229 research outputs found
Noether's stars in gravity
The Noether Symmetry Approach can be used to construct spherically symmetric
solutions in gravity. Specifically, the Noether conserved
quantity is related to the gravitational mass and a gravitational radius that
reduces to the Schwarzschild radius in the limit . We show that it is possible to construct the relation for neutron
stars depending on the Noether conserved quantity and the associated
gravitational radius. This approach enables the recovery of extreme massive
stars that could not be stable in the standard Tolman-Oppenheimer-Volkoff based
on General Relativity. Examples are given for some power law
gravity models.Comment: 7 pages, 4 figures, accepted for publication in Physics Letter
Topological invariant quintessence
The issues of quintessence and cosmic acceleration can be discussed in the
framework of theories of gravity where is the Ricci
curvature scalar and is the Gauss-Bonnet topological invariant. It
is possible to show that such an approach exhausts all the curvature content
related to the Riemann tensor giving rise to a fully geometric approach to dark
energy.Comment: 8 page
Newtonian and relativistic theory of orbits and emission of gravitational waves
This review paper is devoted to the theory of orbits. We start with the
discussion of the Newtonian problem of motion then we consider the relativistic
problem of motion, in particular the PN approximation and the further
gravitomagnetic corrections. Finally by a classification of orbits in
accordance with the conditions of motion, we calculate the gravitational waves
luminosity for different types of stellar encounters and orbits.Comment: 44 pages, 22 figures. arXiv admin note: substantial text overlap with
arXiv:gr-qc/0501041 by other authors without attributio
Quadrupolar gravitational radiation as a test-bed for f(R)-gravity
The debate concerning the viability of f(R)-gravity as a natural extension of
General Relativity could be realistically addressed by using results coming
from binary pulsars like PSR 1913+16. To this end, we develop a quadrupolar
approach to the gravitational radiation for a class of ana- lytic f(R)-models.
We show that experimental results are compatible with a consistent range of
f(R)-models. This means that f(R)-gravity is not ruled out by the observations
and gravitational radiation (in strong field regime) could be a test-bed for
such theories.Comment: 13 pages, 1 figur
Stochastic Background of Relic Scalar Gravitational Waves tuned by Extended Gravity
A stochastic background of relic gravitational waves is achieved by the so
called adiabatically-amplified zero-point fluctuations process derived from
early inflation. It provides a distinctive spectrum of relic gravitational
waves. In the framework of scalar-tensor gravity, we discuss the scalar modes
of gravitational waves and the primordial production of this scalar component
which is generated beside tensorial one. Then we analyze different viable
-gravities towards the Solar System tests and stochastic gravitational
waves background. The aim is to achive experimental bounds for the theory at
local and cosmological scales in order to select models capable of addressing
the accelerating cosmological expansion without cosmological constant but
evading the weak field constraints. It is demonstrated that viable
-gravities under consideration not only satisfy the local tests, but
additionally, pass the PPN-and stochastic gravitational waves bounds for large
classes of parameters.Comment: 6 pages, 1 figure, Dark Energy, Florence 200
Gravitational waves from stellar encounters
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: 14 pages, 3 figure
Noether symmetries in extended gravity quantum cosmology
We summarize the use of Noether symmetries in Minisuperspace Quantum
Cosmology. In particular, we consider minisuperspace models, showing that the
existence of conserved quantities gives selection rules that allow to recover
classical behaviors in cosmic evolution according to the so called Hartle
criterion. Such a criterion selects correlated regions in the configuration
space of dynamical variables whose meaning is related to the emergence of
classical observable universes. Some minisuperspace models are worked out
starting from Extended Gravity, in particular coming from scalar tensor, f(R)
and f(T) theories. Exact cosmological solutions are derived.Comment: 20 pages, Proceedings of "49th Winter School of Theoretical Physics
Cosmology and non-equilibrium statistical mechanics", L{\ka}dek-Zdr\'oj,
Poland, February 10-16, 201
On the universality of MOG weak field approximation at galaxy cluster scale
In its weak field limit, Scalar-tensor-vector gravity theory introduces a
Yukawa-correction to the gravitational potential. Such a correction depends on
the two parameters, which accounts for the modification of the
gravitational constant, and which represents the scale length on
which the scalar field propagates. These parameters were found to be universal
when the modified gravitational potential was used to fit the galaxy rotation
curves and the mass profiles of galaxy clusters, both without Dark Matter. We
test the universality of these parameters using the the temperature
anisotropies due to the thermal Sunyaev-Zeldovich effect. In our model the
intra-cluster gas is in hydrostatic equilibrium within the modified
gravitational potential well and it is described by a polytropic equation of
state. We predict the thermal Sunyaev-Zeldovich temperature anisotropies
produced by Coma cluster, and we compare them with those obtained using the
Planck 2013 Nominal maps. In our analysis, we find and the scale
length, respectively, to be consistent and to depart from their universal
values. Our analysis points out that the assumption of the universality of the
Yukawa-correction to the gravitational potential is ruled out at more than
at galaxy clusters scale, while demonstrating that such a theory of
gravity is capable to fit the cluster profile if the scale dependence of the
gravitational potential is restored.Comment: 8 pages, 3 figures, 2 Tables. Accepted for publication on Physical
Letter
Probing the physical and mathematical structure of gravity by PSR
There are several approaches to extend General Relativity in order to explain
the phenomena related to the Dark Matter and Dark Energy. These theories,
generally called Extended Theories of Gravity, can be tested using observations
coming from relativistic binary systems as PSR . Using a class of
analytical -theories, one can construct the first time derivative of
orbital period of the binary systems starting from a quadrupolar gravitational
emission. Our aim is to set boundaries on the parameters of the theory in order
to understand if they are ruled out, or not, by the observations on PSR
. Finally, we have computed an upper limit on the graviton mass
showing that agree with constraint coming from other observations.Comment: 6 pages, 1 figure, accepted in International Journal of Geometric
Methods in Modern Physic
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