2,243 research outputs found
Back-reaction and effective acceleration in generic LTB dust models
We provide a thorough examination of the conditions for the existence of
back-reaction and an "effective" acceleration (in the context of Buchert's
averaging formalism) in regular generic spherically symmetric
Lemaitre-Tolman-Bondi (LTB) dust models. By considering arbitrary spherical
comoving domains, we verify rigorously the fulfillment of these conditions
expressed in terms of suitable scalar variables that are evaluated at the
boundary of every domain. Effective deceleration necessarily occurs in all
domains in: (a) the asymptotic radial range of models converging to a FLRW
background, (b) the asymptotic time range of non-vacuum hyperbolic models, (c)
LTB self-similar solutions and (d) near a simultaneous big bang. Accelerating
domains are proven to exist in the following scenarios: (i) central vacuum
regions, (ii) central (non-vacuum) density voids, (iii) the intermediate radial
range of models converging to a FLRW background, (iv) the asymptotic radial
range of models converging to a Minkowski vacuum and (v) domains near and/or
intersecting a non-simultaneous big bang. All these scenarios occur in
hyperbolic models with negative averaged and local spatial curvature, though
scenarios (iv) and (v) are also possible in low density regions of a class of
elliptic models in which local spatial curvature is negative but its average is
positive. Rough numerical estimates between -0.003 and -0.5 were found for the
effective deceleration parameter. While the existence of accelerating domains
cannot be ruled out in models converging to an Einstein de Sitter background
and in domains undergoing gravitational collapse, the conditions for this are
very restrictive. The results obtained may provide important theoretical clues
on the effects of back-reaction and averaging in more general non-spherical
models.Comment: Final version accepted for publication in Classical and Quantum
Gravity. 47 pages in IOP LaTeX macros, 12 pdf figure
Exact inhomogeneous cosmologies whose source is a radiation-matter mixture with consistent thermodynamics
We derive a new class of exact solutions of Einstein's equations providing a
physically plausible hydrodynamical description of cosmological matter in the
radiative era (), between nucleosynthesis and decoupling.
The solutions are characterized by the Lema\^{\i}tre-Tolman -Bondi metric with
a viscous fluid source, subjected to the following conditions: (a) the
equilibrium state variables satisfy the equation of state of a mixture of an
ultra-relativistic and a non-relativistic ideal gases, where the internal
energy of the latter has been neglected, (b) the particle numbers of the
mixture components are independently conserved, (c) the viscous stress is
consistent with the transport equation and entropy balance law of Extended
Irreversible Thermodynamics, with the coefficient of shear viscosity provided
by Kinetic Theory for the `radiative gas' model. The fulfilment of (a), (b) and
(c) restricts initial conditions in terms of an initial value function,
, related to the average of spatial gradients of the
fluctuations of photon entropy per baryon in the initial hypersurface.
Constraints on the observed anisotropy of the microwave cosmic radiation and
the condition that decoupling occurs at K yield
an estimated value: which can be associated
with a bound on promordial entropy fluctuations. The Jeans mass at decoupling
is of the same order of magnitude as that of baryon dominated perturbation
models ()Comment: LaTeX with revtex (PRD macros). Contains 9 figures (ps). To be
published in Physics Review
Weighed scalar averaging in LTB dust models, part I: statistical fluctuations and gravitational entropy
We introduce a weighed scalar average formalism ("q-average") for the study
of the theoretical properties and the dynamics of spherically symmetric
Lemaitre-Tolman-Bondi (LTB) dust models models. The "q-scalars" that emerge by
applying the q-averages to the density, Hubble expansion and spatial curvature
(which are common to FLRW models) are directly expressible in terms of
curvature and kinematic invariants and identically satisfy FLRW evolution laws
without the back-reaction terms that characterize Buchert's average. The local
and non-local fluctuations and perturbations with respect to the q-average
convey the effects of inhomogeneity through the ratio of curvature and
kinematic invariants and the magnitude of radial gradients. All curvature and
kinematic proper tensors that characterize the models are expressible as
irreducible algebraic expansions on the metric and 4-velocity, whose
coefficients are the q-scalars and their linear and quadratic local
fluctuations. All invariant contractions of these tensors are quadratic
fluctuations, whose q-averages are directly and exactly related to statistical
correlation moments of the density and Hubble expansion scalar. We explore the
application of this formalism to a definition of a gravitational entropy
functional proposed by Hosoya et al (2004 Phys. Rev. Lett. 92 141302). We show
that a positive entropy production follows from a negative correlation between
fluctuations of the density and Hubble scalar, providing a brief outline on its
fulfillment in various LTB models and regions. While the q-average formalism is
specially suited for LTB and Szekeres models, it may provide a valuable
theoretical insight on the properties of scalar averaging in inhomogeneous
spacetimes in general.Comment: 27 pages in IOP format, 1 figure. Matches version accepted for
publication in Classical and Quantum Gravit
Celebrity culture and public connection: bridge or chasm?
Media and cultural research has an important contribution to make to recent debates about declines in democratic engagement: is for example celebrity culture a route into democratic engagement for those otherwise disengaged? This article contributes to this debate by reviewing qualitative and quantitative findings from a UK project on 'public connection'. Using self-produced diaries (with in-depth multiple interviews) as well as a nationwide survey, the authors argue that while celebrity culture is an important point of social connection sustained by media use, it is not linked in citizens' own accounts to issues of public concern. Survey data suggest that those who particularly follow celebrity culture are the least engaged in politics and least likely to use their social networks to involve themselves in action or discussion about public-type issues. This does not mean 'celebrity culture' is 'bad', but it challenges suggestions of how popular culture might contribute to effective democracy
Two-Component Dust in Spherically Symmetric Motion
Two components of spherically symmetric, inhomogeneous dust penetrating each
other are introduced as a generalization of the well-known Tolman-Bondi dust
solution. The field equations of this model are formulated and general
properties are discussed. inhomogeneous Special solutions with additional
symmetries - an extra Killing- or homothetic vector - and their matching to the
corresponding Tolman-Bondi solution are investigated.Comment: 16 pages, LaTeX, 5 figures, accepted for publication in Class.
Quantum Gra
Ruling Out Chaos in Compact Binary Systems
We investigate the orbits of compact binary systems during the final inspiral
period before coalescence by integrating numerically the second-order
post-Newtonian equations of motion. We include spin-orbit and spin-spin
coupling terms, which, according to a recent study by Levin [J. Levin, Phys.
Rev. Lett. 84, 3515 (2000)], may cause the orbits to become chaotic. To examine
this claim, we study the divergence of initially nearby phase-space
trajectories and attempt to measure the Lyapunov exponent gamma. Even for
systems with maximally spinning objects and large spin-orbit misalignment
angles, we find no chaotic behavior. For all the systems we consider, we can
place a strict lower limit on the divergence time t_L=1/gamma that is many
times greater than the typical inspiral time, suggesting that chaos should not
adversely affect the detection of inspiral events by upcoming
gravitational-wave detectors.Comment: 8 pages, 4 figures, submitted to Phys. Rev. Let
Inhomogeneous cosmologies, the Copernican principle and the cosmic microwave background: More on the EGS theorem
We discuss inhomogeneous cosmological models which satisfy the Copernican
principle. We construct some inhomogeneous cosmological models starting from
the ansatz that the all the observers in the models view an isotropic cosmic
microwave background. We discuss multi-fluid models, and illustrate how more
general inhomogeneous models may be derived, both in General Relativity and in
scalar-tensor theories of gravity. Thus we illustrate that the cosmological
principle, the assumption that the Universe we live in is spatially
homogeneous, does not necessarily follow from the Copernican principle and the
high isotropy of the cosmic microwave background.Comment: 17 pages; to appear in GR
On the thermal footsteps of Neutralino relic gases
Current literature suggests that neutralinos are the dominant cold dark
matter particle species. Assuming the microcanonical definition of entropy, we
examine the local entropy per particle produced between the ``freeze out'' era
to the present. An ``entropy consistency'' criterion emerges by comparing this
entropy with the entropy per particle of actual galactic structures given in
terms of dynamical halo variables. We apply this criterion to the cases when
neutralinos are mosly b-inos and mostly higgsinos, in conjunction with the
usual ``abundance'' criterion requiring that present neutralino relic density
complies with 0.1 < \Omega_{\chic{\tilde\chi^0_1}} < 0.3 for .
The joint application of both criteria reveals that a better fitting occurs for
the b-ino channels, hence the latter seem to be favoured over the higgsino
channels. The suggested methodology can be applied to test other annihilation
channels of the neutralino, as well as other particle candidates of thermal
gases relics.Comment: LaTex AIP style, 8 pages including 1 figure. Final version to appear
in Proceedings of the Mexican School of Astrophysics (EMA), Guanajuato,
M\'exico, July 31 - August 7, 200
Families of Canonical Transformations by Hamilton-Jacobi-Poincar\'e equation. Application to Rotational and Orbital Motion
The Hamilton-Jacobi equation in the sense of Poincar\'e, i.e. formulated in
the extended phase space and including regularization, is revisited building
canonical transformations with the purpose of Hamiltonian reduction. We
illustrate our approach dealing with orbital and attitude dynamics. Based on
the use of Whittaker and Andoyer symplectic charts, for which all but one
coordinates are cyclic in the Hamilton-Jacobi equation, we provide whole
families of canonical transformations, among which one recognizes the familiar
ones used in orbital and attitude dynamics. In addition, new canonical
transformations are demonstrated.Comment: 21 page
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