共查询到20条相似文献，搜索用时 46 毫秒
1.
In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological
models with bouncing behavior such as chameleon cosmology where the boundary of the universe is assumed to be enclosed by
the dynamical apparent horizon. From a thermodynamic point of view, to link between thermodynamic and geometric parameters
in cosmological models, we introduce “entropy rate of change multiplied by the temperature” as a model independent thermodynamic
state parameter together with the well known {r,s} statefinder to differentiate the dark energy models. 相似文献
2.
In this paper interacting chameleon-tachyon model with agegraphic dark energy is revisited. The model in two cases of matter and radiation dominated universe is best fitted with the observational data for distance modulus. Stability of the model is investigated. The model then tested against observational data for Hubble parameter. With respect to the best fitted model parameters, our results show that while both scenarios are in good match with the observational data in low redshifts, the model in radiation dominated case better fits the data in high redshifts. 相似文献
3.
M. Malekjani A. Khodam-Mohammadi N. Nazari-pooya 《Astrophysics and Space Science》2011,332(2):515-524
In this paper, the holographic dark energy model with new infrared cut-off proposed by Granda and Oliveros has been investigated
in spatially non flat universe. The dependency of the evolution of equation of state, deceleration parameter and cosmological
evolution of Hubble parameter on the parameters of new HDE model are calculated. Also, the statefinder parameters r and s in this model are derived and the evolutionary trajectories in s−r plane are plotted. We show that the evolutionary trajectories are dependent on the model parameters of new HDE model. Eventually,
in the light of SNe + BAO + OHD + CMB observational data, we plot the evolutionary trajectories in s−r and q−r planes for best fit values of the parameters of new HDE model. 相似文献
4.
M. Malekjani A. Khodam-Mohammadi N. Nazari-pooya 《Astrophysics and Space Science》2011,334(1):193-201
The generalized Chaplygin gas (GCG) model in spatially flat universe is investigated. The cosmological consequences led by
GCG model including the evolution of EoS parameter, deceleration parameter and dimensionless Hubble parameter are calculated.
We show that the GCG model behaves as a general quintessence model. The GCG model can also represent the pressureless CDM
model at the early time and cosmological constant model at the late time. The dependency of transition from decelerated expansion
to accelerated expansion on the parameters of model is investigated. The statefinder parameters r and s in this model are derived and the evolutionary trajectories in s–r plane are plotted. Finally, based on current observational data, we plot the evolutionary trajectories in s–r and q–r planes for best fit values of the parameters of GCG model. It has been shown that although, there are similarities between
GCG model and other forms of Chaplygin gas in statefinder plane, but the distance of this model from the ΛCDM fixed point
in s–r diagram is shorter compare with standard Chaplygin gas model. 相似文献
5.
Yuri Lyubarsky 《Astrophysics and Space Science》2012,339(1):79-85
Stability analysis of agegraphic dark energy in Brans-Dicke theory is presented in this paper. We constrain the model parameters
with the observational data and thus the results become broadly consistent with those expected from experiment. Stability
analysis of the model without best fitting shows that universe may begin from an unstable state passing a saddle point and
finally become stable in future. However, with the best fitted model, There is no saddle intermediate state. The agegraphic
dark energy in the model by itself exhibits a phantom behavior. However, contribution of cold dark matter on the effective
energy density modifies the state of the universe from phantom phase to quintessence one. The statefinder diagnosis also indicates
that the universe leaves an unstable state in the past, passes the LCDM state and finally approaches the sable state in future. 相似文献
6.
D. J. Scheeres 《Celestial Mechanics and Dynamical Astronomy》1998,70(2):75-98
Starting from the four-body problem a generalization of both the restricted three-body problem and the Hill three-body problem
is derived. The model is time periodic and contains two parameters: the mass ratio ν of the restricted three-body problem
and the period parameter m of the Hill Variation orbit. In the proper coordinate frames the restricted three-body problem
is recovered as m → 0 and the classical Hill three-body problem is recovered as ν → 0. This model also predicts motions described
by earlier researchers using specific models of the Earth–Moon–Sun system. An application of the current model to the motion
of a spacecraft in the Sun perturbed Earth–Moon system is made using Hill's Variation orbit for the motion of the Earth–Moon
system. The model is general enough to apply to the motion of an infinitesimal mass under the influence of any two primaries
which orbit a larger mass.
Using the model, numerical investigations of the structure of motions around the geometric position of the triangular Lagrange
points are performed. Values of the parameter ν range in the neighborhood of the Earth–Moon value as the parameter m increases
from 0 to 0.195 at which point the Hill Variation orbit becomes unstable. Two families of planar periodic orbits are studied
in detail as the parameters m and ν vary. These families contain stable and unstable members in the plane and all have the
out-of-plane stability. The stable and unstable manifolds of the unstable periodic orbits are computed and found to be trapped
in a geometric area of phase space over long periods of time for ranges of the parameter values including the Earth–Moon–Sun
system.
This model is derived from the general four-body problem by rigorous application of the Hill and restricted approximations.
The validity of the Hill approximation is discussed in light of the actual geometry of the Earth–Moon–Sun system.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
We aim in this paper to study Brans-Dicke cosmology in the presence of varying mass fermions and a self-interaction potential. Furthermore, we also probe the entropy corrected holographic dark energy (ECHDE) in the model in two non-interacting and interacting scenarios. The model parameters are constrained by using the recent SNe Ia observational data and tested against observational data of Hubble parameter. For a comparison, we also constrained and tested the cosmological parameters in ΛCDM model with the same observational data. We show that in non of the scenarios the model prediction is better than ΛCDM model. 相似文献
8.
We simultaneously fitted light and velocity data for the star–planet system OGLE-TR-56 with the Wilson–Devinney (WD) binary
star program. We solved for orbital and planet parameters, along with the ephemeris using all currently available observational
data. Parameters for the star (OGLE-TR-56a) were kept fixed at values derived from spectral characteristics and stellar evolutionary
tracks. Our results are in good agreement with parameters obtained by other authors and have slightly smaller errors. We found
no significant change in orbital period that may be due to orbital decay. 相似文献
9.
Anirudh Pradhan Rekha Jaiswal Kanti Jotania Rajeev Kumar Khare 《Astrophysics and Space Science》2012,337(1):401-413
We present two dark energy (DE) models with an anisotropic fluid in Bianchi type-VI
0 space-time by considering time dependent deceleration parameter (DP). The equation of state (EoS) for dark energy ω is found to be time dependent and its existing range for derived models is in good agreement with the recent observations.
Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution
of the universe, the EoS parameter for DE changes from ω>−1 to ω=−1 in first model whereas from ω>−1 to ω<−1 in second model which is consistent with recent observations. The cosmological constant Λ is found to be a positive decreasing
function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results
from recent type Ia supernovae observations. The cosmic jerk parameter in our derived models is also found to be in good agreement
with the recent data of astrophysical observations. The physical and geometric aspects of both the models are also discussed
in detail. 相似文献
10.
《New Astronomy》2021
This study set out to examine the effect of anisotropy on the various dark energy models by using the observational data, including the Sandage-Loeb test, Strongly gravitationally lensing, observational Hubble data, and Baryon Acoustic Oscillations data. In particular, we consider three cases of dark energy models: the cosmological constant model, which is most favored by current observations, the wCDM model where dark energy is introduced with constant w equation of state parameter and in Chevalier-Polarski-Linder parametrization where ω is allowed to evolve with redshift. With an anisotropy framework, a maximum likelihood method to constrain the cosmological parameters was implemented. With an anisotropic universe, we also study the behavior of different cosmological parameters such as Hubble parameter, EoS parameter, and deceleration parameter of dark energy models mentioned. The results indicate that the Bianchi type I model for the dark energy models are consistent with the combined observational data. 相似文献
11.
Annibal Hetem Jr Jane Gregorio-Hetem 《Monthly notices of the Royal Astronomical Society》2007,382(4):1707-1718
The protoplanetary discs of T Tauri and Herbig Ae/Be stars have previously been studied using geometric disc models to fit their spectral energy distribution (SED). The simulations provide a means to reproduce the signatures of various circumstellar structures, which are related to different levels of infrared excess. With the aim of improving our previous model, which assumed a simple flat-disc configuration, we adopt here a reprocessing flared-disc model that assumes hydrostatic, radiative equilibrium. We have developed a method to optimize the parameter estimation based on genetic algorithms (GAs). This paper describes the implementation of the new code, which has been applied to Herbig stars from the Pico dos Dias Survey catalogue, in order to illustrate the quality of the fitting for a variety of SED shapes. The star AB Aur was used as a test of the GA parameter estimation, and demonstrates that the new code reproduces successfully a canonical example of the flared-disc model. The GA method gives a good quality of fit, but the range of input parameters must be chosen with caution, as unrealistic disc parameters can be derived. It is confirmed that the flared-disc model fits the flattened SEDs typical of Herbig stars; however, embedded objects (increasing SED slope) and debris discs (steeply decreasing SED slope) are not well fitted with this configuration. Even considering the limitation of the derived parameters, the automatic process of SED fitting provides an interesting tool for the statistical analysis of the circumstellar luminosity of large samples of young stars. 相似文献
12.
H. Farajollahi A. Ravanpak H. Shojaie M. Abolghasemi 《Astrophysics and Space Science》2014,350(1):325-331
This work is to study the generalized second law (GSL) of thermodynamics in tachyon cosmology where the tachyon field is coupled to the matter Lagrangian while the boundary of universe is assumed to be a dynamical apparent horizon. The two logarithmic and power law corrected entropy on the apparent horizon is also discussed and the conditions for validity of GSL in both scenarios are investigated by using observational data of Sne Ia. In comparison to other research works, since the model is constrained by observational data, the conditions obtained for the dimensionless constant parameter, α in both logarithmic and power law entropy correction of GSL are (physically) meaningful and realistic. The model also predicts an accelerating universe with no phantom crossing in the past or future. 相似文献
13.
R. Vasundhara 《Journal of Astrophysics and Astronomy》1991,12(1):69-87
Results of the observations of mutual eclipses of Galilean satellites observed from the Vainu Bappu Observatory during 1985
are presented. Theoretical models assuming a uniform disc, Lambert’s law and Lommel-Seeliger’s law describing the scattering
characteristics of the surface of the eclipsed satellite were used to fit the observations. Light curves of the 1E2 event
on 1985 September 24 and the 3E1 event on 1985 October 24 observed from VBO and published light curves of the 1E2 event on
1985 September 14, the 3E1 event on 1985 September 26 and the 2E1 event on 1985 October 28 (Arlotet al 1989) were fitted with theoretical light curves using Marquardt’s algorithm. The best fitting was obtained using Lommel-Seeliger’s
law to describe the scattering over the surface of Io and Europa. During the fitting, a parameterδxshift which shifts the theoretical light curve along the direction of relative motion of the eclipsed satellite with respect
to the shadow centre, on the sky plane (as seen from the Sun) was determined along with the impact parameter. In absence of
other sources like prominent surface features or non perfect sky conditions which could lead to asymmetric light curves,δxshift would be a measure of the phase correction (Aksnes, Franklin & Magnusson 1986) with an accuracy as that of the midtime.
Heliocentric Δα cos (δ) and gDδ at mid times derived from fitted impact parameters are reported 相似文献
14.
O. Sergijenko Yu. Kulinich B. Novosyadlyj V. Pelykh 《Kinematics and Physics of Celestial Bodies》2009,25(1):17-27
The evolution of scalar perturbations is studied for 2-component (non-relativistic matter and dark energy) cosmological models
at the linear and non-linear stages. The dark energy is assumed to be the scalar field with either classical or tachyonic
Lagrangian and constant equation-of-state parameter w. The fields and potentials were reconstructed for the set of cosmological parameters derived from observations. The comparison
of the calculated within these models and observational large-scale structure characteristics is made. It is shown that for
w = const such analysis can’t remove the existing degeneracy of the dark energy models.
The article is published in the original. 相似文献
15.
Ewa L. &#;okas 《Monthly notices of the Royal Astronomical Society》2001,327(2):L21-L26
We present predictions for the line-of-sight velocity dispersion profiles of dwarf spheroidal galaxies and compare them to observations in the case of the Fornax dwarf. The predictions are made in the framework of standard dynamical theory of spherical systems with different velocity distributions. The stars are assumed to be distributed according to Sérsic laws with parameters fitted to observations. We compare predictions obtained assuming the presence of dark matter haloes (with density profiles adopted from N -body simulations) with those resulting from Modified Newtonian Dynamics (MOND). If the anisotropy of velocity distribution is treated as a free parameter, observational data for Fornax are reproduced equally well by models with dark matter and with MOND. If stellar mass-to-light ratio of 1 M⊙ /L⊙ is assumed, the required mass of the dark halo is , two orders of magnitude larger than the mass in stars. The derived MOND acceleration scale is . In both cases a certain amount of tangential anisotropy in the velocity distribution is needed to reproduce the shape of the velocity dispersion profile in Fornax. 相似文献
16.
This paper describes a new 2D model for the photospheric evolution of the magnetic carpet. It is the first in a series of
papers working towards constructing a realistic 3D non-potential model for the interaction of small-scale solar magnetic fields.
In the model, the basic evolution of the magnetic elements is governed by a supergranular flow profile. In addition, magnetic
elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. Model parameters for
the emergence of bipoles are based upon the results of observational studies. Using this model, several simulations are considered,
where the range of flux with which bipoles may emerge is varied. In all cases the model quickly reaches a steady state where
the rates of emergence and cancellation balance. Analysis of the resulting magnetic field shows that we reproduce observed
quantities such as the flux distribution, mean field, cancellation rates, photospheric recycle time and a magnetic network.
As expected, the simulation matches observations more closely when a larger, and consequently more realistic, range of emerging
flux values is allowed (4×1016 – 1019 Mx). The model best reproduces the current observed properties of the magnetic carpet when we take the minimum absolute flux
for emerging bipoles to be 4×1016 Mx. In future, this 2D model will be used as an evolving photospheric boundary condition for 3D non-potential modeling. 相似文献
17.
We show that the set of observational characteristics for low-mass X-ray binaries in the optical and X-ray bands can be explained
in terms of the model of an optically thick accretion disk with an atmosphere irradiated by a central X-ray source. We show
that this set of observational data can be successfully used to measure the orbital inclination of a binary, the geometric
parameters of its accretion disk, and the reprocessing time of X-emission to optical one. For the burster GS 1826-238, a low-mass
X-ray binary with a neutron star, we have estimated the binary inclination and the thickness of the disk atmosphere at the
outer edge from the mean optical flux and the amplitude of periodic modulations in the optical light curve: i = 62.5° ± 5.5° and H
d/R
d = 0.145 ± 0.009. The optical response time of the binary to an X-ray burst disagrees with the geometric delay in the propagation
of X-ray photons in the binary. We believe that this points to a finite X-ray reprocessing/reradiation time, 1.0 s ≲ τ
repr ≲ 2.2 s, in the hot atmosphere above the accretion disk. 相似文献
18.
In this paper, we investigate the dynamics of generalized Chaplygin gas (GCG) model with or without viscosity in the w–w′ plane, which is defined by the equation of state parameter and its time derivative with respect to the logarithm of the
scale factor. We show that GCG model without viscosity approaches to a late time de Sitter attractor (w
g
=−1) and behaves like a “freezing” scalar field for the parameter α constrained by the latest observational data. However, introducing viscosity exerts an influence on the evolution of w and affects the location of the late time attractor (w
g
>−1) in viscous GCG model. We also find numerically such a transition from w′>0 to w′<0 as the universe expands in viscous GCG model different from GCG model without viscosity (w′<0) in the w–w′ plane. 相似文献
19.
Anisotropic Bianchi-III cosmological model is investigated with variable gravitational and cosmological constants in the framework of Einstein’s general relativity. The shear scalar is considered to be proportional to the expansion scalar. The dynamics of the anisotropic universe with variable G and Λ are discussed. Without assuming any specific forms for Λ and the metric potentials, we have tried to extract the time variation of G and Λ from the anisotropic model. The extracted G and Λ are in conformity with the present day observations. Basing upon the observational limits, the behavior and range of the effective equation of state parameter are discussed. 相似文献
20.
D. Gerbal H.V. Capelato F. Durret G.B. Lima Neto I. Márquez 《Astrophysics and Space Science》2001,276(2-4):861-868
We suggest that elliptical galaxies, as stellar systems in a stage of quasi-equilibrium, may have a specific entropy. We use
the Sérsic law to describe the light profile. The specific entropy (the Boltzmann–Gibbs definition) is then calculated assuming
that the galaxy behaves as a spherical, isotropic, one-component system. We predict a relation between the three parameters
of the Sérsic law linked to the specific entropy, defining a surface in the parameter space, an ‘entropic plane’. We have
analysed a sample of simulated merging elliptical galaxies (virtual) and a sample of galaxies belonging to the Coma Cluster (real). Both virtual and realgalaxies are: 1) located in their own ‘entropic plane‘ and 2) in this plane, they are located on a straight line, indicating
constant entropy: another physical property A careful examination of the value of the specific entropy indicates a very small
increase in the specific entropy with the generation after merging (virtual sample). Although one cannot distinguish between various generations for real galaxies, the distribution of specific entropy in this sample is very similar to that in the virtual sample.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献