4,295 research outputs found
Generalized Galileon cosmology
We study the cosmology of a generalized Galileon field with five
covariant Lagrangians in which is replaced by general scalar functions
(i=1,...,5). For these theories, the equations of motion remain
at second-order in time derivatives. We restrict the functional forms of
from the demand to obtain de Sitter solutions responsible for
dark energy. There are two possible choices for power-law functions
, depending on whether the coupling with the Ricci
scalar is independent of or depends on . The former
corresponds to the covariant Galileon theory that respects the Galilean
symmetry in the Minkowski space-time. For generalized Galileon theories we
derive the conditions for the avoidance of ghosts and Laplacian instabilities
associated with scalar and tensor perturbations as well as the condition for
the stability of de Sitter solutions. We also carry out detailed analytic and
numerical study for the cosmological dynamics in those theories.Comment: 24 pages, 10 figures, version to appear in Physical Review
Cosmological perturbation in f(R,G) theories with a perfect fluid
In order to classify modified gravity models according to their physical
properties, we analyze the cosmological linear perturbations for f(R,G)
theories (R being the Ricci scalar and G, the Gauss-Bonnet term) with a
minimally coupled perfect fluid. For the scalar type perturbations, we identify
in general six degrees of freedom. We find that two of these physical modes
obey the same dispersion relation as the one for a non-relativistic de Broglie
wave. This means that spacetime is either highly unstable or its fluctuations
undergo a scale-dependent super-luminal propagation. Two other modes correspond
to the degrees of freedom of the perfect fluid, and propagate with the sound
speed of such a fluid. The remaining two modes correspond to the entropy and
temperature perturbations of the perfect fluid, and completely decouple from
the other modes for a barotropic equation of state. We then provide a concise
condition on f(R,G) theories, that both f(R) and R+f(G) do fulfill, to avoid
the de Broglie type dispersion relation. For the vector type perturbation, we
find that the perturbations decay in time. For the tensor type perturbation,
the perturbations can be either super-luminal or sub-luminal, depending on the
model. No-ghost conditions are also obtained for each type of perturbation.Comment: 12 pages, uses RevTe
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
Impulsive gravitational waves of massless particles in extended theories of gravity
We investigate the vacuum pp-wave and Aichelburg-Sexl-type solutions in f(R)
and the modified Gauss-Bonnet theories of gravity with both minimal and
nonminimal couplings between matter and geometry. In each case, we obtain the
necessary condition for the theory to admit the solution and examine it for
several specific models. We show that the wave profiles are the same or
proportional to the general relativistic one
Relativistic Charged Spheres II: Regularity and Stability
We present new results concerning the existence of static, electrically
charged, perfect fluid spheres that have a regular interior and are arbitrarily
close to a maximally charged black-hole state. These configurations are
described by exact solutions of Einstein's field equations. A family of these
solutions had already be found (de Felice et al., 1995) but here we generalize
that result to cases with different charge distribution within the spheres and
show, in an appropriate parameter space, that the set of such physically
reasonable solutions has a non zero measure. We also perform a perturbation
analysis and identify the solutions which are stable against adiabatic radial
perturbations. We then suggest that the stable configurations can be considered
as classic models of charged particles. Finally our results are used to show
that a conjecture of Kristiansson et al. (1998) is incorrect.Comment: revtex, 13 pages. five EPS figures. Accepted by CQ
Can we see naked singularities?
We study singularities which can form in a spherically symmetric
gravitational collapse of a general matter field obeying weak energy condition.
We show that no energy can reach an outside observer from a null naked
singularity. That means they will not be a serious threat to the Cosmic
Censorship Conjecture (CCC). For the timelike naked singularities, where only
the central shell gets singular, the redshift is always finite and they can in
principle, carry energy to a faraway observer. Hence for proving or disproving
CCC the study of timelike naked singularities will be more important. Our
results are very general and are independent of initial data and the form of
the matter.Comment: 10 page
The post-Newtonian limit in C-theories of gravitation
C-theory provides a unified framework to study metric, metric-affine and more
general theories of gravity. In the vacuum weak-field limit of these theories,
the parameterized post-Newtonian (PPN) parameters and can
differ from their general relativistic values. However, there are several
classes of models featuring long-distance modifications of gravity but
nevertheless passing the Solar system tests. Here it is shown how to compute
the PPN parameters in C-theories and also in nonminimally coupled curvature
theories, correcting previous results in the literature for the latter.Comment: 5 pages, no figures; To appear in PRD as a rapid communicatio
Density perturbations in general modified gravitational theories
We derive the equations of linear cosmological perturbations for the general
Lagrangian density , where is a Ricci scalar,
is a scalar field, and is a field kinetic energy. We
take into account a nonlinear self-interaction term recently studied in
the context of "Galileon" cosmology, which keeps the field equations at second
order. Taking into account a scalar-field mass explicitly, the equations of
matter density perturbations and gravitational potentials are obtained under a
quasi-static approximation on sub-horizon scales. We also derive conditions for
the avoidance of ghosts and Laplacian instabilities associated with propagation
speeds. Our analysis includes most of modified gravity models of dark energy
proposed in literature and thus it is convenient to test the viability of such
models from both theoretical and observational points of view.Comment: 17 pages, no figure
Cosmological dynamics of fourth order gravity with a Gauss-Bonnet term
We consider cosmological dynamics in fourth order gravity with both
and correction to the Einstein gravity ( is
the Gauss-Bonnet term). The particular case for which both terms are equally
important on power-law solutions is described. These solutions and their
stability are studied using the dynamical system approach. We also discuss
condition of existence and stability of de Sitter solution in a more general
situation of power-law and .Comment: published version, references update
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
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