89 research outputs found
The Einstein static universe with torsion and the sign problem of the cosmological constant
In the field equations of Einstein-Cartan theory with cosmological constant a
static spherically symmetric perfect fluid with spin density satisfying the
Weyssenhoff restriction is considered. This serves as a rough model of space
filled with (fermionic) dark matter. From this the Einstein static universe
with constant torsion is constructed, generalising the Einstein Cosmos to
Einstein-Cartan theory.
The interplay between torsion and the cosmological constant is discussed. A
possible way out of the cosmological constant's sign problem is suggested.Comment: 8 pages, LaTeX; minor layout changes, typos corrected, one new
equation, new reference [5], completed reference [13], two references adde
Poincar\'e gauge theory with even and odd parity dynamic connection modes: isotropic Bianchi cosmological models
The Poincar\'e gauge theory of gravity has a metric compatible connection
with independent dynamics that is reflected in the torsion and curvature. The
theory allows two good propagating spin-0 modes. Dynamical investigations using
a simple expanding cosmological model found that the oscillation of the 0
mode could account for an accelerating expansion similar to that presently
observed. The model has been extended to include a mode and more
recently cross parity couplings. We investigate the dynamics of this model in a
situation which is simple, non-trivial, and yet may give physically interesting
results that might be observable. We consider homogeneous cosmologies, more
specifically, isotropic Bianchi class A models. We find an effective Lagrangian
for our dynamical system, a system of first order equations, and present some
typical dynamical evolution.Comment: 8 pages, 1 figures, submitted to IARD 2010 Conference Proceedings in
{\em Journal of Physics: Conference Series}, eds. L. Horwitz and M. Land
(2011
Global embedding of D-dimensional black holes with a cosmological constant in Minkowskian spacetimes: Matching between Hawking temperature and Unruh temperature
We study the matching between the Hawking temperature of a large class of
static D-dimensional black holes and the Unruh temperature of the corresponding
higher dimensional Rindler spacetime. In order to accomplish this task we find
the global embedding of the D-dimensional black holes into a higher dimensional
Minkowskian spacetime, called the global embedding Minkowskian spacetime
procedure (GEMS procedure). These global embedding transformations are
important on their own, since they provide a powerful tool that simplifies the
study of black hole physics by working instead, but equivalently, in an
accelerated Rindler frame in a flat background geometry. We discuss neutral and
charged Tangherlini black holes with and without cosmological constant, and in
the negative cosmological constant case, we consider the three allowed
topologies for the horizons (spherical, cylindrical/toroidal and hyperbolic).Comment: 7 pages; ReVTeX
Ideally embedded space-times
Due to the growing interest in embeddings of space-time in higher-dimensional
spaces we consider a specific type of embedding. After proving an inequality
between intrinsically defined curvature invariants and the squared mean
curvature, we extend the notion of ideal embeddings from Riemannian geometry to
the indefinite case. Ideal embeddings are such that the embedded manifold
receives the least amount of tension from the surrounding space. Then it is
shown that the de Sitter spaces, a Robertson-Walker space-time and some
anisotropic perfect fluid metrics can be ideally embedded in a five-dimensional
pseudo-Euclidean space.Comment: layout changed and typos corrected; uses revtex
Einstein-aether theory, violation of Lorentz invariance, and metric-affine gravity
We show that the Einstein-aether theory of Jacobson and Mattingly (J&M) can
be understood in the framework of the metric-affine (gauge theory of) gravity
(MAG). We achieve this by relating the aether vector field of J&M to certain
post-Riemannian nonmetricity pieces contained in an independent linear
connection of spacetime. Then, for the aether, a corresponding geometrical
curvature-square Lagrangian with a massive piece can be formulated
straightforwardly. We find an exact spherically symmetric solution of our
model.Comment: Revtex4, 38 pages, 1 figur
Torsion nonminimally coupled to the electromagnetic field and birefringence
In conventional Maxwell--Lorentz electrodynamics, the propagation of light is
influenced by the metric, not, however, by the possible presence of a torsion
T. Still the light can feel torsion if the latter is coupled nonminimally to
the electromagnetic field F by means of a supplementary Lagrangian of the type
l^2 T^2 F^2 (l = coupling constant). Recently Preuss suggested a specific
nonminimal term of this nature. We evaluate the spacetime relation of Preuss in
the background of a general O(3)-symmetric torsion field and prove by
specifying the optical metric of spacetime that this can yield birefringence in
vacuum. Moreover, we show that the nonminimally coupled homogeneous and
isotropic torsion field in a Friedmann cosmos affects the speed of light.Comment: Revtex, 12 pages, no figure
Gravitation, electromagnetism and the cosmological constant in purely affine gravity
The Eddington Lagrangian in the purely affine formulation of general
relativity generates the Einstein equations with the cosmological constant. The
Ferraris-Kijowski purely affine Lagrangian for the electromagnetic field, which
has the form of the Maxwell Lagrangian with the metric tensor replaced by the
symmetrized Ricci tensor, is dynamically equivalent to the Einstein-Maxwell
Lagrangian in the metric formulation. We show that the sum of the two affine
Lagrangians is dynamically inequivalent to the sum of the analogous Lagrangians
in the metric-affine/metric formulation. We also show that such a construction
is valid only for weak electromagnetic fields. Therefore the purely affine
formulation that combines gravitation, electromagnetism and the cosmological
constant cannot be a simple sum of terms corresponding to separate fields.
Consequently, this formulation of electromagnetism seems to be unphysical,
unlike the purely metric and metric-affine pictures, unless the electromagnetic
field couples to the cosmological constant.Comment: 14 pages, extended and combined with gr-qc/0701176; published versio
Effective Field Equations of Brane-Induced Electromagnetism
Using a covariant embedding formalism, we find the effective field equations
for the Electromagnetism that emerge on branes in the context of
Dvali-Gabadadze-Porrati (DGP) braneworld scenario. Our treatment is essentially
geometrical. We start with Maxwell equations in five-dimensions and project
them into an arbitrary brane. The formalism is quite general and allows us to
consider curved bulk spaces and curved branes whose tension is not necessarily
null. The kinetic electromagnetic term induced on the world volume of the
brane, proper of DGP models, is incorporated in this formulation by means of an
appropriate match condition. We also give an estimate of each term of the
effective field equations and determine the domain in which the
four-dimensional Maxwell equations can be recovered in the brane
On the response of a particle detector in Anti-de Sitter spacetime
We consider the vacuum response of a particle detector in Anti-de Sitter
spacetime, and in particular analyze how spacetime features such as curvature
and dimensionality affect the response spectrum of an accelerated detector. We
calculate useful limits on Wightman functions, analyze the dynamics of the
detector in terms of vacuum fluctuations and radiation reactions, and discuss
the thermalization process for the detector. We also present a generalization
of the GEMS approach and obtain the Gibbons-Hawking temperature of de Sitter
spacetime as an embedded Unruh temperature in a curved Anti-de Sitter
spacetime.Comment: 13 pages, no figures, accepted for publication in Class. Quantum Gra
Isometric Embedding of BPS Branes in Flat Spaces with Two Times
We show how non-near horizon p-brane theories can be obtained from two
embedding constraints in a flat higher dimensional space with 2 time
directions. In particular this includes the construction of D3 branes from a
flat 12-dimensional action, and M2 and M5 branes from 13 dimensions. The
worldvolume actions are determined by constant forms in the higher dimension,
reduced to the usual expressions by Lagrange multipliers. The formulation
affords insight in the global aspects of the spacetime geometries and makes
contact with recent work on two-time physics.Comment: 29 pages, 10 figures, Latex using epsf.sty and here.sty; v2:
reference added and some small correction
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