1,952 research outputs found

### New Spherically Symmetric Solutions in f(R)-gravity by Noether Symmetries

Spherical symmetry for f(R)-gravity is discussed by searching for Noether
symmetries. The method consists in selecting conserved quantities in form of
currents that reduce dynamics of f(R)-models compatible with symmetries. In
this way we get a general method to obtain constants of motion without setting
a priori the form of f(R). In this sense, the Noether symmetry results a
physical criterium. Relevant cases are discussed.Comment: 9 pages, accepted for publication in General Relativity and
Gravitatio

### The post-Minkowskian limit of f(R)-gravity

We formally discuss the post-Minkowskian limit of
$f(R)$-gravity without adopting conformal transformations but developing all
the calculations in the original Jordan frame. It is shown that such an
approach gives rise, in general, together with the standard massless graviton,
to massive scalar modes whose masses are directly related to the analytic
parameters of the theory. In this sense, the presence of massless gravitons
only is a peculiar feature of General Relativity. This fact is never stressed
enough and could have dramatic consequences in detection of gravitational
waves. Finally the role of curvature stress-energy tensor of $f(R)$-gravity is
discussed showing that it generalizes the so called Landau-Lifshitz tensor of
General Relativity. The further degrees of freedom, giving rise to the massive
modes, are directly related to the structure of such a tensor.Comment: 9 page

### Testing metric-affine f(R)-gravity by relic scalar gravitational waves

We discuss the emergence of scalar gravitational waves in metric-affine
f(R)-gravity. Such a component allows to discriminate between metric and
metric-affine theories The intrinsic meaning of this result is that the
geodesic structure of the theory can be discriminated. We extend the formalism
of cross correlation analysis, including the additional polarization mode, and
calculate the detectable energy density of the spectrum for cosmological relic
gravitons. The possible detection of the signal is discussed against
sensitivities of VIRGO, LIGO and LISA interferometers.Comment: 12 pages, 4 figure

### The production of matter from curvature in a particular linearized high order theory of gravity and the longitudinal response function of interferometers

The strict analogy between scalar-tensor theories of gravity and high order
gravity is well known in literature. In this paper it is shown that, from a
particular high order gravity theory known in literature, it is possible to
produce, in the linearized approch, particles which can be seen like massive
scalar modes of gravitational waves and the response of interferometers to this
type of particles is analyzed. The presence of the mass generates a
longitudinal force in addition of the transverse one which is proper of the
massless gravitational waves and the response of an arm of an interferometer to
this longitudinal effect in the frame of a local observer is computed. This
longitudinal response function is directly connected with the function of the
Ricci scalar in the particular action of this high order theory. Important
conseguences from a theoretical point of view could arise from this approach,
because it opens to the possibility of using the signals seen from
interferometers to understand which is the correct theory of gravitation.Comment: Accepted for Journal of Cosmology and Astroparticle Physic

### Probing the dark matter issue in f(R)-gravity via gravitational lensing

For a general class of analytic f(R)-gravity theories, we discuss the weak
field limit in view of gravitational lensing. Though an additional Yukawa term
in the gravitational potential modifies dynamics with respect to the standard
Newtonian limit of General Relativity, the motion of massless particles results
unaffected thanks to suitable cancellations in the post-Newtonian limit. Thus,
all the lensing observables are equal to the ones known from General
Relativity. Since f(R)-gravity is claimed, among other things, to be a possible
solution to overcome for the need of dark matter in virialized systems, we
discuss the impact of our results on the dynamical and gravitational lensing
analyses. In this framework, dynamics could, in principle, be able to reproduce
the astrophysical observations without recurring to dark matter, but in the
case of gravitational lensing we find that dark matter is an unavoidable
ingredient. Another important implication is that gravitational lensing, in the
post-Newtonian limit, is not able to constrain these extended theories, since
their predictions do not differ from General Relativity.Comment: 7 pages, accepted for publication in EPJ

### Running coupling in electroweak interactions of leptons from f(R)-gravity with torsion

The f(R)-gravitational theory with torsion is considered for one family of
leptons; it is found that the torsion tensor gives rise to interactions having
the structure of the weak forces while the intrinsic non-linearity of the f(R)
function provides an energy-dependent coupling: in this way, torsional f(R)
gravity naturally generates both structure and strength of the electroweak
interactions among leptons. This implies that the weak interactions among the
lepton fields could be addressed as a geometric effect due to the interactions
among spinors induced by the presence of torsion in the most general f(R)
gravity. Phenomenological considerations are addressed.Comment: 9 pages. arXiv admin note: text overlap with arXiv:1012.5529 by other
author

### Gravitational Cherenkov Radiation from Extended Theories of Gravity

We linearize the field equations for higher order theories of gravity that
contain scalar invariants other than the Ricci scalar. We find that besides a
massless spin-2 field (the standard graviton), the theory contains also spin-0
and spin-2 massive modes with the latter being, in general, ghost modes. The
rate at which such particles would emit gravitational Cherenkov radiation is
calculated for some interesting physical cases.Comment: 6 pages, to appear in Mod. Phys. Lett. A. arXiv admin note: text
overlap with arXiv:0911.3094, arXiv:1105.619

### A general solution in the Newtonian limit of f(R)- gravity

We show that any analytic $f(R)$-gravity model, in the metric approach,
presents a weak field limit where the standard Newtonian potential is corrected
by a Yukawa-like term. This general result has never been pointed out but often
derived for some particular theories. This means that only $f(R)=R$ allows to
recover the standard Newton potential while this is not the case for other
relativistic theories of gravity. Some considerations on the physical
consequences of such a general solution are addressed.Comment: 5 page

### String Dilaton Fluid Cosmology

We investigate $(n+1)$-dimensional string-dilaton cosmology with effective
dilaton potential in presence of perfect-fluid matter.We get exact solutions
parametrized by the constant \gam of the state equation p=(\gam-1)\rho, the
spatial dimension number $n$, the bulk of matter, and the spatial curvature
constant $k$. Several interesting cosmological behaviours are selected. Finally
we discuss the recovering of ordinary Einstein gravity starting from string
dominated regime and a sort of asymptotic freedom due to string effective
coupling.Comment: 16 pages, Latex, submitted to Int. Jou. Mod. Phys.

### The Cauchy problem for f(R)-gravity: an overview

We review the Cauchy problem for f(R) theories of gravity, in metric and
metric-affine for- mulations, pointing out analogies and differences with
respect to General Relativity. The role of conformal transformations, effective
scalar fields and sources in the field equations is discussed in view of the
well-posedness of the problem. Finally, criteria of viability of the
f(R)-models are considered according to the various matter fields acting as
sources.Comment: 14 page

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