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Horava Gravity 
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#1
May1209, 11:18 AM

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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
May1209, 08:11 PM

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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/...s/lrr200512/ Volovik, http://arxiv.org/abs/0904.4113 Other interesting "emergent" ideas come from: gauge/gravity duality eg. http://arxiv.org/abs/grqc/0602037 condensed matter eg. http://arxiv.org/abs/condmat/0407140 


#3
May1309, 03:15 AM

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Here are the basic ideas:
 Quantum variant of EinsteinHilbert gravity is not renormalizable  Nonrenormalizability 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 ghostfree, but is not Lorentz invariant  At large distances (at which higherderivative terms in the action can be neglected) the theory reduces to the EinsteinHilbert 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 


#4
May1309, 04:01 AM

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Horava Gravity
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 


#5
May1309, 04:08 AM

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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). 


#6
May1309, 04:46 AM

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#7
May1309, 07:05 AM

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I think Marcus can say something better than I do. 


#8
May1309, 08:22 AM

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It certainly isn't something generally accepted. 


#9
May1309, 08:31 AM

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#10
May1309, 09:47 AM

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@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. 


#11
May1309, 10:49 AM

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#12
May1309, 11:02 AM

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



#13
May1409, 03:38 AM

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See e.g. arXiv:0904.1595 


#14
May2609, 12:13 AM

PF Gold
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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 kdeformed 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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