971 research outputs found
Classifying and avoiding singularities in the alternative gravity dark energy models
The future finite-time singularities emerging in alternative gravity dark
energy models are classified and studied in Jordan and Einstein frames. It is
shown that such singularity may occur even in flat spacetime for the specific
choice of the effective potential. The conditions for the avoidance of
finite-time singularities are presented and discussed. The problem is reduced
to the study of a scalar field evolving on an effective potential by using the
conformal transformations. Some viable modified gravity models are analyzed in
detail and the way to cure singularity is considered by introducing the
higher-order curvature corrections. These results maybe relevant for the
resolution of the conjectured problem in the relativistic star formation in
such modified gravity where finite-time singularity is also manifested.Comment: 21 pages, 12 figures, published version in PR
Hydrostatic equilibrium and stellar structure in f(R)-gravity
We investigate the hydrostatic equilibrium of stellar structure by taking
into account the modi- fied La\'e-Emden equation coming out from f(R)-gravity.
Such an equation is obtained in metric approach by considering the Newtonian
limit of f(R)-gravity, which gives rise to a modified Poisson equation, and
then introducing a relation between pressure and density with polytropic index
n. The modified equation results an integro-differential equation, which, in
the limit f(R) \rightarrow R, becomes the standard La\'e-Emden equation. We
find the radial profiles of gravitational potential by solving for some values
of n. The comparison of solutions with those coming from General Relativity
shows that they are compatible and physically relevant.Comment: 9 pages, 1 figur
The Mass-Radius relation for Neutron Stars in gravity
We discuss the Mass -Radius diagram for static neutron star models obtained
by the numerical solution of modified Tolman-Oppenheimer-Volkoff equations in
gravity where the Lagrangians and
are adopted. Unlike the case of the perturbative
approach previously reported, the solutions are constrained by the presence of
an extra degree of freedom, coming from the trace of the field equations. In
particular, the stiffness of the equation of state determines an upper limit on
the central density above which the the positivity condition of
energy-matter tensor trace holds. In the case of
quadratic f(R)-gravity, we find higher masses and radii at lower central
densities with an inversion of the behavior around a pivoting which
depends on the choice of the equation of state. When considering the cubic
corrections, we find solutions converging to the required asymptotic behavior
of flat metric only for . A similar analysis is performed for
considering as the leading parameter. We work
strictly in the Jordan frame in order to consider matter minimally coupled with
respect to geometry. This fact allows us to avoid ambiguities that could emerge
in adopting the Einstein frame.Comment: 10 pages, 6 figures, to appear in Phys. Rev.
Gravitational and electromagnetic emission by magnetized coalescing binary systems
We discuss the possibility to obtain an electromagnetic emission accompanying
the gravitational waves emitted in the coalescence of a compact binary system.
Motivated by the existence of black hole configurations with open magnetic
field lines along the rotation axis, we consider a magnetic dipole in the
system, the evolution of which leads to (i) electromagnetic radiation, and (ii)
a contribution to the gravitational radiation, the luminosity of both being
evaluated. Starting from the observations on magnetars, we impose upper limits
for both the electromagnetic emission and the contribution of the magnetic
dipole to the gravitational wave emission. Adopting this model for the
evolution of neutron star binaries leading to short gamma ray bursts, we
compare the correction originated by the electromagnetic field to the
gravitational waves emission, finding that they are comparable for particular
values of the magnetic field and of the orbital radius of the binary system.
Finally we calculate the electromagnetic and gravitational wave energy outputs
which result comparable for some values of magnetic field and radius.Comment: 9 pages, 3 figures, to appear in Astroph. Sp.Scienc
The Affine Structure of Gravitational Theories: Symplectic Groups and Geometry
We give a geometrical description of gravitational theories from the
viewpoint of symmetries and affine structure. We show how gravity, considered
as a gauge theory, can be consistently achieved by the nonlinear realization of
the conformal-affine group in an indirect manner: due the partial isomorphism
between and the centrally extended ,
we perform a nonlinear realization of the centrally extended (CE) in its semi-simple version. In particular, starting from the bundle
structure of gravity, we derive the conformal-affine Lie algebra and then, by
the non-linear realization, we define the coset field transformations, the
Cartan forms and the inverse Higgs constraints. Finally we discuss the
geometrical Lagrangians where all the information on matter fields and their
interactions can be contained.Comment: 21 pages. arXiv admin note: text overlap with arXiv:0910.2881,
arXiv:0705.460
Gravitational waves from hyperbolic encounters
The emission of gravitational waves from a system of massive objects
interacting on hyperbolic orbits is studied in the quadrupole approximation.
Analytic expressions are derived for the gravitational radiation luminosity,
the total energy output and the gravitational radiation amplitude. An
estimation of the expected number of events towards different targets (i.e.
globular clusters and the center of the Galaxy) is also given. In particular,
for a dense stellar cluster at the galactic center, a rate up to one event per
year is obtained.Comment: 6 pages, 2 figure
Jeans analysis of self-gravitating systems in f(R)-gravity
Dynamics and collapse of collisionless self-gravitating systems is described
by the coupled collisionless Boltzmann and Poisson equations derived from
-gravity in the weak field approximation. Specifically, we describe a
system at equilibrium by a time-independent distribution function
and two potentials and solutions of the modified
Poisson and collisionless Boltzmann equations. Considering a small perturbation
from the equilibrium and linearizing the field equations, it can be obtained a
dispersion relation. A dispersion equation is achieved for neutral
dust-particle systems where a generalized Jeans wave-number is obtained. This
analysis gives rise to unstable modes not present in the standard Jeans
analysis (derived assuming Newtonian gravity as weak filed limit of ).
In this perspective, we discuss several self-gravitating astrophysical systems
whose dynamics could be fully addressed in the framework of -gravity.Comment: 8 pages, 2 figures, Accepted for publication in PR
Axially symmetric solutions in f(R)-gravity
Axially symmetric solutions for f (R)-gravity can be derived starting from
exact spherically sym- metric solutions achieved by Noether symmetries. The
method takes advantage of a complex coordi- nate transformation previously
developed by Newman and Janis in General Relativity. An example is worked out
to show the general validity of the approach. The physical properties of the
solution are also considered.Comment: 13 pages, 1 figure, to appear in Classical and Quantum Gravity 201
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