718 research outputs found
On analytical solutions of f(R) modified gravity theories in FLRW cosmologies
A novel analytical method for f(R) modified theories without matter in
Friedmann-Lemaitre-Robertson-Walker spacetimes is introduced. The equation of
motion for the scale factor in terms of cosmic time is reduced to the equation
for the evolution of the Ricci scalar R with the Hubble parameter H. The
solution of equation of motion for actions of the form of power law in Ricci
scalar R, is presented with a detailed elaboration of the action quadratic in
R. The reverse use of the introduced method is exemplified in finding
functional forms f(R) which lead to specified scale factor functions. The
analytical solutions are corroborated by numerical calculations with excellent
agreement. Possible further applications to the phases of inflationary
expansion and late-time acceleration as well as f(R) theories with radiation
are outlined.Comment: 16 pages, 6 figures. v2: minor changes, references added. v3: minor
changes, more references added. v4: version to appear in IJMPD. v5: DOI and
journal reference adde
f(R) Cosmology from q-Theory
From a macroscopic theory of the quantum vacuum in terms of conserved
relativistic charges (generically denoted by q^{(a)} with label a), we have
obtained, in the low-energy limit, a particular type of f(R) model relevant to
cosmology. The macroscopic quantum-vacuum theory allows us to distinguish
between different phenomenological f(R) models on physical grounds.Comment: 6 pages, v4: this archive version corresponds to the published
version with a few minor correction
A Unified Approach to Variational Derivatives of Modified Gravitational Actions
Our main aim in this paper is to promote the coframe variational method as a
unified approach to derive field equations for any given gravitational action
containing the algebraic functions of the scalars constructed from the Riemann
curvature tensor and its contractions. We are able to derive a master equation
which expresses the variational derivatives of the generalized gravitational
actions in terms of the variational derivatives of its constituent curvature
scalars. Using the Lagrange multiplier method relative to an orthonormal
coframe, we investigate the variational procedures for modified gravitational
Lagrangian densities in spacetime dimensions . We study
well-known gravitational actions such as those involving the Gauss-Bonnet and
Ricci-squared, Kretchmann scalar, Weyl-squared terms and their algebraic
generalizations similar to generic theories and the algebraic
generalization of sixth order gravitational Lagrangians. We put forth a new
model involving the gravitational Chern-Simons term and also give three
dimensional New massive gravity equations in a new form in terms of the Cotton
2-form
Quantifying shear-induced wave transformations in the solar wind
The possibility of velocity shear-induced linear transformations of different
magnetohydrodynamic waves in the solar wind is studied both analytically and
numerically. A quantitative analysis of the wave transformation processes for
all possible plasma- regimes is performed. By applying the obtained
criteria for effective wave coupling to the solar wind parameters, we show that
velocity shear-induced linear transformations of Alfv\'en waves into
magneto-acoustic waves could effectively take place for the relatively
low-frequency Alfv\'en waves in the energy containing interval. The obtained
results are in a good qualitative agreement with the observed features of
density perturbations in the solar wind.Comment: Astrophysical Journal (accepted
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom
Model for screening of resonant magnetic perturbations by plasma in a realistic tokamak geometry and its impact on divertor strike points
This work addresses the question of the relation between strike-point
splitting and magnetic stochasticity at the edge of a poloidally diverted
tokamak in the presence of externally imposed magnetic perturbations. More
specifically, ad-hoc helical current sheets are introduced in order to mimic a
hypothetical screening of the external resonant magnetic perturbations by the
plasma. These current sheets, which suppress magnetic islands, are found to
reduce the amount of splitting expected at the target, which suggests that
screening effects should be observable experimentally. Multiple screening
current sheets reinforce each other, i.e. less current relative to the case of
only one current sheet is required to screen the perturbation.Comment: Accepted in the Proceedings of the 19th International Conference on
Plasma Surface Interactions, to be published in Journal of Nuclear Materials.
Version 2: minor formatting and text improvements, more results mentioned in
the conclusion and abstrac
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