Big Bang

[lucaspa]

Tycho; yes, the theory SSU, is very much alive. checked Google, just to make sure with a million or so hits.

 

Those are hits that mention SSU. But SSU is a falsified theory. There is evidence that simply can't exist if SSU were true.

 

erstanding. not possible and both have many principles which they agree on. i do get upset when a poster is told BBT is the only possible answer and by no mention of another gives a false impression.

 

Sorry, but SSU is not a possible answer. There are proposed alternatives to BB. Ekpyrotic is one, No Boundary is another. However, they both LOOK LIKE BB. That is, they accept all the data that led to BB, but think there is an alternative way to get the same data.

 

SSU cannot produce the data.

[lucaspa]

that gives you the program of the 8-26 jan workshop at KITP about what are the most promising ways to resolve spacetime singularities, especially how to get over the bigbang singularity and conditions before it.

 

http://online.kitp.ucsb.edu/online/singular_m07/

 

Each talk there has VIDEO that you can download and most also have PDF of the lecturer's SLIDES.

 

You can form your own impression. Maybe I'm wrong! Maybe they do have a bunch of stuff focused on colliding branes!

 

There are a couple of titles there that could be discussing 'branes and ekpyrotic. Remember, ekpyrotic is dependent on String Theory.

 

Therefore, the following talks could be ekpyrotic:

Struggling with Spacelike Singularities in String Theory[Podcast][Aud][Cam]

Winding Strings and Spacelike Singularities[slides][Podcast][Aud][Cam]

 

Loop Quantum Gravity -- big rival to String Theory -- was represented:

Non-singular Behavior in Loop Quantum Gravity[slides][Podcast][Aud][Cam]

 

As was No Boundary:

Discussion: No Boundary Proposal and Alpha Parameters[Podcast][Aud][Cam]

 

Now, your claim that the new theories go back to "before" the singularities needs some documentation. I see people trying to explain the singularity and find a theory of quantum gravity, but that still doesn't get us to a "before" the BB. Please cite your sources.

 

Ekpyrotic and No Boundary have a "before" the BB, but you didn't mention those two. So please tell us what you were thinking of.

 

The problem with them, in a nutshell, is that the time-evolution thru the big bang is not DETERMINISTIC. there is a before sequence which is modeled and there is an after sequence, but they don't actually CONNECT. the theory waves its hands at that point:

 

the other models we are talking about evolve the quantum state of the universe in a smooth deterministic way

 

Actually, ekpyrotic does give a smooth transition thru the singularity. Because there is no singularity. Ekpyrotic and No Boundary try to remove the singularity.

 

I also still don't see a "before" sequence in the other quantum gravity theories. Please tell us what you have gleaned is the "before" sequence.

 

Finally, as I mentioned elsewhere, quantum mechanics is inherently not deterministic. Therefore you can't get "the quantum state of the universe in a deterministic way".

 

they also have run a bunch of computer models, simulating the time evolution of the quantum state thru the big bang, in a lot of different cases---different sizes, different matter content, different constants.

 

Nothing like that ever happened with the colliding branes thing, back a few years when some people were into looking at it,

 

The computer model was done, but I don't think Turok needed the different cases. Because the initial conditions were not arbitrary -- there was something pre-existing "before" the collision of the 'branes.

 

However, such modeling of different conditions would be necessary if there was no "before" the BB. So this argues that the different quantum gravity theories don't provide a "before" the BB.

[lucaspa]

that gives you the program of the 8-26 jan workshop at KITP about what are the most promising ways to resolve spacetime singularities, especially how to get over the bigbang singularity and conditions before it.

 

http://online.kitp.ucsb.edu/online/singular_m07/

 

Each talk there has VIDEO that you can download and most also have PDF of the lecturer's SLIDES.

 

You can form your own impression. Maybe I'm wrong! Maybe they do have a bunch of stuff focused on colliding branes!

 

There are a couple of titles there that could be discussing 'branes and ekpyrotic. Remember, ekpyrotic is dependent on String Theory.

 

Therefore, the following talks could be ekpyrotic:

Struggling with Spacelike Singularities in String Theory[Podcast][Aud][Cam]

Winding Strings and Spacelike Singularities[slides][Podcast][Aud][Cam]

 

Loop Quantum Gravity -- String Theory's main rival -- was represented:

Non-singular Behavior in Loop Quantum Gravity[slides][Podcast][Aud][Cam]

 

So was No Boundary:

Discussion: No Boundary Proposal and Alpha Parameters[Podcast][Aud][Cam]

 

You made a claim in an earlier post that there is now a "before" the BB. Please document that. What I have seen are attempts to get a quantum theory of gravity -- to quantize GR. This would remove the "singularity" but still wouldn't give you anything "before" the BB.

 

Ekpyrotic and No Boundary provide a "before" the BB. But you dismiss ekpyrotic out of hand and do not mention No Boundary. So I sincerely want to know where you got the idea that there has not been determined that there was not a t = 0.

[jackson33]

Those are hits that mention SSU. But SSU is a falsified theory. There is evidence that simply can't exist if SSU were true.

 

Sorry, but SSU is not a possible answer. There are proposed alternatives to BB. Ekpyrotic is one, No Boundary is another. However, they both LOOK LIKE BB. That is, they accept all the data that led to BB, but think there is an alternative way to get the same data.

 

SSU cannot produce the data.

 

when trying to explain my understanding of the universe, SSU in its briefest description applies. *whatever the universe is has pretty much always been what it was or will be. regeneration of the old into new the source and conditions the only obstacle to eternal regenerations cannot be changed*.

there is no argument beyond this, that i will argue forward. other SSU theory or those that try to explain BBT principles into SSU are doing an injustice to the basic idea. BBT would seem to that this process had some kind of an exotic beginning, with out a need for or an explanation for why, where or what the singularity in fact was.

 

a google check for BBT or SSU or any particular added word will get you a million plus hits. yes some oppose, offer a variety of alternatives, however this is still true on any search of any theory. the ONLY consistent phrase is the BBT seems or is the accepted view of the scientific community.

[Martin]

There are a couple of titles there that could be discussing 'branes and ekpyrotic. Remember, ekpyrotic is dependent on String Theory.

...

... So I sincerely want to know where you got the idea that there has not been determined that there was not a t = 0.

 

did you WATCH those couple of titles? I mean at least the first few minutes to see if the person was going to discuss ekpyrotic?

Another way to see, if the PDF slides are available, is to check the first few slides.

 

Naturally I'd be interested if you find anything because it would contradict my impression that ekpyrotic has lost most of its interest over past 4 or 5 years.

 

I check arxiv regularly in the hep-th, gr-qc, and astro-ph and that's my sense.

 

As for Hawking, he is of course a celebrity, but I don't see people citing his recent papers. Haven't seen anything about "No Boundary" besides what comes across as perfunctory acknowledgment

 

That could be my own distorted view---I dont have accurate statistics on the current research literature---but I do try to keep my eyes open and that's my impression.

 

The big news in the past two years has been LQC, and that is why David Gross's KITP institute organized the workshop.

 

String theorists have suddenly registered and gotten interested in LQC results.

 

LQC reproduces standard cosmology back to within a few planck times of the start of expansion, and then it does not break down.

 

It continues back in time.

 

A lot of today's thinking is about how can we test LQC versus the standard GR picture (which has a breakdown). So far the observed data supports both equally, since LQC essentially duplicates conventional mainstream cosmology.

 

A lot of computer modeling has been done, running LQC models with various assumptions---the usual result is a bounce (you get a contracting spacetime region that reaches very high matter density of about 0.82 planck density, quantum corrections cause gravity to be repulsive, it then re-expands)

 

Also the LQC model has been solved analytically.

 

LQC was first invented by Martin Bojowald in 2001. It signaled a major sea-change when David Gross KITP invited Bojowald to be CO-ORGANIZER of the January workshop (with string Gary Horowitz). Bojowald drew in a lot of LQC and LQG people (Ashtekar, Dittrich, Thiemann, Pullin) who wouldnt ordinarily come to a KITP workshop. It was a shift.

 

Then KITP appears to have offered Bojowald a position (on his papers he now lists KITP as address, along with Ashtekar's Penn State institute).

 

If you want to see the latest LQC paper, the most authoritative and uptodate would be one dated 29 March 2007 by Bojowald

the "...Physical Solutions of Cosmological Bounces" paper

here is a list of all his papers so you can take your pick.

(the guy is young, PhD 2000)

http://arxiv.org/find/all/1/au:+Bojowald/0/1/0/all/0/1

[dstebbins]

That's not what I have read. Ekpyrotic means the destruction of our universe and the creation of a new one. We're going to need a citation for this paragraph, please.

 

Very well. The source that I am citing is a Science Channel documentary called "Parallel Universe." It was explained near the end of the show, so you may have missed it, but some scientists believe it while others don't. What you read must have been written by one who doesn't believe this.

[Martin]

Very well. The source that I am citing is a Science Channel documentary called "Parallel Universe." It was explained near the end of the show, so you may have missed it, but some scientists believe it while others don't. What you read must have been written by one who doesn't believe this.

 

I would agree with you that Ekpyrotic does not mean the same as "destruction of our universe and creation of another"

 

The name was chosen by Steinhardt and Hurok who first thought up the "colliding branes" scenario. As used by professionals, the term applies specifically to that type scenario.

 

It does not apply more generally to common variations of the "big crunch-bounce-big bang" sort of scenario.

 

The term "ekpyrotic" is a confusing choice because going back to Greek roots it means "from the fire" and evokes the image of the mythical Phoenix bird. So you might THINK it would apply generally to any cosmological scheme involving a bounce (a collapse generating very high density and temperature followed by a reexpansion).

 

So from the Greek you might THINK the term would apply to the Ashtekar/Bojowald LQC model, but it doesnt.

 

As an indication of the shift in focus, this year's APS (Am Phys. Soc.) April meeting in Florida has an invited speaker talk about Ashtekar/Bojowald LQC work. He is P. Singh who has been many times a co-author with both of them.

By contrast, there is no invited talk about the Steinhardt/Hurok "ekpyrotic" colliding branes type of cosmology.

 

I doubt even Steinhardt himself is working on "ekpyrotic" much these days. We can check the archive to make sure.

 

So the upshot is, I'd say you are basically right about what "ekpyrotic" means---which you got from that Science channel program. But there's a big lag (and other sorts of disconnect) between professional research and the popular media, so in general I'd advise caution about believing what you see on the tube.

[lucaspa]

when trying to explain my understanding of the universe, SSU in its briefest description applies. *whatever the universe is has pretty much always been what it was or will be. regeneration of the old into new the source and conditions the only obstacle to eternal regenerations cannot be changed*.

That is not exactly what SSU was. You qualify it by saying "whatever the universe is". That would allow you to accept that the "universe" is two 4-D 'branes in a 5-D space, wouldn't it? Or it would allow the expansion/crunch of the original No Boundary, right?

 

But that isn't what SSU stated. SSU stated that the universe we see -- the observable universe if you will -- is static.

 

 

other SSU theory or those that try to explain BBT principles into SSU are doing an injustice to the basic idea.

 

In that case, you are back to the original SSU. And that has been falsified. Sorry.

 

a google check for BBT or SSU or any particular added word will get you a million plus hits. yes some oppose, offer a variety of alternatives, however this is still true on any search of any theory. the ONLY consistent phrase is the BBT seems or is the accepted view of the scientific community.

 

And how does a theory get to that stage of acceptance? By having the alternative theories falsified!

 

We established in the aquatic ape thread that supporters != support. There is always going to be a few die-hards that cling to falsified theories. Shoot, up until a couple of years ago there was still a Flat Earth Society!

[Martin]

Sorry, but with quantum mechanics, NONE of the universe is strictly deterministic!

 

the quantum state evolves deterministically

we have a semantic disconnect.

the experts, quantum gravitists that run the models that evolve the wavefunction for the universe thru the former singularity

use the word deterministic to describe this.

there is no other word for what they mean, in common use at least

 

they are a bunch of professionals.

 

it is their research area

 

that is what they call it.

 

I don't think that you (or I) have any standing to challenge their use of the word.

 

If you read the papers you will see what they mean---there are graphs showing how the wave function for one of the variables evolves thru the bounce.

[Martin]

I should give some links

Bojowald's papers

http://arxiv.org/find/all/1/au:+Bojowald/0/1/0/all/0/1

there are 76 almost all on Quantum Cosmology (or related BH singularity removal)

He's the guy a science publisher like Elsevier or WorldScientific would go to for a science encyclopedia article on QC, or for a QC chapter in a handbook.

 

Or else they might go to Ashtekar.

http://arxiv.org/find/all/1/au:+Ashtekar/0/1/0/all/0/1

If you only look at the recent Ashtekar they are mostly QC (or general overview Quantum Gravity survey, which he gets asked to do for conferences and such)

 

There is also this by a couple of PHENOMENOLOGISTS (people concerned with testing). Joao Magueijo and Parampreet Singh. Singh works with Ashtekar but Magueijo is more of an outsider, so it gives a different perspective, I found this paper helpful:

http://arxiv.org/abs/astro-ph/0703566

[lucaspa]

did you WATCH those couple of titles? I mean at least the first few minutes to see if the person was going to discuss ekpyrotic?

 

Did you? Do they mention ekpyrotic? I told you I was going by the titles; I didn't have time to watch them.

 

Another way to see, if the PDF slides are available, is to check the first few slides.

 

That I did when slides were available. They are not for most of the talks.

 

Naturally I'd be interested if you find anything because it would contradict my impression that ekpyrotic has lost most of its interest over past 4 or 5 years.

 

Losing interest is not the same as being falsified. Have you found any articles refuting ekpyrotic? I'm not advocating ekpyrotic; I'm simply pointing out that it is a contender.

 

As for Hawking, he is of course a celebrity, but I don't see people citing his recent papers. Haven't seen anything about "No Boundary" besides what comes across as perfunctory acknowledgment

 

A whole paper at the meeting of nothing but "perfunctory acknowledgment"? Does that seem likely.

 

The problem I have heard about No Boundary is that it doesn't lead anywhere to more tests. In order to get No Boundary, parameters where all the space dimensions and time are indistinguishable are picked arbitrarily. They do lead to the universe we see, but there is no compelling reason to pick those parameters except they give the universe we see. So there is no way to test No Boundary.

 

That could be my own distorted view---I dont have accurate statistics on the current research literature---but I do try to keep my eyes open and that's my impression.

 

Martin, I caution you to remember 1) that science is not a popularity contest and 2) don't get emotionally attached to a particular theory. Yes, I see Loop Quantum Cosmology is the big news, but that doesn't mean it is accurate. We need to wait to see that.

 

Since LQC is based on LQG, do you see the same interest in LQG? If not, isn't that strange?

 

String theorists have suddenly registered and gotten interested in LQC results.

 

Well, that may represent the increasing trouble that String Theory is in.

 

LQC reproduces standard cosmology back to within a few planck times of the start of expansion, and then it does not break down.

 

It continues back in time.

 

I'm not sure about that as you are. I found a nice review of LQC written by Bojowald -- http://relativity.livingreviews.org/Articles/lrr-2005-11/

 

Some excerpts relating to LQC and time:

 

"The wave function thus extends to a new branch beyond the classical singularity, i.e., to a classically disconnected region. Intuitively this leads to a picture of a collapsing universe preceding the Big Bang, but one has to keep in mind that this is the picture obtained from internal time where other time concepts are not available. In such a situation it is not clear, intuitive pictures notwithstanding, how this transition would be perceived by observers were they able to withstand the extreme conditions. It can be said reliably that the wave function is defined at both sides, “before” and “after”, and every computation of physical predictions, e.g., using observables, we can do at “our” side can also be done at the other side. In this sense, quantum gravity is free of singularities and provides a transition between the two branches. The more complicated question is what this means for evolution in a literal sense of our usual concept of time (see also [200]). "

 

The article goes on to say that there is a discontinuity in time in the wave functions from "before" to "after":

 

"It is not clear in general that a wave function penetrating a classical singularity enters a new classical regime even if the volume becomes large again. For instance, there can be oscillations on small scales, i.e., violations of pre-classicality, picked up by the wave function when it travels through the classical singularity. As discussed in Section 5.17, the question of what conditions on a wave function to require for a classical regime is still open, but even if one can confidently say that there is such a new classical region does the question arise if time continues during the transition through the pure quantum regime. At least in the special model of a free massless scalar in isotropic cosmology the answer to both questions is affirmative, based on the availability of a physical inner product and quantum observables in this model [24]. "

 

Since Bojowald specifies a "special model", there are other models. I think you are going beyond what LQC says when you say definitively "It goes back in time."

 

The answer I see is "maybe LQC goes smoothly back in time." Or maybe there is a dicontinuity and time "starts" at the BB.

 

A lot of today's thinking is about how can we test LQC versus the standard GR picture (which has a breakdown). So far the observed data supports both equally, since LQC essentially duplicates conventional mainstream cosmology.

 

A lot of computer modeling has been done, running LQC models with various assumptions---the usual result is a bounce (you get a contracting spacetime region that reaches very high matter density of about 0.82 planck density, quantum corrections cause gravity to be repulsive, it then re-expands)

 

Also the LQC model has been solved analytically.

 

A question I have is: where do you get such a contracting spacetime? Black holes? Not the universe in general, since the data is pretty clear now that the universe is going to expand forever.

[lucaspa]

the quantum state evolves deterministically

we have a semantic disconnect.

the experts, quantum gravitists that run the models that evolve the wavefunction for the universe thru the former singularity

use the word deterministic to describe this.

there is no other word for what they mean, in common use at least

 

Partially, we do have a disconnect in usage of terms. As used in LQC, "determinism" has a different meaning than used in philosophy.

 

However, I think you are going beyond what Bojowald states. Again, going back to the review written by Bojowald, he specifically discusses determinism:

http://relativity.livingreviews.org/Articles/lrr-2005-11/

 

"Loosely related to unitarity, but more general, is the concept of determinism. This is usually weakened in quantum mechanics anyway since in general one makes only probabilistic statements. Nevertheless, the wave function is determined at all times by its initial values, which is sometimes seen as the appropriate substitute for deterministic behavior. In loop quantum cosmology the situation again changes slightly since, as discussed in Section 5.18, the wave function may not be determined by the evolution equation everywhere, i.e., not at points of classical singularities, and instead acquire new conditions on its initial values. This could be seen as a form of indeterministic behavior, even though the values of a wave function at classical singularities would not have any effect on the behavior for non-degenerate configurations.2 (If they had such an effect, the evolution would be singular.) In this situation one deals with determinism in a background independent context, which requires a new view. In fact, rather than interpreting the freedom of choosing values at classical singularities as indeterministic behavior, it seems more appropriate to see this as an example for deterministic behavior in a background independent theory. The internal time label first appears as a kinematical object through the eigenvalues of the triad operator (46). It then plays a role in the constraint equation (49) when formulated in the triad representation. Choosing internal time is just made for convenience, and it is the constraint equation that must be used to see if this choice makes sense in order to formulate evolution. This is indeed the case at non-zero where we obtain a difference operator in the evolution parameter. At zero , however, the operator changes and does not allow us to determine the wave function there from previous values. Now, we can interpret this simply as a consequence of the constraint equation rejecting the internal time value . "

 

As I read this, Bojowald is saying that the evolution of the universe is "determined" by the initial wave function. That still leaves the indeterminism of quantum mechanics within the universe intact.

 

HOWEVER, Bojowald is also saying that the initial wave function itself can be indeterministic. It may, or may not, have any connection to previous values (see the last sentence I italicized). If there is no connection to previous values, that breaks the cause-effect chain necessary for determinism in the philosophical sense.

[lucaspa]

I would agree with you that Ekpyrotic does not mean the same as "destruction of our universe and creation of another"

 

That is not what I have read. C Seife, Big bangs's new rival debuts with a splash. Science 292: 189-190, Apr 13, 2001.

 

" It takes place in 11 dimensions, six of which are rolled up and can safely be ignored. In that effectively five-dimensional space float two perfectly flat four-dimensional membranes, like sheets drying on parallel clotheslines. One of the sheets is our universe; the other, a "hidden" parallel universe. Provoked by random fluctuations, our unseen companion spontaneously sheds a membrane that slowly floats toward our universe. As it moves, it flattens out--although quantum fluctuations wrinkle its surface somewhat--and gently accelerates toward our membrane. The floater speeds up and splats into our universe, whereupon some of the energy of the collision becomes the energy and matter that make up our cosmos. ... That is because at any moment another membrane could peel off, float toward us, and destroy our universe. Indeed, Steinhardt says, we might have already seen the signs of impending doom. "Maybe the acceleration of the expansion of the universe is a precursor of such a collision," he says. "It is not a pleasant thought." "

 

Maybe Seife got it wrong. If so, could you please post your source?

 

The name was chosen by Steinhardt and Hurok

 

Why do you keep saying "Hurok" when it is "Turok"?

[lucaspa]

Very well. The source that I am citing is a Science Channel documentary called "Parallel Universe." It was explained near the end of the show, so you may have missed it, but some scientists believe it while others don't. What you read must have been written by one who doesn't believe this.

 

You are assuming I saw the show. I didn't. I was hoping for something in print as a source.

 

See my post above for a description of ekpyrotic. What I cited is a science news story from a scientific meeting plus the original paper discussing ekpyrotic.

 

As I said, ekpyrotic depends on String Theory, and String Theory is in trouble. There have been tests of the "rolled up" dimensions essential to ST at detection levels that early versions of ST predicted they should be detected. Nothing. ST has been modified to lower the detection levels, but new tests are probing those levels and still not seeing what ST says they should be seeing.

 

5. Kaku M, Testing string theory. Discover August 2005 http://www.discover.com/issues/aug-05/cover/

 

To my knowledge, ST has not yet been definitively falsified, but I may have missed it. However, if ST is falsified, then ekpyrotic is also falsified.

[Martin]

YES TUROK, not Hurok, my careless error.

 

Since LQC is based on LQG, do you see the same interest in LQG? If not, isn't that strange?

 

String theorists have suddenly registered and gotten interested in LQC results.

 

Well, that may represent the increasing trouble that String Theory is in.

.

 

These are perceptive comments! Thanks for the interesting enjoyable-to-read answers!

LQC clearly works better than vintage 1995-2000 LQG. You could say "isn't that strange?" but it is a clear indication that the full theory needed a total reworking.

It has gotten several. Spinfoam, GroupFieldTheory, MasterConstraint.

Rather than "strange" it could be seen as clear signs of ferment.

 

Thiemann's MasterConstraint and AlgebraicQG (AQG) completely revolutionizes LQG. The string people showed their interest by inviting Thiemann to KITP Singularities AS WELL AS the LQC people Bojowald and Ashtekar.

 

LQC has also undergone a revolution just in the past 2 years with what Ashtekar calls "improved dynamics". Bojowald has written several times of a "convergence" from three independent directions----he came on a new dynamics which had interesting similarities to what Ashtekar developed and is seen as convergent with Thiemann AQG.

 

It seems that LQC is no longer based exactly on vintage 1990s LQG

=================

 

In what I heard of the KITP talks and comments from audience, the STRING theorists like leader Gary Horowitz were dismissive of Veneziano-pre-BigBang. I don't think any Veniziano rep was invited. That is their business. The workshop showed a lively interest in Thiemann new-LQG and in the Bojo and Asht new-LQC of the last two years.

Thiemann was the only speaker who was given a SECOND HOUR by the chairman Gary Horowitz. That was a signal mark of interest, I thought.

 

=================

 

we are not talking about what is RIGHT or even about my subjective hunches of what approaches could be fruitful.

this workshop is a weathervane of top stringtheorist interest and leaning, the fact that it happend,

David Gross who directs Kitp is top string elite. Gary Horowitz is acting as a younger proxy here.

At the last hour Friday David Gross shows up, almost everybody has left, and Horowitz goes to the blackboard and does an official chairperson 'executive summary' probably for David Gross benefit and to diplomatically reassure the string people that everything was back to normal.

 

anyway, we are not talking about rightness but we have an incredibly informative document about how string thinkers see the picture now.

and they could be wrong. groups of theorists are OFTEN on wrong track it's hardly worth saying. but it is interesting to try to get inside their perspective all the same.

 

LUCASPA very glad someone else here has sampled the Kitp singularities talks!

 

Your observation here is keenly to the point:

*Well, that may represent the increasing trouble that String Theory is in.*

[Martin]

very interesting post!

time-crunch here, busy for the next hour or so

well reply as soon as time permits

============

well, I'm back, but now I can't find the post of yours that I glanced at before and was wanting to respond to.

 

Perhaps it's time to mull things over.

 

Also I would urge looking at most recent Bojowald and Ashtekar papers 2006 and 2007.

 

The Potsdam AEI (Albert Einstein Institute) a branch of Max Planck Institute established that "Living Reviews in Relativity" series and the idea is great---the authors are supposed to continually UPDATE the review. So it is called a "Living" review.

But in our imperfect world, many of the authors don't get around to rewriting the reviews. So i don't know about Bojowalds but to play safe it is a good idea to look at new papers.

 

I will get links to a couple of new ones that I think are representative. I was especially impressed by #2 here

 

 

2. arXiv:gr-qc/0703144 [ps, pdf, other] :

Title: Dynamical coherent states and physical solutions of quantum cosmological bounces

Authors: Martin Bojowald

Comments: 30 pages, 3 figures

 

 

6. arXiv:gr-qc/0611112 [ps, pdf, other] :

Title: Effective constraints of loop quantum gravity

Authors: Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski

Comments: 44 pages, 6 figures

 

7. arXiv:gr-qc/0609057 [ps, pdf, other] :

Title: Hamiltonian cosmological perturbation theory with loop quantum gravity corrections

Authors: Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski

Comments: 24 pages, 1 figure

================================

 

Here are three you might wish to glance at (or scan the abstract and the conclusions paragraph) by Ashtekar. I was impressed by the "improved dynamics" paper, #4 on this list. However #2 is more accessible because it deals with a special case of a spatial finite universe with zero cosm. const. that recollapses and so is "cyclic" I do not consider that realistic, but it is one of a number of cases that need to be studied systematically and it is a good example of the kind of results Ashtekar's group is getting. So perhaps I would recommend this #2 in the list, the "k=1" paper

 

1. arXiv:gr-qc/0702030 [ps, pdf, other] :

Title: An Introduction to Loop Quantum Gravity Through Cosmology

Authors: Abhay Ashtekar

Comments: 20 pages, 4 figures, Introductory Review

 

2. arXiv:gr-qc/0612104 [ps, pdf, other] :

Title: Loop quantum cosmology of k=1 FRW models

Authors: Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh, Kevin Vandersloot

Comments: Typos corrected. To appear in Physical Review D

 

 

4. arXiv:gr-qc/0607039 [ps, pdf, other] :

Title: Quantum Nature of the Big Bang: Improved dynamics

Authors: Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh

Comments: Typos corrected. Revised version to appear in Physical Review D

[Martin]

Ah! I found post #36, the one I remembered wanting to respond to when I had to go out earlier.

 

...question arise if time continues during the transition through the pure quantum regime. At least in the special model of a free massless scalar in isotropic cosmology the answer to both questions is affirmative, based on the availability of a physical inner product and quantum observables in this model [24]. "

 

Since Bojowald specifies a "special model", there are other models. I think you are going beyond what LQC says when you say definitively "It goes back in time."

 

Your source was written in 2005, I think, and they were in the process of checking different cases.

He has a rather careful style and he says "at least in the cases we checked".

 

since then they have checked a lot more cases. Judging from the latest solo Bojowald paper, IN EVERY CASE checked so far, if they start with a classical spacetime, anything remotely like ours, and work back in time THEY GET A BOUNCE. That is how i read the latest paper, you might want to inspect it yourself. I dont understand some of it--I'd be glad of a second pair of eyes.

http://arxiv.org/gr-qc/0703144

Dynamical coherent states and physical solutions of quantum cosmological bounces

 

But they have also found cases (I don't follow the initial conditions) where no bounce occurs! And they have a lot more work to do. They have to RELAX SIMPLIFYING ASSUMPTIONS like homogeneity and isotropy. That work has been begun

 

A question I have is: where do you get such a contracting spacetime? Black holes? Not the universe in general, since the data is pretty clear now that the universe is going to expand forever.

 

Well basically what you get, when you run a model back in time, is whatever you get! You may actually be looking at part of the history of our universe (if you can test the model and it passes enough tests so you take what it says seriously)

 

At present no one is obliged to *believe* things were this way or that way. Quantized models of conventional FRW cosmology (GR-based) are repeatedly showing a bounce and continuing back into the past. But classical GR gives us the classical FRW cosmology which does NOT continue back.

Both fit the observations made so far. For now, there seems no scientific reason to believe that time stops, and also no scientific reason to believe that it does NOT stop, at 14 billion years BP

 

thanks for looking at this stuff with me lucaspa!

I hope you look at that latest Bojowald (a lot has happened since 2005 when I think he wrote what you were reading), and maybe glance at some Ashtekar too. here's link to the Bojowald again

http://arxiv.org/gr-qc/0703144

[Genecks]

I don't believe in the big bang. It seems too simple. I didn't like it a decade ago, and I still don't like it. People keep supporting it and piling theories and math, but I can't really envision it logically happening that way. I somehow feel that the time variable is an illusion. I'm not saying time is an illusion, but perhaps the creation of the universe that long ago is an illusion. Makes you wonder when time actually did start, but I have a feeling it's always been constant. Therefore, if time exists, then the universe exists (or part). Depends on what you consider the universe. I know it gets physical, but I think the philosophical boundaries put constraints on all of it.

 

Sorry, but with quantum mechanics, NONE of the universe is strictly deterministic!

 

Just another reason I don't find quantum mechanics completely logical. I see too much of it as a social construct. Abstract concepts put into math, later put on abstract math, thus trying to make it deterministic. I figure within 50 years that more of this stuff will be sorted out.

 

I know I'm not suppose to go into personal views, but I think this is a dimension within a dimension. You could see the outside dimension as independent from this one, which can't be touched by this one. I figure because the universe wants independence or some form of stability. Also, everything within this dimension is contained. Therefore, the other dimension was first. I call this dimension the "crystal-ball dimension." Imagine a crystal ball on white posterboard. Therefore, we would be limited to understanding this universe and we could not figure out the outside universe. We are blocked from understanding it. Of course, I know I will probably be hit for this, but I think there's a way to understand it. If atomist philosophers were correct, that other dimension would be nothingness or something more extreme.

[joelle]

god created the world...and the galaxiess

and the animals

and humans..

and everything

[lucaspa]

god created the world...and the galaxiess

and the animals

and humans..

and everything

 

Yes, that's your belief. The question is: HOW did God create? Did God create everything in its present form? Or did God create thru the BB and all the material processes that followed?

 

In the previous discussion, what do you see that makes you think God did not create?

[lucaspa]

I don't believe in the big bang. It seems too simple. I didn't like it a decade ago, and I still don't like it. People keep supporting it and piling theories and math, but I can't really envision it logically happening that way.

 

Too bad for your beliefs and logic! I don't mean to be insulting here, but if you do science then "belief" and "logic" have to take a last place to data. The data for the Big Bang is solid. Even Bojowald, Hawking, and others looking at "different" theories accept the data for the BB. All they are trying to do is removed the singularity that is the BB under General Relativity. IOW, they accept BB but want a theory that means BB is not a singularity.

 

I somehow feel that the time variable is an illusion. I'm not saying time is an illusion, but perhaps the creation of the universe that long ago is an illusion.

 

Nope, the universe being that old is not an illusion. It's a theory strongly supported by the data. Also, remember to be careful about separating time from other dimensions. It's spacetime with 4 dimensions: 3 of space and one of time.

 

Just another reason I don't find quantum mechanics completely logical. I see too much of it as a social construct. Abstract concepts put into math, later put on abstract math, thus trying to make it deterministic. I figure within 50 years that more of this stuff will be sorted out.

 

Again, too bad for logic. What you object to is trying to make QM be deterministic. Yes, when people try to make QM what it is not, then the math gets weird. The math is pretty simple if you take QM for what it is.

 

QM itself is based on the data and obsevations. Those observations violate what we consider "logic", but the observations are solid.

 

On a philosophical level, I'm glad the universe is indeterministic. It means the future is open and what I do has real consequences. Think about it. If the universe is strictly deterministic, then everything we do is caused by a prior state and is determined. My writing these sentences has been determined since the beginning of the universe. Your reply has been determined. Why bother? We are just puppets going thru the motions, and it is the instant of the beginning of the universe that is pulling the strings.

 

I know I'm not suppose to go into personal views, but I think this is a dimension within a dimension. You could see the outside dimension as independent from this one, which can't be touched by this one. I figure because the universe wants independence or some form of stability. Also, everything within this dimension is contained. Therefore, the other dimension was first. I call this dimension the "crystal-ball dimension."

 

You do realize that BB says that "before" the universe there was nothing, right? No space, no time, no matter/energy.

 

What I don't see is how the "crystal ball" explains anything. What does it DO? Why hypothesize it in the first place?

 

Of course, I know I will probably be hit for this, but I think there's a way to understand it.

 

If you think the "crystal ball" explains things and can be understood, then work out the math for understanding it, then submit the paper to a physics journal. Let the experts review it for you.

[lucaspa]

Ah! I found post #36, the one I remembered wanting to respond to when I had to go out earlier.

 

Martin, you aren't explaining anything! You give papers but give no quotes from the papers that indicate that they actually do what I asked or explain how they do what I asked. If this discussion is to be more than you assert and I question, I need you to participate more. If it is simply going to be you assert without providing specifics and explanations, then it isn't worth anything.

 

since then they have checked a lot more cases. Judging from the latest solo Bojowald paper, IN EVERY CASE checked so far, if they start with a classical spacetime, anything remotely like ours, and work back in time THEY GET A BOUNCE.

 

What part of the paper gave you that impression?

 

But they have also found cases (I don't follow the initial conditions) where no bounce occurs! And they have a lot more work to do. They have to RELAX SIMPLIFYING ASSUMPTIONS like homogeneity and isotropy. That work has been begun.

 

But that makes no sense. This is sounding like the latest problems with String Theory: you can get lots of cases and there is no compelling reason to get the case that is our universe. What is the reason for thinking you can relax the assumptions? Just getting a universe like ours? That's circular reasoning.

 

Well basically what you get, when you run a model back in time, is whatever you get! You may actually be looking at part of the history of our universe (if you can test the model and it passes enough tests so you take what it says seriously)

 

Martin, that doesn't get to my puzzle. A very dense spacetime gives a "bounce" that obviates the singularity. OK, but how do you get the dense spacetime to being with? Our universe does NOT run backwards in time! It started as very dense spacetime, but that's the problem: the source of the very dense spacetime.

 

Basically, what I hear is that LQC is talking about a cyclic universe of expansions and collapses -- with the collapses giving the dense spacetime for the next expansion. BUT, our universe is not going to collapse! So, either our universe is the last of the cycles (and why would that be so?) or there is another way to get a dense spacetime.

 

At present no one is obliged to *believe* things were this way or that way. Quantized models of conventional FRW cosmology (GR-based) are repeatedly showing a bounce and continuing back into the past.

 

That isn't what Bojowald is saying. He is saying that in LQC time "penetrates" the "pure quantum regime"

 

"does the question arise if time continues during the transition through the pure quantum regime. At least in the special model of a free massless scalar in isotropic cosmology the answer to both questions is affirmative, based on the availability of a physical inner product and quantum observables in this model [24]. "

 

But this isn't the same as "continuing back into the past".

"In this sense, quantum gravity is free of singularities and provides a transition between the two branches. The more complicated question is what this means for evolution in a literal sense of our usual concept of time (see also [200])."

 

Look at the qualifier "in this sense", which means we are not talking about conventional ideas of time and the last sentence. Bojowald doesn't know whether time "continues into the past". All he knows, from the math, is that time doesn't break down because there is no singularity.

 

A question I have is: what is the other "branch"? We have this branch on this side of the BB. Yes, LQC would remove the singularity at the BB, but it does so by making another "branch". Have you come across anything Bojowald has said about the identity of that branch?

 

Both fit the observations made so far. For now, there seems no scientific reason to believe that time stops, and also no scientific reason to believe that it does NOT stop, at 14 billion years BP

 

It's not that "time stops", but rather that "time begins" 13.7 billion years ago. If time does not begin, what is it connecting to?

 

thanks for looking at this stuff with me lucaspa!

 

I'll look more, but I need more help from you, Martin. It's not enough to just give the citations. You need to point us to what caused you to assert what you do about Bojowald.

 

Also, look for things against your assertions. Keep testing what you are saying by looking for things that would be against it. That will help everyone and keep this from being adversarial (which it has gotten pretty close to being in places).

 

I hope you look at that latest Bojowald (a lot has happened since 2005 when I think he wrote what you were reading),

 

That 2005 was the last review Bojowald wrote. Do you know of a more recent review. I didn't find one with my google search.

 

We both need a review, since you admit you don't follow all the mathematics of the situation and I never claimed to follow all the math.

 

http://arxiv.org/gr-qc/0703144

 

"Our general solution (13) for p allows bouncing3 solutions for AB > 0 as well as “singular” solutions for AB < 0 which reach p = 0 in finite time . (Although isotropic loop quantum cosmology is non-singular for any solution [33, 34], additional correction terms become manifest at small volume which are not included here in the solvable model. The model itself thus breaks down before p = 0 is reached. The singularity in our equations only indicates that a deep quantum geometry regime is reached, just as one commonly expects the general singularity problem to be resolved. Nevertheless, we keep solutions with AB < 0 for now since they will be ruled out even within our model shortly.) The internal time variable has just been chosen for convenience of the mathematical description, rather than referring to physical observers. For a solution reaching p = 0 to be considered singular one

must also verify that proper time remains finite. We thus need to interpret our relational solution (p and J as functions of ) as a space-time geometry subject to modified dynamics as it arises from the loop quantization."

 

That first sentence implies that LQG or LQC will also work if time does not go "back".

 

"Fig 2 Two bouncing solutions for the expectation value of ˆp and the spread around it. Generic states have different spread before and after the bounce (dashed lines), while unsqueezed Gaussian initial states lead to solutions which are symmetric around the bounce not only in their expectation values but also in spreads (solid lines)."

 

Bojowald must be using "deterministic" in an unconventional sense, because this says, to me, that there are many possible outcomes from the initial conditions -- "different spread before and after the bounce". This is reinforced by the Abstract:

"Results are evaluated with regard to their implications in cosmology, including a demonstration that in general quantum fluctuations before and after the bounce are unrelated. Thus, even within this solvable model the condition of classicality at late times does not imply classicality at early times before the bounce without further assumptions."

 

Notice that Bojowald can't get from one classical state to another without assumptions.

 

"Although we have proven that the solvable quantum system is exactly described by the effective Hamiltonian

H = h ˆH i = 12i (J − ¯ J) = p sin c

(determining the same equations of motion for p and J as ˆH does for hˆpi and h ˆ Ji) it is possible, depending on the initial state, that the system does not bounce but reaches p = 0 in finite proper time."

 

Eeek! We can have a bounce, in which case we have time "before", or maybe under LQG, there is NO bounce, in which case we are back to having a beginning of time! Which is it in reality?

 

OK, found it.

"As shown explicitly, zero volume is reached when the parameter c1 is negative and large. While this can be achieved respecting the spositivity condition H ≪ H, such a state would never be semiclassical. Thus, any state which is semiclassical at one time will give rise to a bounce."

 

Since our universe is semi-classical, it would have a bounce. Of course, did it have to start out as semi-classical?

 

OOPS! We are back to having no bounce again! I wish Bojowald would make up his mind!

"We end by repeating that any physical statements derived from a single model have to be confirmed by a perturbation analysis around the model. This is feasible in our case, as it is for perturbations around any solvable model, but still requires detailed work which is now in progress. Only such an analysis could justify the transfer of results from single models to our own universe. It may well be that this removes the bounce through back-reaction of quantum variables Ga,n on the expectation values. In particular, it is then conceivable that a state starts out perfectly semiclassically at large volume where its expected volume collapses, evolves for a long time to small volume and all along picks up corrections from quantum back-reaction. Since also quantum variables evolve, it cannot be ruled out without further analysis that the analog of c1 does become negative and large close to the would-be bounce. If this happens, the bounce is avoided for the self-interacting state even if it starts out semiclassically by all possible conditions one could pose."

 

Martin, how much peer-review do articles in arxiv.org undergo?

 

I think the strongest statement that can be made now by people such as you and I is that LQC is being worked on and may represent a way around a beginning to time. Then again, it may not, because 1) it turns out to be wrong and 2) it is possible to still have a beginning of time under some versions of LQC.

 

One other minor thing. Bojowald is single author on the paper but keeps saying "we" all thru the paper! I'm hoping that means tacit acknowledgement of his discussants and not the royal "we".

[Martin]

Martin, how much peer-review do articles in arxiv.org undergo?

 

That is interesting to infer. One can see that a lot of LQC articles are being published in the premier peer-review journal for this type, Physical Review Series D---abbr. PRD.

 

I will indicate how to discover this from the arxiv file.

 

Shorter articles may be published in the related journal PRL (Physical Review Letters).

 

Arxiv is a PREPRINT archive where people put their research immediately or as soon as it is submitted to a peer-review journal. that way their colleagues have access immediately. But IF AND WHEN the article passes editorial and peer-review and is accepted, this may be (and often is) indicated by the author, who can UPDATE the arxiv page.

 

So just by looking at the arxiv page you can often see that a paper (after 6 months or a year of posting on arxiv) has been accepted by, say, PRD or PRL, or is included in some book or conference proceedings.

 

IIRC the announcement is typically in the "comment" field or just below it.

Arxiv recently changed format and it takes getting used to.

 

I am glad you are reading these articles and I'll try to whittle away at responding to your questions!

[Martin]

You asked a general question about peer-review, there is no general answer but I can give you an example. Take Ashtekar's recent papers as a sample and you see that ABOUT HALF will have been subjected to peer-review because published in PRD and PRL. (among the most rigorously reviewed)

 

1. arXiv:gr-qc/0702030 [ps, pdf, other] :

Title: An Introduction to Loop Quantum Gravity Through Cosmology

Authors: Abhay Ashtekar

Comments: 20 pages, 4 figures, Introductory Review

 

2. arXiv:gr-qc/0612104 [ps, pdf, other] :

Title: Loop quantum cosmology of k=1 FRW models

Authors: Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh, Kevin Vandersloot

Comments: Typos corrected. To appear in Physical Review D

 

3. arXiv:gr-qc/0611049 [ps, pdf, other] :

Title: Mechanics of higher-dimensional black holes in asymptotically anti-de Sitter space-times

Authors: Abhay Ashtekar, Tomasz Pawlowski, Chris Van Den Broeck

Comments: Revtex4, 23 pages. Minor typos corrected and one reference added

 

4. arXiv:gr-qc/0607039 [ps, pdf, other] :

Title: Quantum Nature of the Big Bang: Improved dynamics

Authors: Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh

Comments: Typos corrected. Revised version to appear in Physical Review D

 

5. arXiv:physics/0605078 [ps, pdf, other] :

Title: The Issue of the Beginning in Quantum Gravity

Authors: Abhay Ashtekar

Comments: Minor typos corrected, including a factor of $\pi$ in the expression of critical density. 15 pages, 2 figures. History and Philosophy of Physics. Based on an invited talk at the 7th International Conference on the History of General Relativity (HGR7), "Einstein and the Changing World View of Physics, 1905-2005", held at Tenerife, Canary Islands in 2005

 

6. arXiv:gr-qc/0605011 [ps, pdf, other] :

Title: Gravity, Geometry and the Quantum

Authors: Abhay Ashtekar

Comments: Minor typos corrected, including a factor of $\pi$ in the expression of the critical density. 16 pages, 2 figures. To appear in the Proceedings of the `Einstein Century' Conference, 15-22 July, Paris, edited by J-M Alimi et al (American Institute of Physics)

 

7. arXiv:gr-qc/0604013 [ps, pdf, other] :

Title: Quantum Nature of the Big Bang: An Analytical and Numerical Investigation

Authors: Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh

Comments: Revised version to appear in Physical Review D. References added and typos corrected

 

8. arXiv:gr-qc/0602086 [ps, pdf, other] :

Title: Quantum Nature of the Big Bang

Authors: Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh

Comments: Revtex4, 4 Pages, 2 Figures. Minor changes to match the published version in Physical Review Letters

 

If an Ashtekar paper in this list has not been accepted for publication in a peerreview journal it may be because it is too recent (like #1 on this list) or it may have been an invited talk at some conference like the Paris Einstein Centennial.

=============

 

to more fully respond, I should do a similar check with Bojowald

[Martin]

 

Martin, how much peer-review do articles in arxiv.org undergo?

 

 

Lucaspa,

You seemed especially interested in Bojowald, in this connection so I did a check. I ignored his 2007 papers because it is too early to get notice of acceptance for publication, and I took a sample of EIGHT MOST RECENT 2006 papers and found to my pleased surprise that he is

BATTING ONE THOUSAND!

Of these 8, all had been thru the professional journal peer review process and had been accepted. MOST BY PRD AND PRL. (the most prestigeous and presumed rigourous)

 

 

5. PRLarXiv:astro-ph/0611685 [ps, pdf, other] :

Title: Formation and Evolution of Structure in Loop Cosmology

Authors: Martin Bojowald, Hector Hernandez, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski

Comments: 4 pages

 

6. PRDarXiv:gr-qc/0611112 [ps, pdf, other] :

Title: Effective constraints of loop quantum gravity

Authors: Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski

Comments: 44 pages, 6 figures

 

7. PRDarXiv:gr-qc/0609057 [ps, pdf, other] :

Title: Hamiltonian cosmological perturbation theory with loop quantum gravity corrections

Authors: Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski

Comments: 24 pages, 1 figure

 

8. Gen Rel and Graviation journalarXiv:gr-qc/0609034 [ps, pdf, other] :

Title: Loop quantum cosmology and inhomogeneities

Authors: Martin Bojowald

Comments: 25 pages, 1 figure

 

9. PRDarXiv:gr-qc/0608100 [ps, pdf, other] :

Title: Large scale effective theory for cosmological bounces

Authors: Martin Bojowald

Comments: 5 pages, 1 figure; v2: more details on effective equations

 

10. Int. Jrnl Math. Phys.arXiv:gr-qc/0607130 [ps, pdf, other] :

Title: Quantum Geometry and its Implications for Black Holes

Authors: Martin Bojowald

Comments: 16 pages, Plenary talk at ``Einstein's Legacy in the New Millenium,'' Puri, India, December 2005

 

11. Int. Jrnl. Geom. Meth. Math Phys.arXiv:hep-th/0606232 [ps, pdf, other] :

Title: Quantum Gravity and Higher Curvature Actions

Authors: Martin Bojowald, Aureliano Skirzewski

Comments: 28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics ``Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006

 

12. PRDarXiv:gr-qc/0606082 [ps, pdf, other] :

Title: Loop cosmological implications of a non-minimally coupled scalar field

Authors: Martin Bojowald, Mikhail Kagan

Comments: 10 pages, 4 figures

 

It looks to me as if he puts his RESEARCH papers in PRD and the other papers were ones he was invited to give at conferences, which would be more survey-type and aimed at physicists in other specialties, so they wouldnt be right for PRD-----so those would go to the International Journal of Mathematical Physics (still peer-review, but not so prestigeous)