Thursday, February 09, 2006

Dr Dantas has LQG reading list...


Blogger nige said...

For the universe to be a black hole, R = 2GM/c^2.

M = [density].[4/3 Pi R^3].


R = 2G.[density].[4/3 Pi R^3]/c^2

= (8/3)G.[density].Pi(R^3)/c^2


density = (3/8)(c^2)/(Pi.G.R^2)

If Hubble constant H = c/R,

density = (3/8)(H^2)/(Pi.G)

This is the formula for the "critical density" which is about 10 times or so higher than observed density.

The point I'm getting at, is that existing cosmology which uses the standard solution for general relativity implicitly ASSUMES that there is no mechanism for gravity within the universe (ie, it assumes that the universe is a black hole).

If there is a mechanism for gravity which has any analogy to other forces (vector boson exchange between charges in electroweak theory, for instance), then the universe can't be a black hole because it can be considered a single mass.

If gravity is due to exchange of gauge boson radiation, spin-2 gravitons in string theory, for example, then the black hole in the middle of the Milky Way is there because gravitons are being exchanged between it and the surrounding matter.

This can't happen if the whole universe is the black hole, unless you are going to picture a lot more universes around our own!

Gravitons aren't stopped as light is stopped by a black hole. Gravitons, if there are such things, must be exchanged between all masses, including black holes. Therefore, if you have just a single black hole, it will lose energy by radiating away gravitons without exchange (ie without receiving any gravitons back from other masses).

So all this speculation is ignorant of energy conservation, not to say the basic premises of quantum gravity.
Nigel Cook | Homepage | 02.09.06 - 11:34 am | #

9:19 AM  

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