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Singularity and size of universe

  1. Nov 14, 2012 #1
    Hello all .
    what's size of universe before the big bang ?
    We consider big bang after planck time or before planck time ?

    There was no singularity in big bang is that true ? loop quantum cosmology says that
  2. jcsd
  3. Nov 14, 2012 #2


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    We don't know. Our math breaks down and we can no longer make predictions beyond a certain point in time.
  4. Nov 14, 2012 #3


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    LQC is one model, and it is getting a growing amount of attention from researchers. You are right about what it says. The main thing now is to TEST the predictions made by LQC and other non-singular models, about what detailed patterns we expect to see in the Cosmic Microwave Background.

    There are other models which replace the singularity with a bounce and although they aren't as well known (I've seen references to them but haven't studied them, myself) they should all be tested. The LQC bounce should leave its own distinctive mark on the CMB temperature and polarization map. Researchers should be able to tell the difference, as continuing observations pick up finer and finer detail.

    The most recent LQC paper on this came out this month, by Agullo, Ashtekar, Nelson. I will get links.
    All Ashtekar's papers:
    Earlier short paper giving general idea:
    Very concise 4-page paper summarizing their most recent work:
    Long 60-page paper with lots of technical detail, on which the 4-pager is based:

    Ashtekar writes less technical articles for wider audience, from time to time. But this latest work has NOT been popularized yet or explained for general reader. You can always get a certain amount from the overview in the introduction and the conclusions section at the end of any technical paper. So they are not totally useless to non-specialists. You can give it a try and also people here, like myself, can try to interpret.

    there is a nice feature of the LQC bounce which is that it has its own built-in brief "super-inflation" period which happens inevitably because of the mechanism that causes the bounce. As I recall this by itself accomplishes an expansion by a factor of roughly 5000 and it leads into a conventional inflation episode if the right kind of scalar field is present.

    The overall effect of "pre-inflation" LQC expansion is to make it more likely that an adequate conventional inflation will ensue, to achieve the full amount cosmologists expect. So this brief "pre-inflationary" period in the LQC model has gotten the researchers interested and is what is being studied in these recent papers. What kind of quantum fluctuations might arise during it? What kind of imprint might they leave on the CMB?
    Last edited: Nov 14, 2012
  5. Nov 14, 2012 #4


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    Actually that little 4 page paper has a lot in it that is pretty understandable. Here is the abstract summary. What they mean by "extension" is extension back in time, to before the usual inflation scenario:

    A Quantum Gravity Extension of the Inflationary Scenario
    Ivan Agullo, Abhay Ashtekar, William Nelson
    (Submitted on 7 Sep 2012)
    Since the standard inflationary paradigm is based on quantum field theory on classical space-times, it excludes the Planck era. Using techniques from loop quantum gravity, the paradigm is extended to a self-consistent theory from the Planck scale to the onset of slow roll inflation, covering some 11 orders of magnitude in energy density and curvature. This pre-inflationary dynamics also opens a small window for novel effects, e.g. a source for non-Gaussianities, which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
    4 pages, 2 figures

    Expanding the linear scale by a factor of some 5000 would reduce the energy density by a factor of about 50003 which is around 1011. They say reducing the energy density by some "11 orders of magnitude" which is why I gave a linear scale factor of 5000. It's not a lot, but it's nice the bounce gives you that for free. After that, you need a scalar field which these 3 young European researchers (Andrea Dapor and two friends) say could even just be the Higgs field!!! The Andrea Dapor paper is
    Inflation from non-minimally coupled scalar field in loop quantum cosmology
    Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
    (Submitted on 18 Jul 2012)
    The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation.... Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
    14 pages, 5 figures

    So it's not all there yet, but a picture is taking shape, and it (and other bounce models) need to be checked against more detailed observation of the CMB ancient light.
  6. Nov 24, 2012 #5
    Anybody can explain all time periods in big bounce theory ?

    What's initial size of universe in big bounce ? Planck dense ?
  7. Nov 24, 2012 #6


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    Planck density is a good guess. According to the usual LQC picture, quantum gravity effects cause the bounce to occur when the energy density of the U is about 40% of Planck density. You can look up Planck units of energy, volume, density etc. in WikiP.

    That figure (actually slightly over 40%) is in the Ashtekar paper I linked.

    What the volume was, at the bounce, will depend on what you think the volume is NOW.

    The presentday spatial volume of the U is not known, although we have LOWER BOUND estimates. That is, with some degree of confidence we can say that if the U is not infinite volume then it is AT LEAST such and such. And we have estimates of the energy density, now and at various epochs in the early universe. So I guess a size estimate could be worked out. It would be a lower bound estimate on the size of space at the moment of bounce. Not sure I want to do the math, maybe someone else would like to :smile:

    There are much better estimates of the size of the currently observable region of the universe, but of course that is not the whole thing. Maybe that is what you are asking, the size that the currently observable part of it was, at the time the whole thing bounced.
    Last edited: Nov 24, 2012
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