Latent QG and the Quantum Nature of the Big Bang

[marcus]

this is a "connecting the dots" thread about two particular series of papers that I'm watching

In one set of papers, Freidel et al identify a form of Quantum Gravity that is LATENT in (Feynman diagrams of) Quantum Field Theory.
Latent means hidden, which is the word Freidel uses in the titles:

Hidden Quantum Gravity in 3d Feynman diagrams
http://arxiv.org/abs/gr-qc/0604016

Hidden Quantum Gravity in 4d Feynman diagrams: Emergence of spin foams
[To appear]

In essence, Freidel et al have found a form of spinfoam QG that reduces to the usual flat matter QFT when you turn off gravity. In this spinfoam model, if you let the Newtonconstant go to zero (G_N -> 0) you get Feynman diagrams and Feynman graph amplitudes.
Freidel's attitude is that it was the QG model that was "there all along" waiting to be discovered concealed in usual QFT: the basis of the usual Standard Model of matter in flat Minkowski space.

See references therein to earlier Freidel et al papers in this series
(including one by Freidel and Majid earlier this year)

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I would like to see a bridge connecting to this other series of papers by Ashtekar et al, the first of which will appear soon in Physical Review Letters

Quantum Nature of the Big Bang
http://arxiv.org/abs/gr-qc/0602086

Quantum Nature of the Big Bang: An Analytical and Numerical Investigation I
http://arxiv.org/abs/gr-qc/0604013

References therein to related work by Ashtekar, Bojowald and others---in particular Bojowald's Living Reviews article.

 

[f-h]

Baratin has a talk on this at Loops05. http://loops05.aei.mpg.de/index_files/abstract_baratin.html

It seems they have answered an objection/caveat by Baez that this might be unrelated to QG...

I think I finally get what they are trying to do, if feynman graph amplitudes can be expressed as expectation values of *specific* models then knowing which feynman graph amplitudes are present in nature might be a way to constrain the spinfoam dynamics...

"In other words, if one assumes that it is possible to consistently describe quantum gravity amplitudes coupled to matter in terms of a spin foam model, a necessary requirement for the spin foam amplitudes, in order to be physically relevant, is to reduce to the ones appearing in the study of Feynman diagrams when GN → 0. This gives strong restrictions on the admissible, physically viable spin foam models."

no time to study it in more depth at the moment unfortunately :(

 

[marcus]

Baratin has a talk on this at Loops05. http://loops05.aei.mpg.de/index_files/abstract_baratin.html

It seems they have answered an objection/caveat by Baez that this might be unrelated to QG...

I think I finally get what they are trying to do, if feynman graph amplitudes can be expressed as expectation values of *specific* models then knowing which feynman graph amplitudes are present in nature might be a way to constrain the spinfoam dynamics...

"In other words, if one assumes that it is possible to consistently describe quantum gravity amplitudes coupled to matter in terms of a spin foam model, a necessary requirement for the spin foam amplitudes, in order to be physically relevant, is to reduce to the ones appearing in the study of Feynman diagrams when GN ? 0. This gives strong restrictions on the admissible, physically viable spin foam models."

no time to study it in more depth at the moment unfortunately :(

with that Baratin talk I cannot get the audio. for me at least, the link seems to be broken!
I can get the slides but not the audio. What about you?

 

[f-h]

Just downloaded it, works for me... The mime format is of so I actually neede to download it though.

 

[marcus]

Spinfoam Cosmology

thanks F-H, I am glad other people can get the audio of the Loops '05 conference talks. I can only get the slides/lecturenotes. don't understand why. I downloaded a bunch of the audios back in November, but my computer must have changed somehow since then. fortunately I have stored the audios that I downloaded so I can still review much of the conference highlights.

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What I am talking about in this thread is the possible appearance of something called SPIN FOAM COSMOLOGY
which could be based on the LATENT spinfoam model that Freidel and Baratin are developing.

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Ever since 2001 we have had LOOP (quantum) COSMOLOGY----actually since earlier but 2001 was when Bojowald showed that the Big Bang singularity does not occur.

And as long as one stays clear of the Big Bang, Loop Cosmology reproduces usual standard cosmology-------usual is the low-curvature limit of loop. Bojo showed that too.

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So why should one ever bother with "spin foam cosmology"? At most, a reasonable graduate student would say "let's show that Freidel Latent QG is equivalent to canonical Loop QG and then let's show that Loop cosmology derives in a legitimate way from the Full Theory." And so on.

The conventional, almost reflex, way seems to be to change as little as possible and since the symmetry reduced Loop Cosmology has been so successful one wants to keep on using it and just remove some symmetry assumptions and get more general results about non-occurrence of singularities

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But I would like to see some people try to implement spinfoam cosmology.

I think they would have to do it MONTE CARLO style. Analogous to how Renate Loll did with dynamical triangulations. I don't know what you think about this (F-H, if you are reading) but I think that Loll simulations are actually COSMOLOGY. they show a universe coming into existence and expanding some and then contracting and going out of existence.

the brief lifetime of the simulated universe was forced by limited computer resources: only enough CPU time and memory for a small universe----but still she showed the complete history of a whole spacetime, not merely a piece.

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I hope you see some merit in doing this, even though it is not so conventional-----------what I am talking about is building a BRIDGE between Freidel spinfoam QG and Bojowald Ashtekar loop cosmology (e.g. "quantum nature of the big bang" Ashtekar recent paper)