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Friendly discussion: How do you feel the singularity came to be?

  1. Feb 2, 2007 #1
    Hello. It seems from what I’ve gathered on these forums since I’ve read and posted on here, it seems there are only a few possibilities of how the original singularity came to be, which caused the big bang. Those are:

    1.) Oscillating universe – The universe goes through a natural cycle of expanding, then eventually retracting back into a singularity to cause another big bang.
    2.) Possibly black holes – While this idea is far fetched, and most definitely has one of the least (if any) amounts of evidence, a black hole does form a singularity, which could possibly form its own type of big bang.
    3.) Unknown as of yet or “Magically appearing” – I know this one will cause some anger, however it seems this is implied a lot, even on these forums. When most people talk about the big bang, most of the time there lacks an explanation of how or why the singularity appeared. Obviously we know VERY little about the singularity and its implication is due to lack of evidence, which in my opinion is a good reason why not to give a reason. So this explanation is a very good “neutral” standpoint on the subject, and a well respected choice.
    4.) Universal creator – While I know that religion is prohibited on this forum, I will not give any religion at all any credit for this issue. However the truth is a lot of people strongly feel that this is a good reason (although I am neutral about this.) While I may take a neutral position on this, I feel that if one speaks of the big bang without explanation or reason, it lacks the same evidence needed that a universal creator requires, therefore should equally be a choice.

    So this leads to the point of the post. Which of these do you feel is the reason how our current universe began? Please do not bash anyone who feels differently, this is not a debate topic but a topic meant to give each one’s personal opinion. I personally feel that #1 is the answer. In my humble opinion, I don’t see the universe appearing from nothing, and the fact that almost everything in the universe has a cycle of breaking down, and reforming, I am going to say that the universe itself is a big cycle. Another explanation could be a combination of the choices above or if I left out a choice, please feel free to enlighten me and explain!!

    Thanks!!
     
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  3. Feb 2, 2007 #2

    EL

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    The big bang singularity is just a mathematical artifact of General Relativity. It's true that if GR was a universal theory, holding on all scales, the bb-singularity is a must. But since GR has turned out to be incompatible with quantum mechanics, we know it doesn't hold at the very early stages of the universe. Hence there's really no problem with the bb-singularity, simply because it only lives in a theory which we know doesn't work at the time when it predicts the singularity to exist...
     
    Last edited: Feb 2, 2007
  4. Feb 2, 2007 #3

    Garth

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    How do we know?

    Perhaps it is quantum mechanics that does not hold at the very early stages of the universe!

    Garth
     
  5. Feb 2, 2007 #4
    And so the race begins between the quantum theorists and astrophysicists!

    Who will win the coveted prize of figuring out the universe? Stay tuned.
     
  6. Feb 2, 2007 #5

    EL

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    We know quantum mechanics describes small scales very well, and that attempts of quantizing GR lead to a non-renormalizable theory.

    So are you suggesting that at even smaller scales (where even QM may break down) pure classical GR would take over again?
     
  7. Feb 2, 2007 #6

    EL

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    Luckily I'm into particle astrophysics so I'll always win! (And lose...) :rolleyes:
     
  8. Feb 2, 2007 #7

    Garth

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    GR describes the whole universe very well; when the whole universe is at a Planck scale why should the physics of the very small be necessarily superior to the physics of the whole universe? :confused:

    My question was, "How do we know?"

    The fact of the matter is we don't, and await a qg theory.:rolleyes:

    Garth
     
  9. Feb 2, 2007 #8

    EL

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    Yes, but that's my point!:smile:

    Of course there could be a singularity (even though I find it absurd), but then it must be a prediction by the theory unifying gravity and quantum physics, a theory we do not have.
     
  10. Feb 2, 2007 #9

    EL

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    I would rather say GR describes the universe at large (length) scales very well. In the early universe our entire observable universe was very tiny so one would suspect GR not to hold very well. On the other hand there are no reasons why the "physics of the very small" as we know it today shouldn't break down too...
    What we need is a unifying theory. What will come out of that one we can just speculate in at the moment. But, as said, I would find it rather dissapointing if something as unphysical as a singularity popped out...
     
    Last edited: Feb 2, 2007
  11. Feb 2, 2007 #10

    marcus

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    According to all the cosmological data I have seen, our universe is not expected to collapse. The mainstream consensus among cosmologists favors the "LambdaCDM" model, which continues expanding indefinitely.

    (The LamdaCDM universe can be either spatially finite, i.e. closed, or spatially infinite. In either case with the measured parameter values it expands forever.)

    So if we go by the model that the professionals think best fits the observations, we can exclude "Oscillating Universe", your #1, .

    ================

    You don't mention another obvious possibility, that there was a prior spacetime that DID collapse, leading to our big bang. I guess you could call this a "cyclical universe with only one cycle." Your Oscillating picture but not repeating, so there is only one oscillation.

    ================

    I don't understand your comment in #2. It seems to me that the Black Hole bounce picture has AT LEAST as much evidence in its favor as any of the others.

    A prediction was derived from this picture back in 1993, and has stood the test of many observations. Surviving observational tests is a kind of evidence. I would say this is the ONLY picture that has that kind of evidence supporting it.

    Please explain why you say "this idea is far fetched, and most definitely has one of the least (if any) amounts of evidence..."

    I think your assertion needs justification, because it seems to me that of the alternatives this idea has the most supporting evidence and is the opposite of far-fetched. Indeed it was proposed decades ago by the prominent physicist John Archibald Wheeler (Feynman's mentor) and has been studied by quite a few people. It could well turn out to be wrong! But it is a natural possibility for people to study.

    Leonardo Modesto had a recent paper exploring this---it is one of many written since 1993 or so, but as a recent sample I will fetch a link.

    Also there was a top-level workshop at Santa Barbara just this past month, one of whose main themes was "Do new universes emerge from black holes?"
    this was one of the 4 main questions which the chief organizer of the workshop posed at the beginning of the 3-week work period. It is a hot topic right now---do expanding classical spacetime regions evolve from black holes? They had top experts from a lot of places gathered to address this.

    Modesto wasn't at the Santa Barbara workshop though. I will get a link to his paper.
    http://arxiv.org/abs/hep-th/0701239
    Loop quantum gravity and the black hole singularity

    Here is a quote from page 15:

    "...It is interesting to recall that beyond the classical singularity the eigenvalue ... is negative and so
    we can suggest a new universe was born from the black hole formation process. In LQBH scenario
    pure states which fall into black hole emerge in a new universe as pure states and the information
    loss problem is avoided. Information is not lost in the black hole but it exists again in the space-time
    region in the future of the avoided singularity."
     
    Last edited: Feb 2, 2007
  12. Feb 3, 2007 #11

    First of all I don't think there was an 'initial singularity' since that would rather contradict about everything in physics. Observables and physical entities can't take on infinite values.
    Furthermore the most basic property of matter that it is in motion. Motionless matter is an absurdity. If you have matter in motion, which is itself indestructable and increateable, then basically what comes with it is a concept of space and time, which are the effect of matter in motion. I don't see that time and space are "existent" in themselves without matter/motion, that notion makes no sense.

    So basically your question is already wrong.

    Secondly, the cosmological model which applies to the universe as we observe it, and fits best the data, doesn't show a singularity near the beginning, but would lead to some form of inflation that fits the data rather good, and which is potentialy past eternal (once inflation starts, it can go on forever, which also means, it didn't need to have been started at all).

    So, that also means, we don't need an 'initial singularity' since inflation can show that the universe can be infinite in time and past eternal. And in fact such a model not only best fits the data, but also best fits with the foundations of physics.

    Memorize that time itself is not a physical entity or something, so there is no problem for time being past eternal. In fact on a fundamental level we just have matter in motion, which creates time and space, so in fact there is no independend spacetime of some sort. There are just fields (which is: matter in motion).

    And as a PS.
    Your 'universal creator' is some obvious mistake, since - were it existent - it would be part of the universe and thus of the puzzle and we don't need to discuss it seperate then because it is already covered in the discussion on the universe, and were it not, for obvious reason also we don't need to discuss it then.
    Your obviously struck in a 'self-created' unsolvable puzzle, since if at some point you stick to the idea that somehow 'everything' came from 'nothing' and in that matter treat them (which are: being and non-being) as absolutely seperate, then of course, no development / change or begin of anything is possible, and the begin is made incomprehensible. Dialectics sees this more clearly in treating being and non-being as just opposite sides of a dialectical-unity, which is becoming. The becoming of something is just the simultanious end of it's non-being and the start of it's being, which for obvious reasons, can't be seperated. Being and non-being can't be seperated but belong to each other as two sides of a coin or two poles of a magnet.
     
    Last edited: Feb 3, 2007
  13. Feb 3, 2007 #12
    superposition- ALL processes capable of producing the universe and our past exist- the universe is the sum- the probability space would be heavily dominated by something analogous to the simplist Universal Turing Machines-basic boolean logic systems- so simple rule systems almost entirely determine all physical 'laws'- something equivalent to a very simple cellular automaton- or a small network of universal logic gates
     
    Last edited: Feb 3, 2007
  14. Feb 5, 2007 #13

    marcus

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    Maybe you should say "how the beginning of expansion" came to be

    Nexus, I notice that we didn't seem to manage to have a "friendly discussion" and I think it might just be the SEMANTIC IRRITATION of your saying the word SINGULARITY.

    I realize that you don't really mean singularity in the scientific sense (the failure of a particular theory---in this case Gen Rel) you surely must mean something real in nature: the conditions at the beginning of expansion!

    The scientific definition of a singularity is something that does NOT occur in nature, but an artificial thing that happens in theories----they blow up.
    Then further theoretical work can sometimes fix them and get rid of the singularity. A singularity is something (hopefully) to fix and thereby get rid of.

    So maybe we could agree to change your invitation to a friendly discussion to read How do you feel the beginning of expansion came to be?

    Then there are some clear possible answers that have been offered by quantum cosmology work done since 2001, and one can discuss such things without the unnecessary confusion surrounding the word "singularity".
     
  15. Feb 6, 2007 #14

    hellfire

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    The only way to compare models of quantum cosmology is by their theoretical features, because currently there are no experiments that might favour any model against others. The only realistic way to differentiate between models is the analisis of high precision measurements of the anisotropies of the cosmic microwave background. Deviations from Gaussianity or scale-invariance of the power spectrum of density perturbations may be hints about the mechanisms that generated the first seeds of large scale structures, and will be tested in future experiments such as Planck.

    For example, in the inflationary paradigm, a pure de-Sitter exponential expansion would lead to a scale-invariant power spectrum. However, since inflation must slow down, small deviations from scale-invariance are expected, depending on how fast inflation slows down and eventually depending on the shape of the scalar field potential that is responsible for the inflationary expansion. Thus, in the realm of the standard cosmological model (that contains an inflationary epoch), quantum cosmological models can be tested in their ability to provide as output an inflationary period with a specific duration and a specific scalar field with its potential. This is at least an indirect test, because one has to face the problem that inflation washed out any initial conditions leaving only little room for a quantum cosmological imprint.

    From this point of view I can only agree with the reformulation of the question made by marcus as well as with the reference he has posted here. The model of loop quantum cosmology is a very attractive one. It is based on loop quantum gravity, a theory that quantitzes in a standard way (Dirac canonical quantization) the gravitational field as described in general relativity. This road was already explored some decades ago leading the so called canonical quantum gravity, however, in a slightly different approach with different variables describing the gravitational field. Cosmological models based on canonical quantum gravity had very difficult problems to solve such as the initial contidions at the classical singularity and the dynamics at the singularity. Some of these models (such as Hartle-Hawking or Vilenkin models) might be still valid in a semiclassical approximation, explaining some features about the origin of inflation, but they definitely do not tell us anything relevant about the initial classical singularity. The theory of canonical quantum gravitation they were based on was ill defined and with lots of unsolved problems.

    The question that the loop quantum cosmology model tries to answer is basically about the conditions and dynamics of the classical singularity. It predicts repulsive gravitation at high energies and avoids at least partially the problem of initial conditions due to the use of difference equations instead of differential equations. It predicts the origin of the expansion as a consequence of some classical contraction phase. These are genuine quantum cosmological features that none of the models such as Hartle-Hawking or Vilenkin were able to solve succesfully. On the other hand, other issues like the arrow of time, the choice of an quantum mechanical interpretation or the inclusion of inhomogeneities remain still unclear.

    Of course this is not the only model, but other radically different approaches like brane cosmology or cyclic scenarios, some of them based on string theory, do also suggest that our expansion phase is the result of a contration phase. These models do not currently explain how exactly the universe behaved at the classical singularity, but provide an more general picture about the dynamics of the classical expansions embedding it in a series of contration and expansion phases.
     
    Last edited: Feb 6, 2007
  16. Feb 6, 2007 #15

    marcus

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    This is a careful and perceptive summary. Thanks for posting it and for the extra light shed on loop quantum cosmology. Apropos of that, today a new article was posted on arxiv that gives an up-to-date status report on LQC.

    http://arxiv.org/abs/gr-qc/0702030
    An Introduction to Loop Quantum Gravity Through Cosmology
    Abhay Ashtekar
    20 pages, 4 figures, Introductory Review

    "This introductory review is addressed to beginning researchers. Some of the distinguishing features of loop quantum gravity are illustrated through loop quantum cosmology of FRW models. In particular, these examples illustrate: i) how `emergent time' can arise; ii) how the technical issue of solving the Hamiltonian constraint and constructing the \emph{physical} sector of the theory can be handled; iii) how questions central to the Planck scale physics can be answered using such a framework; and, iv) how quantum geometry effects can dramatically change physics near singularities and yet naturally turn themselves off and reproduce classical general relativity when space-time curvature is significantly weaker than the Planck scale."
     
  17. Feb 8, 2007 #16
    Hmmmm.

    What about: very stretchy.... :wink:
     
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