共查询到20条相似文献，搜索用时 31 毫秒
1.
In this paper we present a class of solutions of Einstein's field equations describing two-fluid models of the universe in
a locally rotationally symmetric Bianchi type II space-time. In these models one fluid is the radiation distribution which
represents the cosmic microwave background and the other fluid is the perfect fluid representing the matter content of the
universe. It is found that both the fluids are comoving in the locally rotationally symmetric Bianchi type II space-time.
The behaviour of the radiation density, matter density, the ratio of the matter density to the radiation density and the pressure
has been discussed. A subclass of solutions is found to describe models of a spatially homogeneous and partially isotropic
universe evolving from a radiation dominated era to a pressure free matter dominated era.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
The solution of three new interesting studies,a rotating anisotropic twofluid universe coupled with radiation and a scalar field,are studied here,where the anisotropic pressure is generated by the presence of two non-interacting perfect fluids which are in relative motion with respect to each other.In this problem,special discussion is made of the physically interesting class of models in which one fluid is a perfect comoving radiative fluid which is taken to model the cosmic microwave background and the se... 相似文献
3.
The evolution of a perfect fluid perturbation is considered in the radiation dominated period and in the dust epoch. In the investigation we make use of the general formalism developed in previous papers.It turns out that the evolution tendency is predicted by the state of the cosmic background. The radiation dominated universe does not stimulate growing processes of the perturbation, whereas the dust dominated universe causes a growing tendency of small perfect fluid formations. The results of this work are in accordance with these obtained by the present writers in a previous work. 相似文献
4.
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
5.
We discuss the contribution of the blazar population to the extragalactic background radiation across the electromagnetic (e.m.) spectrum with particular reference to the microwave, hard-X-ray and γ-ray bands. Our estimates are based on a recently derived blazar radio LogN-LogS that was built by combining several radio and multi-frequency surveys. We show that blazar emission integrated over cosmic time gives rise to a considerable broad-band non-thermal cosmic background that dominates the extragalactic brightness in the high-energy part of the e.m. spectrum. We also estimate the number of blazars that are expected to be detected by future planned or hypothetical missions operating in the X-ray and γ-ray energy bands. 相似文献
6.
The thermal evolution of an inter-cluster gas of relativistic electrons heated by quasars with redshifts up toz=3 and 4 is studied in the framework of a Friedmann-Robertson-Walker universe. The gas cools by Compton scattering with the microwave backgroud radiation and by adiabatic cooling due to the universe expansion. Power and exponential laws of cosmological evolution of the comoving density of sources are considered. The obtained temperatures are sensitive to the form of these laws and to the heating epochs. Compared to the nonrelativistic models, the results obtained in the case of the power law present strong differences. These differences decrease when the exponential law is considered. Thermalization times are compared to the characteristic time of variation of the universe energy density and to the time-scales of energy loss by bremsstrahlung radiation and by Compton scattering. It is shown that, in some cases, nonequilibrium effects may be important. The time delay effects in the propagation of electromagnetic waves in cosmic plasma are shown to be very important for the analysis of theoretical models. 相似文献
7.
A. Kogut D. Fixsen S. Fixsen S. Levin M. Limon L. Lowe P. Mirel M. Seiffert J. Singal P. Lubin E. Wollack 《New Astronomy Reviews》2006,50(11-12):925
The absolute radiometer for cosmology, astrophysics, and diffuse emission (ARCADE) is a balloon-borne instrument designed to measure the temperature of the cosmic microwave background at centimeter wavelengths. ARCADE searches for deviations from a blackbody spectrum resulting from energy releases in the early universe. Long-wavelength distortions in the CMB spectrum are expected in all viable cosmological models. Detecting these distortions or showing that they do not exist is an important step for understanding the early universe. We describe the ARCADE instrument design, current status, and future plans. 相似文献
8.
A scheme is proposed for explaining the origin of the observed temperature of the cosmic microwave background (relict) radiation in which this radiation is treated as a product of the decay of primordial vector bosons in the framework of the Hoyle-Narlikar conformal cosmology. 相似文献
9.
J. A. Adams S. Sarkar & D. W. Sciama 《Monthly notices of the Royal Astronomical Society》1998,301(1):210-214
It is attractive to suppose for several astrophysical reasons that the Universe has close to the critical density in light (∼30 eV) neutrinos which decay radiatively with a lifetime of ∼1023 s. In such a cosmology the Universe is re-ionized early and the last scattering surface of the cosmic microwave background significantly broadened. We calculate the resulting angular power spectrum of temperature fluctuations in the cosmic microwave background. As expected, the acoustic peaks are significantly damped relative to the standard case. This would allow a definitive test of the decaying neutrino cosmology with the forthcoming MAP Planck surveyor missions. 相似文献
10.
G. F. R. Ellis W. R. Stoeger 《Monthly notices of the Royal Astronomical Society》2009,398(3):1527-1536
The causal limit usually considered in cosmology is the particle horizon, delimiting the possibilities of causal connection in the expanding Universe. However, it is not a realistic indicator of the effective local limits of important interactions in space–time. We consider here the matter horizon for the Solar system, i.e. the comoving region which has significantly contributed matter to our local physical environment. This lies inside the effective domain of dependence , which (assuming the universe is dominated by dark matter along with baryonic matter and vacuum-energy-like dark energy) consists of those regions that have had a significant active physical influence on this environment through effects such as matter accretion and acoustic waves. It is not determined by the velocity of light c , but by the flow of matter perturbations along their world lines and associated gravitational effects. We emphasize how small a region the perturbations which became our Galaxy occupied, relative to the observable universe – even relative to the smallest scale perturbations detectable in the cosmic microwave background radiation. Finally, looking to the future of our local cosmic domain, we suggest simple dynamical criteria for determining the present domain of influence and the future matter horizon . The former is the radial distance at which our local region is just now separating from the cosmic expansion. The latter represents the limits of growth of the matter horizon in the far future. 相似文献
11.
A. M. Anile 《Astrophysics and Space Science》1974,29(2):415-426
The relativistic transfer equation for polarised radiation is solved in an axisymmetric Bianchi type I universe. Previous
results concerning the linear polarisation induced in the cosmic microwave background radiation by Thomson scattering in an
anisotropically expanding universe are confirmed.
Work partly done at the Osservatorio Astrofisico, Catania (Italia). 相似文献
12.
Koijam Manihar Singh 《Astrophysics and Space Science》2009,323(3):297-306
Presenting some interesting new solutions, rotating models of anisotropic two-fluid universes coupled with a magnetic field
are investigated and studied, where the anisotropic pressure is generated by the presence of two non-interacting perfect fluids
which are in relative motion with respect to each other. Here special discussion is made of the physically interesting class
of models in which one fluid is a comoving radiative perfect fluid which is taken to model the cosmic microwave background
and the second a non-comoving perfect fluid which will model the observed material content of the universe. Besides studying
their physical and dynamical properties the effects of rotation on these models are studied and the reactions of the magnetic
and gravitational fields with respect to the rotational motion are discussed. Analysis on the rotational perturbations are
also made, in the course of which the amount of anisotropy induced in pressure distribution by a small deviation from the
Friedmann metric is also investigated. The models obtained here are found to be theoretically satisfactory and thereby substantiates
the possibilities of existence of such astrophysical objects in this Universe and may be taken as good examples of real astrophysical
situations. 相似文献
13.
A proposal to study the original and new agegraphic dark energy in DGP braneworld cosmology is presented in this work. To verify our model with the observational data, the model is constrained by a variety of independent measurements such as Hubble parameter, cosmic microwave background anisotropies, and baryon acoustic oscillation peaks. The best fitting procedure shows the effectiveness of agegraphic parameter n in distinguishing between the original and new agegraphic dark energy scenarios and subsequent cosmological findings. In particular, the result shows that in both scenarios, our universe enters an agegraphic dark energy dominated phase. 相似文献
14.
15.
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.
A. M.Aliaga V.Quilis J. V.Arnau D.Sáez 《Monthly notices of the Royal Astronomical Society》2002,330(3):625-630
Non-linear evolution of cosmological energy density fluctuations triggers deviations from Gaussianity in the temperature distribution of the cosmic microwave background. A method to estimate these deviations is proposed. N -body simulations – in a Λ cold dark matter cosmology – are used to simulate the strongly non-linear evolution of cosmological structures. It is proved that these simulations can be combined with the potential approximation to calculate the statistical moments of the cosmic microwave background anisotropies produced by non-linear gravity. Some of these moments are computed and the resulting values are different from those corresponding to Gaussianity. 相似文献
17.
We investigate the background dynamics when dark energy is coupled to dark matter in the universe described by loop quantum
cosmology. We consider dark energy of the form modified Chaplygin gas. The dynamical system of equations is solved numerically
and a stable scaling solution is obtained. It henceforth resolves the famous cosmic coincidence problem in modern cosmology.
The statefinder parameters are also calculated to classify this dark energy model. 相似文献
18.
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
19.
A. A. Coley 《Astrophysics and Space Science》1988,140(1):175-189
Exact solutions of Einstein's field equations and the laws of thermodynamics are presented in which both a comoving radiative perfect fluid (modelling the cosmic microwave background) and a non-comoving imperfect fluid (modelling the observed material content of the Universe) act as the source of the gravitational field as represented by the flat FRW line element. The tilting velocity of the imperfect fluid is associated with the peculiar velocity of our local cluster of galaxies relative to the cosmic microwave background. In these relativistic two-fluid cosmological models the temperatures of the radiation and matter fields are equal until hydrogen recombines at 4000 K, after which time thermal contact between the two fluids is broken. The models presented are physically acceptable cosmologies that are shown to give rise to numerical predictions consistent with current observations. 相似文献
20.
A. K. Verma 《Astrophysics and Space Science》2009,321(1):73-77
This paper focuses attention on a qualitative analysis of the evolution of two-fluid flat FRW cosmological models.In the first
model one of the fluid represents matter content of the universe comoving with respect to the another fluid that is the cosmic
microwave background radiation (CMBR), these two fluids are interacting.The first model is most relevant to describe the scenario
before the recombination epoch when matter and radiation were in an interactive phase and the photons was bound to electron
through Thomson scattering. The second model describe two noninteracting fluids where the matter is comoving to the space-time
coordinates and the CMBR is moving axially, relative to the matter thus modeling the relative velocity between galaxies and
the CMBR (Phys. Rev. Lett. 39:898–901, 1977). This model portray the cosmic evolution in the postrecombination epoch when the two-fluid are noninteracting.In this epoch
the photons got themselves free to form the CMBR being observed presently.
相似文献