共查询到20条相似文献,搜索用时 933 毫秒
1.
《New Astronomy》2021
This paper explores the collapsing process of a unidirectional isotropic matter configuration. The junction conditions for a static exterior geometry and the non-static interior geometry are expressed in terms of the cosmological constant. The time-lapse for the appearance of the black hole and the cosmological horizon is calculated. It is observed that in the de-Sitter space, the unidirectional perfect fluid flow does not make the collapse disappear. The vacuum energy of the cosmological constant makes the collapsing process quite slow and affects the time-lapse of horizon formation. Moreover, the presence of string tension increases the time-lapse of horizon formation. 相似文献
2.
《New Astronomy》2016
Using the Darmois–Israel formalism technique, charged thin shell in the presence of a cosmological constant is constructed. An equation governing the behavior of the radial pressure across the junction surface is deduced. The cosmological constant term and the charge term slows down the collapse of matter.The spherical N-shell model with an appropriate initial condition imitates quite well the FRW universe with Λ ≠ 0. 相似文献
3.
In this paper, we investigate spherically symmetric perfect fluid gravitational collapse in metric f(R) gravity. We take non-static spherically symmetric metric in the interior region and static spherically symmetric metric
in the exterior region of a star. The junction conditions between interior and exterior spacetimes are derived. The field
equations in f(R) theory are solved using the assumption of constant Ricci scalar. Inserting their solution into junction conditions, the
gravitational mass is found. Further, the apparent horizons and their time of formation is discussed. We conclude that the
constant scalar curvature term f(R
0) acts as a source of repulsive force and thus slows down the collapse of matter. The comparison with the corresponding results
available in general relativity indicates that f(R
0) plays the role of the cosmological constant. 相似文献
4.
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
5.
The solutions of Einstein’s equations with cosmological constant (Λ) in the presence of a creation field have been obtained for general class of anisotropic cosmological models. We have obtained the cosmological solutions for two different scenarios of average scale factor. In first case, we have discussed three different types of physically viable cosmological solutions of average scale factor for the general class of Bianchi cosmological models by using a special law for deceleration parameter which is linear in time with a negative slope. In second case, we have discussed another three different forms of cosmological solutions by using the average scale factor in three different scenarios like Intermediate scenario, Logamediate scenario and Emergent scenario. All physical parameters are calculated and discussed in each physical viable cosmological model. We examine the nature of creation field and cosmological constant is dominated the early Universe but they do not survive for long time and finally tends to zero for large cosmic time t. We have also discussed the all energy conditions in each cases. 相似文献
6.
In this work, we first obtain the hydrostatic equilibrium equation in dilaton gravity. Then, we examine some of the structural characteristics of a strange quark star in dilaton gravity in the context of Einstein gravity. We show that the variations of dilaton parameter do not affect the maximum mass, but variations in the cosmological constant lead to changes in the structural characteristics of the quark star. We investigate the stability of strange quark stars by applying the MIT bag model with dilaton gravity. We also provide limiting values for the dilaton field parameter and cosmological constant. We also study the effects of dilaton gravity on the other properties of a quark star such as the mean density and gravitational redshift.We conclude that the last reported value for the cosmological constant does not affect the maximum mass of a strange quark star. 相似文献
7.
R. K. Tiwari 《Astrophysics and Space Science》2009,319(1):85-87
We study Bianchi type-III cosmological model filled with perfect fluid in the presence of cosmological constant Λ(t). The Hubble law utilised yields a constant value of deceleration parameter. Physical and Kinematical properties of the model
have also studied.
相似文献
8.
Exact solutions of the field equations for a Bianchi type-I space-time, filled with a viscous fluid and cosmological constant,
are obtained. We utilize the constancy of deceleration parameter to get singular and non-singular solutions. We investigate
a number of solutions with constant and time-varying cosmological constant together with a linear relation between shear viscosity
and expansion scalar. Due to dissipative processes, the mean anisotropy and shear of the model tend to zero at a faster rate. 相似文献
9.
R. K. Tiwari 《Astrophysics and Space Science》2009,321(2):147-150
Einstein field equations with variable gravitational and cosmological constants are considered in the presence of perfect
fluid for Robertson-Walker universe by assuming the cosmological term proportional to the Hubble parameter. This variation
law for vacuum density has recently been proposed by Schützhold on the basis of quantum field estimations in the curved and
expanding background. The cosmological term tends asymptotically to a genuine cosmological constant and the model tends to
a deSitter universe. We obtain that the present universe is accelerating with a large fraction of cosmological density in
the form of cosmological term. 相似文献
10.
Rolando Dünner Pablo A. Araya rés Meza † Andreas Reisenegger 《Monthly notices of the Royal Astronomical Society》2006,366(3):803-811
According to the latest evidence, the Universe is entering an era of exponential expansion, where gravitationally bound structures will get disconnected from each other, forming isolated 'island universes'. In this scenario, we present a theoretical criterion to determine the boundaries of gravitationally bound structures and a physically motivated definition of superclusters as the largest bound structures in the Universe. We use the spherical collapse model self-consistently to obtain an analytical condition for the mean density enclosed by the last bound shell of the structure (2.36 times the critical density in the present Universe, assumed to be flat, with 30 per cent matter and 70 per cent cosmological constant, in agreement with the previous, numerical result of Chiueh & He). N -body simulations extended to the future show that this criterion, applied at the present cosmological epoch, defines a sphere that encloses ≈99.7 per cent of the particles that will remain bound to the structure at least until the scale parameter of the Universe is 100 times its present value. On the other hand, (28 ± 13) per cent of the enclosed particles are in fact not bound, so the enclosed mass overestimates the bound mass, in contrast with the previous, less rigorous criterion of, e.g. Busha and collaborators, which gave a more precise mass estimate. We also verify that the spherical collapse model estimate for the radial infall velocity of a shell enclosing a given mean density gives an accurate prediction for the velocity profile of infalling particles, down to very near the centre of the virialized core. 相似文献
11.
We discuss the condensation phenomena in a very simple model, which allows for transparent calculations. We compare the condensation times of subsystems decoupled from the cosmological expansion in different approaches to Mach-Poincaré gravodynamics with Newtonian mechanics. In all cases contracting subsystems without inner rotation collapse, only the time-scales differ. 相似文献
12.
R. K. Tiwari 《Astrophysics and Space Science》2008,318(3-4):243-247
Einstein field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for Bianchi type-I universe by assuming the cosmological term proportional to the Hubble parameter. This variation law for vacuum density has recently been proposed by Schützhold on the basis of quantum field estimations in the curved and expanding background. The model obtained approaches isotropy. The cosmological term tends asymptotically to a genuine cosmological constant, and the model tends to a deSitter universe. We obtain that the present universe is accelerating with a large fraction of cosmological density in the form of cosmological term. 相似文献
13.
El-Nabulsi Ahmad Rami 《Astrophysics and Space Science》2010,327(2):155-159
We introduce a four-dimensional dilaton-Brans-Dicke cosmological scenario corresponding to multiverse filled with dark energy
or phantom energy having a positive cosmological constant and containing infinite singularities with eternally Big Rip. Many
interesting consequences are revealed and discussed in some details. 相似文献
14.
Dragan Slavkov Hajdukovic 《Astrophysics and Space Science》2012,339(1):1-5
The cosmological constant problem is the principal obstacle in the attempt to interpret dark energy as the quantum vacuum
energy. We suggest that the obstacle can be removed, i.e. that the cosmological constant problem can be resolved by assuming
that the virtual particles and antiparticles in the quantum vacuum have the gravitational charge of the opposite sign. The
corresponding estimates of the cosmological constant, dark energy density and the equation of state for dark energy are in
the intriguing agreement with the observed values in the present day Universe. However, our approach and the Standard Cosmology
lead to very different predictions for the future of the Universe; the exponential growth of the scale factor, predicted by
the Standard Cosmology, is suppressed in our model. 相似文献
15.
Ilian T. Iliev Paul R. Shapiro 《Monthly notices of the Royal Astronomical Society》2001,325(2):468-482
An analytical model is presented for the post-collapse equilibrium structure of virialized objects that condense out of a low-density cosmological background universe, either matter-dominated or flat with a cosmological constant. This generalizes the model we derived previously for an Einstein–de Sitter (EdS) universe. The model is based upon the assumption that cosmological haloes form from the collapse and virialization of 'top-hat' density perturbations, and are spherical, isotropic and isothermal. This leads to the prediction of a unique, non-singular, truncated isothermal sphere (TIS), a particular solution of the Lane–Emden equation (suitably modified when Λ≠0) . The size and virial temperature are unique functions of the mass and redshift of formation of the object for a given background universe. The central density is roughly proportional to the critical density of the universe at the epoch of collapse. This TIS model is in good agreement with observations of the internal structure of dark-matter-dominated haloes on scales ranging from dwarf galaxies to X-ray clusters. It also reproduces many of the average properties of haloes in simulations of the cold dark matter (CDM) model to good accuracy, suggesting that it is a useful analytical approximation for haloes that form from realistic initial conditions. Our TIS model matches the density profiles of haloes in CDM N -body simulations outside the innermost region, while avoiding the steep central cusp of the latter which is in apparent conflict with observations. The TIS model may also be relevant to non-standard CDM models, such as that for self-interacting dark matter, recently proposed to resolve this conflict. 相似文献
16.
Murli Manohar Verma 《Astrophysics and Space Science》2010,330(1):101-105
We propose a time-varying cosmological constant with a fixed equation of state, which evolves mainly through its interaction
with the background during most of the long history of the universe. However, such an interaction does not exist in the very
early- and the late-time universe and this produces the acceleration during these eras when it becomes very nearly a constant.
It is found that after the initial inflationary phase, the cosmological constant, which we call the lambda parameter, rolls
down from a large constant value to another but very small constant value and further dominates the present epoch showing
up in the form of dark energy, driving the acceleration. 相似文献
17.
Nonzero cosmological constant favours cosmological models with larger Hubble constant. The evolution of ionization during
decoupling period in a universe with nonzero cosmological constant is computed by using a corrected recombination coefficient.
Also presented in this paper is the redshift distribution of the last scattering surfaces of the cosmic background photons
while the cosmological constant is nonvanishing. Finally, we give a brief estimation about the influence of He on the last scattering surfaces.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
19.
Antonio Alfonso-Faus 《Astrophysics and Space Science》2008,318(1-2):117-123
In 1937 Dirac proposed the large number hypothesis (LNH). The idea was to explain that these numbers were large because the Universe is old. A time variation of certain “constants” was assumed. So far, no experimental evidence has significantly supported this time variation. Here we present a simplified cosmological model. We propose a new cosmological system of units, including a cosmological Planck’s constant that “absorbs” the well known large number 10120. With this new Planck’s constant no large numbers appear at the cosmological level. They appear at lower levels, e.g. at the quantum world. We note here that Zel’dovich formula, for the cosmological constant Λ, is equivalent to the Weinberg’s relation. The immediate conclusion is that the speed of light c must be proportional to the Hubble parameter H, and therefore decrease with time. We find that the gravitational radius of the Universe and its size are one and the same constant (Mach’s principle). The usual cosmological Ω’s parameters for mass, lambda and curvature turn out to be all constants of order one. The anthropic principle is not necessary in this theory. It is shown that a factor of 1061 converts in this theory a Planck fluctuation (a quantum black hole) into a cosmological quantum black hole: the Universe today. General relativity and quantum mechanics give the same local solution of an expanding Universe with the law a(t)≈const?t. This constant is just the speed of light today. Then the Hubble parameter is exactly H=a(t)′/a(t)=1/t. 相似文献
20.
Sanjay Oli 《Astrophysics and Space Science》2008,314(1-3):89-94
This paper presents anisotropic, homogeneous two-fluid cosmological models in a Bianchi type I space–time with a variable
gravitational constant G and cosmological constant Λ. In the two-fluid model, one fluid represents the matter content of the universe and another fluid is chosen to model the
CMB radiation. We find a variety of solutions in which the cosmological parameter varies inversely with time t. We also discuss in detail the behavior of associated fluid parameters and kinematical parameters. This paper pictures cosmic
history when the radiation and matter content of the universe are in an interactive phase. Here, Ω is closing to 1 throughout
the cosmic evolution.
相似文献