Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session Q14: Quantum Aspects of Gravitation |
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Sponsoring Units: GGR Chair: Jorge Pullin, Louisiana State University Room: Washington 4 |
Monday, February 15, 2010 1:30PM - 1:42PM |
Q14.00001: Gravitational consequences of confined fields in braneworlds Stephen Green In the braneworld scenario, matter fields are usually assumed to be confined to branes in a higher dimensional spacetime, with a stress energy tensor that is sharply peaked on the brane. A field theoretic way to achieve this is by coupling a bulk field to a topological defect which makes up the brane. We point out that in contrast to fermion fields, the stress energy tensor of a confined scalar field contains a term that is of the form of the second derivative of a sharply peaked function. This produces large metric perturbations on the brane. However, the resulting gravitational effect is equivalent to a local self-interaction term, and may be negated by artificially inserting an opposite self-interaction for the underlying theory. We comment on the possible generalization to other bosonic fields as well as the potential observability. [Preview Abstract] |
Monday, February 15, 2010 1:42PM - 1:54PM |
Q14.00002: The Emergence of General Relativity from Loop Quantum Gravity Chun-Yen Lin We show that General Relativity emerges from Loop Quantum Gravity in the relative prescription of gravity against the matter coordinates. The local Dirac observables and coherent states are constructed to explicitly evaluate the dynamics. The dynamics in large scale confirms with General Relativity up to the corrections that appear nearby singularities. [Preview Abstract] |
Monday, February 15, 2010 1:54PM - 2:06PM |
Q14.00003: Linear Response and the Validity of the Semiclassical Approximation in de Sitter Space Paul Anderson, Carmen Molina-Paris, Emil Mottola Linearized fluctuations of quantized matter fields and the spacetime geometry around de Sitter space are considered in the case that the matter fields are conformally invariant. Taking the unperturbed state of the matter to be the de Sitter invariant Bunch-Davies state, the linear variation of the stress tensor about its self-consistent mean value serves as a source for fluctuations in the geometry through the semi-classical Einstein equations. This linear response framework is used to investigate both the importance of quantum backreaction and the validity of the semi-classical approximation in de Sitter space. [Preview Abstract] |
Monday, February 15, 2010 2:06PM - 2:18PM |
Q14.00004: Relational interpretation with Dirac observables and the problem of time in quantum gravity Jorge Pullin, Rodolfo Gambini We show that the use of evolving Dirac observables in conjunction with the conditional probabilities of Page and Wootters correctly predicts the physical propagators in model systems. This eliminates the main objection to that treatment and opens possibilities for correctly handling the problem of time in quantum gravity in terms of observable quantities. [Preview Abstract] |
Monday, February 15, 2010 2:18PM - 2:30PM |
Q14.00005: Spontaneous Dimensional Reduction in Short-Distance Quantum Gravity? Steven Carlip Several lines of evidence hint that quantum gravity at very small distances may be effectively two-dimensional. I will summarize the evidence for such ``spontaneous dimensional reduction,'' and suggest an additional argument coming from the strong-coupling limit of the Wheeler-DeWitt equation. If this description proves to be correct, it suggests an interesting relationship between small-scale quantum spacetime and the behavior of cosmologies near an asymptotically silent singularity. [Preview Abstract] |
Monday, February 15, 2010 2:30PM - 2:42PM |
Q14.00006: Numerical Analysis of Black Hole Evaporation in the CGHS Model Fethi M. Ramazanoglu, Abhay Ashtekar, Frans Pretorius We numerically analyze the black hole formation and evaporation in the CGHS model, in the dynamical background of 1+1 dimensions. Specifically, we investigate the energy flux and the behavior of the asymptotic killing vector $\frac{\partial}{\partial y^-}$ at the future null infinity $\mathcal{I}_R^+$, and their relation to the rate of change of the Bondi mass of the black hole. Emphasis is given to the rapid evolution of the space-time near the point where the last ray reaches $\mathcal{I}_R^+$ and its implications about the information loss in this model. [Preview Abstract] |
Monday, February 15, 2010 2:42PM - 2:54PM |
Q14.00007: Decoherence Functionals for Quantum Cosmology David Craig, Parampreet Singh Decoherence functionals are explicitly constructed for flat cosmological models in both standard Wheeler-DeWitt theory and also loop quantum cosmology, thereby permitting consistent quantum predictions to be made in complete, mathematically precise models of quantum cosmology. Consistent families of quantum histories are exhibited and the prediction of semiclassical behavior for suitable initial states described. Criteria for the resolution of the initial singularity are formulated in a precise way. Singularity avoidance in loop-quantized models is contrasted with the failure of Wheeler-DeWitt quantum cosmological models to avoid the big-bang singularity. The critical role played by decoherence of histories is illustrated in this example by showing that failure to consider decoherence leads to precisely the \emph{opposite} conclusion for a large class of states. [Preview Abstract] |
Monday, February 15, 2010 2:54PM - 3:06PM |
Q14.00008: Asymptotics of the Wigner 9j-Symbol Hal Haggard, Robert Littlejohn We present the asymptotic formula for the Wigner $9j$-symbol, valid when all quantum numbers are large. The formula is a generalization of the well known asymptotic formula of Ponzano and Regge (1968) for the Wigner $6j$-symbol that has played a central role in discrete approaches to $3D$ quantum gravity. Analysis of the classically allowed region of the $9j$-symbol reveals a geometrically rich structure. We conclude with a discussion of the extension of our methods to higher $3nj$-symbols. Asymptotic formulas for the higher $3nj$-symbols will clarify the semiclassical limit of spinfoam approaches to quantum gravity. [Preview Abstract] |
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