1. one by Padmanabhan
http://arxiv.org/gr-qc/0606061
Gravity: A New Holographic Perspective
T. Padmanabhan
Plenary talk at the International Conference on Einstein's Legacy in the New Millennium, December 15 - 22, 2005, Puri, India
2. two by Rovelli
http://arxiv.org/gr-qc/0604044
Graviton propagator in loop quantum gravity
Eugenio Bianchi, Leonardo Modesto, Carlo Rovelli, Simone Speziale
41 pages, 6 figures
"We compute some components of the graviton propagator in loop quantum gravity, using the spinfoam formalism, up to some second order terms in the expansion parameter."
http://arxiv.org/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
7 pages
"We argue that EPR-type correlations do not entail any form of 'non-locality', when viewed in the context of a relational interpretation of quantum mechanics. The abandonment of strict Einstein realism advocated by this interpretation permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."
3. two by Ashtekar
http://arxiv.org/gr-qc/0605011
Gravity, Geometry and the Quantum
Abhay Ashtekar
16 pages, 2 figures. To appear in the Proceedings of the Einstein Century Conference, 15-22 July, Paris, edited by J-M Alimi et al (American Institute of Physics)
"After a brief introduction, basic ideas of the quantum Riemannian geometry underlying loop quantum gravity are summarized. To illustrate physical ramifications of quantum geometry, the framework is then applied to homogeneous isotropic cosmology. Quantum geometry effects are shown to replace the big bang by a big bounce. Thus, quantum physics does not stop at the big-bang singularity. Rather there is a pre-big-bang branch joined to the current post-big-bang branch by a 'quantum bridge'. Furthermore, thanks to the background independence of loop quantum gravity, evolution is deterministic across the bridge."
http://arxiv.org/gr-qc/0602086
Quantum Nature of the Big Bang
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh
4 Pages, 2 Figures. Minor changes to match the published version in Physical Review Letters
Phys.Rev.Lett. 96 (2006) 141301
"Some long standing issues concerning the quantum nature of the big bang are resolved in the context of homogeneous isotropic models with a scalar field. Specifically, the known results on the resolution of the big bang singularity in loop quantum cosmology are significantly extended as follows: i) the scalar field is shown to serve as an internal clock, thereby providing a detailed realization of the 'emergent time' idea; ii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously; iii) the Hamiltonian constraint is solved numerically to show that the big bang is replaced by a big bounce. Thanks to the non-perturbative, background independent methods, unlike in other approaches the quantum evolution is deterministic across the deep Planck regime."
4. two by Baez
http://arxiv.org/abs/gr-qc/0605087
Quantization of strings and branes coupled to BF theory
John C. Baez, Alejandro Perez
"BF theory is a topological theory that can be seen as a natural generalization of 3-dimensional gravity to arbitrary dimensions. Here we show that the coupling to point particles that is natural in three dimensions generalizes in a direct way to BF theory in d dimensions coupled to (d-3)-branes. In the resulting model, the connection is flat except along the membrane world-sheet, where it has a conical singularity whose strength is proportional to the membrane tension. As a step towards canonically quantizing these models, we show that a basis of kinematical states is given by 'membrane spin networks', which are spin networks equipped with extra data where their edges end on a brane."
http://arxiv.org/abs/math.DG/0511710
Higher Gauge Theory
John C. Baez,
Urs Schreiber
"Just as gauge theory describes the parallel transport of point particles using connections on bundles, higher gauge theory describes the parallel transport of 1-dimensional objects (e.g. strings) using 2-connections on 2-bundles. A 2-bundle is a categorified version of a bundle: that is, one where the fiber is not a manifold but a category with a suitable smooth structure. Where gauge theory uses Lie groups and Lie algebras, higher gauge theory uses their categorified analogues: Lie 2-groups and Lie 2-algebras. We describe a theory of 2-connections on principal 2-bundles and explain how this is related to Breen and Messing's theory of connections on nonabelian gerbes. The distinctive feature of our theory is that a 2-connection allows parallel transport along paths and surfaces in a parametrization-independent way. In terms of Breen and Messing's framework, this requires that the "fake curvature" must vanish. In this paper we summarize the main results of our theory without proofs."
5. three by Majid
http://arxiv.org/hep-th/0604132
Algebraic approach to quantum gravity III: noncommmutative Riemannian geometry
Shahn Majid
25 pages, 1 figure, to appear in collection B. Fauser and J. Tolksdorf, eds., Birkhauser
"This is a self-contained introduction to quantum Riemannian geometry based on quantum groups as frame groups, and its proposed role in quantum gravity. Much of the article is about the generalisation of classical Riemannian geometry that arises naturally as the classical limit; a theory with nonsymmetric metric and a skew version of metric compatibilty. Meanwhile, in quantum gravity a key ingredient of our approach is the proposal that the differential structure of spacetime is something that itself must be summed over or 'quantised' as a physical degree of freedom. We illustrate such a scheme for quantum gravity on small finite sets."
http://arxiv.org/hep-th/0604130
Algebraic approach to quantum gravity II: noncommutative spacetime
Shahn Majid
26 pages, 2 figures; book chapter to appear in D. Oriti, ed., Cambridge Univ. Press
"We provide a self-contained introduction to the quantum group approach to noncommutative geometry as the next-to-classical effective geometry that might be expected from any successful quantum gravity theory. We focus particularly on a thorough account of the bicrossproduct model noncommutative spacetimes of the form [t,x_i]=i \lambda x_i and the correct formulation of predictions for it including a variable speed of light. We also study global issues in the Poincaré group in the model with the 2D case as illustration. We show that any off-shell momentum can be boosted to infinite negative energy by a finite Lorentz transformaton."
http://arxiv.org/hep-th/0601004
Noncommutative Harmonic Analysis, Sampling Theory and the Duflo Map in 2+1 Quantum Gravity
L. Freidel, S. Majid
54 pages, 2 figures
"We show that the *-product for U(su
2) arising in [Freidel-Livine] in an effective theory for the Ponzano-Regge quantum gravity model is compatible with the noncommutative bicovariant differential calculus previously proposed for 2+1 Euclidean quantum gravity using quantum group methods in [Batista-Majid]. We show that the effective action for this model essentially agrees with the noncommutative scalar field theory coming out of the noncommutative differential geometry. We show that the required Fourier transform essentially agrees with the previous quantum group Fourier transform. In combining these methods we develop practical tools for noncommutative harmonic analysis for the model including radial quantum delta-functions and Gaussians, the Duflo map and elements of `noncommutative sampling theory' applicable to the bounded SU
2,SO
3 momentum groups. This allows us to understand the bandwidth limitation in 2+1 quantum gravity arising from the bounded momentum. We also argue that the the anomalous extra `time' dimension seen in the noncommutative differential geometry should be viewed as the renormalisation group flow visible in the coarse graining in going from SU
2 to SO
3. Our methods also provide a generalised twist operator for the *-product."Most of these papers have been discussed in current PF threads.
The Majid not, but we had a thread about the Majid papers earlier:
https://www.physicsforums.com/showthread.php?t=118079
