Bulletin of the American Physical Society
APS April Meeting 2012
Volume 57, Number 3
Saturday–Tuesday, March 31–April 3 2012; Atlanta, Georgia
Session X8: General Relativity and Cosmology |
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Sponsoring Units: GGR Chair: Jocelyn Read, University of Mississippi Room: Embassy B |
Tuesday, April 3, 2012 1:30PM - 1:42PM |
X8.00001: Newtonian and Relativistic Cosmologies Stephen Green, Robert Wald Cosmological $N$-body simulations are now being performed using Newtonian gravity on scales larger than the Hubble radius. It is known that a uniformly expanding, homogeneous ball of dust in Newtonian gravity satisfies the Friedmann equations, and also that a correspondence between Newtonian and relativistic dust cosmologies holds in linearized perturbation theory. Nevertheless, it is not obvious that Newtonian gravity can provide a good {\em global} description of an inhomogeneous cosmology with significant nonlinear dynamical behavior at small scales. We investigate this issue in light of a perturbative framework that we have recently developed. We propose a straightforward dictionary---exact at the linearized level---that maps Newtonian dust cosmologies into GR dust cosmologies, and we use our ordering scheme to determine the degree to which the resulting metric and matter distribution solve Einstein's equation. We then find additional corrections needed to satisfy Einstein's equation to ``order 1'' at small scales and to ``order $\epsilon$'' at large scales. We expect that, in realistic Newtonian cosmologies, these additional corrections will be very small; if so, this should provide strong justification for the use of Newtonian simulations to describe GR cosmologies. [Preview Abstract] |
Tuesday, April 3, 2012 1:42PM - 1:54PM |
X8.00002: Measure and Probability in Cosmology Joshua Schiffrin, Robert Wald General relativity has a Hamiltonian formulation, which formally provides a canonical (Liouville) measure on the space of solutions. A number of authors have used the restriction of this measure to the space of homogeneous and isotropic universes with scalar field matter (minisuperspace)---namely, the Gibbons-Hawking-Stewart measure---to make arguments about the likelihood of inflation. We argue here that there are at least four major difficulties with using the measure of general relativity to make probability arguments in cosmology: (1) Equilibration does not occur on cosmological length scales. (2) Even in the minisuperspace case, the measure of phase space is infinite and the computation of probabilities depends very strongly on how the infinity is regulated. (3) The inhomogeneous degrees of freedom must be taken into account even if one is interested only in universes that are very nearly homogeneous. The measure depends upon how the infinite number of degrees of freedom are truncated, and how one defines ``nearly homogeneous''. (4) In a universe where the second law of thermodynamics holds, one cannot make use of our knowledge of the present state of the universe to ``retrodict'' the likelihood of past conditions. [Preview Abstract] |
Tuesday, April 3, 2012 1:54PM - 2:06PM |
X8.00003: Dark matter distribution in the Schwarzschild geometry Laleh Sadeghian, Ferancesc Ferrer, Clifford M. Will The cold dark matter at the galactic centre is redistributed by the presence of the central massive black hole. We consider a range of initial distribution functions for the dark matter and use an adiabatic assumption to ``grow'' the black hole. Unlike the non-relativistic approach of Gondolo and Silk [1], we use a fully general relativistic treatment of the black hole's Schwarzschild geometry. We find significant differences in the final dark matter distribution close to the black hole. We discuss how this could be generalized to the rotating black hole case. \\[4pt] [1] P. Gondolo, J. Silk, Phys. Rev. Lett. 83, 1719-1722 (1999) [Preview Abstract] |
Tuesday, April 3, 2012 2:06PM - 2:18PM |
X8.00004: Measuring luminosity distance and redshift using only gravitational wave observations of binary neutron star coalescences Jocelyn Read, Chris Messenger Detection of gravitational waves from the inspiral phase of binary neutron star coalescence will allow us to measure the effects of the tidal coupling in such systems. Tidal effects provide contributions to the gravitational wave signal that break a degeneracy between the system's mass parameters and redshift and thereby allow the simultaneous measurement of both the effective distance and the redshift for individual sources. These effects will be measurable using 3rd generation gravitational wave detectors, e.g. the Einstein Telescope, which will be capable of detecting inspiralling binary neutron star systems out to redshift z=4. The Einstein Telescope is predicted to measure a population of O($10^3$-$10^7$) binary neutron star systems, allowing the luminosity distance--redshift relation to be probed independently of the cosmological distance ladder and independently of electromagnetic observations. We will discuss current work on cosmological constraints from these effects. [Preview Abstract] |
Tuesday, April 3, 2012 2:18PM - 2:30PM |
X8.00005: New Interpretation for the Observed Cosmological Redshifts and its Implications Branislav Vlahovic The cosmological redshifts z frequently are interpret as a special-relativistic Doppler shift alone. We will show that this interpretation is not correct and that the contribution of gravitational redshift is always present and significant. It will be shown that gravitational redshift is actually about the same magnitude as cosmological redshift, but that only for the cosmological models without dark energy component cosmological and gravitational redshift can be considered to be the same. Significant contribution of the gravitational redshift due to the gravitational field of the Universe, which is ignored in interpretation of observational data, could have considerable impact on cosmological theories. We will first calculate contributions of gravitational redshift to CMB and time dilation of Type Ia supernovae, and later use it to explain the excess redshifts of quasars and active galaxies, and redshifts of companion galaxies of stars. We will show its possible implications on the interpretation of mass density of matter and mass as function of cosmological time. It will be demonstrated that taking into account gravitational redshift allows to interpret luminosity distance and surface brightness of distant galaxies to be consistent with the static universe cosmological models. Finally, it will be considered as a possible explanation for recently found blueshift in angular distribution of quasars spectra and earlier reported anomalies in the low multipoles of the CMB. [Preview Abstract] |
Tuesday, April 3, 2012 2:30PM - 2:42PM |
X8.00006: Dwarf galaxy rotation curves with conformal gravity James O'Brien We continue our study of the application of the conformal gravity theory to galactic rotation curves. Previously we had studied a varied 111 spiral galaxy sample consisting of high surface brightness galaxies, low surface brightness galaxies and dwarf galaxies. With no free parameters other than galactic mass to light ratios, we had found that the theory is able to account for the systematics that is observed in the entire set of galactic rotation curves without the need for any dark matter whatsoever. In the present paper we extend our study to incorporate a further 27 galaxies of which 25 are dwarf galaxies and provide updated studies of 3 additional galaxies that had been in the original sample, and again without dark matter find fully acceptable fits, save only for just a few galaxies that we find to be somewhat troublesome. Since one of the primary ingredients in the theory is a universal contribution to galactic motions coming from matter exterior to the galaxies, and thus independent of them, our study reinforces one of the central concepts of the conformal gravity studies, namely that invoking dark matter should be viewed as being nothing more than an attempt to describe global physics contributions in purely local galactic terms. [Preview Abstract] |
Tuesday, April 3, 2012 2:42PM - 2:54PM |
X8.00007: Quantum fluctuations in the stress energy of a conformally invariant scalar field in de Sitter space Jason Bates, Paul Anderson, H.T. Cho, B.L. Hu In this talk, I will present the results from a calculation of the symmetric two point correlation function for the stress-energy tensor, also known as the noise kernel, for a conformally invariant scalar field in de Sitter space. This calculation differs from previous ones in that the noise kernel is computed for an arbitrary separation of the points including null separation. Additionally, the expression is evaluated in both the spatially flat cosmological coordinates and the static coordinates for de Sitter space. In the static coordinates the behavior of the noise kernel at the cosmological horizon is investigated. [Preview Abstract] |
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