共查询到20条相似文献，搜索用时 125 毫秒
1.
Many have speculated about the presence of a stiff fluid in very early stage of the universe. Such a stiff fluid was first introduced by Zel’dovich. Recently the late acceleration of the universe was studied by taking bulk viscous stiff fluid as the dominant cosmic component, but the age predicted by such a model is less than the observed value. We consider a flat universe with viscous stiff fluid and decaying vacuum energy as the cosmic components and found that the model predicts a reasonable background evolution of the universe with de Sitter epoch as end phase of expansion. More over, the model also predicts a reasonable value for the age of the present universe. We also performed a dynamical system analysis of the model and found that the end de Sitter phase predicted by the model is stable. 相似文献
2.
Atreyee Biswas 《Astrophysics and Space Science》2018,363(10):203
The present work deals with irreversible thermodynamics of universe containing interacting dark fluids. Recent observational evidences reveal that the universe is dominated by two dark components-dark matter and dark energy. The interaction between them leads to spontaneous heat flow between the horizon and the fluid system and as a result the system will no longer be in thermal equilibrium. In this paper dark matter is chosen as pressureless dust while modified Chaplygin gas has been considered as dark energy. In two separate cases we have considered the universe to be bounded by apparent horizon and event horizon and the validity of generalized second law of thermodynamics in the context of irreversible thermodynamics has been studied for both the cases. 相似文献
3.
G. Venturi 《Astroparticle Physics》1993,1(4):417-428
The matter-gravity system is examined in a path integral approach for the case of conformal matter coupled to a Friedman-Robertson-Walker space time. In particular the case of gravitational potentials of interest in cosmology for which the universe tunnels from a small radius is examined. It is observed that in the presence of such gravitational horizons the universe evolves in a complex time and it is shown how a classical time and temperature emerge. Correspondingly, one will have compensating quantum and thermal fluctuations for the matter and gravity system and it is noted that the unstable mode of gravity corresponding to the universe tunneling into existence will be compensated by an analogous mode for matter corresponding to its creation. This last point is examined in a simple De Sitter model with conformal matter and a relation is found between the cosmological constant, the number of matter fields and the self coupling of matter responsable for its instability. 相似文献
4.
《New Astronomy》2021
In this paper, we consider conservation equation in cosmology and propose four possible forms of space-times for universe resulting from dual symmetry between scale factor and energy density in the universe with constant equation of state. We start to describe these four possible types of space-times for universe and it’s possible consequences. Due to the uniformity of the metric signature in Euclidean space-time, for the first time, we introduce a new symmetry for Euclidean space-time and consider it as a transition unstable state between the visible 4D universe and the invisible world with an extra dimension. Finally, we schematically represent these four possible space-times in a unique scenario for the universe. 相似文献
5.
In this work, we have discussed the Maxwell’s electrodynamics in non-linear forms in FRW universe. The energy density and pressure for non-linear electrodynamics have been written in the electro-magnetic universe. The Einstein’s field equations for flat FRW model in loop quantum cosmology have been considered if the universe is filled with the matter and electro-magnetic field. We separately assumed the magnetic universe and electric universe. The interaction between matter and magnetic field have been considered in one section and for some particular form of interaction term, we have found the solutions of magnetic field and the energy density of matter. We have also considered the interaction between the matter and electric field and another form of interaction term has been chosen to solve the field equations. The validity of generalized second law of thermodynamics has been investigated on apparent and event horizons using Gibb’s law and the first law of thermodynamics for magnetic and electric universe separately. 相似文献
6.
In this paper, we have considered a model of our universe containing five components as its constituents. Then, we have done
here the statefinder diagnostics for this model. This model can successfully explain the accelerated expansion of the universe
given that it satisfies a certain condition. Here we have considered the modified Chaplygin gas as the dynamically changing
part of the dark energy component of our universe. Chaplygin gas provides early deceleration and late time acceleration of
the universe. The graphical representation of statefinder parameters shows that the total evolution of the universe starts
from radiation era to phantom model. 相似文献
7.
D. L. Khokhlov 《Astrophysics and Space Science》2011,335(2):577-580
The Einstein static model of the universe as a whole is considered. The Hubble law is explained by the Doppler effect due
to the downward inertial acceleration along a certain radius experienced by an observer in the center of the universe, with
the total acceleration over all radii being equal zero. Evolution of the universe is introduced through the wave function
of the universe dependent on time. This yields the energy density of the universe hence the temperature of the universe dependent
on time. On the contrary, the energy, forth and intensity of radiation are fixed with time that allows to develop the Newtonian
physics in the whole universe. The time-temperature relation of the universe in the model considered is the same as in the
radiation dominated universe in the Friedmann model that allows to explain primordial nucleosynthesis as it is in the standard
scenario. The modern parameters of the universe in the model considered are consistent with the observations. 相似文献
8.
The most recently celebrated cosmological implications of the cosmic microwave background studies with WMAP (2006), though fascinating by themselves, do, however, create some extremely hard conceptual challenges for the present‐day cosmology. These recent extremely refined WMAP observations seem to reflect a universe which was extremely homogeneous at the recombination age and thus is obviously causally closed at the time of the cosmic recombination era. From the very tiny fluctuations apparent at this early epoch the presently observable nonlinear cosmic density structures can, however, only have grown up, if in addition to a mysteriously high percentage of dark matter an even higher percentage of dark energy is admitted as drivers of the cosmic evolution. The required dark energy density, on the other hand, is nevertheless 120 orders of magnitude smaller then the theoretically calculated value. These are outstanding problems of present day cosmology onto which we are looking here under new auspices. We shall investigate in the following, up to what degree a universe simply abolishes all these outstanding problems in case it reveals itself as an universe of constant total energy. As we shall show basic questions like: How could the gigantic mass of the universe of about 1080 proton masses at all become created? – Why is the presently recognized and obviously indispensable cosmic vacuum energy density so terribly much smaller than is expected from quantum theoretical considerations, but nevertheless terribly important for the cosmic evolution? – Why is the universe within its world horizon a causally closed system? –, can perhaps simply be answered, when the assumption is made that the universe has a constant total energy with the consequence that the total mass density of the universe (matter and vacuum) scales with . Such a scaling of matter and vacuum energy abolishes the horizon problem, and the cosmic vacuum energy density can easily be reconciled with its theoretical expectation values. In this model the mass of the universe increases linearly with the world extension Ru and can grow up from a Planck mass as a vacuum fluctuation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
D. L. Khokhlov 《Astrophysics and Space Science》2011,333(1):209-212
The Einstein static model of the universe as a whole is considered in Euclidean space and absolute time of a privileged reference
frame. The universe as a whole is stable that specifies observers at rest relative to the global space of the universe. An
observer in the centre of the universe is assumed to experience downward inertial acceleration along a certain radius due
to the kinetic energy of the universe, with the total acceleration over all radii being equal zero. This yields the Doppler
effect for the photon coming from the distant source that may explain the Hubble law in the static universe. The predictions
of the model are qualitatively in agreement with the observations in the cosmological tests of a standard candle and a standard
rod. Explanation of stretching of the light curve of SN Ia is proposed within the model considered. 相似文献
10.
T. X. Zhang 《Astrophysics and Space Science》2010,330(1):157-165
Modifying slightly the big bang theory, the author has recently developed a new cosmological model called black hole universe.
This new cosmological model is consistent with the Mach principle, Einsteinian general theory of relativity, and observations
of the universe. The origin, structure, evolution, and expansion of the black hole universe have been presented in the recent
sequence of American Astronomical Society (AAS) meetings and published recently in a scientific journal: Progress in Physics.
This paper explains the observed 2.725 K cosmic microwave background radiation of the black hole universe, which grew from
a star-like black hole with several solar masses through a supermassive black hole with billions of solar masses to the present
universe with hundred billion-trillions of solar masses. According to the black hole universe model, the observed cosmic microwave
background radiation can be explained as the black body radiation of the black hole universe, which can be considered as an
ideal black body. When a hot and dense star-like black hole accretes its ambient materials and merges with other black holes,
it expands and cools down. A governing equation that expresses the possible thermal history of the black hole universe is
derived from the Planck law of black body radiation and radiation energy conservation. The result obtained by solving the
governing equation indicates that the radiation temperature of the present universe can be ∼2.725 K if the universe originated
from a hot star-like black hole, and is therefore consistent with the observation of the cosmic microwave background radiation.
A smaller or younger black hole universe usually cools down faster. The characteristics of the original star-like or supermassive
black hole are not critical to the physical properties of the black hole universe at present, because matter and radiation
are mainly from the outside space, i.e., the mother universe. 相似文献
11.
In this paper we have presented a procedure to obtain exact analytical solutions of field equations for spherically symmetric
self-gravitating distribution of anisotropic matter in bimetric theory of gravitation. The solution agrees with the Einstein's
general relativity for a physical system compared to the size of universe such as the solar system.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
In this paper, we investigate the behavior of equation of state parameter and energy density for dark energy in the framework of f(T) gravity. For this purpose, we use anisotropic LRS Bianchi type I universe model. The behavior of accelerating universe is discussed for some well-known f(T) models. It is found that the universe takes a transition between phantom and non-phantom phases for f(T) models except exponential and logarithmic models. We conclude that our results are relativity analogous to the results of FRW universe. 相似文献
13.
In this article we want to answer the cosmologically relevant question what, with some good semantic and physical reason, could be called the massM u of an infinitely extended, homogeneously matter‐filled and expanding universe. To answer this question we produce a space‐like sum of instantaneous cosmic energy depositions surrounding equally each spacepoint in the homogeneous universe. We calculate the added‐up instantaneous cosmic energy per volume around an arbitrary space point in the expanding universe. To carry out this sum we use as basic metrics an analogy to the inner Schwarzschild metric applied to stars, but this time applied to the spacepoint‐related universe. It is then shown that this leads to the added‐up proper energy within a sphere of a finite outer critical radius defining the point‐related infinity. As a surprise this radius turns out to be reciprocal to the square root of the prevailing average cosmic energy density. The equivalent mass of the universe can then also be calculated and, by the expression which is obtained here, shows a scaling with this critical radius of this universe, a virtue of the universe which was already often called for in earlier works by E. Mach, H. Thirring and F. Hoyle and others. This radius on the other hand can be shown to be nearly equal to the Schwarzschild radius of the so‐defined mass M u of the universe. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
14.
This paper deals with the study of dynamical or phase space analysis of Bianchi I universe in Brans-Dicke gravity with chameleon scalar field. For this purpose, the matter contents are taken to be perfect fluid with magnetic field effects described by the non-linear Maxwell Lagrangian density. By taking some ansatz for the field potential and the interaction function in chameleon cosmology, we discuss three cases: Bianchi I universe with perfect fluid, FRW universe with magnetized perfect fluid and Bianchi I universe with magnetized perfect fluid. In all cases, we calculate fixed or critical points and discuss stability of the respective configuration for radiation as well as matter dominated eras. We also evaluate some cosmological parameters in each case for matter dominated era only and investigate their cosmological implications. 相似文献
16.
Wang Wen-fu 《Chinese Astronomy and Astrophysics》1995,19(4):401-404
Wave function of the universe in the superstring theory is discussed and using Vilenkin's boundary condition, the probability density of the scale factor a at a given value of the dilaton field, is obtained. It is shown that when the universe spontaneously nucleates, the minimum value of the scale factor of the classical universe is of the order of the Planck length, that is, quantum effects can prevent the universe from collapsing to a single point. 相似文献
17.
The main goal of this work is investigation of NADE in the cyclic universe scenario. Since, cyclic universe is explained by
a phantom phase (ω<−1), it is shown when there is no interaction between matter and dark energy, ADE and NADE do not produce a phantom phase,
then can not describe cyclic universe. Therefore, we study interacting models of ADE and NADE in the modified Friedmann equation.
We find out that, in the high energy regime, which it is a necessary part of cyclic universe evolution, only NADE can describe
this phantom phase era for cyclic universe. Considering deceleration parameter tells us that the universe has a deceleration
phase after an acceleration phase, and NADE is able to produce a cyclic universe. Also it is found valuable to study generalized
second law of thermodynamics. Since the loop quantum correction is taken account in high energy regime, it may not be suitable
to use standard treatment of thermodynamics, so we turn our attention to the result of Li et al. (Adv. High Energy Phys. 2009:
905705, 2009), which the authors have studied thermodynamics in loop quantum gravity, and we show that which condition can satisfy generalized
second law of thermodynamics. 相似文献
18.
The effect of background dynamics of the universe on formation of large scale structures in the framework of Modified Newtonian Dynamics (MOND) is investigated. A spherical collapse model is used for modeling the formation of the structures. This study is done in two extreme cases: (i) assuming a universe with a low-density baryonic matter without any cold dark matter and dark energy; (ii) a dark energy dominated universe with baryonic matter, without cold dark matter. We show that for the case (ii) the structures virialize at lower redshifts with larger radii compared to the low-density background universe. The dark energy slow downs the collapse of the structures. We show that our results are compatible with recent simulations of the structure formation in MOND. 相似文献
19.
Juan I. Larruquert José Antonio Méndez José Antonio Aznárez Manuela Vidal-Dasilva Sergio García-Cortés Luis Rodríguez-de Marcos Mónica Fernández-Perea 《Astrophysics and Space Science》2011,333(1):305-309
The interaction between Ricci scalar curvature and the baryon number current, dynamically breaks CPT in an expanding universe
and leads to baryon asymmetry. Using this kind of interaction and study the gravitational baryogenesis in the Bianchi type
I universe. We find out the effect of anisotropy of the universe on the baryon asymmetry for the case which the equation of
state parameter, ω, varies with time. 相似文献
20.
Han Ji-chang 《Chinese Astronomy and Astrophysics》1981,5(3):357-364
For the Robertson-Walker metric for a homogeneous and isotropic universe, I have derived explicit solutions of the Einstein-Yang equation (interior torsion-free solutions for a spin-less ideal fluid) and hence found certain solutions of the Einstein field equation for the radiation-dominated phase of the early universe. 相似文献