139 research outputs found
The post-Minkowskian limit of f(R)-gravity
We formally discuss the post-Minkowskian limit of
-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 -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
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
Hamiltonian dynamics and Noether symmetries in Extended Gravity Cosmology
We discuss the Hamiltonian dynamics for cosmologies coming from Extended
Theories of Gravity. In particular, minisuperspace models are taken into
account searching for Noether symmetries. The existence of conserved quantities
gives selection rule to recover classical behaviors in cosmic evolution
according to the so called Hartle criterion, that allows to select correlated
regions in the configuration space of dynamical variables. We show that such a
statement works for general classes of Extended Theories of Gravity and is
conformally preserved. Furthermore, the presence of Noether symmetries allows a
straightforward classification of singularities that represent the points where
the symmetry is broken. Examples of nonminimally coupled and higher-order
models are discussed.Comment: 20 pages, Review paper to appear in EPJ
gravity constrained by PPN parameters and stochastic background of gravitational waves
We analyze seven different viable -gravities towards the Solar System
tests and stochastic gravitational waves background. The aim is to achieve
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. Beside
large scale structure and galactic dynamics, these bounds can be considered
complimentary in order to select self-consistent theories of gravity working at
the infrared limit. It is demonstrated that seven viable -gravities under
consideration not only satisfy the local tests, but additionally, pass the
above PPN-and stochastic gravitational waves bounds for large classes of
parameters.Comment: 23 pages, 8 figure
Oscillatory behavior of closed isotropic models in second order gravity theory
Homogeneous and isotropic models are studied in the Jordan frame of the
second order gravity theory. The late time evolution of the models is analysed
with the methods of the dynamical systems. The normal form of the dynamical
system has periodic solutions for a large set of initial conditions. This
implies that an initially expanding closed isotropic universe may exhibit
oscillatory behaviour.Comment: 16 pages, 3 figures. With some minor improvements. To appear in
General Relativity and Gravitatio
Reconstruction of some cosmological models in f(R,T) gravity
In this paper, we reconstruct cosmological models in the framework of
gravity, where is the Ricci scalar and is the trace of the
stress-energy tensor. We show that the dust fluid reproduces CDM,
phantom-non-phantom era and the phantom cosmology. Further, we reconstruct
different cosmological models including, Chaplygin gas, scalar field with some
specific forms of . Our numerical simulation for Hubble parameter shows
good agreement with the BAO observational data for low redshifts .Comment: 12 pages, 2 figure
Modified f(R) gravity from scalar-tensor theory and inhomogeneous EoS dark energy
The reconstruction of f(R)-gravity is showed by using an auxiliary scalar
field in the context of cosmological evolution, this development provide a way
of reconstruct the form of the function f (R) for a given evolution of the
Hubble parameter. In analogy, f(R)-gravity may be expressed by a perfect fluid
with an inhomogeneous equation of state that depends on the Hubble parameter
and its derivatives. This mathematical equivalence that may confuse about the
origin of the mechanism that produces the current acceleration, and possibly
the whole evolution of the Hubble parameter, is shown here.Comment: 8 page
Viability of Noether symmetry of F(R) theory of gravity
Canonization of F(R) theory of gravity to explore Noether symmetry is
performed treating R - 6(\frac{\ddot a}{a} + \frac{\dot a^2}{a^2} +
\frac{k}{a^2}) = 0 as a constraint of the theory in Robertson-Walker
space-time, which implies that R is taken as an auxiliary variable. Although it
yields correct field equations, Noether symmetry does not allow linear term in
the action, and as such does not produce a viable cosmological model. Here, we
show that this technique of exploring Noether symmetry does not allow even a
non-linear form of F(R), if the configuration space is enlarged by including a
scalar field in addition, or taking anisotropic models into account.
Surprisingly enough, it does not reproduce the symmetry that already exists in
the literature (A. K. Sanyal, B. Modak, C. Rubano and E. Piedipalumbo,
Gen.Relativ.Grav.37, 407 (2005), arXiv:astro-ph/0310610) for scalar tensor
theory of gravity in the presence of R^2 term. Thus, R can not be treated as an
auxiliary variable and hence Noether symmetry of arbitrary form of F(R) theory
of gravity remains obscure. However, there exists in general, a conserved
current for F(R) theory of gravity in the presence of a non-minimally coupled
scalar-tensor theory (A. K. Sanyal, Phys.Lett.B624, 81 (2005),
arXiv:hep-th/0504021 and Mod.Phys.Lett.A25, 2667 (2010), arXiv:0910.2385
[astro-ph.CO]). Here, we briefly expatiate the non-Noether conserved current
and cite an example to reveal its importance in finding cosmological solution
for such an action, taking F(R) \propto R^{3/2}.Comment: 16 pages, 1 figure. appears in Int J Theoretical Phys (2012
Some exact solutions of F(R) gravity with charged (a)dS black hole interpretation
In this paper we obtain topological static solutions of some kind of pure
gravity. The present solutions are two kind: first type is uncharged
solution which corresponds with the topological (a)dS Schwarzschild solution
and second type has electric charge and is equivalent to the
Einstein--conformally invariant Maxwell solution. In other word,
starting from pure gravity leads to (charged) Einstein- solutions
which we interpreted them as (charged) (a)dS black hole solutions of pure
gravity. Calculating the Ricci and Kreschmann scalars show that there is
a curvature singularity at . We should note that the Kreschmann scalar of
charged solutions goes to infinity as , but with a rate slower
than that of uncharged solutions.Comment: 21 pages, 4 figures, generalization to higher dimensions, references
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Some Adventures in the Search for a Modified Gravity Explanation for Cosmic Acceleration
The discovery of cosmic acceleration has raised the intriguing possibility
that we are witnessing the first breakdown of General Relativity on
cosmological scales. In this article I will briefly review current attempts to
construct a theoretically consistent and observationally viable modification of
gravity that is capable of describing the accelerating universe. I will discuss
f(R) models, and their obvious extensions, and the DGP model as an example of
extra-dimensional implementations. I will then briefly describe the Galileon
models and their very recent multifield and curved space extensions - a class
of four-dimensional effective field theories encoding extra dimensional
modifications to gravity. This article is dedicated to the career of my friend
and former colleague, Joshua Goldberg, and is written to appear in his
festschrift.Comment: 17 pages, to appear in a festschrift for Joshua Goldber
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