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  How is the expansion of space itself explained in LQG?

+ 2 like - 0 dislike

To explain the expansion of space I have often heard people saying that space is continuously created. This picture is usually applied to cosmological scales but I`m nevertheless curious if some microscopic description of this process could be found (or probably is already available?).

The first thing which comes into my mind by thinking about this is that in theories like loop quantum gravity, there is an idea that space is quantized. Would space expansion then be explained as the creation of new quanta in loop quantum cosmology? Or are there other different ideas, such as the quanta themselves changing properties? Do people actually working on this already have some interesting results?

asked Dec 28, 2011 in Theoretical Physics by Dilaton (6,240 points) [ revision history ]

1 Answer

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I worked this up the chain to Martin Bojowald, and he sent me this response:

The short answer is: both. The number of edges and vertices as well as their properties (the geometrical excitation level) change. Details have not been analyzed much because the numbers and properties of edges and vertices can be seen only when a full, inhomogeneous graph is used. However, all the explicit models of cosmological expansion in loop quantum gravity rely on a complete reduction to homogeneity, which is so radical that it smears out all edges and vertices and their properties into just one quantum number (or maybe three in anisotropic models). But we do know that the full Hamiltonian generates changes of both the numbers and properties of edges and vertices.

The only reference I can think of is http://arxiv.org/abs/0705.4398; see especially Fig. 1.

Basically, LQC models are framed in terms of large-scale properties of quantum geometry, so nobody knows what happens to the spin networks in very much detail.

This post imported from StackExchange Physics at 2014-03-17 03:29 (UCT), posted by SE-user David Z
answered Jan 27, 2012 by David Z (660 points) [ no revision ]
Thanks a lot for this answer @David Zaslavsky ( and Martin Bojowald ...) :-). To fully appreciate (the technical parts of) the paper which looks exciting, I`ll have to learn some more things from the "First Course" book.

This post imported from StackExchange Physics at 2014-03-17 03:29 (UCT), posted by SE-user Dilaton
Do I understand this right then (with mmy present knowledge...) that in principle the expansion of space can be described by the evolution of the spin networks considering not only "redistributions" which leave the total volume of space constant but by allowing the creation of new space quanta which were not present at the previous state of space? But this can be done only in a very "coarse grained" sense at present?

This post imported from StackExchange Physics at 2014-03-17 03:29 (UCT), posted by SE-user Dilaton
Roughly speaking, I think you're on the right track. But I'd be wary of saying that the "redistributions" keep the total volume of space constant. Volume is an emergent property of the spin network in LQG. To really understand what happens to the the graph, you have to think in terms of the numbers and excitation levels of the vertices and edges.

This post imported from StackExchange Physics at 2014-03-17 03:29 (UCT), posted by SE-user David Z

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