title: A Lorentzian Signature Model for Quantum General Relativity

abstract:
  We give a relativistic spin network model for quantum gravity based on the
Lorentz group and its q-deformation, the Quantum Lorentz Algebra.
  We propose a combinatorial model for the path integral given by an integral
over suitable representations of this algebra. This generalises the state sum
models for the case of the four-dimensional rotation group previously studied
in gr-qc/9709028.
  As a technical tool, formulae for the evaluation of relativistic spin
networks for the Lorentz group are developed, with some simple examples which
show that the evaluation is finite in interesting cases. We conjecture that the
`10J' symbol needed in our model has a finite value.


title: On Gravity and the Uncertainty Principle

abstract:
  Heisenberg showed in the early days of quantum theory that the uncertainty
principle follows as a direct consequence of the quantization of
electromagnetic radiation in the form of photons. As we show here the
gravitational interaction of the photon and the particle being observed
modifies the uncertainty principle with an additional term. From the modified
or gravitational uncertainty principle it follows that there is an absolute
minimum uncertainty in the position of any particle, of order of the Planck
length. A modified uncertainty relation of this form is a standard result of
superstring theory, but the derivation given here is based on simpler and
rather general considerations with either Newtonian gravitational theory or
general relativity theory.


title: Vacuum structure for expanding geometry

abstract:
  We consider gravitational wave modes in the FRW metrics in a de Sitter phase
and show that the state space splits into many unitarily inequivalent
representations of the canonical commutation relations. Non-unitary time
evolution is described as a trajectory in the space of the representations. The
generator of time evolution is related to the entropy operator. The
thermodynamic arrow of time is shown to point in the same direction of the
cosmological arrow of time. The vacuum is a two-mode SU(1,1) squeezed state of
thermo field dynamics. The link between expanding geometry, squeezing and
thermal properties is exhibited.


title:
On local and global measurements of the speed of light on rotating
  platforms

abstract:
  The paper shows that, conceptually and operationally, the speed of light as
measured locally in the inertial comoving frame of a point on the rim of a
rotating disk, is different from the one measured globally for a round trip
along the rim, obtained dividing the length of the rim (as measured in the
''relative space'' of the disk) by the time of flight of the light beam (as
measured by a clock at rest on the disk). As a consequence, contrary to some
recent claims, the anisotropy found in the global value, obtained by the above
procedure, in no way conflicts with the local isotropy, and the internal
consistency of the special relativity theory remains unchallenged.


title: Spectral noncommutative geometry and quantization: a simple example

abstract:
  We explore the relation between noncommutative geometry, in the spectral
triple formulation, and quantum mechanics. To this aim, we consider a dynamical
theory of a noncommutative geometry defined by a spectral triple, and study its
quantization. In particular, we consider a simple model based on a finite
dimensional spectral triple (A, H, D), which mimics certain aspects of the
spectral formulation of general relativity. We find the physical phase space,
which is the space of the onshell Dirac operators compatible with A and H. We
define a natural symplectic structure over this phase space and construct the
corresponding quantum theory using a covariant canonical quantization approach.
We show that the Connes distance between certain two states over the algebra A
(two ``spacetime points''), which is an arbitrary positive number in the
classical noncommutative geometry, turns out to be discrete in the quantum
theory, and we compute its spectrum. The quantum states of the noncommutative
geometry form a Hilbert space K. D is promoted to an operator *D on the direct
product *H of H and K. The triple (A, *H, *D) can be viewed as the quantization
of the family of the triples (A, H, D).


title: On variations in teleparallelism theories

abstract:
  The variation procedure on a teleparallel manifold is studied. The main
problem is the non-commutativity of the variation with the Hodge dual map. We
establish certain useful formulas for variations and restate the master formula
due to Hehl and his collaborates. Our approach is different and sometimes
easier for applications. By introducing the technique of the variational matrix
we find necessary and sufficient conditions for commutativity
(anti-commutativity) of the variation derivative with the Hodge dual operator.
A general formula for the variation of the quadratic-type expression is
obtained. The described variational technique are used in two viable field
theories: the electro-magnetic Lagrangian on a curved manifold and the Rumpf
Lagrangian of the translation invariant gravity.


title: Super-energy tensor for space-times with vanishing scalar curvature

abstract:
  A four-index tensor is constructed with terms both quadratic in the Riemann
tensor and linear in its second derivatives, which has zero divergence for
space-times with vanishing scalar curvature. This tensor reduces in vacuum to
the Bel-Robinson tensor. Furthermore, the completely timelike component
referred to any observer is positive, and zero if and only if the space-time is
flat (excluding some unphysical space-times). We also show that this tensor is
the unique that can be constructed with these properties. Such a tensor does
not exist for general gravitational fields. Finally, we study this tensor in
several examples: the Friedmann-Lema\^{\i}tre-Robertson-Walker space-times
filled with radiation, the plane-fronted gravitational waves, and the Vaidya
radiating metric.


title: Equivalence Principle and Clocks

abstract:
  String theory suggests the existence of gravitational-strength scalar fields
("dilaton" and "moduli") whose couplings to matter violate the equivalence
principle. This provides a new motivation for high-precision clock experiments,
as well as a generic theoretical framework for analyzing their significance.


title: Strings in gravity with torsion

abstract:
  A theory of gravitation in 4D is presented with strings used in the material
action in $U_4$ spacetime. It is shown that the string naturally gives rise to
torsion. It is also shown that the equation of motion a string follows from the
Bianchi identity, gives the identical result as the Noether conservation laws,
and follows a geodesic only in the lowest order approximation. In addition, the
conservation laws show that strings naturally have spin, which arises not from
their motion but from their one dimensional structure.


title: 4D quantum black hole physics from 2D models?

abstract:
  Minimally coupled 4D scalar fields in Schwarzschild space-time are
considered. Dimensional reduction to 2D leads to a well known anomaly induced
effective action, which we consider here in a local form with the introduction
of auxiliary fields. Boundary conditions are imposed on them in order to select
the appropriate quantum states (Boulware, Unruh annd Israel-Hartle-Hawking).
The stress tensor is then calculated and its comparison with the expected 4D
form turns out to be unsuccessful. We also critically discuss in some detail a
recent controversial result appeared in the literature on the same topic.


title: Can primordial wormholes be induced by GUTs at the early Universe?

abstract:
  Using large N, 4d anomaly induced one-loop effective action for conformally
invariant matter (typical GUT multiplet) we study the possibility to induce the
primordial spherically symmetric wormholes at the early Universe. The
corresponding effective equations are obtained in two different coordinate
frames. The numerical investigation of these equations is done for matter
content corresponding to ${\cal N}=4$ SU(N) super Yang-Mills theory. For some
choice of initial conditions, the induced wormhole solution with increasing
throat radius and increasing red-shift function is found.


title: Gauges, Holes, and their `Connections'

abstract:
  The purpose of this paper is to present a generalized hole argument for gauge
field theories and their geometrical setting in terms of fiber bundles. The
generalized hole argument is motivated and extended from the spacetime hole
arguments which appear in spacetime theories based on differentiable manifolds
such as general relativity. Analogously, the generalized hole argument rules
out fiber bundle substantivalism and, thus, a relationalistic interpretation of
the geometry of fiber bundle spaces is favoured. Along the way, the concept of
gauge field theories will be analyzed via considering the gauge principle and
thereby hopefully clarifying certain terminological ambiguities.


title:
Exact Solutions to Sourceless Charged Massive Scalar Field Equation on
  Kerr-Newman Background

abstract:
  The separated radial part of a sourceless massive complex scalar field
equation on the Kerr-Newman black hole background is shown to be a generalized
spin-weighted spheroidal wave equation of imaginary number order. While the
separated angular part is an ordinary spheroidal wave equation. General exact
solutions in integral forms and in power series expansion as well as several
special solutions with physical interest are given for the radial equation in
the non-extreme case. In the extreme case, power series solution to the radial
equation is briefly studied. Recurrence relations between coefficients in power
series expansion of general solutions and connection between the radial
equation are discussed in both cases.


title:
Curvature conditions for the occurrence of a class of spacetime
  singularities

abstract:
  It has previously been shown [W. Rudnicki, Phys. Lett. A 224, 45 (1996)] that
a generic gravitational collapse cannot result in a naked singularity
accompanied by closed timelike curves. An important role in this result plays
the so-called inextendibility condition, which is required to hold for certain
incomplete null geodesics. In this paper, a theorem is proved that establishes
some relations between the inextendibility condition and the rate of growth of
the Ricci curvature along incomplete null geodesics. This theorem shows that
the inextendibility condition may hold for a much more general class of
singularities than only those of the strong curvature type. It is also argued
that some earlier cosmic censorship results obtained for strong curvature
singularities can be extended to singularities corresponding to the
inextendibility condition.


title: Spacelike Fluctuations of the Stress Tensor for de Sitter Vacuum

abstract:
  The two-point function characterizing the stress tensor fluctuations of a
massless minimally coupled field for an invariant vacuum state in de Sitter
spacetime is discussed. This two-point function is explicitly computed for
spacelike separated points which are geodesically connected. We show that these
fluctuations are as important as the expectation value of the stress tensor
itself. These quantum field fluctuations will induce fluctuations in the
geometry of de Sitter spacetime. This paper is a first step towards the
computation of such metric fluctuations, which may be of interest for
large-scale structure formation in cosmology. The relevance of our results in
this context is briefly discussed.


title: Evolutionary sequences of irrotational binary neutron stars

abstract:
  We present results of numerical computations of quasiequilibrium sequences of
binary neutron stars with zero vorticity, in the general relativistic
framework. The Einstein equations are solved under the assumption of a
conformally flat spatial 3-metric (Wilson-Mathews approximation). The evolution
of the central density of each star is monitored as the orbit shrinks in
response to gravitational wave emission. For a compactification ratio M/R=0.14,
the central density remains rather constant (with a slight increase, below
0.1%) before decreasing. For a higher compactification ratio M/R=0.17 (i.e.
stars closer to the maximum mass configuration), a very small density increase
(at most 0.3%) is observed before the decrease. This effect remains within the
error induced by the conformally flat approximation. It can be thus concluded
that no substantial compression of the stars is found, which would have
indicated a tendency to individually collapse to black hole prior to merger.
Moreover, no turning point has been found in the binding energy or angular
momentum along evolutionary sequences, which may indicate that these systems do
not have any innermost stable circular orbit (ISCO).


title:
The Shapiro Conjecture: Prompt or Delayed Collapse in the head-on
  collision of neutron stars?

abstract:
  We study the question of prompt vs. delayed collapse in the head-on collision
of two neutron stars. We show that the prompt formation of a black hole is
possible, contrary to a conjecture of Shapiro which claims that collapse is
delayed until after neutrino cooling. We discuss the insight provided by
Shapiro's conjecture and its limitation. An understanding of the limitation of
the conjecture is provided in terms of the many time scales involved in the
problem. General relativistic simulations in the Einstein theory with the full
set of Einstein equations coupled to the general relativistic hydrodynamic
equations are carried out in our study.


title: Ambiguities in Quantizing a Classical System

abstract:
  One classical theory, as determined by an equation of motion or set of
classical trajectories, can correspond to many unitarily {\em in}equivalent
quantum theories upon canonical quantization. This arises from a remarkable
ambiguity, not previously investigated, in the construction of the classical
(and hence the quantized) Hamiltonian or Lagrangian. This ambiguity is
illustrated for systems with one degree of freedom: An arbitrary function of
the constants of motion can be introduced into this construction. For example,
the nonrelativistic and relativistic free particles follow identical classical
trajectories, but the Hamiltonians or Lagrangians, and the canonically
quantized versions of these descriptions, are inequivalent. Inequivalent
descriptions of other systems, such as the harmonic oscillator, are also
readily obtained.


title: Impact of a Multi-TeraFlop Machine to Gravitational Physics

abstract:
  A multi-TeraFlop/TeraByte machine will enable the application of the Einstein
theory of gravity to realistic astrophysical processes. Without the
computational power, the complexity of the Einstein theory restricts most
studies based on it to the quasi static/linear near-Newtonian regime of the
theory.
  The application of the Einstein theory to realistic astrophysical processes
is bound to bring deep and far-reaching scientific discoveries, and produce
results that will inspire the general public. It is an essential component in
developing the new frontier of gravitational wave astronomy - an exciting new
window to observe our universe.
  The computational requirements of carrying out numerical simulations based on
the Einstein theory is discussed with an explicit example, the coalescence of a
neutron star binary.
  This document is prepared for presentation at the National Computational
Science Alliance User Advisory Council Meeting at NSF, June 1998, in support of
the funding of a NSF TeraFlop computer.


title: Electrostatics in a Schwarzschild black hole pierced by a cosmic string

abstract:
  We explicitly determine the expression of the electrostatic potential
generated by a point charge at rest in the Schwarzschild black hole pierced by
a cosmic string. We can then calculate the electrostatic self-energy. From
this, we find again the upper entropy bound for a charged object by employing
thermodynamics of the black hole.


title:
Einstein equations in the null quasi-spherical gauge III: numerical
  algorithms

abstract:
  We describe numerical techniques used in the construction of our 4th order
evolution for the full Einstein equations, and assess the accuracy of
representative solutions. The code is based on a null gauge with a
quasi-spherical radial coordinate, and simulates the interaction of a single
black hole with gravitational radiation. Techniques used include spherical
harmonic representations, convolution spline interpolation and filtering, and
an RK4 "method of lines" evolution. For sample initial data of "intermediate"
size (gravitational field with 19% of the black hole mass), the code is
accurate to 1 part in 10^5, until null time z=55 when the coordinate condition
breaks down.


title:
The Role of Energy and a New Approach to Gravitational Waves in General
  Relativity

abstract:
  The energy localization hypothesis of the author that energy is localized in
non-vanishing regions of the energy-momentum tensor implies that gravitational
waves do not carry energy in vacuum. If substantiated, this has significant
implications for current research. Support for the hypothesis is provided by a
re-examination of Eddington's classic calculation of energy loss by a spinning
rod. It is emphasized that Eddington did not monitor the entire Tolman energy
integral, concentrating solely upon the change of the 'kinetic' part of the
energy . The 'quadrupole formula' is thus seen to measure the kinetic energy
change. When the derivative of the missing stress-trace integral is computed,
it is seen to cancel the Eddington term and hence the energy of the rod is
conserved, in support of the localization hypothesis. The issue of initial and
final states is addressed.


title: The Birkhoff theorem for topologically massive gravity

abstract:
  We derive the general $\Sigma_2\times S$ solution of topologically massive
gravity in vacuum and in presence of a cosmological constant. The field
equations reduce to three-dimensional Einstein equations and the solution has
constant Ricci tensor. We briefly discuss the emergence of non-Ricci flat
solutions when spin is introduced.


title: Well-posed forms of the 3+1 conformally-decomposed Einstein equations

abstract:
  We show that well-posed, conformally-decomposed formulations of the 3+1
Einstein equations can be obtained by densitizing the lapse and by combining
the constraints with the evolution equations. We compute the characteristics
structure and verify the constraint propagation of these new well-posed
formulations. In these formulations, the trace of the extrinsic curvature and
the determinant of the 3-metric are singled out from the rest of the dynamical
variables, but are evolved as part of the well-posed evolution system. The only
free functions are the lapse density and the shift vector. We find that there
is a 3-parameter freedom in formulating these equations in a well-posed manner,
and that part of the parameter space found consists of formulations with causal
characteristics, namely, characteristics that lie only within the lightcone. In
particular there is a 1-parameter family of systems whose characteristics are
either normal to the slicing or lie along the lightcone of the evolving metric.


title:
Complete Classification of the String-like Solutions of the Gravitating
  Abelian Higgs Model

abstract:
  The static cylindrically symmetric solutions of the gravitating Abelian Higgs
model form a two parameter family. In this paper we give a complete
classification of the string-like solutions of this system. We show that the
parameter plane is composed of two different regions with the following
characteristics: One region contains the standard asymptotically conic cosmic
string solutions together with a second kind of solutions with Melvin-like
asymptotic behavior. The other region contains two types of solutions with
bounded radial extension. The border between the two regions is the curve of
maximal angular deficit of $2\pi$.


title: Relativistic Charged Balls

abstract:
  It is proven that the relativistic charged ball with its charge less than its
mass (in natural units) cannot have a non-singular static configuration while
its radius approaches its external horizon size. This conclusion does not
depend on the details of charge distribution and the equation of state. The
involved assumptions are (1) the ball is made of perfect fluid, (2) the energy
density is everywhere non-negative.


title: Quantum corrected geodesics

abstract:
  We compute the graviton-induced corrections to the trajectory of a classical
test particle. We show that the motion of the test particle is governed by an
effective action given by the expectation value (with respect to the graviton
state) of the classical action. We analyze the quantum corrected equations of
motion for the test particle in two particular backgrounds: a Robertson Walker
spacetime and a 2+1 dimensional spacetime with rotational symmetry. In both
cases we show that the quantum corrected trajectory is not a geodesic of the
background metric.


title: Ultrarelativistic fluid dynamics

abstract:
  This is the first of two papers examining the critical collapse of
spherically symmetric perfect fluids with the equation of state P = (Gamma
-1)rho. Here we present the equations of motion and describe a computer code
capable of simulating the extremely relativistic flows encountered in critical
solutions for Gamma <= 2. The fluid equations are solved using a
high-resolution shock-capturing scheme based on a linearized Riemann solver.


title: Renormalization of Long-wavelength Solution of Einstein Equation

abstract:
  Using the renormalization group method, we improved the first order solution
of the long-wavelength expansion of the Einstein equation. By assuming that the
renormalization group transformation has the property of Lie group, we can
regularize the secular divergence caused by the spatial gradient terms and
absorb it to the background seed metric. The solution of the renormalization
group equation shows that the renormalized metric describes the behavior of
gravitational collapse in the expanding universe qualitatively well.


title:
Apparent Horizon Formation and Hoop Conjecture in Non-axisymmetric
  Spaces

abstract:
  We investigate the validity of Thorne's hoop conjecture in non-axisymmetric
spacetimes by examining the formation of apparent horizons numerically. If
spaces have a discrete symmetry about one axis, we can specify the boundary
conditions to determine an apparent horizon even in non-axisymmetric spaces. We
implement, for the first time, the ``hoop finder'' in non-axisymmetric spaces
with a discrete symmetry. We construct asymptotically flat vacuum solutions at
a moment of time symmetry. Two cases are examined: black holes distributed on a
ring, and black holes on a spherical surface. It turns out that calculating
${\cal C}$ is reduced to solving an ordinary differential equation. We find
that even in non-axisymmetric spaces the existence or nonexistence of an
apparent horizon is consistent with the inequality: ${\cal C} \siml 4\pi M$.


title: On the Causal Propagation of Fields

abstract:
  By using geometric methods and superenergy tensors, we find new simple
criteria for the causal propagation of physical fields in spacetimes of any
dimension. The method can be applied easily to many different theories and to
arbitrary fields (such as scalar or electromagnetic ones). In particular, it
provides a conservation theorem of the free gravitational field in all
N-dimensional spacetimes conformally related to Einstein spaces (including
vacuum solutions). In the case of general relativity, our criteria provide
simple proofs and a unified treatment of conservation theorems for neutrinos,
photons, electrons and all other massless and massive free spin n/2 fields. The
uniqueness of the solution to the field equations also follows from our
treatment under certain circumstances.


title:
Cut-off free finite zero-point vacuum energy and the cosmological
  missing mass problem

abstract:
  As the mass-energy is universally self-gravitating, the gravitational binding
energy must be subtracted self-consistently from its bare mass value so as to
give the physical gravitational mass. Such a self-consistent gravitational
self-energy correction can be made non-perturbatively by the use of a
gravitational `charging' technique, where we calculate the incremental change
$dm$ of the physical mass of the cosmological object, of size $r_o$ due to the
accretion of a bare mass $dM$, corresponding to the gravitational coupling-in
of the successive zero-point vacuum modes, i.e., of the Casimir energy, whose
bare value $\Sigma_{\bf k} \hbar ck$ is infinite. Integrating the `charging'
equation, $dm = dM - (3\alpha/5)Gm\Delta M/r_o c^2$, we get a gravitational
mass for the cosmological object that remains finite even in the limit of the
infinite zero-point vacuum energy, i.e., without any ultraviolet cut-off
imposed. Here $\alpha$ is a geometrical factor of order unity. Also, setting
$r_o = c/H$, the Hubble length, we get the corresponding cosmological density
parameter $\Omega \simeq 1$, without any adjustable parameter. The cosmological
significance of this finite and unique contribution of the otherwise infinite
zero-point vacuum energy to the density parameter can hardly be overstated.


title: Experimental Tests of Relativistic Gravity

abstract:
  The confrontation between Einstein's gravitation theory and experimental
results, notably binary pulsar data, is summarized and its significance
discussed. Experiment and theory agree at the 10^{-3} level or better. All the
basic structures of Einstein's theory (coupling of gravity to matter;
propagation and self-interaction of the gravitational field, including in
strong-field conditions) have been verified. However, the theoretical
possibility that scalar couplings be naturally driven toward zero by the
cosmological expansion suggests that the present agreement between Einstein's
theory and experiment might be compatible with the existence of a long-range
scalar contribution to gravity (such as the dilaton field, or a moduli field,
of string theory). This provides a new theoretical paradigm, and new
motivations for improving the experimental tests of gravity.


title: Textures in Dilaton Gravity

abstract:
  We examine the field equations of a self-gravitating texture in low-energy
superstring gravity, allowing for an arbitrary coupling of the texture field to
the dilaton. Both massive and massless dilatons are considered. For the
massless dilaton non-singular spacetimes exist, but only for certain values of
the coupling, dependent on the gravitational strength of the texture; moreover,
this non-singular behaviour exists only in a certain frame. For the massive
dilaton, the texture induces a long-range dilaton cloud, but we expect the
gravitational behaviour of the defect to be similar to that found in Einstein
theory. We compare these results with those found for other global topological
defects.


title:
Construction of Astrophysical Initial Data for Perturbations of
  Relativistic Stars

abstract:
  We develop a framework for constructing initial data sets for perturbations
about spherically symmetric matter distributions. This framework facilitates
setting initial data representing astrophysical sources of gravitational
radiation involving relativistic stars. The procedure is based on
Lichnerowicz-York's conformal approach to solve the constraints in Einstein's
equations. The correspondence of these initial data sets in terms of the
standard gauge perturbation variables in the Regge-Wheeler perturbation
variables is established, and examples of initial data sets of merging neutron
stars under the close-limit approximation are presented.


title:
2+1-Gravity and The Embedding its Dynamiycal Symmetry and
  Para-Supersymmetry into SO(4,c) Group

abstract:
  Some special solutions of the Einstein-Maxwell action with a non-negative
cosmological constant and a very heavy point mass particle have been obtained.
The solutions correspond to static spacetime of locally constant curvature in
its spatial part and a constant magnetic field of a magnetic monopole together
with deficit of angle at the location of point mass. The quantum mechanics of a
point particle in these spacetimes in the absence of angular deficit has been
solved algebraically both relativistically and non-relativistically. It has
been also shown that these 2-dimensional Hamiltonians have the degeneracy group
of GL(2,c) type and para-supersymmetry of arbitrary order or shape invariance,
which is originated from a SO(4,c) group.


title: The Generalized Second Law of Thermodynamics in Cosmology

abstract:
  A classical and quantum mechanical generalized second law of thermodynamics
in cosmology implies constraints on the effective equation of state of the
universe in the form of energy conditions, obeyed by many known cosmological
solutions, and is compatible with entropy bounds which forbid certain
cosmological singularities. In string cosmology the second law provides new
information about the existence of non-singular solutions, and the nature of
the graceful exit transition from dilaton-driven inflation.


title: A Classical Sequential Growth Dynamics for Causal Sets

abstract:
  Starting from certain causality conditions and a discrete form of general
covariance, we derive a very general family of classically stochastic,
sequential growth dynamics for causal sets. The resulting theories provide a
relatively accessible ``half way house'' to full quantum gravity that possibly
contains the latter's classical limit (general relativity). Because they can be
expressed in terms of state models for an assembly of Ising spins living on the
relations of the causal set, these theories also illustrate how
non-gravitational matter can arise dynamically from the causal set without
having to be built in at the fundamental level. Additionally, our results bring
into focus some interpretive issues of importance for causal set dynamics, and
for quantum gravity more generally.


title: On the degenerate phase boundaries

abstract:
  The structure of the phase boundary between degenerate and non-degenerate
regions in Ashtekar's gravity has been studied by Bengtsson and Jacobson who
conjectured that the "phase boundary" should always be null. In this paper, we
reformulate the reparametrization procedure in the mapping language and
distinguish a phase boundary from its image. It is shown that the image has to
be null, while the nullness of the phase boundary requries more suitable
criterion.


title:
Gravitational Radiation from a Naked Singularity -- Odd-Parity
  Perturbation --

abstract:
  It has been suggested that a naked singularity may be a good candidate for a
strong gravitational wave burster. The naked singularity occurs in the generic
collapse of an inhomogeneous dust ball. We study odd-parity mode of
gravitational waves from a naked singularity of the Lema\^{\i}tre-Tolman-Bondi
space-time. The wave equation for gravitational waves are solved by numerical
integration using the single null coordinate. The result is that the naked
singularity is not a strong source of the odd-parity gravitational radiation
although the metric perturbation grows in the central region. Therefore, the
Cauchy horizon in this space-time would be marginally stable against odd-parity
perturbations.


title: Stretched Horizon for Non-Supersymmetric Black Holes

abstract:
  We review the idea of stretched horizon for extremal black holes in
supersymmetric string theories, and we compute it for non-supersymmetric black
holes in four dimensions. Only for small masses of the order of the Veneziano
wavelength is the stretched horizon bigger than the event horizon.


title: Quadratic reheating

abstract:
  The reheating process for the inflationary scenario is investigated
phenomenologically. The decay of the oscillating massive inflaton field into
light bosons is modeled after an out of equilibrium mixture of interacting
fluids within the framework of irreversible thermodynamics. Self-consistent,
analytic results for the evolution of the main macroscopic magnitudes like
temperature and particle number densities are obtained. The models for linear
and quadratic decay rates are investigated in the quasiperfect regime. The
linear model is shown to reheat very slowly while the quadratic one is shown to
yield explosive particle and entropy production. The maximum reheating
temperature is reached much faster and its magnitude is comparable with the
inflaton mass.


title: Spacetime metric from linear electrodynamics

abstract:
  The Maxwell equations are formulated on an arbitrary (1+3)-dimensional
manifold. Then, imposing a (constrained) linear constitutive relation between
electromagnetic field $(E,B)$ and excitation $({\cal D},{\cal H})$, we derive
the metric of spacetime therefrom.


title:
Quantum-Gravitational Diffusion and Stochastic Fluctuations in the
  Velocity of Light

abstract:
  We argue that quantum-gravitational fluctuations in the space-time background
give the vacuum non-trivial optical properties that include diffusion and
consequent uncertainties in the arrival times of photons, causing stochastic
fluctuations in the velocity of light ``in vacuo''. Our proposal is motivated
within a Liouville string formulation of quantum gravity that also suggests a
frequency-dependent refractive index of the particle vacuum. We construct an
explicit realization by treating photon propagation through quantum excitations
of $D$-brane fluctuations in the space-time foam. These are described by
higher-genus string effects, that lead to stochastic fluctuations in couplings,
and hence in the velocity of light. We discuss the possibilities of
constraining or measuring photon diffusion ``in vacuo'' via $\gamma$-ray
observations of distant astrophysical sources.


title: Quantum Space-Time and Reference Frames in ADM Canonical Gravity

abstract:
  The quantum space-time model which accounts material Reference Frames (RF)
quantum effects considered for flat space-time and ADM canonical gravity. As
was shown by Aharonov for RF - free material object its c.m. nonrelativistic
motion in vacuum described by Schrodinger wave packet evolution which modify
space coordinate operator of test particle in this RF and changes its
Heisenberg uncertainty relations. In the relativistic case we show that Lorentz
transformations between two RFs include the quantum corrections for RFs
momentum uncertainty and in general can be formulated as the quantum space-time
transformations. As the result for moving RF its Lorentz time boost acquires
quantum fluctuations which calculated solving relativistic Heisenberg equations
for the quantum clocks models. It permits to calculate RF proper time for the
arbitrary RF quantum motion including quantum gravity metrics fluctuations.
Space-time structure of canonical Quantum Gravity and its observables evolution
for RF proper time discussed in this quantum space-time transformations
framework.


title: A spacetime characterization of the Kerr metric

abstract:
  We obtain a characterization of the Kerr metric among stationary,
asymptotically flat, vacuum spacetimes, which extends the characterization in
terms of the Simon tensor (defined only in the manifold of trajectories) to the
whole spacetime. More precisely, we define a three index tensor on any
spacetime with a Killing field, which vanishes identically for Kerr and which
coincides in the strictly stationary region with the Simon tensor when
projected down into the manifold of trajectories. We prove that a stationary
asymptotically flat vacuum spacetime with vanishing spacetime Simon tensor is
locally isometric to Kerr. A geometrical interpretation of this
characterization in terms of the Weyl tensor is also given. Namely, a
stationary, asymptotically flat vacuum spacetime such that each principal null
direction of the Killing form is a repeated principal null direction of the
Weyl tensor is locally isometric to Kerr.


title:
Comment on: Topological Invariants, instantons, and the chiral anomaly
  on spaces with torsion

abstract:
  In Riemann-Cartan spacetimes with torsion only its axial covector piece $A$
couples to massive Dirac fields. Using renormalization group arguments, we show
that besides the familiar Riemannian term only the Pontrjagin type four-form
$dA\wedge dA$ does arise additionally in the chiral anomaly, but not the
Nieh-Yan term $d^\star A$, as has been claimed in a recent paper [PRD 55, 7580
(1997)].


title: Towards a theory of Warm Inflation of the Universe

abstract:
  The warm inflation scenario is an alternative mechanism which can explain the
isotropic and homogeneous Universe which we are living in. In this work I
extend a previously introduced formalism, without the restriction of slow -
roll regime. Quantum to classical transition of the fluctuations is studied by
means of the "transition function" here introduced. I found that the
fluctuations of radiation energy density decrease with time and the thermal
equilibrium at the end of inflation holds.


title:
Nakedness and curvature strength of shell-focusing singularity in the
  spherically symmetric space-time with vanishing radial pressure

abstract:
  It was recently shown that the metric functions which describe a spherically
symmetric space-time with vanishing radial pressure can be explicitly
integrated. We investigate the nakedness and curvature strength of the
shell-focusing singularity in that space-time. If the singularity is naked, the
relation between the circumferential radius and the Misner-Sharp mass is given
by $R\approx 2y_{0} m^{\beta}$ with $ 1/3<\beta\le 1$ along the first radial
null geodesic from the singularity. The $\beta$ is closely related to the
curvature strength of the naked singularity. For example, for the outgoing or
ingoing null geodesic, if the strong curvature condition (SCC) by Tipler holds,
then $\beta$ must be equal to 1. We define the ``gravity dominance condition''
(GDC) for a geodesic. If GDC is satisfied for the null geodesic, both SCC and
the limiting focusing condition (LFC) by Kr\'olak hold for $\beta=1$ and
$y_{0}\ne 1$, not SCC but only LFC holds for $1/2\le \beta <1$, and neither
holds for $1/3<\beta <1/2$, for the null geodesic. On the other hand, if GDC is
satisfied for the timelike geodesic $r=0$, both SCC and LFC are satisfied for
the timelike geodesic, irrespective of the value of $\beta$. Several examples
are also discussed.


title: G\"odel-type Spacetimes in Induced Matter Gravity Theory

abstract:
  A five-dimensional (5D) generalized G\"odel-type manifolds are examined in
the light of the equivalence problem techniques, as formulated by Cartan. The
necessary and sufficient conditions for local homogeneity of these 5D manifolds
are derived. The local equivalence of these homogeneous Riemannian manifolds is
studied. It is found that they are characterized by three essential parameters
$k$, $m^2$ and $\omega$: identical triads $(k, m^2, \omega)$ correspond to
locally equivalent 5D manifolds. An irreducible set of isometrically
nonequivalent 5D locally homogeneous Riemannian generalized G\"odel-type
metrics are exhibited. A classification of these manifolds based on the
essential parameters is presented, and the Killing vector fields as well as the
corresponding Lie algebra of each class are determined. It is shown that the
generalized G\"odel-type 5D manifolds admit maximal group of isometry $G_r$
with $r=7$, $r=9$ or $r=15$ depending on the essential parameters $k$, $m^2$
and $\omega$. The breakdown of causality in all these classes of homogeneous
G\"odel-type manifolds are also examined. It is found that in three out of the
six irreducible classes the causality can be violated. The unique generalized
G\"odel-type solution of the induced matter (IM) field equations is found. The
question as to whether the induced matter version of general relativity is an
effective therapy for these type of causal anomalies of general relativity is
also discussed in connection with a recent article by Romero, Tavakol and
Zalaletdinov.


title: Causal structure and degenerate phase boundaries

abstract:
  Timelike and null hypersurfaces in the degenerate space-times in the Ashtekar
theory are defined in the light of the degenerate causal structure proposed by
Matschull. Using the new definition of null hypersufaces, the conjecture that
the "phase boundary" separating the degenerate space-time region from the
non-degenerate one in Ashtekar's gravity is always null is proved under certain
circumstances.


title:
Generalization of the model of Hawking radiation with modified high
  frequency dispersion relation

abstract:
  The Hawking radiation is one of the most interesting phenomena predicted by
the theory of quantum field in curved space. The origin of Hawking radiation is
closely related to the fact that a particle which marginally escapes from
collapsing into a black hole is observed at the future infinity with infinitely
large redshift. In other words, such a particle had a very high frequency when
it was near the event horizon. Motivated by the possibility that the property
of Hawking radiation may be altered by some unknowned physics which may exist
beyond some critical scale, Unruh proposed a model which has higher order
spatial derivative terms. In his model, the effects of unknown physics are
modeled so as to be suppressed for the waves with a wavelength much longer than
the critical scale, $k_0^{-1}$. Surprisingly, it was shown that the thermal
spectrum is recovered for such modified models. To introduce such higher order
spatial derivative terms, the Lorentz invariance must be violated because one
special spatial direction needs to be chosen. In previous works, the rest frame
of freely-falling observers was employed as this special reference frame. Here
we give an extension by allowing a more general choice of the reference frame.
Developing the method taken by Corley, % and especially focusing on subluminal
case, we show that the resulting spectrum of created particles again becomes
the thermal one at the Hawking temperature even if the choice of the reference
frame is generalized. Using the technique of the matched asymptotic expansion,
we also show that the correction to the thermal radiation stays of order
$k_0^{-2}$ or smaller when the spectrum of radiated particle around its peak is
concerned.


title: Critical Phenomena Inside Global Monopoles

abstract:
  The gravitational collapse of a triplet scalar field is examined assuming a
hedgehog ansatz for the scalar field. Whereas the seminal work by Choptuik with
a single, strictly spherically symmetric scalar field found a discretely
self-similar (DSS) solution at criticality with echoing period $\Delta=3.44$,
here a new DSS solution is found with period $\Delta=0.46$. This new critical
solution is also observed in the presence of a symmetry breaking potential as
well as within a global monopole. The triplet scalar field model contains
Choptuik's original model in a certain region of parameter space, and hence his
original DSS solution is also a solution. However, the choice of a hedgehog
ansatz appears to exclude the original DSS.


title: Relativistic contraction and related effects in noninertial frames

abstract:
  Although there is no relative motion among different points on a rotating
disc, each point belongs to a different noninertial frame. This fact, not
recognized in previous approaches to the Ehrenfest paradox and related
problems, is exploited to give a correct treatment of a rotating ring and a
rotating disc. Tensile stresses are recovered, but, contrary to the prediction
of the standard approach, it is found that an observer on the rim of the disc
will see equal lengths of other differently moving objects as an inertial
observer whose instantaneous position and velocity are equal to that of the
observer on the rim. The rate of clocks at various positions, as seen by
various observers, is also discussed. Some results are generalized for
observers arbitrarily moving in a flat or a curved spacetime. The generally
accepted formula for the space line element in a non-time-orthogonal frame is
found inappropriate in some cases. Use of Fermi coordinates leads to the result
that for any observer the velocity of light is isotropic and is equal to $c$,
providing that it is measured by propagating a light beam in a small
neighborhood of the observer.


title: Quantization of discretized spacetimes and the correspondence principle

abstract:
  An algebraic quantization procedure for discretized spacetime models is
suggested based on the duality between finitary substitutes and their incidence
algebras. The provided limiting procedure that yields conventional manifold
characteristics of spacetime structures is interpreted in the algebraic quantum
framework as a correspondence principle.


title: Averaging from a global point of view

abstract:
  We study the averaging problem from a point of view of variation of spatial
volume $V$. We show that in the space of spherically symmetric dust solutions
which are regular on the spatial manifold $S^3$ the variation $\delta V$
vanishes at the Friedmann-Lemaitre-Robertson-Walker (FLRW) solution in an
appropriate sense, which supports the validity of the FLRW solution as the
averaged solution. We also present the second variation $\delta^2 V$, giving
the leading effect of the deviation from the FLRW solution.


title: Finslerian spaces possessing local relativistic symmetry

abstract:
  It is shown that the problem of a possible violation of the Lorentz
transformations at Lorentz factors $\gamma >5\times 10^{10} ,$ indicated by the
situation which has developed in the physics of ultra-high energy cosmic rays
(the absence of the GZK cutoff), has a nontrivial solution. Its essence
consists in the discovery of the so-called generalized Lorentz transformations
which seem to correctly link the inertial reference frames at any values of
$\gamma .$ Like the usual Lorentz transformations, the generalized ones are
linear, possess group properties and lead to the Einstein law of addition of
3-velocities. However, their geometric meaning turns out to be different: they
serve as relativistic symmetry transformations of a flat anisotropic Finslerian
event space rather than of Minkowski space. Consideration is given to two types
of Finsler spaces which generalize locally isotropic Riemannian space-time of
relativity theory, e. g. Finsler spaces with a partially and entirely broken
local 3D isotropy. The investigation advances arguments for the corresponding
generalization of the theory of fundamental interactions and for a specific
search for physical effects due to local anisotropy of space-time.


title: Lightcone fluctuations in flat spacetimes with nontrivial topology

abstract:
  The quantum lightcone fluctuations in flat spacetimes with compactified
spatial dimensions or with boundaries are examined. The discussion is based
upon a model in which the source of the underlying metric fluctuations is taken
to be quantized linear perturbations of the gravitational field. General
expressions are derived, in the transverse trace-free gauge, for the summation
of graviton polarization tensors, and for vacuum graviton two-point functions.
Because of the fluctuating light cone, the flight time of photons between a
source and a detector may be either longer or shorter than the light
propagation time in the background classical spacetime. We calculate the mean
deviations from the classical propagation time of photons due to the changes in
the topology of the flat spacetime. These deviations are in general larger in
the directions in which topology changes occur and are typically of the order
of the Planck time, but they can get larger as the travel distance increases.


title: Black Holes and Wormholes in 2+1 Dimensions

abstract:
  Vacuum Einstein theory in three spacetime dimensions is locally trivial, but
admits many solutions that are globally different, particularly if there is a
negative cosmological constant. The classical theory of such locally "anti-de
Sitter" spaces is treated in an elementary way, using visualizable models.
Among the objects discussed are black holes, spaces with multiple black holes,
their horizon structure, closed universes, and the topologies that are
possible.


title: ``Natural'' Vacua in Hyperbolic Friedmann-Robertson-Walker Spacetimes

abstract:
  Recent evidence indicates that the Universe is open, i.e., spatially
hyperbolic, longstanding theoretical preferences to the contrary
notwithstanding. This makes it possible to select a vacuum state, Fock space,
and particle definition for a quantized field, by requiring concordance with
ordinary flat-spacetime theory at late times. The particle-number basis states
thus identified span the physical state space of the field at all times. This
construction is demonstrated here explicitly for a massive, minimally coupled,
linear scalar field in an open, radiation-dominated Friedmann-Robertson-Walker
spacetime.


title: Some General Expressions for the Coefficient of the 14th Chern Form

abstract:
  Some general expressions are given for the coefficient of the 14th Chern form
in terms of the Riemann-Christoffel curvature tensor and some of its
concomitants (e.g., Pontrjagin's characteristic tensors) for n-dimensional
differentiable manifolds having a general linear connection.


title:
General Expressions for the Coefficients of Chern Forms Up to the 13th
  Order in Curvature

abstract:
  General expressions are given for the coefficients of Chern forms up to the
13th order in curvature in terms of the Riemann-Christoffel curvature tensor
and some of its concomitants (e.g., Pontrjagin's characteristic tensors) for
n-dimensional differentiable manifolds having a general linear connection.


title: A General Expression for the 14th Chern Form

abstract:
  A general expression is given for the 14th Chern form in terms of simple
polynomial concomitants of the curvature 2-form for n-dimensional
differentiable manifolds having a general linear connection.


title: General Expressions for Chern Forms Up to the 13th Order in Curvature

abstract:
  General expressions are given for Chern forms up to the 13th order in
curvature in terms of simple polynomial concomitants of the curvature 2-form
for n-dimensional differentiable manifolds having a general linear connection.


title: The World Function in Robertson-Walker Spacetime

abstract:
  A method for finding the world function of Robertson-Walker spacetimes is
presented. It is applied to find the world function for the $k=0, \ga=2$,
solution. The close point approximation for the Robertson-Walker world function
is calculated upto fourth order.


title: Imploding Scalar Fields

abstract:
  Static spherically symmetric uncoupled scalar space-times have no event
horizon and a divergent Kretschmann singularity at the origin of the
coordinates. The singularity is always present so that non-static solutions
have been sought to see if the singularities can develop from an initially
singular free space-time. In flat space-time the Klein-Gordon equation
$\Box\ph=0$ has the non-static spherically symmetric solution $\ph=\si(v)/r$,
where $\si(v)$ is a once differentiable function of the null coordinate $v$. In
particular the function $\si(v)$ can be taken to be initially zero and then
grow, thus producing a singularity in the scalar field. A similar situation
occurs when the scalar field is coupled to gravity via Einstein's equations;
the solution also develops a divergent Kretschmann invariant singularity, but
it has no overall energy. To overcome this Bekenstein's theorems are applied to
give two corresponding conformally coupled solutions. One of these has positive
ADM mass and has the properties: i) it develops a Kretschmann invariant
singularity, ii)it has no event horizon, iii)it has a well-defined source,
iv)it has well-defined junction condition to Minkowski space-time, v)it is
asymptotically flat with positive overall energy. This paper presents this
solution and several other non-static scalar solutions. The properties of these
solutions which are studied are limited to the following three: i)whether the
solution can be joined to Minkowski space-time, ii)whether the solution is
asymptotically flat, iii)and if so what the solutions' Bondi and ADM masses
are.


title:
The optimal entropy bound and the self-energy of test objects in the
  vicinity of a black hole

abstract:
  Recently Bekenstein and Mayo conjectured an entropy bound for charged
rotating objects. On the basis of the No-Hair principle for black holes, they
speculate that this bound cannot be improved generically based on knowledge of
other ``quantum numbers'', e.g. baryon number, which may be borne by the
object. Here we take a first step in the proof of this conjecture. The proof
make use of a gedanken experiment in which a massive object endowed with a
scalar charge is lowered adiabatically towards a Schwarzschild's black hole and
than dropped into the black hole from some proper distance above the horizon.
Central to the proof is the intriguing fact that the self-energy of the
particle receives no contribution from the scalar charge. Thus the energy with
which the object is assimilated consists of its gravitational energy alone.
This of course agrees with the No-scalar-Hair principle for black holes: after
the object is assimilated into the black hole, any knowledge of the scalar
field properties is lost. Using the GSL, we reach the conclusion that the
original entropy bound was not improved by the knowledge of the scalar charge.
At the end we speculate on whether or not massive vector fields may serve in
the tightening of the entropy bound.


title: On Signature Transition and Compactification in Kaluza-Klein Cosmology

abstract:
  We consider an empty (4+1) dimensional Kaluza-Klein universe with a negative
cosmological constant and a Robertson-Walker type metric. It is shown that the
solutions to Einstein field equations have degenerate metric and exhibit
transitioins from a Euclidean to a Lorentzian domain. We then suggest a
mechanism, based on signature transition which leads to compactification of the
internal space in the Lorentzian region as $a \sim |\Lambda|^{1/2}$. With the
assumption of a very small value for the cosmological constant we find that the
size of the universe $R$ and the internal scale factor $a$ would be related
according to $Ra\sim 1$ in the Lorentzian region. The corresponding
Wheeler-DeWitt equation has exact solution in the mini-superspace giving rise
to a quantum state which peaks in the vicinity of the classical solutions
undergoing signature transition.


title:
New Equations for Gravitation with Riemann Tensor and 4-Index
  Energy-Momentum Tensors for Gravitation and Matter

abstract:
  A generalized version of the Einstein equations in the 4-index form,
containing the Riemann tensor linearly, is derived. It is shown, that the
gravitational energy-momentum density tensor outside a source is represented
across the Weyl tensor vanishing at the 2-index contraction. The 4-index
energy-momentum density tensor for matter also is constructed.


title: Global monopoles and massless dilatons in Einstein-Cartan gravity

abstract:
  A global monopole in dilatonic Einstein-Cartan gravity is presented. A
linearized solution representing a global monopole interacting with a massless
dilaton is found where Cartan torsion does not interact with the monopole Higgs
field.Computation of the geodesic equation shows that the monopole-dilaton
system generates a repulsive gravitational field.The solution is shown to break
the linear approximation for certain values of torsion.


title: Null particle solutions in three-dimensional (anti-) de Sitter spaces

abstract:
  We obtain a class of exact solutions representing null particles moving in
three-dimensional (anti-) de Sitter spaces by boosting the corresponding static
point source solutions given by Deser and Jackiw. In de Sitter space the
resulting solution describes two null particles moving on the (circular)
cosmological horizon, while in anti-de Sitter space it describes a single null
particle propagating from one side of the universe to the other. We also boost
the BTZ black hole solution to the ultrarelativistic limit and obtain the
solution for a spinning null particle moving in anti-de Sitter space. We find
that the ultrarelativistic geometry of the black hole is exactly the same as
that resulting from boosting the Deser-Jackiw solution when the angular
momentum of the hole vanishes. A general class of solutions is also obtained
which represents several null particles propagating in the Deser-Jackiw
background. The differences between the three-dimensional and four-dimensional
cases are also discussed.


title: Fluctuations of the Hawking Flux

abstract:
  The fluctuations of the flux radiated by an evaporating black hole will be
discussed. Two approaches to this problem will be adopted. In the first, the
squared flux operator is defined by normal ordering. In this case, both the
mean flux and the mean squared flux are well defined local quantites. It is
shown that the flux undergoes large fluctuations on a time scale of the order
of the black hole's mass. Thus the semiclassical theory of gravity, in which a
classical gravitational field is coupled to the expectation value of the stress
tensor, breaks down below this time scale. In the second approach, one does not
attempt to give meaning to the squared flux as a local quantity, but only as a
time-averaged quantity. In both approaches, the mean squared mass minus the
square of the mean mass grows linearly in time, but four times as fast in the
second approach as in the first.


title:
Inflation from an effective standard model of particle physics for
  curved space-time

abstract:
  Beginning from an effective theory in eight dimensions, Macias, Camacho and
Matos proposed an effective model for the electroweak part of the Standard
Model of particles in curved spacetime. Using this model, we investigate the
cosmological consequences of the electroweak interaction in the early universe.
We use the approximation that, near the Planck epoch, the Yang-Mills fields
behave like a perfect fluid. Then we recover the field equations of
inflationary cosmology, with the Higgs field directly related to the inflaton.
We present some qualitative discussion about this and analyse the behavior of
isospin space using some known exact solutions.


title: Flat foliations of spherically symmetric geometries

abstract:
  We examine the solution of the constraints in spherically symmetric general
relativity when spacetime has a flat spatial hypersurface. We demonstrate
explicitly that given one flat slice, a foliation by flat slices can be
consistently evolved. We show that when the sources are finite these slices do
not admit singularities and we provide an explicit bound on the maximum value
assumed by the extrinsic curvature. If the dominant energy condition is
satisfied, the projection of the extrinsic curvature orthogonal to the radial
direction possesses a definite sign. We provide both necessary and sufficient
conditions for the formation of apparent horizons in this gauge which are
qualitatively identical to those established earlier for extrinsic time
foliations of spacetime, Phys. Rev. D56 7658, 7666 (1997) which suggests that
these conditions possess a gauge invariant validity.


title: Thermodynamic equilibrium in the expanding universe

abstract:
  We show that a relativistic gas may be at ``global'' equilibrium in the
expanding universe for any equation of state $0 < p \leq \rho /3$, provided
that the gas particles move under the influence of a self-interacting,
effective one-particle force in between elastic binary collisions. In the
force-free limit we recover the equilibrium conditions for ultrarelativistic
matter which imply the existence of a conformal timelike Killing vector.


title: Do H_0 and q_0 really have the values we believe they have?

abstract:
  We present an example where a justified modification of the law of
propagation of light in a Robertson-Walker model of the universe leads to an
identification of H_0 and q_0 different from that corresponding to the usual
law of propagation along null geodesics. We conclude from this example that
observed values which we would associate with the values of H_0 and q_0 with
the usual interpretation correspond in fact to the values of 2H_0 and $1/2
(q_0-1)$. It is therefore possible that observed values that we usually
interpret as corresponding to a moderately aged universe with accelerating
expansion may in fact correspond a much older universe with a decelerating
expansion.


title: Charged Scalar-Tensor Boson Stars: Equilibrium, Stability and Evolution

abstract:
  We study charged boson stars in scalar-tensor (ST) gravitational theories. We
analyse the weak field limit of the solutions and analytically show that there
is a maximum charge to mass ratio for the bosons above which the weak field
solutions are not stable. This charge limit can be greater than the GR limit
for a wide class of ST theories. We numerically investigate strong field
solutions in both the Brans Dicke and power law ST theories. We find that the
charge limit decreases with increasing central boson density. We discuss the
gravitational evolution of charged and uncharged boson stars in a cosmological
setting and show how, at any point in its evolution, the physical properties of
the star may be calculated by a rescaling of a solution whose asymptotic value
of the scalar field is equal to its initial asymptotic value. We focus on
evolution in which the particle number of the star is conserved and we find
that the energy and central density of the star decreases as the cosmological
time increases. We also analyse the appearance of the scalarization phenomenon
recently discovered for neutron stars configurations and, finally, we give a
short discussion on how making the correct choice of mass influences the
argument over which conformal frame, the Einstein frame or the Jordan frame, is
physical.


title:
End-to-end algorithm for hierarchical area searches for long-duration GW
  sources for GEO 600

abstract:
  We describe a hierarchical, highly parallel computer algorithm to perform
searches for unknown sources of continuous gravitational waves -- spinning
neutron stars in the Galaxy -- over wide areas of the sky and wide frequency
bandwidths. We optimize the algorithm for an observing period of 4 months and
an available computing power of 20 Gflops, in a search for neutron stars
resembling millisecond pulsars. We show that, if we restrict the search to the
galactic plane, the method will detect any star whose signal is stronger than
15 times the $1\sigma$ noise level of a detector over that search period. Since
on grounds of confidence the minimum identifiable signal should be about 10
times noise, our algorithm does only 50% worse than this and runs on a computer
with achievable processing speed.


title: Operators for quantized directions

abstract:
  Inspired by the spin geometry theorem, two operators are defined which
measure angles in the quantum theory of geometry. One operator assigns a
discrete angle to every pair of surfaces passing through a single vertex of a
spin network. This operator, which is effectively the cosine of an angle, is
defined via a scalar product density operator and the area operator. The second
operator assigns an angle to two ``bundles'' of edges incident to a single
vertex. While somewhat more complicated than the earlier geometric operators,
there are a number of properties that are investigated including the full
spectrum of several operators and, using results of the spin geometry theorem,
conditions to ensure that semiclassical geometry states replicate classical
angles.


title: A Spin Network Primer

abstract:
  Spin networks, essentially labeled graphs, are ``good quantum numbers'' for
the quantum theory of geometry. These structures encompass a diverse range of
techniques which may be used in the quantum mechanics of finite dimensional
systems, gauge theory, and knot theory. Though accessible to undergraduates,
spin network techniques are buried in more complicated formulations. In this
paper a diagrammatic method, simple but rich, is introduced through an
association of 2 by 2 matrices to diagrams. This spin network diagrammatic
method offers new perspectives on the quantum mechanics of angular momentum,
group theory, knot theory, and even quantum geometry. Examples in each of these
areas are discussed.


title: Graviton-Graviton Scattering, Bel-Robinson and Energy (Pseudo)-Tensors

abstract:
  Motivated by recent work involving the graviton-graviton tree scattering
amplitude, and its twin descriptions as the square of the Bel-Robinson tensor,
$B_{\m\n\a\b}$, and as the "current-current interaction" square of
gravitational energy pseudo-tensors $t_{\a\b}$,we find an exact tensor-square
root equality $B_{\mn\a\b} = \pa^2_\mn t_{\a\b}$, for a combination of Einstein
and Landau-Lifschitz $t_\ab$, in Riemann normal coordinates. In the process, we
relate, on-shell, the usual superpotential basis for classifying pseudo-tensors
with one spanned by polynomials in the curvature.


title:
Isomorphism between Non-Riemannian gravity and Einstein-Proca-Weyl
  theories extended to a class of Scalar gravity theories

abstract:
  We extend the recently proved relation between certain models of
Non-Riemannian gravitation and Einstein- Proca-Weyl theories to a class of
Scalar gravity theories. This is used to present a Black-Hole Dilaton solution
with non-Riemannian connection.


title: Time-frequency detection of Gravitational Waves

abstract:
  We present a time-frequency method to detect gravitational wave signals in
interferometric data. This robust method can detect signals from poorly modeled
and unmodeled sources. We evaluate the method on simulated data containing
noise and signal components. The noise component approximates initial LIGO
interferometer noise. The signal components have the time and frequency
characteristics postulated by Flanagan and Hughes for binary black hole
coalescence. The signals correspond to binaries with total masses between $45
M_\odot$ to $70 M_\odot$ and with (optimal filter) signal-to-noise ratios of 7
to 12. The method is implementable in real time, and achieves a coincident
false alarm rate for two detectors $\approx$ 1 per 475 years. At this false
alarm rate, the single detector false dismissal rate for our signal model is as
low as 5.3% at an SNR of 10. We expect to obtain similar or better detection
rates with this method for any signal of similar power that satisfies certain
adiabaticity criteria. Because optimal filtering requires knowledge of the
signal waveform to high precision, we argue that this method is likely to
detect signals that are undetectable by optimal filtering, which is at present
the best developed detection method for transient sources of gravitational
waves.


title: Energy Conservation and the Unruh Effect

abstract:
  In this paper it is explicitly demonstrated that the energy conservation law
is kept when a detector uniformly accelerated in the Minkowski vacuum is
excited and emits a particle. This fact had been hidden in conventional
approaches in which detectors were considered to be forced on trajectories. To
lift the veil we suggest a detector model written in terms of the Minkowski
coordinates. In this model the Hamiltonian of the detector involves a classical
potential term instead of the detector's fixed trajectory. The transition rate
agrees with the corresponding conventional one in the limit of an infinite mass
detector though even then the recoil remains.


title: Nonexistence theorems for traversable wormholes

abstract:
  Gauss-Bonnet formula is used to derive a new and simple theorem of
nonexistence of vacuum static nonsingular lorentzian wormholes. We also derive
simple proofs for the nonexistence of lorentzian wormhole solutions for some
classes of static matter such as, for instance, real scalar fields with a
generic potential obeying $\phi V'(\phi) \ge 0$ and massless fermions fields.


title: The Detection of Gravitational Waves with LIGO

abstract:
  Gravitational wave emission is expected to arise from a variety of
astrophysical phenomena. A new generation of detectors with sensitivity
consistent with expectation from such sources is being developed. The Laser
Interferometer Gravitational-Wave Observatory (LIGO), one of these ambitious
undertakings, is being developed by a Caltech-MIT collaboration. It consists of
two widely separated interferometers, which will be used in coincidence to
search for sources from compact binary systems, spinning neutron stars,
supernovae and other astrophysical or cosmological phenomena that emit
gravitational waves. The construction of LIGO is well underway and preparations
are being made for the commissioning phase. In this lecture, I review the
underlying physics of gravitational waves, review possible astrophysical and
cosmological sources and discuss the LIGO interferometer status and plans.


title:
Constructing a Mass-Current Radiation-Reaction Force For Numerical
  Simulations

abstract:
  We present a new set of 3.5 Post-Newtonian equations in which Newtonian
hydrodynamics is coupled to the nonconservative effects of gravitational
radiation emission. Our formalism differs in two significant ways from a
similar 3.5 Post-Newtonian approach proposed by Blanchet (1993, 1997). Firstly
we concentrate only on the radiation-reaction effects produced by a
time-varying mass-current quadrupole $S_{ij}$. Secondly, we adopt a gauge in
which the radiation-reaction force densities depend on the fourth time
derivative of $S_{ij}$, rather than on the fifth, as in Blanchet's approach.
This difference makes our formalism particularly well-suited to numerical
implementation and could prove useful in performing fully numerical simulations
of the recently discovered $r$-mode instability for rotating neutron stars
subject to axial perturbations.


title:
Quasitemporal structure and symmetries in histories-based generalized
  quantum mechanics in curved spacetime

abstract:
  The formalism for histories-based generalized quantum mechanics developed in
two earlier papers is applied to the treatment of histories (of particles or
fields or more general objects) in curved spacetimes (which need not admit
foliation in spacelike hypersurfaces). The construction of the space of
temporal supports (a partial semigroup generalizing the space of finite time
sequences employed in traditional temporal description of histories) employs
spacelike subsets of spacetime having dimensionality less than or equal to
three. Definition of symmetry is sharpened by the requirement of continuity of
mappings (employing topological partial semigroups). It is shown that with this
proviso, a symmetry in our formalism implies a conformal isometry of the
spacetime metric.


title:
Dynamical measure and field theory models free of the cosmological
  constant problem

abstract:
  Summary of abstract Field theory models including gauge theories with SSB are
presented where the energy density of the true vacuum state (TVS) is zero
without fine tuning. The above models are constructed in the gravitational
theory where a measure of integration \Phi in the action is not necessarily
\sqrt{-g} but it is determined dynamically through additional degrees of
freedom. The ratio \Phi/\sqrt{-g} is a scalar field which can be solved in
terms of the matter degrees of freedom due to the existence of a constraint. We
study a few explicit field theory models where it is possible to combine the
solution of the cosmological constant problem with: 1) possibility for
inflationary scenario for the early universe; 2) spontaneously broken gauge
unified theories (including fermions). The models are free from the well known
problem of the usual scalar-tensor theories in what is concerned with the
classical GR tests. The only difference of the field equations in the Einstein
frame from the canonical equations of the selfconsistent system of Einstein's
gravity and matter fields, is the appearance of the effective scalar field
potential which vanishes in TVS without fine tuning.


title: The Anti-de Sitter Gott Universe: A Rotating BTZ Wormhole

abstract:
  Recently it has been shown that a 2+1 dimensional black hole can be created
by a collapse of two colliding massless particles in otherwise empty anti-de
Sitter space. Here we generalize this construction to the case of a non-zero
impact parameter. The resulting spacetime, which may be regarded as a Gott
universe in anti-de Sitter background, contains closed timelike curves. By
treating these as singular we are able to interpret our solution as a rotating
black hole, hence providing a link between the Gott universe and the BTZ black
hole. When analyzing the spacetime we see how the full causal structure of the
interior can be almost completely inferred just from considerations of the
conformal boundary.


title:
Non-perturbative effects of vacuum energy on the recent expansion of the
  universe

abstract:
  We show that the vacuum energy of a free quantized field of very low mass can
significantly alter the recent expansion of the universe. The effective action
of the theory is obtained from a non-perturbative sum of scalar curvature terms
in the propagator. We numerically investigate the semiclassical Einstein
equations derived from it. As a result of non-perturbative quantum effects, the
scalar curvature of the matter-dominated universe stops decreasing and
approaches a constant value. The universe in our model evolves from an open
matter-dominated epoch to a mildly inflating de Sitter expansion. The Hubble
constant during the present de Sitter epoch, as well as the time at which the
transition occurs from matter-dominated to de Sitter expansion, are determined
by the mass of the field and by the present matter density. The model provides
a theoretical explanation of the observed recent acceleration of the universe,
and gives a good fit to data from high-redshift Type Ia supernovae, with a mass
of about 10^{-33} eV, and a current ratio of matter density to critical
density, Omega_0 <0.4 . The age of the universe then follows with no further
free parameters in the theory, and turns out to be greater than 13 Gyr. The
model is spatially open and consistent with the possibility of inflation in the
very early universe. Furthermore, our model arises from the standard
renormalizable theory of a free quantum field in curved spacetime, and does not
require a cosmological constant or the associated fine-tuning.


title: Godel-type Universes in String-inspired Charged Gravity

abstract:
  We consider a string-inspired, gravitational theory of scalar and
electromagnetic fields and we investigate the existence of axially-symmetric,
G\"{o}del-type cosmological solutions. The neutral case is studied first and an
"extreme" G\"{o}del-type rotating solution, that respects the causality, is
determined. The charged case is considered next and two new configurations for
the, minimally-coupled to gravity, electromagnetic field are presented. Another
configuration motivated by the expected distribution of currents and charges in
a rotating universe is studied and shown to lead to a G\"{o}del-type solution
for a space-dependent coupling function. Finally, we investigate the existence
of G\"{o}del-type cosmological solutions in the framework of the one-loop
corrected superstring effective action and we determine the sole configuration
of the electromagnetic field that leads to such a solution. It turns out that,
in all the charged cases considered, Closed Timelike Curves do appear and the
causality is always violated.


title: How much negative energy does a wormhole need?

abstract:
  It is known that traversible wormholes require negative energy density. We
here argue how much negative energy is needed for wormholes, using a local
analysis which does not assume any symmetry. and in particular allows dynamic
(non-stationary) but non-degenerate wormholes. We find that wormholes require
two constraints on the energy density, given by two independent components of
the Einstein equation.


title: Black hole radiation with high frequency dispersion

abstract:
  We consider one model of a black hole radiation, in which the equation of
motion of a matter field is modified to cut off high frequency modes. The
spectrum in the model has already been analytically derived in low frequency
range, which has resulted in the Planckian distributin of the Hawking
temperature. On the other hand, it has been numerically shown that its spectrum
deviates from the thermal one in high frequency range. In this paper, we
analytically derive the form of the deviation in the high frequency range. Our
result can qualitatively explain the nature of the numerically calculated
spectrum. The origin of the deviation is clarified by a simple discussion.


title: Mirror Images of String Cosmologies

abstract:
  A discrete symmetry of the four-dimensional string effective action is
employed to derive spatially homogeneous and inhomogeneous string cosmologies
from vacuum solutions of general relativity that admit two commuting spacelike
Killing vectors. In particular, a tilted Bianchi type V cosmology is generated
from a vacuum type VI_h solution and a plane wave solution with a bounded and
oscillating dilaton field is found from a type ${\rm VII}_h$ model. Further
applications are briefly discussed.


title: Four-Index Energy-Momentum Tensors for Gravitation and Matter

abstract:
  The 4-index energy-momentum tensors for gravitation and matter are analyzed
on the basis of new equations for the gravitational field with the Riemann
tensor. Some properties of the such defined gravitational energy are discussed.


title: Possible Wormhole Solutions in (4+1) Gravity

abstract:
  We extend previous analyses of soliton solutions in (4+1) gravity to new
ranges of their defining parameters. The geometry, as studied using invariants,
has the topology of wormholes found in (3+1) gravity. In the induced-matter
picture, the fluid does not satisfy the strong energy conditions, but its
gravitational mass is positive. We infer the possible existance of (4+1)
wormholes which, compared to their (3+1) counterparts, are less exotic.


title: Twisted spinors on black holes

abstract:
  We remark that the standard black hole topology admits twisted configurations
of spinor field due to existence of the twisted spinor bundles and analyse them
using the Schwarzschild black hole as an example. This is physically linked
with the natural presence of Dirac monopoles on black holes and entails marked
modification of the Hawking radiation for spinor particles.


title: The Asymmetric Merger of Black Holes

abstract:
  We study event horizons of non-axisymmetric black holes and show how features
found in axisymmetric studies of colliding black holes and of toroidal black
holes are non-generic and how new features emerge. Most of the details of black
hole formation and black hole merger are known only in the axisymmetric case,
in which numerical evolution has successfully produced dynamical space-times.
The work that is presented here uses a new approach to construct the geometry
of the event horizon, not by locating it in a given spacetime, but by direct
construction. In the axisymmetric case, our method produces the familiar
pair-of-pants structure found in previous numerical simulations of black hole
mergers, as well as event horizons that go through a toroidal epoch as
discovered in the collapse of rotating matter. The main purpose of this paper
is to show how new - substantially different - features emerge in the
non-axisymmetric case. In particular, we show how black holes generically go
through a toroidal phase before they become spherical, and how this fits
together with the merger of black holes.


title:
Analytic Solutions of Teukolsky Equation in Kerr-de Sitter and
  Kerr-Newman-de Sitter Geometries

abstract:
  The analytic solution of Teukolsky equation in Kerr-de Sitter and
Kerr-Newman-de Sitter geometries is presented and the properties of the
solution are examined. In particular, we show that our solution satisfies the
Teukolsky-Starobinsky identities explicitly and fix the relative normalization
between solutions with the spin weight $s$ and $-s$.


title:
Particle creation, renormalizability conditions and the mass-energy
  spectrum in gravity theories of quadratic Lagrangians

abstract:
  Massive scalar particle production, due to the anisotropic evolution of a
five-dimensional spacetime, is considered in the context of a quadratic
Lagrangian theory of gravity. Those particles, corresponding to field modes
with non-vanishing momentum component along the fifth dimension, are created
mostly in the neighbourhood of a singular epoch where only their high-frequency
behaviour is of considerable importance. At the 1-loop approximation level,
general renormalizability conditions on the physical quantities relevant to
particle production are derived and discussed. Exact solutions of the resulting
Klein-Gordon field equation are obtained and the mass-energy spectrum
attributed to the scalar field due to the cosmological evolution is being
investigated further. Finally, analytic expressions regarding the number and
the energy density of the created particles at late times, are also derived and
discussed.


title:
Higher-dimensional models in gravitational theories of
  quarticLagrangians

abstract:
  Ten-dimensional models, arising from a gravitational action which includes
terms up to the fourth order in curvature tensor, are discussed. The spacetime
consists of one timelike dimension and two maximally symmetric subspaces,
filled with matter in the form of an anisotropic fluid. Numerical integration
of the cosmological field equations indicates that exponential, as well as
power law, solutions are possible. We carry out a dynamical study of the
results in the H_{ext} - H_{int} plane and confirm the existence of
"attractors" in the evolution of the Universe. Those attracting points
correspond to "extended De Sitter" spacetimes, in which the external space
exhibits inflationary expansion, while the internal one contracts.


title:
On the adiabatic expansion of the visible space in a higher-dimensional
  Cosmology

abstract:
  In the context of higher-dimensional cosmologies with isotropic visible and
internal space and multi-perfect fluid matter, we study the conditions under
which adiabatic expansion of the visoble external space is possible, when a
time-dependent internal space is present. The analysis is based on a
reinterpretation of the four-dimensional stress-energy tensor in the presence
of the extra dimensions. This modifies the usual adiabatic energy conservation
laws for the visible Universe, leading to a new type of cosmological evolution
which includes large-scale entropy production in four dimensions.


title: Kinetic description of particle interaction with a gravitational wave

abstract:
  The interaction of charged particles, moving in a uniform magnetic field,
with a plane-polarized gravitational wave is considered using the
Fokker-Planck- Kolmogorov (FPK) approach. By using a stochasticity criterion,
we determine the exact locations in phase space, where resonance overlapping
occurs. We investigate the diffusion of orbits around each primary resonance of
order (m) by deriving general analytical expressions for an effective diffusion
coeficient. A solution to the corresponding diffusion equation (Fokker-Planck
equation) for the static case is found. Numerical integration of the full
equations of motion and subsequent calculation of the diffusion coefficient
verifies the analytical results.


title: Stellar footprints of a variable G

abstract:
  Theories with varying gravitational constant $G$ have been studied since long
time ago. Among them, the most promising candidates as alternatives of the
standard General Relativity are known as scalar-tensor theories. They provide
consistent descriptions of the observed universe and arise as the low energy
limit of several pictures of unified interactions. Therefore, an increasing
interest on the astrophysical consequences of such theories has been sparked
over the last few years. In this essay we comment on two methodological
approaches to study evolution of astrophysical objects within a varying-$G$
theory, and the particular results we have obtained for boson and white dwarf
stars.


title: Fock-Lorentz transformations and time-varying speed of light

abstract:
  The theory of relativity was built up on linear Lorentz transformation.
However, in his fundamental work "Theory of Space, Time and Gravitation"
V.A.Fock shows that the general form of the transformation between the
coordinates in the two inertial frames could be taken to be linear fractional.
The implicit form of this transformation contains two constants of different
space-time dimensions. They can be reduced to the constant "c" with the
dimension of speed ("speed of light"), and to the constant "R" with the
dimension of length (an invariant radius of the visible part of the Universe).
The geometry of the "light cones" shows that "R" is a fundamental constant, but
"c" depends on the time of transformation.


title:
Non-Existence of Time-Periodic Solutions of the Dirac Equation in an
  Axisymmetric Black Hole Geometry

abstract:
  We prove that, in the non-extreme Kerr-Newman black hole geometry, the Dirac
equation has no normalizable, time-periodic solutions. A key tool is
Chandrasekhar's separation of the Dirac equation in this geometry. A similar
non-existence theorem is established in a more general class of stationary,
axisymmetric metrics in which the Dirac equation is known to be separable.
These results indicate that, in contrast with the classical situation of
massive particle orbits, a quantum mechanical Dirac particle must either
disappear into the black hole or escape to infinity.


title: Search for Quantum Gravity

abstract:
  A satisfactory theory of quantum gravity may necessitate a drastic
modification of our perception of space-time, by giving it a foamy structure at
distances comparable to the Planck length. It is argued in this essay that the
experimental detection of such structures may be a realistic possibility in the
foreseeable future. After a brief review of different theoretical approaches to
quantum gravity and the relationships between them, we discuss various possible
experimental tests of the quantum nature of space-time. Observations of photons
from distant astrophysical sources such as Gamma-Ray Bursters and laboratory
experiments on neutral kaon decays may be sensitive to quantum-gravitational
effects if they are only minimally suppressed. Experimental limits from the
Whipple Observatory and the CPLEAR Collaboration are already probing close to
the Planck scale, and significant increases in sensitivity are feasible.


title: Relativistic quantum field inertia and vacuum field noise spectra

abstract:
  The main theme of this survey is the equivalence statements for quantum
scalar field vacuum states that have been recognized over the last couple of
decades as a powerful line of reasoning when discussing the highly academic
thermal-like Hawking effect and Unruh effect. An important ingredient in this
framework is the concept of vacuum field noise spectrum by which one can obtain
information about the curvature invariants of classical worldlines
(relativistic classical trajectories). It is argued, in the spirit of the free
fall type universality, that the preferred quantum field vacua with respect to
accelerated worldlines should be chosen in the class of all those possessing
stationary spectra for their quantum fluctuations. For scalar quantum field
vacua there are six stationary cases as shown by Letaw some time ago, these are
reviewed here. However, the non-stationary vacuum noises are not out of reach
and can be processed by a few mathematical methods that are mentioned as well.
Since the information about the kinematical curvature invariants of the
worldlines is of radiometric origin, hints are given on a more useful
application of such an academic formalism to radiation and beam radiometric
standards at high energy accelerators and in astrophysics. The survey ends up
with a quick look to related axiomatic quantum field topics and a few other
recent works


title: Experimental Test for Kinetic Quantum Gravity Theory

abstract:
  A experimental test based on the inverse Compton effect (photon-electron
collision) is proposed to check the Kinetic Quantum Gravity Theory. The
experimental set-up point out a possible propulsionsystem, based on gravity
control.


title: The classical solutions of two-dimensional gravity

abstract:
  The solutions of two-dimensional gravity following from a non-linear
Lagrangian L = f(R) are classified, and their symmetry and singularity
properties are described. Then a conformal transformation is applied to rewrite
these solutions as analogous solutions of two-dimensional Einstein-dilaton
gravity and vice versa.


title:
Effective time variation of G in a model universe with variable space
  dimension

abstract:
  Time variation of Newtonian gravitational constant, $G$, is studied in the
model universe with variable space dimension proposed recently.
  Using the Lagrangian formulation of these models, we find the effective
gravitational constant as a function of time. To compare it with observational
data, a test theory for the time variation of $G$ is formulated. We have
assumed a power law behavior of the time variation of $G$ where the exponent
$\beta$ is itself time dependent. Within this test theory we are able to
restrict the free parameter of the theories under consideration and give upper
bounds for the space dimension at the Planck era. The time variation of $G$ at
earlier times, such as the time of nucleosynthesis is also predicted which
express the needs to look for related observational data.


title: Thermodynamic Geometry and Locally Anisotropic Black Holes

abstract:
  Thermodynamic properties of locally anisotropic (2+1)-black holes are studied
by applying geometric methods. We consider a new class of black holes with a
constant in time elliptical event horizon which is imbedded in a generalized
Finsler like spacetime geometry induced from Einstein gravity. The
corresponding thermodymanic systems are three dimensional with entropy S being
a hypersurface function on mass M, anisotropy angle $\theta$ and eccentricity
of elliptic deformations $\epsilon$. Two-dimensional curved thermodynamic
geometries for locally anistropic deformed black holes are constructed after
integration on anisotropic parameter $\theta$. Two approaches, the first one
based on two-dimensional hypersurface parametric geometry and the second one
developed in a Ruppeiner-Mrugala-Janyszek fashion, are analyzed. The
thermodynamic curvatures are computed and the critical points of curvature
vanishing are defined.


title: Parametric resonant acceleration of particles by gravitational waves

abstract:
  We study the resonant interaction of charged particles with a gravitational
wave propagating in the non-empty interstellar space in the presence of a
uniform magnetic field. It is found that this interaction can be cast in the
form of a parametric resonance problem which, besides the main resonance,
allows for the existence of many secondary ones. Each of them is associated
with a non-zero resonant width, depending on the amplitude of the wave and the
energy density of the interstellar plasma. Numerical estimates of the
particles' energisation and the ensuing damping of the wave are given.


title:
Geodesic motions versus hydrodynamic flows in a gravitating perfect
  fluid: Dynamical equivalence and consequences

abstract:
  Stimulated by the methods applied for the observational determination of
masses in the central regions of the AGNs, we examine the conditions under
which, in the interior of a gravitating perfect fluid source, the geodesic
motions and the general relativistic hydrodynamic flows are dynamically
equivalent to each other. Dynamical equivalence rests on the functional
similarity between the corresponding (covariantly expressed) differential
equations of motion and is obtained by conformal transformations. In this case,
the spaces of the solutions of these two kinds of motion are isomorphic. In
other words, given a solution to the problem "hydrodynamic flow in a perfect
fluid", one can always construct a solution formally equivalent to the problem
"geodesic motion of a fluid element" and vice versa. Accordingly, we show that,
the observationally determined nuclear mass of the AGNs is being overestimated
with respect to the real, physical one. We evaluate the corresponding
mass-excess and show that it is not always negligible with respect to the mass
ofthe central dark object, while, under circumstances, can be even larger than
the rest-mass of the circumnuclear gas involved.


title: Quantum Cosmology and Open Universes

abstract:
  Quantum creation of Universes with compact spacelike sections that have
curvature $k$ either closed, flat or open, i.e. $k=\pm1,0$ are studied. In the
flat and open cases, the superpotential of the Wheeler De Witt equation is
significantly modified, and as a result the qualitative behaviour of a typical
wavefunction differs from the traditional closed case. Using regularity
arguments, it is shown that the only consistent state for the wavefunction is
the Tunneling one. By computing the quantum probabilities for the curvature of
the sections, it is shown that quantum cosmology actually favours that the
Universe be open, $k=-1$. In all cases sufficient inflation $\sim 60$
e-foldings is predicted: this is an improvement over classical measures that
generally are ambiguous as to whether inflation is certain to occur.


title:
(Super)^n-Energy for arbitrary fields and its interchange: Conserved
  quantities

abstract:
  Inspired by classical work of Bel and Robinson, a natural purely algebraic
construction of super-energy tensors for arbitrary fields is presented, having
good mathematical and physical properties. Remarkably, there appear quantities
with mathematical characteristics of energy densities satisfying the dominant
property, which provides super-energy estimates useful for global results and
helpful in other matters. For physical fields, higher order (super)^n-energy
tensors involving the field and its derivatives arise. In Special Relativity,
they provide infinitely many conserved quantities. The interchange of
super-energy between different fields is shown. The discontinuity propagation
law in Einstein-Maxwell fields is related to super-energy tensors, providing
quantities conserved along null hypersurfaces. Finally, conserved super-energy
currents are found for any minimally coupled scalar field whenever there is a
Killing vector.


title:
Fully general relativistic simulation of merging binary clusters --
  Spatial gauge condition --

abstract:
  We have carried out simulations of the coalescence between two relativistic
clusters of collisionless particles using a 3D numerical relativity code. We
have adopted a new spatial gauge condition obtained by slightly modifying the
minimum distortion gauge condition proposed by Smarr and York and resulting in
a simpler equation for the shift vector. Using this gauge condition, we have
performed several simulations of the merger between two identical clusters in
which we have varied the compaction, the type of internal motion in the
clusters, and the magnitude of the orbital velocity. As a result of the
coalescence, either a new rotating cluster or a black hole is formed. In the
case in which a black hole is not formed, simulations could be carried out for
a time much longer than the dynamical time scale, and the resulting
gravitational waveforms were calculated fairly accurately: In these cases, the
amplitude of gravitational waves emitted can be $\sim
10^{-18}(M/10^6M_{\odot})$ at a distance 4000Mpc, and $\sim 0.5%$ of the rest
mass energy may be dissipated by the gravitational wave emission in the final
phase of the merger. These results confirm that the new spatial gauge condition
is promising in many problems at least up to the formation of black holes. In
the case in which a black hole is formed, on the other hand, the gauge
condition seems to be less adequate, but we suggest a strategy to improve it in
this case. All of the results obtained confirm the robustness of our
formulation and the ability of our code for stable evolution of strong
gravitational fields of compact binaries.


title: On the definition of cylindrical symmetry

abstract:
  The standard definition of cylindrical symmetry in General Relativity is
reviewed. Taking the view that axial symmetry is an essential pre-requisite for
cylindrical symmetry, it is argued that the requirement of orthogonal
transitivity of the isometry group should be dropped, this leading to a new,
more general definition of cylindrical symmetry. Stationarity and staticity in
cylindrically symmetric spacetimes are then defined, and these issues are
analysed in connection with orthogonal transitivity, thus proving some new
results on the structure of the isometry group for this class of spacetimes.


title: Weyl-type Fields with Geodesic Lines of Force

abstract:
  The static electrogravitational equations are studied and it is shown that an
aligned type D metric which has a Weyl-type relationship between the
gravitational and electric potential has shearfree geodesic lines of force. All
such fields are then found and turn out to be the fields of a charged sphere,
charged infinite rod and charged infinite plate. A further solution is also
found with shearing geodesic lines of force. This new solution can have $m>|e|$
or $m<|e|$, but cannot be in the Majumdar-Papapetrou class (in which $m =
|e|$). It is algebraically general and has flat equipotential surfaces.


title: Topology change and context dependence

abstract:
  The non-classical features of quantum mechanics are reproduced using models
constructed with a classical theory - general relativity. The inability to
define complete initial data consistently and independently of future
measurements, non-locality, and the non-Boolean logical structure are
reproduced by these examples. The key feature of the models is the role of
topology change. It is the breakdown of causal structure associated with
topology change that leads to the apparently non-classical behaviour. For
geons, topology change is required to describe the interaction of particles. It
is therefore natural to regard topology change as an essential part of the
measurement process. This leads to models in which the measurement imposes
additional non-redundant boundary conditions. The initial state cannot be
described independently of the measurement and there is a causal connection
between the measurement and the initial state.