337 research outputs found
Modification of the Coulomb potential from a Kaluza-Klein model with a Gauss-Bonnet term in the action
In four dimensions a Gauss-Bonnet term in the action corre- sponds to a total
derivative, and it does not contribute to the classical equations of motion.
For higher-dimensional geometries this term has the interesting property
(shared with other dimensionally continued Euler densities) that when the
action is varied with respect to the metric, it gives rise to a symmetric,
covariantly conserved tensor of rank two which is a function of the metric and
its first and second order derivatives. Here we review the unification of
General Relativity and electromagnetism in the classical five-dimen- sional,
restricted (with g_55 = 1) Kaluza-Klein model. Then we discuss the
modifications of the Einstein-Maxwell theory that results from adding the
Gauss-Bonnet term in the action. The resulting four-dimensional theory
describes a non-linear U(1) gauge theory non-minimally coupled to gravity. For
a point charge at rest, we find a perturbative solution for large distances
which gives a mass-dependent correction to the Coulomb potential. Near the
source we find a power-law solution which seems to cure the short-distance
divergency of the Coulomb potential. Possible ways to obtain an experimen- tal
upper limit to the coupling of the hypothetical Gauss- Bonnet term are also
considered.Comment: 27 pages, compressed and uuencoded postscript file with unpacking
instructions; major revision to section IV.D.2 on pages 15-16 ("Corrections
to the Coulomb potential at short distances") and to the figure on page 27,
revised unpacking instruction; to be published in The Annals of Physics (NY),
NORDITA 94/5
Weighing the Milky Way
We describe an experiment to measure the mass of the Milky Way galaxy. The
experiment is based on calculated light travel times along orthogonal
directions in the Schwarzschild metric of the Galactic center. We show that the
difference is proportional to the Galactic mass. We apply the result to light
travel times in a 10cm Michelson type interferometer located on Earth. The mass
of the Galactic center is shown to contribute 10^-6 to the flat space component
of the metric. An experiment is proposed to measure the effect.Comment: 10 pages, 1 figur
Bounds on the basic physical parameters for anisotropic compact general relativistic objects
We derive upper and lower limits for the basic physical parameters
(mass-radius ratio, anisotropy, redshift and total energy) for arbitrary
anisotropic general relativistic matter distributions in the presence of a
cosmological constant. The values of these quantities are strongly dependent on
the value of the anisotropy parameter (the difference between the tangential
and radial pressure) at the surface of the star. In the presence of the
cosmological constant, a minimum mass configuration with given anisotropy does
exist. Anisotropic compact stellar type objects can be much more compact than
the isotropic ones, and their radii may be close to their corresponding
Schwarzschild radii. Upper bounds for the anisotropy parameter are also
obtained from the analysis of the curvature invariants. General restrictions
for the redshift and the total energy (including the gravitational
contribution) for anisotropic stars are obtained in terms of the anisotropy
parameter. Values of the surface redshift parameter greater than two could be
the main observational signature for anisotropic stellar type objects.Comment: 18 pages, no figures, accepted for publication in CQ
Minimum mass-radius ratio for charged gravitational objects
We rigorously prove that for compact charged general relativistic objects
there is a lower bound for the mass-radius ratio. This result follows from the
same Buchdahl type inequality for charged objects, which has been extensively
used for the proof of the existence of an upper bound for the mass-radius
ratio. The effect of the vacuum energy (a cosmological constant) on the minimum
mass is also taken into account. Several bounds on the total charge, mass and
the vacuum energy for compact charged objects are obtained from the study of
the Ricci scalar invariants. The total energy (including the gravitational one)
and the stability of the objects with minimum mass-radius ratio is also
considered, leading to a representation of the mass and radius of the charged
objects with minimum mass-radius ratio in terms of the charge and vacuum energy
only.Comment: 19 pages, accepted by GRG, references corrected and adde
No-go theorem for false vacuum black holes
We study the possibility of non-singular black hole solutions in the theory
of general relativity coupled to a non-linear scalar field with a positive
potential possessing two minima: a `false vacuum' with positive energy and a
`true vacuum' with zero energy. Assuming that the scalar field starts at the
false vacuum at the origin and comes to the true vacuum at spatial infinity, we
prove a no-go theorem by extending a no-hair theorem to the black hole
interior: no smooth solutions exist which interpolate between the local de
Sitter solution near the origin and the asymptotic Schwarzschild solution
through a regular event horizon or several horizons.Comment: 16 pages, 1 figure, Latex, some references added, to appear in
Classical and Quantum Gravit
Inflationary Cosmologies in an Anisotropic Brane World
A new cosmological solution of the gravitational field equations in the
generalized Randall-Sundrum model for an anisotropic brane with Bianchi I
geometry and with perfect fluid as matter sources is presented. The matter is
described by a scalar field. The solution admits inflationary era and at a
later epoch the anisotropy of the universe washes out. We obtain two classes of
cosmological scenario, in the first case universe evolves from singularity and
in the second case universe expands without singularity.Comment: 15 pages, no figures, LaTe
Gradient expansion(s) and dark energy
Motivated by recent claims stating that the acceleration of the present
Universe is due to fluctuations with wavelength larger than the Hubble radius,
we present a general analysis of various perturbative solutions of fully
inhomogeneous Einstein equations supplemented by a perfect fluid. The
equivalence of formally different gradient expansions is demonstrated. If the
barotropic index vanishes, the deceleration parameter is always positive
semi-definite.Comment: 17 pages, no figure
k-Essence, superluminal propagation, causality and emergent geometry
The k-essence theories admit in general the superluminal propagation of the
perturbations on classical backgrounds. We show that in spite of the
superluminal propagation the causal paradoxes do not arise in these theories
and in this respect they are not less safe than General Relativity.Comment: 34 pages, 5 figure
Cosmic Acceleration Driven by Mirage Inhomogeneities
A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X
S_5 bulk is introduced. Although there is no special points in the bulk, the
brane Universe has a center and is isotropic around it. The model has an
accelerating expansion and its effective cosmological constant is inversely
proportional to the distance from the center, giving a possible geometrical
origin for the smallness of a present-day cosmological constant. Besides, if
our model is considered as an alternative of early time acceleration, it is
shown that the early stage accelerating phase ends in a dust dominated FRW
homogeneous Universe. Mirage-driven acceleration thus provides a dark matter
component for the brane Universe final state. We finally show that the model
fulfills the current constraints on inhomogeneities.Comment: 14 pages, 1 figure, IOP style. v2, changed style, minor corrections,
references added, version accepted in Class. Quant. Gra
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