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Prior to the Big Bang Question

  1. Mar 20, 2009 #1
    What was the force holding the matter together before the Big Bang?
    Gravity, electromagnetic forces, or something completely different?
     
  2. jcsd
  3. Mar 20, 2009 #2
    Big Bang is NOT an explosion of something into empty space
    Before Big Bang there was neither time nor space, so question is meaningless
     
  4. Mar 20, 2009 #3

    marcus

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    Paul, if you want to ask questions about conditions before the Big Bang then you have to go to quantum gravity models that deal with that.

    The main line of research that is currently dealing with conditions before and at the BB time is called Loop Quantum Cosmology.

    The way you should ask the question is "what caused the temporarily very high density at the Big Bang moment according to LQC?"

    If you ask that question in the context of according to classical non-quantum cosmology it cannot be answered because classical cosmology does not say.

    Here is a keyword search of professional articles since 2006, keyword "quantum cosmology".

    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    The papers are listed with the most highly cited ones first. These tend to be the most important papers.

    The top 20 papers are almost all about explaining the Big Bang as a bounce, or rebound from a prior collapse. A mechanism has been conjectured that makes this happen and there has been a surge of interest in studying this.

    In this context it is very easy to answer your question! The extremely high density only occured briefly during the bounce, and was produced by the prior collapse. And the large but finite density that was achieved can be calculated within the context of the LQC bounce model.

    Feel free to ask if you have more questions and I will see if I can answer---others may as well. I didn't give you that Stanford technical research link expecting that you would read the articles, but just so you can see what the professional literature on this subject looks like.
     
    Last edited by a moderator: May 4, 2017
  5. Mar 21, 2009 #4
    Marcus, I remember I red when I was young (in 80x :) ) that as(if) entropy is preserved during the bounce, then the entropy after the Big Bang should be ery high, and it is not cosistent with the obserations. So, what is different now?
     
  6. Mar 21, 2009 #5

    marcus

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    Thermodynamic entropy requires the viewpoint of an observer who decides what is microstates and what is macrostates----so how many microstates are subsumed in one macro.
    The Second Law is something that holds as long as you have a consistent classification of states. And it rules out various things, that no observer shall be able to see, and machines no observer shall be allowed to build:biggrin:

    But isn't it naive to suppose that two observers, one Before the crunch and one After the bang, are really the same observer and share exactly the same map of state space?

    I don't think the Second Law is even meaningful in this situation.
    whose map of state space, whose micro/macro distinction? Mr Before's or Mr After's?
    Who is going to build a perpetual machine to extract illegal work and violate the law?

    In other words the operational meanings of the second law that we are used to do not apply when the whole universe collapses and then re-expands.

    That's my guess anyway, Dima. I'd like to hear your opinion.
     
  7. Mar 22, 2009 #6
    Thank you. I understand your logic.

    When I first read about the 'bounce' I had another explanation: imagine an infinite chess board. When you increase density (put more and more figures) the number of combinations increases but then it starts to decrase when density >50%. Finally, when all squares are filled there is only 1 possible state so all history is lost and entropy is 'reset'

    However, that was before I read about the black hole informational paradox - so it appears that information and entropy are believed to be so fundamental that they are believed to survive even the singularity... No now I am not sure that our both explanations are valid.

    For example, I can critisize your "Who is going to build a perpetual machine to extract illegal work and violate the law?" by saying that in your case we cant talk about the entropy during the inflation era and soon after that, because there could not be any obserers and machines, in principle (empty space or too hot)
     
  8. Mar 22, 2009 #7

    Chronos

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    Even logic fails prior to the big bang. Until we have observational evidence affirming or denying the propositions, such discussions are philosophy, not science. Not that I object, just wish to keep it in perspective.




    w
     
  9. Mar 22, 2009 #8

    apeiron

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    Yes, but. The flocking behaviour of post-docs is a subject in itself. How many times in the past 30 years have such examples of enthusiasm born concrete fruit?

    The big bounce story has to contend with a certain difficulty called dark energy. If there is an acceleration (as observation suggests) then how does gravity achieve the recollapse?

    Of course, there is a frantic search going on to find a mechanism to change the sign of the acceleration. And perhaps they may even find it.

    The other major reason to object to the big bounce is the second law. Again, what we actually observe is accelerative expansion and an asymptotic approach to a heat death. So any discussion of a collapse, and the thermodynamic consequences of a collapse are at the moment complete speculation.
     
  10. Mar 22, 2009 #9
    Yes, an yet another thing: fine tuning that our universe is almost flat. And that fine tuning had to be extremely precise!
     
  11. Mar 22, 2009 #10

    marcus

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    Heh. Mr After has pretty thick skin, Dima. He tolerates high temperature in any regime mild enough for temperature to be meaningfully defined. But seriously, we ourselves are representative of the observers after the big bang, and we can talk about entropy during inflation. Our thermodynamic perspective can extend continuously all the way back through the presumed inflation episode. All the stuff that happened since the start of expansion can be pictured as a continuous trajectory in our state space. We define the macro and micro states from our ("After") perspective. Remember that inflation is described within a classical geometry framework.

    I am skeptical of the Second Law only within a few Planck times of the bounce, because no classical geometry exists there. I suspect there can be no continuity between the perspective of someone before and someone after.

    BTW this is model dependent, certainly! We don't know conditions around the start of expansion, all we have is some models. I think you were asking about LQC, so we have to look at that model and try to guess how entropy would be calculated.

    In that model the convergence to classical GR is very fast. A few Planck times after the bounce (and the definition of time is problematical, but very quickly) the LQC model closely approximates GR. It also gives a good approximation of classical spacetime up to shortly before the bounce. Perhaps i should say classical geometry rather than spacetime.

    There is a brief hiatus---sometimes called the quantum regime---when it is suspected that there is no geometry, concepts like dimensionality would presumably not be defined. I don't see how a macro quantity like temperature could be defined either.

    Obviously I am not an authority about the LQC bounce model. We could write email to someone knowledgeable and ask about this, or do a literature search and it might turn out that I'm wrong. But mainly I am just skeptical that thermodynamics can be well-defined during that brief quantum regime.

    LQC is fairly new, there may be research on this very question that I just don't know about.

    Why is that a relevant question, Apeiron? LQC does not predict a recollapse. It describes a prior collapse, which could depending on which model, be a unique one-time event. There are some interesting questions connected with collapse in the case of positive cosmological constant and Ashtekar has a paper in preparation about this, co-authored with Tomasz Pawlowski. It has been cited in something I saw but I haven't seen the paper yet. I hope if you are interested that we can discuss it when it comes out.
     
    Last edited: Mar 22, 2009
  12. Mar 22, 2009 #11

    marcus

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    Are we still talking about the LQC model (or models)? I hope so. I don't fully understand how your comment applies. Certainly it is true.
    The usual solution to that puzzle is inflation.

    LQC accommodates inflation, they recently (2008) started putting that in and running computer models of the bounce with inflation. A recent review article would cover that, if you want I'll fetch a link.

    Again, I am skeptical of the need for inflation. There could be some other reason for the amazing flatness of the early universe. But I can't offer one so if you think you need inflation to get flatness you can certainly have inflation in the LQC context.

    Keep in mind that LQC is quite new. A revolution occurred in 2006 where a new dynamics was introduced, it has no name except "the improved dynamics". The picture of the bounce changed quite a bit. They are still busy filling in the picture.

    I should really get a link to the latest Ashtekar review paper, something 2008 or 2009, and we should look it over to see where they are at the moment.
    Ashtekar is the representative author, when one is talking about LQC. To make sure we are all talking about the same thing here are links to some of his recent papers.

    In looking back, I see that one of the recent papers happens to involve entropy. So I took the liberty of including the abstract. It shows that Ashtekar has been thinking about that as well, in the LQC context.

    http://arXiv.org/abs/0903.3397
    Loop quantum cosmology of Bianchi I models
    Abhay Ashtekar, Edward Wilson-Ewing

    [Dima, notice that they are extending LQC to cover less symmetric cases. The isotropy condition is being relaxed.]

    http://arXiv.org/abs/0901.0933
    Quantum field theory on a cosmological, quantum space-time
    Abhay Ashtekar, Wojciech Kaminski, Jerzy Lewandowski

    http://arXiv.org/abs/0812.4703
    Singularity Resolution in Loop Quantum Cosmology: A Brief Overview
    Abhay Ashtekar

    http://arXiv.org/abs/0805.3511
    The covariant entropy bound and loop quantum cosmology
    Abhay Ashtekar, Edward Wilson-Ewing
    15 pages, 3 figures; Phys.Rev.D78:064047,2008
    (Submitted on 22 May 2008)
    "We examine Bousso's covariant entropy bound conjecture in the context of radiation filled, spatially flat, Friedmann-Robertson-Walker models. The bound is violated near the big bang. However, the hope has been that quantum gravity effects would intervene and protect it. Loop quantum cosmology provides a near ideal setting for investigating this issue. For, on the one hand, quantum geometry effects resolve the singularity and, on the other hand, the wave function is sharply peaked at a quantum corrected but smooth geometry which can supply the structure needed to test the bound. We find that the bound is respected. We suggest that the bound need not be an essential ingredient for a quantum gravity theory but may emerge from it under suitable circumstances."
     
    Last edited: Mar 22, 2009
  13. Mar 22, 2009 #12

    marcus

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    Since Dima's initial question to me was about entropy and the Second Law in LQC context, I will quote what Ashtekar said on page 24 of this paper:

    http://arXiv.org/abs/0812.0177
    Loop Quantum Cosmology: An Overview
    Abhay Ashtekar
    To appear in the Proceedings of the Bad Honef Workshop entitled Quantum Gravity: Challenges and Perspectives, dedicated to the memory of John A. Wheeler
    (Submitted on 30 Nov 2008)
    "A brief overview of loop quantum cosmology of homogeneous isotropic models is presented with emphasis on the origin of and subtleties associated with the resolution of big bang and big crunch singularities. These results bear out the remarkable intuition that John Wheeler had. Discussion is organized at two levels. The the main text provides a bird's eye view of the subject that should be accessible to non-experts. Appendices address conceptual and technical issues that are often raised by experts in loop quantum gravity and string theory."

    Dmitry, this passage starting on page 23 has some relevance to your question since it deals with the concepts, although I don't think it provides a conclusive answer. The highlighting for emphasis here is mine, not Ashtekar's :biggrin:

    "...This result suggests the following overall viewpoint on the entropy bound. Recall that the bound is strongly motivated by the generalized second law of thermodynamics (which also does not have a sharp, definitive formulation). Now, already the standard second law of thermodynamics is a deep fact of Nature but it has a ‘fuzziness’ which is not shared by other deep laws such as energy-momentum conservation. In particular, the second law requires a coarse graining in an essential way. [Dima, my point was "whose coarse-graining?" Does God have a favorite one? or do we use Mr Before and Mr After's coarse-graining in discontinuous alternation?]

    It is not a statement about the evolution of micro-states; in a fundamental theory their dynamics is always time reversible (leaving aside, for simplicity, quantum measurements). Rather, it is a statement about how the number of micro-states compatible with a pre-specified coarse graining changes in time. For specific processes, the increase of entropy can be calculated using statistical mechanics. But this entropy has little relevance to the fundamental dynamics of micro-states and is not an input in the construction of statistical mechanics. In the same vein, it seems unlikely that covariant entropy bounds would be essential ingredients in the construction of a quantum theory of gravity. It seems more natural to expect the covariant entropy bound should emerge from a fundamental quantum gravity theory under suitable conditions. Returning to the LQC calculation..."

    My intuitive feeling is that for a few Planck times around the bounce, for that brief interval of quantum regime, there are only microstates. No meaningful coarse-graining exists. Classical geometry has ceased to exist and will re-emerge momentarily (a voice on the loudspeaker says "we apologize for the interruption of service and hope that it causes you no inconvenience" :biggrin:)
     
    Last edited: Mar 22, 2009
  14. Mar 22, 2009 #13
    Marcus, but are you feeling comfortable about the whole idea of the bounce?

    The only Big Bang and our Universe - I am ok with it.
    Multiverse with the colliding branes, creating new universes with different parameters again and again - no problem.

    But the bounce... Our expansion is accelerating. Hence, our expansion is the last one. What was before?

    An infinite or finite sequences of smaller bounces? Or even a stranger solution where before the rebounce there was a contraction from t= - infinity?

    I would say, God does not like the rebounce :)
     
  15. Mar 22, 2009 #14

    marcus

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    Dima, I enjoy your comment very much! Your perspective is different from mine. My bias is toward the empirical observational as opposed to aesthetic.

    I would like to see formal predictions about CMB and gravitation waves derived from Lqc bounce models and compared with data. There have been some papers moving in that direction. The Lqc bounce must have a signature.

    Right now I do not feel the urge to ask "what came before" the bounce. At least not long before. The origin of the universe is a remote question that doesn't concern me at the moment.
    I only think "what replaces the singularity?"

    The singularity must be wrong, it is just a failure of theory. The bounce is the simplest of all what can replace it. It requires no extra dimensions and elaborate machinery (like branes which moreover do not work very well).

    With the bounce you do not have to "make up" or fantasize anything. You essentially just quantize Friedman (the equation already in use by cosmologists) and run it back in time and you find it does not break down but instead evolves back to a phase of contraction.

    It seems to me this is obviously the thing to look at and test FIRST. Because by far the simplest and least exotic. So I am happy to hear from Apeiron about postdocs flocking to help study it!

    I don't know about postdocs specifically but I see papers by established senior people, relativists/cosmologists, who already have a publication record outside Lqc. I see a general movement of people into the field---including of course postdocs. I would call this rational focus on the proper order of business, not mindless enthusiasm. Maybe some of them can find a way to empirically disprove the Lqc bounce. That would also be progress.

    So you asked if I was comfortable. Yes very much. And it is from a pragmatic empirical point of view rather than one of aesthetic and cosmogony.

    We have to see what observations confirm. Only then do we ask what came before the bounce and how did it happen.
     
  16. Mar 22, 2009 #15
    Well, as I understand it is very difficult to find any observational evidence because, as you say, loop gravity converges into GR after few plank times.

    I have some questions about how loop gravity explains isothropy ofthe universe without an inflation, but I need to read the articles first. You know, there is time to post in the forum and time to read longer articles, when nobody interrupts...

    Thank you for so many links...What is the best to begin with?

    P.S.
    How do you know that Dmitry=Dima? :)

    P.P.S
    Cant stop thinking about the bounce...
    Sorry for the pure speculation, just trying to find something goodon the intuitive level.

    Say, there is a contraction era from t=-infinity to t=0, and our era from 0 to infinity.
    During the contraction era entropy decreases, so for the observers it is not a contraction, but an expansion! If solution is symmetric for the sign of t, then there are not 2 worlds, but just one...
     
  17. Mar 22, 2009 #16

    marcus

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    I know. It is always a fruitful time for me when the modem breaks or the ISP interrupts service. Reading things on actual paper.

    I think I gave only two links to general survey papers by Ashtekar. I would look at the shorter of the two. And then possibly the longer---following up references if any excite curiosity.

    The longer overview paper is this:

    http://arXiv.org/abs/0812.0177
    Loop Quantum Cosmology: An Overview
    Abhay Ashtekar
    To appear in the Proceedings of the Bad Honef Workshop entitled Quantum Gravity: Challenges and Perspectives, dedicated to the memory of John A. Wheeler
    (Submitted on 30 Nov 2008)
    "A brief overview of loop quantum cosmology of homogeneous isotropic models is presented with emphasis on the origin of and subtleties associated with the resolution of big bang and big crunch singularities. These results bear out the remarkable intuition that John Wheeler had. Discussion is organized at two levels. The the main text provides a bird's eye view of the subject that should be accessible to non-experts. Appendices address conceptual and technical issues that are often raised by experts in loop quantum gravity and string theory."

    As I recall the first few pages are historical/philosophical and I would be inclined to skip over them and go to where he summarizes recent research results.

    The shorter essay is this:
    http://arXiv.org/abs/0812.4703
    Singularity Resolution in Loop Quantum Cosmology: A Brief Overview
    Abhay Ashtekar
    12 pages

    I can't think of a better place to start than with a recent short review paper written for nonspecialists (e.g. classic relativists unfamiliar with any Lqg or Lqc specifics.)

    Let me know if this doesn't work for you. This would teach me something, I think.
     
    Last edited: Mar 22, 2009
  18. Mar 23, 2009 #17

    apeiron

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    If there is an elephant in the room, it is always a relevant fact to mention.

    If LQC can only give a big crunch followed by a big bang, then all you have done is held up a mirror and doubled the original mystery. You may have got rid of the singularity in between, but quantum foam thinking would do that for you. The aim of LQC is of course to move to a cyclical bounce model (as the admirably lucid Ashtekar brief overview outlines). So it would be relevant unless you really are content with something so ugly as a single origin-less crunch being followed by a single destiny-less bang.

    And the dark energy/cosmological constant issue would be relevant even to this horribly limited project. If dark energy exists in the big bang era, it must have some analog in the big crunch era (or else, why not?).

    If the dark energy is positive, then it would seem there could be no crunch. It would overwhelm gravitational collapse. And if dark enery is for some reason negative, an accelerating contraction, then we need a reason for its sign to be switched. And a reason why gravity is not also switched around to become a repulsive factor. Etc.

    Ashtekar does discuss the ramifications of positive dark energy but - tellingly - becomes uncharacteristally opaque at this point of the paper.

    But perhaps you can unravel the argument being sketched here.....http://arxiv.org/abs/0812.4703

    • The case of a positive cosmological constant is more subtle [33]. Now, as in the _ = 0
    case, the classical theory admits two types of trajectories. One starts with a big bang and
    expands to infinity while the other starts out with infinite volume and contracts into a big
    crunch. But, in contrast to the _ = 0 case, they attain an infinite volume at a finite value
    φmax of φ. The energy density ρ|φ at the ‘internal time’ φ goes to zero at φmax. Because
    the φ ‘evolution’ is unitary in LQC, it yields a natural extension of the classical solution
    beyond φmax. States which are semi-classical in the low ρ|φ regime again follow an effective
    trajectory. Since ρ|φ remains bounded, it is convenient to draw these trajectories in the
    ρφ-φ plane (rather than v-φ plane). They agree with the classical trajectories in the low ρφ
    regime and analytically continue the classical trajectories beyond ρφ = 0.
     
  19. Mar 23, 2009 #18

    marcus

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    Hi Apeiron,
    (BTW since apeiron was the favorite idea of Anaximander of Miletus, you might be interested to know that Carlo Rovelli has competed a book about Anaximander---Rovelli has history of science as a sideline. A link to the draft copy is at http://www.cpt.univ-mrs.fr/~rovelli/ )

    For me the most meaningful thing in that paragraph of Ashtekar you quote is "...[33]..."
    It refers to a paper "in preparation" by Ashtekar and Tom Pawlowski. They are addressing the positive cosmo constant case.

    I think of the human enterprise of understanding nature as a process of successive approximation.
    The classical cosmo singularity represents (imho) zero understanding. Replacing it with a simple mechanism like the Lqc bounce is extremely interesting and could be the correct next step.

    I'm confident that predictions will be derived and checked, and we will proceed from there.
    The bounce could pass its first tests or it could be rejected, either way we learn.

    I don't need to have all the questions I can think of answered immediately. Like, were there a lot of prior bounces, or just one? That's speculative and aesthetic, not practical.

    Since 1917 we have the deSitter universe which has a positive cosmological constant and something resembling a bounce. It has the "hour-glass" shape of contraction-rebound-expansion. That is certainly not the same thing as a Lqc bounce, but it is one route to thinking about the case with positive Lambda.

    More significant, i think, is the question what is Lambda? We don't know what dark energy is yet. And of course we don't know if it is really constant or if it changes. It may be a cumulative quantum geometry correction and not an energy (Bojowald has explored that in the Lqg context.) Martin Bojowald has been invited to edit a special issue of some journal that will gather together many experts' ideas about what dark energy could be.

    Basically I think it is simply not a fruitful time to be thinking about the entire history of the universe (which for simplicity sake I consider eternal, there is no scientific reason to introduce the extra complication of a beginning or an end.)

    What seems to be fruitful is to focus on the one thing we know is wrong---the classical singularity. When that has been replaced with something that survives empirical tests, then we may profitably look further back. Meanwhile, fundamental concepts will have changed...
     
    Last edited: Mar 23, 2009
  20. Mar 23, 2009 #19

    apeiron

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    Hey thanks for that. Shame its in Italian but I will follow that up. I've of course studied Anaximander intensively and he is a remarkably misunderstood and underappreciated philosopher.

    I understand your point but from my perspective, there are multiple avenues to explore, LQC is not the only game in town for me. So I want to look down an avenue and gauge how likely it is to reach the destination.

    You yourself are enthusiastic about no go theorems. And dark energy would be a red flag for bounce cosmology at the moment. I am much less bothered by second law issues as the second law has only really been formulated for "within worlds" use, not in a wider form.

    For those making a career in a particular field like LQC, of course they will have their heads down and plough through, having already made their choice.

    Personally, I see the story as the universe dissolving to a "quantum foam" rather than arriving at a singularity. So quantum gravity approaches are a good avenue in that regard. But it seems "obvious" to me that a quantum foam is now a vague state, an apeiron indeed, and not a crisp state. Therefore if you can indeed project the correct calculations beyond the planckscale first moment, you would have to get "something even foamier" rather than instead the foam for some reason turning crisply into a mirror universe the other side.

    If Rovelli is into Anaximander, perhaps he too has this thought in mind?
     
  21. Mar 25, 2009 #20

    marcus

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    I can't say about the specifics but I do know that Rovelli is an Anaximander fan.

    He's kind of a hero. He was at the beginning of Greek science (was it circa 600 BC) where they struggled to find some mechanistic explanation rather than "god did it" or "the gods want it that way" and "it sits on the back of a giant turtle".

    I think Rovelli believes we have a lot to thank Ax'der for. The idea of physical law. The idea of dynamics, and that you could explain in an unfanciful way, or at least try. A critical standard of rational explanation.

    I know Rovelli also thinks Anaximander is underappreciated, as you also say.
     
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