Big Bang

[Martin]

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.

 

this seems to be another question I can encapsulate and reply to. BTW I'm glad you asked about peer-review and got me to check. It hadn't realized Bojowald was so successful. He is getting essentially 100% of his research papers into the top American Physical Society (APS) venue, namely Physical Review D (PRD). He got his PhD in 2000 so he's still quite young. It's impressive.

 

Also Ashtekar is getting most of his research into PRD and PRL, but he does a lot of general survey and invited conference talks, not so many purely research papers.

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

 

Anyway Ashtekar's two main papers last year have been about two main cases, which he called the k=0 and k=1 case, and which I can roughly characterize this way:

 

1. in the whole history of the universe there is only one bounce. the expanding phase we are now in extends indefinitely into the future and the prior contracting phase (figuratively mirroring this) could extend indefinitely into the past.

 

2. there is a series of bounces going back indefinitely into the past and indefinitely into the future

 

I am not sure I understand whether or not you see a problem with this, or what problem there could be.

 

Here are the two papers

REPEATED BOUNCE

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

 

SINGLE BOUNCE

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

 

The single bounce case is more realistic since our universe is not expected to recollapse. But BOTH cases should be explored, and indeed I would say there are many different papers to be written here, examining many cases, all worth exploring!

 

The single bounce case also happened to be technically easier. Probably for this reason they treated it first. The repeated bounce case had a technical difficulty which was pointed out by William Unruh several years ago, however they finally overcame it.

 

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

About cases. Mainstream cosmologists doing classical (non-quantum) modeling consider numerous cases. I see no reason why the quantum cosmologists should not also examine (and try their analytical methods out on) several different cases. Indeed they seem to be.

 

this list of things to consider barely scratches the surface

single bounce

repeated bounce

spatial finite

spatial infinite

positive Lambda

zero Lambda

inhomogeneous matter distribution

 

there is lots and lots of work to be done!

 

Lucaspa, thanks for asking questions like this. Please REask question you think especially important to help me focus my response.

[VikingF]

Anyway Ashtekar's two main papers last year have been about two main cases, which he called the k=0 and k=1 case, and which I can roughly characterize this way:

 

1. in the whole history of the universe there is only one bounce. the expanding phase we are now in extends indefinitely into the future and the prior contracting phase (figuratively mirroring this) could extend indefinitely into the past.

 

2. there is a series of bounces going back indefinitely into the past and indefinitely into the future

 

I am not sure I understand whether or not you see a problem with this, or what problem there could be.

 

 

What I don't get is this: If the "single bounce" scenario is correct, then the universe is (more or less) the same on each side of the Big Bang/Bounce, and hence the universe we have now is almost the same as the universe 13.7 billion years before the bounce. But if this is the case, then the universe must have started exactly like it will end in the future, that is with a "inverse Big Rip". How can a (inverse) Big Rip be the begining of the universe?

 

Or have I misunderstood completely?

[Martin]

VikingF, what impressed me about your post was that you do have an approximate picture of what they are talking about in that k = 0 case.

You may be imagining it in more detail than we can speculate about, at least for now. but a collapsing classical spacetime does rougly resemble the expanding one we know about, but run backwards.

========

But it wouldn't necessarily involve a Big Rip at the end of our expanding phase, or anything reverse-corresponding at the other side. the "Big Rip" was was not a real prediction, more something people speculated about but couldn't with confidence predict

 

having it happen depends on assuming that some parameters (which so far look constant) are not constant but evolve with time.

 

(one of these is the dark energy equation of state parameter w.)

 

the most recent observations (WMAP 3-rd year data and some recent supernova data) tend to support the idea that we are NOT looking at big rip as a real possibility

 

they tend to confirm the belief that the cosmological constant really is that (and the associated w = -1)

 

this means indefinite expansion getting darker colder and lonelier as stars burn out, but our galaxy for example does NOT get torn apart. nor does ordinary matter get torn apart (as used to be imagined in Rip scenarios)

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

 

we can't really predict, and we can only try to measure the parameters more and more accurately, the parameters that go into determining the past and future of our universe.

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

 

Nor can we really look back very well to times before the Bounce, and say what it was like. The models which probe before are still very provisional and iffy.

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

[lucaspa]

1. in the whole history of the universe there is only one bounce. the expanding phase we are now in extends indefinitely into the future and the prior contracting phase (figuratively mirroring this) could extend indefinitely into the past.

 

2. there is a series of bounces going back indefinitely into the past and indefinitely into the future

 

I am not sure I understand whether or not you see a problem with this, or what problem there could be.

 

1. The problem here is where did the first universe come from that contracted for the bounce? "Extending indefinitely into the past" doesn't tell us where that universe that is indefinitely contracting came from. All we've done is move the same mystery -- origin of our universe -- off to another hypothetical universe we are not even sure exists.

 

2. The indefinite cyclic universe was found to be impossible in the 1960s. The problem was entropy. Too much entropy -- what Bojowald would call the entire quantum state -- is "lost". Therefore, since our universe is "determined" by the conditions pre-bounce, in a very few (< 5 if I remember correctly), the entropy state would ensure that you could not get a universe like we see today. So there goes the "indefinitely".

 

Otherwise, running our universe backwards in time to a "bounce" doesn't really tell us anything.

 

Then, of course, there are the quotes I gave you from Bojowald's paper showing that a bounce is not mandatory! There may not have been a bounce at all. In which case LQC serves the purpose of eliminating the singularity, but still doesn't give us anything "before" the universe and we are still stuck with the question: why is there a universe at all?

 

At least ekpyrotic answered that. Even it it turns out to be an incorrect answer.

[lucaspa]

Here are the two papers

REPEATED BOUNCE

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

 

SINGLE BOUNCE

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, can you cut and paste the webpage address in there, please? Otherwise, I'm having a difficult time finding the articles.

 

And Martin? You still aren't being all that helpful yet. You still aren't pointing us to the portions of the paper that you think explain what you are trying to say.

[lucaspa]

but a collapsing classical spacetime does rougly resemble the expanding one we know about, but run backwards.

 

So where did that collapsing spacetime come from? Do the LQC people ever speculate about that?

 

the most recent observations (WMAP 3-rd year data and some recent supernova data) tend to support the idea that we are NOT looking at big rip as a real possibility

 

Which means, without the "big rip", there is no origin for that collapsing spacetime.

 

Nor can we really look back very well to times before the Bounce, and say what it was like. The models which probe before are still very provisional and iffy.

 

That is different from your earlier claims. You said that, since time didn't stop at the BB, that we could know "before". Does this represent some newfound caution on your part?

 

BTW, you never really answered whether LQG was also gaining in popularity. Bojowald makes it very clear that LQC is dependent on LQG. You went off into a diversion about the "new" LQG being different from the old, but were never specific about that. Also, you never answered the question: is LQG enjoying the same increase in "popularity" as evidenced in publications in the physics literature? If so, why is Smolin writing his book deriding the monolith of SST? After all, LQG is Smolin's.

[lucaspa]

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.

 

IOW, articles in arxiv receive NO peer-review. I inferred it from some grammatical mistakes in Bojowald's paper, plus the contradiction about whether there is a bounce or not. The Discussion was rambling and contradictory, which would be fixed by the reviewers.

 

Also, Bojowald "batting 1,000" for getting articles published is not that hard. I have the same batting average for my publications! What you need to look at are any differences between the arxiv papers and the peer-reviewed ones.

[Jacques]

I just thought of something time slow when gravity increase. In the early universe maybe the gravity was high enought to stop time.

Just a thinking...

[Martin]

 

 

Here are the two papers

REPEATED BOUNCE

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

 

SINGLE BOUNCE

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, can you cut and paste the webpage address in there, please? Otherwise, I'm having a difficult time finding the articles.

Martin, can you cut and paste the webpage address in there, please? Otherwise, I'm having a difficult time finding the articles.

...

Sorry you were having trouble. I'll explain how to do it and then do it in these cases.

When you see something like "arXiv:gr-qc/0607039"

the trick is to type "http://arxiv.org/" and then the preprint designation.

 

So the complete URL is

http://arxiv.org/gr-qc/0607039

 

In the case of the other paper, the complete URL is formed the same way

http://arxiv.org/gr-qc/0612104

 

One of these treats the single bounce case with a spatially flat universe

 

The other treats the repeated bounce case.

 

I think one could also have a single bounce case with a universe that was spatially [/b]nearly[/b] flat and spatially finite, having a positive cosmological constant. But I havent seen them deal with that case yet.

 

I'm familiar with the SECOND LAW argument against repeated bounce (read it presented by Penrose and also heard him give it in person last year as part of a quantum cosmology talk) but I don't think it holds water. Idea of entropy seems to require an observer and an observed system.

Observer A in a prebounce collapsing universe sees a region with high entropy. Observer B in postbounce expanding universe sees a region with low entropy. not the same observer, not the same region (only tenuous connection). No reason entropy should be related. Second law of questionable applicability.

 

So I still think the repeat bounce case is interesting to study (tho may well not correspond to our universe) and not physically ruled out by Second Law considerations.

[Martin]

So where did that collapsing spacetime come from? Do the LQC people ever speculate about that?

...

 

Never heard that kind of speculation from LQC people.

 

Objective does not seem to be to answer the (philosophical or near-philos.) question of "Where did it all come from?"

 

I think instead the aim is just to run the model back smoothly past where the singularity used to be.

 

General attitude of physicists is singularities do not occur in nature but are instances where a theory fails. So a singularity in an old theory like GR represents a challenge to try to devise a new theory that will NOT fail, but will be just as good as the old theory in other situations.