4,165 research outputs found
Observational Bounds on Modified Gravity Models
Modified gravity provides a possible explanation for the currently observed
cosmic accelaration. In this paper, we study general classes of modified
gravity models. The Einstein-Hilbert action is modified by using general
functions of the Ricci and the Gauss-Bonnet scalars, both in the metric and in
the Palatini formalisms. We do not use an explicit form for the functions, but
a general form with a valid Taylor expansion up to second order about redshift
zero in the Riemann-scalars. The coefficients of this expansion are then
reconstructed via the cosmic expansion history measured using current
cosmological observations. These are the quantities of interest for theoretical
considerations relating to ghosts and instabilities. We find that current data
provide interesting constraints on the coefficients. The next-generation dark
energy surveys should shrink the allowed parameter space for modifed gravity
models quite dramatically.Comment: 23 pages, 5 figures, uses RevTe
Neutron stars in generalized f(R) gravity
Quartic gravity theory is considered with the Einstein-Hilbert Lagrangean
being Ricci\'s tensor and R
the curvature scalar. The parameters and are taken of order 1 km
Arguments are given which suggest that the effective theory so obtained may be
a plausible approximation of a viable theory. A numerical integration is
performed of the field equations for a free neutron gas. As in the standard
Oppenheimer-Volkoff calculation the star mass increases with increasing central
density until about 1 solar mass and then decreases. However a dramatic
difference exists in the behaviour of the baryon number, which increases
monotonically. The calculation suggests that the theory allows stars in
equilibrium with arbitrary baryon number, no matter how large.Comment: Keywords: stars, neutron stars; gravity; modified gravity Accepted in
Astrophysics and Space Scienc
Variational approach to gravitational theories with two independent connections
A new variational approach for general relativity and modified theories of
gravity is presented. In addition to the metric tensor, two independent affine
connections enter the action as dynamical variables. In the matter action the
dependence upon one of the connections is left completely unspecified. When the
variation is applied to the Einstein-Hilbert action the Einstein field
equations are recovered. However when applied to and Scalar-Tensor
theories, it yields gravitational field equations which differ from their
equivalents obtained with a metric or Palatini variation and reduce to the
former ones only when no connections appear in the matter action.Comment: 11 pages, no figure
Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009
The last solar minimum activity period, and the consequent minimum modulation
conditions for cosmic rays, was unusual. The highest levels of galactic protons
were recorded at Earth in late 2009 in contrast to expectations. Proton spectra
observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the
Resurs-DK1 satellite are presented together with the solutions of a
comprehensive numerical model for the solar modulation of cosmic rays. The
model is used to determine what mechanisms were mainly responsible for the
modulation of protons during this period, and why the observed spectrum for
2009 was the highest ever recorded. From mid-2006 until December 2009 we find
that the spectra became significantly softer because increasingly more low
energy protons had reached Earth. To simulate this effect, the rigidity
dependence of the diffusion coefficients had to decrease significantly below ~3
GeV. The modulation minimum period of 2009 can thus be described as relatively
more "diffusion dominated" than previous solar minima. However, we illustrate
that drifts still had played a significant role but that the observable
modulation effects were not as well correlated with the waviness of the
heliospheric current sheet as before. Protons still experienced global gradient
and curvature drifts as the heliospheric magnetic field had decreased
significantly until the end of 2009, in contrast to the moderate decreases
observed during previous minimum periods. We conclude that all modulation
processes contributed to the observed increases in the proton spectra for this
period, exhibiting an intriguing interplay of these major mechanisms
Generalized Brans-Dicke theories
In Brans-Dicke theory a non-linear self interaction of a scalar field allows
a possibility of realizing the late-time cosmic acceleration, while recovering
the General Relativistic behavior at early cosmological epochs. We extend this
to more general modified gravitational theories in which a de Sitter solution
for dark energy exists without using a field potential. We derive a condition
for the stability of the de Sitter point and study the background cosmological
dynamics of such theories. We also restrict the allowed region of model
parameters from the demand for the avoidance of ghosts and instabilities. A
peculiar evolution of the field propagation speed allows us to distinguish
those theories from the LCDM model.Comment: 14 pages, 4 figures, version to appear in JCA
Spinning test particles and clock effect in Kerr spacetime
We study the motion of spinning test particles in Kerr spacetime using the
Mathisson-Papapetrou equations; we impose different supplementary conditions
among the well known Corinaldesi-Papapetrou, Pirani and Tulczyjew's and analyze
their physical implications in order to decide which is the most natural to
use. We find that if the particle's center of mass world line, namely the one
chosen for the multipole reduction, is a spatially circular orbit (sustained by
the tidal forces due to the spin) then the generalized momentum of the test
particle is also tangent to a spatially circular orbit intersecting the center
of mass line at a point. There exists one such orbit for each point of the
center of mass line where they intersect; although fictitious, these orbits are
essential to define the properties of the spinning particle along its physical
motion. In the small spin limit, the particle's orbit is almost a geodesic and
the difference of its angular velocity with respect to the geodesic value can
be of arbitrary sign, corresponding to the spin-up and spin-down possible
alignment along the z-axis. We also find that the choice of the supplementary
conditions leads to clock effects of substantially different magnitude. In
fact, for co-rotating and counter-rotating particles having the same spin
magnitude and orientation, the gravitomagnetic clock effect induced by the
background metric can be magnified or inhibited and even suppressed by the
contribution of the individual particle's spin. Quite surprisingly this
contribution can be itself made vanishing leading to a clock effect
undistiguishable from that of non spinning particles. The results of our
analysis can be observationally tested.Comment: IOP macros, eps figures n. 12, to appear on Classical and Quantum
Gravity, 200
Modified Special Relativity on a fluctuating spacetime
It was recently proposed that deformations of the relativistic symmetry, as
those considered in Deformed Special Relativity (DSR), can be seen as the
outcome of a measurement theory in the presence of non-negligible (albeit
small) quantum gravitational fluctuations [1,2]. In this paper we explicitly
consider the case of a spacetime described by a flat metric endowed with
stochastic fluctuations and, for a free particle, we show that DSR-like
nonlinear relations between the spaces of the measured and classical momenta,
can result from the average of the stochastic fluctuations over a scale set be
the de Broglie wavelength of the particle. As illustrative examples we consider
explicitly the averaging procedure for some simple stochastic processes and
discuss the physical implications of our results.Comment: 7 pages, no figure
- …