Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session D11: Theoretical and Quantum Gravity |
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Sponsoring Units: GGR Chair: David Garfinkle, Oakland University Room: Plaza Court 1 |
Saturday, May 2, 2009 3:30PM - 3:42PM |
D11.00001: Unitarity and Holography in Gravitational Physics Donald Marolf Because the gravitational Hamiltonian is a pure boundary term on-shell, asymptotic gravitational fields store information in a manner not possible in local field theories. This fact has consequences for both perturbative and non-perturbative quantum gravity. In perturbation theory about an asymptotically flat collapsing black hole, the algebra generated by asymptotic fields on future null infinity within any neighborhood of spacelike infinity contains a complete set of observables. Assuming that the same algebra remains complete at the non-perturbative quantum level, we argue that either 1) the S-matrix is unitary or 2) the dynamics in the region near timelike, null, and spacelike infinity is not described by perturbative quantum gravity about flat space. We also consider perturbation theory about a collapsing asymptotically anti-de Sitter (AdS) black hole, where we show that the algebra of boundary observables within any neighborhood of any boundary Cauchy surface is similarly complete. Whether or not this algebra continues to be complete non-perturbatively, the assumption that the Hamiltonian remains a boundary term implies that information available at the AdS boundary at any one time $t_1$ remains present at this boundary at any other time $t_2$. [Preview Abstract] |
Saturday, May 2, 2009 3:42PM - 3:54PM |
D11.00002: Causal Dynamical Triangulations Rajesh Kommu An overview of the Causal Dynamical Triangulations (CDT) approach to quantum gravity is presented. CDT is a non-perturbative approach defined as a state sum over causal geometries. Some important results that have been obtained are presented and discussed, including the phase structure of the model, dimensionality of spacetime, etc. [Preview Abstract] |
Saturday, May 2, 2009 3:54PM - 4:06PM |
D11.00003: Gauge invariant observables in de Sitter Ian Morrison The construction of gauge-invariant observables is a longstanding problem in quantum gravity. Cosmologically relevant spacetimes are a particularly interesting context in which to study such observables. Using the technique of group averaging we construct gauge-invariant observables in the case where spacetime is perturbatively global de Sitter. In the appropriate limit these observables reduce to local observables of quantum field theory; additionally, they reproduce the local observable algebra. Bounds on the locality of these observables are briefly discussed. [Preview Abstract] |
Saturday, May 2, 2009 4:06PM - 4:18PM |
D11.00004: Effective Four-Dimensional Actions in Braneworld Scenarios Jolyon Bloomfield, Eanna Flanagan We discuss a general method to efficiently derive four dimensional effective actions from higher dimensional models with branes in a long wavelength limit, and discuss some specific applications to models that yield a coupling between dark energy and dark matter. [Preview Abstract] |
Saturday, May 2, 2009 4:18PM - 4:30PM |
D11.00005: Particle emission from a static black-hole on a tense codimension-2 brane Usama al-Binni, George Siopsis The introduction of finite brane tension to the study of mini black-hole evaporation in brane-world models has been recently shown to modify possible observables that might be seen at the LHC. We present an analytical study of grey-body factors for Hawking radiation emitted by Schwarzschild black-holes localized on a tensional 3-brane in a 6-dimensional bulk. The calculations are done for low frequencies and for large imaginary frequencies for various types of perturbation, and the results are then compared with exact numerical results. [Preview Abstract] |
Saturday, May 2, 2009 4:30PM - 4:42PM |
D11.00006: Quantum Corrections to the Mass of a Black Hole coupled to $N$scalars Martin Schaden Einstein gravity coupled minimally or conformally to $N$scalar fields has well-known static and spherically symmetric classical black hole solutions of Schwarzschild and extremal Reissner-Nordstr\"{o}m type,respectively. These classical solutions depend on a single integration constant corresponding to their Schwarzschild radius $NR$. Assuming that this system can be considered in isolation and/or other mass scales may be neglected, the mass $m$ of such a configuration is of the form $m(N\sim \infty )=\frac{c^2NR}{2G}+\chi \frac{\hbar }{cR}+O\left( {{G\hbar ^2} \mathord{\left/ {\vphantom {{G\hbar ^2} {(c^4NR^3)}}} \right. \kern-\nulldelimiterspace} {(c^4NR^3)}} \right)$, where$\ell _P =\sqrt {G\hbar \mathord{\left/ {\vphantom {\hbar {c^3}}} \right. \kern-\nulldelimiterspace} {c^3}} $is the Planck length and $R$corresponds to the Schwarzschild radius for a single scalar.Only the first two terms of the expansion are relevant in the formal asymptotic limit of an infinite number of only gravitationally interacting scalars forming a black hole whose mass essentially is proportional to the number of degrees of freedom,. The correction to the classical mass that is inversely proportional to$R$ may be interpreted as due to the change in vacuum energy caused by forming a black hole of radius $NR$, i.e. as a Casimir effect. The dimensionless constant $\chi $escribing this correction is estimated semi-classically using periodic orbit theory. The value (and sign) of $\chi $ in this approximation is determined by the unstable classical periodic orbits on the photon sphere of the black hole. [Preview Abstract] |
Saturday, May 2, 2009 4:42PM - 4:54PM |
D11.00007: Anisotropic Evolution of $D$-dimensional FRW Spacetime Chad Middleton We examine the evolution of the $D$-dimensional Einstein field equations subject to a flat, anisotropic Friedmann-Robertson-Walker (FRW) metric. By choosing equations of state relating the 4- and $d$-dimensional pressures to the density, we obtain an expression relating the scale factors to an integration constant. For certain special cases, we obtain exact solutions to the field equations. When the integration constant is set to zero, we obtain the dynamical compactification scenario of Mohammedi et. al. When the volume of the $D$-dimensional spacetime is held constant, we find a late-time accelerated expansion of the 4-dimensional Universe without a cosmological constant. [Preview Abstract] |
Saturday, May 2, 2009 4:54PM - 5:06PM |
D11.00008: Self Coupled Gravity: Another Approach to General Relativity James Crawford In 1960, Arnowitt, Deser, and Misner published a paper arguing (in the context of General Relativity) that if one includes the gravitational self-energy in the total energy of a point charge, the total energy is rendered finite. This paper is also discussed in Ashtekar's book where he gives a more ``simple-minded calculation.'' Motivated by this work, I consider a ``toy'' field theoretic model of gravity where the static gravitational energy density is assumed to be in electrostatic form and where this (negative) energy density contributes as a gravitational source. In the case of a charged point particle, the additional gravitational energy density renders the total energy finite. In addition, the self coupling results in a non-Newtonian form for the gravitational field of a massive body, giving rise to perihelion precession. The amount of precession depends on the precise form of the relativistic force law, but for theoretically reasonable choices it gives the same order of magnitude as the General Relativistic value. Finally, I comment on the form that a fully relativistic version (in particular, including time dependence) of this model should take, and rediscover, among other possibilities, the Einstein-Hilbert action. Work Supported in part by a grant from the Eberly Science Fund -- Penn State Fayette. [Preview Abstract] |
Saturday, May 2, 2009 5:06PM - 5:18PM |
D11.00009: The Geometry of Time in General Relativity Alexander Mayer A conceptual model of special relativity that rests on relative temporal geometry motivated by H. Minkowski, rather than relative temporal rate motivated by H. Lorentz, provides a more robust intellectual foundation for the synthesis of special relativity with accelerated reference frames than Einstein's perspective yielded in 1909-1915. Minkowski's geometry implies local orthogonality of space and time dimensions for a freefalling reference frame; for an idealized region of ``flat'' spacetime devoid of gravitational influence, all local time coordinates associated with the neighborhood of all distinct points in space are parallel. Geometric deformation of spacetime due to the presence of mass implies that no two of these local time coordinates in the ``curved'' spacetime remain parallel. Relativistic effects between points in the gravitational field are most accurately described, both qualitatively and quantitatively, in terms of the relative angular displacement of local time coordinates associated with those points. In addition to reifying general relativity, temporal geometry provides a means of calculating transverse gravitational redshift, a subtle relativistic gravitational effect implied by first principles that was previously overlooked in gravitational theory. Calculations (presented in Session E, Poster Session 1) accurately predict empirical observation of the effect. [Preview Abstract] |
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