86 research outputs found
Universal behavior in complex-mediated reactions: Dynamics of S(1D)+ o-D2 --> D + SD at low collision energies
Reactive and elastic cross-sections, and rate coefficients, have been
calculated for the S(1D)+ D2 (v=0, j=0) reaction using a modified
hyperspherical quantum reactive scattering method. The considered collision
energy ranges from the ultracold regime, where only one partial wave is open,
up to the Langevin regime, where many of them contribute. This work presents
the extension of the quantum calculations, which were compared with the
experimental results in a previous work, down to energies in the cold and
ultracold domains. Results are analyzed and compared with the universal case of
the quantum defect theory by Jachymski et al. [Phys. Rev. Lett. 110, 213202
(2013)]. State-to-state integral and differential cross sections are also shown
covering the ranges of low-thermal, cold and ultracold collision energy
regimes. It is found that at E/k_B T < 1 K there are substantial departures
from the expected statistical behavior, and that dynamical features become
increasingly important with decreasing collision energy, leading to vibrational
excitation.Comment: Submitted to Journal of Chemical Physic
Beyond universality: parametrizing ultracold complex-mediated reactions using statistical assumptions
We have calculated accurate quantum reactive and elastic cross-sections for
the prototypical barrierless reaction D + H(=0, =0) using the
hyperspherical scattering method. The considered kinetic energy ranges from the
ultracold to the Langevin regimes. The availability of accurate results for
this system allows to test the quantum theory by Jachymski et al. [Phys. Rev.
Lett. 110, 213202 (2013)] in a nonuniversal case. The short range reaction
probability is rationalized using statistical model assumptions and related to
a statistical factor. This provides a means to estimate one of the parameters
that characterizes ultracold processes from first principles. Possible
limitations of the statistical model are considered
From geodesics of the multipole solutions to the perturbed Kepler problem
A static and axisymmetric solution of the Einstein vacuum equations with a
finite number of Relativistic Multipole Moments (RMM) is written in MSA
coordinates up to certain order of approximation, and the structure of its
metric components is explicitly shown. From the equation of equatorial
geodesics we obtain the Binet equation for the orbits and it allows us to
determine the gravitational potential that leads to the equivalent classical
orbital equations of the perturbed Kepler problem. The relativistic corrections
to Keplerian motion are provided by the different contributions of the RMM of
the source starting from the Monopole (Schwarzschild correction). In
particular, the perihelion precession of the orbit is calculated in terms of
the quadrupole and 2-pole moments. Since the MSA coordinates generalize the
Schwarzschild coordinates, the result obtained allows measurement of the
relevance of the quadrupole moment in the first order correction to the
perihelion frequency-shift
Exterior Differential System for Cosmological G2 Perfect Fluids and Geodesic Completeness
In this paper a new formalism based on exterior differential systems is
derived for perfect-fluid spacetimes endowed with an abelian orthogonally
transitive G2 group of motions acting on spacelike surfaces. This formulation
allows simplifications of Einstein equations and it can be applied for
different purposes. As an example a singularity-free metric is rederived in
this framework. A sufficient condition for a diagonal metric to be geodesically
complete is also provided.Comment: 27 pages, 0 figures, LaTeX2e, to be published in Classical and
Quantum Gravit
Reconstruction of the equation of state for the cyclic universes in homogeneous and isotropic cosmology
We study the cosmological evolutions of the equation of state (EoS) for the
universe in the homogeneous and isotropic
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) space-time. In particular, we
reconstruct the cyclic universes by using the Weierstrass and Jacobian elliptic
functions. It is explicitly illustrated that in several models the universe
always stays in the non-phantom (quintessence) phase, whereas there also exist
models in which the crossing of the phantom divide can be realized in the
reconstructed cyclic universes.Comment: 29 pages, 8 figures, version accepted for publication in Central
European Journal of Physic
Screening of cosmological constant for De Sitter Universe in non-local gravity, phantom-divide crossing and finite-time future singularities
We investigate de Sitter solutions in non-local gravity as well as in
non-local gravity with Lagrange constraint multiplier. We examine a condition
to avoid a ghost and discuss a screening scenario for a cosmological constant
in de Sitter solutions. Furthermore, we explicitly demonstrate that three types
of the finite-time future singularities can occur in non-local gravity and
explore their properties. In addition, we evaluate the effective equation of
state for the universe and show that the late-time accelerating universe may be
effectively the quintessence, cosmological constant or phantom-like phases. In
particular, it is found that there is a case in which a crossing of the phantom
divide from the non-phantom (quintessence) phase to the phantom one can be
realized when a finite-time future singularity occurs. Moreover, it is
demonstrated that the addition of an term can cure the finite-time future
singularities in non-local gravity. It is also suggested that in the framework
of non-local gravity, adding an term leads to possible unification of the
early-time inflation with the late-time cosmic acceleration.Comment: 42 pages, no figure, version accepted for publication in General
Relativity and Gravitatio
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