Friday, December 05, 2008

KK description of Black Holes?

Still no second part of the spectral action post. But instead a litte puzzle that came up over coffee yesterday: What is the Kaluza-Klein description of a black hole?

To be more explicit: Take pure gravity on R^4xK for compact K and imagine that it is large (some parsec in diameter say). Then you could imagine you have something that looks like a blackhole in this total space-time. What is its four dimenional description in KK theory?

With KK theory I mean the 4d theory with an infinite number of fields. I want to include the whole KK tower. This theory should be equivalent to the higher dimensional one since both are related by a (generalised) Fourier transform on K. One might be worried that a black hole is so singular that this Fourier transform has problems, does not converge or something. But if that is your worry, take a black hole that is not eternal but one that is formed by the collision of graviational waves say. In the past, those waves came in from infinity and if you siufficiently go back in time all fields are weak. This weak field configuration should have no problem being described in KK language and then the evolution is done in the 4d perspective. What would the 4d observer see when the black hole forms in higher dimensions?

The question I would be most interested in is if there is always a black hole in terms of the 4d gravity or if the 4d gravity can remain weak and the action can be entirely in the other fields.

One scenario I could imagine is as follows: The 4d theory has besides the metric some gauge fields and some dilatons. If the black hole is well localised in K then many higher Fourier modes of K will participate. From the 4d perspective, the KK momentum is the charge under the gauge fields and the unit is dependent on the dilaton. So could it be that there is a gauge theory black hole, i.e. a charged configuration that is confined to small region of space time where the coupling is strong with all the causality implications of black holes in gravity?

Mark A. Thomas said...

Interesting and highly speculative. As a layman I played around with the idea of a 'gauge theory' black hole. One can look at such an object as a cover of a 'gauge theory' VeV on a sphere (temporarily ignoring the event horizon). As you state a KK tower gains you access to the description of the 4D fields in a semi-classical action. Having the dilaton and evaporative time ensures the running of couplings including the gravitational constant. This means that the 'gauge theory' VeV goes to higher values as the sphere gets smaller. I think a trick would be to find intervals where the phase transitions occur. As long as the black hole is in the semi-classical regime the gravitational fields are weak because of the initial large exponential suppressions 1/Mpl^2 that are at the low energy end of things. The cross over into the Planck regime (the quantum corrections) is the really tricky part.

Robert said...

I have to admit I have no idea what you are talking about. What I have in mind is a classical description of the BH from a 4d perspective. There is nothing quantum about, it is not semi-classical, it is classical and I want to ignore quantum corrections completely.

Mark A. Thomas said...

Robert,
I interpreted the weak field 4d black hole you describe as evaporating as per the semi-classical physics (wrongly). The higher dimension black hole formation you mention I thought was reference toward the Planckian (again apparently wrong). I am not trained in the field and maybe I just should keep my mouth shut.
mark

Robert said...

No, on the contrary. Please keep commenting! Just explain yourself a bit more.