121 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
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
Exponential Metric Fields
The Laser Interferometer Space Antenna (LISA) mission will use advanced
technologies to achieve its science goals: the direct detection of
gravitational waves, the observation of signals from compact (small and dense)
stars as they spiral into black holes, the study of the role of massive black
holes in galaxy evolution, the search for gravitational wave emission from the
early Universe. The gravitational red-shift, the advance of the perihelion of
Mercury, deflection of light and the time delay of radar signals are the
classical tests in the first order of General Relativity (GR). However, LISA
can possibly test Einstein's theories in the second order and perhaps, it will
show some particular feature of non-linearity of gravitational interaction. In
the present work we are seeking a method to construct theoretical templates
that limit in the first order the tensorial structure of some metric fields,
thus the non-linear terms are given by exponential functions of gravitational
strength. The Newtonian limit obtained here, in the first order, is equivalent
to GR.Comment: Accepted for publication in Astrophysics and Space Science, 17 page
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