共查询到20条相似文献，搜索用时 125 毫秒
1.
2.
3.
Einstein field equations are considered for zero-curvature Robertson-Walker models in the case of a viscous fluid distribution interacting with zero-rest-mass scalar fields. Exact solutions are obtained for two different phases of the early universe viz. the inflationary phase and the radiation-dominated phase, by using the 'gamma-law' equation of state p = (-1). The index describing the material content varies continuously with cosmological time. The gravitational 'constant' and bulk viscosity are both allowed to depend on the cosmic time. Some physical properties of the cosmological models are also discussed. 相似文献
4.
C.P. Singh 《Astrophysics and Space Science》2001,275(4):377-383
A spatially homogeneous and isotropic Robertson-Walker model withzero-curvature of the universe is studied within the frame-work of Lyra'smanifold. The gauge-function in Lyra's manifold is taken to betime-dependent. Exact solutions of Einstein equations are obtained for twodifferent early phases of the universe viz. Inflationary phase andradiation-dominated phase by using `gamma-law' equation of statep = ( - 1) . The -index, describing the material content,varies continuously with cosmic time so that in the course of itsevolution, the universe goes through a transition from an inflationaryphase to a radiation-dominated phase. The physical properties of themodels are also discussed. 相似文献
5.
The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory
is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously
as the Universe expands, are obtained by using the power-law relation φ = bR
n
, which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation
dominated era of the universe. The physical properties of cosmological solutions are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
F. Darabi 《Astrophysics and Space Science》2012,338(1):171-177
We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant.
Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational
coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value
of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological
coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration
of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence
problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s
cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum
density as ρ∼a
−4. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing
exotic type matter like cosmic strings. 相似文献
7.
Sanjay Oli 《Astrophysics and Space Science》2008,314(1-3):95-103
In this paper we present anisotropic, homogeneous two-fluid cosmological models in a Bianchi I space-time. These classes of
cosmological models picture two different scenarios of cosmic history; viz., when the radiation and matter content of the
universe are in interactive phase and another when the two are non-interacting. The universe is highly anisotropic in the
initial stages, however, anisotropy tapers out to insignificance in due course of cosmic evolution. In every model the anisotropy
of the space-time is determined by the density parameter Ω0 at the present epoch. For Ω0=1, the anisotropy is washed out before long. An interesting class of models, having an inflationary epoch in finite future,
is discovered.
相似文献
8.
J. V. Narlikar 《Journal of Astrophysics and Astronomy》1984,5(1):67-78
The concept of the steady-state universe discussed by Hoyle & Narlikar two decades ago is revived in the light of the present
discussions of the phase transition in the early big-bang universe. It is shown that with suitable scaling the bubble universe
solution bears a striking similarity to the inflationary scenarios being discussed today. The currently discussed idea of
cosmic baldness was also anticipated in the C-field cosmology of the steady-state universe.
An erratum to this article is available at . 相似文献
9.
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
10.
We consider a spatially homogeneous and isotropic flat Robertson-Walker model filled with a scalar (or tachyonic) field minimally
coupled to gravity in the framework of higher derivative theory. We discuss the possibility of the emergent universe with
normal and phantom scalar fields (or normal and phantom tachynoic fields) in higher derivative theory. We find the exact solution
of field equations in normal and phantom scalar fields and observe that the emergent universe is not possible in normal scalar
field as the kinetic term is negative. However, the emergent universe exists in phantom scalar field in which the model has
no time-like singularity at infinite past. The model evolves into an inflationary stage and finally admits an accelerating
phase at late time. The equation of state parameter is found to be less than −1 in early time and tends to −1 in late time
of the evolution. The scalar potential increases from zero at infinite past to a flat potential in late time. More precisely,
we discuss the particular case for phantom field in detail. We also carry out a similar analysis in case of normal and phantom
tachyonic field and observe that only phantom tachyonic field solution represents an emergent universe. We find that the coupling
parameter of higher order correction affects the evolution of the emergent universe. The stability of solutions and their
physical behaviors are discussed in detail. 相似文献
11.
I. S. Shklovskii 《Journal of Astrophysics and Astronomy》1984,5(1):13-18
It is argued that the iron nucleosynthesis rate in the universe due to SNI outbursts is dependent on the mass function of
star formation. Since the mass function depends on the chemical composition and since the masses of SNI precursors have upper
limits, the iron nucleosynthesis rate was low at an earlier evolutionary epoch of the universe when mainly massive stars were
formed. The iron nucleosynthesis rate should reach a maximum near z ∼ 0.5. At such or similar value of z the well-known ‘step’
in the cosmic γ-ray background spectrum may be explained by the presence of γ-gray quanta accompanying the radioactive56Co →56Fe decay. An argument is presented against the identification of the hidden mass of the universe with black-hole remnants
of ‘type III’ stars. 相似文献
12.
In this letter, we have considered a flat FRW universe. Instead of considering only one candidate for the dark energy, we
have considered the interaction between phantom field and modified Chaplygin gas. It has been shown that the potential of
the phantom field increases from a lower value with evolution of the universe. It has been observed that the field has an
increasing tendency and the potential has also an increasing tendency with passage of cosmic time. In the evolution of the
universe the crossing of w=−1 has been realized by this interacting model. 相似文献
13.
Hypersurface–homogeneous cosmological models containing a bulk viscous fluid with time varying G and Λ have been presented. We have shown that the field equations are solvable for any arbitrary cosmic scale function. The
viscosity coefficient of bulk viscous fluid is assumed to be a power function of the energy density. Exact solutions of Einstein’s
field equations are obtained which represent an expanding, shearing and accelerating/decelerating models of the universe.
The physical and kinematical behaviours of the models are also discussed. 相似文献
14.
In this paper we compare outcomes of some extended phantom-like cosmologies with each other and also with ΛCDM and ΛDGP. We
focus on the variation of the luminosity distances, the age of the universe and the deceleration parameter versus the redshift
in these scenarios. In a dynamical system approach, we show that the accelerating phase of the universe in the f(R)-DGP scenario is stable if one consider the curvature fluid as a phantom scalar field in the equivalent scalar-tensor theory, otherwise it is a transient and unstable phenomenon. Up
to the parameters values adopted in this paper, the extended F(R,ϕ)-DGP scenario is closer to the ΛCDM scenario than other proposed models. All of these scenarios explain the late-time cosmic
speed-up in their normal DGP branches, but the redshift at which transition to the accelerating phase occurs are different:
while the ΛDGP model transits to the accelerating phase much earlier, the F(R,ϕ)-DGP model transits to this phase much later than other scenarios. Also, within the parameter spaces adopted in this paper,
the age of the universe in the f(R)-DGP model is larger than ΛCDM, but this age in F(G,ϕ)-DGP is smaller than ΛCDM. 相似文献
15.
A. Kogut D.T. Chuss D. Fixsen G.F. Hinshaw M. Limon S.H. Moseley N. Phillips E. Sharp E.J. Wollack K. U-Yen N. Cao T. Stevenson W. Hsieh M. Devlin S. Dicker C. Semisch K. Irwin 《New Astronomy Reviews》2006,50(11-12):1009
The primordial anisotropy polarization pathfinder array (PAPPA) is a balloon-based instrument to measure the polarization of the cosmic microwave background and search for the signal from gravity waves excited during an inflationary epoch in the early universe. PAPPA will survey a 20° × 20° patch at the North Celestial Pole using 32 pixels in 3 passbands centered at 89, 212, and 302 GHz. Each pixel uses MEMS switches in a superconducting microstrip transmission line to combine the phase modulation techniques used in radio astronomy with the sensitivity of transition-edge superconducting bolometers. Each switched circuit modulates the incident polarization on a single detector, allowing nearly instantaneous characterization of the Stokes I, Q, and U parameters. We describe the instrument design and status. 相似文献
16.
We examine the warm inflationary universe model in the presence of generalized cosmic Chaplygin gas and standard scalar field. We assume the generalized dissipative coefficient and corresponding weak and strong dissipative regimes. In this framework, we analyze the characteristics of inflationary dynamics under the slow-roll approximations. Under these approximations, we formulate the important inflationary parameters, such as scalar spectral index, scalar and tensor power spectrum, tensor-to-scalar ratio etc. It is interesting to note that our results for these inflationary parameters are well corroborated with the recent observational data like WMAP7, WMAP9 and Planck data. 相似文献
17.
U. Kasper 《Astronomische Nachrichten》1988,309(4):259-261
Scalar fields are an important ingredient of modern cosmological models describing the very early universe. If they are of the Higgs field type, scalar fields offer a possibility to understand why the cosmological constant is such a small quantity. This is because of the fact that different ground states are possible for a Higgs field. The unstable ground state gives an inflationary stage of the cosmic evolution and a large cosmological constant whereas the stable ground state has a vanishing cosmological constant and is decisive for the late time behaviour with an Einstein-De Sitter — like expansion law. 相似文献
18.
Corrado Massa 《Astrophysics and Space Science》2006,305(4):377-384
Quantum theory in Robertson – Walker spacetime suggests the existence of a minimal energy ε of the order of 10−45 erg. Reasonable forms for ε give the expansion factor R=R(t)(t= the cosmic time) with no need of gravitational field equations.Einstein's theory should be modified in gravitational fields of strength less than ε c/ħ ∼ 10−8 cm/s2 where c is the speed of light and ħ is the reduced Planck constant. The cosmological term λ is expected to decrease as the universe expands.In the Appendix, ε is derived from a big bang – big crunch Newtonian cosmology. 相似文献
19.
Radiation from the Magellanic Clouds is discussed from the point of view of near future possibilities in observational γ-ray
astronomy. The γ-ray fluxes expected according to the metagalactic and galactic theories of the origin of cosmic rays are
compared. It is shown that the strongest test of the metagalactic hypothesis will be provided by a determination of the ratio
of γ-ray fluxes from SMC and LMC. The γ-ray luminosity of a typical young supernova remnant that can generate sufficient antiprotons
is estimated. It is shown that such remnants must have a short phase during which they are very powerful γ-ray emitters. 相似文献
20.
Spatially homogeneous and isotropic Robertson-Walker model of the universe is studied in Barber's second self-creation theory of gravitation in the presence of perfect fluid by using gamma-law equation of state p =(-1). The parameter gamma varies continuously with cosmological time. Exact solutions of the field equations are obtained for inflationary period and radiation-dominated era by using the power law relation Rn-3 = B. Some physical properties of the models are also discussed. 相似文献