Properties of the volume operator in loop quantum gravity: I. Results (original) (raw)

We analyze the spectral properties of the volume operator of Ashtekar and Lewandowski in Loop Quantum Gravity, which is the quantum analogue of the classical volume expression for regions in three dimensional Riemannian space. Our analysis considers for the first time generic graph vertices of valence greater than four. Here we find that the geometry of the underlying vertex characterizes the spectral properties of the volume operator, in particular the presence of a 'volume gap' (a smallest non-zero eigenvalue in the spectrum) is found to depend on the vertex embedding. We compute the set of all non-spatially diffeomorphic non-coplanar vertex embeddings for vertices of valence 5-7, and argue that these sets can be used to label spatial diffeomorphism invariant states. We observe how gauge invariance connects vertex geometry and representation properties of the underlying gauge group in a natural way. Analytical results on the spectrum on 4-valent vertices are included, for which the presence of a volume gap is proved. This paper presents our main results; details are provided by a companion paper . * brunnemann@math.uni-hamburg.de † d.rideout@imperial.ac.uk 1 With the term 'background independence' we imply independence on a choice of fixed background geometry. In order to allow for the possibility of topology change in quantum gravity, it would be desirable to have the theory in its final formulation be independent of the topology of the underlying manifold, however to our knowledge this has not been achieved so far within LQG.