203 research outputs found
Spherical Gravitating Systems of Arbitrary Dimension
We study spherically symmetric solutions to the Einstein field equations
under the assumption that the space-time may possess an arbitrary number of
spatial dimensions. The general solution of Synge is extended to describe
systems of any dimension. Arbitrary dimension analogues of known four
dimensional solutions are also presented, derived using the above scheme.
Finally, we discuss the requirements for the existence of Birkhoff's theorems
in space-times of arbitrary dimension with or without matter fields present.
Cases are discussed where the assumptions of the theorem are considerably
weakened yet the theorem still holds. We also discuss where the weakening of
certain conditions may cause the theorem to fail.Comment: 14 pages with one fugure. Uses AMS fonts and Prog. Theor. Phys. style
files. Added section on neutron star and anisotropic fluid star as well as
Comments on Buchdahl's theorem and more analysis regarding the Birkhoff's
theorem. Accepted for publication in Prog. Theor. Phy
Inhomogeneous cosmologies with tachyonic dust as dark matter
A cosmology is considered driven by a stress-energy tensor consisting of a
perfect fluid, an inhomogeneous pressure term (which we call a ``tachyonic
dust'' for reasons which will become apparent) and a cosmological constant. The
inflationary, radiation dominated and matter dominated eras are investigated in
detail. In all three eras, the tachyonic pressure decreases with increasing
radius of the universe and is thus minimal in the matter dominated era. The
gravitational effects of the dust, however, may still strongly affect the
universe at present time. In case the tachyonic pressure is positive, it
enhances the overall matter {\em density} and is a candidate for dark matter.
In the case where the tachyonic pressure is negative, the recent acceleration
of the universe can be understood without the need for a cosmological constant.
The ordinary matter, however, has positive energy density at all times. In a
later section, the extension to a variable cosmological term is investigated
and a specific model is put forward such that recent acceleration and future
re-collapse is possible.Comment: 23 pages, four figures. Updated version incorporates some changes in
the introduction. Clarification on why the term tachyonic is used. References
added. Accepted for publication in General Relativity and Gravitatio
Gravitational Effects of Quantum Fields in the Interior of a Cylindrical Black Hole
The gravitational back-reaction is calculated for the conformally invariant
scalar field within a black cosmic string interior with cosmological constant.
Using the perturbed metric, the gravitational effects of the quantum field are
calculated. It is found that the perturbations initially strengthen the
singularity. This effect is similar to the case of spherical symmetry (without
cosmological constant). This indicates that the behaviour of quantum effects
may be universal and not dependent on the geometry of the spacetime nor the
presence of a non-zero cosmological constant.Comment: 13 pages, 1 figure, uses AMS package. D.E. solution corrected. Some
qualitative results are change
The gravitating perfect fluid-scalar field equations: quintessence and tachyonic
The system consisting of a self gravitating perfect fluid and scalar field is
considered in detail. The scalar fields considered are the quintessence and
``tachyonic'' forms which have important application in cosmology. Mathematical
properties of the general system of equations are studied including the
algebraic and differential identities as well as the eigenvalue structure. The
Cauchy problem for both quintessence and the tachyon is presented. We discuss
the initial constraint equations which must be satisfied by the initial data. A
Cauchy evolution scheme is presented in the form of a Taylor series about the
Cauchy surface. Finally, a simple numerical example is provided to illustrate
this scheme.Comment: 15 pages, 2 figures. Revised version contains more references.
Accepted for publication in General Relativity and Gravitatio
Wormhole throats in <i> R <sup>m</sup> </i> gravity
We consider wormhole geometries subject to a gravitational action consisting
of non-linear powers of the Ricci scalar. Specifically, wormhole throats are
studied in the case where Einstein gravity is supplemented with a Ricci-squared
and inverse Ricci term. In this modified theory it is found that static
wormhole throats respecting the weak energy condition can exist. The analysis
is done locally in the vicinity of the throat, which eliminates certain
restrictions on the models introduced by considering the global topology.Comment: 12 pages, 3 figures. Update includes more references and
clarification that "zero tidal force" class of models are considered.
Accepted for publication in Classical and Quantum Gravit
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