2,228 research outputs found
Alternative mechanism of avoiding the big rip or little rip for a scalar phantom field
Depending on the choice of its potential, the scalar phantom field
(the equation of state parameter ) leads to various catastrophic fates of
the universe including big rip, little rip and other future singularity. For
example, big rip results from the evolution of the phantom field with an
exponential potential and little rip stems from a quadratic potential in
general relativity (GR). By choosing the same potential as in GR, we suggest a
new mechanism to avoid these unexpected fates (big and little rip) in the
inverse-\textit{R} gravity. As a pedagogical illustration, we give an exact
solution where phantom field leads to a power-law evolution of the scale factor
in an exponential type potential. We also find the sufficient condition for a
universe in which the equation of state parameter crosses divide. The
phantom field with different potentials, including quadratic, cubic, quantic,
exponential and logarithmic potentials are studied via numerical calculation in
the inverse-\textit{R} gravity with correction. The singularity is
avoidable under all these potentials. Hence, we conclude that the avoidance of
big or little rip is hardly dependent on special potential.Comment: 9 pages,6 figure
Solar system tests for realistic models with nonminimal torsion-matter coupling
In the previous paper, we have constructed two models with nonminimal
torsion-matter coupling extension, which are successful in describing the
evolution history of the Universe including the radiation-dominated era, the
matter-dominated era, and the present accelerating expansion. Meantime, the
significant advantage of these models is that they could avoid the cosmological
constant problem of CDM. However, the nonminimal coupling between
matter and torsion will affect the tests of Solar system. In this paper, we
study the effects of Solar system in these models, including the gravitation
redshift, geodetic effect and perihelion preccesion. We find that Model I can
pass all three of the Solar system tests. For Model II, the parameter is
constrained by the measure of the perihelion precession of Mercury.Comment: 10 page
Finite temperature properties of clusters by replica exchange metadynamics: the water nonamer
We introduce an approach for the accurate calculation of thermal properties
of classical nanoclusters. Based on a recently developed enhanced sampling
technique, replica exchange metadynamics, the method yields the true free
energy of each relevant cluster structure, directly sampling its basin and
measuring its occupancy in full equilibrium. All entropy sources, whether
vibrational, rotational anharmonic and especially configurational -- the latter
often forgotten in many cluster studies -- are automatically included. For the
present demonstration we choose the water nonamer (H2O)9, an extremely simple
cluster which nonetheless displays a sufficient complexity and interesting
physics in its relevant structure spectrum. Within a standard TIP4P potential
description of water, we find that the nonamer second relevant structure
possesses a higher configurational entropy than the first, so that the two free
energies surprisingly cross for increasing temperature.Comment: J. Am. Chem. Soc. 133, 2535-2540 (2011
チュウゴク ノ ニッチュウ カンケイ ケンキュウ ジッタイ ト トクチョウ 1990-2012 スウリョウテキ ブンセキ
大阪大学中国文化论坛 讨论文件Discussion Papers in Contemporary China Studies, Osaka University Forum on Chin
Casimir pistons with hybrid boundary conditions
The Casimir effect giving rise to an attractive or repulsive force between
the configuration boundaries that confine the massless scalar field is
reexamined for one to three-dimensional pistons in this paper. Especially, we
consider Casimir pistons with hybrid boundary conditions, where the boundary
condition on the piston is Neumann and those on other surfaces are Dirichlet.
We show that the Casimir force on the piston is always repulsive, in contrast
with the same problem where the boundary conditions are Dirichlet on all
surfaces.Comment: 8 pages, 4 figures,references added, minor typos correcte
8-(4-Chlorobenzylidene)-4-(4-chlorophenyl)-2-phenyl-5,6,7,8-tetrahydroquinoline
In the crystal structure of the title compound, C28H21Cl2N, π–π interactions link pairs of molecules into centrosymmetric dimers with a distance of 3.756 (3) Å between the centroids of the pyridine rings. Weak intermolecular C—H⋯Cl hydrogen bonds further link these dimers into chains propagating along [01]. The pyridine ring forms dihedral angles of 21.52 (1) and 55.87 (2)°, respectively, with the phenyl ring and the 4-chlorophenyl ring
- …