1,153 research outputs found
Gravitational waves production from stellar encounters around massive black holes
The emission of gravitational waves from a system of massive objects
interacting on elliptical, hyperbolic and parabolic orbits is studied in the
quadrupole approximation. Analytical expressions are then derived for the
gravitational wave luminosity, the total energy output and gravitational
radiation amplitude. A crude estimate of the expected number of events towards
peculiar targets (i.e. globular clusters) is also given. In particular, the
rate of events per year is obtained for the dense stellar cluster at the
Galactic Center.Comment: 6 pages, 3 figures; Multifrequency Behaviour of High-Energy Cosmic
Sources, Vulcano Workshop 200
Coalescing binaries as possible standard candles
Gravitational waves detected from well-localized inspiraling binaries would
allow to determine, directly and independently, both binary luminosity and
redshift. In this case, such systems could behave as "standard candles"
providing an excellent probe of cosmic distances up to and thus
complementing other indicators of cosmological distance ladder.Comment: 6 pages, 5 figures. accepted in Astroparticle Physic
Stochastic background of gravitational waves "tuned" by gravity
The cosmological background of gravitational waves can be tuned by Extended
Theories of Gravity. In particular, it can be shown that assuming a generic
function of the Ricci scalar gives a parametric approach to control
the evolution and the production mechanism of gravitational waves in the early
Universe.Comment: 5 pages, to appear in the Proceedings of the 3rd Stueckelberg
Workshop, July 2008, Pescara - Ital
First Order Extended Gravity and the Dark Side of the Universe: the General Theory
General Relativity is not the definitive theory of Gravitation due to several
shortcomings which are coming out both from theoretical and experimental
viewpoints. At large scales (astrophysical and cosmological scales) the
attempts to match it with the today observational data lead to invoke Dark
Energy and Dark Matter as the bulk components of the cosmic fluid. Since no
final evidence, at fundamental level, exists for such ingredients, it is clear
that General Relativity presents shortcomings at infrared scales. On the other
hand, the attempts to formulate theories more general than the Einstein one
give rise to mathematical difficulties that need workarounds which, in turn,
generate problems from the interpretative viewpoint. We present here a
completely new approach to the mathematical objects in terms of which a theory
of Gravitation may be written in a first-order `a la Palatini formalism, and
introduce the concept of Dark Metric which could completely bypass the
introduction of disturbing concepts as Dark Energy and Dark Matter.Comment: Proceedings of the Conference "The Invisible Universe" Paris, June
29-July 3, 2009 10 page
The physical foundations for the geometric structure of relativistic theories of gravitation. From General Relativity to Extended Theories of Gravity through Ehlers-Pirani-Schild approach
We discuss in a critical way the physical foundations of geometric structure
of relativistic theories of gravity by the so-called Ehlers-Pirani-Schild
formalism. This approach provides a natural interpretation of the observables
showing how relate them to General Relativity and to a large class of Extended
Theories of Gravity. In particular we show that, in such a formalism, geodesic
and causal structures of space-time can be safely disentangled allowing a
correct analysis in view of observations and experiment. As specific case, we
take into account the case of f(R) gravity.Comment: 11 pages, 2 figure
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
MOND's acceleration scale as a fundamental quantity
Some quantum-cosmic scaling relations indicate that the MOND acceleration
parameter a_0 could be a fundamental quantity ruling the self-gravitating
structures, ranging from stars and globular clusters up to superclusters of
galaxies and the whole observed universe. We discuss such coincidence relations
starting from the Dirac quantization condition ruling the masses of primordial
black holes.Comment: 6 page
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
Constraining f(R) gravity with PLANCK data on galaxy cluster profiles
Models of gravity that introduce corrections to the Newtonian
potential in the weak field limit are tested at the scale of galaxy clusters.
These models can explain the dynamics of spiral and elliptical galaxies without
resorting to dark matter. We compute the pressure profiles of 579 galaxy
clusters assuming that the gas is in hydrostatic equilibrium within the
potential well of the modified gravitational field. The predicted profiles are
compared with the average profile obtained by stacking the data of our cluster
sample in the Planck foreground clean map SMICA. We find that the resulting
profiles of these systems fit the data without requiring a dominant dark matter
component, with model parameters similar to those required to explain the
dynamics of galaxies. Our results do not rule out that clusters are dynamically
dominated by Dark Matter but support the idea that Extended Theories of Gravity
could provide an explanation to the dynamics of self-gravitating systems and to
the present period of accelerated expansion, alternative to the concordance
cosmological model.Comment: 10 pages, 5 figures, accepted for publication in MNRA
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