413 research outputs found
Critical collapse of a massive vector field
We perform numerical simulations of the critical gravitational collapse of a
massive vector field. The result is that there are two critical solutions. One
is equivalent to the Choptuik critical solution for a massless scalar field.
The other is periodic.Comment: 7 pages, 4 figure
Radiation and String Atmosphere for Relativistic Stars
We extend the Vaidya radiating metric to include both a radiation field and a
string fluid. Assuming diffusive transport for the string fluid, we find new
analytic solutions of Einstein's field equations. Our new solutions represent
an extention of Xanthopoulos superposition.Comment: To appear in Phys. Rev. D, Rapid Communicatio
Dynamics of a string coupled to gravitational waves II - Perturbations propagate along an infinite Nambu-Goto string
The perturbative modes propagating along an infinite string are investigated
within the framework of the gauge invariant perturbation formalism on a
spacetime containing a self-gravitating straight string with a finite
thickness. These modes are not included in our previous analysis. We
reconstruct the perturbation formalism to discuss these modes and solve the
linearized Einstein equation within the first order with respect to the string
oscillation amplitude. In the thin string case, we show that the oscillations
of an infinite string must involve the propagation of cosmic string traveling
wave.Comment: 4 pages (2 columns), no figure, revtex with multicol.sty. To appear
in Physical Review
Numerical Ricci-flat metrics on K3
We develop numerical algorithms for solving the Einstein equation on
Calabi-Yau manifolds at arbitrary values of their complex structure and Kahler
parameters. We show that Kahler geometry can be exploited for significant gains
in computational efficiency. As a proof of principle, we apply our methods to a
one-parameter family of K3 surfaces constructed as blow-ups of the T^4/Z_2
orbifold with many discrete symmetries. High-resolution metrics may be obtained
on a time scale of days using a desktop computer. We compute various geometric
and spectral quantities from our numerical metrics. Using similar resources we
expect our methods to practically extend to Calabi-Yau three-folds with a high
degree of discrete symmetry, although we expect the general three-fold to
remain a challenge due to memory requirements.Comment: 38 pages, 10 figures; program code and animations of figures
downloadable from http://schwinger.harvard.edu/~wiseman/K3/ ; v2 minor
corrections, references adde
Pair Creation of Dilaton Black Holes
We consider dilaton gravity theories in four spacetime dimensions
parametrised by a constant , which controls the dilaton coupling, and
construct new exact solutions. We first generalise the C-metric of
Einstein-Maxwell theory () to solutions corresponding to oppositely
charged dilaton black holes undergoing uniform acceleration for general . We
next develop a solution generating technique which allows us to ``embed" the
dilaton C-metrics in magnetic dilaton Melvin backgrounds, thus generalising the
Ernst metric of Einstein-Maxwell theory. By adjusting the parameters
appropriately, it is possible to eliminate the nodal singularities of the
dilaton C-metrics. For (but not for ), it is possible to further
restrict the parameters so that the dilaton Ernst solutions have a smooth
euclidean section with topology , corresponding to
instantons describing the pair production of dilaton black holes in a magnetic
field. A different restriction on the parameters leads to smooth instantons for
all values of with topology .Comment: 22 pages, EFI-93-51, FERMILAB-Pub-93/272-A, UMHEP-393. (Asymptotics
of Ernst solutions clarified, typos repaired
Dilatonic Black Holes in Higher Curvature String Gravity
We give analytical arguments and demonstrate numerically the existence of
black hole solutions of the Effective Superstring Action in the presence
of Gauss-Bonnet quadratic curvature terms. The solutions possess non-trivial
dilaton hair. The hair, however, is of ``secondary" type", in the sense that
the dilaton charge is expressed in terms of the black hole mass. Our solutions
are not covered by the assumptions of existing proofs of the ``no-hair"
theorem. We also find some alternative solutions with singular metric
behaviour, but finite energy. The absence of naked singularities in this system
is pointed out.Comment: 22 pages, Latex file, 7 Latex figures already include
Flat World of Dilatonic Domain Walls
We study dilatonic domain walls specific to superstring theory.
Along with the matter fields and metric the dilaton also changes its value in
the wall background. We found supersymmetric (extreme) solutions which in
general interpolate between isolated superstring vacua with non-equal value of
the matter potential; they correspond to the static, planar domain walls with
{\it flat} metric in the string (sigma model) frame.
We point out similarities between the space-time of dilatonic walls and that
of charged dilatonic black holes. We also comment on non-extreme solutions
corresponding to expanding bubbles.Comment: 11 pgs (+2 figures available upon request), UPR-560-
Exact SU(2)*U(1) Stringy Black Holes
Extreme magnetic dilaton black holes are promoted to exact solutions of
heterotic string theory with unbroken supersymmetry. With account taken of
alpha' corrections this is accomplished by supplementing the known solutions
with SU(2) Yang-Mills vectors and scalars in addition to the already existing
Abelian U(1) vector field. The solution has a simple analytic form and includes
multi-black-holes. The issue of exactness of other black-hole-type solutions,
including extreme dilaton electrically charged black holes and Taub-NUT
solutions is discussed.Comment: 10 pages, SU-ITP-94-27 and QMW-PH-94-34 (version accepted for
publication in Phys. Rev., contains a discussion of (4.1) supersymmetry of
the black hole sigma model
Vanishing Hawking Radiation from a Uniformly Accelerated Black Hole
We consider quantum fields around uniformly accelerated black holes. At a
particular value of the acceleration, the Bogolubov transformation which would
be responsible for the late-time Hawking radiation, is found to be trivial.
When this happens, Hawking's thermal radiation, Doppler-shifted or not, is
absent to the asymptotic inertial observers despite the nonzero Hawking
temperature, while the co-moving observers find the black hole radiance exactly
balanced by the acceleration heat bath. After a brief comparison to the
classical system of a uniformly accelerated charge, we close with two important
comments. (Phys. Rev. Lett. 75 (1995) 382)Comment: LaTeX, 10pages, 2 figures (a typo in Eq.(3) corrected; minor
revisions to accomodate the length limitation of the journal
The Principle of Non-Gravitating Vacuum Energy and some of its consequences
For Einstein's General Relativity (GR) or the alternatives suggested up to
date the vacuum energy gravitates. We present a model where a new measure is
introduced for integration of the total action in the D-dimensional space-time.
This measure is built from D scalar fields . As a consequence of
such a choice of the measure, the matter lagrangian can be changed by
adding a constant while no gravitational effects, like a cosmological term, are
induced. Such Non-Gravitating Vacuum Energy (NGVE) theory has an infinite
dimensional symmetry group which contains volume-preserving diffeomorphisms in
the internal space of scalar fields . Other symmetries contained
in this symmetry group, suggest a deep connection of this theory with theories
of extended objects. In general {\em the theory is different from GR} although
for certain choices of , which are related to the existence of an
additional symmetry, solutions of GR are solutions of the model. This is
achieved in four dimensions if is due to fundamental bosonic and
fermionic strings. Other types of matter where this feature of the theory is
realized, are for example: scalars without potential or subjected to nonlinear
constraints, massless fermions and point particles. The point particle plays a
special role, since it is a good phenomenological description of matter at
large distances. de Sitter space is realized in an unconventional way, where
the de Sitter metric holds, but such de Sitter space is supported by the
existence of a variable scalar field which in practice destroys the maximal
symmetry. The only space - time where maximal symmetry is not broken, in a
dynamical sense, is Minkowski space. The theory has non trivial dynamics in 1+1
dimensions, unlike GR.Comment: 23 page
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