2,934 research outputs found
PPN limit and cosmological gravitational waves as tools to constrain f(R)-gravity
We discuss the PPN Solar-System constraints and the GW stochastic background
considering some recently proposed gravity models which satisfy both
cosmological and stability conditions. Using the definition of PPN-parameters
and in terms of -models and the definition of scalar
GWs, we compare and discuss if it is possible to search for parameter ranges of
-models working at Solar System and GW stochastic background scale.Comment: 4 pages, Grassmann Meeting 2009, Annalen der Physi
Noether symmetric minisuperspace model of cosmology
We study the metric cosmology using Noether symmetry approach by
utilizing the behavior of the corresponding Lagrangian under infinitesimal
generators of the desired symmetry. The existence of Noether symmetry of the
cosmological minisuperspace helps us to find out the form of
function for which such symmetry exist. It is shown that the resulting form for
yields a power law expansion for the cosmic scale factor. We also show
that in the corresponding Noether symmetric quantum model, the solutions to the
Wheeler-DeWitt equation can be expressed as a superposition of states of the
form . It is shown that in terms of such wavefunctions the classical
trajectories can be recovered.Comment: 5 pages, Talk given at "The Grassmannian Conference in Fundamental
Cosmology (Grasscosmofun'09)", Szczecin, Poland, 14-19th September 200
Physical non-equivalence of the Jordan and Einstein frames
We show, considering a specific f(R)-gravity model, that the Jordan frame and
the Einstein frame are physically non-equivalent, although they are connected
by a conformal transformation which yields a mathematical equivalence. Since
all the calculations are performed analytically, this non-equivalence is shown
in an unambiguous way.Comment: 7 pages, 3 figure
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
-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
Scalar-tensor cosmology with R^{-1} curvature correction by Noether Symmetry
We discuss scalar-tensor cosmology with an extra correction by the
Noether Symmetry Approach. The existence of such a symmetry selects the forms
of the coupling , of the potential and allows to obtain
physically interesting exact cosmological solutions.Comment: 7 page
Palatini f(R) gravity and Noether symmetry
We study Palatini f(R) cosmology using Noether symmetry approach for the
matter dominated universe. In order to construct a point-like Lagrangian in the
flat FRW space time, we use the dynamical equivalence between f(R) gravity and
scalar-tensor theories. The existence of Noether symmetry of the cosmological
f(R) Lagrangian helps us to find out the form of f(R) and the exact solutions
for cosmic scale factor. We show that this symmetry always exist for f(R)~R^n
and the Noether constant is a function of the Newton's gravitational constant
and the current matter content of the universe.Comment: 4 pages,to appear in physics letters
Curvature Quintessence
The issues of quintessence and cosmic acceleration can be discussed in the
framework of higher order theories of gravity. We can define effective pressure
and energy density directly connected to the Ricci scalar of curvature of a
generic fourth order theory and then ask for the conditions to get an
accelerated expansion. Exact accelerated expanding solutions can be achieved
for several fourth order theories so that we get an alternative scheme to the
standard quintessence scalar field, minimally coupled to gravity, usually
adopted. We discuss also conformal transformations in order to see the links of
quintessence between the Jordan and Einstein frames.Comment: 10 pages, LATEX files, to appear on IJMP
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
Noether symmetry in cosmology
The Noether symmetry of a generic cosmological model is investigated
by utilizing the behavior of the corresponding Lagrangian under the
infinitesimal generators of the desired symmetry. We explicitly calculate the
form of for which such symmetries exist. It is shown that the resulting
form of yields a power law expansion for the cosmological scale factor.
We also obtain the effective equation of state parameter for the corresponding
cosmology and show that our model can provide a gravitational alternative to
the quintessence.Comment: 9 pages,no figures,typos corrected,references added, to appear in PL
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