共查询到20条相似文献，搜索用时 31 毫秒
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Few years ago, Cho and Vilenkin have proposed that topological defects can arise in symmetry breaking models without having degenerate vacua. These types of defects are known as vacuumless defects. In the present work, the gravitational field of a vacuumless global string and global monopole have been investigated in the context of Lyra geometry. We find the metric of the vacuumless global string and global monopole in the weak field approximations. It has been shown that the vacuumless global string can have repulsive whereas global monopole exerts attractive gravitational effects on a test particle. It is dissimilar to the case studied in general relativity. 相似文献
3.
Farook Rahaman 《Astrophysics and Space Science》2002,280(4):337-343
We present an approximate solution of global monopole based on Lyra geometry retaining terms of the order 1/3
2 in the energy momentum tensor for a triplet scalar field. Also the gravitational field of the monopole solution has been
considered.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
《天文和天体物理学研究(英文版)》2016,(12)
We investigate plane symmetric spacetime filled with perfect fluid in the C-field cosmology of Hoyle and Narlikar. A new class of exact solutions has been obtained by considering the creation field C as a function of time only. To get the deterministic solution, it has been assumed that the rate of creation of matter-energy density is proportional to the strength of the existing C-field energy density. Several physical aspects and geometrical properties of the models are discussed in detail, especially showing that some of our solutions of C-field cosmology are free from singularity in contrast to the Big Bang cosmology. A comparative study has been carried out between two models, one singular and the other nonsingular, by contrasting the behaviour of the physical parameters. We note that the model in a unique way represents both the features of the accelerating as well as decelerating universe depending on the parameters and thus seems to provide glimpses of the oscillating or cyclic model of the universe without invoking any other agent or theory in allowing cyclicity. 相似文献
5.
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
6.
It is shown that, on account of the commutability of the order of derivation of the electromagnetic 4-potential, the first pair of Maxwell equations, when written in a four dimensional notation, is incompatible with the existence of the classical magnetic monopole. It is hypothesized that the appearance in the past of magnetic monopoles could be accepted only if, in exceptional occurrences, it came to be locally lacking the space homogeneity and isotropy on which the commutability of space derivatives is based. The situation is more complex for massive cosmological monopoles. 相似文献
7.
A. A. Saharian 《Astrophysics》2004,47(2):260-272
A study is made of the vacuum expectation values for the energy-momentum tensor of a massive scalar field that satisfy a Robin mixed boundary condition on a spherical surface with a background gravitational field from a D+1-dimensional global monopole. Expressions are derived for the Wightman function, vacuum expectation of the square of the field, vacuum energy density, and the radial and azimuthal pressures inside the spherical surface. The regularization procedure involves using the generalized Abel-Plana formula for series in terms of the zeroes of cylindrical functions. This formula makes it possible to separate the part owing to the gravitational field of a global monopole without boundaries from the vacuum expectation and to represent the parts induced by the boundary in the form of exponentially converging integrals which are especially convenient for numerical calculations. The asymptotic behavior of the vacuum averages is studied at the center of the sphere and near its surface. It is shown that for small values of the parameter describing the solid-angle deficit in the geometry of a global monopole, the vacuum stresses induced by the boundary are highly anisotropic. 相似文献
8.
In this paper, a general FRW cosmological model has been constructed in f(R,T) gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman's law of constant deceleration parameter q = m-1. The possible decelerating and accelerating solutions have been investigated. For(q 0) we get a stable flat decelerating radiation-dominated universe at q = 1. For(q 0) we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed. 相似文献
9.
J. P. Vallée 《Astrophysics and Space Science》1988,140(1):119-123
Of the various proposed mechanisms to maintain spiral arms in spiral galaxies, three have been supported by observations, statistics, or theories (bar, companion, extended solid-body rotation curve). It is shown here that in the presence of a central bar or oval distirtion to maintain spiral arms, the global magnetic field lines also follow the spiral shape of the arms. Excluding then barred galaxies, it is confirmed that in the presence of a companion galaxy to maintain spiral arms, the global magnetic lines in a spiral galaxy will either follow thespiral shape of the arms (when tides are larger), or else will follow thering shape of the orbit of matter crossing spiral arms (when tides are small). In the presence of an extended solid-body rotation curve to maintain spiral arms within the solid-body rotation region, the global magnetic field lines also follow the spiral shape of the arms.The results above do not favour the hypothesis that a weak intergalactic magnetic field could have been amplified enough by gravitational contraction of a protogalaxy to give rise to the observed strength of galactic magnetic fields. On the contrary, leakage of galactic magnetic fields into intergalactic/cosmological space is expected. 相似文献
10.
The effect of C-field in high density matter has been studied. We find that the negative energy and negative pressure of the
C-field helps in formation of massive compact stable neutron stars of mass ∼ 0.5 solar mass which is in the range of 0.01
to 1.0 solar mass of recently observed dwarf stars. 相似文献
11.
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
12.
Tilted Bianchi Type I cosmological model for perfect fluid distribution in presence of magnetic field, is investigated. To
get a determinate solution, it has been assumed that the universe is filled with stiff perfect fluid distribution together
with A=(BC)
n where A,B,C are metric potentials and n is a constant. The behaviour of the model in presence and absence of magnetic field is discussed. The various physical and
geometrical aspects of the model, is also discussed. It has been shown that tilted nature of the Bianchi Type I model is preserved
due to magnetic field.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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14.
Radiation by a monopole which revolves round a neutron star is investigated in detail. Using radiant probabilityW
p
t
(k), we solve the radiant kinetic equation and proliferative equation of a monopole having the distribution functionf
p, and obtain the solution of wave numberN
k
t
(r) and distribution functionf
p. From these solutions, we know when the monopole moves round the neutron star; the exponential radiation is yielded with change of the motive orbit radiusr. As radiusr increases, the radiation appears as a saturated effect. 相似文献
15.
Mauricio Cataldo Fabiola Ar��valo Patricio Mella 《Astrophysics and Space Science》2011,333(1):287-293
It is well known that Kasner-type cosmologies provide a useful framework for analyzing the three-dimensional anisotropic expansion because of the simplification of the anisotropic dynamics. In this paper relativistic multi-fluid Kasner-type scenarios are studied. We first consider the general case of a superposition of two ideal cosmic fluids, as well as the particular cases of non-interacting and interacting ones, by introducing a phenomenological coupling function q(t). For two-fluid cosmological scenarios there exist only cosmological scaling solutions, while for three-fluid configurations there exist not only cosmological scaling ones, but also more general solutions. In the case of triply interacting cosmic fluids we can have energy transfer from two fluids to a third one, or energy transfer from one cosmic fluid to the other two. It is shown that by requiring the positivity of energy densities there always is a matter component which violates the dominant energy condition in this kind of anisotropic cosmological scenarios. 相似文献
16.
An exact cosmological solution for Friedman Robertson Walker (FRW)metric is obtained with a classical scalar field φ along
with a potential in the presence of a causal viscous fluid. Assuming the scale factor to be a function of the scalar field
we have obtained solutions in both the truncated and the full causal theory. It is shown that an inflationary model can be
obtained by imposing certain constraints on some constants. The radiative bulk viscosity during the decoupling era has also
been studied as a special case.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
《New Astronomy》2019
We have constructed a model in Lyra manifold and time varying cosmological constant with perfect fluid using LVDP (Linear Varying Deceleration Parameter). Bianchi type-III metric is used as source of investigation. To get a deterministic solution of the field equation the expansion scalar (θ) is considered as proportional to the shear scalar (σ). The cosmological constant is found to be positive which satisfies the result obtained by supernova Type-Ia Observations [1999]. Here we analyse the behaviour of pressure and deceleration parameter by using different form of dark energy(DE). In addition to it, some physical and geometrical properties of the solutions are studied. 相似文献
18.
J -L. Atteia 《Astrophysics and Space Science》1995,231(1-2):11-18
We discuss the use of selected characteristics of the burster population (e.g. the distribution of peak fluxes, or durations) to derive the GRB distance scale within the framework of cosmological models. The effects of the cosmological expansion on GRBs are briefly noted and it is shown that intrinsic GRB properties may strongly complicate the search for (and the interpretation of) purely cosmological effects. In this context, bright GRBs provide a sample of reference; they are almost free of cosmological influences and they have been studied for a long time. We also emphasize the need for a GRB distance indicator which could be used for individual events; several recent studies suggest that such a quantity may exist. 相似文献
19.
In the standard Friedmann cosmology the black-body radiation spectrum is usually taken (without explicit proof as far as we know) to have the same familiarT
4-form that it has in a flat space. With explicit use of the equation of motion of a quantized massless field propagating in a curved background Robertson-Walker metric we show (for the readily tractable scalar field case) that the assumption is in fact true for an open Universe. For a closed Universe, we find that there is an in principle modification to theT
4-law. Unfortunately, the correction turns out to be too small to be experimentally detectable. In passing, we also obtain a simple derivation for the cosmological red shift of frequencies. 相似文献
20.
M. D. Roberts 《Astrophysics and Space Science》1993,200(2):331-335
It is known that the requirement of asymptotic flatness places restrictions on spherically-symmetric solutions to field equations. Here it is shown that the most general solution to the static spherically-symmetric massless scalar Einstein equations with zero cosmological constant is asymptotically flat; furthermore, the general solution is derived and shown to be identical to a solution previusly found by M. Wyman. 相似文献