683 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.
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 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
Reconstructing the universe history, from inflation to acceleration, with phantom and canonical scalar fields
We consider the reconstruction technique in theories with a single or
multiple (phantom and/or canonical) scalar fields. With the help of several
examples, it is demonstrated explicitly that the universe expansion history,
unifying early-time inflation and late-time acceleration, can be realized in
scalar-tensor gravity. This is generalized to the theory of a scalar field
coupled non-minimally to the curvature and to a Brans-Dicke-like theory.
Different examples of unification of inflation with cosmic acceleration, in
which de Sitter, phantom, and quintessence type fields play the fundamental
role--in different combinations--are worked out. Specifically, the frame
dependence and stability properties of de Sitter space scalar field theory are
studied. Finally, for two-scalar theories, the late-time acceleration and
early-time inflation epochs are successfully reconstructed, in realistic
situations in which the more and more stringent observational bounds are
satisfied, using the freedom of choice of the scalar field potential, and of
the kinetic factor.Comment: 22 pages, revtex, no figures, version to appear in Phys. Rev.
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