73 research outputs found
Gravitational Lensing and f(R) theories in the Palatini approach
We investigate gravitational lensing in the Palatini approach to the f(R)
extended theories of gravity. Starting from an exact solution of the f(R) field
equations, which corresponds to the Schwarzschild-de Sitter metric and, on the
basis of recent studies on this metric, we focus on some lensing observables,
in order to evaluate the effects of the non linearity of the gravity
Lagrangian. We give estimates for some astrophysical events, and show that
these effects are tiny for galactic lenses, but become interesting for
extragalactic ones.Comment: 7 Pages, RevTex, 1 eps figure; references added; revised to match the
version accepted for publication in General Relativity and Gravitatio
Classical Euclidean wormhole solutions in Palatini cosmology
We study the classical Euclidean wormholes in the context of extended
theories of gravity. With no loss of generality, we use the dynamical
equivalence between gravity and scalar-tensor theories to
construct a point-like Lagrangian in the flat FRW space time. We first show the
dynamical equivalence between Palatini gravity and the
Brans-Dicke theory with self-interacting potential, and then show the dynamical
equivalence between the Brans-Dicke theory with self-interacting potential and
the minimally coupled O'Hanlon theory. We show the existence of new Euclidean
wormhole solutions for this O'Hanlon theory and, for an special case, find out
the corresponding form of having wormhole solution. For small
values of the Ricci scalar, this is in agreement with the
wormhole solution obtained for higher order gravity theory .Comment: 11 page
One-loop f(R) gravity in de Sitter universe
Motivated by the dark energy issue, the one-loop quantization approach for a
family of relativistic cosmological theories is discussed in some detail.
Specifically, general gravity at the one-loop level in a de Sitter
universe is investigated, extending a similar program developed for the case of
pure Einstein gravity. Using generalized zeta regularization, the one-loop
effective action is explicitly obtained off-shell, what allows to study in
detail the possibility of (de)stabilization of the de Sitter background by
quantum effects. The one-loop effective action maybe useful also for the study
of constant curvature black hole nucleation rate and it provides the plausible
way of resolving the cosmological constant problem.Comment: 25 pages, Latex file. Discussion enlarged, new references added.
Version accepted in JCA
Equilibrium hydrostatic equation and Newtonian limit of the singular f(R) gravity
We derive the equilibrium hydrostatic equation of a spherical star for any
gravitational Lagrangian density of the form . The Palatini
variational principle for the Helmholtz Lagrangian in the Einstein gauge is
used to obtain the field equations in this gauge. The equilibrium hydrostatic
equation is obtained and is used to study the Newtonian limit for
. The same procedure is carried out for the more
generally case giving a good
Newtonian limit.Comment: Revised version, to appear in Classical and Quantum Gravity
Accelerated Cosmological Models in Ricci squared Gravity
Alternative gravitational theories described by Lagrangians depending on
general functions of the Ricci scalar have been proven to give coherent
theoretical models to describe the experimental evidence of the acceleration of
universe at present time. In this paper we proceed further in this analysis of
cosmological applications of alternative gravitational theories depending on
(other) curvature invariants. We introduce Ricci squared Lagrangians in minimal
interaction with matter (perfect fluid); we find modified Einstein equations
and consequently modified Friedmann equations in the Palatini formalism. It is
striking that both Ricci scalar and Ricci squared theories are described in the
same mathematical framework and both the generalized Einstein equations and
generalized Friedmann equations have the same structure. In the framework of
the cosmological principle, without the introduction of exotic forms of dark
energy, we thus obtain modified equations providing values of w_{eff}<-1 in
accordance with the experimental data. The spacetime bi-metric structure plays
a fundamental role in the physical interpretation of results and gives them a
clear and very rich geometrical interpretation.Comment: New version: 26 pages, 1 figure (now included), Revtex
A covariant formalism for Chern-Simons gravity
Chern--Simons type Lagrangians in dimensions are analyzed from the
point of view of their covariance and globality. We use the transgression
formula to find out a new fully covariant and global Lagrangian for
Chern--Simons gravity: the price for establishing globality is hidden in a
bimetric (or biconnection) structure. Such a formulation allows to calculate
from a global and simpler viewpoint the energy-momentum complex and the
superpotential both for Yang--Mills and gravitational examples.Comment: 12 pages,LaTeX, to appear in Journal of Physics
Consistent modified gravity: dark energy, acceleration and the absence of cosmic doomsday
We discuss the modified gravity which includes negative and positive powers
of the curvature and which provides the gravitational dark energy. It is shown
that in GR plus the term containing negative power of the curvature the cosmic
speed-up may be achieved, while the effective phantom phase (with less than
-1) follows when such term contains the fractional positive power of the
curvature. The minimal coupling with matter makes the situation more
interesting: even 1/R theory coupled with the usual ideal fliud may describe
the (effective phantom) dark energy. The account of term (consistent
modified gravity) may help to escape of cosmic doomsday.Comment: LaTeX file, 9 pages, based on the talk given by S.D. Odintsov (Int.
Conference Mathematical Methods in Physics, Rio de Janeiro, Augest, 2004), to
appear in CQG, Letter
Transdermal delivery of probenecid: the effects of vehicles and enhancers on permeation through pig skin
Vehicles and enhancers effect on in vitro probenecid permeation through dermatomed abdominal
pig skin was investigated. The permeability of different probenecid percentages dispersed in vehicles
as vaseline, carbopol/ethanol/water and carbopol/propylene glycol was tested. The 1.3% L-menthol
addition, as permeation enhancer, over probenecid/vaseline formulations showed the highest values for
both, flux and permeation coefficient. Permeation experiments of the probenecid formulations in carbopol/
propylene glycol showed that the carbopol/probenecid concentration relation is the most important
issue to be considered. Comparatively to lipophilic vehicle (vaseline), carbopol dispersions seen to be more
convenient as vehicle for topical administration of probenecid. The results obtained from this study may
be helpful in the development of a probenecid transdermal drug delivery system.Colegio de Farmacéuticos de la Provincia de Buenos Aire
New binary solid dispersion of indomethacin and croscarmellose sodium: physical characterization and in vitro dissolution enhancement
Solid dispersions (SDx) containing Indomethacin (IND), a poorly water-soluble drug, and the disintegrant excipient sodium croscarmellose (SC) were prepared by a co-drying method and characterized by Infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). An FT-IR analysis performed on IND-SC solid dispersion and their physical mixtures indicated that IND does not interact with SC in the solid state. An analysis of the information produced by
DSC, XRD, and SEM confirmed that the crystalline α-form of IND was homogeneously incorporated into SDx. IND release from SDx was significantly greater than that from its corresponding physical mixtures with
the high homogeneous molecular dispersion and the crystalline modification of IND appearing to be the cause. This behavior may have a beneficial effect on the biopharmaceutical performance of this drug
Hamiltonian, Energy and Entropy in General Relativity with Non-Orthogonal Boundaries
A general recipe to define, via Noether theorem, the Hamiltonian in any
natural field theory is suggested. It is based on a Regge-Teitelboim-like
approach applied to the variation of Noether conserved quantities. The
Hamiltonian for General Relativity in presence of non-orthogonal boundaries is
analysed and the energy is defined as the on-shell value of the Hamiltonian.
The role played by boundary conditions in the formalism is outlined and the
quasilocal internal energy is defined by imposing metric Dirichlet boundary
conditions. A (conditioned) agreement with previous definitions is proved. A
correspondence with Brown-York original formulation of the first principle of
black hole thermodynamics is finally established.Comment: 29 pages with 1 figur
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