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1.
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
2.
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.
It is well known that a C-field, generated by a certain source equation leads to interesting changes in the cosmological solutions
of Einstein's equations. It is argued that different types of topological objects may have been created by the vacuum phase
transitions in the early universe. In the cosmological arena, the defects have been put forward as a possible mechanism for
structure formation. A global monopole is a heavy object formed in the phase transition of a system composed of a self coupling
scalar field triplet φa whose original global 0(3) symmetry is spontaneously broken to U(1). In this article, we find a special solution for the
space-time of a global monopole in presence of C-field. It is shown that the nature of the solution remains the same as in
general relativity case i.e. monopole exerts no gravitational force on the matter surrounding it but space around it has a
deficit solid angle.
Pacs Nos: 98.80cq, 04.20jb, 04.50 相似文献
4.
5.
Farook Rahaman 《Astrophysics and Space Science》2003,283(2):155-159
W.A. Hiscock (1990, Class. Quantum Gravitation
7, L235) obtained the vacuum expectation value of the stress-energy tensor of an arbitrary collection of conformal massless
free quantum fields (scalar, spinor and vectors) in the space-time of a static global monopole. With this stress-energy tensor,
the semi-classical Einstein equations are solved retaining terms up to first order in ħ in Lyra geometry.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
The influence on the rate of angular momentum loss from the Sun of magnetic geometries which are not spherically symmetric is estimated. Departures from spherical symmetry are expected to influence significantly the loss rate by two effects - the presence of closed magnetic field regions with no loss and also the variability in the radial distance to the Alfvénic point, as stressed by Mestel (1968).The loss rate is calculated for an MHD solar wind model with a solar magnetic field whose normal component at the surface is that of a north-south dipole. In contrast to Mestel's work, where the field was assumed dipolar within a certain surface and radial outside, the coupling between the solar wind and magnetic field is here taken into account exactly. For equivalent boundary conditions at the surface, the resulting field configuration yields an angular momentum loss rate which is only 15% of that for the monopole field normally used in angular momentum loss estimates. If, instead of equating boundary conditions at the Sun, one equates the two losses at the equator to that observed at 1 AU by spacecraft, then the ratio of the total loss for the distended dipole to that for the monopole is about 40%.On Leave from the Department of Applied Mathematics, The University, St. Andrews, Scotland.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
7.
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
8.
9.
The gravitational field of a global monopole in the context of Einstein–Cartan theory is investigated. A comparison is made
with the corresponding results predicted by general relativity.
PACS Nos: 98.80cq; 04.20 jb; 04.50 相似文献
10.
11.
M. Kutschera 《Monthly notices of the Royal Astronomical Society》2003,345(1):L1-L5
Einstein's general relativity predicts that pressure, in general stresses, plays a similar role to energy density, ε=ρ c 2 (with ρ being the corresponding mass density), in generating gravity. The source of gravitational field, the active gravitational mass density, sometimes referred to as Whittaker's mass density, is ρgrav =ρ+ 3 p / c 2 , where p is pressure in the case of an ideal fluid. Whittaker's mass is not conserved, hence its changes can propagate as monopole gravitational waves. Such waves can be generated only by astrophysical sources with varying gravitational mass. Here we show that relativistic fireballs, considered in modelling gamma-ray burst phenomena, are likely to radiate monopole gravitational waves from high-pressure plasma with varying Whittaker's mass. Also, ejection of a significant amount of initial mass-energy of the progenitor contributes to the monopole gravitational radiation. We identify monopole waves with h 11 + h 22 waves of Eddington's classification which propagate (in the z -direction) together with the energy carried by massless fields. We show that the monopole waves satisfy Einstein's equations, with a common stress-energy tensor for massless fields. The polarization mode of monopole waves is Φ22 , i.e. these are perpendicular waves which induce changes of the radius of a circle of test particles only (breathing mode). The astrophysical importance of monopole gravitational waves is discussed. 相似文献
12.
Using the shape model of Mars GTM090AA in terms of spherical harmonics complete to degree and order 90 and gravitational field
model of Mars GGM2BC80 in terms of spherical harmonics complete to degree and order 80, both from Mars Global Surveyor (MGS)
mission, the geometry (shape) and gravity potential value of reference equipotential surface of Mars (Areoid) are computed based on a constrained optimization problem. In this paper, the Areoid is defined as a reference equipotential surface, which best fits to the shape of Mars in least squares sense. The estimated gravity potential value of the Areoid from this study, i.e. W
0 = (12,654,875 ± 69) (m2/s2), is used as one of the four fundamental gravity parameters of Mars namely, {W
0, GM, ω, J
20}, i.e. {Areoid’s gravity potential, gravitational constant of Mars, angular velocity of Mars, second zonal spherical harmonic
of gravitational field expansion of Mars}, to compute a bi-axial reference ellipsoid of Somigliana-Pizzetti type as the hydrostatic approximate figure of Mars. The estimated values of semi-major and semi-minor axis of the computed
reference ellipsoid of Mars are (3,395,428 ± 19) (m), and (3,377,678 ± 19) (m), respectively. Finally the computed Areoid
is presented with respect to the computed reference ellipsoid. 相似文献
13.
Vladimir V. Sokolov 《Astrophysics and Space Science》1992,197(1):87-108
In the bounds of a theoretical scheme treating consistently gravitational interaction as dynamical (gauge) field in flat space-time, an expression was obtained for the density of energy-momentum-tension of gravitational field in vacuum around a collapsed object. A case was studied of an interacting static spherically-symmetric field of a collapsar in vacuum with taking into account of input of all the possible components (spin states of virtual gravitons) into the energy for the symmetric tensor of second rank
ik
. The radius of the sphere filled by matter for the collapsar of massM may achieve values up toGM/c
2. 相似文献
14.
Quasinormal modes of a black hole with quintessence-like matter and a deficit solid angle: scalar and gravitational perturbations 总被引：1，自引：0，他引：1
Ping Xi 《Astrophysics and Space Science》2009,321(1):47-51
In the previous paper (Li et al. in Phys. Lett. B 666:125–130, 2008), we show the solutions of Einstein equations with static spherically-symmetric quintessence-like matter surrounding a global
monopole. Furthermore, this monopole become a black hole with quintessence-like matter and a deficit solid angle when it is
swallowed by an ordinary black hole. We study its quasinormal modes by WKB method in this paper. The numerical results show
that both the real part of the quasinormal frequencies and the imaginary part decrease as the state parameter w, for scalar and gravitational perturbations. And we also show variations of quasinormal frequencies of scalar and gravitational
fields via different ε (deficit solid angel parameter) and different ρ
0 (density of static spherically-symmetric quintessence-like matter at r=1), respectively. 相似文献
15.
The scalar equations of infinitesimal elastic gravitational motion for a rotating, slightly elliptical Earth are always used
to study the Earth's nutation and tides theoretically, while the determination of the integration of the equations depends,
to a certain extent, on the choice of a set of appropriate boundary conditions. In this paper, a continuity quantity related
to the displacement is first transformed from the elliptical reference boundary to the corresponding effective spherical domain,
and then converted from a vector (or tensor) form to a scalar form by generalized surface spherical harmonics expansion. All
the related components, including the displacement vector field (or the stress tensor field), are then decomposed into the
poloidal and toroidal field using the symmetry restrictions on the normal mode eigenfunctions. After truncation, the boundary
conditions are finally derived, in a scalar ordinary differential form. The process of the derivation is second order in ellipticity
and in full detail. Moreover, the other boundary conditions are also presented as second order in ellipticity at the end of
this paper.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
Z. Horák 《Astrophysics and Space Science》1984,100(1-2):1-11
It is shown that Archimedes' principle can be generalized for external gravitational fields due to stationary macroscopic bodies. For instance, a particle immersed in a homogeneous fluid at the centre of spherical symmetry of the fluid, or anywhere in an unbounded homogenous fluid, experiences — in an external field — a force that it would experience in a vacuum if it had an apparent mass less than the actual one by the mass of displaced fluid. Inversely, if one immerses a particle into a symmetrically arranged homogeneous fluid apart from its centre of symmetry, the particle and the fluid produce, at the centre of symmetry of the fluid, a gravitational field that would be produced in vacuo by a particle of the same size and shape but having apparent mass. Simple laboratory experiments, suitable to verify this inverse theorem, are described. On the other hand, the gravitational force between two particles in an infinite homogeneous fluid is reduced by a factor proportional to the product of their apparent masses which can be positive or negative. Two particles with opposite apparent masses repel each other. The results obtained imply corrections to vacuum of the order of (10–5–10–4) G of the gravitational constant,G, measured by the common laboratory methods. 相似文献
17.
This review provides explanations of how geodesy, rotation and gravity can be addressed using radioscience data of an orbiter around a planet or of the lander on its surface.The planet Mars is the center of the discussion.The information one can get from orbitography and radioscience in general concerns the global static gravitational field, the time variation of the gravitational field induced by mass exchange between the atmosphere and the ice caps, the time variation of the gravitational field induced by... 相似文献
18.
Recently, a renormalizable gravity theory has been proposed by Hořava, and it might be an ultraviolet completion of general
relativity or its infrared modification. Particular limit of the theory allows for the Minkowski vacuum. A spherical asymptotically
flat black hole solution that represents the analogy of Schwarzschild solution of general relativity has been obtained. It
will be very interesting to find the difference between traditional general relativity and Hořava-Lifshitz gravity theory.
The three classical tests of general relativity including gravitational red-shift, perihelion precession of the planet Mercury,
and light deflection in gravitational field in the spherical asymptotically flat black hole solution of infrared modified
Hořava-Lifshitz gravity are investigated. The first order corrections from the standard general relativity is obtained. The
result can be used to limit the parameters in Hořava-Lifshitz gravity and to show the viability of the theory. 相似文献
19.
The paper discusses the possibilty of interpreting the magnetic fields of astronomical bodies in the framework of a unified field theory.Using one of the solutions of the generalized field theory, a direct relation between the polar magnetic field, the angular velocity and the gravitational potential of the body considered, is obtained. The geometric model used for applications has spherical symmetry and is of the type (FIGI).The predictions of the theoretical formula, obtained from the model, are compared with available observational data, and with the empirical formula of Blackett. The theoretical formula gives a possible interpretation of a seed magnetic field which will develop and produce the large-scale magnetic field observed for celestial objects. The formula shows that the field is generated as a result of rotation of a massive object. 相似文献
20.
Green's function methods for potential magnetic fields 总被引：1，自引：0，他引：1
Takashi Sakurai 《Solar physics》1982,76(2):301-321
The Green's function method to calculate potential magnetic field on the Sun, which was first established by Schmidt (1964) in the case that the field component normal to a flat boundary plane is specified, is extended to the following three cases: (a) The field component along the line of sight, which is not generally normal to the flat boundary plane, is specified; (b) the line of sight component on a spherical boundary surface is specified; (c) the normal component on a spherical surface is specified, together with the condition that the field becomes approximately radial on an outer spherical surface (the so-called source surface). Properties of these Green's functions are examined, and the applicability of these methods to solar magnetic data is discussed.On leave of absence from Department of Astronomy, University of Tokyo. 相似文献