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- Thread starter Digitalism
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martinbn

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There is just one bounce.

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marcus

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Nothing wrong with the one-bounce picture. That is the most common in the LQC research lit.

You can set it up, with zero cosmological curvature constant , so that it repeatedly bounces, but that is not the most common case to consider.

It seems to me that the theory does not address questions like "why does existence exist?" or "where did it all come from?". It studies what could have happened to geometry and matter right around the bounce. What implications for inflation? What features might carry over from prior collapsing region? What might be an OBSERVABLE signature of the bounce?

You might say the attitude is first let's try to understand what really happened instead of that classical failure called "singularity". And let's try to TEST our model by determining its footprint on the CMB polarization map.

So it's very focused on what should replace the classical theory breakdown. When the classical theory is replaced by a quantum theory that recovers the classical shortly before and after its failure, they find that in the new version quantum effects at high density cause gravity to be repellent. The brief episode where gravity is repellent involves a period of faster than exponential growth called "super-inflation", which precedes the ordinary inflation era. Ordinary inflation has constant or slowly declining H, super-inflation has rapidly increasing H, driving H to around the Planck frequency. (H is hubble growth rate, units of reciprocal time). Considerable inhomogeneity, or "structure" is wiped out during the period where quantum effects dominate and gravity is repellent instead of attractive.

there are some recent papers on the pre-inflationary period in LQC. In case you are interested I'll get some links. there's also the interesting question of

BTW you are quite right that it pushes the frontier back a step. that is how science works I guess.

Model and test, figure out what was actually going on at the start of expansion, and immediately before. then, if you can do that, you still have another problem:what came before that. But I wouldn't call that *merely* pushing one step back in time. I think it is potentially a valuable accomplishment and very worthwhile striving for. (Though it fails to explain why existence exists )

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marcus

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I'm not an expert, or active researcher, I simply follow research in QG and cosmology with interest. So this is FWIW, just opinion.

I tend to think that LQG does not explain ANYTHING until it has made some predictions about, say, features of ancient light (CMB) that are then found. And a small preliminary confirmation would even so still be very iffy and provisional.

I can't think how observational astrophysicists would ever confirm what LQG says about *inside* Black Holes. Maybe they might see some signature in Gammaray Bursts (GRB). I'm don't think the theory has a definitive account of the BH *inside*, although there have been several proposals. That would look to a researcher, I imagine, like an attractive area where there are interesting *unsolved* problems.

I get the impression that you want to better understand what attracts researchers to LQG and to Loop cosmology in particular. I imagine that what attracts young researchers, in general, is when a field has interesting *unsolved* problems, which are accessible and which are probably solvable, and which if solved, the solution might turn out to be important or consequential in some way. It's been a growing field, fast developing, for 7 or 8 years now, probably because of these factors.

Right now I think I see a significant rival competing for attention from young researchers: this is called Shape Dynamics. SD burst on the scene in 2011 when the Loops 2011 conference organizers devoted a day to it. The first day of the weeklong conference as I recall. Now in 2013 it will probably make a strong showing at Loops 2013. It is different from LQG but the Loop people have generally had their conferences be open to other background independent approaches (QG where you don't have a fixed geometric setup to start with).

They have hosted CDT (causal dynam. triang.) and AS (asymp. safety) QG in the past. Also have invited string theorists to give talks. I don't know how well SD will thrive in the longer run.

For some papers you could look down the list for the FIRST QUARTER 2013 MIP POLL.

https://www.physicsforums.com/showthread.php?t=681598

It's not all Loop but it has a bunch of interesting recent QG papers.

I'll get some links too, later today.

I tend to think that LQG does not explain ANYTHING until it has made some predictions about, say, features of ancient light (CMB) that are then found. And a small preliminary confirmation would even so still be very iffy and provisional.

I can't think how observational astrophysicists would ever confirm what LQG says about *inside* Black Holes. Maybe they might see some signature in Gammaray Bursts (GRB). I'm don't think the theory has a definitive account of the BH *inside*, although there have been several proposals. That would look to a researcher, I imagine, like an attractive area where there are interesting *unsolved* problems.

I get the impression that you want to better understand what attracts researchers to LQG and to Loop cosmology in particular. I imagine that what attracts young researchers, in general, is when a field has interesting *unsolved* problems, which are accessible and which are probably solvable, and which if solved, the solution might turn out to be important or consequential in some way. It's been a growing field, fast developing, for 7 or 8 years now, probably because of these factors.

Right now I think I see a significant rival competing for attention from young researchers: this is called Shape Dynamics. SD burst on the scene in 2011 when the Loops 2011 conference organizers devoted a day to it. The first day of the weeklong conference as I recall. Now in 2013 it will probably make a strong showing at Loops 2013. It is different from LQG but the Loop people have generally had their conferences be open to other background independent approaches (QG where you don't have a fixed geometric setup to start with).

They have hosted CDT (causal dynam. triang.) and AS (asymp. safety) QG in the past. Also have invited string theorists to give talks. I don't know how well SD will thrive in the longer run.

For some papers you could look down the list for the FIRST QUARTER 2013 MIP POLL.

https://www.physicsforums.com/showthread.php?t=681598

It's not all Loop but it has a bunch of interesting recent QG papers.

I'll get some links too, later today.

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- #7

marcus

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Digitalism, this was a really nice question! After thinking for a while about it "LQG and entropy" I decided I would just give you the list of links to the top papers in the first quarter QG poll! That way it comes not just from me alone but from 14 of us who participated. It turned out that the top paper was about geometric ENTROPY and not specifically about LQG. I don't recommend reading it or any of these in particular, could be way too technical. This list is more to get an idea of what topics interest the nonstring qg researchers these days. An overview of the qg "research scene". Any paper you want to look at, just click the link. But the main thing is to kind of scan the lay of the land.

Six votes

http://arxiv.org/abs/1303.5612

**A Gravitational Entropy Proposal**

Timothy Clifton, George F R Ellis, Reza Tavakol

four votes

http://arxiv.org/abs/1302.0724

**Death and resurrection of the zeroth principle of thermodynamics**

Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1301.1264

**Inflation as a prediction of loop quantum cosmology**

Linda Linsefors, Aurelien Barrau

three votes

http://arxiv.org/abs/1303.7139

**Symmetry and Evolution in Quantum Gravity**

Sean Gryb, Karim Thebault

http://arxiv.org/abs/1303.2773

**BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models**

J.Manuel Garcia-Islas

http://arxiv.org/abs/1302.5265

**The loop quantum gravity black hole**

Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1301.5859

**Hamiltonian spinfoam gravity**

Wolfgang M. Wieland

Two votes

http://arxiv.org/abs/1303.6157

**Loop quantum dynamics of the gravitational collapse**

Yaser Tavakoli, Joao Marto, Andrea Dapor

http://arxiv.org/abs/1303.4989

**Loop Quantum Gravity and the The Planck Regime of Cosmology**

Abhay Ashtekar

http://arxiv.org/abs/1302.7142

**Holonomy Operator and Quantization Ambiguities on Spinor Space**

Etera R. Livine

http://arxiv.org/abs/1302.0254

**The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations**

Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1301.6210

**Embedding loop quantum cosmology without piecewise linearity**

Jonathan Engle

http://arxiv.org/abs/1301.3480

**Gauge networks in noncommutative geometry**

Matilde Marcolli, Walter D. van Suijlekom

Thanks to everybody who participated: Atyy, Chemist@, Chronos, Devils, Jason_0, John, Martin, Nonlinearity, Nullius, Sam, Skydive, Tom and Vasu. It really gives a clarified 3D vision to have all our various viewpoints blended in, instead of just one person's.

Six votes

http://arxiv.org/abs/1303.5612

Timothy Clifton, George F R Ellis, Reza Tavakol

four votes

http://arxiv.org/abs/1302.0724

Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1301.1264

Linda Linsefors, Aurelien Barrau

three votes

http://arxiv.org/abs/1303.7139

Sean Gryb, Karim Thebault

http://arxiv.org/abs/1303.2773

J.Manuel Garcia-Islas

http://arxiv.org/abs/1302.5265

Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1301.5859

Wolfgang M. Wieland

Two votes

http://arxiv.org/abs/1303.6157

Yaser Tavakoli, Joao Marto, Andrea Dapor

http://arxiv.org/abs/1303.4989

Abhay Ashtekar

http://arxiv.org/abs/1302.7142

Etera R. Livine

http://arxiv.org/abs/1302.0254

Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1301.6210

Jonathan Engle

http://arxiv.org/abs/1301.3480

Matilde Marcolli, Walter D. van Suijlekom

Thanks to everybody who participated: Atyy, Chemist@, Chronos, Devils, Jason_0, John, Martin, Nonlinearity, Nullius, Sam, Skydive, Tom and Vasu. It really gives a clarified 3D vision to have all our various viewpoints blended in, instead of just one person's.

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- #8

MTd2

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Is there any relation between LQC's bounce and the end of an Aeon, from Penrose?

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ok, thank you so much marcus! I will definitely peruse the links as time permits.

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