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1.
We explore flat ΛCDM models with bulk viscosity, and study the role of the bulk viscosity in the evolution of these universe
models. The dynamical equations for these models are obtained and solved for some cases of bulk viscosity. We obtain differential
equations for the Hubble parameter H and the energy density of dark matter ρ
m
, for which we give analytical solutions for some cases and for the general case we give a numerical solution. Also we calculate
the statefinder parameters for these models and display them in the s–r-plane. 相似文献
2.
We study the fundamental modes of radiation hydrodynamic linear waves that arise from one-dimensional small-amplitude initial fluctuations with wave number k in a radiating and scattering grey medium by taking into account the gravitational effects. The equation of radiative acoustics is derived from three hydrodynamic equations, Poisson’s equation, and two moment equations of radiation, by assuming a spherical symmetry for the matter and radiation and by using the Eddington approximation. We solve the dispersion relation as a quintic function of angular frequency ω, the wave number k being a real parameter. Numerical results reveal that wave patterns of five solutions are distinguished into three types: the radiation-dominated, type 1, and type 2 matter-dominated cases. In the case of no gravitaional effects (Kaneko et al., 2005), the following wave modes appear: radiation wave, conservative radiation wave, entropy wave, Newtonian-cooling wave, opacity-damped and cooling-damped waves, constant-volume and constant-pressure diffusions, adiabatic sound wave, cooling-damped and drag-force-damped isothermal sound waves, isentropic radiation-acoustic wave, and gap mode. Meanwhile, the gravitaional effects being taken into account, the growing gravo-diffusion mode newly arises from the constant-pressure diffusion at the point that k agrees with Jeans’ wave number specified by the isothermal sound speed. This mode changes to the growing radiation-acoustic gravity mode near the point that k becomes Jeans’ wave number specified by the isentropic radiation-acoustic speed. In step with a transition between them, the isentropic radiation-acoustic wave splits into the damping radiation-acoustic gravity mode and constant-volume diffusion. The constant-volume diffusion emerges twice if the gravitational effects are taken into account. Since analytic solutions are derived for all wave modes, we discuss their physical significance. The critical conditions are given which distinguish between radiation-dominated and type 1 matter-dominated cases, and between type 1 and type 2 matter-dominated cases. Waves in a self-gravitating scattering grey medium are also analyzed, which provides us some hints for the effects of energy and momentum exchange between matter and radiation. 相似文献
3.
We study quarkand strangequarkmatter in the contextof generalrelativity.For this purpose,we solve Einstein's field equations for quark and strange quark matter in spherical symmetric space-times. We analyze strange quark matter for the different equations of state (EOS) in the spherical symmetric space-times,thus we are able to obtain the space-time geometries of quark and strange quark matter. Also,we discuss the features of the obtained solutions. The obtained solutions are consistent with the results of Brookhaven Laboratory,i.e. the quark-gluon plasma has a vanishing shear (i.e. quark-gluon plasma is perfect). 相似文献
4.
Radio-emission of pulsars is investigated through computer simulation. We assume that electronpositron plasma is thermal and beam particles have
b
in the plasma rest frame. The main purpose of this study is to search for unstable electromagnetic waves which may be caused by beam-plasma interactions. We obtain the dominant waves in the range of radio frequencies in the observer frame. The results show that transverse electromagnetic waves grow rapidly, while electrostatic waves have very slow growth. 相似文献
5.
We state the time-dependent problem in the Jordan-Brans-Dicke tensor-scalar theory of gravitation. We show that in the conformally corresponding space the equations of the problem have the same form as the Einstein equations with an extra source in the form of the components of the energy-momentum tensor of the scalar field. We discuss the question whether Birkhoff's theorem can hold in the Jordan-Brans-Dicke theory. We simultaneously obtain a formula that expresses the mass of an isolated static object as a function of the distribution of pressure of matter. We also discuss the integrability of the vacuum time-dependent equations of the Einstein theory.Translated fromAstrofizika, Vol. 37, No. 3, 1994.The authors are grateful to the participants in the seminar of the Department of Theoretical Physics of Erevan State University for helpful discussions. Two of the authors (VVP and GGA) worked with partial support from grant RY 6000 of the International Science Fund (Soros Foundation). 相似文献
6.
R. A. Krikorian 《Astrophysics》1999,42(3):338-343
The field-to-particle method of H. P. Robertson as applied by Noonan, in order to obtain the general relativistic equations
describing the trajectory of a photon in a refractive medium, is compared with Synge’s general relativistic Hamiltonian theory
of waves and rays. For a photon in vacuum it is known that both approaches yield the same equation for the trajectory, i.e.,
a null geodesic. However for a photon in a medium, in contradistinction to the Hamiltonian theory, the field-to-particle method
(a) yields equations of the photon trajectory valid only in a nondispersive medium, (b) the time component u0
of the tangent to the ray remains an undetermined quantity, (c) agreement with the Hamiltonian theory is achieved by substituting
into Noonan’s equations the Hamiltonian expression for u
0.
Published in Astrofizika, Vol. 42, No. 3, pp. 449–455, July–September, 1999. 相似文献
7.
A set of fluid type equations is derived to describe the macroscopic behaviour of a plasma consisting of a mixture of matter and antimatter. The equations are written in a form which displays the full symmetry of the medium with respect to particle charge and mass, a symmetry absent in normal plasmas. This symmetry of the equations facilitates their manipulation and solution, and by way of illustration the equations are used to analyze the propagation of electromagnetic and acoustic waves through a matter-antimatter plasma. Some differences from the propagation of such waves in a normal plasma are noted. 相似文献
8.
A weakly nonlinear theory of wave propagation in superposed fluids in the presence of magnetic fields is presented in this paper. We derive the equations governing the evolution of the amplitude of the progressive as well as the standing waves. It is demonstrated that the waves can be unstable against modulation in the presence of the magnetic fields. We also obtain the nonlinear cut off wave number which separates the region of stability from instability. 相似文献
9.
G. Jovanović 《Solar physics》2014,289(11):4085-4104
We derive the dispersion equation for gravito-magnetohydrodynamical (MHD) waves in an isothermal, gravitationally stratified plasma with a horizontal inhomogeneous magnetic field. Sound and Alfvén speeds are constant. Under these conditions, it is possible to derive analytically the equations for gravito-MHD waves. The high values of the viscous and magnetic Reynolds numbers in the solar atmosphere imply that the dissipative terms in the MHD equations are negligible, except in layers around the positions where the frequency of the MHD wave equals the local Alfvén or slow wave frequency. Outside these layers the MHD waves are accurately described by the equations of ideal MHD. We consider waves that propagate energy upward in the atmosphere. For the plane boundary, z=0, between two isothermal plasma regions with horizontal but different magnetic fields, we discuss the boundary conditions and derive the equations for the reflection and transmission coefficients. In the simpler case of a gravitationally stratified plasma without magnetic field, these coefficients describe the reflection and transmission properties of gravito-acoustic waves. 相似文献
10.
Dieter Lorenz-Petzold 《Astrophysics and Space Science》1984,106(2):409-413
We discuss the Brans-Dicke-Bianchi type-VIIh field equations. Exact solutions are given for the vacuum case as well as for the stiff matter case. The derived solutions are the generalizations of the GRT-solutions first given by Doroshkevichet al. (1973) and Lukash (1974a). In addition we present a new BDT-stiff matter solution which has no analogy in the GRT. 相似文献
11.
We obtain the possible ellipsoidal and hyperboloidal equilibrium figures of the light subsystem with internal fluxes of matter of constant vorticity inside a gravitating homogeneous triaxial ellipsoid. We obtain the domains of stability of these equilibrium figures with respect to second-form oscillations.Translated fromAstrofizika, Vol. 38, No. 2, 1995. 相似文献
12.
The linear theory and N-body simulations are used to present a new, alternative model of the galaxy A0035-324 (the “Cartwheel”), which is the most
striking example of the relatively small class of ring galaxies. The model is based on the gravitational Jeans-type instability
of both axisymmetric (radial) and nonaxisymmetric (spiral) small-amplitude gravity perturbations (e.g., those produced by
spontaneous disturbances) of a dynamically cold subsystem (identified as the gaseous component) of an isolated disk galaxy.
The simplified model of a galaxy is used in which stars (and a dark matter, if it exists at all) do not participate in the
disk collective oscillations and just form a background charge. In the theory presented here, a case for both purely radial
solutions and purely spiral solutions to the equations of motion of an infinitesimally thin gaseous disk is made, which is
associated with both a radial density wave and a dominant spiral density wave which propagate outwards creating a rough ring
and a number of spiral arms. Through three-dimensional numerical simulation of a collisionless set of many particles, I associate
these gravitationally unstable axisymmetric waves and nonaxisymmetric waves with growing clumps of matter which take on the
appearance of a ring and spokes of mass blobs. 相似文献
13.
We investigate nonadiabatic hydrodynamic waves in a nongrey, radiating, thermally conducting, homogeneous atmosphere in LTE with a finite mean free path of photons. Avoiding the Eddington approximation the remaining simplifications in the basic equations are discussed, the generalized dispersion relation is analysed, and some wave properties in a grey model are studied. The properties of waves in a stratified atmosphere are analysed as well. In connection with the predicted properties of the nonadiabatic waves we discuss observations ofp-modes by measuring brightness fluctuations. 相似文献
14.
A parametric survey on the propagation characteristics of the dust ion-acoustic (DIA) shock waves showing the effect of nonextesivity with nonextensive electrons in a dissipative dusty plasma system has been carried out using the reductive perturbation technique. We have considered continuity and momentum equations for inertial ions, q-distributed nonextensive electrons, and stationary charged dust grains, to derive the Burgers equation. It has been found that the basic features of DIA shock waves are significantly modified by the effects of electron nonextensivity and ion kinematic viscosity. Depending on the degree of nonextensivity of electrons, the dust ion-acoustic shock structures exhibit compression and rarefaction. The implications of our results would be useful to understand some astrophysical and cosmological scenarios like stellar polytropes, hadronic matter and quark-gluon plasma, protoneutron stars, dark-matter halos, etc., where effects of nonextensivity can play the significant roles. 相似文献
15.
Omar Benhar Emanuele Berti Valeria Ferrari 《Monthly notices of the Royal Astronomical Society》1999,310(3):797-803
We discuss the dependence of the pulsation frequencies of the axial quasi-normal modes of a non-rotating neutron star upon the equation of state describing the star interior. The continued fraction method has been used to compute the complex frequencies for a set of equations of state based on different physical assumptions and spanning a wide range of stiffness. The numerical results show that axial gravitational waves carry relevant information on both the structure of neutron star matter and the nature of the hadronic interactions. 相似文献
16.
A reconstruction of sunspot numbers for the last 1000 years was obtained using a sum of sine waves derived from spectral analysis of the time series of sunspot number R
z for the period 1700–1999. The time series was decomposed in frequency levels using the wavelet transform, and an iterative regression model (ARIST) was used to identify the amplitude and phase of the main periodicities. The 1000-year reconstructed sunspot number reproduces well the great maximums and minimums in solar activity, identified in cosmonuclides variation records, and, specifically, the epochs of the Oort, Wolf, Spörer, Maunder, and Dalton Minimums as well the Medieval and Modern Maximums. The average sunspot number activity in each anomalous period was used in linear equations to obtain estimates of the solar radio flux F
10.7, solar wind velocity, and the southward component of the interplanetary magnetic field. 相似文献
17.
In the context of the Kaluza-Klein theory we obtain the external solution for the gravitational field generated by a static spherically symmetric distribution of neutral and charged matter. We obtain a formula that connects the mass of the central body with the integral of the pressure distribution inside it.Translated fromAstrofizika, Vol. 38, No. 3, 1995. 相似文献
18.
Time-dependent wormhole solutions are found which evolve in a cosmological background. Solutions are presented both for GR
and Brans-Dicke field equations. Conditions are derived for the supporting matter to be non-exotic. The traceless energy-momentum
tensor needed to support the geometry is in the form of an anisotropic fluid. Far from the wormhole, the equation of state
rapidly approaches that of an isotropic perfect fluid with p = 1/3 ρ. For the BD wormholes we obtain ρ = 0everywhere, except for the π = const. limit, in which case the GR results are
reproduced.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
António Vale José P. S. Lemos 《Monthly notices of the Royal Astronomical Society》2001,325(3):1197-1204
There is now evidence that the cosmological constant Λ has a non-zero positive value. Alternative scenarios to a pure cosmological constant model are provided by quintessence, an effective negative pressure fluid permeating the Universe. Recent results indicate that the energy density ρ and the pressure p of this fluid are constrained by − ρ ≤ p ≲−0.6 ρ . As p =− ρ is equivalent to the pure cosmological constant model, it is appropriate to analyse this particular, but important, case further.
We study the linear theory of perturbations in a Friedmann–Robertson–Walker universe with a cosmological constant. We obtain the equations for the evolution of the perturbations in the fully relativistic case, for which we analyse the single-fluid and two-fluid cases. We obtain solutions to these equations in appropriate limits. We also study the Newtonian approximation. We find that for a positive cosmological constant universe (i) the perturbations will grow more slowly in the relativistic regime for a two-fluid composed of dark matter and radiation, and (ii) in the Newtonian regime the perturbations stop growing. 相似文献
We study the linear theory of perturbations in a Friedmann–Robertson–Walker universe with a cosmological constant. We obtain the equations for the evolution of the perturbations in the fully relativistic case, for which we analyse the single-fluid and two-fluid cases. We obtain solutions to these equations in appropriate limits. We also study the Newtonian approximation. We find that for a positive cosmological constant universe (i) the perturbations will grow more slowly in the relativistic regime for a two-fluid composed of dark matter and radiation, and (ii) in the Newtonian regime the perturbations stop growing. 相似文献
20.
The comoving-frame equations of radiative transfer and moment equations to accurate terms of all orders inv/c are derived in the modified Lagrangian form. The equations exactly describe the interaction of radiation with matter in a relativistically moving medium in flat or curved spacetime. Two specialized sets of equations are presented: (1) the equation of radiative transfer and moment equations accurate to terms of second order (v
2/c
2), and (2) the transfer equation and moment equations for a radial flow in curved spacetime with the Schwarzschild-type metric. 相似文献