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Physics does not end at singularities (Abhay at MG12)

  1. Aug 2, 2009 #1


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    The Marcel Grossmann meeting is a major international conference held every three years--on "recent developments in theoretical and experimental general relativity, astrophysics and relativistic field theories." The twelfth in the series--MG12--was in Paris last month.

    Abhay Ashtekar chaired the July 14 plenary session (at which Laurent Freidel, Herbert Hamber, and Juan Maldacena spoke) and then later, on July 16, Ashtekar gave this presentation:
    "Recent advances in Loop Quantum Cosmology"

    If you want to know where that field is and where it's going, that is obviously the PDF file to check out.

    Marcel Twelve was attended by 886 scientists. It is a premier event and definitive of a lot of things. So it's worth paying attention to some of the talks.
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  3. Aug 2, 2009 #2


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    Much of the presentation gives details of recent results. The last two slides summarize what he has been discussing and give a kind of overview, which I'll quote with highlighting to call attention to some specifics. Note progress towards the inclusion of more realistic matter. Earlier LQC involved only simple (one could say 'toy') matter fields, now that is beginning to change (work by Ashtekar himself, by Lewandowski, and others.)

    3. Summary: Singularity Resolution

    • Quantum geometry creates a brand new repulsive force in the Planck regime, replacing the big-bang by a quantum bounce. Repulsive force rises and dies very quickly but makes dramatic changes to classical dynamics. (Origin: Planck scale non-locality of quantum Einstein’s equations.)
    New paradigm: Physics does not end at singularities.
    Quantum space-times may be vastly larger than Einstein’s.

    • Singularities treated are of direct physical interest. Long standing questions have been answered in LQC. In particular, the ultra-violet/infra-red tension has been resolved.

    • Detailed analysis in specific models but taken together with the BKL conjecture, results suggest that all space-like, strong curvature singularities may be resolved by the quantum geometry effects of LQG.

    3. Summary: Other Developments

    • Path integrals open up two frontiers: i) Effective theories not tied to specific quantum states; and, ii) Detailed explorations of spin foams in simple models.

    First results on the phenomenological front have already appeared.
    Inhomogeneities are now being incorporated. Growing exchange between cosmologists and the LQC community.

    Beginning of QFT on cosmological, quantum space-times (AA, Kaminski, Lewandowski).
    Tension between relational time of LQC and proper/conformal time of QFT in CST resolved by systematic simplifications to descend from the first to the second. Fertile area for conceptual, mathematical and phenomenological developments.(Puchta)

    • Several other issues I could not mention in the talk: Entropy considerations (AA); Barbero Immrizi field (Calcagni, Mercuri, Taveras, Yunes), measures on spaces of solutions to calculate a priori [event] probabilities...(Sloan), ...

    For more information and talks from the Marcel Twelve meeting:
    It is particularly revealing to compare three talks given at the plenary session which Ashtekar chaired, by Freidel, Hamber, and Maldacena:
    Last edited: Aug 2, 2009
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