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Size of universe at Big Bang without space?

  1. Aug 21, 2011 #1
    I have heard statements made that at the instant of the Big Bang, the entire universe was condensed into an incredibly tiny space (a "singularity"). But I've also heard it said that space-time began with the Big Bang. So the singularity did not exist within a Newtonian absolute space. How, then, can it make sense to talk about how big the universe was?

    Perhaps this is related to the question of how the universe could expand if there is no Newtonian space into which it expands?
  2. jcsd
  3. Aug 21, 2011 #2


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    you'd probably find it informative to read the FAQs here in cosmology
  4. Aug 22, 2011 #3
    Thanks, phinds. I read the FAQs and found them somewhat helpful. Still, I am trying to figure out how to dismiss a commitment to a Newtonian absolute space within which the universe's initial state would have to be situated.

    Suppose that prior to (assuming temporal concepts can be applied in this way) the Big Bang, there was quantum vacuum state subject to random fluctuations. Since a fluctuation is a change of state, it seems some kind of temporal predicates must be applicable here. And if there are multiple fluctuations going on withing a space-time foam, I can't see how to avoid imposing some kind of spatial metric on it.

    But more basically, if space-time and matter coalesce at the singularity, I don't see how there can be a meaningful description of anything as having size when there is nothing to serve as a yardstick relative which size would be reckoned.

    Apologies if I've overlooked some important FAQ or earlier post that addresses this, but I didn't find anything in my initial searches.

    But more basically,
  5. Aug 22, 2011 #4


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    I'm no expert on this at all and find it all very confusing myself. I am getting closer to, but have not yet fully arrived at, that point where I no longer run off screaming obscenities when I read about the next impossible thing in QM or cosmology.

    The consensus seems to be that it just doesn't make any sense, in physics as it is currently understood, to talk about "before the big bang". I completely understand your point of view and can say only that this seems to be one of those many instances where human intuition and "common sense" just DO NOT APPLY and the logical points that you make are part of what doesn't apply.

    I WOULD say however that your statement " if space-time and matter coalesce at the singularity" does not reflect what I believe is correct physics. Nothing "coalesced" at the big bang, since that presumes a prior existance of the stuff that coalesced and that's not what seems to have happened. Space and time CAME INTO BEING at the singluarity. There ARE other theories but the seriously prevalent one is the big bang.

    I found one helpful explanation in my early reading that went some thing like this: The term "big bang" really has two meanings. One is the "singularity" (t=0) and the other is the events FOLLOWING the singularity (t>0). The singularity is not really thought of in physical terms but rather is the place where the current theories break down and make no sense. It is not really helpful to discuss the singularity with current knowledge since it doesn't make any sense. We need a better theory.

    One plank time AFTER the singularity there starts what could more properly be called "the big bang PERIOD" and that IS very well understood and conforms to LOTS of serious math and physics that is widely accepted. How long this "period" lasts is not something I remember seening any specific duration on although certainly it goes on no longer than the first 400,000 years and some discussions seem to limit it to the time when inflation ended (and that is some miniscule fraction of a second) and others to the first 3 or 4 minutes.

    Now, let me reiterate that the two paragraphs above are my interpretation of what I have read and I don't guarantee that it's exactly right but it's what I believe is a correct interpretation/restatement of what I have read, and that in turn is, I believe, an accurate summation of current thought in physics regarding the big bang (at a high level of explanation).
  6. Aug 23, 2011 #5
    This thread brings to mind a related question that's been bugging me...

    If the universe is infinite, which seems to be quite possible, then it must have been infinite at birth as opposed to "growing" to infinity later -- right?

    If right, it becomes difficult for me to grasp that the universe began as a tiny singularity, like the size of an atom or even a watermellon. Makes my brain hurt. Can someone relieve my pain?
  7. Aug 23, 2011 #6


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    The problem here seems to arise from the fact that time and space STARTED at the singularity, in the "normal" model of cosmology, and matter expanded from there, BUT it was always ALL OF SPACE. I have never manage to really get my head around this confustion of "started small but infinite in size", so I'm just regurgitating what I have read.

    The main thing to take into consideration is that if the universe was not infinite at t=0 then it will never be infinite since you cannot "grow", in any sense, from finite to infinite, so if it's infinite now (and that's not proven) it always was.
    Last edited: Aug 23, 2011
  8. Aug 23, 2011 #7
    A lot of your article above is very will written, just a few minor clarifications: We can only really discuss t>0 with any accuracy; more specifically t>planck time. I believe you outlined this in a previous post though - just want to make you aware as this sort of minor mistake can really confuse total amateurs - I was confused for weeks on here!

    A universe expanding at a finite rate for an infinite amount of time becomes infinite in size at an infinite point in the future. I know this seems like word play but has crucial implications when talking about infinities - such as mapping a finite observer time to an infinite coordinate time (A freestanding observer watching an infalling observer of a BH)

    So I would amend this with the caveat that; "if the universe is not infinite now, it was not infinite at its "beggining", and if is not infinite now it could become infinite at an infinite time in the future."

    All very interesting!
    Last edited: Aug 23, 2011
  9. Aug 23, 2011 #8


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    How is this ANY different than what I said in the middle of post #2 ?

  10. Aug 23, 2011 #9


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  11. Aug 23, 2011 #10
    Yes I went on to say you had previously noted the difference, just important to be clear.

    Yes thanks for pointing out my grammar - I have amended this. What I am trying to do is generate a bit of food for thought for the OP. Yes nothing will ever get to infinity, that is my point; if the Universe is truly infinite now and was truly infinite at the big bang then it was the matter dense pertubations within expanding spacetime that would have been infinite - these than geometrically scaling as the scale factor increases due to expansion. Space is not seperable from spacetime, spacetime is the metric for U. That was my point which I probably didnt make too well.

    Thanks for noticing, sometimes in a rush at work! :smile:
    Last edited: Aug 23, 2011
  12. Aug 23, 2011 #11


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    Gotcha ... somehow I though you were refering to something similar I had said in a whole different thread and I didn't realize that you were in fact just reinforcing what I had said in THIS thread.

    Yeah, I know what you mean about getting rushed.
  13. Aug 23, 2011 #12


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    Problems like this usually start with the words used to translate technical/math models.

    "Singularity" can be infinite in extent. It should not evoke the idea of "tiny" in your mind.
    Technically it means where a theory breaks down.

    Expansion means a pattern of increasing distances. Space does not have to be finite in order for distances within it to increase according to some overall pattern. The word "expansion" should not evoke in your mind the picture of a finite body expanding within some surrounding empty space.

    The idea of "tiny" comes in with popularizers who want to convey the picture of extreme *density*. The entire chunk of the U that we are now seeing with our telescopes concentrated into the hind end of a gnat. Wow! Pretty dense, right?
    It does not mean that the universe was, at that time, tiny. It could have been infinite in size. Just the part we are now looking at was concentrated into a small volume.

    The standard model does not yet tell us finite or infinite, or if finite whether large or small.
    What the model tells us is expansion of distances started in a state of very high density.
    It is carefully designed to run without making unnecessary assumptions. Hopefully, as more data comes in, we may learn answers to more questions like finite/infinite and if finite then how big at suchandsuch time. For now, best to avoid mentally picturing more than we know. We think we know high density---OK, picture being in a space of very high density. Don't picture it surrounded by some other space or having some definite size because we don't know about that. Those things are outside the purview of the current math model.

    What? You say the current math model is incomplete? Yes of course it is incomplete. That is one of beauties of the math language. You can intentionally make a model spare so that it will fit the available data without overcommitting you to too much extra mental baggage.

    In English, by contrast, all the words have extra connotations that that drag in with them a confusion of unwanted expectations. Like "bang", what does that make you think of?
    Try to ditch the extra baggage and think of very high density with no overall size, with very high percentage rate of increase in distances initiated somehow. (that could for instance have been initiated by a contraction and bounce, what initiated it is still an open question that people are trying to answer.)

    this is basically just my two cents worth based on my own experience trying to understand. maybe it will help.
  14. Aug 23, 2011 #13
    Marcus, during the expansion of the universe from very high density to the present time, do we know or perhaps we just assume that the all known characteristics and physical constants of vacuum of free space have remained unchanged? I am thinking here of the balloon analogy where as two points separate the rubber of the balloon changes in various ways as it stretches.
    Last edited: Aug 23, 2011
  15. Aug 23, 2011 #14


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    By the tine the four fundamental forces 'separated' around t = E-12, there is good evidence all of the constants and rules of engagement in physics [as we know them] were fixed and have persisted unchanged to this day. It is, however, not impossible that certain aspects [both known and unknown] of the universe may have continued evolving over time by means not yet understood.
  16. Aug 23, 2011 #15
    Thanks Chronos, am I right that dark energy is one example of something that has changed in magnitude during the history of the Universe?
  17. Aug 25, 2011 #16
    Is the big bang possibly like virtual particles occuring in a vacum? So then space has always existed but without something happening within it, scales of distance and time are impossible to measure from within. This might also account for the distinct lack of observable antimatter in our universe. As it is there in the larger vacum just not in our space at this moment.
  18. Aug 25, 2011 #17


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    No, this does not fit with the currently accepted picture of what happened. Space-time (which mean BOTH space AND time) came into being at the singularity. Neither existed prior to that. Does this make any "sense" ? Not to me it doesn't, but I don't have any good alternate theories.

    There ARE alternate theories such as the one where two "branes" collided and caused the big bang inside of existing space-time, but I find those even harder to swallow.
  19. Aug 25, 2011 #18
    I was reading a book which states that many Physicists had great trouble believing in the existance of quarks because it is not possible to separate one and observe it, unlike other theories in the past. With such theories it is necessary to make accurate predictions to build up confidence in a model over time. I suspect that we can never be 100% certain that such a model really describes reality.
  20. Aug 25, 2011 #19


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    That puzzles me since quarks are fundamental to the Standard Model and that is widely stated to be the most experimentally solid theory in the history of physics. Who wrote that book? What QM phyiscists was it referring to?
  21. Aug 25, 2011 #20
    Steven Hawking :

    http://en.wikipedia.org/wiki/The_Grand_Design_(book [Broken])
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