1,975 research outputs found
The post-Newtonian limit in C-theories of gravitation
C-theory provides a unified framework to study metric, metric-affine and more
general theories of gravity. In the vacuum weak-field limit of these theories,
the parameterized post-Newtonian (PPN) parameters and can
differ from their general relativistic values. However, there are several
classes of models featuring long-distance modifications of gravity but
nevertheless passing the Solar system tests. Here it is shown how to compute
the PPN parameters in C-theories and also in nonminimally coupled curvature
theories, correcting previous results in the literature for the latter.Comment: 5 pages, no figures; To appear in PRD as a rapid communicatio
Perturbations in electromagnetic dark energy
It has been recently proposed that the presence of a temporal electromagnetic
field on cosmological scales could explain the phase of accelerated expansion
that the universe is currently undergoing. The field contributes as a
cosmological constant and therefore, the homogeneous cosmology produced by such
a model is exactly the same as that of CDM. However, unlike a
cosmological constant term, electromagnetic fields can acquire perturbations
which in principle could affect CMB anisotropies and structure formation. In
this work, we study the evolution of inhomogeneous scalar perturbations in this
model. We show that provided the initial electromagnetic fluctuations generated
during inflation are small, the model is perfectly compatible with both CMB and
large scale structure observations at the same level of accuracy as
CDM.Comment: 12 pages, 3 figures. Added new comments to match the published
versio
String cosmological model in the presence of a magnetic flux
A Bianchi type I string cosmological model in the presence of a magnetic flux
is investigated. A few plausible assumptions regarding the parametrization of
the cosmic string and magneto-fluid are introduced and some exact analytical
solutions are presented.Comment: 9 pages, 4 Figure
Shining Light on Modifications of Gravity
Many modifications of gravity introduce new scalar degrees of freedom, and in
such theories matter fields typically couple to an effective metric that
depends on both the true metric of spacetime and on the scalar field and its
derivatives. Scalar field contributions to the effective metric can be
classified as conformal and disformal. Disformal terms introduce gradient
couplings between scalar fields and the energy momentum tensor of other matter
fields, and cannot be constrained by fifth force experiments because the
effects of these terms are trivial around static non-relativistic sources. The
use of high-precision, low-energy photon experiments to search for conformally
coupled scalar fields, called axion-like particles, is well known. In this
article we show that these experiments are also constraining for disformal
scalar field theories, and are particularly important because of the difficulty
of constraining these couplings with other laboratory experiments.Comment: 20 pages, 10 figures. v2: Matches version accepted by JCAP;
additional discussion of the strong coupling scale. Conclusions unchange
Unifying Einstein and Palatini gravities
We consider a novel class of gravity theories where the connection is
related to the conformally scaled metric with
a scaling that depends on the scalar curvature only. We call them
C-theories and show that the Einstein and Palatini gravities can be obtained as
special limits. In addition, C-theories include completely new physically
distinct gravity theories even when . With nonlinear ,
C-theories interpolate and extrapolate the Einstein and Palatini cases and may
avoid some of their conceptual and observational problems. We further show that
C-theories have a scalar-tensor formulation, which in some special cases
reduces to simple Brans-Dicke-type gravity. If matter fields couple to the
connection, the conservation laws in C-theories are modified. The stability of
perturbations about flat space is determined by a simple condition on the
lagrangian.Comment: 17 pages, no figure
Dirac fields in f(R)-gravity with torsion
We study f(R)-gravity with torsion in presence of Dirac massive fields. Using
the Bianchi identities, we formulate the conservation laws of the theory and we
check the consistency with the matter field equations. Further, we decompose
the field equations in torsionless and torsional terms: we show that the
non-linearity of the gravitational Lagrangian reduces to the presence of a
scalar field that depends on the spinor field; this additional scalar field
gives rise to an effective stress-energy tensor and plays the role of a scale
factor modifying the normalization of Dirac fields. Problems for fermions
regarding the positivity of energy and the particle-antiparticle duality are
discussed.Comment: 14 page
Growth of perturbations in dark matter coupled with quintessence
We consider the evolution of linear perturbations in models with a nonminimal
coupling between dark matter and scalar field dark energy. Growth of matter
inhomogeneities in two examples of such models proposed in the literature are
investigated in detail. Both of these models are based on a low-energy limit of
effective string theory action, and have been previously shown to naturally
lead to late acceleration of the universe. However, we find that these models
can be ruled out by taking properly into account the impact of the scalar field
coupling on the formation of structure in the dark matter density. In
particular, when the transition to acceleration in these models begins, the
interaction with dark energy enchances the small scale clustering in dark
matter much too strongly. We discuss the the role of an effective small scale
sound speed in such models with a coupled dark sector.Comment: 14 pages, 10 figures; v2: references adde
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
Covariant conservation of energy momentum in modified gravities
An explicit proof of the vanishing of the covariant divergence of the
energy-momentum tensor in modified theories of gravity is presented. The
gravitational action is written in arbitrary dimensions and allowed to depend
nonlinearly on the curvature scalar and its couplings with a scalar field. Also
the case of a function of the curvature scalar multiplying a matter Lagrangian
is considered. The proof is given both in the metric and in the first-order
formalism, i.e. under the Palatini variational principle. It is found that the
covariant conservation of energy-momentum is built-in to the field equations.
This crucial result, called the generalized Bianchi identity, can also be
deduced directly from the covariance of the extended gravitational action.
Furthermore, we demonstrate that in all of these cases, the freely falling
world lines are determined by the field equations alone and turn out to be the
geodesics associated with the metric compatible connection. The independent
connection in the Palatini formulation of these generalized theories does not
have a similar direct physical interpretation. However, in the conformal
Einstein frame a certain bi-metricity emerges into the structure of these
theories. In the light of our interpretation of the independent connection as
an auxiliary variable we can also reconsider some criticisms of the Palatini
formulation originally raised by Buchdahl.Comment: 8 pages. v2: more discussio
- …