843 research outputs found
Solar System experiments do not yet veto modified gravity models
The dynamical equivalence between modified and scalar-tensor gravity theories
is revisited and it is concluded that it breaks down in the limit to general
relativity. A gauge-independent analysis of cosmological perturbations in both
classes of theories lends independent support to this conclusion. As a
consequence, the PPN formalism of scalar-tensor gravity and Solar System
experiments do not veto modified gravity, as previously thought.Comment: 7 pages, latex, submitted to Phys. Rev.
Phantom Field from Conformal Invariance
We establish a correspondence between a conformally invariant complex scalar
field action (with a conformal self-interaction potential) and the action of a
phantom scalar field minimally coupled to gravity (with a cosmological
constant). In this correspondence, the module of the complex scalar field is
used to relate conformally the metrics of both systems while its phase is
identified with the phantom scalar field. At the level of the equations, the
correspondence allows to map solution of the conformally non-linear
Klein-Gordon equation with vanishing energy-momentum tensor to solution of a
phantom scalar field minimally coupled to gravity with cosmological constant
satisfying a massless Klein-Gordon equation. The converse is also valid with
the advantage that it offers more possibilities owing to the freedom of
rewriting a metric as the conformal transformation of another metric. Finally,
we provide some examples of this correspondence.Comment: 5 pages, two column
A Singularity-Free Cosmological Model with a Conformally Coupled Scalar Field
We explore the possibility of describing our universe with a
singularity--free, closed, spatially homogeneous and isotropic cosmological
model, using only general relativity and a suitable equation of state which
produces an inflationary era. A phase transition to a radiation--dominated era
occurs as a consequence of boundary conditions expressing the assumption that
the temperature cannot exceed the Planck value. We find that over a broad range
of initial conditions, the predicted value of the Hubble parameter is
approximately km~s~Mpc. Inflation is driven by a
scalar field, which must be conformally coupled to the curvature if the
Einstein equivalence principle has to be satisfied. The form of the scalar
field potential is derived, instead of being assumed a priori.Comment: 19 pages, figures and tables available from the author
A viability criterion for modified gravity with an extra force
A recently proposed theory of modified gravity with an explicit ``anomalous''
coupling of the Ricci curvature to matter is discussed, and an inequality is
derived which expresses a necessary and sufficient condition to avoid the
notorius Dolgov-Kawasaki instability.Comment: 4 latex pages, to appear in Phys. Rev.
Scalar-tensor cosmology at the general relativity limit: Jordan vs Einstein frame
We consider the correspondence between the Jordan frame and the Einstein
frame descriptions of scalar-tensor theory of gravitation. We argue that since
the redefinition of the scalar field is not differentiable at the limit of
general relativity the correspondence between the two frames is lost at this
limit. To clarify the situation we analyse the dynamics of the scalar field in
different frames for two distinct scalar-tensor cosmologies with specific
coupling functions and demonstrate that the corresponding scalar field phase
portraits are not equivalent for regions containing the general relativity
limit. Therefore the answer to the question whether general relativity is an
attractor for the theory depends on the choice of the frame.Comment: 16 pages, 8 figures, version appeared in PR
Scalar-tensor cosmologies: fixed points of the Jordan frame scalar field
We study the evolution of homogeneous and isotropic, flat cosmological models
within the general scalar-tensor theory of gravity with arbitrary coupling
function and potential. After introducing the limit of general relativity we
describe the details of the phase space geometry. Using the methods of
dynamical systems for the decoupled equation of the Jordan frame scalar field
we find the fixed points of flows in two cases: potential domination and matter
domination. We present the conditions on the mathematical form of the coupling
function and potential which determine the nature of the fixed points
(attractor or other). There are two types of fixed points, both are
characterized by cosmological evolution mimicking general relativity, but only
one of the types is compatible with the Solar System PPN constraints. The phase
space structure should also carry over to the Einstein frame as long as the
transformation between the frames is regular which however is not the case for
the latter (PPN compatible) fixed point.Comment: 21 pages, 4 figures, some comments and references adde
Position and frequency shifts induced by massive modes of the gravitational wave background in alternative gravity
Alternative theories of gravity predict the presence of massive scalar,
vector, and tensor gravitational wave modes in addition to the standard
massless spin~2 graviton of general relativity. The deflection and frequency
shift effects on light from distant sources propagating through a stochastic
background of gravitational waves, containing such modes, differ from their
counterparts in general relativity. Such effects are considered as a possible
signature for alternative gravity in attempts to detect deviations from
Einstein's gravity by astrophysical means.Comment: 9 pages, 1 figur
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