共查询到20条相似文献,搜索用时 31 毫秒
1.
Ahmad Rami El-Nabulsi 《Astrophysics and Space Science》2010,327(1):111-115
We investigate five-dimensional Brans–Dicke cosmology with spacetime described by the homogeneous, anisotropic and flat spacetime
with the topology M
1×R
3×S
1 where S
1 is taken in the form of a circle. We conjecture throughout this letter that the extra-dimension compactifies as the visible
dimensions expand like b(t)≈a
−1(t) and that the non-minimal coupling between the scalar field and the matter is of the form f(φ)∝
φ
2. The model gives rise to a transition from a decelerated epoch to an accelerated epoch for large values of the Brans–Dicke
parameter ω. The model predicts crossing of the phantom divided barrier unless the universe is governed by a growing matter field. 相似文献
2.
El-Nabulsi Ahmad?Rami 《Astrophysics and Space Science》2011,332(2):491-495
We investigate the late-time dynamics of a four-dimensional universe based on modified scalar field gravity in which the standard
Einstein-Hilbert action R is replaced by f(φ)R+f(R) where f(φ)=φ
2 and f(R)=AR
2+BR
μν
R
μν,(A,B)∈ℝ. We discussed two independent cases: in the first model, the scalar field potential is quartic and for this special form
it was shown that the universe is dominated by dark energy with equation of state parameter w≈−0.2 and is accelerated in time with a scale factor evolving like a(t)∝t
5/3 and B+3A≈0.036. When, B+3A→∞ which corresponds for the purely quadratic theory, the scale factor evolves like a(t)∝t
1/2 whereas when B+3A→0 which corresponds for the purely scalar tensor theory we found when a(t)∝t
1.98. In the second model, we choose an exponential potential and we conjecture that the scalar curvature and the Hubble parameter
vary respectively like
R=hH[(f)\dot]/f,h ? \mathbbRR=\eta H\dot{\phi}/\phi,\eta\in\mathbb{R} and
H=g[(f)\dot]c,(g,c) ? \mathbbRH=\gamma\dot{\phi}^{\chi},(\gamma,\chi)\in\mathbb{R}. It was shown that for some special values of χ, the universe is free from the initial singularity, accelerated in time, dominated by dark or phantom energy whereas the
model is independent of the quadratic gravity corrections. Additional consequences are discussed. 相似文献
3.
E. Kh. Danielian 《Astrophysics》2011,54(2):250-268
New methods are proposed for finding the Ambartsumyan functions φ(η) for a half space and φ(η, τ) and ψ(η, τ) for finite layers,
as well as their analogs with complete frequency redistribution, X (z, τ) and Y (z, τ). Substantial simplifications are obtained for monochromatic conservative scattering. Besides the Ambartsumyan functions,
expressions for several of their angular moments are obtained directly in terms of the basis functions u
±. A system of differential equations is obtained for the basis functions. A system of equations without the characteristic
pseudosingularities is obtained for φ(η, τ) and ψ(η, τ) instead of the classical system of nonlinear equations. Some aspects
of the numerical realization of the proposed method are also discussed. 相似文献
4.
In this paper, we have considered a model for FRW space-time in the presence of coupled scalar field φ and potential V(φ) with causal viscous fluid and polytropic fluid. We have shown that irrespective of fluid the causality theory provides late
time acceleration of the universe. In all cases, the potential always decreases due to evolution of the universe. 相似文献
5.
The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory
is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously
as the Universe expands, are obtained by using the power-law relation φ = bR
n
, which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation
dominated era of the universe. The physical properties of cosmological solutions are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Assuming the time-dependent equation of state p=λ(t)ρ, five dimensional cosmological models with viscous fluid for an open universe (k=−1) and flat universe (k=0) are presented. Exact solutions in the context of the rest mass varying theory of gravity proposed by Wesson (Astron. Astrophys.
119, 145, 1983) are obtained. It is found that the phenomenon of isotropisation takes place in this theory, i.e. the mass scale factor A(t) which characterizes the rest mass of a typical particle is evolving with cosmic time just as the spatial scale factor R(t). It is further found that rest mass is approximately constant in the present universe. 相似文献
7.
Mohamed E. Hassani 《Astrophysics and Space Science》2010,326(1):33-38
In this work we propose cyclical reversible transitions as the scenario in which the universe evolves, through a series consisting of reversible expansion, temporary stability,
and contraction. Our model is based on the comparison between local and global time-dependent densities {ρ
0(τ
0),ρ(τ)} instead of the critical density ρ
c, local and global time-dependent Hubble parameters {H
0(τ
0),H(τ)}, and the variations {Δρ(τ),ΔH(τ)} due to cosmological chaotic fluctuations, which are generally ignored in certain oscillating models. By taking into account
all these factors, a rate equation in the form of (H
0/H)2
∝(ρ
0/ρ) has been established, and from it we derive some others, to provide a mechanism that is responsible for the cyclical reversible
transitions. Also, the problems of singularities, black hole overproduction, and the second law of thermodynamics arising
in oscillating universe models are conceptually resolved. 相似文献
8.
Antonio Alfonso-Faus 《Astrophysics and Space Science》2012,337(1):367-372
Combining the kinematical definitions of the two dimensionless parameters, the deceleration q(x) and the Hubble t
0
H(x), we get a differential equation (where x=t/t
0 is the age of the universe relative to its present value t
0). First integration gives the function H(x). The present values of the Hubble parameter H(1) [approximately t
0
H(1)≈1], and the deceleration parameter [approximately q(1)≈−0.5], determine the function H(x). A second integration gives the cosmological scale factor a(x). Differentiation of a(x) gives the speed of expansion of the universe. The evolution of the universe that results from our approach is: an initial
extremely fast exponential expansion (inflation), followed by an almost linear expansion (first decelerated, and later accelerated).
For the future, at approximately t≈3t
0 there is a final exponential expansion, a second inflation that produces a disaggregation of the universe to infinity. We
find the necessary and sufficient conditions for this disaggregation to occur. The precise value of the final age is given
only with one parameter: the present value of the deceleration parameter [q(1)≈−0.5]. This emerging picture of the history of the universe represents an important challenge, an opportunity for the
immediate research on the Universe. These conclusions have been elaborated without the use of any particular cosmological
model of the universe. 相似文献
9.
In this paper we compare outcomes of some extended phantom-like cosmologies with each other and also with ΛCDM and ΛDGP. We
focus on the variation of the luminosity distances, the age of the universe and the deceleration parameter versus the redshift
in these scenarios. In a dynamical system approach, we show that the accelerating phase of the universe in the f(R)-DGP scenario is stable if one consider the curvature fluid as a phantom scalar field in the equivalent scalar-tensor theory, otherwise it is a transient and unstable phenomenon. Up
to the parameters values adopted in this paper, the extended F(R,ϕ)-DGP scenario is closer to the ΛCDM scenario than other proposed models. All of these scenarios explain the late-time cosmic
speed-up in their normal DGP branches, but the redshift at which transition to the accelerating phase occurs are different:
while the ΛDGP model transits to the accelerating phase much earlier, the F(R,ϕ)-DGP model transits to this phase much later than other scenarios. Also, within the parameter spaces adopted in this paper,
the age of the universe in the f(R)-DGP model is larger than ΛCDM, but this age in F(G,ϕ)-DGP is smaller than ΛCDM. 相似文献
10.
E. Kh. Danielian 《Astrophysics》2010,53(2):268-283
The basic concepts for developing a system of analytic solutions for the standard problems of radiative transfer theory are
discussed. These solutions, which are found using Ambartsumyan’s layer addition method in Sobolev’s probabilistic interpretation
for radiative diffusion problems, are maximally compact and easily used in numerical computations. New expressions are obtained
for the resolvents and the resolvent functions, as well as a unified structure for the form of an integral representation
for solving different radiative transfer problems in semi-infinite media and in finite layers. Block diagrams of the sequence
of stages for solving these problems are provided, where the Ambartsumyan function φ(η) (more precisely, 1/φ(η)) plays a fundamental
role in the case of semi-infinite media while the functions a(η, τ0 ) and b(η, τ0) play an analogous role for finite layers. 相似文献
11.
K. S. Adhav 《Astrophysics and Space Science》2011,335(2):611-617
The Bianchi type-V cosmological model with variable modified Chaplygin gas having the equation of state p=Aρ−B/ρ
α
, where 0≤α≤1, A is a positive constant and B is a positive function of the average scale factor a(t) of the universe [i.e. B=B(a)] has been studied. While studying its role in accelerated phase of the universe, it is observed that the equation of state
of the variable modified Chaplygin gas interpolates from radiation dominated era to quintessence dominated era. The statefinder
diagnostic pair {r,s} is adopted to characterize different phases of the universe. 相似文献
12.
Solar rotation rate has been measured using the sunspot positions recorded by W.C. Bond during the period 1847 – 1849 at the
Harvard College Observatory. From the drawings carried out by Bond we have selected the sunspots and groups of sunspots with
more reliable positions presented in three or more drawings on successive days. We have calculated from the positions of the
selected sunspots (41 in total) a synodic rotation rate of ω=[(12.92±0.08)−(1.5±1.0)sin 2
φ] degrees/day, where φ is the heliographic latitude. This rate, although slightly lower, is similar to the actual solar rotation rate, confirming
no important changes in the solar rotation during the last 160 years. 相似文献
13.
R.R. Reddy Y. Nazeer Ahammed B. Sasikala Devi K. Rama Gopal P. Abdul Azeem T.V.R. Rao 《Astrophysics and Space Science》2002,281(4):729-741
The true experimental potential energy curves for the electronic ground states of astrophysically important TaO, TaS, ZrS
and SiO+molecules are constructed by using the Rydberg–Klein–Rees method as modified by Vanderslice et al. The ground state dissociation
energies are determined by curve fitting techniques using the five parameters Hulburt-Hirschfelder (H-H) function. The estimated
dissociation energies are 8.19 ± 0.17, 6.9 ±0.14, 5.89 ± 0.12 and 5.75 ± 0.12 eVfor TaO, TaS, ZrS and SiO+ respectively. These values are in good agreement with the literature values. The r-centriods and Franck–Condon factors (FC
Factors) for the bands of K
2
φ
5/2
- X
2
Δ
3/2
(K-X) system of TaO, A-X
2 Δ (A-X) and B-X
2 Δ (B-X)systems of TaS, B
1 Π - X1 Σ+
(B-X) system of ZrS and B
2Σ+ - X2 Σ+
(B-X) and A
2 Π - X2 Σ+
(A-X)systems of SiO+ molecules have been calculated. The Franck–Condon factors (FC factors) are evaluated by the approximate analytical methods
of Jarmain and Fraser. The absence of the bands of these systems is explained.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
A new class of exact solutions of Einstein’s field equations with a bulk viscous fluid for an LRS Bianchi type-Ia obtained
by using a time dependent deceleration parameter and cosmological term Λ. The coefficient of bulk viscosity is assumed to
be a power function of mass density (ξ=ξ
0
ρ
n
). We have obtained a general solution of the field equations from which six models of the universe are derived: exponential,
polynomial and sinusoidal form respectively. The behaviour of these models of the universe are also discussed in the frame
of reference of recent supernovae Ia observations.
相似文献
15.
It is surprising that we hardly know only 4% of the universe. Rest of the universe is made up of 73% of dark-energy and 23%
of dark-matter. Dark-energy is responsible for acceleration of the expanding universe; whereas dark-matter is said to be necessary
as extra-mass of bizarre-properties to explain the anomalous rotational-velocity of galaxy. Though the existence of dark-energy
has gradually been accepted in scientific community, but the candidates for dark-matter have not been found as yet and are
too crazy to be accepted. Thus, it is obvious to look for an alternative theory in place of dark-matter. Milgrom (Astrophys.
J. 270:365, 1983a; 270:371, 1983b) has suggested a ‘Modified Newtonian Dynamics (MOND)’ which appears to be highly successful for explaining the anomalous
rotational-velocity. But unfortunately MOND lacks theoretical support. The MOND, in-fact, is (empirical) modification of Newtonian-Dynamics
through modification in the kinematical acceleration term ‘a’ (which is normally taken as
a=\fracv2ra=\frac{v^{2}}{r}) as effective kinematic acceleration
aeffective = a m(\fracaa0)a_{\mathit{effective}} = a \mu(\frac{a}{a_{0}}), wherein the μ-function is 1 for usual-values of accelerations but equals to
\fracaa0 ( << 1)\frac{a}{a_{0}} (\ll1) if the acceleration ‘a’ is extremely-low lower than a critical value a
0(10−10 m/s2). In the present paper, a novel variant of MOND is proposed with theoretical backing; wherein with the consideration of universe’s
acceleration a
d
due to dark-energy, a new type of μ-function on theoretical-basis emerges out leading to
aeffective = a(1 -K \fraca0a)a_{\mathit{effective}} = a(1 -K \frac{a_{0}}{a}). The proposed theoretical-MOND model too is able to fairly explain ‘qualitatively’ the more-or-less ‘flat’ velocity-curve
of galaxy-rotation, and is also able to predict a dip (minimum) on the curve. 相似文献
16.
The study of the variation of equivalent width in a Rayleighscattering planetary atmosphere along the intensity equator and
along the mirror meridian on whichμ =μ
0 shows that the equivalent widths decrease monotonically towards the poles, the limb and the terminator with the following
characteristics: (i) the weakest lines exhibit the maximum change; (ii) theI
e
r component shows more change than theI
e
r component; (iii) the decrease towards the limb or the terminator is not as sharp as that towards the poles; (iv)I
e
r component shows more decrease towards the limb whileI
e
r component shows more decrease towards the terminator; and (v) the relationW (μ, φ;μ
0,φ
0)= W (μ
0,φ
0;μ, φ) holds for the total intensity. These results are qualitatively in agreement with the observations of absorption bands in
the spectra of Venus, Jupiter and Saturn. 相似文献
17.
How does inflation depend upon the nature of fluids filling up the universe in brane world scenario?
Sudeshna Mukerji Ritabrata Biswas Nairwita Mazumder Subenoy Chakraborty 《Astrophysics and Space Science》2012,337(1):425-437
By constructing different parameters which are able to give us the information about our universe during inflation, (specially
at the start and the end of the inflationary universe) a brief idea of brane world inflation is given in this work. What will
be the size of the universe at the end of inflation, i.e., how many times will it grow than the original size is been speculated
and analysed thereafter. Different kinds of fluids are taken to be the matter inside the brane. It is observed that in the
case of highly positive pressure giving gas like polytropic, the size of the universe at the end of inflation is comparatively
smaller. Whereas for negative pressure creators (like Chaplygin gas) this size is much bigger. Except these two cases, inflation
has been studied for barotropic fluid and linear red shift parametrization ω(z)=ω
0+ω
1
z too. For them the size of the universe after inflation is much more high. We also have seen that this size does not depend
upon the potential energy at the end of the inflation. On the contrary, there is a high impact of the initial potential energy
upon the size of inflation. 相似文献
18.
Corrado Massa 《Astrophysics and Space Science》2006,305(4):377-384
Quantum theory in Robertson – Walker spacetime suggests the existence of a minimal energy ε of the order of 10−45 erg. Reasonable forms for ε give the expansion factor R=R(t)(t= the cosmic time) with no need of gravitational field equations.Einstein's theory should be modified in gravitational fields of strength less than ε c/ħ ∼ 10−8 cm/s2 where c is the speed of light and ħ is the reduced Planck constant. The cosmological term λ is expected to decrease as the universe expands.In the Appendix, ε is derived from a big bang – big crunch Newtonian cosmology. 相似文献
19.
The cosmological reconstruction of modified F(R) and F(G)F(\mathcal{G}) gravities with agegraphic dark energy (ADE) model in a spatially flat universe without matter field is investigated by using
e-folding “N” as a forward way. After calculating a consistent F(R) in ADE’s framework, we obtain conditions for effective equation of state parameter w
eff, and see that reconstruction is possible for both phantom and non-phantom era. These calculations also are done for F(G)F(\mathcal{G}) gravity and the condition for a consistent reconstruction is obtained. 相似文献
20.
In this paper, we solve the field equations in metric f(R) gravity for Bianchi type VI
0 spacetime and discuss evolution of the expanding universe. We find two types of non-vacuum solutions by taking isotropic
and anisotropic fluids as the source of matter and dark energy. The physical behavior of these solutions is analyzed and compared
in the future evolution with the help of some physical and geometrical parameters. It is concluded that in the presence of
isotropic fluid, the model has singularity at [(t)\tilde]=0\tilde{t}=0 and represents continuously expanding shearing universe currently entering into phantom phase. In anisotropic fluid, the
model has no initial singularity and exhibits the uniform accelerating expansion. However, the spacetime does not achieve
isotropy as t→∞ in both of these solutions. 相似文献