
#1
Sep1507, 08:47 PM

Astronomy
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PF Gold
P: 22,809

Bojowald's What happened before the Big Bang? article in the August 2007 issue of NATURE PHYSICS is available to nonsubscribers
To get the article directly in HTML http://npg.nature.com/nphys/journal/.../nphys654.html Or you can go thru the Table of Contents and see the other articles in the journal. http://npg.nature.com/nphys/journal/v3/n8/index.html During August it was accessible only if you paid, but they just made it free. The link gives the TOC for the August issue. Scroll halfway down the page to where it says LETTERS and click on the PDF for the Bojo piece. In this piece Bojowald derives (in the context of his quantum cosmology model) some definite limitations on what can be known about the universe prior to the beginning of expansion. Even though the singularity is no longer there, in the Loop Cosmology model, a Heisenberglike principle of indeterminacy limits knowledge in some (but not all) respects. Bojowald was just awarded the Xanthopoulos prize for his work in quantum cosmology. This was presented at the international conference of General Relativity and Gravitation (GRG) people that is held every three years. In past years (1993 and 2001) famous string theorists have been awarded the prize. I think it is nice to see Bojowald (and Thiemann who was honored along with him) get the recognitionthey do nonstring quantum gravity. I suppose part of the news about the Nature Physics article is that it is prominent among peerreviewed journals specializing in physics. Has the highest IPI impact ratinga measure of citations per article. (It's a subsidiary of the weekly NATURE.) It's a good place to publishprestige and visibilitywise. Bojowald normally publishes in Physical Review Letters (PRL) and in Physical Review series D (PRD). These are good solid journals but his piece in Nature Physics caused a bigger splash. ===commentary on the Bojo article=== Carlo Rovelli is one of the founders of Loop Quantum Gravity and the author of the book Quantum Gravity published in 2004 by Cambridge University Press. He had some comments on Martin Bojowald's article in the August issue of NP. Here is Rovelli's commentary, just over one page http://npg.nature.com/nphys/journal/.../nphys690.html Here is a sample: Science has frontiers; sometimes these frontiers move. One of the most impressive of science's frontiers is the Big Bang, and now a quantum theory of gravity — loop quantum gravity — is providing equations with which to explore it. Although these equations are still tentative, and rely on drastic approximations, they introduce a definite method of exploration, and are capable of describing the Universe not only close to the Big Bang but also beyond it. It is in this context that Martin Bojowald reports, in this issue, on the possibility of a peculiar limitation to our ability to observe fully the 'other side' of the Big Bang — whatever that expression might mean (Nature Phys. 3, 523–525; 2007). 



#2
Sep1907, 03:36 PM

Astronomy
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PF Gold
P: 22,809

Here is one way to track the impact of the article
http://adsabs.harvard.edu/abs/2007NatPh...3..523B The harvard abstract service gives a citation count. So far essentially nothing, since it is new. It isn't on arxiv, so the usual way to check citations doesnt work. Couldn't find it on Spires either, so must resort to harvard. 



#3
Sep2007, 02:10 AM

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#4
Oct2107, 05:26 PM

Astronomy
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PF Gold
P: 22,809

Bounce replaces bang: articles in *Nature Physics*
Ashtekar and some people from his group have a three paper series on this in the works, which challenges some of Bojowald's findings, and makes some new headway (including inflationand their version of the solvable LQC model)
Ashtekar and his group have published numerous papers about quantum bounce cosmology in the past two yearsit has been an active time. But I think this paper has special importance so I'll give a link to it. http://arxiv.org/abs/0710.3565 On the robustness of key features of loop quantum cosmology Abhay Ashtekar, Alejandro Corichi, Parampreet Singh 28 pages, 1 figure (Submitted on 18 Oct 2007) "A small simplification based on well motivated approximations is shown to make loop quantum cosmology of the k=0 FRW model (with a massless scalar field) exactly soluble. Analytical methods are then used i) to show that the quantum bounce is generic; ii) to establish that the matter density has an absolute upper bound which, furthermore, equals the critical density that first emerged in numerical simulations and effective equations; iii) to bring out the precise sense in which the Wheeler DeWitt theory approximates loop quantum cosmology and the sense in which this approximation fails; and iv) to show that discreteness underlying LQC is fundamental. Finally, the model is compared to analogous discussions in the literature and it is pointed out that some of their expectations do not survive a more careful examination. An effort has been made to make the underlying structure transparent also to those who are not familiar with details of loop quantum gravity." Last year their computer models of the big bounce kept showing that the bounce occurs when the density reaches 82 percent Planck, which is 4.2 x 10^{96} kilograms per cubic meter Now the solvable model has confirmed that analytically and given an algebraic expression for it. A second paper, by Ashtekar, Singh, and Pawlowski (in preparation) includes "phenomenologically viable" INFLATION that is an inflation that gives enough magnification to fit astronomical data. It had to be checked that the bounce model was compatible with this. See page 24 of the current paper. A third paper is in preparation by Corichi and Singh. It is called "Quantum Bounce and Cosmic Recall". It challenges Bojowald's finding of a serious loss of information during the bouncean effect which he called "cosmic forgetfulness". See page 4 of the current Ashtekar et al paper. =================== the point of including a massless scalar field is simply to have SOME matter in the universeand this is the simplest kind to include. the practice of doing that goes back to the very first papers about the LQC bounce, around 2001. Having a completely empty universe would be too unrealistic, so you put in the simplest matterfield you can. 



#5
Oct2107, 09:15 PM

P: 640

My questions/comments
http://arxiv.org/abs/0710.3565 On the robustness of key features of loop quantum cosmology Authors: Abhay Ashtekar, Alejandro Corichi, Parampreet Singh (Submitted on 18 Oct 2007) I. INTRODUCTION Specifically, when there is at least one massless scalar field present, the physical sector of the theory was constructed in detail and then used to show that the big bang is replaced by a quantum bounce.  What are we talking about … one massless scalar field present,? Is this a fifth force that does not interact with the fermions? (The fundamental matter particles (quarks and leptons, as well as most composite particles, such as protons and neutrons) are fermions.) Is this massless scalar field the “building block” of spacetime?  p. 21 …. where in the last step we have used the value γ ≈ 0.24 for the BarberoImmirzi parameter that led to ρcrit ≈ 0.82ρpl in [3, 4, 13]. We wish to emphasize that this is an absolute bound on ρ on the entire physical Hilbert space Hphy; there is no restriction that the states be, e.g., semiclassical. Note also that a factor of 10 in the value of γ would change ρsup (and ρcrit) by a factor of 103. The fact that the value of γ obtained from the entropy calculation yields ρsup ∼ ρPl points to an overall coherence of LQG.  I’ve note this (factor of 10) in my blog.  p.23 Not only is there no a priori reason for the theory to make sense as the area gap is shrunk to zero, but within the LQG framework it would be hard to make physical sense of the limit if it existed. At the Planck scale, discreteness is a fundamental and essential ingredient of the theory. The continuum emerges on coarse graining; ignoring the fine structure of quantum geometry because one is not interested in phenomena at the Planck scale is very different from taking the naive continuum limit λ → 0 which corresponds to washing out quantum geometry at all scales.  In his calculations, the area gap cannot go to less than minimum length. As I have shown in my blog the minimum size of a sphere will be one that has a surface area of 24 units.  p. 24 Recent analysis has shown that its bounce picture is also robust with respect to the inclusion of a phenomenologically viable inflationary potential [31]. Thus, the suggestion (see, e.g., [12, 14]) that the bounce would not persist once a potential is included has turned out to be incorrect.  Is this arm waving? … the inclusion of a phenomenologically viable inflationary potential [31]  p. 25 The new element —which may also be useful in a suitably gauge fixed version of full LQG— is that the loop along which the holonomy is defined is shrunk not to zero but only till it encloses an area ao of Planck size.  I think that this statement violates minimum length. As I have shown in my blog the minimum size of a sphere will be one that has a surface area of 24 units. Simple calculations show that if the surface area is 24 Planck units then the enclosed area will not be Planck size.  jal 



#6
Oct2107, 10:33 PM

Astronomy
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PF Gold
P: 22,809

The sentence you quote is not an argument, it is a brief announcement of a paper in the works. Since the sentence is not an argument, it is not handwaving. So the answer to your question is no. To evaluate their argument that they can include adequate inflation, you simply have to wait until reference [31] appears. I will flag it for sure. Then if you wish, you can read it and decide if you think the arguments are trustworthy or handwaving. For now we just have a headsup to expect the paper [31] (by Ashtekar, Pawlowski, Singh). The amount of inflation usually considered adequate is 60 efolds, that is distance increases by a factor of e^{60}. That is what folks call phenomenologically adequate (essentially meaning consistent with observations) All they are saying (without any handwaving) is simply that in one of their forthcoming papers they include what is normally considered adequate inflation and the model still works. For help with other points, please reread my post #4. Hope this helps. 



#7
Oct2207, 12:07 PM

P: 640

Hi Marcus!
You are taking this into unknown territories. Therefore, I'll extend it .... The Schwarzschild radius of our universe is greater than the present size of our universe. It does not expand. It was always the same size. Changing the position of the particles inside the Scharzschild radius does not change the radius. What those particles are doing does not change the size of the radius. So, if you want to pretend that they all got together and made a big bang or that they all got together and bounced it still does not change the radius of the gravity. (Scharzschild radius) What is important is how far away is another universe. After all, none of them have come crashing into our universe. (A black hole) There is no evidence that even one particle is falling into our universe. Or, ???? is there? (Fred Hoyle would have liked to know.) The universe is expanding into it’s Scharzschild radius and by black hole logic it cannot expand any farther.  Reference from David M. Harrison: For a mass of 2.5 x 1053 kg, i.e. a 2 and a 5 followed by 52 zeroes kg, the Schwarzschild radius is about 17 billion light years. This huge mass is an estimate for the total mass of the universe. Also, given that the age of the universe is 15 billion years or so, 17 billion light years is awfully close to the size of the universe. Does this mean that the universe itself is a black hole?  jal 



#8
Oct2207, 12:17 PM

Astronomy
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PF Gold
P: 22,809

Hello all, I would appreciate it if we could keep this thread on the topic of the LQC bounce and the current research papers on it.
The main thing that has come up recently is a series of three papers by Ashtekar's group. Where we have one (see the abstract here) and it cites two more in the works. 



#10
Oct2407, 07:41 PM

Astronomy
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PF Gold
P: 22,809

Ashtekar's LQC bounce paper for which the link and abstract was given a post or two back indicated it was the first of a series of three papers about the bounce.
Although these authors are the same Penn State group as Bojowald, and are using virtually the same formalism, there are some key differences. So they find a result that differs from the conclusion in Bojowald's Nature Physics article. It will be useful to compare http://arxiv.org/abs/0710.4543 Quantum bounce and cosmic recall Alejandro Corichi, Parampreet Singh 4 pages (Submitted on 24 Oct 2007) "Loop quantum cosmology predicts that, in simple models, the big bang singularity of classical general relativity is replaced by a quantum bounce. Because of the extreme physical conditions near the bounce, a natural question is whether the universe can retain, after the bounce, its memory about the previous epoch. More precisely, does the universe recall various properties of the state after evolving unitarily through the bounce or does it suffer from cosmic amnesia as has been recently suggested? Here we show that this issue can be answered unambiguously by means of an exactly solvable model, derived from a small simplification of loop quantum cosmology, for which there is full analytical control on the quantum theory. We show that if there exists a semiclassical state at late times on one side, peaked around a pair of canonically conjugate variables, then there are very strong bounds on the fluctuations on the other side of the bounce, implying semiclassicality. For a model universe which grows to a size of 1 megaparsec at late times, the change in relative fluctuations of the only nontrivial observable of the model across the bounce is less than 10^{57} (becoming smaller for universes which grow larger). The universe maintains (an almost) total recall. ======================== EDIT to reply to next post. The model normally used in cosmology is the (1923) Friedmann model which is SYMMETRYREDUCED. It is simplified by assuming HOMOGENEOUS AND ISOTROPIC. LQC takes over that assumptionwhich makes things easy to solve. But you don't know if the singularity would still be resolved it you removed the assumption of uniformity and had a very lopsided screwedup nonuniform universe collapse. Maybe it wouldnt bounce. Maybe it wouldn't end up afterwards looking like our rather uniform universe. In that sense, it is at least a logical possibility tthat the removal of the singularity could be an "artifact" or ARTIFICIAL RESULT of the (unrealistically restrictive) assumptions. So there is lots more to do. Relaxing the assumptions, removing restrictions, generalizing the results. Several papers recently by Bojowald and others are working in that direction (removing symmetries little by little to see what happens) Corichi does not refer to that recent work, but he points to the need for it. 



#11
Oct2507, 10:50 AM

P: 640

http://arxiv.org/abs/0710.4543
Quantum bounce and cosmic recall Alejandro Corichi, Parampreet Singh 4 pages (Submitted on 24 Oct 2007) Another question... In his opening statement, he says, "Whether singularity resolution is an artifact of the symmetries or a generic feature of the quantum cosmology is an open question." What does he mean by artifact of the symetries? 



#12
Oct2507, 07:27 PM

Astronomy
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PF Gold
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http://arxiv.org/abs/0710.4919
Harmonic cosmology: How much can we know about a universe before the big bang? Martin Bojowald 16 pages (Submitted on 25 Oct 2007) "Quantum gravity may remove classical spacetime singularities and thus reveal what a universe at and before the big bang could be like. In loop quantum cosmology, an exactly solvable model is available which allows one to address precise dynamical coherent states and their evolution in such a setting. It is shown here that quantum fluctuations before the big bang are generically unrelated to those after the big bang. A reliable determination of prebig bang quantum fluctuations would require exceedingly precise observations. " Bojo takes up the challenge and replies. 



#13
Oct2507, 07:46 PM

Mentor
P: 8,287

Incidentally, do you know when the arxiv is updated? This definitely wasn't there this morning when I scanned through! 



#14
Oct2507, 08:24 PM

Astronomy
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PF Gold
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tends to be around midnight GMT 



#16
Oct2507, 09:46 PM

Astronomy
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PF Gold
P: 22,809

There is an allout debate going onas I read from the past couple of articles. Bojowald is not yielding any ground and from careful reading I don't see him softening his punches.
I hope this does not cause a meltdown at Penn State. It is a uniquely valuable group of quantum cosmology people there. Bojowald argues that one can use the simplified solvable model pessimistically in the sense that if one can show that even in this simple model our knowledge of the prebounce phase is severely limited, then in any more realistic model (with less symmetry and with fluctuation in more degrees of freedom) the limitation of knowledge will be even worse. He gives arguments why the kind of information we would need about the present, in order to infer back, is not realistic to hope ever to acquire. It is a strong argument IMHO. It gives an upper bound on what we can theoretically know because the nice symmetric solvable model, if the universe would follow that, is in some sense the best possible situation for us! Even in that situation we can't infer back 100 %! (figuratively, Nature encrypts the prior quantum state, by making it depend on obscure unobvious features of the present). So in any real case we can expect things to be worse. this is what he means by the "pessimistic" use of the solvable model ================ in some sense, isn't this the way science is supposed to be done? hardhitting debate also it is the kind of thing that I was expecting to come to the fore across the board in QG because people's theoretical approaches are converging as they get closer to the goal of a comprehensive QG theory. One might think that convergence would a time of harmony and sweet amity, but what I expect is that as theories come closer to each other it becomes more urgent to thrash out any differences, and in this there can never be compromise. One sees this with the new spinfoam formulasFreidel and Rovelli are very close, and therefore feelings can be almost violent (paradoxically because of this very closeness!) so I think the eruption of disputes can be a good signif a little nervewracking 



#17
Oct2707, 04:17 PM

P: 21

___________ "Ask not what came before the Big Bang!" (said by the previous pope) 



#18
Oct2707, 04:48 PM

Astronomy
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PF Gold
P: 22,809

But in case anyone is confused, the estimates of the size, mass and Schwarzschild radius that were given above do not correspond to prevailing estimates you'd find in the professional literature AFAIK and need to be taken skeptically. Ask about those things in Cosmology forum if you want to know more. We constantly observe stuff that is estimated 45 billion LY distant. That is where the CMB photons came from, they are redshift z = 1100. So universe must be AT LEAST 45 billion LY radius. This is the present distance that corresponds to redshift 1100. Cosmologists do not know whether the universe is finite or infinite and they typically treat it as if it were infinite. Infinite radius, infinite mass. (Be sure you distinguish between models of the whole universe and the piece of it that is currently observable, from which light has reached us. The observable piece is necessarily finite, and constantly increasing.) The usual LambdaCDM model comes in two main versionsthe infinite one and a finite "best fit" positive curved model (technically corresponding say to Omega=1.011 as in Wright's January 2007 paper*) which has a circumference of about 800 billion LY. Since 1998 in professional cosmology there has essentially been no talk of the universe collapsing. The expectation is for it to expand indefinitely. A PF poster can sometimes talk about the "Schwarzschild radius of the universe" but it does not necessarily refer to anything mathematically defined. So not to worry. If you were just joking, grosquet, then I didn't have to say all that * eg. see Wright for the Omega = 1.011 figure corresponding to finite universe http://www.slac.stanford.edu/spires/...ww?irn=7049200 


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