848 research outputs found
Robertson-Walker fluid sources endowed with rotation characterised by quadratic terms in angular velocity parameter
Einstein's equations for a Robertson-Walker fluid source endowed with
rotation Einstein's equations for a Robertson-Walker fluid source endowed with
rotation are presented upto and including quadratic terms in angular velocity
parameter. A family of analytic solutions are obtained for the case in which
the source angular velocity is purely time-dependent. A subclass of solutions
is presented which merge smoothly to homogeneous rotating and non-rotating
central sources. The particular solution for dust endowed with rotation is
presented. In all cases explicit expressions, depending sinusoidally on polar
angle, are given for the density and internal supporting pressure of the
rotating source. In addition to the non-zero axial velocity of the fluid
particles it is shown that there is also a radial component of velocity which
vanishes only at the poles. The velocity four-vector has a zero component
between poles
Born-Infeld black hole in the isolated horizon framework
In this work we probe the Born-Infeld (BI) black hole in the isolated horizon
framework. It turns out that the BI black hole is consistent with the heuristic
model for colored black holes proposed by Ashtekar et al [(2001)
Class.Quant.Grav. v. 18, 919-940]. The model points to the unstability of the
BI black hole.Comment: 4 pages, 5 figure
Hubble flow variance and the cosmic rest frame
We characterize the radial and angular variance of the Hubble flow in the
COMPOSITE sample of 4534 galaxies, on scales in which much of the flow is in
the nonlinear regime. With no cosmological assumptions other than the existence
of a suitably averaged linear Hubble law, we find with decisive Bayesian
evidence (ln B >> 5) that the Hubble constant averaged in independent spherical
radial shells is closer to its asymptotic value when referred to the rest frame
of the Local Group, rather than the standard rest frame of the Cosmic Microwave
Background. An exception occurs for radial shells in the range 40/h-60/h Mpc.
Angular averages reveal a dipole structure in the Hubble flow, whose amplitude
changes markedly over the range 32/h-62/h Mpc. Whereas the LG frame dipole is
initially constant and then decreases significantly, the CMB frame dipole
initially decreases but then increases. The map of angular Hubble flow
variation in the LG rest frame is found to coincide with that of the residual
CMB temperature dipole, with correlation coefficient -0.92. These results are
difficult to reconcile with the standard kinematic interpretation of the motion
of the Local Group in response to the clustering dipole, but are consistent
with a foreground non-kinematic anisotropy in the distance-redshift relation of
0.5% on scales up to 65/h Mpc. Effectively, the differential expansion of space
produced by nearby nonlinear structures of local voids and denser walls and
filaments cannot be reduced to a local boost. This hypothesis suggests a
reinterpretation of bulk flows, which may potentially impact on calibration of
supernovae distances, anomalies associated with large angles in the CMB
anisotropy spectrum, and the dark flow inferred from the kinematic
Sunyaev-Zel'dovich effect. It is consistent with recent studies that find
evidence for a non-kinematic dipole in the distribution of distant radio
sources.Comment: 37 pages, 9 tables, 13 figures; v2 adds extensive new analysis
(including additional subsections, tables, figures); v3 adds a Monte Carlo
analysis (with additional table, figure) which further tightens the
statistical robustness of the dipole results; v4 adds further clarifications,
small corrections, references and discussion of Planck satellite results; v5
typos fixed, matches published versio
Kolmogorov cosmic microwave background sky
A new map of the sky representing the degree of randomness in the cosmic
microwave background (CMB) temperature has been obtained. The map based on
estimation of the Kolmogorov stochasticity parameter clearly distinguishes the
contribution of the Galactic disk from the CMB and reveals regions of various
degrees of randomness that can reflect the properties of inhomogeneities in the
Universe. For example, among the high randomness regions is the southern
non-Gaussian anomaly, the Cold Spot, with a stratification expected for the
voids. Existence of its counterpart, a Northern Cold Spot with almost identical
randomness properties among other low-temperature regions is revealed. By its
informative power, Kolmogorov's map can be complementary to the CMB temperature
and polarization sky maps.Comment: A & A (in press), to match the published version, 4 pages, 5 figs, 2
Table
An inhomogeneous universe with thick shells and without cosmological constant
We build an exact inhomogeneous universe composed of a central flat Friedmann
zone up to a small redshift , a thick shell made of anisotropic matter, an
hyperbolic Friedmann metric up to the scale where dimming galaxies are observed
() that can be matched to a hyperbolic Lema\^{i}tre-Tolman-Bondi
spacetime to best fit the WMAP data at early epochs. We construct a general
framework which permits us to consider a non-uniform clock rate for the
universe. As a result, both for a uniform time and a uniform Hubble flow, the
deceleration parameter extrapolated by the central observer is always positive.
Nevertheless, by taking a non-uniform Hubble flow, it is possible to obtain a
negative central deceleration parameter, that, with certain parameter choices,
can be made the one observed currently. Finally, it is conjectured a possible
physical mechanism to justify a non-uniform time flow.Comment: Version published in Class. Quantum gra
Isotropy, shear, symmetry and exact solutions for relativistic fluid spheres
The symmetry method is used to derive solutions of Einstein's equations for
fluid spheres using an isotropic metric and a velocity four vector that is
non-comoving. Initially the Lie, classical approach is used to review and
provide a connecting framework for many comoving and so shear free solutions.
This provides the basis for the derivation of the classical point symmetries
for the more general and mathematicaly less tractable description of Einstein's
equations in the non-comoving frame. Although the range of symmetries is
restrictive, existing and new symmetry solutions with non-zero shear are
derived. The range is then extended using the non-classical direct symmetry
approach of Clarkson and Kruskal and so additional new solutions with non-zero
shear are also presented. The kinematics and pressure, energy density, mass
function of these solutions are determined.Comment: To appear in Classical and Quantum Gravit
Smarr's formula for black holes with non-linear electrodynamics
It is known that for nonlinear electrodynamics the First Law of Black Hole
Mechanics holds, however the Smarr's formula for the total mass does not. In
this contribution we discuss the point and determine the corresponding
expressions for the Bardeen black hole solution that represents a nonlinear
magnetic monopole. The same is done for the regular black hole solution derived
by Ayon-Beato and Garcia, showing that in the case that variations of the
electric charge are involved, the Smarr's formula does not longer is valid.Comment: 10 pages, 3 figures.Contribution to the Festscrift of Prof. A. Garci
Cosmology, cohomology, and compactification
Ashtekar and Samuel have shown that Bianchi cosmological models with compact
spatial sections must be of Bianchi class A. Motivated by general results on
the symmetry reduction of variational principles, we show how to extend the
Ashtekar-Samuel results to the setting of weakly locally homogeneous spaces as
defined, e.g., by Singer and Thurston. In particular, it is shown that any
m-dimensional homogeneous space G/K admitting a G-invariant volume form will
allow a compact discrete quotient only if the Lie algebra cohomology of G
relative to K is non-vanishing at degree m.Comment: 6 pages, LaTe
Rotating Solution of Einstein-Maxwell Dilaton Gravity with Unusual Asymptotics
We study electrically charged, dilaton black holes, which possess
infinitesimal angular momentum in the presence of one or two Liouville type
potentials. These solutions are neither asymptotically flat nor (anti)-de
Sitter. Some properties of the solutions are discussed.Comment: 11 pages, Accepted (Int. J. Theor. Phys.
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