135 research outputs found
The Force of Gravity from a Lagrangian containing Inverse Powers of the Ricci Scalar
We determine the gravitational response to a diffuse source, in a locally de
Sitter background, of a class of theories which modify the Einstein-Hilbert
action by adding a term proportional to an inverse power of the Ricci scalar.
We find a linearly growing force which is not phenomenologically acceptable.Comment: 10 pages, LaTeX 2 epsilon, no figures. Minor conceptual change
A generic problem with purely metric formulations of MOND
We give a simple argument to show that no purely metric-based, relativistic
formulation of Milgrom's Modified Newtonian Dynamics (MOND) whose energy
functional is stable (in the sense of being quadratic in perturbations) can be
consistent with the observed amount of gravitational lensing from galaxies. An
important part of the argument is the fact that reproducing the MOND force law
requires any completely stable, metric-based theory of gravity to become
conformally invariant in the weak field limit. We discuss the prospects for a
formulation with a very weak instability.Comment: 4 pages, revtex4, no figure
Marketing in developing countries : an audit on the state of marketing activities and marketing orientation
According to the two schools of thought on marketing and economic development, marketing can either spur economic development or be spurred by it. As such, there is a hypothesized link between economic development and the level of marketing orientation and activities in developing countries. This thesis aims to determine the level of marketing orientation and marketing activities in developing countries and seeks to investigate the correlation between these levels and the variables that the literature argues are key for economic development. To this end, a model is created based on the literature on economic development and marketing in developing countries--with the independent variables resembling [what is known in emerging market circles as] the 'Washington Consensus'. A survey is carried out using Kotlers' Marketing Audit as the investigative tool and sent to members of the American Marketing Association in developing countries. The data is analyzed by means of factor analysis and multivariate regression analysis. Descriptive statistics and a summary of the Marketing Audit results provide insight into the state of marketing in developing countries. Evidence was found supporting the hypothesis that the variables linked to the economic development of developing countries may also be used to predict the level of marketing orientation and activities in these countries. The implications of this research for academics and practitioners are discussed. Future research suggestions are also provided
The Axial Anomaly in D=3+1 Light-Cone QED
We consider -dimensional, Dirac electrons of arbitrary mass,
propagating in the presence of electric and magnetic fields which are both
parallel to the axis. The magnetic field is constant in space and time
whereas the electric field depends arbitrarily upon the light-cone time
parameter . We present an explicit solution to the
Heisenberg equations for the electron field operator in this background. The
electric field results in the creation of electron-positron pairs. We compute
the expectation values of the vector and axial vector currents in the presence
of a state which is free vacuum at . Both current conservation and the
standard result for the axial vector anomaly are verified for the first time
ever in -dimensional light-cone QED. An interesting feature of our
operator solution is the fact that it depends in an essential way upon
operators from the characteristic at , in addition to the usual
dependence upon operators at . This dependence survives even in the
limit of infinite . Ignoring the operators leads to a progressive loss
of unitarity, to the violation of current conservation, to the loss of
renormalizability, and to an incorrect result for the axial vector anomaly.Comment: 31 pages, LaTeX 2 epsilon, no figures, some typoes corrected for
publicatio
Gravitational waves and lensing of the metric theory proposed by Sobouti
We investigate in detail two physical properties of the metric f(R) theory
developed by Sobouti (2007). We first look for the possibility of producing
gravitational waves that travel at the speed of light. We then check the
possibility of producing extra bending in the lenses produced by the theory. We
do this by using standard weak field approximations to the gravitational field
equations that appear in Sobouti's theory. We show in this article that the
metric theory of gravitation proposed by Sobouti (2007) predicts the existence
of gravitational waves travelling at the speed of light in vacuum. In fact,
this is proved in general terms for all metric theories of gravity which can be
expressed as powers of Ricci's scalar. We also show that an extra additional
lensing as compared to the one predicted by standard general relativity is
produced. These two points are generally considered to be of crucial importance
in the development of relativistic theories of gravity that could provide an
alternative description to the dark matter paradigm.Comment: 10 pages, 2 figures. Added a comment on the recent article by Saffari
(arXiv:0704.3345v1) and small typos as well as general comments in the
introuduction and conclusio
A Generic Test of Modified Gravity Models which Emulate Dark Matter
We propose a generic test for models in which gravity is modified to do away
with dark matter. These models tend to have gravitons couple to a different
metric than ordinary matter. A strong test of such models comes from comparing
the arrival time of the gravitational wave pulse from a cosmological event such
as a supernova with the arrival times of the associated pulses of neutrinos and
photons. For SN 1987a we show that the gravity wave would have arrived 5.3 days
after the neutrino pulse.Comment: 4 pages, uses RevTex, no figure
On the Newtonian limit of Generalized Modified Gravity Models
We consider the Newtonian limit of modified theories of gravity that include
inverse powers of the curvature in the action in order to explain the cosmic
acceleration. It has been shown that the simplest models of this kind are in
conflict with observations at the solar system level. In this letter we point
out that when one adds to the action inverse powers of curvature invariants
that do not vanish for the Schwarzschild geometry one generically recovers an
acceptable Newtonian limit at small distances. Gravity is however modified at
large distances. We compute the first correction to the Newtonian potential in
a quite general class of models. The characteristic distance entering in these
modifications is of the order of 10pc for the Sun and of the order of 10^2 kpc
for a galaxy.Comment: 7 pages, late
f(R) Gravities \`a la Brans-Dicke
We extend f(R) theories via the addition of a fundamental scalar field. The
approach is reminiscent of the dilaton field of string theory and the
Brans-Dicke model. f(R) theories attracted much attention recently in view of
their potential to explain the acceleration of the universe. Extending f(R)
models to theories with scalars can be motivated from the low energy effective
action of string theory. There, a fundamental scalar (the dilaton), has a
non-minimal coupling to the Ricci scalar. Furthermore beyond tree level actions
will contain terms having higher (or lower) powers of R compared to the
canonical Einstein-Hilbert term. Theories with f(R) will contain an extra
scalar degree on top of the ad-hoc dilaton and mixing of these two modes around
a stable solution is a concern. In this work we show that no mixing condition
mandates the form for the action
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