3,940 research outputs found
Tracing a relativistic Milky Way within the RAMOD measurement protocol
Advancement in astronomical observations and technical instrumentation
implies taking into account the general relativistic effects due the
gravitational fields encountered by the light while propagating from the star
to the observer. Therefore, data exploitation for Gaia-like space astrometric
mission (ESA, launch 2013) requires a fully relativistic interpretation of the
inverse ray-tracing problem, namely the development of a highly accurate
astrometric models in accordance with the geometrical environment affecting
light propagation itself and the precepts of the theory of measurement. This
could open a new rendition of the stellar distances and proper motions, or even
an alternative detection perspective of many subtle relativistic effects
suffered by light while it is propagating and subsequently recorded in the
physical measurements.Comment: Proceeding for "Relativity and Gravitation, 100 Years after Einstein
in Prague" to be published by Edition Open Access, revised versio
Quasiblack holes with pressure: General exact results
A quasiblack hole is an object in which its boundary is situated at a surface
called the quasihorizon, defined by its own gravitational radius. We elucidate
under which conditions a quasiblack hole can form under the presence of matter
with nonzero pressure. It is supposed that in the outer region an extremal
quasihorizon forms, whereas inside, the quasihorizon can be either nonextremal
or extremal. It is shown that in both cases, nonextremal or extremal inside, a
well-defined quasiblack hole always admits a continuous pressure at its own
quasihorizon. Both the nonextremal and extremal cases inside can be divided
into two situations, one in which there is no electromagnetic field, and the
other in which there is an electromagnetic field. The situation with no
electromagnetic field requires a negative matter pressure (tension) on the
boundary. On the other hand, the situation with an electromagnetic field
demands zero matter pressure on the boundary. So in this situation an
electrified quasiblack hole can be obtained by the gradual compactification of
a relativistic star with the usual zero pressure boundary condition. For the
nonextremal case inside the density necessarily acquires a jump on the
boundary, a fact with no harmful consequences whatsoever, whereas for the
extremal case the density is continuous at the boundary. For the extremal case
inside we also state and prove the proposition that such a quasiblack hole
cannot be made from phantom matter at the quasihorizon. The regularity
condition for the extremal case, but not for the nonextremal one, can be
obtained from the known regularity condition for usual black holes.Comment: 18 pages, no figures; improved introduction, added references,
calculations better explaine
The Formation of non-Keplerian Rings of Matter about Compact Stars
The formation of energetic rings of matter in a Kerr spacetime with an
outward pointing acceleration field does not appear to have previously been
noted as a relativistic effect. In this paper we show that such rings are a
gravimagneto effect with no Newtonian analog, and that they do not occur in the
static limit. The energy efficiency of these rings can, depending of the
strength of the acceleration field, be much greater than that of Keplerian
disks. The rings rotate in a direction opposite to that of compact star about
which they form. The size and energy efficiency of the rings depend on the
fundamental parameters of the spacetime as well as the strength the
acceleration field.Comment: 19 pages, 7 figures, 1 diagram. Figures are included in the text
using the "graphicx" package. If you do not have this package you can use
epsfig, or another package as long as you alter the tex file appropriately.
Alternatively you could print the figures out seperatel
Reissner-Nordstrom and charged gas spheres
The main point of this paper is a suggestion about the proper treatment of
the photon gas in a theory of stellar structure and other plasmas. This problem
arises in the study of polytropic gas spheres, where we have already introduced
some innovations. The main idea, already advanced in the contextof neutral,
homogeneous, polytropic stellar models, is to base the theory firmly on a
variational principle. Another essential novelty is to let mass distribution
extend to infinity, the boundary between bulk and atmosphere being defined by
an abrupt change in the polytropic index, triggered by the density. The logical
next step in this program is to include the effect of radiation, which is a
very significant complication since a full treatment would have to include an
account of ionization, thus fieldsrepresenting electrons, ions, photons,
gravitons and neutral atoms as well. In way of preparation, we consider models
that are charged but homogeneous, involving only gravity, electromagnetism and
a single scalar field that represents both the mass and the electric charge; in
short, anon-neutral plasma. While this work only represents a stage in the
development of a theory of stars, without direct application to physical
systems, it does shed some light on the meaning of the Reissner-Nordstrom
solution of the modified Einstein-Maxwell equations., with an application to a
simple system.Comment: 19 pages, plain te
A note on the factorization conjecture
We give partial results on the factorization conjecture on codes proposed by
Schutzenberger. We consider finite maximal codes C over the alphabet A = {a, b}
with C \cap a^* = a^p, for a prime number p. Let P, S in Z , with S = S_0 +
S_1, supp(S_0) \subset a^* and supp(S_1) \subset a^*b supp(S_0). We prove that
if (P,S) is a factorization for C then (P,S) is positive, that is P,S have
coefficients 0,1, and we characterize the structure of these codes. As a
consequence, we prove that if C is a finite maximal code such that each word in
C has at most 4 occurrences of b's and a^p is in C, then each factorization for
C is a positive factorization. We also discuss the structure of these codes.
The obtained results show once again relations between (positive)
factorizations and factorizations of cyclic groups
Cosmological perturbations in -essence model
Subhorizon approximation is often used in cosmological perturbation theory.
In this paper, however, it is shown that the subhorizon approximation is not
always a good approximation at least in case of -essence model. We also show
that the sound speed given by -essence model exerts a huge influence on the
time evolution of the matter density perturbation, and the future observations
could clarify the differences between the CDM model and -essence
model.Comment: 21 pages, sentences and equations are corrected, conclusions are
changed a littl
Cosmological constraints on extended Galileon models
The extended Galileon models possess tracker solutions with de Sitter
attractors along which the dark energy equation of state is constant during the
matter-dominated epoch, i.e. w_DE = -1-s, where s is a positive constant. Even
with this phantom equation of state there are viable parameter spaces in which
the ghosts and Laplacian instabilities are absent. Using the observational data
of the supernovae type Ia, the cosmic microwave background (CMB), and baryon
acoustic oscillations, we place constraints on the tracker solutions at the
background level and find that the parameter s is constrained to be s=0.034
(-0.034,+0.327) (95% CL) in the flat Universe. In order to break the degeneracy
between the models we also study the evolution of cosmological density
perturbations relevant to the large-scale structure (LSS) and the
Integrated-Sachs-Wolfe (ISW) effect in CMB. We show that, depending on the
model parameters, the LSS and the ISW effect is either positively or negatively
correlated. It is then possible to constrain viable parameter spaces further
from the observational data of the ISW-LSS cross-correlation as well as from
the matter power spectrum.Comment: 17 pages, 9 figures, uses RevTeX4-
Black holes in scalar-tensor gravity
Hawking has proven that black holes which are stationary as the endpoint of
gravitational collapse in Brans--Dicke theory (without a potential) are no
different than in general relativity. We extend this proof to the much more
general class of scalar-tensor and f(R) gravity theories, without assuming any
symmetries apart from stationarity.Comment: v1: 4 pages; v2: typos corrected, published versio
Collimation of a spherical collisionless particles stream in Kerr space-time
We examine the propagation of collisionless particles emitted from a
spherical shell to infinity. The number distribution at infinity, calculated as
a function of the polar angle, exhibits a small deviation from uniformity. The
number of particles moving from the polar region toward the equatorial plane is
slightly larger than that of particles in the opposite direction, for an
emission radius in extreme Kerr space-time. This means that the black
hole spin exerts an anti-collimation effect on the particles stream propagating
along the rotation axis. We also confirm this property in the weak field limit.
The quadrupole moment of the central object produces a force toward the
equatorial plane. For a smaller emission radius , the absorption of
particles into the black hole, the non-uniformity and/or the anisotropy of the
emission distribution become much more important.Comment: 11 pages, 8 figures; accepted for publication in CQ
Generalized Brans-Dicke theories
In Brans-Dicke theory a non-linear self interaction of a scalar field allows
a possibility of realizing the late-time cosmic acceleration, while recovering
the General Relativistic behavior at early cosmological epochs. We extend this
to more general modified gravitational theories in which a de Sitter solution
for dark energy exists without using a field potential. We derive a condition
for the stability of the de Sitter point and study the background cosmological
dynamics of such theories. We also restrict the allowed region of model
parameters from the demand for the avoidance of ghosts and instabilities. A
peculiar evolution of the field propagation speed allows us to distinguish
those theories from the LCDM model.Comment: 14 pages, 4 figures, version to appear in JCA
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