Bounce replaces bang: articles in *Nature Physics*

In summary, Abhay Ashtekar and his group have shown that the key features of loop quantum cosmology are robust and can be explained in a way that is analogous to string theory.
  • #1
marcus
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Bojowald's What happened before the Big Bang? article in the August 2007 issue of NATURE PHYSICS is available to non-subscribers

To get the article directly in HTML
http://npg.nature.com/nphys/journal/v3/n8/full/nphys654.html [Broken]

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 [Broken]
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 Heisenberg-like 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 recognition----they do nonstring quantum gravity.

I suppose part of the news about the Nature Physics article is that it is prominent among peer-reviewed journals specializing in physics. Has the highest IPI impact rating--a measure of citations per article. (It's a subsidiary of the weekly NATURE.) It's a good place to publish---prestige and visibility-wise. 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/v3/n8/full/nphys690.html [Broken]

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).
 
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  • #2
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 doesn't work. Couldn't find it on Spires either, so must resort to harvard.
 
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  • #3
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).
I haved read the paper but I did not understand this. It is known that inflation washes out initial conditions because it stretches them out of the observable universe. Is the limitation to observe fully the other side of the big bang of different nature?
 
  • #4
Ashtekar weighs in on the bounce

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 inflation---and their version of the solvable LQC model)

Ashtekar and his group have published numerous papers about quantum bounce cosmology in the past two years---it 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 1096 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 bounce---an 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 universe----and 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.
 
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  • #5
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?
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p. 21
…. where in the last step we have used the value γ ≈ 0.24 for the Barbero-Immirzi 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., semi-classical. 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.
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I’ve note this (factor of 10) in my blog.
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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.
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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.
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Is this arm waving? … the inclusion of a phenomenologically viable inflationary potential [31]
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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.
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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
jal said:
Is this arm waving? … the inclusion of a phenomenologically viable inflationary potential [31]
Arguments are called hand-waving if they leave out steps, are unrigorous, appeal to intuition and hopeful suggestion instead of solid math and clear logic.

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 hand-waving. 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 heads-up to expect the paper [31] (by Ashtekar, Pawlowski, Singh).

The amount of inflation usually considered adequate is 60 e-folds, that is distance increases by a factor of e60. 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 re-read my post #4.

Hope this helps.
 
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  • #7
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
 
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  • #8
Keeping on the LQC bounce topic

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.

marcus said:
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 inflation---and their version of the solvable LQC model)

Ashtekar and his group have published numerous papers about quantum bounce cosmology in the past two years---it 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 1096 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 bounce---an 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 universe----and 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.

In case anyone still does not get it---we are talking RECENT research by the PENN STATE LQC GROUP which contains Ashtekar, Bojowald, Corichi, Singh, Pawlowski and which studies the LQC bounce in quantum cosmology. If you have trouble telling whether a paper is relevant to topic or not, a handy way to check is to see if it is recent, that is 2007, and if it is from that LQC group of people who mostly are at Penn State, or close collaborators.
 
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  • #9
You started... not me!
 
  • #10
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 semi-classical 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 semi-classicality. For a model universe which grows to a size of 1 megaparsec at late times, the change in relative fluctuations of the only non-trivial 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 SYMMETRY-REDUCED. It is simplified by assuming HOMOGENEOUS AND ISOTROPIC.
LQC takes over that assumption---which 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 screwed-up non-uniform universe collapse. Maybe it wouldn't 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.
 
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  • #11
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
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 space-time 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 pre-big bang quantum fluctuations would require exceedingly precise observations. "

Bojo takes up the challenge and replies.:biggrin:
 
  • #13
marcus said:
Bojo takes up the challenge and replies.:biggrin:

Something tells me that Bojowald knew about Corichi and Singh's papers before it was put on the arxiv!

Incidentally, do you know when the arxiv is updated? This definitely wasn't there this morning when I scanned through!
 
  • #14
cristo said:
Incidentally, do you know when the arxiv is updated? This definitely wasn't there this morning when I scanned through!

5 PM pacific
tends to be around midnight GMT
 
  • #15
marcus said:
5 PM pacific
tends to be around midnight GMT

Ahh, ok. Thanks
 
  • #16
There is an all-out debate going on---as 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 pre-bounce 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?
hard-hitting 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 formulas----Freidel 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 sign---if a little nerve-wracking
 
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  • #17
jal said:
The universe is expanding into its Schwarzschild 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

Would the universe go into a reverse bounce, once it expands close enough to its Schwarzschild radius?
___________
"Ask not what came before the Big Bang!" (said by the previous pope)
 
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  • #18
grosquet said:
Would the universe go into a reverse bounce, once it expands close enough to the Schwarzschild radius?
I don't know if you are asking that seriously, grosquet, or just mocking the statement. I assume you are asking "tongue in cheek" (that is, ironically.)
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 versions---the 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 :smile:

* eg. see Wright for the Omega = 1.011 figure corresponding to finite universe
http://www.slac.stanford.edu/spires/find/hep/www?irn=7049200 [Broken]
 
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  • #19
Marcus!
I wasn't joking.
I'm learning from all of you. I got this info from someone who is suppose to know more than me. I said that I cannot do the calculations for the Schwarzschild radius of the universe.
I couldn't understand the paper you mentioned and the relationship to the Schwarzschild radius of the universe. Your comment definitely indicates a conflict if the Schwarzschild radius is less than 45 BLY distance.
At the very least you know from my comments that there is at least one person interested in reading the links that you provide.
jal
 
  • #20
jal said:
At the very least you know from my comments that there is at least one person interested in reading the links that you provide.

make that at least 2.
 
  • #21
Does a bounce model require inflation when you can have the Schwarzschild radius do the job?

How to get rid of the Schwarzschild radius. Without inflation you cannot get out of the Schwarzschild radius. The Schwarzschild radius is given by
Rs = 2GM/c^2
where G is the gravitational constant, m is the mass of the object, and c is the speed of light.
-------------
http://www.astronomynotes.com/cosmolgy/chindex.htm
Astronomy Notes by Nick Strobel
--------------
http://www.astro.ucla.edu/~wright/density.html
How do Astronomer's Measure the Density of the Universe?
-------------------
http://www.physlink.com/Education/AskExperts/ae252.cfm?CFID=3897344&CFTOKEN=27312663
How was the critical density of the universe calculated?
------------------
http://www.damtp.cam.ac.uk/user/gr/public/inf_lowden.html
Low Density Inflationary Universes
-------------
http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/age.html
How do we measure the size and the age of the Universe?
-----------
 
  • #22
I would like to clear the air.
I am not a pimply faced teenager trying to jerk your chain.
I did not think that you were a child that I had to hold your hand through every step to investigate a new concept, a problem, or an ambiguity that I have seen in a paper that under a discussion.
I do not like to be referred to in a demeaning way, “a PF poster”.
I do not expect you to have the answer to all of my questions. Other readers might have the answer. Ignoring my questions is equivalent to giving me the silent treatment and signaling that I should go away and go play with the children.
I have made it clear that I’m learning and that I’m looking for answers.
-----------
Now back to the subject of the bouncing model.
How was the Schwarzschild radius not created as you decreased the volume of the universe? Those 10^80 particles are always creating a “standing” Schwarzschild radius. How big is that radius? How was that barrier overcome in the expanding bounce phase?
Is the answer a "speculative" force that still needs to be detected or is it still a mathematical construct?
jal

ps Glad the topic got moved. Someone here might be able to answer the bounce model questions.
 
  • #23
jal, what precisely do you mean when you talk about the "Schwarzschild radius of the universe?"
 
  • #25
I see that Chris Hillman has done a lot of explanations on black holes.

I pulled out a few quotes from Black-hole boundaries by Ivan Booth
p. 10
Finally, before moving on to nonisolated horizons, we note that just as for Killing horizons, the phase space of isolated horizons includes more than just outer black-hole horizons. For example, all horizons in the fully extended Reissner–Nordström–deSitter spacetime qualify as isolated horizons —the outer black-hole horizon, the inner Cauchy horizon, the corresponding horizons associated with white holes, and even the cosmological horizons.
Further, since all Killing horizons are also isolated horizons, the examples of non-black-hole Killing horizons discussed in Sect. 2.2 are also examples of isolated horizons. Thus, while the isolated horizons conditions are sufficient to capture many of the properties of equilibrium black holes they are not exclusively black holes. In the next section, as we classify (potentially) nonisolated horizons, we will also consider the extra conditions that are necessary to distinguish between the various subclasses of horizons.
p. 17
It is also of some interest to consider the physical circumstances under which the horizon “jumps” can occur. Again it is shown in ref. 35 that these jumps occur when the infalling matter is dense relative to 1/A, where A is the surface area of the horizon. For a solar-mass black hole this means that jumps occur only when the infalling matter is at least as dense as a neutron star while it is only when one gets to galactic-mass black holes that jumps could be generated by matter with the density of water. This suggests an answer to the question posed in relation to the Vaidya and Oppenheimer–Snyder spacetimes.
Both evolutions can occur but “jumps” only happen under very extreme conditions (for small black holes). Essentially they correspond to new horizons forming outside of old ones.
note: The bigger the black hole the less is the density inside to keep the Schwarzschild radius
p. 20
4.2. Laws of expansion
As was discussed in Sect. 2.1, one of the best known properties of classical, causally defined, black holes is that, given the null energy condition, they never decrease in area.
…. It is also clear that if the null-energy condition is violated (as it would be by Hawking radiation) then we could have L_θ(_) > 0 and so a FOTH that decreases in area.
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What is so interesting with the bounce model is that it gives a whole new prospective on the universe.

Whether you call the spacetime fabric pixels, waves, strings, nodes, solitons, plaquettes, ZPE, quarks, preons, kernels or dark energy you still need a mechanism to make them.
By studying the Schwarzschild radius of the universe you might be able to get a mechanism.
First, let’s think of a microwave oven. You turn the power on and you get a pattern of standing waves. Turn the power off and the pattern disappears. The pattern appears because there is a continuous “feed” of the same wavelength to the cavity and the shape of the cavity determines the pattern. At first the pattern is chaotic but it quickly becomes a stable pattern.
Now … think of the Schwarzschild radius of the universe. It is sending out a continuous “feed” of the same wavelength to the interior of the universe. We are now into its stable pattern phase. This phase could have started many bounces ago and will continue for many more bounces. I don’t know what would be the wavelength that the Schwarzschild radius of the universe would be sending out to bounce around inside and to create a stable pattern. That’s a job for the “math kids”.
The stable pattern that exist must be between 10^-15 and 10^-18 because we cannot see them and quarks need to see them and interact with them.
Let’s go to another simple example; A guitar string. It is fixed at both ends. Start a vibration at one end. If the initial vibrations that you start are at both ends at the same time then there will be the well understood wave interferences and the standing wave/soliton pattern will arise in the string.
You need, in your model, the Schwarzschild radius of the universe to set up a Simple Quantum Structured Spacetime.
Only a range of Schwarzschild radius will produce our spacetime that is at a quantum range of 10^-15 to 10-^18. It must be just about right size.
Let’s see what the “math kids” can come up with.

There is enough material here to write a good paper. (Even make a holographic universe.)
jal

This post can also be found in my blog
 
  • #26
I couldn't read the article you link to, since I don't have subscription. However, I've read the post above, and you still don't seem to define the "schwarzschild radius of the universe." Are you saying that the universe is a black hole?
 
  • #27
'What happened before the Big Bang?'

'What is north of the north pole?'
 
  • #28
I have been able to find
http://eprintweb.org/S/authors/All/my/Y_Myung/10 [Broken]
Black hole and holographic dark energy
Yun Soo Myung

Received. 06 February 2007 Last updated. 11 April 2007
Abstract. We discuss the connection between black hole and holographic dark energy. We examine the issue of the equation of state (EOS) for holographic energy density as a candidate for the dark energy carefully. This is closely related to the EOS for black hole, because the holographic dark energy comes from the black hole energy density. In order to derive the EOS of a black hole, we may use its dual (quantum) systems. Finally, a regular black hole without the singularity is introduced to describe an accelerating universe inside the cosmological horizon. Inspired by this, we show that the holographic energy density with the cosmological horizon as the IR cutoff leads to the dark energy-dominated universe with $ømega_{ m Lambda}=-1$.
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I would assume that, eventually, that there will be calculations made which include the bounce model.
If you "click" on his name you will get 106 papers.
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jal
 
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  • #29
Schwarzschild radius = black hole = The Cosmic Horizon
Fulvio Melia is not a "crank" and he is proposing a model of the universe that has a Schwarzschild radius.

http://arxiv.org/find/astro-ph/1/au:+Melia_F/0/1/0/all/0/1?skip=0&query_id=9c0202a79c0b7582
Showing results 1 through 25 (of 74 total) for au:Melia_F

http://arxiv.org/abs/0711.4181
The Cosmic Horizon
Authors: Fulvio Melia
(Submitted on 27 Nov 2007)

Birkhoff’s theorem states that the metric inside an empty spherical cavity, at the center of a spherically symmetric system, must be equivalent to the flat-space Minkowski metric.
Space must be flat in a spherical cavity even if the system is infinite. It matters not what the constituents of the medium outside the cavity are, as long as the medium is spherically symmetric.
If one then imagines placing a spherically symmetric mass at the center of this cavity, according to Birkhoff’s theorem and its corollary, the metric between this mass and the edge of the cavity is necessarily of the Schwarzschild type.
This consequence of the corollary to Birkhoff’s theorem is so important—and critical to the discussion in this paper—that it merits re-statement: the spacetime curvature of a wordline linking any point in the universe to an observer a distance R away may be determined by calculating the mass-energy enclosed within a sphere of radius R centered at the origin (i.e., at the location of the observer). The mass-energy outside of this volume has a net zero effect on observations made within the sphere.
There is little doubt that a cosmic horizon exists.
It is required by the application of the corollary to Birkhoff’s theorem to an infinite, homogeneous medium, and there is some evidence that we have already observed phenomena close to it. However, it may be that observational cosmology is not entirely consistent with the condition R0 ≈ ct in the current epoc. If not, there must be some other reason for this apparent coincidence. Perhaps the assumption of an infinite, homogeneous universe is incorrect. Whatever the case may be, the answer could be even more interesting than the one we have explored here.
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There is no reason for me to use my words when it is said much better by Fulvio Melia. Therefore, I’m quoting, for those who do not want to take the time to read all of his paper and arguments.

http://arxiv.org/abs/0711.4810
Dark Energy in Light of the Cosmic Horizon
Authors: Fulvio Melia
(Submitted on 29 Nov 2007)
In this paper, we examine the role played by our cosmic horizon
R0 in our interrogation of the data, and reach the rather firm conclusion that
the existence of a cosmological constant is untenable.
The observations are
telling us that R0 ≈ ct0, where t0 is the perceived current age of the Universe,
yet a cosmological constant would drive R0 towards ct (where t is the cosmic
time) only once, and that would have to occur right now. In contrast, scaling
solutions simultaneously eliminate several conundrums in the standard model,
including the ‘coincidence’ and ‘flatness’ problems, and account very well for
the fact that R0 ≈ ct0. We show in this paper that for such dynamical dark
energy models, either R0 = ct for all time (thus eliminating the apparent
coincidence altogether), or that what we believe to be the current age of the
universe is actually the horizon time th ≡ R0/c, which is always shorter than
t0. Our best fit to the Type Ia supernova data indicates that t0 would then
have to be ≈ 16.9 billion years. Though surprising at first, an older universe
such as this would actually eliminate several other long-standing problems in
cosmology, including the (too) early appearance of supermassive black holes
(at a redshift > 6) and the glaring deficit of dwarf halos in the local group.

Dark energy is often thought to be the manifestation of a cosmological constant, _, though no reasonable explanation has yet been offered as to why such a fixed, universal density ought to exist at this scale. It is well known that if _ is associated with the energy of the vacuum in quantum theory, it should have a scale representative of phase transitions in the early Universe—many, many orders of magnitude larger than _c.
Nonetheless, though many in the cosmology community suspect that some sort of dynamics is responsible for the appearance of dark energy, until now the sensitivity of current observations has been deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant _ (see, e.g., Corasaniti et al. 2004). This conclusion, however, appears to be premature, given that the role of our cosmic horizon has not yet been fully folded into the interrogation of current observations.

… it is now possible to accurately calculate the radius of our cosmic horizon, R0, defined by the condition …
… This is the radius at which a sphere encloses sufficient mass-energy to turn it into a Schwarzschild surface for an observer at the origin of the coordinates …

… What we infer to be the time since the Big Bang, is instead the “horizon” time th ≡ R0/c, which must be shorter than t0. As discussed in Melia (2008), this has some important consequences that may resolve several long-standing conflicts in cosmology.
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The “math kids” have already done the calculations.
I hope you enjoy the mental stimulation.
 
  • #30
Schwarzschild radius of the universe?

jal said:
Hi Marcus!
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.

Since you mentioned my name, maybe I can chime in here with a rather fundamental observation: the familiar Carter-Penrose diagrams illustrating the causal structure of black hole models in gtr generally assume that we are modeling an isolated massive object in an asymptotically flat spacetime (e.g. Schwarzschild vacuum), or perhaps an isolated massive object in a (perturbed) de Sitter model (e.g. Kottler or Schwarzschild-de Sitter lambdavacuum). The "mass of the universe" cannot simply be plugged into compute a "Schwarzschild radius" because the universe is not an isolate object. This is just one reason why attempts to describe "the universe" as something like a "black hole" are open to grave suspicion; at the very least, authors trying to construct such analogies have a great deal of very careful explaining to do.

For a nontechnical discussion (predating the revelation of an apparently nonzero cosmological constant, but still valid and still relevant here), see the second and third sections of this UseNet Physics FAQ article. (The author of that article, Phil Gibbs, long ago made his own suggestion in an eprint which--- to his amusement--- was one of those ripped off in the recent Turkish Plagiarism Ring scandal at the arXiv; see http://eventsymmetry.blogspot.com/2007/08/white-hole-model-of-big-bang-in-recent.html [Broken].)
 
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1. What is the main concept behind "Bounce replaces bang" in *Nature Physics*?

The concept of "Bounce replaces bang" refers to a new theory proposed by scientists that suggests the universe may have gone through a series of bounces instead of a big bang, leading to its expansion.

2. How does this theory differ from the traditional big bang theory?

This theory challenges the traditional big bang theory, which states that the universe began with a single, massive explosion. Instead, "Bounce replaces bang" suggests that the universe underwent a series of expansions and contractions, with each bounce leading to a new cycle of expansion.

3. What evidence supports this theory?

Scientists have found evidence in the cosmic microwave background radiation, which is the leftover heat from the big bang. This radiation shows patterns that could be explained by the bounces in the "Bounce replaces bang" theory.

4. How does this theory impact our understanding of the universe?

If this theory is proven to be true, it would significantly change our understanding of the universe's origins and evolution. It could also help explain some of the unanswered questions in physics, such as dark energy and dark matter.

5. What are the next steps in researching this theory?

Scientists are currently conducting more studies and experiments to gather more evidence and support for the "Bounce replaces bang" theory. They are also working on developing new models and simulations to better understand the implications of this theory on the universe.

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