3,712 research outputs found
General models of Einstein gravity with a non-Newtonian weak-field limit
We investigate Einstein theories of gravity, coupled to a scalar field \vphi
and point-like matter, which are characterized by a scalar field-dependent
matter coupling function e^{H(\vphi)}. We show that under mild constraints on
the form of the potential for the scalar field, there are a broad class of
Einstein-like gravity models -characterized by the asymptotic behavior of H-
which allow for a non-Newtonian weak-field limit with the gravitational
potential behaving for large distances as ln r. The Newtonian term GM/r appears
only as sub-leading. We point out that this behavior is also shared by gravity
models described by f(R) Lagrangians. The relevance of our results for the
building of infrared modified theories of gravity and for modified Newtonian
dynamics is also discussed.Comment: 9 page
Didáctica básica para la construcción de la esfera basada en la estructura del cubo: una lección elemental en la enseñanza aprendizaje de los sólidos para todos los estudiantes de dibujo
El objetivo de este artículo es explicar un
proceso de dibujo que involucra el estudio de los elementos
estructurales de la esfera.
Es un planteamiento dirigido a los estudiantes de los
cursos de dibujo quienes deben aprender a percibir las
cualidades volumétricas del objeto real y representarlo
por medio de los elementos estructurales intrínsecos.
Se definen dos maneras de abordar la representación
de la esfera, una para estudiantes novatos, y otra para
estudiantes avanzados. Se repasa la estructura del cubo,
y los conceptos básicos de la perspectiva, el método
de visado, el dominio de las proporciones del cubo, la
relación de los ángulos del cubo respecto de los lados,
la estructura portante, los elementos básicos de la estructura
del cubo. Se estudia además la construcción
de las elipses en los tres planos cartesianos, y las diferencias
de calidad tonal que se deben de utilizar. Todos
estos procedimientos se enumeran en el presente artículo,
explicados paso a paso, por medio de ilustraciones.
También, se muestran varios ejemplos de la esfera en
posiciones sencillas y extremas que demuestran la variación
de los ejes básicos de acuerdo a su posición y
dirección en el espaci
Precise Measurements of the Kilohertz Quasi-Periodic Oscillations in 4U 1728-34
We have analyzed seventeen observations of the low-mass X-ray binary and
atoll source 4U 1728-34, carried out by the Rossi X-ray Timing Explorer in 1996
and 1997. We obtain precise measurements of the frequencies of the two
simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source.
We show that the frequency separation between the two QPO, , is
always significantly smaller than the frequency of the nearly-coherent
oscillations seen in this source during X-ray bursts, even at the lowest
inferred mass accretion rate, when seems to reach its maximum
value. We also find that decreases significantly, from Hz to Hz, as the frequency of the lower frequency kHz QPO
increases from 615 to 895 Hz. This is the first time that variations of the kHz
QPO peak separation are measured in a source which shows nearly-coherent
oscillations during bursts.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses
AAS LaTex v4.0 (5 pages plus 4 postscript figures
Statistical entropy of the Schwarzschild black hole
We derive the statistical entropy of the Schwarzschild black hole by
considering the asymptotic symmetry algebra near the boundary of
the spacetime at past null infinity. Using a two-dimensional description and
the Weyl invariance of black hole thermodynamics this symmetry algebra can be
mapped into the Virasoro algebra generating asymptotic symmetries of anti-de
Sitter spacetime. Using lagrangian methods we identify the stress-energy tensor
of the boundary conformal field theory and we calculate the central charge of
the Virasoro algebra. The Bekenstein-Hawking result for the black hole entropy
is regained using Cardy's formula. Our result strongly supports a non-local
realization of the holographic principleComment: 3 pages no figure
Effect of nonadiabatic switching of dynamic perturbations in 1d Fermi systems
We study a two-dimensional fermionic QFT used to model 1D strongly correlated
electrons in the presence of a time-dependent impurity that drives the system
out of equilibrium. In contrast to previous investigations, we consider a
dynamic barrier switched on at a finite time. We compute the total energy
density (TED) of the system and establish two well defined regimes in terms of
the relationship between the frequency of the time-dependent perturbation
and the electron energy . Finally, we derive a relaxation time
such that for times shorter than the finite-time switching
process is relevant.Comment: 9 pages, 4 figures. Changed title. Added comments on backscattering.
Added result for electrical current. Version accepted in PR
Saturation effect for dependence of the electrical conductivity of planar oriented nematic liquid crystal 6CB on the concentration of Cu7PS6 nanoparticles
The influence of Cu7PS6 nanoparticles with the average size 117 nm on the dielectric properties of planar oriented nematic liquid crystal 6CB has been investigated within the frequency range 10(1) ...10(6) Hz and at the temperature 293 K. It has been shown that when changing the concentration of nanoparticles within the range 0 to 1 wt.%, the conductivity of the liquid crystal changes stronger than its dielectric permittivity. It has been shown that the electrical conductivity increases monotonously with increasing the concentration of nanoparticles. However, for this dependence a saturation effect is observed. The mechanism of this effect was proposed.info:eu-repo/semantics/publishedVersio
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