1,357 research outputs found
Vainshtein mechanism in Gauss-Bonnet gravity and Galileon aether
We derive field equations of Gauss-Bonnet gravity in 4 dimensions after
dimensional reduction of the action and demonstrate that in this scenario
Vainshtein mechanism operates in the flat spherically symmetric background. We
show that inside this Vainshtein sphere the fifth force is negligibly small
compared to the gravitational force. We also investigate stability of the
spherically symmetric solution, clarify the vocabulary used in the literature
about the hyperbolicity of the equation and the ghost-Laplacian stability
conditions. We find superluminal behavior of the perturbation of the field in
the radial direction. However, because of the presence of the non linear terms,
the structure of the space-time is modified and as a result the field does not
propagate in the Minkowski metric but rather in an "aether" composed by the
scalar field . We thereby demonstrate that the superluminal behavior
does not create time paradoxes thank to the absence of Causal Closed Curves. We
also derive the stability conditions for Friedmann Universe in context with
scalar and tensor perturbations.Comment: 9 pages, 5 figures, references added, more details on the
cosmological analysis included, results and conclusions unchanged, final
version to appear in PR
Notes on the post-Newtonian limit of massive Brans-Dicke theory
We consider the Post-Newtonian limit of massive Brans-Dicke theory and we
make some notes about the Post-Newtonian limit of the case . This
case is dynamically equivalent to the metric theory. It is known that
this theory can be compatible with the solar system tests if Chameleon
mechanism occurs. Also, it is known that this mechanism is because of the
non-linearity in the field equations produced by the largeness of the local
curvature relative to the background curvature. Thus, the linearization of the
field equations breaks down. On the other hand we know that Chameleon mechanism
exists when a coupling between the matter and the scalar field exists. In the
Jordan frame of Brans-Dicke theory, we have not such a coupling. But in the
Einstein frame this theory behaves like a Chameleon scalar field. By confining
ourselves to the case , we show that "Chameleon-like" behavior can
exist also in the Jordan frame but it has an important difference compared with
the Chameleon mechanism. Also we show that the conditions which lead to the
existence of "Chameleon-like" mechanism are consistent with the conditions in
the Post-Newtonian limit which correspond to a heavy scalar filed at the
cosmological scale and a small effective cosmological constant. Thus, one can
linearize field equations to the Post-Newtonian order and this linearization
has not any contradiction with the existence of "Chameleon-like" behavior.Comment: to appear in CQ
Region of magnetic dominance near a rotating black hole
This is a brief contribution in which a simplified criterion of the relevance
of the test-particle approximation describing motion of material near a
magnetized black hole is discussed. Application to processes of the dissipative
collimation of astronomical jets (as proposed by de Felice and Curir, 1992) is
mentioned.Comment: 11 pages, to appear in General Relativity and Gravitation, also
available (with additional illustrations) at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
An extended scheme for fitting X-ray data with accretion disk spectra in the strong gravity regime
Accreting black holes are believed to emit X-rays which then mediate
information about strong gravity in the vicinity of the emission region. We
report on a set of new routines for the Xspec package for analysing X-ray
spectra of black-hole accretion disks. The new computational tool significantly
extends the capabilities of the currently available fitting procedures that
include the effects of strong gravity, and allows one to systematically explore
the constraints on more model parameters than previously possible (for example
black-hole angular momentum). Moreover, axial symmetry of the disk intrinsic
emissivity is not assumed, although it can be imposed to speed up the
computations. The new routines can be used also as a stand-alone and flexible
code with the capability of handling time-resolved spectra in the regime of
strong gravity. We have used the new code to analyse the mean X-ray spectrum
from the long XMM--Newton 2001 campaign of the Seyfert 1 galaxy MCG--6-30-15.
Consistent with previous findings, we obtained a good fit to the broad Fe K
line profile for a radial line intrinsic emissivity law in the disk which is
not a simple power law, and for near maximal value of black hole angular
momentum. However, equally good fits can be obtained also for small values of
the black hole angular momentum. The code has been developed with the aim of
allowing precise modelling of relativistic effects. Although we find that
current data cannot constrain the parameters of black-hole/accretion disk
system well, the approach allows, for a given source or situation, detailed
investigations of what features of the data future studies should be focused on
in order to achieve the goal of uniquely isolating the parameters of such
systems.Comment: Accepted for publication in ApJ S
Gyroscopic Precession and Inertial Forces in Axially Symmetric Stationary Spacetimes
We study the phenomenon of gyroscopic precession and the analogues of
inertial forces within the framework of general relativity. Covariant
connections between the two are established for circular orbits in stationary
spacetimes with axial symmetry. Specializing to static spacetimes, we prove
that gyroscopic precession and centrifugal force both reverse at the photon
orbits. Simultaneous non-reversal of these in the case of stationary spacetimes
is discussed. Further insight is gained in the case of static spacetime by
considering the phenomena in a spacetime conformal to the original one.
Gravi-electric and gravi-magnetic fields are studied and their relation to
inertial forces is established.Comment: 21 pages, latex, no figures, http://202.41.67.76/~nayak/gpifass.te
Cosmological zoo -- accelerating models with dark energy
ecent observations of type Ia supernovae indicate that the Universe is in an
accelerating phase of expansion. The fundamental quest in theoretical cosmology
is to identify the origin of this phenomenon. In principle there are two
possibilities: 1) the presence of matter which violates the strong energy
condition (a substantial form of dark energy), 2) modified Friedmann equations
(Cardassian models -- a non-substantial form of dark matter). We classify all
these models in terms of 2-dimensional dynamical systems of the Newtonian type.
We search for generic properties of the models. It is achieved with the help of
Peixoto's theorem for dynamical system on the Poincar{\'e} sphere. We find that
the notion of structural stability can be useful to distinguish the generic
cases of evolutional paths with acceleration. We find that, while the
CDM models and phantom models are typical accelerating models, the
cosmological models with bouncing phase are non-generic in the space of all
planar dynamical systems. We derive the universal shape of potential function
which gives rise to presently accelerating models. Our results show explicitly
the advantages of using a potential function (instead of the equation of state)
to probe the origin of the present acceleration. We argue that simplicity and
genericity are the best guide in understanding our Universe and its
acceleration.Comment: RevTeX4, 23 pages, 10 figure
Accelerating Universe and Cosmological Perturbation in the Ghost Condensate
In the simplest Higgs phase of gravity called ghost condensation, an
accelerating universe with a phantom era (w<-1) can be realized without ghost
or any other instabilities. In this paper we show how to reconstruct the
potential in the Higgs sector Lagrangian from a given cosmological history
(H(t), \rho(t)). This in principle allows us to constrain the potential by
geometrical information of the universe such as supernova distance-redshift
relation. We also derive the evolution equation for cosmological perturbations
in the Higgs phase of gravity by employing a systematic low energy expansion.
This formalism is expected to be useful to test the theory by dynamical
information of large scale structure in the universe such as cosmic microwave
background anisotropy, weak gravitational lensing and galaxy clustering.Comment: 30 pages; typos corrected; version accepted for publication in JCA
Linearized f(R) Gravity: Gravitational Radiation & Solar System Tests
We investigate the linearized form of metric f(R)-gravity, assuming that f(R)
is analytic about R = 0 so it may be expanded as f(R) = R + a_2 R^2/2 + ... .
Gravitational radiation is modified, admitting an extra mode of oscillation,
that of the Ricci scalar. We derive an effective energy-momentum tensor for the
radiation. We also present weak-field metrics for simple sources. These are
distinct from the equivalent Kerr (or Schwarzschild) forms. We apply the
metrics to tests that could constrain f(R). We show that light deflection
experiments cannot distinguish f(R)-gravity from general relativity as both
have an effective post-Newtonian parameter \gamma = 1. We find that planetary
precession rates are enhanced relative to general relativity; from the orbit of
Mercury we derive the bound |a_2| < 1.2 \times 10^18 m^2. Gravitational wave
astronomy may be more useful: considering the phase of a gravitational waveform
we estimate deviations from general relativity could be measurable for an
extreme-mass-ratio inspiral about a 10^6 M_sol black hole if |a_2| > 10^17 m^2,
assuming that the weak-field metric of the black hole coincides with that of a
point mass. However Eot-Wash experiments provide the strictest bound |a_2| < 2
\times 10^-9 m^2. Although the astronomical bounds are weaker, they are still
of interest in the case that the effective form of f(R) is modified in
different regions, perhaps through the chameleon mechanism. Assuming the
laboratory bound is universal, we conclude that the propagating Ricci scalar
mode cannot be excited by astrophysical sources.Comment: 19 pages, 1 figure; typos in Sec. VIII. A. correcte
Complexity Characterization in a Probabilistic Approach to Dynamical Systems Through Information Geometry and Inductive Inference
Information geometric techniques and inductive inference methods hold great
promise for solving computational problems of interest in classical and quantum
physics, especially with regard to complexity characterization of dynamical
systems in terms of their probabilistic description on curved statistical
manifolds. In this article, we investigate the possibility of describing the
macroscopic behavior of complex systems in terms of the underlying statistical
structure of their microscopic degrees of freedom by use of statistical
inductive inference and information geometry. We review the Maximum Relative
Entropy (MrE) formalism and the theoretical structure of the information
geometrodynamical approach to chaos (IGAC) on statistical manifolds. Special
focus is devoted to the description of the roles played by the sectional
curvature, the Jacobi field intensity and the information geometrodynamical
entropy (IGE). These quantities serve as powerful information geometric
complexity measures of information-constrained dynamics associated with
arbitrary chaotic and regular systems defined on the statistical manifold.
Finally, the application of such information geometric techniques to several
theoretical models are presented.Comment: 29 page
Measurement of boron and carbon fluxes in cosmic rays with the PAMELA experiment
The propagation of cosmic rays inside our galaxy plays a fundamental role in
shaping their injection spectra into those observed at Earth. One of the best
tools to investigate this issue is the ratio of fluxes for secondary and
primary species. The boron-to-carbon (B/C) ratio, in particular, is a sensitive
probe to investigate propagation mechanisms. This paper presents new
measurements of the absolute fluxes of boron and carbon nuclei, as well as the
B/C ratio, from the PAMELA space experiment. The results span the range 0.44 -
129 GeV/n in kinetic energy for data taken in the period July 2006 - March
2008
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