Horava Gravity

  1. tom.stoer

    tom.stoer 5,489
    Science Advisor

    Can anybody explain in brief what the content of this new research program is?
    - where does it come from?
    - what are the main ideas and ingredients?
    - what are the common aspects / main differences compared to other approaches?
    - ...
    - is there already some kind of review, e.g. an article, slides, a talk, ...?

    It's interesting that this new approach attracts so many people in a very short time (4 month from the first paper in arxiv); there IS something and I would like to understand the main ideas w/o going through all these papers.

    Thanks in advance
     
  2. jcsd
  3. atyy

    atyy 10,078
    Science Advisor

    Is "background independence" or even Lorentz invariance really necessary? Some previous lines of work trying to get gravity to "emerge" from a theory with a fixed background are:
    Barcelo et al, http://relativity.livingreviews.org/Articles/lrr-2005-12/
    Volovik, http://arxiv.org/abs/0904.4113

    Other interesting "emergent" ideas come from:
    gauge/gravity duality eg. http://arxiv.org/abs/gr-qc/0602037
    condensed matter eg. http://arxiv.org/abs/cond-mat/0407140
     
    Last edited: May 12, 2009
  4. Demystifier

    Demystifier 5,023
    Science Advisor

    Here are the basic ideas:
    - Quantum variant of Einstein-Hilbert gravity is not renormalizable
    - Non-renormalizability could be overcomed if the propagators had higher powers of k^2 in the denominator, because then loop integrals could become less divergenet or even finite
    - Higher powers of k^2 correspond to an effective action with higher derivatives
    - Due to Lorentz invariance the higher derivatives occur not only in space but also in time
    - Higher derivatives in time imply that the theory has additional degrees of freedom - ghosts - that make troubles
    - This leads to the Horava's idea - let us propose an action which has higher derivatives in space, but not in time - such a theory of gravity is renormalizable and ghost-free, but is not Lorentz invariant
    - At large distances (at which higher-derivative terms in the action can be neglected) the theory reduces to the Einstein-Hilbert theory, which makes the theory consistent with observations

    Now we have a NEW classical action for gravity the solutions of which can be studied even at the classical level, without actually quantizing it. This fact makes it relatively EASY to calculate various new stuff, which is one of the main reasons why so many papers appeared in a very short time. In addition to being easy, it is also INTERESTING, because it is related to a theory that at the quantum level is renormalizable.

    Further advantages with respect to other theories:
    Compared to string theory - much simpler and works in 3+1 dimensions
    Compared to LQG - the classical limit is not a problem
     
    Last edited: May 13, 2009
  5. Demystifier

    Demystifier 5,023
    Science Advisor

    My own contribution:
    The absence of Lorentz invariance helps to solve the problem of quantum particles in classical gravitational backgrounds:
    http://xxx.lanl.gov/abs/0904.3412
     
  6. Haelfix

    Haelfix 1,735
    Science Advisor

    I agree with Demystifier that it appears to be an interesting theory precisely b/c you can calculate a lot of things in it. Those observables in turn might be related by universality to something more fundamentally sound, at least thats the hope.

    Much like other modified gravity proposals that don't work for one reason or another, you can usually extract valuable insights from them.

    The downside with this theory is the maximal amount of finetuning thats required (in general this is always the case when you break Lorentz invariance; upon RG flow operators that were normally under control can in principle recieve enormous corrections that can only be tamed by finetuning).
     
  7. Demystifier

    Demystifier 5,023
    Science Advisor

    Yes, that's the weakness of the theory, at least in its present formulation.
     
  8. MTd2

    MTd2 1,980
    Gold Member

    But isn't there strong evidence that it is non perturbative renormalizable? I mean, this is one of recent discussions here, because it seems that Horava Gravity coincides with assymptotic safety in many aspects, like the continuous dimensions transition.

    I think Marcus can say something better than I do.
     
  9. Demystifier

    Demystifier 5,023
    Science Advisor

    I'm not an expert, but I think it is questionable whether this evidence is "strong" enough.
    It certainly isn't something generally accepted.
     
  10. MTd2

    MTd2 1,980
    Gold Member

    I know what you mean. It is questionable since until sometime ago there was no non-divergent term, from the perturbative expantion, included in the truncation. This is one of the things that Jacques Distler found fishy. But, recently, one such term was included, and shown that the method still yields a non perturbative renormalizable result, without significant changes to what was found without it. Well, as I said, Marcus can tell us more about this.
     
  11. tom.stoer

    tom.stoer 5,489
    Science Advisor

    @Demystifier: Thanks a lot for the summary!

    The major weakness seems to be that there's no guiding principle which terms are to be included. That's like f(R) gravity: you can include a lot of stuff and fit nearly everything after approriate adjusting and finetuning.

    So the approach is nice but lacks a guidiung principle, e.g. a unique theory which produces Horava gravity in some appropriate limit.
     
  12. Demystifier

    Demystifier 5,023
    Science Advisor

    Well, Horava used the detailed balance principle to fix the action uniquely. But this principle does not seem to be necessary.
     
  13. tom.stoer

    tom.stoer 5,489
    Science Advisor

    what do you mean by "... does not seem to be necessary"? Is there another way to fix the action?
     
  14. Demystifier

    Demystifier 5,023
    Science Advisor

    Actions that do not satisfy the detailed balance principle have also been proposed.
    See e.g. arXiv:0904.1595
     
  15. MTd2

    MTd2 1,980
    Gold Member

    I was checking the citations for that article that says that a scalar field does not decouple from the theory, and I found this:

    http://arxiv.org/abs/0905.3423v1

    It uses k-deformed algebra to fix that problem, which is kind of interesting given that it has been appearing whenever there is a fractal spacetime, a la Modesto, being consistent with assymptotic safety and LQG.

    This another one also says that there is a way to get rid of the scalar:

    http://arxiv.org/abs/0905.3821v1

    It also cites the one above, but doesnt analyze much of it since it was posted on arxiv just 2 days after.

    The funny thing it is that, although I understand little, it seems that with small corrections, that scalar can easily dealt with.
     
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