
#19
Dec2011, 12:09 AM

P: 171

It's right there in the maths! You don't need other peoples opinions  do you think the maths works out? If you don't, why? If you don't understand the maths, learn it!
I am saying that in GR, an arbitrary amount of energy can exist in an arbitrary volume. Can I ask why you're so hung up on the Planck area thing? Think of it another way  if you think of energy in terms of photons, which are bosons, then you can have an arbitrary amount of them in whatever volume you want, due to Bose enhancement. (AKA the lack of Pauli Blocking) 



#20
Dec2011, 12:12 AM

P: 3

a book I read says the universe began as a bubble sitting in a liquid. The liquid is straight string energy at absolute zero with a quiver of waves moving through it. The first strings jumped into the bubble and went BANG and continued. This raised the temperature of the inside surface of the bubble which decayed the liquid into circular string inside the bubble forming all the particles. A black hole with lots of gravity is this energy trying to revert to it's previous liquid form. Dark energy is the gravitational pull of the surrounding liquid on all matter and is why it is accelerating. It would account for the even distribution of galaxies. Einstein's GR black hole centre defining space and energy as infinity + would also define the liquid.




#21
Dec2011, 12:14 AM

P: 171





#22
Dec2011, 12:51 AM

Astronomy
Sci Advisor
PF Gold
P: 22,809

I can't believe you read a reputable account saying "billions of galaxies" energy fitting into a planck volume. You have to be kidding or totally naive. In a standard discussion, you don't expect GR to apply when density is above planck. Planck density is only a few micrograms of mass per planck volume. A few millionths of a gram, nowhere near the mass of a planet or a galaxy, let alone billions of galaxies. There is no need to take the online source quoted by BAR.GOUM seriously because it is talking about the density limit in general relativity. Nobody expects GR to apply at very high density so it does not matter what limit it has or does not have. I think bar.goum is sophisticated and realizes that a theoretical limit based on GR does not mean much. But a naive reader could get the idea that this is a real physical limit applicable to nature (not just something in a manmade theory). What is far more relevant is what people calculate as the density limit in QUANTUM GR. Quantum effects are expected to be important at high density, so forget classical GR. There are several different approaches to getting a quantum theory of GR and in several of them something like the Heisenberg Uncertainty Principle (HUP) takes over at very high density. HUP says quantum fields resist narrow constraint as to location. If you try to nail down position then momentum becomes highly uncertain. So as you might imagine, in a quantum theory of gravity the quantum corrections dominate and make gravity repel at high density. So you never even get as high as planck density. A minor collapse like a stellar mass BH might just settle down into an equilibrium (we don;t know yet). A major collapse could conceivably bounce out the back door. Basically we don't knowstuff has to be tested observationally. An important item on the agenda is to examine the CMB for traces of a bounce, as described in one or more of the theoretical models. There is brief simple discussion of this and some links at the "einsteinonline" essay I mentioned earlier. but current research papers dealing with the testing issue are more to the point. Here are 40some recent research papers related to the observational testing of a type of quantum gravity big bounce model. I.e. was the expansion we see initiated by collapse of a prior space: http://inspirehep.net/search?ln=en&l...100&sc=0&of=hb You really should look at actual peerreview research and not just exclaim "HOW INCREDIBLE!!!" about unverified secondhand gossip. 



#23
Dec2011, 01:21 AM

P: 171

Ouch man, that's a bit harsh. I don't think I was using "unverified secondhand gossip" when I was talking about local Lorentz invariance. I was careful to point out that the above was only true in the limit where GR is OK, and you'd have to consider QG in the limit as t>0. It was to illustrate the point that even in classical theories, arbitrary energy densities are possible. I don't think there's any issue with my statements when taken in that light. If there is, do let me know.
Insofar as Glyde was asking whether arbitrary energy densities are possible, my reply was that "Yes, in GR, they are". Since we have no theory of QG, it seems stupid to ask whether there would be a density limit in it. However, I must admit to some ignorance in this area  why is the HUP going to limit the possible energy density (by making gravity repel) in a QG situation? In classical QM arbitrary energy densities are possible, HUP or not. Nevertheless, it all seems rather speculative, since we have no theory of QG. 



#24
Dec2011, 02:08 AM

Astronomy
Sci Advisor
PF Gold
P: 22,809

I wish it were customary for science popularizers to add a caveat. "this incredible thing I'm telling you is simply the mathematical consequence of a manmade theory which the experts are pretty sure does not apply here." Like the myth of the "singularity", which is just the breakdown of a theory and shows it is not applicable. Not something one expects to exist in nature. This should be made carefully clear. It is an interesting research task to relax assumptions of isotropy and homogeneity and vary parameters and see if the bounce still happens and see at what density. And what possible observable signatures in the CMB, from different cases. It is not stupid, I expect you realize Bar.Goum. It is the way science is done. You get a model that fits current observation, you work back in time. It happens to show a bounce (which you did not put in) so you study various cases. Then you look for ways to test when we get higher resolution CMB data, what kinds of "footprint" if this bounce actually occurred. And maybe then you can falsify! So you can throw out the model which said there was a bounce and predicted the "footprint". It is pretty straightforward. If you want to find out more about the early universe phenomenology related to QG cosmology you can look up papers on arxiv e.g. by Aurelien Barrau Julien Grain Wen Zhao Jakub Mielczarek Here is a search that gets some of their papers and papers by others along related lines. http://inspirehep.net/search?ln=en&l...100&sc=0&of=hb These are papers which appeared 20082011. You can change the dates in the search if you want to go back earlier, but most of this QG early universe phenomenology research is rather recent so you will not find much. Don't bother to think about the HUP except as a preliminary analogy suggesting that quantum effects in geometry might resist collapse. Effects in quantum geometry that you can think of as in some sense analogous to how matter behaves. It's an intuitive handle people can relate to because they have heard of HUP. But if you want to see the actual math, look at a recent review paper by Abhay Ashtekar. 



#25
Dec2011, 02:23 AM

P: 171

Fair enough. I would say that I was saying something about nature insofar as our theories correspond to nature, to the best of our ability. Certainly QM and GR have provided the most accurate correspondence to measurements of any theory. What are we up to, 14 sig fig in QM, or something crazy like that?
But, you are right, and caveats should be given. I still see no reason that energy densities of arbitrary values cannot exist in nature. I should have perhaps said "we have no good theory of QG". But yes, there is no reason not to do research into the consequences of LQG on the CMB. Apart from the fact that it's using an unproven theory on as yet uncollected data. Generally speaking, I do have reservations about the search for "signatures" of any kind in the CMB  remember that paper by Penrose a couple of years ago? It was widely regarded as pretty dodgy. I think that the search for signatures in the CMB needs to be *very carefully* done  you can find almost anything in noise if you look closely enough. Thanks for the author links. I must admit, early universe QG cosmology isn't something I look into much, I mostly focus on experimental nuclear astrophysics, which is practically on the other side of the scale. 



#26
Dec2011, 06:44 AM

P: 275

It is stated: "At ultrahigh densities, with the whole of the observable universe squeezed into a volume much smaller than that of an atom" In other words, billions and billions of galaxies in energy form can fit the volumn smaller than the size of a single atom? Do you agree? If so, then planck volume can fit the energy of at least one galaxy like the milky way. Agree or disagree? 



#27
Dec2011, 07:21 AM

P: 366

Why does the planck volume hold such significance in your opinion? 



#28
Dec2011, 07:51 AM

Sci Advisor
P: 1,570

Sure, such high densities have not been tested in the lab. But we understand the equation of state of radiation, and we have lots of observational evidence that supports the Friedmann model. Extrapolation of these physical theories into untested regimes (as long as the theory is appropriate to these regimes) is a perfectly reasonable and substantiated practice. 



#29
Dec2011, 10:27 AM

Astronomy
Sci Advisor
PF Gold
P: 22,809

But second and perhaps more significantly, what you quote from the article is exactly where it is explaining why cosmologists suspect that the classical model may not apply. There is NOT a professional consensus that a singularity actually can or did occur in nature. (E.g. because quantum effects may be expected to dominate.) ==quote== Whether or not there really was a big bang singularity is a totally different question. Most cosmologists would be very surprised if it turned out that our universe really did have an infinitely dense, infinitely hot, infinitely curved beginning. Commonly, the fact that a model predicts infinite values for some physical quantity indicates that the model is too simple and fails to include some crucial aspect of the real world. In fact, we already know what the usual cosmological models fail to include: At ultrahigh densities, with the whole of the observable universe squeezed into a volume much smaller than that of an atom, we would expect quantum effects to become crucially important. But the cosmological standard models do not include full quantum versions of space, time and geometry  they are not based on a quantum theory of gravity. However, at the present time we do not yet have a reliable theory of quantum gravity. While there are promising candidates for such a theory, none are developed far enough to yield reliable predictions for the very early universe. ==endquote== Even this essay, which was written in 2006* or earlier, is now somewhat out of date. Unfortunately I don't happen to know a comparable more recent piece written for general audience. (*I first saw it online in 2006, and there have been only minor changes. I see there is a new copyright date of 2011.) 



#30
Dec2011, 04:35 PM

P: 275

Anyway. The head of a spin contains billions of atoms. So do you agree the following is correct: "Q. How many galaxies (in energy form) can fit the head of a pin? A. Billions and billions of galaxies or the entire universe (in energy form) assuming the detectable size of 43 billion light years" 



#31
Dec2011, 05:02 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

Comments on this thread remind me that LQC has progressed quite a bit in the past 2 or 3 years and it is not so easy to keep abreast. So I want to call attention to a Abhay Ashtekar's recent review papera kind of status reportand quote some interesting passages just to get the latest stuff out on the table.
http://arxiv.org/abs/1108.0893 Loop Quantum Cosmology: A Status Report Abhay Ashtekar, Parampreet Singh (Submitted on 3 Aug 2011) The goal of this article is to provide an overview of the current state of the art in loop quantum cosmology for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general; and, cosmologists who wish to apply loop quantum cosmology to probe modifications in the standard paradigm of the early universe. An effort has been made to streamline the material so that, as described at the end of section I, each of these communities can read only the sections they are most interested in, without a loss of continuity. 138 pages, 15 figures. Invited Topical Review, To appear in Classical and Quantum Gravity. ======================= The paper is long and covers many topics. We already had a discussion here at PF forum of one of the key equations. Equation (5.7) as I recall. I forget who was asking about it. Yes! It was (5.7) on page 73. This is the modified form of the Friedman equation which comes out of quantizing it and it shows clearly why you get gravity repelling at high density and causing a rebound (with an interval of superexponential expansion called superinflation). That however is not new, one sees that modified Friedman derived already in 2007 basic LQC papers. (Together with a figure for the critical density at which the quantum corrections dominate.) So I won't copy that here. What I want to take note of is some more recent stuff about generalizing and extending the model that they go into around page 67. Don't have time right now but hope to get back to this later today. 



#32
Dec2011, 06:42 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

It is as you know customary to make predictions which can be tested by FUTURE data and this on the whole is what is being done. I expect some more constraints to accrue from the European Planck spacecraft observations now in progress. But higher resolution (especially polarization of CMB) will be needed, again see Wen Zhao's paper. It's hardly a criticism to note that a lot of the testing literature is aimed at future possible data collectionalthough of course some relates to past and current. Here is the link again. http://inspirehep.net/search?ln=en&l...100&sc=0&of=hb The one by Wen Zhao I mentioned is as I recall #22 on the list. 



#33
Dec2011, 07:41 PM

P: 275

Markus, the universe in a grain of sand is assuming classing GR.. but quantum GR says the density can't be high so not only can all energy of the universe in planck volume not possible, even atomic volume not possible. So based on your reading and experience, what is the surest bet of the minimize size of the initial universe. Do you think it was once maybe about a Ping Pong ball size or a baseball size or building size or the size of Texas? What is your estimate from non classical GR calculations and theoretical projection of rewinding the universe down to smaller and smaller size?




#34
Dec2011, 09:45 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

Their number was more precise than 10. I am just speaking approximately. Their lower bound was roughly that. I can get the link to the report if you want. It's online. Many cosmologists think of the U as spatially infinite, and therefore it would be spatially infinite at the start of expansion. And they do their calculations based on that assumption. You get approximately the same fit to the data whether you say infinite or finitebutverylarge. So the first thing is always to remember that when people talk cosmology OBSERVABLE universe is just a small portion of the full universe that one has to model with the equations or the computer simulator. What one models is the full thing and this can be spatially infinite (even already at "bang" time) or in any case very large. Don't confuse observable universe with the whole thing. I'm sure you know this, but people forget. It has to be made explicit to avoid confusion. In standard cosmology, as you probably know, the universe has no edge or boundary, and matter is distributed approximately evenly throughout. So if space is infinite volume then matter must be infinitebecause it is throughout all space. =================== That is just preliminaries. Are you OK with all that? 



#35
Dec2011, 09:55 PM

P: 275

Good you emphasize on the observable universe vs actual extent. Anyway. Do you know how many planck volume can fit in say the hydrogen atom up to the electron orbital? You really think that if the observable universe energy were contained in the hydrogen atom. The planck volume would merely hold a few micrograms. This would make the planck scale unimaginably small. I wonder if your analogy is valid (ignoring quantum gravity and HUP). Or for a radius of 40 Billion light years, how many meters or miles across would be the planck length? Any ideas? 



#36
Dec2011, 10:05 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

This is around 100 thousand times smaller than the atom, if I remember right. 


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