共查询到20条相似文献,搜索用时 625 毫秒
1.
Utpal Mukhopadhyay Partha Pratim Ghosh Saibal Ray 《International Journal of Theoretical Physics》2010,49(7):1622-1627
Two phenomenological models of Λ, viz. L ~ ([(a)\dot]/a)2\Lambda \sim (\dot{a}/a)^{2} and L ~ [(a)\ddot]/a\Lambda \sim \ddot{a}/a are studied under the assumption that G is a time-variable parameter. Both models show that G is inversely proportional to time as suggested earlier by others including Dirac. The models considered here can be matched
with observational results by properly tuning the parameters of the models. Our analysis shows that L ~ [(a)\ddot]/a\Lambda \sim \ddot{a}/a model corresponds to a repulsive situation and hence correlates with the present status of the accelerating Universe. The
other model L ~ ([(a)\dot]/a)2\Lambda \sim (\dot{a}/a)^{2} is, in general, attractive in nature. Moreover, it is seen that due to the combined effect of time-variable Λ and G the Universe evolved with acceleration as well as deceleration. This later one indicates a Big Crunch. 相似文献
2.
Saibal Ray Maxim Khlopov Partha Pratim Ghosh Utpal Mukhopadhyay 《International Journal of Theoretical Physics》2011,50(3):939-951
Among various phenomenological Λ models, a time-dependent model [(L)\dot] ~ H3\dot{\Lambda}\sim H^{3} is selected here to investigate the Λ-CDM cosmology. The model can follow from dynamics, underlying the origin of Λ. Using
this model the expressions for the time-dependent equation of state parameter ω and other physical parameters are derived. It is shown that in H
3 model accelerated expansion of the Universe takes place at negative energy density, but with a positive pressure. It has
also been possible to obtain the change of sign of the deceleration parameter q during cosmic evolution. 相似文献
3.
G. S. Khadekar Rupali Wanjari Cenap Ozel 《International Journal of Theoretical Physics》2009,48(9):2550-2557
In this study we have analyzed the Kaluza-Klein type Robertson Walker (RW) cosmological model by considering variable cosmological
constant term Λ of the form:
,
and Λ∼ρ in the presence of strange quark matter with domain wall. The various physical aspects of the model are also discussed. 相似文献
4.
Bianchi Type I bulk viscous barotropic fluid cosmological with varying Λ is investigated. We have also assumed a functional
relation on Hubble parameter as H(R)=a(R
−n
+1), n>1, a>0, H the Hubble constant, R being scale factor and H = [(R)\dot]/RH = \dot{R}/R. The physical and geometrical aspects of the model related with astronomical observations are discussed. 相似文献
5.
Saibal Ray P. C. Ray M. Khlopov P. P. Ghosh U. Mukhopadhyay P. Chowdhury 《International Journal of Theoretical Physics》2009,48(9):2499-2510
Choosing the three phenomenological models of the dynamical cosmological term Λ, viz.,
,
and Λ∼ρ where a is the cosmic scale factor, it has been shown by the method of numerical analysis for the considered non-linear differential
equations that the three models are equivalent for the flat Universe k=0 and for arbitrary non-linear equation of state. The evolution plots for dynamical cosmological term Λ vs. time t and also the cosmic scale factor a vs. t are drawn here for k=0,+1. A qualitative analysis has been made from the plots which supports the idea of inflation and hence expanding Universe. 相似文献
6.
Linsen Zhang Puxun Wu Hongwei Yu 《The European Physical Journal C - Particles and Fields》2011,71(3):1588
In this paper, two modified Ricci models are considered as the candidates of unified dark matter–dark energy. In model one,
the energy density is given by rMR=3Mpl(aH2+b[(H)\dot])\rho_{\mathrm{MR}}=3M_{\mathrm{pl}}(\alpha H^{2}+\beta\dot{H}), whereas, in model two, by
rMR=3Mpl(\fraca6 R+g[(H)\ddot]H-1)\rho_{\mathrm{MR}}=3M_{\mathrm{pl}}(\frac{\alpha}{6} R+\gamma\ddot{H}H^{-1}). We find that they can explain both dark matter and dark energy successfully. A constant equation of state of dark energy
is obtained in model one, which means that it gives the same background evolution as the wCDM model, while model two can give an evolutionary equation of state of dark energy with the phantom divide line crossing
in the near past. 相似文献
7.
Lorenzo Iorio 《General Relativity and Gravitation》2010,42(1):199-208
Analytical and numerical calculations show that a putative temporal variation of the speed of light c, with the meaning of space-time structure constant c
ST, assumed to be linear over timescales of about one century, would induce a secular precession of the longitude of the pericenter
v{\varpi} of a test particle orbiting a spherically symmetric body. By comparing such a predicted effect to the corrections D[(v)\dot]{\Delta\dot\varpi} to the usual Newtonian/Einsteinian perihelion precessions of the inner planets of the Solar System, recently estimated by
E.V. Pitjeva by fitting about one century of modern astronomical observations with the standard classical/relativistic dynamical
force models of the EPM epehemerides, we obtained [(c)\dot]/c = (0.5±2)×10-7 yr-1{\dot c/c =(0.5\pm 2)\times 10^{-7} {\rm yr}^{-1}} . Moreover, the possibility that [(c)\dot]/c 1 0{\dot c/c\neq 0} over the last century is ruled out at 3−12σ level by taking the ratios of the perihelia for different pairs of planets. Our results are independent of any measurement
of the variations of other fundamental constants which may be explained by a variation of c itself (with the meaning of electromagnetic constant c
EM). It will be important to repeat such tests if and when other teams of astronomers will estimate their own corrections to
the standard Newtonian/Einsteinian planetary perihelion precessions with different ephemerides. 相似文献
8.
In this paper we have analyzed the Kaluza-Klein type Robertson Walker (RW) cosmological models by considering three different
forms of variable Λ:
,
and Λ∼ρ. It is found that, the connecting free parameters of the models with cosmic matter and vacuum energy density parameters are
equivalent, in the context of higher dimensional space time. The expression for the look back time, luminosity distance and
angular diameter distance are also derived. This work has thus generalized to higher dimensions the well-known results in
four dimensional space time. It is found that there may be significant difference in principle at least, from the analogous
situation in four dimensional space time. 相似文献
9.
We study the two-dimensional Gross-Pitaevskii theory of a rotating Bose gas in a disc-shaped trap with Dirichlet boundary
conditions, generalizing and extending previous results that were obtained under Neumann boundary conditions. The focus is
on the energy asymptotics, vorticity and qualitative properties of the minimizers in the parameter range |log ε|≪Ω≲ε
−2|log ε|−1 where Ω is the rotational velocity and the coupling parameter is written as ε
−2 with ε≪1. Three critical speeds can be identified. At
\varOmega = \varOmegac1 ~ |loge|\varOmega=\varOmega_{\mathrm{c_{1}}}\sim |\log\varepsilon| vortices start to appear and for
|loge| << \varOmega < \varOmegac2 ~ e-1|\log\varepsilon|\ll\varOmega< \varOmega_{\mathrm{c_{2}}}\sim \varepsilon^{-1} the vorticity is uniformly distributed over the disc. For
\varOmega 3 \varOmega c2\varOmega\geq\varOmega _{\mathrm{c_{2}}} the centrifugal forces create a hole around the center with strongly depleted density. For Ω≪ε
−2|log ε|−1 vorticity is still uniformly distributed in an annulus containing the bulk of the density, but at
\varOmega = \varOmegac3 ~ e-2|loge|-1\varOmega=\varOmega_{\mathrm {c_{3}}}\sim\varepsilon ^{-2}|\log\varepsilon |^{-1} there is a transition to a giant vortex state where the vorticity disappears from the bulk. The energy is then well approximated
by a trial function that is an eigenfunction of angular momentum but one of our results is that the true minimizers break
rotational symmetry in the whole parameter range, including the giant vortex phase. 相似文献
10.
Using Brownian hydrodynamic simulation techniques, we study single polymers in shear. We investigate the effects of hydrodynamic
interactions, excluded volume, chain extensibility, chain length and semiflexibility. The well-known stretching behavior with
increasing shear rate [(g)\dot] \dot{{\gamma}} is only observed for low shear [(g)\dot] \dot{{\gamma}} < [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} , where [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} is the shear rate at maximum polymer extension. For intermediate shear rates [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} < [(g)\dot] \dot{{\gamma}} < [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} the radius of gyration decreases with increasing shear with minimum chain extension at [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} . For even higher shear [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} < [(g)\dot] \dot{{\gamma}} the chain exhibits again shear stretching. This non-monotonic stretching behavior is obtained in the presence of excluded-volume
and hydrodynamic interactions for sufficiently long and inextensible flexible polymers, while it is completely absent for
Gaussian extensible chains. We establish the heuristic scaling laws [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} ∼ N
-1.4 and [(g)\dot]min \dot{{\gamma}}^{{\min}}_{} ∼ N
0.7 as a function of chain length N , which implies that the regime of shear-induced chain compression widens with increasing chain length. These scaling laws
also imply that the chain response at high shear rates is not a universal function of the Weissenberg number Wi = [(g)\dot] \dot{{\gamma}}
t \tau anymore, where t \tau is the equilibrium relaxation time. For semiflexible polymers a similar non-monotonic stretching response is obtained. By
extrapolating the simulation results to lengths corresponding to experimentally studied DNA molecules, we find that the shear
rate [(g)\dot]max \dot{{\gamma}}^{{\max}}_{} to reach the compression regime is experimentally realizable. 相似文献
11.
J. M. Albornoz A. Parravano 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,74(2):251-263
Using a set of heterogeneous competing systems with intra-system cooperation and inter-system aggression, we show how the
coevolution of the system parameters (degree of organization and conditions for aggression) depends on the rate of supply
of resources [(S)\dot]\dot{S}. The model consists of a number of units grouped into systems that compete for the resource S; within each system several
units can be aggregated into cooperative arrangements whose size is a measure of the degree of organization in the system.
Aggression takes place when the systems release inhibitors that impair the performance of other systems. Using a mean field
approximation we show that i) even in the case of identical systems there are stable inhomogeneous solutions; ii) a system
steadily producing inhibitors needs large perturbations to leave this regime; and iii) aggression may give comparative advantages.
A discrete model is used in order to examine how the particular configuration of the units within a system determines its
performance in the presence of aggression. We find that full-scale, one sided aggression is only profitable for less-organized
systems, and that systems with a mixture of degrees of organization exhibit robustness against aggression. By using a genetic
algorithm we find that, in terms of the full-occupation resource supply rate [(S)\dot]F\dot{S}_{F}, the coevolution of the set of systems displays the following behavior: i) for [(S)\dot] < [(S)\dot]F/10\dot{S}< \dot{S}_{F}/10 aggressions are irrelevant and most systems exhibit a high degree of organization; ii) For [(S)\dot]F/10 < [(S)\dot] < [(S)\dot]F/3\dot{S}_{F}/10 < \dot{S} < \dot{S}_{F}/3 aggressions are frequent, making systems with a low degree of organization competitive; iii) for [(S)\dot]F/3 < [(S)\dot] < [(S)\dot]F/2\dot{S}_{F}/3 < \dot{S} < \dot{S}_{F}/2 the systems display global evolutive transitions between periods of calm (few aggressions and high degree of organization)
and periods of belligerence (frequent aggressions and low degree of organization); iv) for $ \dot{S} > \dot{S}_{F}/2$ \dot{S} > \dot{S}_{F}/2 the periods of aggression becomes progressively rarer and shorter. Finally, when [(S)\dot]\dot{S} approaches [(S)\dot]F\dot{S}_{F} the selection pressure on the cooperativity and the aggression between systems disappears. This kind of model can be useful
to analyse the interplay of the cooperation/competition processes that can be found in some social, economic, ecological and
biochemical systems; as an illustration we refer to the competition between drug-selling gangs. 相似文献
12.
G. H. Arakelyan A. B. Kaidalov C. Merino Yu. M. Shabelski 《Physics of Atomic Nuclei》2011,74(3):426-436
The WA89 Collaboration experimental data on production of Λ, Σ−, Σ+, Ξ−, Ω− baryons, $
\bar \Lambda
$
\bar \Lambda
and $
\bar \Xi ^ +
$
\bar \Xi ^ +
antibaryons in Σ− collisions with C and Cu targets at 345 GeV/c ($
\sqrt {s_{\Sigma N} }
$
\sqrt {s_{\Sigma N} }
≈ 25.5 GeV) in the frame of the Quark-Gluon String Model is described. The comparison of the theoretical results with the
experimental data is discussed. Finally, some relations among the values of the model parameters obtained with the help of
quark combinatorics are presented. 相似文献
13.
We make the cosmological constant, Λ, into a field and restrict the variations of the action with respect to it by causality.
This creates an additional Einstein constraint equation. It restricts the solutions of the standard Einstein equations and
is the requirement that the cosmological wave function possess a classical limit. When applied to the Friedmann metric it
requires that the cosmological constant measured today, t
U
, be L ~ tU-2 ~ 10-122{\Lambda \sim t_{U}^{-2} \sim 10^{-122}} , as observed. This is the classical value of Λ that dominates the wave function of the universe. Our new field equation
determines Λ in terms of other astronomically measurable quantities. Specifically, it predicts that the spatial curvature
parameter of the universe is Wk0 o -k/a02H2=-0.0055{\Omega _{\mathrm{k0}} \equiv -k/a_{0}^{2}H^{2}=-0.0055} , which will be tested by Planck Satellite data. Our theory also creates a new picture of self-consistent quantum cosmological
history. 相似文献
14.
Anil Kumar Yadav 《International Journal of Theoretical Physics》2011,50(5):1664-1670
The present study deals with dissipative future universe without big rip in context of Eckart formalism. The generalised Chaplygin
gas, characterised by equation of state
p=-\fracAr\frac1ap=-\frac{A}{\rho^{\frac{1}{\alpha}}}, has been considered as a model for dark energy due to its dark-energy-like evolution at late time. It is demonstrated that,
if the cosmic dark energy behaves like a fluid with equation of state p=ωρ; ω<−1 as well as Chaplygin gas simultaneously then the big rip problem does not arise and the scale factor is found to be regular
for all time. 相似文献
15.
J. F. Cariñena 《The European physical journal. Special topics》2008,160(1):51-60
Milne–Pinney equation
[(x)\ddot]=-w2(t)x+ k/x3\ddot x=-\omega^2(t)x+ k/{x^3}
is usually studied together with the time-dependent harmonic oscillator
[(y)\ddot]+w2(t) y=0\ddot y+\omega^2(t) y=0
and the system is called Ermakov system, and actually Pinney showed in a short paper that the general solution of the first
equation can be written as a superposition of two solutions of the associated harmonic oscillator. A recent generalization
of the concept of Lie systems for second order differential equations and the usual techniques of Lie systems will be used
to study the Ermakov system. Several applications of Ermakov systems in Quantum Mechanics as the relation between Schroedinger
and Milne equations or the use of Lewis–Riesenfeld invariant will be analysed from this geometric viewpoint. 相似文献
16.
Y. F. Liu X. H. He D. H. Shi J. F. Sun 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2011,61(2):349-353
Quasi-classical trajectory (QCT) method is carried out to calculate the
dynamics of the H + LiH (v = 0, j = 0) ?\to H2 + Li reaction and its
isotopic variants based on the potential energy surface of the lowest 2A¢^2A' electronic state reported by Prudente et al. [Chem. Phys. Lett. 474, 18 (2009)]. The reaction cross-section,
product rotational alignment parameter áP2\langle P_2 ([(j¢)\vec]\vec{j'}
·\cdot
[(k)\vec])?\vec{k})\rangle and
one generalized polarization-dependent differential cross-section (2π/σ)(ds00d\sigma_{00}/d
wt\omega_t)
are calculated. We found that
different collision energies and mass factors show driving influence on the
process of the reactions and product molecules H2 (HD, D2)
polarization distribution, and the trend of the isotopic effects in the high
collision energy range is different to that in the low collision energy
range. The calculations are also interpreted in relation to the features of
the underlying potential energy surface. A comparison between the title
reactions and a barrier-less reaction F + HBr ?\to FH + Br has been
discussed in detail. 相似文献
17.
José Ignacio Rosado 《Foundations of Physics》2011,41(7):1200-1213
The quantum state of a d-dimensional system can be represented by a probability distribution over the d
2 outcomes of a Symmetric Informationally Complete Positive Operator Valued Measure (SIC-POVM), and then this probability distribution
can be represented by a vector of
\mathbb Rd2-1\mathbb {R}^{d^{2}-1} in a (d
2−1)-dimensional simplex, we will call this set of vectors Q\mathcal{Q}. Other way of represent a d-dimensional system is by the corresponding Bloch vector also in
\mathbb Rd2-1\mathbb {R}^{d^{2}-1}, we will call this set of vectors B\mathcal{B}. In this paper it is proved that with the adequate scaling B=Q\mathcal{B}=\mathcal{Q}. Also we indicate some features of the shape of Q\mathcal{Q}. 相似文献
18.
M. Tsamados 《The European physical journal. E, Soft matter》2010,32(2):165-181
Many amorphous glassy materials exhibit complex spatio-temporal mechanical response and rheology, characterized by an intermittent
stress strain response and a fluctuating velocity profile. Under quasistatic and athermal deformation protocols this heterogeneous
plastic flow was shown to be composed of plastic events of various sizes, ranging from local quadrupolar plastic rearrangements
to system spanning shear bands. In this paper, through numerical study of a 2D Lennard-Jones amorphous solid, we generalize
the study of the heterogeneous dynamics of glassy materials to the finite shear rate ( [(g)\dot] \dot{{\gamma}}
1 \neq 0 and temperature case (T
1 \neq 0 . In practice, we choose an effectively athermal limit (T ∼ 0 and focus on the influence of shear rate on the rheology of the glass. In line with previous works we find that the model
Lennard-Jones glass follows the rheological behavior of a yield stress fluid with a Herschel-Bulkley response of the form,
s \sigma = sY \sigma_{{Y}}^{} + c
1
[(g)\dot]b \dot{{\gamma}}^{{\beta}}_{} . The global mechanical response obtained through the use of Molecular Dynamics is shown to converge in the limit [(g)\dot] \dot{{\gamma}}
? \rightarrow 0 to the quasistatic limit obtained with an energy minimization protocol. The detailed analysis of the plastic deformation
at different shear rates shows that the glass follows different flow regimes. At sufficiently low shear rates the mechanical
response reaches a shear-rate-independent regime that exhibits all the characteristics of the quasistatic response (finite-size
effects, cascades of plastic rearrangements, yield stress, ...). At intermediate shear rates the rheological properties are
determined by the externally applied shear rate and the response deviates from the quasistatic limit. Finally at higher shear
the system reaches a shear-rate-independent homogeneous regime. The existence of these three regimes is also confirmed by
the detailed analysis of the atomic motion. The computation of the four-point correlation function shows that the transition
from the shear-rate-dominated to the quasistatic regime is accompanied by the growth of a dynamical cooperativity length scale
x \xi that is shown to diverge with shear rate as x \xi
μ \propto
[(g)\dot]-n \dot{{\gamma}}^{{-\nu}}_{} , with n \nu ∼ 0.2 -0.3. This scaling is compared with the prediction of a simple model that assumes the diffusive propagation of plastic
events. 相似文献
19.
We present the measurement of directed flow (v
1) for the identified particles, namely, Λ, $
\bar \Lambda
$
\bar \Lambda
and K
s
0, as a function of rapidity and centrality in Au+Au collisions at $
\sqrt {s_{NN} }
$
\sqrt {s_{NN} }
= 200 GeV and 62.4 GeV. The measurement is based on the run IV data obtained by the STAR experiment at RHIC. In order to
enhance event plane resolution, we use tracks reconstructed from the Forward Time Projection Chambers (FTPCs), together with
the sideward deflection of spectator neutrons measured by the STAR’s Shower Maximum Detector at Zero Degree Calorimeters (ZDC-SMDs).
We find that for 200 GeV, proton and antiproton v
1 is less than 1%, the K
s
0
Λ, $
\bar \Lambda
$
\bar \Lambda
v
1 is less than 2%; for 62 GeV, proton v
1 is less than 1% and antiproton is less than 2%, v
1 for K
s
0, Λ, $
\bar \Lambda
$
\bar \Lambda
is less than 2% in Au+Au collisions at 200 GeV. 相似文献
20.
Vladimir Rabinovich 《Russian Journal of Mathematical Physics》2012,19(1):107-120
The propagation of electromagnetic waves issued by modulated moving sources of the form j( t,x ) = a( t )e - iw0 t [(x)\dot]0 ( t )d( x - x0 ( t ) )j\left( {t,x} \right) = a\left( t \right)e^{ - i\omega _0 t} \dot x_0 \left( t \right)\delta \left( {x - x_0 \left( t \right)} \right) is considered, where j(t, x) stands for the current density vector, x = (x
1, x
2, x
3) ∈ ℝ3 for the space variables, t ∈ ℝ for time, t → x
0(t) ∈ ℝ3 for the vector function defining the motion of the source, ω
0 for the eigenfrequency of the source, a(t) for a narrow-band amplitude, and δ for the standard δ function. Suppose that the media under consideration are dispersive. This means that the electric and magnetic permittivity
ɛ(ω), μ(ω) depends on the frequency ω. We obtain a representation of electromagnetic fields in the form of time-frequency oscillating integrals whose phase contains
a large parameter λ > 0 characterizing the slowness of the change of the amplitude a(t) and the velocity [(x)\dot]0 ( t )\dot x_0 \left( t \right) and a large distance between positions of the source and the receiver. Applying the two-dimensional stationary phase method
to the integrals, we obtain explicit formulas for the electromagnetic field and for the Doppler effects. As an application
of our approach, we consider the propagation of electromagnetic waves produced by moving source in a cold nonmagnetized plasma
and the Cherenkov radiation in dispersive media. 相似文献