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Bojo in Nature Physics July issue, probing before bigbang with quantum cosmology

  1. Jul 2, 2007 #1


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    Here is the Penn State press release

    called "What happened before the big bang"

    the press release evidently covers much of what you need a subscription to read in Nature Physics magazine, or else have to get it at your university library. Here's another report based on Bojowald's article

    Here's a short quote from the beginning of the press release
    What happened before the Big Bang?

    New discoveries about another universe whose collapse appears to have given birth to the one we live in today will be announced in the early on-line edition of the journal Nature Physics on 1 July 2007 and will be published in the August 2007 issue of the journal's print edition. "My paper introduces a new mathematical model that we can use to derive new details about the properties of a quantum state as it travels through the Big Bounce, which replaces the classical idea of a Big Bang as the beginning of our universe," said Martin Bojowald, assistant professor of physics at Penn State. Bojowald's research also suggests that, although it is possible to learn about many properties of the earlier universe, we always will be uncertain about some of these properties because his calculations reveal a "cosmic forgetfulness" that results from the extreme quantum forces during the Big Bounce.

    The idea that the universe erupted with a Big Bang explosion has been a big barrier in scientific attempts to understand the origin of our expanding universe, although the Big Bang long has been considered by physicists to be the best model. As described by Einstein's Theory of General Relativity, the origin of the Big Bang is a mathematically nonsensical state -- a "singularity" of zero volume that nevertheless contained infinite density and infinitely large energy. Now, however, Bojowald and other physicists at Penn State are exploring territory unknown even to Einstein -- the time before the Big Bang -- using a mathematical time machine called Loop Quantum Gravity. This theory, which combines Einstein's Theory of General Relativity with equations of quantum physics that did not exist in Einstein's day, is the first mathematical description to systematically establish the existence of the Big Bounce and to deduce properties of the earlier universe from which our own may have sprung. For scientists, the Big Bounce opens a crack in the barrier that was the Big Bang.

    Here's a version with a JPG image---computer generated schematic of a bounce with quantum fluctuations getting amplified (wave function spreads out some during bounce). Relates to something he was saying further down in the article.

    Here's a derivative Space Daily report:
    Last edited: Jul 2, 2007
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  3. Jul 2, 2007 #2


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    Marcus the cosmic forgetfulness, does this mean infinite possibilities, or some thing like my cakes, even though i use the same ingredients, quantities
    baking time, they never come out identical.
  4. Jul 2, 2007 #3


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    I like the down-to-earth evocative analogy.

    It's a good one for what he seems to be talking about, which is a complementarity between two quantities, one relating to the size* of the prior, collapsing, region
    and the other relating to the size of the expanding region it gives rise to.

    Complementarity is a familiar idea from way back---the Heisenberg uncertainty principle: position and momentum examplify a pair of complementary quantities because the more accurately you try to pin down one the more uncertain the other gets (you can't measure them both simultaneously)

    We should probably wait to see what Bojowald actually SAYS in his article in Nature Physics, and listen carefully to how he qualifies it.

    It would not be fair to him to take seriously what a journalist thinks he or she heard him say, or how the journalist interprets and quotes out of context.

    It is probably just the GERM of an idea at the moment. He has an improved model of the bounce now (which still hasnt been observationally verified although it looks increasingly interesting ) and in this particular model he sees two quantities which you can't get a grip on simultaneously

    (and that is inevitably going to excite a quantum physicist because of the historical importance of such pairs, with an uncertainty tradeoff)

    so he gets excited, naturally.

    but we have to just sit it out and wait for what comes out in legitimate peer-review journals, which it will

    What he has in Nature Physics is a "letter", and I can't get access to it because I don't have a subscription to the magazine. It comes out in hardcopy print form in August, but for July it is only subscription-only e-journal

    *I don't know what measure of "size" is meant. Does it mean mass or energy-content or some geometrical parameter or what?
    Last edited: Jul 2, 2007
  5. Jul 2, 2007 #4


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    The Nature Physics website
    now shows the table of contents of the July 2007 issue, including this

    Letter by Bojowald

    Can we ever know what happened before the Big Bang? It may have been only a stage in the existence of our Universe rather than its beginning, but analysis suggests the Big Bang is a barrier beyond which we may never see with clarity.

    SciAm July issue
    Last edited: Jul 2, 2007
  6. Jul 4, 2007 #5

    George Jones

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    Sean Carroll is less than enamoured with Bojowald's ideas.
  7. Jul 4, 2007 #6


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    He doesn't seem to know much about Bojowald's research, or LQC research in general, does he?

    He has tried several lines of objection but nothing seems to be very convincing.

    His latest attempt on that thread is here:
    It is quite different from the argument he tried earlier, which was based on second law of thermo.
    Here he tries a new tack (but says he is repeating himself). The new critique is misleading in several ways, or simply misinformed.

    ==quote Sean==

    Lee, I never expressed doubt that the formulation was well defined, only that there’s any reason to expect it to relate to the real world. At least, no such reason is given. You can’t restrict to the spatially homogeneous case, and then claim there is no fine tuning. That is an infinite amount of fine tuning, which needs to be justified.

    I seem to be saying the same thing over and over, but I’ll try one more time. Unlike cosmologies in which the Big Bang is a boundary condition, bounce cosmologies feature a pre-bounce contracting phase. You need to tell me what happens during that phase, and why. Are there perturbations that are in their growing mode as they approach the bounce? If no, why in the world not? Generic gravitational collapse is expected to be highly non-linear and inhomogeneous, what is so special about this? And if yes, why don’t the perturbations grow and destroy the smoothness? Why in the world would we expect a homogeneous expanding cosmology to emerge from the other side?

    These are not annoying technical issues that can be addressed later. They are the Whole Big Problem that must be confronted by any attempt to honestly address the issue of initial conditions.

    Bojo recent work is not limited to homogeneous case (Carroll hint to contrary)
    Recent Lqc papers, Bojo et al, do perturbative analysis, and indeed show inhomogeneities growing (Carroll to contrary)
    and they determine *what happens* in various pre-bounce contracting phase cases (Carroll to contrary)
    and they even find cases where the prior conditions are such that you DON'T get a bounce, beginning to answer the question about what makes cases where you get a bounce (in this model) different.

    So here we have Sean lecturing Lee that these things Sean lists are not to be put off but must be addressed---and in fact if you read the last couple of years Lqc they ARE being addressed and quite a lot of progress made----even if you just glance at this most recent (7 June) paper on arxiv, and refs therein, you can see this.
    Last edited: Jul 4, 2007
  8. Jul 4, 2007 #7


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    If anyone wants to catch up on recent LQC, there is this paper from 7 June

    Effective equations for isotropic quantum cosmology including matter

    it is not limited to the homogeneous case (as Carroll seems to think) and it does a perturbative analysis (which Carroll was calling for, as if it was not already in progress). Maybe Sean Carroll just needs to get up to speed on the subject, before launching crits.

    that 7 June paper is 42 pages with a lot of detail, and I think it probably overlaps quite a bit with Bojowald's Nature Physics piece. I see overlap with what came through in the press releases (if you subtract the journalism).

    So the longer paper is probably a good source, and has equations in it that help me make sense out of the verbal interpretation that came out in the media

    If anyone wants blog coverage and discussion, I wouldn't recommend Sean's (I don't think his critques hold water) but I WOULD point to Phil Plait's BAD ASTRONOMY BLOG.
    I thought his piece on the Bojo thing was great.

    Maybe the gist is this. Bojowald and friends are analysing a MODEL, and extending its generality to cover more and more realistic cases.
    this model has not yet been TESTED.
    Phil Plait the BadAsstronomer makes this point very clearly. It will be tested and it may prove wrong! Or it may pass observational tests. Either way it's good science and Phil likes it----he has learned quite a lot about LQC which he explains in clear simple terms, and he sees the point.

    For some reason, Sean Carroll doesn't do this. He tries to DISMISS the work on theoretical (thermodynamics) grounds. Take the superior stance that this work that all these other people are engaged in CAN'T POSSIBLY BE RIGHT :smile:.
    But that didn't work so well, there were even some comments before Lee's. So, again for some reason I don't understand, Sean leaves off the thermo reasoning and starts to simply DENIGRATE the research by misleading innuendo hinting things that are not actually true. (restricted to homog, does not study perturbations, does not say what happens in contracting phase.) I don't get it. Why wouldnt he just report developments? Some rivalry? Is he mad about something? Puzzling. Here is webpage with photo, recent real research, usual academic info http://preposterousuniverse.com/self.html
    Last edited: Jul 4, 2007
  9. Jul 4, 2007 #8


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  10. Jul 4, 2007 #9


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    Lee S had a nice clear comment on LQC in that thread at Carroll's blog


    45. Lee Smolin on Jul 4th, 2007 at 9:41 pm

    Dear Sean and Dan,

    I have not worked on loop quantum cosmology, of which there is now a long and technical literature, but I can help with a few points in answer to questions above.

    0) Most of these models are gravity plus some matter fields, massless scalars, scalars with various potentials, with and without inflation etc. have all been studied in detail.

    1) In all the models in question, classical FRW cosmology is always recovered when the curvature of spacetime is small in Planck units. That is the symmetry reduction of the Einstein equations coupled to matter is derived as the low curvature limit of the same dynamics in which singularities are replaced by bounces. So one cannot say that these models do not contain the appropriate form of the Einstein equations. Furthermore, in the full theory that these models are restrictions of, with spin foam dynamics, sufficient components of the graviton propagator have been calculated and Newton’s law is recovered. Hence, LQG in general is a theory of gravity. Granted there are open issues in the relations between the full theory in this form and the models studied in LQC, but it is not correct to say that “these models do not have gravity in them.”

    2) The question of at what scale the bounces take place has been studied in detail, and the conclusion is that bounces happen when the spacetime curvature becomes Planck scale. Once the models are chosen there are no fine tunings. You have the wrong impression from 0608100. To get a correct impression read the review paper arXiv:gr-qc/0601085, or the many papers it cites. For a somewhat different approach to these models that leads to the same conclusions there is the recent paper of Ashtekar et al arXiv:gr-qc/0612104.

    3) Why do bounces happen? Because of quantum corrections to the Einstein’s equations that become of the same order as the classical terms when the curvature approaches Planck scales. This does not contradict the gravitational force dominating at low curvature, as indeed it is shown they do.

    4) The claims that these solutions always bounce are not based on gr-qc/0608100. That has been demonstrated previously in many models and papers, either analytically or numerically. The point of that paper is to set up and study a scenario in which an effective field theory can be derived and used to reproduce some aspects of the exact theories, which have been already solved.

    5) Homogeneous quantum cosmological models have been studied for decades, and most previous results were restricted to the semiclassical level. I am not aware of any test for well definidness, or correspondence with classical GR in appropriate limits, that these models have not passed.



  11. Jul 5, 2007 #10


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    Anthony Aguirre, Sean, and Jacques Distler had clear focused comments on the "contra bounce" thread of Sean's blog: comments 49-52:

    49. Anthony A. on Jul 5th, 2007 at 11:37 am


    I share your intuition that a gravitational collapse to the planck density is very unlikely to bounce into a homogeneous region. But I think “watcher” has a good point here: don’t you think a very similar objection could be leveled at the nucleation of baby universes? There, we must rely on inflation to take a rare baby universe that is large enough and homogeneous enough to inflate, and turn it into a large or infinite homogeneous region. So I think there are two separate questions:

    1) Might a ‘bojowald bouce’ lead to a universe with one single FRW-region a (perhaps cyclicly repeating) bounce replacing the BB-singularity? (My guess is no, this will not make sense for just the reasons you put forward).

    2) Might some actually realistic version of a ‘bojowald bounce’ take future singularities and allow them (while still increasing entropy) to create [new] regions that are homogeneous enough to inflate, and thus provide a new mechanism of creating baby universe? (My guess is maybe, who knows?)

    In either case, though, it seems completely clear to me also that until a non-homogeneous analysis has been done, these results don’t really address either question in a meaningful way.

    50. Sean on Jul 5th, 2007 at 12:20 pm

    Anthony, I think there are plenty of reasonable objections to baby-universe nucleation, but the one that I’m raising against bouncing cosmologies is not one of them. The defining feature of a bounce is the existence of a pre-bounce contracting phase. (Otherwise it’s not really a bounce, is it?) And then the problem is that either the entropy is decreasing during that collapse, for no good reason and in contradiction with everything we think we know about gravitational dynamics, or it is increasing during the collapse, yet supposedly gives rise to an extraordinarily low-entropy condition on the other side, for no good reason and in contradiction with everything we think we know about unitarity and thermodynamics.

    My suspicion is that there isn’t any good way out of this dilemma, and bounces of that sort aren’t part of the real world. But I’d be happy to change my mind, if anyone would offer a plausible response to these objections, or even an outline of what such a response might look like. I haven’t heard any, although someone might have one.

    51. Anthony A. on Jul 5th, 2007 at 1:37 pm


    I agree with your concern about the bounces. What I’m getting at is that perhaps in a less contrived version in which there is some sort of bounce, but in a way that increases entropy and leads to an irregular — but expanding — universe, we might have a picture very similar to the baby universe picture.

    In terms of baby inverses, what I meant was that if you proposed baby universes as a way to generate a new universe out of a fluctuation, but made no mention of inflation, I think you would encounter an analogous (but I agree not identical) argument: how would a fluctuation possibly lead to a baby homogeneous universe? The answer is that it would not, but we can appeal to inflation to fix this.

    So again, I also don’t buy the ‘bounce into homogeneity’ either, but I would not rule out that bounces might be a way to lead into some initial state that, say, might inflate. The argument that such an initial state (that will give rise to inflation) is super-low entropy is also a concern for baby universes. It seems to me that in either case the only hope is that we’re only taking a tiny set of the degrees of freedom of the pre-existing space to create the baby (or bounced region). If we take *all* of the d.o.f. and force them into the low-entropy ‘initial’ state, then we run into awful problems in either case.

    52. Jacques Distler on Jul 5th, 2007 at 2:00 pm

    If you don’t buy that, then you don’t buy a single word of Bojowald et al’s mini-superspace analysis of bouncing cosmologies.

    If inhomogeneities are not magically suppressed, then there is no way they can be neglected when the universe reaches the Planckian densities characteristic of the bounce. Inflation, you are right, provides a mechanism for erasing primordial inhomogeneities after the fact.

    But it can’t render an otherwise nonsensical mini-superspace analysis sensible.


    Distler's comment may not show him at his best (he seems inclined to dismiss Bojowald's model as "nonsense") but has the virtues of clarity and brevity. The comments by the other two, Anthony and Sean, are more interesting and show them making an effort to understand the LQC bounce picture on their own terms. Any of the points raised in comments 50 and 51 would, I suspect, provide a subject for fruitful discussion.
    There was a typo in comment 49 where Anthony typed create no (which didn't make sense in context) where I think meant create new.

    It is interesting to see these people coming at these issues (cosmological bounce related) from their different backgrounds of knowledge and perspective.
    Actually I think it IS quite an important issue, so I am glad to see them coming to grips with it and being, in several cases, mentally open to new possibilities.
    Last edited: Jul 5, 2007
  12. Jul 6, 2007 #11


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    Cosmic forgetfulness" shrouds time before the Big Bang
    Even if we do accept LQG and Bojowald's conclusions, the fact still remains that "cosmic forgetfulness" renders discussion of a pre-BB state outside the remit of observable and testable science.

    In other words be sceptical, very sceptical.

  13. Jul 6, 2007 #12


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    That is your spin on your reading of a journalist's account, I take it. :smile:
    It seems like nobody has read the actual article, or?

    I liked Wolram's take on it, which came in the form of a question:
    My take is based on reading the 40-page technical article that came out in June, immediately before the piece in Nature Physics. My nickname for it is "1057"
    It gives a mathematical description of a kind of indeterminacy trade-off between certain observables. We've seen that sort of thing in quantum mechanics before, haven't we?

    My guess is that the Nature Physics piece just takes selected things out of the "1057" article and amplifies them for an audience of non-specialists, giving a little historical background for people who haven't been following the field.

    I may not see the Nature Physics article until it comes out in hardcopy and is on the physics department shelves. If anyone else gets it before I do I hope they will share some exerpts.

    As for what Bojowald means by "cosmic forgetfulness", it seems pretty clear from "1057" that his and the other researchers' grip on the pre-bounce collapsing region is getting firmer and that the "forgetfulness" they are talking about is partial and narrowly defined, not total. But that comes from reading the technical article, not the Nature Physics letter.

    My response to Wolram's question essentially comes down to
    Last edited: Jul 6, 2007
  14. Jul 6, 2007 #13


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    I'll try to give a general perspective on the current progress in LQC
    based on what I have, the June 2007 paper I call "1057"

    The research currently going on is centered around TESTABILITY
    (using e.g. observations of supernovae, CMB and galaxy survey structure formation).

    There are a halfdozen very active researchers, and a dozen or so if you count those who have published in the past year. could be more I havent made a careful count.

    Bojowald et al "1057" can be seen as responding in part to a call for more precise prediction made here
    by Magueijo and Singh
    This will be published in Physical Review D
    The message from Magueijo et al was essentially to say "if you get such and so details of the bounce worked out we will be able to derive a distinct signature in the formation of structure, which we can look for and test your theory---also constrain some constants for you."

    The preceding paper by Bojowald had to do with accelerated expansion (supernova observations) and was also strongly oriented toward testing.
    The analysis there is preliminary but if it is confirmed by more detailed calculation it should provide a very distinctive acceleration history---agreeing with what has been observed for recent (low redshift) supernovae but differing from other scenarios where data is still to be taken.

    There have been a number of papers discussing LQC effects on inflation and possible observability in the CMB. An early example was co-authored with Singh by a couple of well-known cosmologists: Roy Maartens of Portsmouth, and Shinji Tsujikawa.
    Loop quantum gravity effects on inflation and the CMB
    This was published in Classical and Quantum Gravity.

    Making precise predictions that can be tested by observation is essentially what the recent "1057" paper is about. In it Bojowald et al are constructing an exact solvable model with additional parameters which has the possibility of serving as a basis for perturbation analysis (so that inhomongeneities can be included). More general kinds of matter can be included.

    The fact that the improved model contains a BOUNCE is almost incidental :biggrin: The fact is that the vast majority of LQC since 2001 has shown a bounce or eliminated the cosmo singularity in one way or another.
    It just happened to have gotten public attention this time, and stirred up a little controversy.

    In the process of building this improved model, in "1057", with the basic aim of more precise predictions leading to testability, Bojowald happened to come across this determinacy limitation he christened "forgetfulness" and that seems to have led to the present article in Nature Physics.
    Last edited: Jul 6, 2007
  15. Jul 16, 2007 #14
    I jump in just in your latest post of this thread, but could have done this somewhere else.

    What happened before the Big-Bang?

    Very glad that you Marcus pointed so many times in the direction of Bojowald’s c.s. research and I think Bojowald’s letter published 1 July 2007 in Nature physics is a milestone. I have read this letter and also read all the posts of this thread. I must admit that I cannot follow everything, certainly not, but I tried and are still trying to get a better understanding while also taking into account what I have read, and partly, understood from physicist like R.Penrose.
    You will not be surprised that I still have my questions and remarks while I have even difficulties to formulate these.
    Here are some of them.
    1). R. Penrose in his book “Road to Reality” learned to me that, however physics laws are symmetric, entropy grows with time while it decreases back in time (see Fig. 27.7 page 698). At the time of the BB the state of our observable universe was extremely special (but still equally probable as each of all other, almost infinity, possible states). Indeed if history continued to go back in the past, as we observed so, then the farther back in time in the pre- bounce (pre-bang) state the entropy should have been lower and lower and thus more and more special. I suppose physics provides us (only) in principle with a (restricted) possibility to ‘calculate’ the future, but history, because of the second law of thermodynamics, can only be observed or approached by calculated guesses? IMO future can not be observed.
    2) I wonder if and where, in the past and in the future eventually, the second law stops being valid or being reversed. This wondering has to do with my idea that existence (or if you like autonomous reality) is not created and never ends, it is only transforming. To me for real infinity to be valid, somewhere in space-time, entropy’s trend(s) must be reversed (though, if not so then, still as an improbable way out, every possible very, very, very special state remains possible)! But, could it eventually be seen from the mathematics of LQC(G) as presented by Bojowald that during the contraction phase the entropy was decreasing in stead of increasing with the future, or was it still increasing just as happens now in our observable universe while expanding (after the bounce)?
    3) One reason why I am so happy with Bojowald’s research is that he, once more, showed that singularities (as well as notions such as begin and end) are not belonging to the physics domain and that in (full) reality only phase transformations exist. This fits to my intuition and reminds me of A.L de Lavoisier who stated : « rien ne se creé rien ne se perds tout est transformation ».
    4) Two difficulties I have with Bojowald’s letter are that he is talking about the ‘volume of the (a) bounce’. A) Does that mean, where I have great difficulty to accept, that he simply “sees” just only one bounce surrounded by really (non existing) nothing? B) Or does that means that at a real very large scale ‘the cosmological principle’ does not hold anymore, in other words, locally (in our observable universe and even in a large space(time) surrounding it), this principle holds but further away density could be more and more different? Certainly IMO one can not speak of baby universes and at the same time maintaining the cosmological principle? Why did Bojowald not express his formulas in energy density or did he?
    Ashtekar does, see your thread “the Ashtekar density”.
    5) At first sight I thought “forgetfulness” had to do with chaos which is deterministic but not predictable. Apparently those behaviours are shared but indeed forgetfulness seems to be different because it is said to be due to QM. Will forgotten mean only forgotten but not erased? If finally Einstein will be right because of “hidden variables” (but non local?) then I would say there will nothing be forgotten even not during a bounce, QM being incomplete?
    Is forgetfulness a notion which will only relates to a pre-bounce?
    6) It is stated that the bounce occurs when density during pre-bounce came down to Planckian or Ashtekar’s. Is that really a necessity or is it imaginable that the density during a bounce can be some orders of magnitude lower than Ashtekar’s. However special a transform from pre-bounce to after-bounce might be, should it indeed only be the most special one can suppose. In a less special bounce I think there will be more freedom for fluctuations/variations? Further more, more different bounces would be possible at other places in the university?
    7) Why still taking inflation into account (or talking about it) if “begin” doesn’t exist. Without “begin” there might be plenty of time for communication, so IMO the (unnecessarily invented) horizon problem can then be considered as a solution for a non problem?
    8) That our observable universe can not be part of a black-hole seems to be ‘proved’ in case of a (presumed) begin does exist (http:/math.ucr.edu/home/baez/physics/Relativity/blackholes/universe.html). But, if one can’t take singularities into real consideration, the arguments seem not to be valid. In that case it might be imaginable to me that real universe is filled up with infinitely many black/white-holes, (“bounces”) (each of them appearing and disappearing/evaporating?) with their event horizons. Those holes embedded in energy, our observable universe (and an environment of it) being part of a black/white-hole? Furthermore, maybe a bounce with its contraction and subsequent expansion might only happen mainly in a kernel within such an enormous black/white-hole?
    9) Is it possible that Bojowald’s LQC theory also applies to black-holes in our observable universe, can bounces happen within their event horizons? If so then they might be apt to indirect observations!
    If my questions are (at least partly) reasonable but if no body can give the answers should we, (I?), then ask Martin Bojowald, or should I pose them somewhere else (e.g. in a blog?). You understand that I am curious about further implications of Bojowald’s theory.
  16. Jul 16, 2007 #15


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    the definition of entropy depends on the observer's map of the macrostates as regions of phase space. the second law only holds if one keeps the same map (the same definition of entropy) and therefore one cannot change maps (or observers) in the middle and expect the law to hold.

    a pre-bounce observer watches gravitational collapse and sees increasing entropy, analogous to that of a black hole.
    a post-bounce observer looks back to his bigbang event and sees entropy increasing from a very low initial level.
    this is not inconsistent because it is two observers, for whom different degrees of freedom a measurable and meaningful---they define different macrostates

    in between there is a planck regime which is not observable by any observer.
    so nobody every gets to see a violation of the second law.
    hope this satisfies.

    I believe that entropy is increasing in the pre-bounce era, and also in the post-bounce era. And there is a discontinuity at the bounce, where one changes the point of view and the map of phasespace, hence the very definition of entropy
    Bojowald's analysis indicates there is a degree of uncertainty---one observer's inherently limited knowledge of what the other sees.

    to paraphrase Lavoisier: there is no beginning, there is no end, everything is transformation----this is a classic Western European view of time. I believe that Newton would have agreed that time extends from minus infinity to plus infinity.

    I too am happy with Bojowald's analysis which does away with the cosmological singularity and the idea of a "beginning of time". It helps to bring us back to the normal scientific view.
    There has only been a short period of some 50 or 60 years when the notion of "time beginning" prevailed in cosmology. It has always been controversial and a bit suspect, I think, perhaps because of its philosophical awkwardness.

    A)-a finite space which is topologically S3 can have a finite volume, and yet have no other space surrounding it.
    Such a space has no boundary and the distribution of matter in it can satisfy the cosmological principle. there is no contradiction.

    I believe the cosmological principle can be satisfied in a baby universe. Do you disagree? If so, maybe you would explain.

    We still have to get a copy of the Nature Physics article. It will appear in August 2007 in print.

    I think now that one cannot take this result too seriously. Ashtekar's group ran many different computer simulations and always the bounce came at close to Planck density. I meant it only half-seriously to call this the "Ashtekar density". A lot more research is needed both to verify the LQC model and also to explore the consequences---one still has not looked at enough different cases. The researchers, Bojowald and others, are working to remove simplifying assumptions. Maybe in the end it will be seen that a bounce occurs in some cases but not others. I think the bounce that they are studying now is not the only behavior that can replace the classical singularity. So the answer to your question is I don't know.

    YES! This is quite possible! there is a recent paper by Magueijo and Singh that says the "horizon problem" goes away in LQC. this is fairly obvious to them so they don't waste much time talking about it, and they move on to consider the "structure formation problem"
    Today I came across a paper by a Brazilian that argued that structure formation is also taken care of in the case of a bounce. Magueijo and Singh were not yet sure about that. There paper is on arxiv.

    I don't want to speculate about this. It seems to be an open question whether our spacetime could have come from a blackhole collapse in another region. If some well-formulated model is constructed that has this feature, it will have to be tested by what OTHER predictions it makes.

    I don't know and can't answer. It could be that bounce can happen in the plunge of a black hole. Some people talk about this at conferences but almost no papers about it are published. Bojowald has been studying this. His analysis might show that it cannot happen!
    Or it might show that it can happen. And then his models still must be checked observationally.
    Last edited: Jul 16, 2007
  17. Jul 17, 2007 #16
    Thank you for all the replies

    I suppose my education on topology is by far not sufficient. What I meant was mainly for the full real infinite universe. Though I can see the possibility that on average, just as in our observable universe the cosmological principle can hold (but indeed not locally). The space in between baby universes must have a different (energy-)density than in the baby universe.
    But also in a baby universe (or in whatever else "universe") I can imagine that density slows down in the direction of its event horizon and that it only looks like to be valid as long as the observer finds himselves far enough from its even horizon.

    kind regards
  18. Jul 17, 2007 #17


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    I don't think its a question of education in topology (I had only quite simple pictures in mind, that you wouldn't need to take a course to understand.)
    It is more the limitations of the discussion board. drawing pictures on the computer would be a bother for me and take a lot of time just with the computer formalities. the problem is we are picturing things differently.

    roger penrose draws good pictures
    take a look at his slides for this talk at Cambridge given 7 November 2005
    if you want, just look at the Smolin part of the FIRST SLIDE that comes up when you click.

    I don't know what you mean by the "full real infinite". In penrose discussion space is pictured as a large 3D sphere---which would be the surface of a 4D ball except the ball inside isn't there--the sphere is hollow. because he cant draw it on the transparency, he draws instantaneous space as a RING (the disc inside the ring doesnt exist, there is only the ring) and that is supposed to be analogous to the higher-dim. sphere.

    I don't know what you mean by "the space in between".

    In my mental picture there is no surrounding space.

    In my mental picture there is no event horizon that we can be "far" from or near to.

    If you and I are living on a planet in a baby universe, that started from a blackhole bounce somewhere else, (as we may be, who knows?)

    then there is no event horizon anywhere having to do with that blackhole bounce---that is an event in our PAST. It is not present in today's space.

    Asking the distance to it is like asking how far it is to the 'big bang' in conventional cosmology. It is not some distance away, it is 14 billion years AGO.

    A very common picture of our universe looks like a horn, with the bigbang at the tip.
    Space, in that picture is just a one-dimensional ring. Space is a ringshape SLICE. The horn, or conical witches hat if you like, shows the expansion of the ring----because the slices grow as you move along in time.

    time goes along the horn or conical hat, starting from the tip

    if you could picture the analogous thing with a 2-sphere instead of a ring, then that would be the "expanding balloon" picture of the universe.

    even more realistic, but impossible to draw, would be a kind of horn or conical thing where the slices are 3-spheres---the 3d analog of the surface of a balloon.

    Penrose, in his lecture sketches, doesnt try to draw the realistic picture, he just draws a universe as a horn (so space at any given instant is a ring, a slice thru the horn)

    he draws baby universes as horns budding off of horns---a kind of cactus or tree-like picture.

    in his picture there is not supposed to be any surrounding space (I couldnt even say what dimension it should be)----there is only the single horn---or if it has offspring then there is the branching horn

    that is a lot of words, when one picture would do if I could draw pictures.

    BTW in Penrose talk he does not ADVOCATE this picture, he draws a lot of different ways to picture the universe and ends up advocating a somewhat different scheme---still a reproductive scheme with "babies" but only after all the black holes in sight have evaporated. what he has to say is, for me, too complicated to talk about, and his arguments against the simpler picture don't hold up, but at least he draws that kind of cactus picture of horns budding off of horns (where the budding point is the TIP, representing a bigbang event)
    Last edited: Jul 17, 2007
  19. Jul 18, 2007 #18
    Marcus, I now have the Penrose's slides.
    Before I am going to respond to you I will start with some consideration and explanation below. Later on I like to come back.


    Maybe I should not have taken baby universes as a means to look critical to the cosmological-principle, it makes me feel obliged to go into details of baby universes while I must admit that I know nothing about them. In trying to explain my thoughts or intuitions I grasp around and use language and notions which I at first sight think are useful for me. This time I think I was triggered by Sean Carrol’s “Why is the past different from the future” which link I found in George Jones thread post #1 “Penrose, Carroll and inflation”. I think I will have to go back to Sean’s presentation and study it. His presentation seems to me good looking.
    I allow myself here a short intermezzo in order to make myself better understandable.
    I think indeed that a good picture of our observable universe in evolution can be given by a time axis and a perpendicular space axis, where in a cylinder-symmetric situation around the time axis, just one slice through the time axis can do. Let’s take both axis almost logarithmic and the units for time and length Plancktime and Plancklenght respectively. Let the time-axis begin with log1(=0) and the length with roughly log(R=10E60)=60 (dimension of the observable universe at Planck-time, which I can eventually more secure using the number of baryons in our OU and adding DM and DE to it). Further more lets mirror around (0.60).
    What I have seen lately in literature (around WMAP) is a picture which starts with a needle which after a short time grows very thick because of inflation where about it is almost glued to a horn.
    What irritates me is the needle with its point and the inflation. In my mind the needle should be broken off and the horn should be glued against a symmetric one, one horn being the pre-bang the other the after-bang.
    But this picture only refers to our observable situation. To continue I propose to talk about “verses” in case not the (complete infinite) universe is meant.
    In my mind I “see” the universe filled up with an infinite number of verses of the kind that I just have described. In my mind those verses each have an event horizon which already existed before their bounces within took place. In between those event horizons there still must exist (low-density) high energy, this because nothing itself can not exist (contradiction in terminus).

    kind regards,
  20. Jul 19, 2007 #19
    1.Distance TODAY between us and the furthest observable star: 46 bill. LY
    (will increase in the future)

    2.Distance TODAY between us and the furthest star in the universe: unknown but for sure much bigger than 1., maybe infinite ?
    Not enough precision on the measurement of Omega to make a precise statement.

    3.Distance TODAY between us and the furthest star that will ever receive a signal emmitted today from us : 16 bill. LY
    (will decrease in the future !)

    Basis : LamdaCDM Omega M = 0,3 , Omega Lambda = 0,7, H0 = 70 s-1
  21. Jul 19, 2007 #20
    sorry, the above was supposed to go to another post, please ignore it here...
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