共查询到20条相似文献，搜索用时 31 毫秒
1.
Differential equations are derived, following the methods ofLifshitz (1946) andLifshitz andKhalatnikov (1963), for density perturbations in isotropic, spatially homogeneous cosmological models of arbitrary space curvature. The unperturbed models contain both matter and radiation. An explicit third-order equation is obtained for the time development of the perturbations, and it is shown that one of the solutions is not covariantly defined. The two remaining solutions are compared with existing solutions for the limiting cases of radiation-filled and dust-filled models. The results ofBonnor's (1957) Newtonian analysis are shown to be a valid limiting case of our equation when the pressurep is finite, but small compared with the densityp timesc
2.A detailed analysis is given of a model containing coupled radiation (p=pc
2/3) and dust (p=0). It is shown that density perturbations with long wavelengths are unstable (with slow growth rate) for all time. The instability exists because for a long-wavelength disturbance, the time scale governing the propagation of pressure effects (which stabilize perturbations) is longer than the time scale for which pressure falls to the point of ineffectiveness. The present value of the critical wavelength is 60 Mpc in models based on flat space sections in which the present background radiation temperature is 3 °K.The research reported herein was supported in part by the Atomic Energy Commission under contract number AT(11-1)-34, Project Agreement No. 125, and by the National Science Foundation, under Grant GP-4975. 相似文献
2.
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
3.
P. P. Malovichko 《Kinematics and Physics of Celestial Bodies》2014,30(1):22-31
The work is devoted to the study of the behavior of dispersive Alfvén waves in astrophysical plasma of finite and high pressure. All the main wave characteristics were obtained, namely, the dispersion, fading, polarization, density perturbations, and charge density perturbations. The effect of the parameters of the space environment on the behavior and properties of dispersive Alfvén waves was analyzed. The wave behavior in finite and high-pressure plasmas is shown to differ appreciably from the behavior in very low, intermediate, and low-pressure plasmas. 相似文献
4.
Perturbations of the matter density in a homogeneous and isotropic cosmological model which leads to the formation of galaxies should, at later stages of evolution, cause spatial fluctuations of relic radiation. Silk assumed that an adiabatic connection existed between the density perturbations at the moment of recombination of the initial plasma and fluctuations of the observed temperature of radiation δT/T=δ ?m /3 ?m . It is shown in this article that such a simple connection is not applicable due to:
- The long time of recombination;
- The fact that when regions withM<1015 M ⊙ become transparent for radiation, the optical depth to the observer is still large due to Thompson scattering;
- The spasmodic increase of δ ?m/?m in recombination.
5.
This paper studies libration dynamics and stability of deorbiting nano-satellites by short and bare electrodynamic tethers. A critical aspect of satellite deorbit by an electrodynamic tether is to maintain the tether aligned with the local vertical and stable while subjected to external perturbations. The dynamics of electrodynamic tether system in deorbit application is divided into the orbital motion of the center of system’s mass and the tether libration motion relative to that center. Major space environmental perturbations including the current-induced electrodynamic force, atmospheric drag, oblateness effect of the Earth, irregularity of geomagnetic field, variable plasma density, solar radiation pressure, and lunisolar gravitational attractions are considered in the dynamic analysis. Quantitative analyses are provided in order to characterize the order of the perturbative torques during the deorbit process. A single index is derived from the libration energy to stabilize the libration motion by regulating the current in the tether through simple on-off switching. Numerical results show that the libration dynamics of an electrodynamic tether has significant impacts on the deorbit process and the electrodynamic tether cannot effectively deorbit satellites without libration stability control. The proposed current regulation strategy is simple and very effective in stabilizing libration motion of an electrodynamic tether. 相似文献
6.
Adi Nusser 《Monthly notices of the Royal Astronomical Society》2005,361(3):977-982
We discuss the stability of galactic discs in which the energy of interstellar clouds is gained in encounters with expanding supernova (SN) remnants and lost in inelastic collisions. Energy gain and loss processes introduce a phase difference between the pressure and density perturbations, making discs unstable on small scales for several recipes of star formation. This is in contrast to the standard stability analysis in which small-scale perturbations are stabilized by pressure. In the limit of small scales, the dispersion relation for the growth rate reduces to that of thermal instabilities in a fluid without gravity. If instabilities lead to star formation, then our results imply a secondary mode of star formation that operates on small scales and feeds on the existence of a primary mode on intermediate scales. This may be interpreted as positive feedback. Further, the standard stability criterion on intermediate scales is significantly modified. 相似文献
7.
Previous analyses of the orbits of spherical balloon satellites have attempted to satisfy residuals in observed perturbations in Keplerian elements, assumed to be caused by diffuse radiation pressure, by introducing small variations ins, the parameter representing the reflection characteristics of the satellite's surface. It is difficult to distinguish, however, between those perturbations caused by diffuse radiation and those caused by reflected radiation, as a result of the deformation of the assumed sphere. Following the derivation by Lucas of exact expressions for both incident and reflected radiation forces on a prolate spheroidal satellite, and the subsequent work of Aksnes pertaining to spherical satellites, the theory is extended to include the effects of diffuse radiation whilst at the same time qualifying the assumption that the radiation force acting along the Sun-satellite line can be taken as parallel to the Sun-Earth line. 相似文献
8.
The purpose of this study is to analyze the dynamical role of a radiation field on the growth rate of the unstable Kelvin-Helmholtz (KH) perturbations. As a first step toward this purpose, the analyze is done in a general way, irrespective of applying the model to a specific astronomical system. The transition zone between the two layers of the fluid is ignored. Then, we perform a linear analysis and by imposing suitable boundary conditions and considering a radiation field, we obtain appropriate dispersion relation. Unstable modes are studied by solving the dispersion equation numerically, and then growth rates of them are obtained. By analyzing our dispersion relation, we show that for a wide range of the input parameters, the radiation field has a destabilizing effect on KH instability. In eruptions of the galaxies or supermassive stars, the radiation field is dynamically important and because of the enhanced KH growth rates in the presence of the radiation; these eruptions can inject more momentum and energy into their environment and excite more turbulent motions. 相似文献
9.
Ravi Subrahmanyan 《Journal of Astrophysics and Astronomy》1997,18(4):251-255
The discovery of the 3K microwave background radiation (MBR) and its interpretation as a relict of the hot big bang was probably the most important
observation that led to the elevation of the hot big bang model to the status of a ‘Standard Model’. The temperature of this
background is consistent with the primordial nucleosynthesis hypothesis. Detailed measurements of the spectrum and angular
anisotropy of this radiation background have been found — within the measurement errors - to be consistent with the expectations
of the Standard Model and with the formation of structure from the gravitational growth of primordial seed density perturbations
within this framework. 相似文献
10.
P. Moore 《Celestial Mechanics and Dynamical Astronomy》1979,20(2):125-142
The orbital accelerations of certain balloon satellites exhibit marked oscillations caused by solar radiation impinging on the surface of the satellites, which, once spherical, have assumed a spheroidal shape producing a component of force at right-angles to the Sun-satellite direction. Given the characteristics and orientation of the satellite, the equations of force are determined by the formulae of Lucas. Otherwise the phase-angle and magnitude of the right-angle force are determined by trial and error, or best-fit techniques. Using a variation of the approach developed by Aksnes, a semi-analytical algorithm is presented for evaluating the perturbations of the Keplerian elements by direct solar radiation pressure on a spheroidal satellite. The perturbations are obtained by summing over the sunlit part of each orbit and allow for a linear variation in the phase-angle. The algorithm is used to determine the orbital accelerations of 1963-30D due to direct solar radiation pressure, and these results are compared to the observed values over two separate periods of the satellite's lifetime. 相似文献
11.
Properties of dispersive Alfvén waves: 1. Kinetics (very low, intermediate, and low density plasmas)
P. P. Malovichko 《Kinematics and Physics of Celestial Bodies》2013,29(6):269-284
The work is devoted to the study of the behavior of dispersive Alfvén waves, including inertial and kinetic Alfvén waves, in astrophysical plasma of very low, intermediate, and low pressure. New and full solutions were obtained. The solutions for “ordinary” and inertial Alfvén waves were shown to be particular cases of the general solution. The effect of the parameters of the astrophysical medium on the behavior and properties of dispersive Alfvén waves was analyzed. All the main wave characteristics were obtained, namely, the dispersion, fading, polarization, density perturbations, and charge density perturbations, whose consideration is essential for observation and detection of these waves and for more adequate understanding of their role in different astrophysical processes that occur in the space environment. 相似文献
12.
J. P. Mücket 《Astronomische Nachrichten》1987,308(1):1-7
The evolution of small-scale density perturbations on the background of increasing large-scale perturbations of supercluster size will be considered. In the case that the characteristic length scales of both perturbation modes differ significantly, the interaction between both modes has to be taken into account already within lowest order of approximation. It will be shown that in this case an effective amplification for the smaller-scale perturbations occurs. For these perturbations the characteristic times of evolution decreases in dependence on the considered mass-scales more or less rapidly. Therefore, the growth of adiabatic density perturbations on mass-scales up to galaxy masses seems to be triggered by the density evolution of superclusters which the smaller-mass perturbations are embedded in. A model for the formation of observed condensed matter distribution will be proposed. 相似文献
13.
O. Bschorr 《Solar physics》1988,116(2):401-404
In a gas heat transport is accompanied by the transport of momentum. The momentum change that accompanies a spatial change in heat flow - this is the radiometer force - results in a pressure gradient. This effect is analogous to the radiation pressure of wavemechanical energy transport. The radiometer pressure increases with temperature and temperature gradient but is independent of the gas density. In the transition zone and in the solar corona the radiometer forces have a definite effect on the pressure balance within the solar atmosphere. In this note the relationship between the radiometer pressure and the acoustic radiation pressure in the solar atmosphere is derived. 相似文献
14.
Kanan K. Datta Somnath Bharadwaj T. Roy Choudhury 《Monthly notices of the Royal Astronomical Society》2007,382(2):809-818
Linear transient phenomena induced by flow non-normality in thin self-gravitating astrophysical discs are studied using the shearing sheet approximation. The considered system includes two modes of perturbations: vortex and (spiral density) wave. It is shown that self-gravity considerably alters the vortex mode dynamics; its transient (swing) growth may be several orders of magnitude stronger than in the non-self-gravitating case and two to three times larger than the transient growth of the wave mode. Based on this finding, we comment on the role of vortex mode perturbations in a gravitoturbulent state. We also describe the linear coupling of the perturbation modes, caused by the differential character of disc rotation. The coupling is asymmetric: vortex mode perturbations are able to excite wave mode perturbations, but not vice versa. This asymmetric coupling lends additional significance to the vortex mode as a participant in spiral density waves and shock manifestations in astrophysical discs. 相似文献
15.
D. J. Heath 《Astrophysics and Space Science》1991,175(1):35-50
In this paper we review the gravitational instability theory of galaxy formation. Closed-form expressions for the rate of growth of density perturbations in the regime prior to recombination are presented in special cases in which the cosmological constant is non-zero. After recombination, the fate of perturbations in Lemaître universes is studied with the aid of exact solutions to the density perturbation equation orginally derived by Bonnor. 相似文献
16.
P. P. Malovichko 《Kinematics and Physics of Celestial Bodies》2014,30(5):223-233
The behavior of dispersive Alfven waves (DAWs) in astrophysical plasmas of finite and high pressure, which have not been considered thus far, is studied in the hydrodynamic approximation. The results are analyzed and compared with those obtained in the kinetic approach. It is shown that one general solution for DAWs in plasmas of finite and high pressure can be obtained using the hydrodynamic approach in contrast to the kinetic one. Kinetic and hydrodynamic solutions correspond to each other very well in a domain with weakly damped DAWs; however, solutions may differ appreciably in some parameter domains, especially in high-pressure plasma. The effect of parameters of the astrophysical medium on the DAW behavior and properties is analyzed. All the main wave characteristics were determined: dispersion, damping, polarization, density perturbations, and charge density perturbations. Since finite-pressure plasma is one of the most frequently encountered states of astrophysical plasma, it is very important to take into account specific features in behavior of these waves for their detecting and a more correct understanding of their behavior and the role they play in different astrophysical processes that occur in space environment. 相似文献
17.
Alexander Wittig Camilla Colombo Roberto Armellin 《Celestial Mechanics and Dynamical Astronomy》2017,128(4):435-452
This paper introduces and combines for the first time two techniques to allow long-term density propagation in astrodynamics. First, we introduce an efficient method for the propagation of phase space densities based on differential algebra (DA) techniques. Second, this DA density propagator is used in combination with a DA implementation of the averaged orbital dynamics through semi-analytical methods. This approach combines the power of orbit averaging with the efficiency of DA techniques. While the DA-based method for the propagation of densities introduced in this paper is independent of the dynamical system under consideration, the particular combination of DA techniques with averaged equations of motion yields a fast and accurate technique to propagate large clouds of initial conditions and their associated probability density functions very efficiently for long time. This enables the study of the long-term behavior of particles subjected to the given dynamics. To demonstrate the effectiveness of the proposed approach, the evolution of a cloud of high area-to-mass objects in Medium Earth Orbit is reproduced considering the effects of solar radiation pressure, the Earth’s oblateness and luni-solar perturbations. The method can propagate 10,000 random fragments and their density for 1 year within a few seconds on a common desktop PC. 相似文献
18.
Net radiative flux measurements by instruments on the Pioneer Venus Day, North, and Night probes are too large below 30 km to be consistent with present estimates of atmospheric opacity. We evaluate the only known mechanisms which could potentially have caused significant errors in the deep atmosphere, namely, (1) radiation field perturbations behind each probe due to its thermal wake, (2) cloud particle deposition on the sensor windows, and (3) thermal perturbations within the radiation sensor produced by gas flow through the sensor window retainers. Thermal analysis of the wake effect shows that temperature perturbations are not large enough to produce significant flux perturbations when gas opacity and sensor field-of-view characteristics are taken into account. The particle deposition effect is rejected because it requires a signature in the measured radiation profile which is not observed. The absence of such a feature also implies that mode 3 cloud particles are either not sulfuric acid or are far less numerous than previously reported. We find that the third mechanism is the most likely source of the large net flux measurements. However, this error is not sufficiently constrained by laboratory data to allow rigorous corrections to the measured flux profiles. If we use radiative transfer calculations to constrain the fluxes at 14 km and limited laboratory data to estimate the altitude dependence of the error, then we obtain a plausible set of corrected flux profiles which are roughly consistent with reasonable H2O mixing ratios below the clouds. 相似文献
19.
The growth of two-dimensional axisymmetric perturbations in the motion of a neutral shell formed in the interstellar medium
when an ionization-shock front exits at the surface of a cloud is simulated numerically. The perturbations are assumed to
emerge when the shock ahead of the ionization front reaches the cloud boundary. For long-wavelength perturbations, the accumulation
of mass has been found to take place in radially oriented condensations in the shape of a rod pointed toward the star and
widened at the opposite end as a result of instability. The shell fragmentation is accompanied by supersonic spouting of a
hot plasma into a low-density medium. Flow nonstationarity is shown to affect significantly the gas density and velocity distributions
both inside and in the immediate vicinity of the condensation. As one recedes from the ionization front, the density of charged
particles changes only slightly, which is inconsistent with the power law of density decrease with increasing distance from
the condensation center commonly used in interpreting observations. 相似文献
20.
David Arnas Daniel Casanova Eva Tresaco 《Celestial Mechanics and Dynamical Astronomy》2017,128(2-3):197-219
The aim of the time distribution methodology presented in this paper is to generate constellations whose satellites share a set of relative trajectories in a given time, and maintain that property over time without orbit corrections. The model takes into account a series of orbital perturbations such as the gravitational potential of the Earth, the atmospheric drag, the Sun and the Moon as disturbing third bodies and the solar radiation pressure. These perturbations are included in the design process of the constellation. Moreover, the whole methodology allows to design constellations with multiple relative trajectories that can be distributed in a minimum number of inertial orbits. 相似文献