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B Questions regarding pre Big Bang

  1. Feb 10, 2016 #1
    Okay, first off let me say hello, I am new here.

    I am creating this thread in the hope that I can get some answers to some questions that are bugging me. Please keep in mind that I am not educated in physics so some of my questions might seem silly. Also, feel free to write responses as lenghty as you feel, the more precise you explain it, the better.

    1) What does the B.G.V theorem tells us?
    a. Does it say "our Universe" had a beginning or "The Universe" (aka "everything") had a beginning?
    b.One common argument I hear is the the B.G.V theorem uses classical space-time and since we don't have a theory of quantum gravity, the theorem does not hold. But I think Alexander Vilenkin addressed this.
    c. I remember that Sean Carroll got Alan Guth to say that he thinks the Universe is in his view Eternal. You might say that is his opinion but how can one of the authors of a paper that shows the Universe had a beginning say that?

    2) From what state does the Universe begin to exist in the B.G.V theorem?
    a. Is it timeless?
    b. Can it be eternal?
    c. Does it need a cause? Can it just BE?
    d. Is it immaterial, spaceless, timeless ?

    3) What is the "physical nothing" Lawrence Krauss talks about?
    a. Again. Can it be or is it eternal or timeless ?

    4) If our Universe began to exist and had a cause does that cause have to be immaterial, timeless and spaceless?

    5) Could I get an opinion on the following "The definition of “singularity in the past” is not really the same as “had a beginning” — it means that some geodesics must eventually come to an end. (Others might not.) Most importantly, I don’t think that any result dealing with classical spacetimes can teach us anything definitive about the beginning of the universe. The moment of the Big Bang is, if anything is, a place where quantum gravity is supremely important. The Borde-Guth-Vilenkin results are simply not about quantum gravity." - Sean Carroll

    6)I heard that hartle-hawking no boundary proposal shows that our Universe could have a beginning but no cause. Can someone explain how that works?

    Feel free to answer my questions in whatever format you want.
  2. jcsd
  3. Feb 10, 2016 #2
    I don't think the Borde-Guth-Vilenkin singularity theorem has ever been peer-reviewed by any reputable groups, so I would be skeptical to give it any credibility unless you can point to a peer-reviewed article in a journal such as Nature or something.
  4. Feb 10, 2016 #3


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    The answer requires knowledge of conditions at the time of the big bang. Unfortunately, we have virtually zero knowledge in this arena, only speculation - some more informed than others. It currently appears an understanding of quantum gravity is needed to answer many of the questions about the very early universe. This is an active area of research which has only enticed us with a few clues, but, no real 'facts'. We may never know the answer with any certainty, but, hope blooms eternal.
  5. Feb 10, 2016 #4
    I believe I am wrong. At least they have published something in Physical Review Letters, vol. 90, Issue 15, id. 151301.
  6. Feb 10, 2016 #5


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    Although the OP is phrased as a bunch of questions about the BGV singularity theorem, the Sean Carroll blog from which it quotes, http://www.preposterousuniverse.com/blog/2012/09/25/let-the-universe-be-the-universe/ , has a good explanation of why a specific technical result such as this one is not an OMG Final Answer to Everything. Keep in mind that we don't even have any solid evidence that inflation existed, or that any particular model of inflation is right. Therefore a result like BGV is relevant only within a very narrow set of theoretical assumptions.

    The OP would in my opinion be well advised to back off from all of these overly specific questions about an overly specific technical result and ask questions about big bang singularities and cosmological models in general, especially simpler models. All of these questions phrased in nonmathematical language ("From what state does the Universe begin to exist in the B.G.V theorem? a. Is it timeless? b. Can it be eternal?") are basically meaningless in the form in which they were posed. In order to gain understanding the OP would need to start by learning how to pose simpler questions and refine them into meaningful simple questions.
  7. Apr 5, 2016 #6
    The BGV theorem was designed to address the question can inflation avoid the singularity ? At first glance it appears that inflation can avoid the singularity as it violates the energy condition of the Penrose Hawking singularity theorem. However the BGV theorem shows that inflation alone cannot avoid the singularity . But that does not mean there really is a singularity/beginning of time. You can extend the space-time to before simply violating the condition of the theorem e.g. have a prior contracting space-time. So the BGV theorem is interesting but does not prove the universe has a beginning.
  8. Apr 6, 2016 #7


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    At the time of the big bang, all physical theories break down. So we can have no knowledge of 'before'.
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