1,545 research outputs found
Global Monopole in Palatini f(R) gravity
We consider the space-time metric generated by a global monopole in an
extension of General Relativity (GR) of the form
. The theory is formulated in
the metric-affine (or Palatini) formalism and exact analytical solutions are
obtained. For , one finds that the solution has the same
characteristics as the Schwarzschild black hole with a monopole charge in
Einstein's GR. For , instead, the metric is more closely related to
the Reissner-Nordstr\"{o}m metric with a monopole charge and, in addition, it
possesses a wormhole-like structure that allows for the geodesic completeness
of the space-time. Our solution recovers the expected limits when
and also at the asymptotic far limit. The angular deflection of light in this
spacetime in the weak field regime is also calculated.Comment: 15 pages, version accepted to PR
Nonlinear -models in the Eddington-inspired Born-Infeld Gravity
In this paper we consider two different nonlinear -models minimally
coupled to Eddington-inspired Born-Infeld gravity. We show that the resultant
geometries represent minimal modifications with respect to those found in GR,
though with important physical consequences. In particular, wormhole structures
always arise, though this does not guarantee by itself the geodesic
completeness of those space-times. In one of the models, quadratic in the
canonical kinetic term, we identify a subset of solutions which are regular
everywhere and are geodesically complete. We discuss characteristic features of
these solutions and their dependence on the relationship between mass and
global charge.Comment: 19 pages, 5 figure
Maghemite Nanofluid Based on Natural Ester: Cooling and Insulation Properties Assessment
The objective of this work is to study the effect that the addition of magnetic nanoparticles to a natural ester has on its properties and its cooling capacity. Some samples of ferrofluid (natural ester with maghemite) have been prepared using different concentrations. These have been characterized by measuring their thermo-hydraulic and dielectric properties, to find an optimal concentration. Then, the cooling capacities of the optimal nanofluid and the base fluid have been tested in a transformer immersed in these liquids. The experimental platform allowed the measurement of temperatures in different locations at different load levels. Parallel simulations of these tests have been carried out with a Computational Fluid Dynamics model of the experimental platform. The results show an improvement of the insulating capacity of the base fluid with the addition of maghemite nanoparticles, and an enhanced cooling capacity.
The nearly Newtonian regime in Non-Linear Theories of Gravity
The present paper reconsiders the Newtonian limit of models of modified
gravity including higher order terms in the scalar curvature in the
gravitational action. This was studied using the Palatini variational principle
in [Meng X. and Wang P.: Gen. Rel. Grav. {\bf 36}, 1947 (2004)] and
[Dom\'inguez A. E. and Barraco D. E.: Phys. Rev. D {\bf 70}, 043505 (2004)]
with contradicting results. Here a different approach is used, and problems in
the previous attempts are pointed out. It is shown that models with negative
powers of the scalar curvature, like the ones used to explain the present
accelerated expansion, as well as their generalization which include positive
powers, can give the correct Newtonian limit, as long as the coefficients of
these powers are reasonably small. Some consequences of the performed analysis
seem to raise doubts for the way the Newtonian limit was derived in the purely
metric approach of fourth order gravity [Dick R.: Gen. Rel. Grav. {\bf 36}, 217
(2004)]. Finally, we comment on a recent paper [Olmo G. J.: Phys. Rev. D {\bf
72}, 083505 (2005)] in which the problem of the Newtonian limit of both the
purely metric and the Palatini formalism is discussed, using the equivalent
Brans--Dicke theory, and with which our results partly disagree.Comment: typos corrected, replaced to match published versio
The cosmic snap parameter in f(R) gravity
We derive the expression for the snap parameter in f(R) gravity. We use the
Palatini variational principle to obtain the field equations and regard the
Einstein conformal frame as physical. We predict the present-day value of the
snap parameter for the particular case f(R)=R-const/R, which is the simplest
f(R) model explaining the current acceleration of the universe.Comment: 9 pages; published versio
Torsion and accelerating expansion of the universe in quadratic gravitation
Several exact cosmological solutions of a metric-affine theory of gravity
with two torsion functions are presented. These solutions give a essentially
different explanation from the one in most of previous works to the cause of
the accelerating cosmological expansion and the origin of the torsion of the
spacetime. These solutions can be divided into two classes. The solutions in
the first class define the critical points of a dynamical system representing
an asymptotically stable de Sitter spacetime. The solutions in the second class
have exact analytic expressions which have never been found in the literature.
The acceleration equation of the universe in general relativity is only a
special case of them. These solutions indicate that even in vacuum the
spacetime can be endowed with torsion, which means that the torsion of the
spacetime has an intrinsic nature and a geometric origin. In these solutions
the acceleration of the cosmological expansion is due to either the scalar
torsion or the pseudoscalar torsion function. Neither a cosmological constant
nor dark energy is needed. It is the torsion of the spacetime that causes the
accelerating expansion of the universe in vacuum. All the effects of the
inflation, the acceleration and the phase transformation from deceleration to
acceleration can be explained by these solutions. Furthermore, the energy and
pressure of the matter without spin can produce the torsion of the spacetime
and make the expansion of the universe decelerate as well as accelerate.Comment: 20 pages. arXiv admin note: text overlap with gr-qc/0604006,
arXiv:1110.344
Static quantum corrections to the Schwarzschild spacetime
We study static quantum corrections of the Schwarzschild metric in the
Boulware vacuum state. Due to the absence of a complete analytic expression for
the full semiclassical Einstein equations we approach the problem by
considering the s-wave approximation and solve numerically the associated
backreaction equations. The solution, including quantum effects due to pure
vacuum polarization, is similar to the classical Schwarzschild solution up to
the vicinity of the classical horizon. However, the radial function has a
minimum at a time-like surface close to the location of the classical event
horizon. There the g_{00} component of the metric reaches a very small but
non-zero value. The analysis unravels how a curvature singularity emerges
beyond this bouncing point. We briefly discuss the physical consequences of
these results by extrapolating them to a dynamical collapsing scenario.Comment: 10 pages; Talk given at QG05, Cala Gonone (Italy), September 200
Two-point functions with an invariant Planck scale and thermal effects
Nonlinear deformations of relativistic symmetries at the Planck scale are
usually addressed in terms of modified dispersion relations. We explore here an
alternative route by directly deforming the two-point functions of an
underlying field theory. The proposed deformations depend on a length parameter
(Planck length) and preserve the basic symmetries of the corresponding theory.
We also study the physical consequences implied by these modifications at the
Planck scale by analyzing the response function of an accelerated detector in
Minkowski space, an inertial one in de Sitter space, and also in a black hole
spacetime.Comment: 12 pages, no figure
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