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A Brief History Of Time (Stephen Hawking) FAQs -- Big Bang & Multiverse question

  1. Jan 3, 2015 #1
    If the universe was created by the Big Bang, then how did Multiverse come into existence?
     
  2. jcsd
  3. Jan 3, 2015 #2

    phinds

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    There is absolutely ZERO evidence that there IS a multiverse. Many of us think it is an unnecessary mathematical fiction.
     
  4. Jan 3, 2015 #3

    Garth

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    I absolutely agree with you phinds, however many in the community do not and for them the multiverse is as real, and as a consequence of, inflation.

    And thereby hangs a tale!

    Garth
     
    Last edited: Jan 3, 2015
  5. Jan 3, 2015 #4

    phinds

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    Oh, yeah there are notable physicists who contend that the multiverse is inevitable but I think they are carried away by the implication of quantum mechanics and the thesis that "if something CAN happen then it HAS TO happen", which I don't agree with.
     
  6. Jan 3, 2015 #5

    HallsofIvy

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    What does this have to do with 'A Brief History of Time'?
     
  7. Jan 3, 2015 #6

    Chalnoth

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    The big bang is fiction. It arises from carrying certain models of the universe back in time to higher energies than those models can possibly describe with any accuracy.

    As far as the multiverse, there are a number of different multiverse ideas. You'll have to be more specific.
     
  8. Jan 3, 2015 #7

    martinbn

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    So what is the energy limit that GR can model with accuracy? Why should there be any limit? There aren't any experiments or observations to show that.
     
  9. Jan 3, 2015 #8

    phinds

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    The problem is taking it to the point where the energies become infinite. It is infinities that are generally called "singularities" in physics because they are not believed to represent reality but rather a place where the theory breaks down and the math gives an answer that is not likely to represent physical reality.

    Some care is needed in interpreting Chalnoth's statement "the big bang is fiction". There are two distinct meanings to the phrase "big bang". One is the "big bang singularity" which is what he is talking about and the other is the "Big Bang Theory" which is a description of the evolution of the universe starting at about one Plank time after the singularity. the Big Bang Theory is not fiction at all, although it still has some holes (is "inflation" real or not)
     
  10. Jan 3, 2015 #9

    martinbn

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    Right, I had the big bang singularity in mind as well. But in that case the infinities are only in the limit. There are no points on the space-time manifold where anything is infinite. There are incompete geodesics and things get to infinity along them in finite proper time, which is strange but mathematically perfectly fine, no energies that are infinite at any point of the manifold.
     
  11. Jan 3, 2015 #10

    marcus

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    For a long time the inflation theorists couldn't come up with any mechanism by which
    1) an adequate inflation episode was likely to occur, but
    2) the mechanism wouldn't run hog wild and generate an infinity of universes.

    So as Garth points out many in the community thought they had to accept multiverse if they wanted to have inflation. But several researchers (including V. Mukhanov, a major world class figure in theoretical physics and cosmology) have now come up with versions of inflation that don't need to repeat ad infinitum i.e. inflation without "self-reproduction."
    http://arxiv.org/abs/1409.2335
    Inflation without Selfreproduction
    Viatcheslav Mukhanov
    (Submitted on 8 Sep 2014)
    We find a rather unique extension of inflationary scenario which avoids selfreproduction and thus resolves the problems of multiverse, predictability and initial conditions. In this theory the amplitude of the cosmological perturbations is expressed entirely in terms of the total duration of inflation.

    The nickname for Viatcheslav is "Slava", so you often see him referred to as Slava Mukhanov. He's the author of a well-regarded textbook called The Physical Foundations of Cosmology.
    As he points out, if you think you are forced to accept the "multiverse" idea, you have to give up trying to understand and explain some essential features of the universe we actually see, and live in. It weakens predictability and diminishes the challenge to physicists to explain the way our universe is. So Slava's September 2014 paper is heartening to those who are disgusted by the "multiverse" cop-out.

    To agree with what Phinds said: AFAIK there is zero evidence that a "multiverse" actually exists AND furthermore, if you believe Slava, you can posit inflation without accepting a proliferating cosmic multiplicity.
     
    Last edited: Jan 3, 2015
  12. Jan 3, 2015 #11

    marcus

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    Priyank, there is no evidence that the universe was "created" by the start of expansion. There are models which fit the observational data just as well according to which the current expansion was preceded in time by a contraction of the universe, which rebounded.

    Time does not necessarily stop, and density does not necessarily blow up, as you run a model back in time. That could just be the fault of the model. The classical model failed around the start of expansion (which temporarily convinced a lot of people that must be "the creation of the universe" and "the beginning of time") but other people saw that as a symptom that the model needed improvement, so they fixed the model. Some new models don't fail. It could simply be that one needs to quantize the cosmic model, since quantum nature resists being pinned down and confined by infinitely precise restrictions.
    Heres a recent paper by two young researchers as an example:
    http://arxiv.org/abs/1412.2914
    A ΛCDM bounce scenario
    Yi-Fu Cai, Edward Wilson-Ewing
    (Submitted on 9 Dec 2014)
    We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
    14 pages, 8 figures
    If you would like to read the paper, the pdf is at http://arxiv.org/pdf/1412.2914v1.pdf
     
    Last edited: Jan 3, 2015
  13. Jan 4, 2015 #12

    Chalnoth

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    It's not completely clear. Certainly it can't go above the Planck scale, though there's a fair chance it starts to become inaccurate before that.

    The big bang requires that GR be accurate to infinite energy, however.
     
  14. Jan 4, 2015 #13

    martinbn

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    What's the reason for that? As it stands it seems as an expectation, which may turn out to be correct or not, but is there an argument for it.

    That's what I don't understand! I would phrase it as the big bang requires that GR is accurate to arbitrarily large energies. But I wouldn't say infinite.
     
  15. Jan 4, 2015 #14

    ChrisVer

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    GR is a classical theory. At the Planck Scale however, the gravity in the elementary particles level becomes comparable to the rest of fundamental forces (or GUT). So you'd expect for quantum gravity effects to rule over the classical predictions of GR...as a result, it starts not giving accurate results (we don't have a quantum gravity theory)

    Infinite is just a word... you could translate it as any arbitrary "large" number if you like it more that way. But "arbitrary" can send you as much high as you can imagine...
     
  16. Jan 4, 2015 #15

    martinbn

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    Exactly, it is what I would expect, but is only an expectation. It need not be the case.

    I don't mind the word. What I meant was that there are no infinities. Here is an oversimplified example the function [tex]y=x^2[/tex] is defined for all real numbers and does not become infinite at any real number. It grows to infinity, it can have as large values as you wish, but it is never infinite. So there's absolutely no problem with this function. Same with the space-time manifold (for the FLWR case or any other) all fields have finite values at any point of the manifold. Some may grow unbounded in some directions but that is not the same as being infinite (having an infinite value) at a given point.
     
  17. Jan 4, 2015 #16
    I read all of your views, but honestly speaking, I understood very little. I am very keen about this universe and its functioning.
    My next question to you all is : Can Tachyonic world bear some organisms having v=c. Can they come to The Einsteinian world, the world in which we live...?
     
  18. Jan 4, 2015 #17

    phinds

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    If you are interested in the universe and how it works, why not focus on the REAL universe instead of hypothetical things that probably don't even exist? I can assure you there are plenty of interesting thing to focus on in the real world.
     
  19. Jan 4, 2015 #18

    ChrisVer

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    What other scenarios are you talking about? It's in fact how you define the Planck scale... by setting the gravitational force equal to the rest... It's more than an expectation, it's a definition.

    There is no such a value as infinite.... I think you are misunderstanding the concept of infinite.... Infinite is the arbitrary large number... by following the sequence that let's you choose [itex]10^{20}GeV[/itex] or [itex]10^{50}GeV [/itex] or [itex]10^{1000} GeV[/itex] and so on, you reach infinite...Infinity in physics only makes sense in limits and not in equalities.
     
  20. Jan 4, 2015 #19
    #phinds, please give me the real topics which you are telling abt?
     
  21. Jan 4, 2015 #20

    phinds

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    Start by reading the entries in the cosmology FAQ on this forum.
     
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