Francesca's spinfoam cosmology talk is online (clear slides, haven't tried the audio)

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Main Question or Discussion Point

Spinfoam cosmology is a fast-growing field, since it represents the application of the full LQG theory to the BB and expanding universe. In the early years loop cosmology addressed a restricted model with just a small number of degrees of freedom. Going from oldfashioned LQC loop cosmology to 2010 spinfoam cosmology is a radical change.

Here are Francesca Vidotto's slides.
http://relativity.phys.lsu.edu/ilqgs/vidotto020111.pdf

She is one of the first people to do spinfoam cosmology. She got into LQG by attending the 2007 Zakopane QG school which was about the same time of year (Feb-March) as the same school this year---ski season. Soon after March 2007 she showed up at Marseille. That is fast, to get in a new research line.

The QG school format seems to me to be a good one. It boosts people into leading edge research, if they work hard, or so it looks to me.

The slides are clear and well thought out, IMHO. I haven't put the audio on yet.

Battisti and Marciano have a recent paper where they prove bounce in a simple spinfoam cosmo case.

The main spinfoam cosmology paper is still the March 2010 Bianchi Rovelli Vidottto. If anybody is interested that is the paper to read, I think.

I will listen to the seminar and tell you my reaction to the audio. Her native language is Italian so there is always the language issue---ILQGS seminars are held in English.
http://relativity.phys.lsu.edu/ilqgs/
 
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marcus
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It's good! at least the first 30 minutes--I have listened halfway through. I chose the AIF audio instead of the WAV
http://relativity.phys.lsu.edu/ilqgs/vidotto020111.aif

Francesca is well prepared not only to give the lecture but to respond to questions
I am up to slide 9 and equation 14.

Sometimes someone who first language is not English can give a good presentation but gets confused by questions, because they do not understand the question being asked in English, so it derails the talk. This does not happen here. F. has full command of the material in English including possible questions. That is my impression from the 30 minutes so far. Also the material is quite interesting. The inclusion of the cosmological constant in spinfoam quantum cosmology....and she shows some of the calculations...and they get a semiclassical limit of deSitter space.

So it is working out quite well so far. I am rooting. She is still only a grad student, AFAIK.

I also tried the other audio resource, the WAV. It quit after the first 10 or 12 minutes. This could have been my computer's fault. You can try the WAV audio. The quality is better than AIF, more bandwidth, and I usually have no trouble with it. But if you have difficulty go back to AIF. Here is WAV:
http://relativity.phys.lsu.edu/ilqgs/vidotto020111.wav

I want to listen to the second half now because most of the hard questions in the ILQGS seminar come at the end and I want to hear how Francesca handles them. So far she is doing a really fine job. Ooops, Param Singh (at Perimeter) has asked an awkward question about sign of Lambda (and Euclidean versus Lorentzian version). Now Eugenio Bianchi at Marseille is coming in. And I have to go to supper!

Also now my computer is having trouble. So I can't tell about the second 30 minutes of the audio yet.
===================

It went through OK. There was a lot of interest, Someone with a slight UK accent, I think it may have been Neil Turok, the director of Perimeter, asked a number of questions. Lee Smolin asked what seemed like 3 or 4 questions. Param Singh. And then the seminar host Jorge Pullin (Louisiana State) asked 2 or 3. They kept Francesca jumping and pushed out into areas that she and co-authors had not looked at. So the second half was a bit of a cliff-hanger, but I felt it came off generally quite well.

I think some of the audio problems were with my wireless connection, so I switched to a wireful computer and the AIF audio had no more problems.
 
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Just a quick question, in a Big Bounce cosmological model I understand that quantum effects near a singularity cause gravity to become highly repulsive thus motivating a "Bounce". But what cause the universe to re-collapse on its self in LQC?
 
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Just a quick question, in a Big Bounce cosmological model I understand that quantum effects near a singularity cause gravity to become highly repulsive thus motivating a "Bounce". But what cause the universe to re-collapse on its self in LQC?
Standard cosmology models the universe at large scale with a simple diff. eqn. called the Friedmann eqn. (e.g. wikipedia). It is derived from the GR equation by making simplifying assumptions (approx uniform distribution of matter at large scale).

You get a variety of behavior (such as recollapse or not) depending on a few parameters.

LQC reproduces Friedmann eqn behavior. After the bounce it quickly goes back to semiclassical and classical behavior. So you can get whatever behavior you got with the classical. You can arrange things so the U does NOT recollapse if you want.
You can arrange it so there is only one collapse bounce and re-expansion in the whole history.

(In that case the infinitely long collapse, pre-bounce, roughly speaking mirrors the infinitely long expansion post-bounce.)

Or you can arrange it so you get an infinite sequence of bounces.

So to answer your question:
But what cause the universe to re-collapse on its self in LQC?
.

The exact same thing that causes it to do that in classical mainstream cosmo: setting aside the cosmo constant question, that would be an average density (measured at any moment in history) that exceeds the critical density (calculated from the expansion rate measured at that moment.) It's own gravity, if large enough, causes expansion to eventually stop and recollapse to begin.

If there is only one bounce in the whole history (similar to the hourglass shape of a deSitter U) then there is no "recollapse". What causes it is, I guess you could say, initial conditions at time minus infinity---just how it is. Same as with the classical deSitter. The classical deS has an inf'ly long contraction followed by inf'ly long expansion. It is just one way that existence can be. What causes existence to exist? :biggrin: We are still at the beginning of our journey on that one.

Willem deSitter discovered that solution to the Einstein eqn. around 1917.
http://en.wikipedia.org/wiki/De_Sitter_space
They say that Levi-Civita discovered it independently the same year.

It has a bounce, but the bounce is not singular. It is just a classical turnaround (possible at lower density than our early U had) caused by the cosmo constant itself.
 
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So how would one identify if a Big Bounce occurred?
 
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So how would one identify if a Big Bounce occurred?
Look in arxiv for the papers of Aurelien Barrau.

What you are asking is a phenomenology question (How could we test...)
There are around 10 phenomenologists working on this, that I know of. Most have co-authored with Barrau, or with another guy in the UK called Wen Zhao.

The short answer is that a bounce should leave a "footprint" in the CMB.

The short answer also is that QG bounce (as modeled by LQG) should interact with inflation (if both occurred) and that should leave some trace. Barrau has a paper about that too.

Phenomenologists job is not to take sides with this or that theory (Barrau has studied several early universe models) but to find ways to test, and if possible rule out.

LQG cosmology has reached the stage where it is attracting this kind of professional tester.

All I can do is point you to the literature. It is fairly new (2009-2010). Do you want links or can you use the arxiv search?
 
  • #7
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I found some papers, thanks!
 

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