265 research outputs found
Symmetries of asymptotically flat electrovacuum spacetimes and radiation
Symmetries compatible with asymptotic flatness and admitting gravitational
and electromagnetic radiation are studied by using the Bondi-Sachs-van der Burg
formalism. It is shown that in axially symmetric electrovacuum spacetimes in
which at least locally a smooth null infinity in the sense of Penrose exists,
the only second allowable symmetry is either the translational symmetry or the
boost symmetry. Translationally invariant spacetimes with in general a straight
"cosmic string" along the axis of symmetry are non-radiative although they can
have a non-vanishing news function. The boost-rotation symmetric spacetimes are
radiative. They describe "uniformly accelerated charged particles" or black
holes which in general may also be rotating - the axial and an additional
Killing vector are not assumed to be hypersurface orthogonal. The general
functional forms of both gravitational and electromagnetic news functions, and
of the mass aspect and total mass of asymptotically flat boost-rotation
symmetric spacetimes at null infinity are obtained. The expressions for the
mass are new even in the case of vacuum boost-rotation symmetric spacetimes
with hypersurface orthogonal Killing vectors. In Appendices some errors
appearing in previous works are corrected.Comment: 23 pages, RevTeX, to appear in JM
Null limits of generalised Bonnor-Swaminarayan solutions
The Bonnor-Swaminarayan solutions are boost-rotation symmetric space-times
which describe the motion of pairs of accelerating particles which are possibly
connected to strings (struts). In an explicit and unified form we present a
generalised class of such solutions with a few new observations. We then
investigate the possible limits in which the accelerations become unbounded.
The resulting space-times represent spherical impulsive gravitational waves
with snapping or expanding cosmic strings. We also obtain an exact solution for
a snapping string of finite length.Comment: 13 pages LaTeX 2e. To appear in Gen. Rel. Gra
Electromagnetic sources distributed on shells in a Schwarzschild background
In the Introduction we briefly recall our previous results on stationary
electromagnetic fields on black-hole backgrounds and the use of spin-weighted
spherical harmonics. We then discuss static electric and magnetic test fields
in a Schwarzschild background using some of these results. As sources we do not
consider point charges or current loops like in previous works, rather, we
analyze spherical shells with smooth electric or magnetic charge distributions
as well as electric or magnetic dipole distributions depending on both angular
coordinates. Particular attention is paid to the discontinuities of the field,
of the 4-potential, and their relation to the source.Comment: dedicated to Professor Goldberg's 86th birthday, accepted for
publication in Gen. Relat. Gravit., 12 page
The fields of uniformly accelerated charges in de Sitter spacetime
The scalar and electromagnetic fields of charges uniformly accelerated in de
Sitter spacetime are constructed. They represent the generalization of the Born
solutions describing fields of two particles with hyperbolic motion in flat
spacetime. In the limit Lambda -> 0, the Born solutions are retrieved. Since in
the de Sitter universe the infinities I^+- are spacelike, the radiative
properties of the fields depend on the way in which a given point of I^+- is
approached. The fields must involve both retarded and advanced effects: Purely
retarded fields do not satisfy the constraints at the past infinity I^-.Comment: 5 pages, 3 figures, RevTeX; Slightly expanded version of the paper
published in Physical Review Letters. (The published version can be generated
from the same TeX source.); problem with the postscript fixe
The Newtonian limit of spacetimes for accelerated particles and black holes
Solutions of vacuum Einstein's field equations describing uniformly
accelerated particles or black holes belong to the class of boost-rotation
symmetric spacetimes. They are the only explicit solutions known which
represent moving finite objects. Their Newtonian limit is analyzed using the
Ehlers frame theory. Generic spacetimes with axial and boost symmetries are
first studied from the Newtonian perspective. The results are then illustrated
by specific examples such as C-metric, Bonnor-Swaminarayan solutions,
self-accelerating "dipole particles", and generalized boost-rotation symmetric
solutions describing freely falling particles in an external field. In contrast
to some previous discussions, our results are physically plausible in the sense
that the Newtonian limit corresponds to the fields of classical point masses
accelerated uniformly in classical mechanics. This corroborates the physical
significance of the boost-rotation symmetric spacetimes
Disk sources of the Kerr and Tomimatsu-Sato spacetimes: construction and physical properties
We construct the disk sources matched to the exact vacuum Kerr and to the two
classes of Tomimatsu-Sato spacetimes. We analyze two models of the matter
forming these disks. At each radius we consider either a rotating massive ring
with pressure or two counter-rotating streams of particles in circular geodesic
motion. Dragging effects present in such spacetimes lead either to rotation of
rings or asymmetry of both streams. We demonstrate that the model of rotating
rings is general enough to describe all axisymmetric stationary disk sources
with vanishing radial pressure which satisfy weak energy condition, and that
centrifugal effects present in the disk sources of spacetimes with large
angular momentum prevent the construction of highly compact sources made of
counter-rotating streams of geodesic particles. We illustrate the radial
distribution of the mass inside the disks and the angular velocities of both
geodesic streams
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