66,115 research outputs found
Hamilton-Jacobi Approach for Power-Law Potentials
The classical and relativistic Hamilton-Jacobi approach is applied to the
one-dimensional homogeneous potential, , where and
are continuously varying parameters. In the non-relativistic case, the
exact analytical solution is determined in terms of , and the total
energy . It is also shown that the non-linear equation of motion can be
linearized by constructing a hypergeometric differential equation for the
inverse problem . A variable transformation reducing the general problem
to that one of a particle subjected to a linear force is also established. For
any value of , it leads to a simple harmonic oscillator if , an
"anti-oscillator" if , or a free particle if E=0. However, such a
reduction is not possible in the relativistic case. For a bounded relativistic
motion, the first order correction to the period is determined for any value of
. For , it is found that the correction is just twice that one
deduced for the simple harmonic oscillator (), and does not depend on the
specific value of .Comment: 12 pages, Late
Cosmographic constraints on a class of Palatini f(R) gravity
Modified gravity, known as gravity, has presently been applied to
Cosmology as a realistic alternative to dark energy. For this kind of gravity
the expansion of the Universe may accelerate while containing only baryonic and
cold dark matter. The aim of the present investigation is to place cosmographic
constraints on the class of theories of the form within
the Palatini approach. Although extensively discussed in recent literature and
confronted with several observational data sets, cosmological tests are indeed
inconclusive about the true signal of in this class of theories. This is
particularly important to define which kind of corrections (infra-red or
high-energy) to general relativity this class of theory indeed represent. We
shed some light on this question by examining the evolution of the deceleration
parameter for these theories. We find that for a large range of
, models based on gravity in the Palatini
approach can only have positive values for , placing thus a broad
restriction on this class of gravity.Comment: 4 pages, 2 figures, Latex, Submitte
Is CDM an effective CCDM cosmology?
We show that a cosmology driven by gravitationally induced particle
production of all non-relativistic species existing in the present Universe
mimics exactly the observed flat accelerating CDM cosmology with just
one dynamical free parameter. This kind of scenario includes the creation cold
dark matter (CCDM) model [Lima, Jesus & Oliveira, JCAP 011(2010)027] as a
particular case and also provides a natural reduction of the dark sector since
the vacuum component is not needed to accelerate the Universe. The new cosmic
scenario is equivalent to CDM both at the background and perturbative
levels and the associated creation process is also in agreement with the
universality of the gravitational interaction and equivalence principle.
Implicitly, it also suggests that the present day astronomical observations
cannot be considered the ultimate proof of cosmic vacuum effects in the evolved
Universe because CDM may be only an effective cosmology.Comment: 6 pages, 2 figures, changes in the abstract, introduction, new
references and typo correction
New coupled quintessence cosmology
A component of dark energy has been recently proposed to explain the current
acceleration of the Universe. Unless some unknown symmetry in Nature prevents
or suppresses it, such a field may interact with the pressureless component of
dark matter, giving rise to the so-called models of coupled quintessence. In
this paper we propose a new cosmological scenario where radiation and baryons
are conserved, while the dark energy component is decaying into cold dark
matter (CDM). The dilution of CDM particles, attenuated with respect to the
usual scaling due to the interacting process, is characterized by a
positive parameter , whereas the dark energy satisfies the equation
of state (). We carry out a joint statistical
analysis involving recent observations from type Ia supernovae, baryon acoustic
oscillation peak, and Cosmic Microwave Background shift parameter to check the
observational viability of the coupled quintessence scenario here proposed.Comment: 7 pages, 7 figures. Minor corrections to match published versio
Pair correlation function of short-ranged square-well fluids
We have performed extensive Monte Carlo simulations in the canonical (NVT)
ensemble of the pair correlation function for square-well fluids with well
widths ranging from 0.1 to 1.0, in units of the diameter
of the particles. For each one of these widths, several densities and
temperatures in the ranges and
, where is the
critical temperature, have been considered. The simulation data are used to
examine the performance of two analytical theories in predicting the structure
of these fluids: the perturbation theory proposed by Tang and Lu [Y. Tang and
B. C.-Y. Lu, J. Chem. Phys. {\bf 100}, 3079, 6665 (1994)] and the
non-perturbative model proposed by two of us [S. B. Yuste and A. Santos, J.
Chem. Phys. {\bf 101}, 2355 (1994)]. It is observed that both theories
complement each other, as the latter theory works well for short ranges and/or
moderate densities, while the former theory does for long ranges and high
densities.Comment: 10 pages, 10 figure
Comment on "Theory and computer simulation for the equation of state of additive hard-disk fluid mixtures"
A flaw in the comparison between two different theoretical equations of state
for a binary mixture of additive hard disks and Monte Carlo results, as
recently reported in C. Barrio and J. R. Solana, Phys. Rev. E 63, 011201
(2001), is pointed out. It is found that both proposals, which require the
equation of state of the single component system as input, lead to comparable
accuracy but the one advocated by us [A. Santos, S. B. Yuste, and M. L\'{o}pez
de Haro, Mol. Phys. 96, 1 (1999)] is simpler and complies with the exact limit
in which the small disks are point particles.Comment: 4 pages, including 1 figur
Nonlinear viscosity and velocity distribution function in a simple longitudinal flow
A compressible flow characterized by a velocity field is
analyzed by means of the Boltzmann equation and the Bhatnagar-Gross-Krook
kinetic model. The sign of the control parameter (the longitudinal deformation
rate ) distinguishes between an expansion () and a condensation ()
phenomenon. The temperature is a decreasing function of time in the former
case, while it is an increasing function in the latter. The non-Newtonian
behavior of the gas is described by a dimensionless nonlinear viscosity
, that depends on the dimensionless longitudinal rate . The
Chapman-Enskog expansion of in powers of is seen to be only
asymptotic (except in the case of Maxwell molecules). The velocity distribution
function is also studied. At any value of , it exhibits an algebraic
high-velocity tail that is responsible for the divergence of velocity moments.
For sufficiently negative , moments of degree four and higher may diverge,
while for positive the divergence occurs in moments of degree equal to or
larger than eight.Comment: 18 pages (Revtex), including 5 figures (eps). Analysis of the heat
flux plus other minor changes added. Revised version accepted for publication
in PR
On FRW Model in Conformal Teleparallel Gravity
In this paper we use the conformal teleparallel gravity to study an isotropic
and homogeneous Universe which is settled by the FRW metric. We solve the field
equations and we obtain the behavior of some cosmological parameters such as
scale factor, deceleration parameter and the energy density of the perfect
fluid which is the matter field of our model. The field equations, that we
called modified Friedmann equations, allow us to define a dark fluid, with dark
energy density and dark pressure, responsible for the acceleration in the
Universe.Comment: Accepted in EPJ
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