title: Implications of a Quantum Mechanical Treatment of the Universe

abstract:
  We attempt to treat the very early Universe according to quantum mechanics.
Identifying the scale factor of the Universe with the width of the wave packet
associated with it, we show that there cannot be an initial singularity and
that the Universe expands. Invoking the correspondence principle, we obtain the
scale factor of the Universe and demonstrate that the causality problem of the
standard model is solved.


title: Cosmic String Created from Vacuum Fluctuaions

abstract:
  The possibility of the cosmic string creation by the vacuum fluctuations of
quantum fields in the self-consistent semiclassical theory of gravity is
discussed. We use the approximate method for obtaining vacuum expectation value
of the renormalized stress-energy tensor of conformally invariant quantum
fields in static cylindrically symmetric spacetimes. We have obtained the
particular solutions of Einstein equations for the different boundary
conditions at the cylinder symmetry axis.


title: New Higgs Field Ansatz for Effective Gravity in Flat Space Time

abstract:
  Regarding Pauli's matrices as proper Higgs fields one can deduce an
effective(!) approximation for gravity in flat space. In this work we extend
this approximation up to the second order. Reaching complete agreement in the
special case of gravitational waves. Unification in view, we introduce
isospinorial degrees of freedom. In this way the mass spectrum and chiral
asymmetry can be generated with the help of an additional scalar Higgs field.
The Higgs modes corresponding to gravity are discussed.


title:
Neutron stars in scalar-tensor theories of gravity and catastrophe
  theory

abstract:
  We investigate neutron stars in scalar-tensor theories. We examine their
secular stability against spherically symmetric perturbations by use of a
turning point method. For some choices of the coupling function contained in
the theories, the number of the stable equilibrium solutions changes and the
realized equilibrium may change discontinuously as the asymptotic value of the
scalar field or total baryon number is changed continuously. The behaviour of
the stable equilibrium solutions is explained by fold and cusp catastrophes.
Whether the cusp catastrophe appears or not depends on the choices of the
coupling function. These types of the catastrophes are structurally stable.
Recently discovered spontaneous scalarization, which is non-perturbative
strong-field phenomenon due to the presence of the gravitational scalar field,
is well described in terms of the cusp catastrophe.


title: Structure and stability of cold scalar-tensor black holes

abstract:
  We study the structure and stability of the recently discussed spherically
symmetric Brans-Dicke black-hole type solutions with an infinite horizon area
and zero Hawking temperature, existing for negative values of the coupling
constant $\omega$. These solutions split into two classes: B1, whose horizon is
reached by an infalling particle in a finite proper time, and B2, for which
this proper time is infinite. Class B1 metrics are shown to be extendable
beyond the horizon only for discrete values of mass and scalar charge,
depending on two integers $m$ and $n$. In the case of even $m-n$ the space-time
is globally regular; for odd $m$ the metric changes its signature at the
horizon. All spherically symmetric solutions of the Brans-Dicke theory with
$\omega<-3/2$ are shown to be linearly stable against spherically symmetric
\pns. This result extends to the generic case of the Bergmann-Wagoner class of
scalar-tensor theories of gravity with the coupling function $\omega(\phi) <
-3/2$.


title:
Microscopic Fields and Macroscopic Averages in Einstein's Unified Field
  Theory

abstract:
  The relation between microscopic and macroscopic entities in the generally
covariant theories is considered, and it is argued that a sensible definition
of the macroscopic averages requires a restriction of the allowed
transformations of coordinates. Spacetime averages of the geometric objects of
Einstein's unified field theory are then defined, and the reconstruction of
some features of macroscopic reality from hypothetic microscopic structures is
attempted. It is shown how a fluctuating microscopic behaviour of the metric
field can rule the constitutive relation for electromagnetism both in vacuo and
in nondispersive material media. Moreover, if both the metric and the skew
tensor density that represents the electric displacement and the magnetic field
are assumed to possess a wavy microscopic structure, nonvanishing generalized
force densities can appear in the continuum. They originate from a resonance
process, in which at least three waves need to be involved. This process only
occurs if the wavevectors fulfil the three-wave resonance condition, so
ubiquitous in quantum physics. The wavy behaviour of the metric is essential
for the occurrence of this resonance phenomenon.


title:
Interpretation of the Siklos solutions as exact gravitational waves in
  the anti-de Sitter universe

abstract:
  The Siklos class of solutions of Einstein's field equations is investigated
by analytical methods. By studying the behaviour of free particles we reach the
conclusion that the space-times represent exact gravitational waves propagating
in the anti-de Sitter universe. The presence of a negative cosmological
constant implies that the 'background' space is not asymptotically flat and
requires a 'rotating' reference frames in order to fully simplify and view the
behaviour of nearby test particles. The Kaigorodov space-time, which is the
simplest representative of the Siklos class, is analyzed in more detail. It is
argued that it may serve as a 'cosmological' analogue of the well-known
homogeneous pp-waves in the flat universe.


title: Relativistic Disks as Sources of Kerr-Newman Fields

abstract:
  If an appropriate region of Kerr-Newman space-time is removed and suitable
identifications are made, the resulting space-time can be interpreted as an
infinitely thin disk producing the original electromagnetic and gravitational
fields. We choose the shape of the regions removed in such a way that radial
pressures in the disks vanish. Even the very inner parts such as ergoregions
may exist around the disks. The surface energy-momentum tensor of the disks is
checked to satisfy the weak and strong energy conditions. To emphasize the
reality of these sources two models of the disks are presented: (i) rotating
conductive charged rings which are supported against collapse by their internal
pressure, (ii) two counter-rotating streams of charged particles moving along
circular electro-geodesics. All these disk sources form a three-dimensional
parameter space with specific electric charge Q/M, angular momentum a/M and the
size of the excluded region being the parameters.


title: New examples of sandwich gravitational waves and their impulsive limit

abstract:
  Non-standard sandwich gravitational waves are constructed from the
homogeneous pp vacuum solution and the motions of free test particles in the
space-times are calculated explicitly. They demonstrate the caustic property of
sandwich waves. By performing limits to impulsive gravitational wave it is
demonstrated that the resulting particle motions are identical regardless of
the ''initial'' sandwich.


title: Yang-Mills analogues of the Immirzi ambiguity

abstract:
  We draw parallels between the recently introduced ``Immirzi ambiguity'' of
the Ashtekar-like formulation of canonical quantum gravity and other
ambiguities that appear in Yang-Mills theories, like the $\theta$ ambiguity. We
also discuss ambiguities in the Maxwell case, and implication for the loop
quantization of these theories.


title:
Hot, Rotating Disks In General Relativity: Collisionless Equilibrium
  Models

abstract:
  We present a method for constructing equilibrium disks with net angular
momentum in general relativity. The method solves the relativistic Vlasov
equation coupled to Einstein's equations for the gravitational field. We apply
the method to construct disks that are relativistic versions of Newtonian
Kalnajs disks. In Newtonian gravity these disks are analytic, and are stable
against ring formation for certain ranges of their velocity dispersion. We
investigate the existence of fully general relativistic equilibrium sequences
for differing values of the velocity dispersion. These models are the first
rotating, relativistic disk solutions of the collisionless Boltzman equation.


title:
On the perturbed Schwarzschild geometry for determination of particle
  motion

abstract:
  A novel method for calculation of the motion and radiation reaction for the
two-body problem (body plus particle, the small parameter m/M being the ratio
of the masses) is presented. In the background curvature given by the
Schwarzschild geometry rippled by gravitational waves, the geodesic equations
insure the presence of radiation reaction also for high velocities and strong
field. The method is generally applicable to any orbit, but radial fall is of
interest due to the non-adiabatic regime (equality of radiation reaction and
fall time scales), in which the particle locally and immediately reacts to the
emitted radiation. The energy balance hypothesis is only used (emitted
radiation equal to the variation in the kinetic energy) for determination of
the 4-velocity via the Lagrangian and normalization of divergencies. The
solution in time domain of the Regge-Wheeler-Zerilli-Moncrief radial wave
equation determines the metric tensor expressing the polar perturbations, in
terms of which the geodesic equations are written and shown herein.


title: Quantum gravity effects in the CGHS model of collapse to a black hole

abstract:
  We show that only a sector of the classical solution space of the CGHS model
describes formation of black holes through collapse of matter. This sector has
either right or left moving matter. We describe the sector which has left
moving matter in canonical language. In the nonperturbative quantum theory all
operators are expressed in terms of the matter field operator which is
represented on a Fock space. We discuss existence of large quantum fluctuations
of the metric operator when the matter field is approximately classical. We end
with some comments which may pertain to Hawking radiation in the context of the
model.


title:
Late-Time Tails in Gravitational Collapse of a Self-Interacting
  (Massive) Scalar-Field and Decay of a Self-Interacting Scalar Hair

abstract:
  We study analytically the initial value problem for a self-interacting
(massive) scalar-field on a Reissner-Nordstr\"om spacetime. Following the
no-hair theorem we examine the dynamical physical mechanism by which the
self-interacting (SI) hair decays. We show that the intermediate asymptotic
behaviour of SI perturbations is dominated by an oscillatory inverse power-law
decaying tail. We show that at late-times the decay of a SI hair is slower than
any power-law. We confirm our analytical results by numerical simulations.


title:
Late-Time Evolution of Charged Gravitational Collapse and Decay of
  Charged Scalar Hair - III. Nonlinear Analysis

abstract:
  We study the nonlinear gravitational collapse of a charged massless
scalar-field. We confirm the existence of oscillatory inverse power-law tails
along future timelike infinity, future null infinity and along the future
outer-horizon. The nonlinear dumping exponents are in excellent agreement with
the analytically predicted ones. Our results prove the analytic conjecture
according to which a charged hair decays slower than a neutral one and also
suggest the occurrence of mass-inflation along the Cauchy horizon of a
dynamically formed charged black-hole.


title: Modelling the dynamics of global monopoles

abstract:
  A thin wall approximation is exploited to describe a global monopole coupled
to gravity. The core is modelled by de Sitter space; its boundary by a thin
wall with a constant energy density; its exterior by the asymptotic
Schwarzschild solution with negative gravitational mass $M$ and solid angle
deficit, $\Delta\Omega/4\pi = 8\pi G\eta^2$, where $\eta$ is the symmetry
breaking scale. The deficit angle equals $4\pi$ when $\eta=1/\sqrt{8\pi G}
\equiv M_p$. We find that: (1) if $\eta <M_p$, there exists a unique globally
static non-singular solution with a well defined mass, $M_0<0$. $M_0$ provides
a lower bound on $M$. If $M_0<M<0$, the solution oscillates. There are no
inflating solutions in this symmetry breaking regime. (2) if $\eta \ge M_p$,
non-singular solutions with an inflating core and an asymptotically
cosmological exterior will exist for all $M<0$. (3) if $\eta$ is not too large,
there exists a finite range of values of $M$ where a non-inflating monopole
will also exist. These solutions appear to be metastable towards inflation. If
$M$ is positive all solutions are singular. We provide a detailed description
of the configuration space of the model for each point in the space of
parameters, $(\eta, M)$ and trace the wall trajectories on both the interior
and the exterior spacetimes. Our results support the proposal that topological
defects can undergo inflation.


title:
Comments on the Unification of Electromagnetism and Gravitation through
  "Generalized Einstein manifolds"

abstract:
  H. Akbar-Zadeh has recently proposed (J Geom Phys 17 (1995) 342) a new
geometric formulation of Einstein-Maxwell system with source in terms of what
are called "Generalized Einstein manifolds". We show that, contrary to the
claim, Maxwell equations have not been derived in this formulation and that,
the assumed equations can be identified only as source-free Maxwell equations
in the proposed geometric set up. A genuine derivation of source-free Maxwell
equations is presented within the same framework. We draw a conclusion that the
proposed unification scheme can pertain only to source-free situations.


title: Boson Stars: Early History and Recent Prospects

abstract:
  Boson stars are descendants of the so-called geons of Wheeler, except that
they are built from scalar particles instead of electromagnetic fields. If
scalar fields exist in nature, such localized configurations kept together by
their self-generated gravitational field can form within Einstein's general
relativity. In the case of complex scalar fields, an absolutely stable branch
of such non-topological solitons with conserved particle number exists. Our
present surge stems from the speculative possibility that these compact objects
could provide a considerable fraction of the non-baryonic part of dark matter.
In any case, they may serve as a convenient "laboratory" for studying
numerically rapidly rotating bodies in general relativity and the generation of
gravitational waves.


title: Simplest cosmological model with the scalar field

abstract:
  We investigate the simplest cosmological model with the massive real scalar
non-interacting inflaton field minimally coupled to gravity. The classification
of trajectories in closed minisuperspace Friedmann-Robertson-Walker model is
presented.The fractal nature of a set of infinitely bounced trajectories is
discussed. The results of numerical calculations are compared with those
obtained byperturbative analytical calculations around the exactly solvable
minisuperspace cosmological model with massless scalar field.


title: Primordial Density Fluctuations in a Dual Supergravity Cosmology

abstract:
  We analyse the spectrum of energy density fluctuations of a dual supergravity
model where the dilaton and the moduli are stabilized and sucessful inflation
is achieved inside domain walls that separate different vacua of the theory.
Constraints on the parameters of the superpotential are derived from the
amplitude of the primordial energy density fluctuations as inferred from COBE
and it is shown that the scale dependence of the tensor perturbations nearly
vanishes.


title: Birth of a Closed Universe of Negative Spatial Curvature

abstract:
  We propose a modified form of the spontaneous birth of the universe by
quantum tunneling. It proceeds through topology change and inflation, to
eventually become a universe with closed spatial sections of negative curvature
and nontrivial global topology.


title:
The imposition of Cauchy data to the Teukolsky equation II: Numerical
  comparison with the Zerilli-Moncrief approach to black hole perturbations

abstract:
  We revisit the question of the imposition of initial data representing
astrophysical gravitational perturbations of black holes. We study their
dynamics for the case of nonrotating black holes by numerically evolving the
Teukolsky equation in the time domain. In order to express the Teukolsky
function Psi explicitly in terms of hypersurface quantities, we relate it to
the Moncrief waveform phi_M through a Chandrasekhar transformation in the case
of a nonrotating black hole. This relation between Psi and phi_M holds for any
constant time hypersurface and allows us to compare the computation of the
evolution of Schwarzschild perturbations by the Teukolsky and by the Zerilli
and Regge-Wheeler equations. We explicitly perform this comparison for the
Misner initial data in the close limit approach. We evolve numerically both,
the Teukolsky (with the recent code of Ref. [1]) and the Zerilli equations,
finding complete agreement in resulting waveforms within numerical error. The
consistency of these results further supports the correctness of the numerical
code for evolving the Teukolsky equation as well as the analytic expressions
for Psi in terms only of the three-metric and the extrinsic curvature.


title:
Energy and angular momentum of the weak gravitational waves on the
  Schwarzschild background -- quasilocal gauge-invariant formulation

abstract:
  It is shown that the axial and polar perturbations of the spherically
symmetric black hole can be described in a gauge-invariant way. The reduced
phase space describing gravitational waves outside of the horizon is described
by the gauge-invariant quantities. Both degrees of freedom fulfill generalized
scalar wave equation. For the axial degree of freedom the radial part of the
equation corresponds to the Regge-Wheeler result (Phys. Rev. 108, 1063-1069
(1957)) and for the polar one we get Zerilli result (Phys. Rev. D2, 2141-2160
(1970)), see also Chandrasekhar (The Mathematical Theory of Black
Holes,(Clarendon Press Oxford, 1983)), Moncrief (Annals of Physics 88, 323-342
(1974)) for both. An important ingredient of the analysis is the concept of
quasilocality which does duty for the separation of the angular variables in
the usual approach. Moreover, there is no need to represent perturbations by
normal modes (with time dependence $\exp(-ikt)$), we have fields in spacetime
and the Cauchy problem for them is well defined outside of the horizon. The
reduced symplectic structure explains the origin of the axial and polar
invariants. It allows to introduce an energy and angular momentum for the
gravitational waves which is invariant with respect to the gauge
transformations. Both generators represent quadratic approximation of the ADM
nonlinear formulae in terms of the perturbations of the Schwarzschild metric.
We also discuss the boundary-initial value problem for the linearized Einstein
equations on a Schwarzschild background outside of the horizon.


title:
Stable characteristic evolution of generic 3-dimensional
  single-black-hole spacetimes

abstract:
  We report new results which establish that the accurate 3-dimensional
numerical simulation of generic single-black-hole spacetimes has been achieved
by characteristic evolution with unlimited long term stability. Our results
cover a selection of distorted, moving and spinning single black holes, with
evolution times up to 60,000M.


title: Cauchy-characteristic matching

abstract:
  This paper gives a detailed pedagogic presentation of the central concepts
underlying a new algorithm for the numerical solution of Einstein's equations
for gravitation. This approach incorporates the best features of the two
leading approaches to computational gravitation, carving up spacetime via
Cauchy hypersurfaces within a central worldtube, and using characteristic
hypersurfaces in its exterior to connect this region with null infinity and
study gravitational radiation. It has worked well in simplified test problems,
and is currently being used to build computer codes to simulate black hole
collisions in 3-D.


title:
Curvature-based Gauge-Invariant Perturbation Theory for Gravity: A New
  Paradigm

abstract:
  A new approach to gravitational gauge-invariant perturbation theory begins
from the fourth-order Einstein-Ricci system, a hyperbolic formulation of
gravity for arbitrary lapse and shift whose centerpiece is a wave equation for
curvature. In the Minkowski and Schwarzschild backgrounds, an intertwining
operator procedure is used to separate physical gauge-invariant curvature
perturbations from unphysical ones. In the Schwarzschild case, physical
variables are found which satisfy the Regge-Wheeler equation in both odd and
even parity. In both cases, the unphysical "gauge'' degrees of freedom are
identified with violations of the linearized Hamiltonian and momentum
constraints, and they are found to evolve among themselves as a closed
subsystem. If the constraints are violated, say by numerical
finite-differencing, this system describes the hyperbolic evolution of the
constraint violation. It is argued that an underlying raison d'\^etre of causal
hyperbolic formulations is to make the evolution of constraint violations
well-posed.


title: Gravity and Duality between Coordinates and Matter Fields

abstract:
  We use the duality between the local Cartezian coordinates and the solutions
of the Klein-Gordon equation to parametrize locally the spacetime in terms of
wave functions and prepotentials. The components of metric, metric connection,
curvature as well as the Einstein equation are given in this parametrization.
We also discuss the local duality between coordinates and quantum fields and
the metric in this later reparametrization.


title: Uniqueness of the mass in the radiating regime

abstract:
  The usual approaches to the definition of energy give an ambiguous result for
the energy of fields in the radiating regime. We show that for a massless
scalar field in Minkowski space-time the definition may be rendered unambiguous
by adding the requirement that the energy cannot increase in retarded time. We
present a similar theorem for the gravitational field, proved elsewhere, which
establishes that the Trautman-Bondi energy is the unique (up to a
multiplicative factor) functional, within a natural class, which is monotonic
in time for all solutions of the vacuum Einstein equations admitting a smooth
``piece'' of conformal null infinity Scri.


title:
The dynamical structure of four-dimensional Chamseddine's gauge theory
  of gravity

abstract:
  We perform the Dirac hamiltonian analysis of a four-dimensional gauge theory
of gravity with an action of topological type, which generalizes some
well-known two-dimensional models. We show that, in contrast with the
two-dimensional case, the theory has a non-vanishing number of dynamical
degrees of freedom and that its structure is very similar to higher-dimensional
Chern-Simons gravity.


title: Geometry of the extreme Kerr black holes

abstract:
  Geometrical properties of the extreme Kerr black holes in the topological
sectors of nonextreme and extreme configurations are studied. We find that the
Euler characteristic plays an essential role to distinguish these two kinds of
extreme black holes. The relationship between the geometrical properties and
the intrinsic thermodynamics are investigated.


title: A duality relation for fluid spacetime

abstract:
  We consider the electromagnetic resolution of gravitational field. We show
that under the duality transformation, in which active and passive electric
parts of the Riemann curvature are interchanged, a fluid spacetime in comoving
coordinates remains invariant in its character with density and pressure
transforming, while energy flux and anisotropic pressure remaining unaltered.
Further if fluid admits a barotropic equation of state, $p = (\gamma - 1) \rho$
where $1 \leq \gamma \leq 2$, which will transform to $p = (\frac{2 \gamma}{3
\gamma - 2} - 1) \rho$. Clearly the stiff fluid and dust are dual to each-other
while $\rho + 3 p =0$, will go to flat spacetime. However the n $(\rho - 3 p =
0)$ and the deSitter $(\rho + p = 0$) universes ar e self-dual.


title: The Cauchy Horizon in Higher-derivative Gravity Theories

abstract:
  A class of exact solutions of the field equations with higher derivative
terms is presented when the matter field is a pressureless null fluid plus a
Maxwellian static electric component. It is found that the stable solutions are
black holes in anti de Sitter background. The issue of the stability of the
Cauchy horizon is discussed.


title:
Numerical Evidence that the Singularity in Polarized U(1) Symmetric
  Cosmologies on $T^3 \times R$ is Velocity Dominated

abstract:
  Numerical evidence supports the conjecture that polarized U(1) symmetric
cosmologies have asymptotically velocity term dominated singularities.


title: Particle-Like Solutions of the Einstein-Dirac Equations

abstract:
  The coupled Einstein-Dirac equations for a static, spherically symmetric
system of two fermions in a singlet spinor state are derived. Using numerical
methods, we construct an infinite number of soliton-like solutions of these
equations. The stability of the solutions is analyzed. For weak coupling (i.e.,
small rest mass of the fermions), all the solutions are linearly stable (with
respect to spherically symmetric perturbations), whereas for stronger coupling,
both stable and unstable solutions exist. For the physical interpretation, we
discuss how the energy of the fermions and the (ADM) mass behave as functions
of the rest mass of the fermions. Although gravitation is not renormalizable,
our solutions of the Einstein-Dirac equations are regular and well-behaved even
for strong coupling.


title: Quantum Black Hole Entropy

abstract:
  We derive an exact formula for the dimensionality of the Hilbert space of the
boundary states of SU(2) Chern-Simons theory, which, according to the recent
work of Ashtekar et al, leads to the Bekenstein-Hawking entropy of a four
dimensional Schwarzschild black hole. Our result stems from the relation
between the (boundary) Hilbert space of the Chern-Simons theory with the space
of conformal blocks of the Wess-Zumino model on the boundary 2-sphere.


title:
Scattering on Dislocations and Cosmic Strings in the Geometric Theory of
  Defects

abstract:
  We consider scattering of elastic waves on parallel wedge dislocations in the
geometric theory of defects or, equivalently, scattering of point particles and
light rays on cosmic strings. Dislocations are described as torsion
singularities located on parallel lines, and trajectories of phonons are
assumed to be the corresponding extremals. Extremals are found for arbitrary
distribution of the dislocations in the monopole, dipole, and quadrupole
approximation and the scattering angle is obtained. Examples of continuous
distribution of wedge and edge dislocations are considered. We have found that
for deficit angles close to -2\pi a star located behind a cosmic string may
have any even number of images, 2,4,6,... The close relationship between
dislocations and conformal maps is elucidated in detail.


title:
Simplest cosmological model with the scalar field II. Influence of
  cosmological constant

abstract:
  Continuing the investigation of the simplest cosmological model with the
massive real scalar non-interacting inflaton field minimally coupled to gravity
we study an influence of the cosmological constant on the behaviour of
trajectories in closed minisuperspace Friedmann-Robertson-Walker model. The
transition from chaotic to regular behaviour for large values of cosmological
constant is discussed. Combining numerical calculations with qualitative
analysis both in configuration and phase space we present a convenient
classification of trajectories.


title: Renormalization Group Approach in Newtonian Cosmology

abstract:
  We apply the renormalization group (RG) method to examine the observable
scaling properties in Newtonian cosmology. The original scaling properties of
the equations of motion in our model are modified for averaged observables on
constant time slices. In the RG flow diagram, we find three robust fixed
points: Einstein-de Sitter, Milne and Quiescent fixed points. Their stability
(or instability) property does not change under the effect of fluctuations.
Inspired by the inflationary scenario in the early Universe, we set the
Einstein-de Sitter fixed point with small fluctuations, as the boundary
condition at the horizon scale. Solving the RG equations under this boundary
condition toward the smaller scales, we find generic behavior of observables
such that the density parameter $\Omega$ decreases, while the Hubble parameter
$H$ increases for smaller averaging volume. The quantitative scaling properties
are analyzed by calculating the characteristic exponents around each fixed
point. Finally we argue the possible fractal structure of the Universe beyond
the horizon scale.


title: On the Canonical Formalism for a Higher-Curvature Gravity

abstract:
  Following the method of Buchbinder and Lyahovich, we carry out a canonical
formalism for a higher-curvature gravity in which the Lagrangian density ${\cal
L}$ is given in terms of a function of the salar curvature $R$ as ${\cal
L}=\sqrt{-\det g_{\mu\nu}}f(R)$. The local Hamiltonian is obtained by a
canonical transformation which interchanges a pair of the generalized
coordinate and its canonical momentum coming from the higher derivative of the
metric.


title:
The Dynamical Instability of Static, Spherically Symmetric Solutions in
  Nonsymmetric Gravitational Theories

abstract:
  We consider the dynamical stability of a class of static,
spherically-symmetric solutions of the nonsymmetric gravitational theory. We
numerically reproduce the Wyman solution and generate new solutions for the
case where the theory has a nontrivial fundamental length scale \mu^{-1}. By
considering spherically symmetric perturbations of these solutions we show that
the Wyman solutions are generically unstable.


title: Null weak singularities in plane-symmetric spacetimes

abstract:
  We construct a new class of plane-symmetric solutions possessing a curvature
singularity which is null and weak, like the spacetime singularity at the
Cauchy horizon of spinning (or charged) black holes. We then analyse the
stability of this singularity using a rigorous non-perturbative method. We find
that within the framework of (linearly-polarized) plane-symmetric spacetimes
this type of null weak singularity is locally stable. Generically, the
singularity is also scalar-curvature. These observations support the new
picture of the null weak singularity inside spinning (or charged) black holes,
which is so far established primarily on the perturbative approach.


title:
A 3+1 Computational Scheme for Dynamic Spherically Symmetric Black Hole
  Spacetimes -- I: Initial Data

abstract:
  When using the black hole exclusion (horizon boundary condition) technique,
$K$ is usually nonzero and spatially variable, so none of the special cases of
York's conformal-decomposition algorithm apply, and the full 4-vector nonlinear
York equations must be solved numerically.
  We discuss the construction of dynamic black hole initial data slices using
this technique: We perturb a known black hole slice via some Ansatz, apply the
York decomposition (using another Ansatz for the inner boundary conditions) to
project the perturbed field variables back into the constraint hypersurface,
and finally optionally apply a numerical 3-coordinate transformation to (eg)
restore an areal radial coordinate.
  In comparison to other initial data algorithms, the key advantage of this
algorithm is its flexibility: $K$ is unrestricted, allowing the use of whatever
slicing is most suitable for (say) a time evolution.
  We have implemented this algorithm for the spherically symmetric scalar field
system. We present numerical results for a number of Eddington-
Finkelstein--like initial data slices containing black holes surrounded by
scalar field shells. Using 4th order finite differencing with resolutions of
$\Delta r/r \approx 0.02$ (0.01) near the (Gaussian) perturbations, the
numerically computed energy and momentum constraints for the final slices are
$\ltsim 10^{-8}$ ($10^{-9}$) and $\ltsim 10^{-9}$ ($10^{-10}$) in magnitude.
  Finally, we briefly discuss the errors incurred when interpolating data from
one grid to another, as in numerical coordinate transformation or horizon
finding. We show that for the usual moving-local-interpolation schemes, even
for smooth functions the interpolation error is not smooth.


title: New varieties of Gowdy spacetimes

abstract:
  Gowdy spacetimes are generalized to admit two commuting spatial "local"
Killing vectors, and some new varieties of them are presented, which are all
closely related to Thurston's geometries. Explicit spatial compactifications,
as well as the boundary conditions for the metrics are given in a systematic
way. A short comment on an implication to their dynamics toward the initial
singularity is made.


title: Gravitational Conical Bremsstrahlung and Differential Structures

abstract:
  Differential properties of a spin 2 boson field $\psi_{\mu\nu}$ describing
propagation of gravitational perturbations on a straight cosmic string's
space-time background are studied by means of methods of the differential
spaces theory. It is shown that this field is a smooth one in the interior of
cosmic string's space-time and looses this property at the singular boundary
except for cosmic string space-times with the following deficits of angle:
$\Delta=2\pi (1-1/n) $, $n=1,2,...$. A relationship between smoothness of
$\psi_{\mu\nu}$ at the singularity and the gravitational conical bremsstrahlung
effect is discussed. A physical interpretation of the smoothness notion is
given. It is also argued that the assumption of smoothness of $\psi_{\mu\nu}$
at the singularity plays an equivalent role to the Aliev and Gal'tsov
"quantization" condition.


title: Dualities and Hidden Supersymmetry in String Quantum Cosmology

abstract:
  A supersymmetric approach to string quantum cosmology based on the
non-compact, global duality symmetries of the effective action is developed. An
N=2 supersymmetric action is derived whose bosonic component is the
Neveu-Schwarz/Neveu-Schwarz sector of the $(d+1)$--dimensional effective action
compactified on a $d$--torus. A representation for the supercharges is found
and the form of the zero-and one-fermion quantum states is determined. The
purely bosonic component of the wavefunction is unique and manifestly invariant
under the symmetry of the action. The formalism applies to a wide class of
non-linear sigma-models.


title: Instability of three dimensional conformally dressed black hole

abstract:
  The three dimensional black hole solution of Einstein equations with negative
cosmological constant coupled to a conformal scalar field is proved to be
unstable against linear circularly symmetric perturbations.


title: Lensing and caustic effects on cosmological distances

abstract:
  We consider the changes which occur in cosmological distances due to the
combined effects of some null geodesics passing through low-density regions
while others pass through lensing-induced caustics. This combination of effects
increases observed areas corresponding to a given solid angle even when
averaged over large angular scales, through the additive effect of increases on
all scales, but particularly on micro-angular scales; however angular sizes
will not be significantly effected on large angular scales (when caustics
occur, area distances and angular-diameter distances no longer coincide). We
compare our results with other works on lensing, which claim there is no such
effect, and explain why the effect will indeed occur in the (realistic)
situation where caustics due to lensing are significant. Whether or not the
effect is significant for number counts depends on the associated angular
scales and on the distribution of inhomogeneities in the universe. It could
also possibly affect the spectrum of CBR anisotropies on small angular scales,
indeed caustics can induce a non-Gaussian signature into the CMB at small
scales and lead to stronger mixing of anisotropies than occurs in weak lensing.


title: On the Ultrarelativistic Limit of General Relativity

abstract:
  As is well-known, Newton's gravitational theory can be formulated as a
four-dimensional space-time theory and follows as singular limit from
Einstein's theory, if the velocity of light tends to the infinity. Here
'singular' stands for the fact, that the limiting geometrical structure differs
from a regular Riemannian space-time. Geometrically, the transition Einstein to
Newton can be viewed as an 'opening' of the light cones. This picture suggests
that there might be other singular limits of Einstein's theory: Let all light
cones shrink and ultimately become part of a congruence of singular world
lines. The limiting structure may be considered as a nullhypersurface embedded
in a five-dimensional spacetime. While the velocity of light tends to zero
here, all other velocities tend to the velocity of light. Thus one may speak of
an ultrarelativistic limit of General Relativity. The resulting theory is as
simple as Newton's gravitational theory, with the basic difference, that
Newton's elliptic differential equation is replaced by essentially ordinary
differential equations, with derivatives tangent to the generators of the
singular congruence. The Galilei group is replaced by the Carroll group
introduced by L\'evy-Leblond. We suggest to study near ultrarelativistic
situations with a perturbational approach starting from the singular structure,
similar to post-Newtonian expansions in the $c \to \infty$ case.


title: Particle production and classical condensates in de Sitter space

abstract:
  The cosmological particle production in a $k=0$ expanding de Sitter universe
with a Hubble parameter $H_0$ is considered for various values of mass or
conformal coupling of a free, scalar field. One finds that, for a minimally
coupled field with mass $0 \leq m^2 < 9 H_0^2/4$ (except for $m^2= 2H_0^2$),
the one-mode occupation number grows to unity soon after the physical
wavelength of the mode becomes larger than the Hubble radius, and afterwards
diverges as $n(t) \sim O(1)(\lambda_{phys}(t)/H_0^{-1})^{2\nu}$, where $\nu
\equiv [9/4 - m^2/H_0^2]^{1/2}$. However, for a field with $m^2 > 9H_0^2/4$,
the occupation number of a mode outside the Hubble radius is rapidly
oscillating and bounded and does not exceed unity. These results, readily
generalized for cases of a nonminimal coupling, provide a clear argument that
the long-wavelength vacuum fluctuations of low-mass fields in an inflationary
universe do show classical behavior, while those of heavy fields do not. The
interaction or self-interaction does not appear necessary for the emergence of
classical features, which are entirely due to the rapid expansion of the de
Sitter background and the upside-down nature of quantum oscillators for modes
outside the Hubble radius.


title: Gravitational Analog of the Electromagnetic Poynting Vector

abstract:
  The gravitational analog of the electromagnetic Poynting vector is
constructed using the field equations of general relativity in the Hilbert
gauge. It is found that when the gravitational Poynting vector is applied to
the solution of the linear mass quadrupole oscillator, the correct
gravitational quadrupole radiation flux is obtained. Further to this, the
Maxwell-like gravitational Poynting vector gives rise to Einstein's quadrupole
radiation formula. The gravitational energy-momentum (pseudo) tensor obtained
is symmetric and traceless. The former property allows the definition of
angular momentum for the free gravitational field.


title: Statistical Entropy of De Sitter Space

abstract:
  Quantum gravity in 2+1 dimensions with a positive cosmological constant can
be represented as an SL(2,C) Chern-Simons gauge theory. The symmetric vacuum of
this theory is a degenerate configuration for which the gauge fields and
spacetime metric vanish, while de Sitter space corresponds to a highly excited
thermal state. Carlip's approach to black hole entropy can be adapted in this
context to determine the statistical entropy of de Sitter space. We find that
it equals one-quarter the area of the de Sitter horizon, in agreement with the
semiclassical formula.


title: Ultrarelativistic limits of boosted dilaton black holes

abstract:
  We investigate the ultrarelativistic limits of dilaton black holes, black
$p$-branes (strings), multi-centered dilaton black hole solutions and black
$p$-brane (string) solutions when the boost velocity approaches the speed of
light. For dilaton black holes and black $p$-branes (boost is along the
transverse directions), the resulting geometries are gravitational shock wave
solutions generated by a single particle and membrane. For the multi-centered
dilaton black hole solutions and black $p$-brane solutions (boost is along the
transverse directions), the limiting geometries are shock wave solutions
generated by multiple particles and membranes. When the boost is along the
membrane directions, for the black $p$-brane and multi-centered black $p$-brane
solution, the resulting geometries describe general plane-fronted waves
propagating along the membranes. The effect of the dilaton on the limit is
considered.


title:
Propagation of vacuum polarized photons in topological black hole
  spacetimes

abstract:
  The one-loop effective action for QED in curved spacetime contains
equivalence principle violating interactions between the electromagnetic field
and the spacetime curvature. These interactions lead to the dependence of
photon velocity on the motion and polarization directions. In this paper we
investigate the gravitational analogue to the electromagnetic birefringence
phenomenon in the static and radiating topological black hole backgrounds,
respectively. For the static topological black hole spacetimes, the velocity
shift of photons is the same as the one in the Reissner-Nordstr\"om black
holes. This reflects that the propagation of vacuum polarized photons is not
sensitive to the asymptotic behavior and topological structure of spacetimes.
For the massless topological black hole and BTZ black hole, the light cone
condition keeps unchanged. In the radiating topological black hole backgrounds,
the light cone condition is changed even for the radially directed photons. The
velocity shifts depend on the topological structures. Due to the null fluid,
the velocity shift of photons does no longer vanish at the apparent horizons as
well as the event horizons. But the ``polarization sum rule'' is still valid.


title:
Analyticity properties and thermal effects for general quantum field
  theory on de Sitter space-time

abstract:
  We propose a general framework for quantum field theory on the de Sitter
space-time (i.e. for local field theories whose truncated Wightman functions
are not required to vanish). By requiring that the fields satisfy a weak
spectral condition, formulated in terms of the analytic continuation properties
of their Wightman functions, we show that a geodesical observer will detect in
the corresponding ``vacuum'' a blackbody radiation at temperature T=1/(2 \pi
R). We also prove the analogues of the PCT, Reeh-Schlieder and
Bisognano-Wichmann theorems.


title: Non-Relativistic Spacetimes with Cosmological Constant

abstract:
  Recent data on supernovae favor high values of the cosmological constant.
Spacetimes with a cosmological constant have non-relativistic kinematics quite
different from Galilean kinematics. De Sitter spacetimes, vacuum solutions of
Einstein's equations with a cosmological constant, reduce in the
non-relativistic limit to Newton-Hooke spacetimes, which are non-metric
homogeneous spacetimes with non-vanishing curvature. The whole non-relativistic
kinematics would then be modified, with possible consequences to cosmology, and
in particular to the missing-mass problem.


title: On the multipole expansion of the gravitational field

abstract:
  This paper constructs the multipole expansion (in general relativity) of the
gravitational field generated by a slowly-moving isolated source. We introduce
some definitions for the source multipole moments, valid to all orders in a
post-Newtonian expansion, and depending in a well-defined way on the total
stress-energy pseudo-tensor of the material and gravitational fields.
Previously obtained expressions of the source multipole moments are recovered
in the appropriate limits. The source moments parametrize the linearized
approximation of the gravitational field exterior to the source, as computed by
means of a specific post-Minkowskian algorithm defined in a previous work.
Since the radiative multipole moments parametrizing the radiation field far
from the source can be obtained as non-linear functionals of the source
moments, the present paper permits relating the radiation field far from a
slowly-moving source to the stress-energy pseudo-tensor of the source. This
should be useful when comparing to theory the future observations of
gravitational radiation by the LIGO and VIRGO experiments.


title:
Gravitational-wave bursts from soft gamma-ray repeaters: Can they be
  detected?

abstract:
  In this letter we suggest a scenario for simultaneous emission of
gravitational-wave and $\gamma$-ray bursts (GRBs) from soft gamma-ray repeaters
(SGRs). we argue that both of the radiations can be generated by a
super-Eddington accreting neutron stars in X-ray binaries. In this model a
supercritical accretion transient takes back onto the remnant star the disk
leftover by the hydrodynamic instability phase of a low magnetized, rapidly
rotating neutron star in a X-ray binary system. We estimate the rise timescale
$\Delta t_c = 0.21 ms$, minimum mass accretion rate needed to trigger the
$\gamma$-ray emission, $\dot{M}_\lambda = 4.5 \times 10^{28} g$, and its
effective associated temperature $T_{eff} = 740 keV$, and the timescale for
repeating a burst of $\gamma$-rays $\Delta \tau_R = 11.3 yr$. Altogether, we
find the associated GW amplitude and frequency to be $h_c = 2.7 \times
10^{-23}/{(Hz)}^{1/2}$ and $f_{gw} = 966 Hz$, for a source distance $\sim 55
kpc$. Detectability of the pulses by t he forthcoming GW anntenas is discussed
and found likely.


title: Stationary generalized Kerr-Schild spacetimes

abstract:
  In this paper we have applied the generalized Kerr-Schild transformation
finding a new family of stationary perfect-fluid solutions of the Einstein
field equations. The procedure used combines some well-known techniques of null
and timelike vector fields, from which some properties of the solutions are
studied in a coordinate-free way. These spacetimes are algebraically special
being their Petrov types II and D. This family includes all the classical
vacuum Kerr-Schild spacetimes, excepting the plane-fronted gravitational waves,
and some other interesting solutions as, for instance, the Kerr metric in the
background of the Einstein Universe. However, the family is much more general
and depends on an arbitrary function of one variable.


title: Unruh effect with back reaction--A first quantized treatment

abstract:
  We present a first-quantized treatment of the back reaction on an accelerated
particle detector. The accelerated detector is described as a first quantized,
charged, pointlike particle in a constant field with internal energy levels. A
quantum version of canonically conjugate future and past Rindler horizon
operators is introduced to facilitate the calculation. The evaluated transition
amplitude for detection agrees with previously obtained results.


title:
On the gravitational, dilatonic and axionic radiative damping of cosmic
  strings

abstract:
  We study the radiation reaction on cosmic strings due to the emission of
dilatonic, gravitational and axionic waves. After verifying the (on average)
conservative nature of the time-symmetric self-interactions, we concentrate on
the finite radiation damping force associated with the half-retarded minus
half-advanced ``reactive'' fields. We revisit a recent proposal of using a
``local back reaction approximation'' for the reactive fields. Using
dimensional continuation as convenient technical tool, we find, contrary to
previous claims, that this proposal leads to antidamping in the case of the
axionic field, and to zero (integrated) damping in the case of the
gravitational field. One gets normal positive damping only in the case of the
dilatonic field. We propose to use a suitably modified version of the local
dilatonic radiation reaction as a substitute for the exact (non-local)
gravitational radiation reaction. The incorporation of such a local
approximation to gravitational radiation reaction should allow one to complete,
in a computationally non-intensive way, string network simulations and to give
better estimates of the amount and spectrum of gravitational radiation emitted
by a cosmologically evolving network of massive strings.


title: Zero Energy Configurations in General Relativity

abstract:
  We investigate the ratio of gravitational binding energy to rest mass in
general relativity. For N pointlike masses, an upper bound on the magnitude of
this ratio can be derived using the second law of black hole dynamics. Only as
N approaches infinity can it approach one. A configuration that saturates the
$N = \infty$ bound is a thin spherical shell as its radius is taken to zero.
This system provides, in principle, a perfectly efficient mechanism for
converting rest mass into energy. We mention possible implications for the
black hole information problem.


title: Midi-Superspace Quantization of Non-Compact Toroidally Symmetric Gravity

abstract:
  We consider the quantization of the midi-superspace associated with a class
of spacetimes with toroidal isometries, but without the compact spatial
hypersurfaces of the well-known Gowdy models. By a symmetry reduction, the
phase space for the system at the classical level can be identified with that
of a free massless scalar field on a fixed background spacetime, thereby
providing a simple route to quantization. We are then able to study certain
non-perturbative features of the quantum gravitational system. In particular,
we examine the quantum geometry of the asymptotic regions of the spacetimes
involved and find some surprisingly large dispersive effects of quantum
gravity.


title:
Dynamics of Relativistic Interacting Gases : from a Kinetic to a Fluid
  Description

abstract:
  Starting from a microscopic approach, we develop a covariant formalism to
describe a set of interacting gases. For that purpose, we model the collision
term entering the Boltzmann equation for a class of interactions and then
integrate this equation to obtain an effective macroscopic description. This
formalism will be useful to study the cosmic microwave background
non-perturbatively in inhomogeneous cosmologies. It should also be useful for
the study of the dynamics of the early universe and can be applied, if one
considers fluids of galaxies, to the study of structure formation.


title: Conical Space-times: a Distributional Theory Approach

abstract:
  We consider the problem of calculating the Gaussian curvature of a conical
2-dimensional space by using concepts and techniques of distribution theory. We
apply the results obtained to calculate the Riemannian curvature of the
4-dimensional conical space-time. We show that the method can be extended for
calculating the curvature of a special class of more general space-times with
conical singularity.


title: Stable 3-level leapfrog integration in numerical relativity

abstract:
  The 3-level leapfrog time integration algorithm is an attractive choice for
numerical relativity simulations since it is time-symmetric and avoids
non-physical damping. In Newtonian problems without velocity dependent forces,
this method enjoys the advantage of long term stability. However, for more
general differential equations, whether ordinary or partial, delayed onset
numerical instabilities can arise and destroy the solution. A known cure for
such instabilities appears to have been overlooked in many application areas.
We give an improved cure ("deloused leapfrog") that both reduces memory demands
(important for 3+1 dimensional wave equations) and allows for the use of
adaptive timesteps without a loss in accuracy. We show both that the
instability arises and that the cure we propose works in highly relativistic
problems such as tightly bound geodesics, spatially homogeneous spacetimes, and
strong gravitational waves. In the gravitational wave test case (polarized
waves in a Gowdy spacetime) the deloused leapfrog method was five to eight
times less CPU costly at various accuracies than the implicit Crank-Nicholson
method, which is not subject to this instability.


title: Relation Between Einstein And Quantum Field Equations

abstract:
  We show that there exists a choice of scalar field modes, such that the
evolution of the quantum field in the zero-mass and large-mass limits is
consistent with the Einstein equations for the background geometry. This choice
of modes is also consistent with zero production of these particles and thus
corresponds to a preferred vacuum state preserved by the evolution. In the
zero-mass limit, we find that the quantum field equation implies the Einstein
equation for the scale factor of a radiation-dominated universe; in the
large-mass case, it implies the corresponding Einstein equation for a
matter-dominated universe. Conversely, if the classical radiation-dominated or
matter-dominated Einstein equations hold, there is no production of scalar
particles in the zero and large mass limits, respectively. The suppression of
particle production in the large mass limit is over and above the expected
suppression at large mass. Our results hold for a certain class of conformally
ultrastatic background geometries and therefore generalize previous results by
one of us for spatially flat Robertson-Walker background geometries. In these
geometries, we find that the temporal part of the graviton equations reduces to
the temporal equation for a massless minimally coupled scalar field, and
therefore the results for massless particle production hold also for gravitons.
Within the class of modes we study, we also find that the requirement of zero
production of massless scalar particles is not consistent with a non-zero
cosmological constant. Possible implications are discussed.


title:
The Causal Interpretation of Conformally Coupled Scalar Field Quantum
  Cosmology

abstract: This paper has been withdrawn due to crucial erros in equations (24)--(31).


title: A numerical simulation of pre-big bang cosmology

abstract:
  We analyse numerically the onset of pre-big bang inflation in an
inhomogeneous, spherically symmetric Universe. Adding a small dilatonic
perturbation to a trivial (Milne) background, we find that suitable regions of
space undergo dilaton-driven inflation and quickly become spatially flat
($\Omega \to 1$). Numerical calculations are pushed close enough to the big
bang singularity to allow cross checks against previously proposed analytic
asymptotic solutions.


title: First Order Phase Transitions in Gravitational Collapse

abstract:
  In recent numerical simulations of spherically symmetric gravitational
collapse a new type of critical behaviour, dominated by a sphaleron solution,
has been found. In contrast to the previously studied models, in this case
there is a finite gap in the spectrum of black-hole masses which is reminiscent
of a first order phase transition. We briefly summarize the essential features
of this phase transition and describe the basic heuristic picture underlying
the numerical phenomenology.


title: Quantum-to-classical transition for fluctuations in the early Universe

abstract:
  According to the inflationary scenario for the very early Universe, all
inhomogeneities in the Universe are of genuine quantum origin. On the other
hand, looking at these inhomogeneities and measuring them, clearly no specific
quantum mechanical properties are observed. We show how the transition from
their inherent quantum gravitational nature to classical behaviour comes about
-- a transition whereby none of the successful quantitative predictions of the
inflationary scenario for the present-day universe is changed. This is made
possible by two properties. First, the quantum state for the spacetime metric
perturbations produced by quantum gravitational effects in the early Universe
becomes very special (highly squeezed) as a result of the expansion of the
Universe (as long as the wavelength of the perturbations exceeds the Hubble
radius). Second, decoherence through the environment distinguishes the field
amplitude basis as being the pointer basis. This renders the perturbations
presently indistinguishable from stochastic classical inhomogeneities.


title: Approximate black holes from a variable cosmological constant

abstract:
  The small or zero cosmological constant, $\Lambda$, probably results from a
macroscopic cancellation mechanism of the zero-point energies. However, nearby
horizon surfaces any macroscopic mechanism is expected to result in imperfect
cancellations. A phenomenological description is given for the residual
variable cosmological constant. In the static, spherically symmetric case it
produces approximate black holes. The model describes the case of exponential
decay by $\Box\ln\Lambda=-3a$, were $a$ is a positive constant.


title: Caustics of Compensated Spherical Lens Models

abstract:
  We consider compensated spherical lens models and the caustic surfaces they
create in the past light cone. Examination of cusp and crossover angles
associated with particular source and lens redshifts gives explicit lensing
models that confirm previous claims that area distances can differ by
substantial factors from angular diameter distances even when averaged over
large angular scales. `Shrinking' in apparent sizes occurs, typically by a
factor of 3 for a single spherical lens, on the scale of the cusp caused by the
lens; summing over many lenses will still leave a residual effect.


title:
A New Method for Solving the Initial Value Problem with Application to
  Multiple Black Holes

abstract:
  This work consists of two distinct parts. In the first part we present a new
method for solving the initial value problem of general relativity. Given any
spatial metric with a surface orthogonal Killing field and two freely specified
components of the extrinsic curvature we solve for extrinsic curvature's
remaining components. For the second part, after noting that initial data for
the Kerr spacetime can be derived within our formalism we construct data for
axisymmetric configurations of spinning black holes. Though our method is
limited to axisymmetry, it offers an advantage over the Bowen-York proceedure
that our data approach those for Kerr holes in the limit of large separations
and in the close limit.


title:
Exact metric for the exterior of a global string in the Brans-Dicke
  theory

abstract:
  We determine in closed form the general static solution with cylindrical
symmetry to the Brans-Dicke equations for an energy-momentum tensor
corresponding to the one of the straight U(1) global string outside the core
radius assuming that the Goldstone boson field takes its asymptotic value


title: Quantum field theory and time machines

abstract:
  We analyze the "F-locality condition" (proposed by Kay to be a mathematical
implementation of a philosophical bias related to the equivalence principle, we
call it the "GH-equivalence principle"), which is often used to build a
generalization of quantum field theory to non-globally hyperbolic spacetimes.
In particular we argue that the theorem proved by Kay, Radzikowski, and Wald to
the effect that time machines with compactly generated Cauchy horizons are
incompatible with the F-locality condition actually does not support the
"chronology protection conjecture", but rather testifies that the F-locality
condition must be modified or abandoned. We also show that this condition
imposes a severe restriction on the geometry of the world (it is just this
restriction that comes into conflict with the existence of a time machine),
which does not follow from the above mentioned philosophical bias. So, one need
not sacrifice the GH-equivalence principle to "emend" the F-locality condition.
As an example we consider a particular modification, the "MF-locality
condition". The theory obtained by replacing the F-locality condition with the
MF-locality condition possesses a few attractive features. One of them is that
it is consistent with both locality and the existence of time machines.


title: The Chern-Simons state for the non-diagonal Bianchi IX model

abstract:
  The Bianchi IX mixmaster model is quantized in its non-diagonal form,
imposing spatial diffeomorphism, time reparametrization and Lorentz invariance
as constraints on physical state vectors before gauge-fixing. The result turns
out to be different from quantizing the diagonal model obtained by gauge-fixing
already on the classical level. For the non-diagonal model a generalized
9-dimensional Fourier transformation over a suitably chosen manifold connects
the representations in metric variables and in Ashtekar variables. A space of
five states in the metric representation is generated from the single physical
Chern-Simons state in Ashtekar variables by choosing five different integration
manifolds, which cannot be deformed into each other. For the case of a positive
cosmological constant $\Lambda$ we extend our previous study of these five
states for the diagonal Bianchi IX model to the non-diagonal case. It is shown
that additional discrete (permutation) symmetries of physical states arise in
the quantization of the non-diagonal model, which are satisfied by two of the
five states connected to the Chern-Simons state. These have the characteristics
of a wormhole groundstate and a Hartle-Hawking `no-boundary' state,
respectively. We also exhibit a special gauge-fixing of the time
reparametrization invariance of the quantized system and define an associated
manifestly positive scalar product. Then the wormhole ground state is left as
the only normalizable physical state connected to the Chern-Simons state.


title: Is There a General Area Theorem for Black Holes?

abstract:
  The general validity of the area law for black holes is still an open
problem. We first show in detail how to complete the usually incompletely
stated text-book proofs under the assumption of piecewise $C^2$-smoothness for
the surface of the black hole. Then we prove that a black hole surface
necessarily contains points where it is not $C^1$ (called ``cusps'') at any
time before caustics of the horizon generators show up, like e.g. in merging
processes. This implies that caustics never disappear in the past and that
black holes without initial cusps will never develop such. Hence black holes
which will undergo any non-trivial processes anywhere in the future will always
show cusps. Although this does not yet imply a strict incompatibility with
piecewise $C^2$ structures, it indicates that the latter are likely to be
physically unnatural. We conclude by calling for a purely measure-theoretic
proof of the area theorem.


title:
Cauchy-perturbative matching and outer boundary conditions I: Methods
  and tests

abstract:
  We present a new method of extracting gravitational radiation from
three-dimensional numerical relativity codes and providing outer boundary
conditions. Our approach matches the solution of a Cauchy evolution of
Einstein's equations to a set of one-dimensional linear wave equations on a
curved background. We illustrate the mathematical properties of our approach
and discuss a numerical module we have constructed for this purpose. This
module implements the perturbative matching approach in connection with a
generic three-dimensional numerical relativity simulation. Tests of its
accuracy and second-order convergence are presented with analytic linear wave
data.


title: Particle-Like Solutions of the Einstein-Dirac-Maxwell Equations

abstract:
  We consider the coupled Einstein-Dirac-Maxwell equations for a static,
spherically symmetric system of two fermions in a singlet spinor state.
Soliton-like solutions are constructed numerically. The stability and the
properties of the ground state solutions are discussed for different values of
the electromagnetic coupling constant. We find solutions even when the
electromagnetic coupling is so strong that the total interaction is repulsive
in the Newtonian limit. Our solutions are regular and well-behaved; this shows
that the combined electromagnetic and gravitational self-interaction of the
Dirac particles is finite.


title: Casimir driven evolution of the universe

abstract:
  For a Friedman-Robertson-Walker space-time in which the only contribution to
the stress-energy tensor comes from the renormalised zero-point energy (i.e.
the Casimir energy) of the fundamental fields the evolution of the universe
(the scale factor) depends upon whether the universe is open, flat or closed
and upon which fundamental fields inhabit the space-time. We calculate this
"Casimir effect" using the heat kernel method, and the calculation is thus
non-perturbative. We treat fields of spin $0,1/2,1$ coupled to the
gravitational background only. The heat kernels and/or zeta-functions for the
various spins are related to that of a non-minimally coupled one. A WKB
approximation is used in obtaining the radial part of that heat kernel. The
simulations of the resulting equations of motion seem to exclude the
possibility of a closed universe, $K=+1$, as these turn out to have an
overwhelming tendency towards a fast collapse - the details such as the rate of
this collapse depends on the structure of the underlying quantum degrees of
freedom: a non-minimal coupling to curvature accelerates the process. Only
$K=-1$ and K=0 will in general lead to macroscopic universes, and of these
$K=-1$ seems to be more favourable. The possibility of the scale factor being a
concave rather than a convex function potentially indicates that the problem of
the large Hubble constant is non-existent as the age of the universe need not
be less than or equal to the Hubble time. Note should be given to the fact,
however, that we are not able to pursue the numerical study to really large
times neither do simulations for a full standard model.


title: Newtonian-like and anti-Newtonian universes

abstract:
  In an irrotational dust universe, the locally free gravitational field is
covariantly described by the gravito-electric and gravito-magnetic tensors
$E_{ab}$ and $H_{ab}$. In Newtonian theory, $H_{ab}=0$ and $E_{ab}$ is the
tidal tensor. Newtonian-like dust universes in general relativity (i.e. with
$H_{ab}=0$, often called `silent') have been shown to be inconsistent in
general and unlikely to extend beyond the known spatially homogeneous or
Szekeres examples. Furthermore, they are subject to a linearization
instability. Here we show that `anti-Newtonian' universes, i.e. with purely
gravito-magnetic field, so that $E_{ab} = 0\neq H_{ab}$, are also subject to
severe integrability conditions. Thus these models are inconsistent in general.
We show also that there are no anti-Newtonian spacetimes that are linearized
perturbations of Robertson-Walker universes. The only $E_{ab}=0\neq H_{ab}$
solution known to us is not a dust solution, and we show that it is
kinematically G\"{o}del-like but dynamically unphysical.


title: Dilatonic global strings

abstract:
  We examine the field equations of a self-gravitating global string in low
energy superstring gravity, allowing for an arbitrary coupling of the global
string to the dilaton. Massive and massless dilatons are considered. For the
massive dilaton the spacetime is similar to the recently discovered
non-singular time-dependent Einstein self-gravitating global string, but the
massless dilaton generically gives a singular spacetime, even allowing for
time-dependence. We also demonstrate a time-dependent non-singular
string/anti-string configuration, in which the string pair causes a
compactification of two of the spatial dimensions, albeit on a very large
scale.


title:
Indeterministic Quantum Gravity and Cosmology IX. Nonreality of
  Many-Place Gravitational Autolocalization: Why a Ball Is Not Located in
  Different Places at Once

abstract:
  This paper is a sequel to the series of papers [gr-qc/9409010, gr-qc/9505034,
gr-qc/9603022, gr-qc/9609035, gr-qc/9609046, gr-qc/9704033, gr-qc/9704038,
gr-qc/9708014], being an immediate continuation and supplement to the last of
them, where gravitational autolocalization of a body has been considered. A
resulting solution, which describes a one-place location, has been called
gravilon. Here it is shown that a gravilon is the only solution, i.e., that
many-place gravitational autolocalization is unreal. This is closely related to
nonreality of tunneling in the conditions under consideration.


title: Matters of Gravity, the newsletter of the APS TG on gravitation

abstract:
  Contents:
  *News:
  Topical group news, by Jim Isenberg
  April 1997 Joint APS/AAPT Meeting, GTG program, by Abhay Ashtekar
  Formation of the Gravitational-Wave International Committee, by Sam Finn
  The 1997 Xanthopoulos award, by Abhay Asthekar
  We hear that..., by Jorge Pullin
  *Research Briefs:
  LIGO project update, by David Shoemaker
  The search for frame-dragging by NS and BH's, by Sharon Morsink
  Gamma-ray bursts, recent developments, by Peter Meszaros
  Status of the Binary Black hole Grand Challenge, by Richard Matzner
  *Conference Reports:
  Quantum gravity at GR15, by Don Marolf
  GR Classical, by John Friedman
  An Experimentalist's Idiosyncratic Report on GR15, by Peter Saulson
  Bangalore gravitational wave meeting, by Sharon Morsink
  Bangalore quantum gravity meeting, by Domenico Giulini
  Cleveland cosmology-topology workshop, by Neil Cornish
  Quantum Gravity in the Southern Cone II, by Carmen Nunez
  Baltimore AMS meeting, by Kirill Krasnov


title: Viscoelastic effects in a spherical Gravitational Wave antenna

abstract:
  Internal friction effects are responsible for line widening of the resonance
frequencies in spherical gravitational wave detectors, and result in
exponentially damped oscillations of its eigenmodes with a decay time which is
proportional to the quality factor of the mode and to its inverse frequency. We
study the solutions to the equations of motion for a viscoelastic spherical GW
detector based on various different assumptions about the material's
constituent equations. Quality factor dependence on mode frequency is
determined in each case, and a discussion of its applicability to actual
detectors is made.


title: Light deflection by gravitational waves from localized sources

abstract:
  We study the deflection of light (and the redshift, or integrated time delay)
caused by the time-dependent gravitational field generated by a localized
material source lying close to the line of sight. Our calculation explicitly
takes into account the full, near-zone, plus intermediate-zone, plus wave-zone,
retarded gravitational field. Contrary to several recent claims in the
literature, we find that the deflections due to both the wave-zone 1/r
gravitational wave and the intermediate-zone 1/r^2 retarded fields vanish
exactly. The leading total time-dependent deflection caused by a localized
material source, such as a binary system, is proven to be given by the
quasi-static, near-zone quadrupolar piece of the gravitational field, and
therefore to fall off as the inverse cube of the impact parameter.


title: Sources of radiation from neutron stars

abstract:
  I give a brief introduction to the problem of detecting gravitational
radiation from neutron stars. After a review of the mechanisms by which such
stars may produce radiation, I consider the different search strategies
appropriate to the different kinds of sources: isolated known pulsars, neutron
stars in binaries, and unseen neutron stars. The problem of an all-sky survey
for unseen stars is the most taxing one that we face in analysing data from
interferometers. I describe the kinds of hierarchical methods that are now
being investigated to reach the maximal sensitivity, and I suggest a
replacement for standard Fourier-transform search methods that requires fewer
floating-point operations for Fourier-based searches over large parameter
spaces, and in addition is highly parallelizable, working just as well on a
loosely coupled network of workstations as on a tightly coupled parallel
computer.


title: Singular shells of quark-gluon matter

abstract:
  The spherically symmetric thin shells of the macroscopically stable
quark-gluon matter are considered within the frameworks of the bag model and
theory of discontinuities in general relativity. The equation of state for the
two-dimensional matter is suggested, and its features are discussed. The exact
equations of motion of such shells are obtained. Distinguishing the two cases,
circumstellar and microscopical shells, we calculate the parameters of
equilibrium configurations, including the conditions of decay (deconfinement).


title:
Indeterministic Quantum Gravity and Cosmology X. Probability-Theoretic
  Aspect: A Hidden Selector for Quantum Jumps, or How the Universe Plays the
  Game of Chance

abstract:
  This paper is a sequel to the series of papers [gr-qc/9409010, gr-qc/9505034,
gr-qc/9603022, gr-qc/9609035, gr-qc/9609046, gr-qc/9704033, gr-qc/9704038,
gr-qc/9708014, gr-qc/9802016]. The problem of the meaning of objective a priori
probability for individual random trials without repetition is considered. A
sequence of such trials, namely quantum jumps, is realized in indeterministic
dynamics of the universe. A hidden selector for the quantum jumps is
constructed.


title: Spontaneous Decay of the Effective Cosmological Constant

abstract:
  We discuss the notion that quantum fields may induce an effective
time-dependent cosmological constant which decays from a large initial value.
It is shown that such cosmological models are viable in a non-de Sitter
spacetime.


title:
Quantum implementation of the cosmic censorship conjecture for toroidal
  black holes

abstract:
  We consider some aspects of quantum field theory of a conformally coupled
scalar field on the singular background obtained in the massless limit of a
class of toroidal black holes. The stress-tensor and its back-reaction on the
metric are computed using the point-splitting regularization, in the cases of
transparent, Neumann and Dirichlet boundary conditions. We find that the
quantum fluctuations generate an event horizon which hides the singularity. The
resulting object can be interpreted as a long lived remnant. We discuss the
relevance of this result in the context of the cosmic censorship conjecture,
and in connection to the end-point of the quantum evaporation process.


title: Hamilton-Jacobi Solutions for Strongly-Coupled Gravity and Matter

abstract:
  A Green's function method is developed for solving strongly-coupled gravity
and matter in the semiclassical limit. In the strong-coupling limit, one
assumes that Newton's constant approaches infinity. As a result, one may
neglect second order spatial gradients, and each spatial point evolves like an
homogeneous universe. After constructing the Green's function solution to the
Hamiltonian constraint, the momentum constraint is solved using functional
methods in conjunction with the superposition principle for Hamilton-Jacobi
theory. Exact and approximate solutions are given for a dust field or a scalar
field interacting with gravity.


title: Nordtvedt effect in rotational motion

abstract:
  Rotational motion of extended celestial bodies is discussed in the framework
of the Parametrized Post-Newtonian (PPN) formalism with the two parameters
$\gamma$ and $\beta$. A local PPN reference system of a massive extended body
being a member of a system of N massive extended bodies is constructed. In the
local PPN reference system the external gravitational field manifests itself
only in the form of tidal potentials. Rotational equations of motion, which are
then derived in the local reference system, reveal a special term in the torque
analogous to the Nordtvedt effect in the translational equations of motion: it
is proportional to $4\beta-\gamma-3$, to the acceleration of the body relative
to the global PPN reference system and to some quantity characterizing the
distribution of inertial gravitational energy within the body. This term is a
direct consequence of the violation of the Strong Equivalence Principle in
alternative theories of gravitation.


title: Curvature-based Hyperbolic Systems for General Relativity

abstract:
  We review curvature-based hyperbolic forms of the evolution part of the
Cauchy problem of General Relativity that we have obtained recently. We
emphasize first order symmetrizable hyperbolic systems possessing only physical
characteristics.


title: Entanglement thermodynamics

abstract:
  Entanglement entropy is a statistical entropy measuring information loss due
to coarse-graining corresponding to a spatial division of a system. In this
paper we construct a thermodynamics (entanglement thermodynamics) which
includes the entanglement entropy as the entropy variable, for a massless
scalar field in Minkowski, Schwarzschild and Reissner-Nordstr{\"o}m spacetimes
to understand the statistical origin of black-hole thermodynamics. It is shown
that the entanglement thermodynamics in Minkowski spacetime differs
significantly from black-hole thermodynamics. On the contrary, the entanglement
thermodynamics in Schwarzschild and Reissner-Nordstr{\"o}m spacetimes has close
relevance to black-hole thermodynamics.


title: Dynamical Vacuum in Quantum Cosmology

abstract:
  By regarding the vacuum as a perfect fluid with equation of state p=-rho, de
Sitter's cosmological model is quantized. Our treatment differs from previous
ones in that it endows the vacuum with dynamical degrees of freedom. Instead of
being postulated from the start, the cosmological constant arises from the
degrees of freedom of the vacuum regarded as a dynamical entity, and a time
variable can be naturally introduced. Taking the scale factor as the sole
degree of freedom of the gravitational field, stationary and wave-packet
solutions to the Wheeler-DeWitt equation are found. It turns out that states of
the Universe with a definite value of the cosmological constant do not exist.
For the wave packets investigated, quantum effects are noticeable only for
small values of the scale factor, a classical regime being attained at
asymptotically large times.


title: Non-uniqueness of the third post-Newtonian binary point-mass dynamics

abstract:
  It is shown that the recently found non-uniqueness of the third
post-Newtonian binary point-mass ADM-Hamiltonian is related to the
non-uniqueness at the third post-Newtonian approximation of the applied
ADM-coordinate conditions.


title: Radiation Damping in FRW Space-times with Different Topologies

abstract:
  We study the role played by the compactness and the degree of connectedness
in the time evolution of the energy of a radiating system in the
Friedmann-Robertson-Walker (FRW) space-times whose $t=const $ spacelike
sections are the Euclidean 3-manifold ${\cal R}^3$ and six topologically
non-equivalent flat orientable compact multiply connected Riemannian
3-manifolds. An exponential damping of the energy $E(t)$ is present in the
${\cal R}^3$ case, whereas for the six compact flat 3-spaces it is found
basically the same pattern for the evolution of the energy, namely relative
minima and maxima occurring at different times (depending on the degree of
connectedness) followed by a growth of $E(t)$. Likely reasons for this
divergent behavior of $E(t)$ in these compact flat 3-manifolds are discussed
and further developments are indicated. A misinterpretation of Wolf's results
regarding one of the six orientable compact flat 3-manifolds is also indicated
and rectified.


title:
Can nonextremal Reissner-Nordstrom black hole become extremal by
  assimilating infalling charged particle and shell?

abstract:
  By using the gedanken experiments suggested by Bekenstein and Rosenzweig, we
have shown that nonextremal Reissner-Nordstrom black hole cannot turn into
extremal one by assimilating infalling charged particle and charged spherical
shell.


title: Instability of 1-loop superstring cosmology

abstract:
  A stability analysis is made in the context of the previously discovered
non-singular cosmological solution from 1-loop corrected superstring effective
action. We found that this solution has an instability in graviton mode, which
is shown to have a close relation to the avoidance of initial singularity via
energy condition. We also estimate the condition for the breakdown of the
background solution due to the overdominance of the graviton.


title: p-Adic physics below and above Planck scales

abstract:
  We present a rewiew and also new possible applications of $p$-adic numbers to
pre-spacetime physics. It is shown that instead of the extension $R^n\to
Q_p^n$, which is usually implied in $p$-adic quantum field theory, it is
possible to build a model based on the $R^n\to Q_p$, where p=n+2 extension and
get rid of loop divergences. It is also shown that the concept of mass
naturally arises in $p$-adic models as inverse transition probability with a
dimensional constant of proportionality.


title:
Particle Creation in the Marginally Bound, Self Similar Collapse of
  Inhomogeneous Dust

abstract:
  We consider the evaporation of the (shell focusing) naked singularity formed
during the self-similar collapse of a marginally bound inhomogeneous dust
cloud, in the geometric optics approximation. We show that, neglecting the back
reaction of the spacetime, the radiation on $\scrip$ tends to infinity as the
Cauchy Horizon is approached. Two consequences can be expected from this
result: (a) that the back reaction of spacetime will be large and eventually
halt the formation of a naked singularity thus preserving the Cosmic Censorship
Hypothesis and (b) matter attempting to collapse into a naked singularity will
radiate away energy at an intense rate, thereby possibly providing experimental
signatures of quantum effects in curved spacetimes.


title: Equation of motion for a domain wall coupled to gravitational field

abstract:
  The equation of motion for a domain wall coupled to gravitational field is
derived from the Nambu-Goto action. The domain wall is treated as a source of
gravitational field. The perturbed equation is also obtained with gravitational
back reaction on the wall motion taken into account. For general spherically
symmetric background case, the equation is expressed in terms of the
gauge-invariant variables.


title: The model of the universe with two spaces

abstract:
  The model of the homogenous and isotropic universe with two spaces is
considered. The background space is a coordinate system of reference and
defines the behaviour of the universe. The other space characterizes the
gravity of the matter of the universe. In the presented model, the first
derivative of the scale factor of the universe with respect to time is equal to
the velocity of light. The density of the matter of the universe changes from
the Planckian value at the Planck time to the modern value at the modern time.
The model under consideration describes the universe from the Planck time to
the modern time and avoids the problems of the Friedman universe such as the
flatness problem and the horizon problem.


title: Quantum Creation of an Open Inflationary Universe

abstract:
  We discuss a dramatic difference between the description of the quantum
creation of an open universe using the Hartle-Hawking wave function and the
tunneling wave function. Recently Hawking and Turok have found that the
Hartle-Hawking wave function leads to a universe with Omega = 0.01, which is
much smaller that the observed value of Omega > 0.3. Galaxies in such a
universe would be about $10^{10^8}$ light years away from each other, so the
universe would be practically structureless. We will argue that the
Hartle-Hawking wave function does not describe the probability of the universe
creation. If one uses the tunneling wave function for the description of
creation of the universe, then in most inflationary models the universe should
have Omega = 1, which agrees with the standard expectation that inflation makes
the universe flat. The same result can be obtained in the theory of a
self-reproducing inflationary universe, independently of the issue of initial
conditions. However, there exist two classes of models where Omega may take any
value, from Omega > 1 to Omega << 1.


title: Quantum Fluctuations for Gravitational Impulsive Waves

abstract:
  Quantum fluctuations for a massless scalar field in the background metric of
spherical impulsive gravitational waves through Minkowski and de Sitter spaces
are investigated. It is shown that there exist finite fluctuations for de
Sitter space.


title: Causality properties of topologically nontrivial space-time models

abstract:
  Some problems of the space-time causal structure are discussed using models
with traversable wormholes. For this purpose the conditions of traversable
wormhole matching with the exterior space-time are considered in detail and a
mixed boundary problem for the Einstein equations is formulated and analyzed.
The influence of these matching conditions on the space-time properties and
causal structure is analyzed. These conditions have a non-dynamical nature and
cannot be determined by any physical process. So, the causality violation
cannot be a result of dynamical evolution of some initial hypersurface. It is
also shown that the same conditions which determine the wormhole joining with
the outer space provide the self-consistency of solutions and the absence of
paradoxes in the case of causality violation.


title:
Barotropic thin shells with linear EOS as models of stars and
  circumstellar shells in general relativity

abstract:
  The spherically symmetric thin shells of the barotropic fluids with the
linear equation of state are considered within the frameworks of general
relativity. We study several aspects of the shells as completely relativistic
models of stars, first of all the neutron stars and white dwarfs, and
circumstellar shells. The exact equations of motion of the shells are obtained.
Also we calculate the parameters of the equilibrium configurations, including
the radii of static shells. Finally, we study the stability of the equilibrium
shells against radial perturbations.


title: Mass of perfect fluid black shells

abstract:
  The spherically symmetric singular perfect fluid shells are considered for
the case of their radii being equal to the event horizon (the black shells). We
study their observable masses, depending at least on the three parameters,
viz., the square speed of sound in the shell, instantaneous radial velocity of
the shell at a moment when it reaches the horizon, and integration constant
related to surface mass density. We discuss the features of black shells
depending on an equation of state.


title:
Evolution of the Bianchi I, the Bianchi III and the Kantowski-Sachs
  Universe: Isotropization and Inflation

abstract:
  We study the Einstein-Klein-Gordon equations for a convex positive potential
in a Bianchi I, a Bianchi III and a Kantowski-Sachs universe. After analysing
the inherent properties of the system of differential equations, the study of
the asymptotic behaviors of the solutions and their stability is done for an
exponential potential. The results are compared with those of Burd and Barrow.
In contrast with their results, we show that for the BI case isotropy can be
reached without inflation and we find new critical points which lead to new
exact solutions. On the other hand we recover the result of Burd and Barrow
that if inflation occurs then isotropy is always reached. The numerical
integration is also done and all the asymptotical behaviors are confirmed.


title: Broken Weyl-Invariance and the Origin of Mass

abstract:
  A massless Weyl-invariant dynamics of a scalar, a Dirac spinor, and
electromagnetic fields is formulated in a Weyl space, $W_4$, allowing for
conformal rescalings of the metric and of all fields with nontrivial Weyl
weight together with the associated transformations of the Weyl vector fields
$\ka_\mu$ representing the D(1) gauge fields with D(1) denoting the dilatation
group. To study the appearance of nonzero masses in the theory the
Weyl-symmetry is broken explicitly and the corresponding reduction of the Weyl
space $W_4$ to a pseudo-Riemannian space $V_4$ is investigated assuming the
breaking to be determined by an expression involving the curvature scalar $R$
of the $W_4$ and the mass of the scalar, selfinteracting field. Thereby also
the spinor field acquires a mass proportional to the modulus $\Phi$ of the
scalar field in a Higgs-type mechanism formulated here in a Weyl-geometric
setting with $\Phi$ providing a potential for the Weyl vector fields $\ka_\mu$.
After the Weyl-symmetry breaking one obtains generally covariant and U(1) gauge
covariant field equations coupled to the metric of the underlying $V_4$. This
metric is determined by Einstein's equations, with a gravitational coupling
constant depending on $\Phi$, coupled to the energy-momentum tensors of the now
massive fields involved together with the (massless) radiation fields.


title: Einstein-Yang-Mills black hole solutions in three dimensions

abstract:
  In this paper, a procedure which gives euclidean solutions of 3-dimensional
Einstein-Yang-Mills equations when one has solutions of the Einstein equations
is proposed. The method is based on reformulating Yang-Mills theory in such a
way that it becomes a gravity. It is applied to find black hole solutions of
the coupled Einstein-Yang-Mills equations.


title: Dynamic wormholes, anti-trapped surfaces, and energy conditions

abstract:
  Adapting and extending a suggestion due to Page, we define a wormhole throat
to be a marginally anti-trapped surface, that is, a closed two-dimensional
spatial hypersurface such that one of the two future-directed null geodesic
congruences orthogonal to it is just beginning to diverge. Typically a dynamic
wormhole will possess two such throats, corresponding to the two orthogonal
null geodesic congruences, and these two throats will not coincide, (though
they do coalesce into a single throat in the static limit). The divergence
property of the null geodesics at the marginally anti-trapped surface
generalizes the ``flare-out'' condition for an arbitrary wormhole. We derive
theorems regarding violations of the null energy condition (NEC) at and near
these throats and find that, even for wormholes with arbitrary time-dependence,
the violation of the NEC is a generic property of wormhole throats. We also
discuss wormhole throats in the presence of fully antisymmetric torsion and
find that the energy condition violations cannot be dumped into the torsion
degrees of freedom. Finally by means of a concrete example we demonstrate that
even temporary suspension of energy-condition violations is incompatible with
the flare-out property of dynamic throats.


title:
Physical interpretation of constants in the solutions to the Brans-Dicke
  equations

abstract:
  Using an energy-momentum complex we give a physical interpretation to the
constants in the well-known static spherically symmetric asymptotically flat
vacuum solution to the Brans-Dicke equations. The positivity of the tensor mass
puts a bound on parameters in the solution.


title: The null energy condition in dynamic wormholes

abstract:
  We extend previous proofs that violations of the null energy condition (NEC)
are a generic and universal feature of traversable wormholes to completely
non-symmetric time-dependent wormholes. We show that the analysis can be
phrased purely in terms of local geometry at and near the wormhole throat, and
do not have to make any technical assumptions about asymptotic flatness or
other global properties. A key aspect of the analysis is the demonstration that
time-dependent wormholes have two throats, one for each direction through the
wormhole, and that the two throats coalesce only for the case of a static
wormhole.


title: A Maximum Mass-to-Size Ratio in Scalar-Tensor Theories of Gravity

abstract:
  We derive a modified Buchdahl inequality for scalar-tensor theories of
gravity. In general relativity, Buchdahl has shown that the maximum value of
the mass-to-size ratio, $2M/R$, is 8/9 for static and spherically symmetric
stars under some physically reasonable assumptions. We formally apply
Buchdahl's method to scalar-tensor theories and obtain theory-independent
inequalities. After discussing the mass definition in scalar-tensor theories,
these inequalities are related to a theory-dependent maximum mass-to-size
ratio. We show that its value can exceed not only Buchdahl's limit, 8/9, but
also unity, which we call {\it the black hole limit}, in contrast to general
relativity. Next, we numerically examine the validity of the assumptions made
in deriving the inequalities and the applicability of our analytic results. We
find that the assumptions are mostly satisfied and that the mass-to-size ratio
exceeds both Buchdahl's limit and the black hole limit. However, we also find
that this ratio never exceeds Buchdahl's limit when we impose the further
condition, $\rho-3p\ge0$, on the density, $\rho$, and pressure, $p$, of the
matter.


title: Stress-energy tensor in the Bel-Szekeres space-time

abstract:
  In a recent work an approximation procedure was introduced to calculate the
vacuum expectation value of the stress-energy tensor for a conformal massless
scalar field in the classical background determined by a particular colliding
plane wave space-time. This approximation procedure consists in appropriately
modifying the space-time geometry throughout the causal past of the collision
center. This modification in the geometry allows to simplify the boundary
conditions involved in the calculation of the Hadamard function for the quantum
state which represents the vacuum in the flat region before the arrival of the
waves. In the present work this approximation procedure is applied to the
non-singular Bel-Szekeres solution, which describes the head on collision of
two electromagnetic plane waves. It is shown that the stress-energy tensor is
unbounded as the killing-Cauchy horizon of the interaction is approached and
its behavior coincides with a previous calculation in another example of
non-singular colliding plane wave space-time.


title: Galaxy Formation -- a Condensation Process just after Recombination

abstract:
  A scenario of galaxy formation is put forward which is a process of sudden
condensation just after recombination. It is essentially based on the fact that
the cosmic matter gas after recombination is a general relativistic Boltzmann
gas which runs within a few $10^6$ years into a state very close to
collision--dominated equilibrium. The mass spectrum of axially symmetric
condensation "drops" extends from the lower limit M about $10^5$ solar masses
to the upper limit M about $10^{12}$ solar masses. The lower limit masses are
spheres whereas the upper limit masses are extremely thin pancakes. These
pancakes contract within a time of about $2.5 \cdot 10^9 y$ to fastly rotating
spiral galaxies with ordinary proportions. In this final state they have a
redshift z about 3. At an earlier time during their contraction they are higly
active and are observed with a redshift z about 5.


title: Effects of anisotropy and spatial curvature on the pre-big bang scenario

abstract:
  A class of exact, anisotropic cosmological solutions to the vacuum
Brans-Dicke theory of gravity is considered within the context of the pre-big
bang scenario. Included in this class are the Bianchi type III, V and VI_h
models and the spatially isotropic, negatively curved
Friedmann-Robertson-Walker universe. The effects of large anisotropy and
spatial curvature are determined. In contrast to negatively curved
Friedmann-Robertson-Walker model, there exist regions of the parameter space in
which the combined effects of curvature and anisotropy prevent the occurrence
of inflation. When inflation is possible, the necessary and sufficient
conditions for successful pre-big bang inflation are more stringent than in the
isotropic models. The initial state for these models is established and
corresponds in general to a gravitational plane wave.


title: Scale Factor Dualities in Anisotropic Cosmologies

abstract:
  The concept of scale factor duality is considered within the context of the
spatially homogeneous, vacuum Brans-Dicke cosmologies. In the Bianchi class A,
it is found that duality symmetries exist for the types I, II, VI_0, VII_0, but
not for types VIII and IX. The Kantowski-Sachs and locally rotationally
symmetric Bianchi type III models also exhibit a scale factor duality, but no
such symmetries are found for the Bianchi type V. In this way anisotropy and
spatial curvature may have important effects on the nature of such dualities.


title: Gravitational evolution and stability of boson halos

abstract:
  We investigate the gravitational evolution of dark matter halos made up of a
massless bosonic field. The coupled Einstein-Klein-Gordon equations are solved
numerically, showing that such a boson halo is stable and can be formed under a
large class of initial conditions. We also present an analytical proof that
such objects are stable in the Newtonian limit. In the context of boson stars
made of massive scalar fields, we introduce new solutions with an oscillatory
scalar field, similar to boson halos. We find that these solutions are
unstable.


title:
Vacuum polarization in the Szekeres Class of colliding plane wave
  space-times

abstract:
  We study the quantization of a scalar field on a classical background given
by the Szekeres Class of solutions, which represent the collision of two
gravitational plane waves with constant polarization. These solutions consist
of two approaching gravitational plane waves moving in a flat background and an
interaction region which always contains a curvature singularity. Following a
suitable approximate procedure, introduced in a previous paper, we propose a
way to compute the vacuum expectation value of the stress-energy tensor
throughout the causal past region of the collision center in the quantum state
which corresponds to the vacuum before the arrival of the waves.