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Video lecture on Loop Quantum Cosmology

  1. Jun 18, 2008 #1


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    LQC is the dominant approach to quantum cosmology these days, if you gauge by recent peer-review publication and citations in the professional literature.

    So anybody who wants to know about quantum cosmology could be well advised to get an introduction to LQC.

    Parampreet Singh is one of the experts and he is currently at Perimeter Institute, where they have the PIRSA online video seminar resource. Singh just gave a talk on LQC:

    it is in a nice windows format with split screen, so you see him talking and also see the current slide being projected.

    LQC is the approach where after General Relativity is quantized, at the Friedmann equation level, it turns out that gravity repels at high density and you get a bounce. The big bang is replaced by a bounce, from a prior contraction.

    A lot of people have been joining the LQC research effort recently, so there are a lot of new authors and papers. Parampreet Singh is one of the leaders, along with Ashtekar, Bojowald and a few others.

    If you want to get some perspective on it. Look at the Spires list, with keyword quantum cosmology, date > 2005, ordered by citation count. Most of the top 10, or even top 20, papers are LQC.

    So it is something you need to know about if you follow developments in cosmology. And hopefully this video talk by Singh can help fill the need for some kind of accessible introduction to the subject.

    There is no perfect introduction as yet. The talk by Singh is only 20 minutes long! He has to talk very fast, and even with amplification one can't always understand.

    Another good resource is the July 2008 Scientific American article by Renate Loll team
    "Self-organizing Quantum Universe" pages 42-49.
    This makes the triangulations approach to quantum gravity more intuitive, and has excellent graphics illustrating some of the non-trivial points.
    Last edited: Jun 18, 2008
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  3. Jun 18, 2008 #2
    Hi, thanks for posting the video :)

    It is a happy coincidence that only yesterday I posted that the Schwarzschild solution predicts the same thing. https://www.physicsforums.com/showpost.php?p=1769733&postcount=137

  4. Jun 18, 2008 #3


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    Hi kev, I'm glad someone is interested in introductory descriptions of quantum gravity models. For balance, so as to keep an eye on all the horses in the race, I hope you read the 7-page article in Scientific American by Renate Loll as well:
    Another good resource is the July 2008 Scientific American article by Renate Loll team
    "Self-organizing Quantum Universe" pages 42-49.

    This field is going gangbusters. It is a very active research front. Still too early to pick winners.
    The SciAm article is exceptionally well-written, I think, with very helpful graphic illustrations. It is a different model from both what you are talking about and LQC, but it may interest you that they also derive a kind of bounce behavior.

    As yet they have not included matter in their model. They still have a lot to do.

    The good thing is that they don't put any spacetime in at the beginning to start with. They just put in a bunch of simple "quantum bits" or "geometry atoms" and tell them to join and interact according to some simple rules.

    So the whole swarm of bits self-organizes into thousands of different unsmooth spacetimes, which average out to be a smooth deSitter spacetime (plus quantum fluctuations around that ideal shape). So one can say that that the deSitter emerges from an incoherent chaos of possibilities.

    And the standard version deSitter does involve a kind of bounce. It has a narrow throat or waist-----shrinking down to maximal constriction and then expanding out again.

    You really should look at the SciAm article by Loll and friends.

    Andy Randono has a new article also showing a bounce, but deriving an interesting addtional detail: both the shrinking down, and the subsequent expanding out, proceed in little stepwise ripples.
    Here is the paper
    The essential thing is to look at Figure 2, where he plots the ripples. It shows the size of the universe decreasing and then increasing in quasi-jumps.
    Last edited: Jun 18, 2008
  5. Jun 18, 2008 #4
    Hi marcus,
    I do find new quantum models exciting and interesting but I think I may have missed that issue of Scientic American. I did however see an article by Amanda Gefter in the May 3rd 2008 issue of New Scientist that listed the top 5 TOE contenders as: (in no particular order)

    Loop Quantum Gravity.
    Causal Dynamic Triangulations.
    Quantum Einstein Gravity.
    Quantum Graphity.
    Internal Relativity.

    They sounded particularly enthusiastic about the Causal Dynamic Triangulations. (Strings or Loops are replaced by Triangles :P) The trouble is, with so many competing theories it is difficult to be motivated to study them in detail, until the the scientific world says "THIS IS THE ONE!" especially as there are so many competing and promising theories and trying to learn any one of them is like taking your first swimming lesson by being dropped in the middle of the Atlantic Ocean by helicopter, without water wings :O There seems to be only two types of scientific books. Those that have no equations at all and those that are full of extremely complex equations with no explanation of the symbols used. Maybe it is just the inherent nature of quantum physics that there can be no middle ground.

    This sounds a lot like the "New Kind Of Science" cellular automata idea that was being touted as the way forward by Wolfram the creator of Mathmatica software. Just maybe he WAS on to something! See http://www.goertzel.org/dynapsyc/2002/WolframReview.htm

    Do you get the impression that God is smart but a bit lazy? Everytime you look closely at something, for example living things, you discover it is made of self organising elements. He just puts the seeds there and lets the universe build itself! Oops.. just a bit of fun :) ...not meant to be a religious point ;)

    That last diagram shows the "classic" model as smoothly turning around before hitting infinite density which seems to contradict what was said in the LQG video that is the topic of this thread. Maybe they are talking about different "classic" models?
  6. Jun 18, 2008 #5


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    Try this link!

    "Self-organizing Quantum Universe"
    This has the July 2008 pages 42-49 article free online. Does the link work on your computer?

    Yes I read Amanda Gefter in the New Scientist too. She put Loll's CDT first and gave it the most attention. Definitely particularly enthusiastic about that one.

    Yes, and evolution by natural selection in biology. Not to take the metaphor of a creator seriously, nature does seem to operate a lot by having complexity arise from simple elements by some kind of self-organizing process. If it is a creator it is a very laid-back one :biggrin: Just joking. Let's keep on topic.

    Randono explains in the text why at that very point you shouldn't take the superimposed ripple seriously. It doesnt hit infinite density. There's an imperfect approximation at the turning point. See last paragraph of section 4, on page 8.

    "The WKB analysis is not valid near the classical turning point so the peculiar behavior of the effective scale factor at tau = 0 can be discarded..."

    Good for you for noticing this! It is something he will have to deal with in the followup research, which he mentions.
  7. Jun 18, 2008 #6
    Yep. Works fine :approve: I will read that at my leisure. Thanks for the link! :cool:

    Well, there was that, but there was the fact that in LQG video the presenter said that "classic" GR predicts that if a universe crunches that it does not bounce. He showed two curves that crossed over and asymptotically aproached the vertical axis with no connection between the two curves, while LQG predicts a single curve with a smooth turnaround. The Randano text on the other hand shows the classic model as smoothly turning around while the mesoscopic quantum model turns around in a "wavey" fashion. Did he mean classical GR or classical quantum cosmology? If he meant classical GR why does it contradict the curve shown in the LQC video?
  8. Jun 18, 2008 #7
    I read the article and I like it! CDT operates with just 3 space dimensions and one time dimension and an assumption that causality is an inherent part of the structure of the universe and not an emergent quality. It rules out worm holes and baby universes and does not require lots of additional unseen dimensions. That's a relief! I never did buy into all that mumbo jumbo. If CDT can show that black holes are hollow shells of mass and not singularites of infinite mass, I will be really impressed! hehe :devil:
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