". And by the way, I agree w/ all comments above that say we do not know whether the universe is infinite or not.
  13. May 2, 2013 #12
    Ok, its all clear now, thanks a lot!!!
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