title: Observables for Two-Dimensional Black Holes

abstract:
  We consider the most general dilaton gravity theory in 1+1 dimensions. By
suitably parametrizing the metric and scalar field we find a simple expression
that relates the energy of a generic solution to the magnitude of the
corresponding Killing vector. In theories that admit black hole solutions, this
relationship leads directly to an expression for the entropy $S=2\pi \tau_0/G$,
where $\tau_0$ is the value of the scalar field (in this parametrization) at
the event horizon. This result agrees with the one obtained using the more
general method of Wald. Finally, we point out an intriguing connection between
the black hole entropy and the imaginary part of the ``phase" of the exact
Dirac quantum wave functionals for the theory.


title: Fluctuations of the Unruh Temperature

abstract:
  Using the influence functional formalism, the problem of an accelerating
detector in the presence of a scalar field in its ground state is considered in
Minkowski space. As is known since the work of Unruh, to a quantum mechanical
detector following a definite, classical acceleration, the field appears to be
thermally excited. We relax the requirement of perfect classicality for the
trajectory and substitute it with one of {\it derived} classicality through the
criteria of decoherence. The ensuing fluctuations in temperature are then
related with the time and the amplitude of excitation in the detector's
internal degree of freedom.


title: Relevance of Induced Gauge Interactions in Decoherence

abstract:
  Decoherence in quantum cosmology is shown to occur naturally in the presence
of induced geometric gauge interactions associated with particle production.A
new 'gauge '-variant form of the semiclassical Einstein equations is also
presented which makes the non-gravitating character of the vacuum polarisation
energy explicit.


title:
Spherically Symmetric Event Horizons and Trapped Surfaces Developing
  {}from Innociuous Data

abstract:
  In this paper we show the existence of a large class of spherically symmetric
data $d$ (on a spacelike hypersurface $S$), from which a perfect fluid
spacetime (surrounded by vacuum) develops. This spacetime contains an event
horizon (with trapped surfaces behind it). The data $d$ are regular and {\it
innociuous}, i.e. the data--surface $S$ does not contain any point of the
horizon or of the trapped surface area.
  We give auxiliary data on an auxiliary hypersurface $H$ and also on the star
boundary; then we solve Einstein's equations for perfect fluid in the future
and past of $H$. Our solution induces the above mentioned data $d$ on some
chosen spacelike hypersurface $S$ in the past of $H$. By construction $H$ turns
out to be the matter part of the horizon, once we attach a vacuum to our matter
spacetime. Obviously, from these data $d$ on $S$ it develops (into the future)
the event horizon $H$.
  We solve the constraint equations for the auxiliary data posed on the
null--surface $H$. This reduces the choice of these data to the choice of the
density $\rho$ and $R:=[\text{curvature}]^{-1/2}$. Our data fulfil positivity
of $\rho$, $2m/R=1$ (at the star boundary) and other properties. This is
archieved by an algorithm, which for given $\rho $ yields $R$ (from an input
parameter function $h \in C^1( \left[0,1\right],\left]0,-\infty \right[)$).


title:
Spontaneous excitation of an accelerated atom: The contributions of
  vacuum fluctuations and radiation reaction

abstract:
  We consider an atom in interaction with a massless scalar quantum field. We
discuss the structure of the rate of variation of the atomic energy for an
arbitrary stationary motion of the atom through the quantum vacuum. Our main
intention is to identify and to analyze quantitatively the distinct
contributions of vacuum fluctuations and radiation reaction to the spontaneous
excitation of a uniformly accelerated atom in its ground state. This gives an
understanding of the role of the different physical processes underlying the
Unruh effect. The atom's evolution into equilibrium and the Einstein
coefficients for spontaneous excitation and spontaneous emission are
calculated.


title: On the stability of black hole event horizons

abstract:
  In this work we study a {\it gedanken} experiment constructed in order to
test the cosmic censorship hypothesis and the second law of black hole
thermo-dynamics. Matter with a negative gravitating energy is imagined added to
a near extremal $U(1)$-charged static black hole in Einstein-Maxwell theory.
The dynamics of a similar process is studied and the thermo-dynamical
properties of the resulting black hole structure is discussed. A new mechanism
which stabilizes black hole event horizons is shown to operate in such
processes.


title: Quantum State of Wormholes and Topological Arrow of Time

abstract:
  This paper studies the time-symmetry problem in quantum gravity. The issue
depends critically on the choice of the quantum state and has been considered
in this paper by restricting to the case of quantum wormholes. It is seen that
pure states represented by a wave functional are time symmetric. However, a
maximal analytic extension of the wormhole manifold is found that corresponds
to a mixed state describable by a nondegenerate density-matrix functional that
involves an extra quantum uncertainty for the three-metric, and is free from
the divergences encountered so far in statistical states formulated in quantum
gravity. It is then argued that, relative to one asymptotic region, the
statistical quantum state of single Euclidean wormholes in semiclassical
approximation is time-asymmetric and gives rise to a {\it topological} arrow of
time which will reflect in the set of all quantum fields at low energies of the
asymptotic flat region (To appear in Int. J. Mod. Phys. D)


title: Wormholes and Cosmic Strings

abstract:
  A gravitational scenario is proposed where the euclidean action is invariant
under the isotropic and homogeneous version of the euclidean {\it U(1)} group
of local transformations of the scale factor and scalar matter field,
interpreting the the trace of the second fundamental form as the gauge field.
The model allows spontaneous breakdown of some involved symmetries, including
possibly diffeomorphism invariance, and leads to the formation of topologiocal
deffects. In particular, we consider here the case of the wormhole-induced
formation of cosmic strings. (To appear in the Proceedings of the International
Workshop on "Quantum Systems: New Trends and Methods", World Scientific)


title: The Theory of Everything vs the Theory of Anything

abstract:
  To what extent can our limited set of observations be used to pin down the
specifics of a ``Theory of Everything''? In the limit where the links are
arbitrarily tenuous, a ``Theory of Everything'' might become a ``Theory of
Anything''. A clear understanding of what we can and can not expect to learn
about the universe is particularly important in the field of particle
cosmology. The aim of this article is to draw attention to some key issues
which arise in this context, in the hopes of fostering further discussion. In
particular, I explore the idea that a variety of different inflaton potentials
may contribute to worlds ``like ours''. A careful examination of the
conditional probability questions we can ask might give a physical measure of
what is ``natural'' for an inflation potential which is quite different from
those previously used.


title: Quantum cosmology and the value of $\Lambda$

abstract:
  We analyse a simple model with just a $\Lambda$ term present. Differing
results are obtained depending on the boundary conditions applied. HH boundary
conditions give the factor exp(1/Lambda) but in agreement with Rubakov et al.
is badly behaved for negative lambda. Tunneling boundary conditions suggests a
large initial lambda. If only a Lorentzian region is considered all boundary
conditions suggest a large value of Lambda. This differs from the result of
Strominger for such models.


title: Inflationary cosmology with a non-analytic Lagrangian

abstract:
  We consider an alternative higher order gravity theory which is non-analytic
in the Ricci curvature. It has power-law inflationary behaviour. This is
further evidence that inflation is a general property of higher derivative
gravity theories.


title: Canonical measure and the flatness of a FRW universe

abstract:
  We consider the claim of Hawking and Page that the canonical measure applied
to a FRW universe with a massive scalar field can solve the flatness problem
regardless of inflation. By considering a general potential we are able to
understand how the ambiguity for $\Omega$ found by Page in the $R^2$ theory is
present when the potential is bounded from above. We suggests reasons why such
potentials are realistic. The ambiguity for the possibility of inflation
present in the Gibbons- Hawking- Stewart measure can be resolved by an input
from Quantum cosmology. We contrast the resulting measure obtained in this way
with those obtained in the more usual approaches to quantum cosmology. The
measure agrees with that found from a "classical" signature change.


title: What is and What should be Time?

abstract:
  The notions of time in the theories of Newton and Einstein are reviewed so
that certain of their assumptions are clarified. These assumptions will be seen
as the causes of the incompatibility between the two different ways of
understanding time, and seen to be philosophical hypotheses, rather than purely
scientific ones. The conflict between quantum mechanics and (general)
relativity is shown to be a consequence of retaining the Newtonian conception
of time in the context of quantum mechanics. As a remedy for this conflict, an
alternative definition of time -- earlier presented in Kitada 1994a and 1994b
-- is reviewed with less mathematics and more emphasis on its philosophical
aspects. Based on this revised understanding of time it is shown that quantum
mechanics and general relativity are reconciled while preserving the current
mathematical formulations of both theories.


title:
New Algebraic Methods for Calculating the Heat Kernel and Effective
  Action in Quantum Gravity and Gauge Theories

abstract:
  An overview about recent progress in the calculation of the heat kernel and
the one-loop effective action in quantum gravity and gauge theories is given.
We analyse the general structure of the standard Schwinger-De Witt asymptotic
expansion and discuss the applicability of that to the case of strongly curved
manifolds and strong background fields. We argue that the low-energy limit in
gauge theories and quantum gravity, when formulated in a covariant way, should
be related to background fields with covariantly constant curvature, gauge
field strength and potential term. It is shown that the condition of the
covariant constancy of the background curvatures brings into existence some Lie
algebra. The heat kernel operator for the Laplace operator is presented then as
an average over the corresponding Lie group with some nontrivial Gaussian
measure. Using this representation the heat kernel diagonal is obtained. The
result is expressed purely in terms of curvature invariants and is explicitly
covariant. Related topics concerning the structure of symmetric spaces and the
calculation of the effective action are discussed.


title: Quantum Behavior in Asymmetric, Weyl-Like Cartan Geometries

abstract:
  This discussion examines recent developments in the theory of a Weyl-like,
Cartan geometry with natural Schr\"odinger field behavior proposed previously.
In that model, very nearly exactly a coupled Einstein-Maxwell- Schr\"odinger,
classical field theory emerges from a gauge invariant, purely geometric action
based solely on variations of the electromagnetic potentials and the metric. In
spite of this, only slight differences appear between the resulting
Schr\"odinger part, and the conventional theory of the Schr\"odinger field.
Close examination of the differences reveals that most are general relativistic
effects which are unobservable in flat spacetime, and which are estimated to
interact significantly only via their gravitational fields, or on scales
comparable with neutrino interaction cross sections. The only remaining
difference is that the wavefunction obeying the conjugate wave equation is not
always restricted to be exactly the complex conjugate of the primary
wavefunction. Generalizations of the model lead naturally to spinlike
phenomena, a possible new mechanism for a theory of rest mass, and spinor
connections containing the form of an SU(2) potential.


title:
Maupertuis principle, Wheeler's superspace and an invariant criterion
  for local instability in general relativity

abstract:
  It is tempting to raise the issue of (metric) chaos in general relativity
since the Einstein equations are a set of highly nonlinear equations which may
exhibit dynamically very complicated solutions for the space-time metric.
However, in general relativity it is not easy to construct indicators of chaos
which are gauge-invariant. Therefore it is reasonable to start by investigating
- at first - the possibility of a gauge-invariant description of local
instability. In this paper we examine an approach which aims at describing the
dynamics in purely geometrical terms. The dynamics is formulated as a geodesic
flow through the Maupertuis principle and a criterion for local instability of
the trajectories may be set up in terms of curvature invariants (e.g. the Ricci
scalar) of the manifold on which geodesic flow is generated. We discuss the
relation of such a criterion for local instability (negativity of the Ricci
scalar) to a more standard criterion for local instability and we emphasize
that no inferences can be made about global chaotic behavior from such local
criteria.


title:
Kaluza -- Klein Quantum Cosmology with Primordial Negative Cosmological
  Constant

abstract:
  In many interesting models, including superstring theories, a negative vacuum
energy is predicted. Although this effect is usually regarded as undesirable
from a cosmological point of view, we show that this can be the basis for a new
approach to the cosmology of the early Universe. In the framework of quantum
cosmology (in higher dimensions) when we consider a negative cosmological
constant and matter that could be dust or, alternatively, coherent excitations
of a scalar field, the role of cosmic time can be understood. Then we can
predict the existence of a ``quantum inflationary phase'' for some dimensions
and a simultaneous ``quantum deflationary phase'' for the remaining dimensions.
We discuss how it may be possible to exit from this inflation-compactification
era to a phase with zero cosmological constant which allows a classical
description at late times.


title:
Gravitation and Cosmology in Generalized (1+1)-dimensional dilaton
  gravity

abstract:
  The actions of the ``$R=T$'' and string-inspired theories of gravity in (1+1)
dimensions are generalized into one single action which is characterized by two
functions. We discuss differing interpretations of the matter stress-energy
tensor, and show how two such different interpretations can yield two different
sets of field equations from this action. The weak-field approximation,
post-Newtonian expansion, hydrostatic equilibrium state of star and
two-dimensional cosmology are studied separately by using the two sets of field
equations. Some properties in the ``$R=T$'' and string-inspired theories are
shown to be generic in the theory induced by the generalized action.


title:
Chern-Simons functional and the no-boundary proposal in Bianchi IX
  quantum cosmology

abstract:
  The Chern-Simons functional $S_{\rm CS}$ is an exact solution to the
Ashtekar-Hamilton-Jacobi equation of general relativity with a nonzero
cosmological constant. In this paper we consider $S_{\rm CS}$ in Bianchi type
IX cosmology with $S^3$ spatial surfaces. We show that among the classical
solutions generated by~$S_{\rm CS}$, there is a two-parameter family of
Euclidean spacetimes that have a regular NUT-type closing. When two of the
three scale factors are equal, these spacetimes reduce to a one-parameter
family within the Euclidean Taub-NUT-de~Sitter metrics. For a nonzero
cosmological constant, $\exp(iS_{\rm CS})$ therefore provides a semiclassical
estimate to the Bianchi~IX no-boundary wave function in Ashtekar's variables.


title: A Comment on Junction and Energy Conditions in Thin Shells

abstract:
  This comment contains a suggestion for a slight modification of Israel's
covariant formulation of junction conditions between two spacetimes, placing
both sides on equal footing with normals having uniquely defined orientations.
The signs of mass energy densities in thin shells at the junction depend not
only on the orientations of the normals and it is useful therefore to discuss
the sign separately. Calculations gain in clarity by not choosing the
orientations in advance. Simple examples illustrate our point and complete
previous classifications of spherical thin shells in spherically symmetric
spacetimes relevant to cosmology.


title:
Exact solutions of Einstein and Einstein-scalar equations in 2 + 1
  dimensions

abstract:
  A nonstatic and circularly symmetric exact solution of the Einstein equations
(with a cosmological constant $\Lambda$ and null fluid) in $2+1$ dimensions is
given. This is a nonstatic generalization of the uncharged spinless BTZ metric.
For $\Lambda = 0 $, the spacetime is though not flat, the Kretschmann invariant
vanishes. The energy, momentum, and power output for this metric are obtained.
Further a static and circularly symmetric exact solution of the
Einstein-massless scalar equations is given, which has a curvature singularity
at $r =0$ and the scalar field diverges at $r=0$ as well as at infinity .


title:
Calculation of gravitational wave forms from black hole collisions and
  disk collapse: Applying perturbation theory to numerical spacetimes

abstract:
  Many simulations of gravitational collapse to black holes become inaccurate
before the total emitted gravitational radiation can be determined. The main
difficulty is that a significant component of the radiation is still in the
near-zone, strong field region at the time the simulation breaks down. We show
how to calculate the emitted waveform by matching the numerical simulation to a
perturbation solution when the final state of the system approaches a
Schwarzschild black hole. We apply the technique to two scenarios: the head-on
collision of two black holes, and the collapse of a disk to a black hole. This
is the first reasonably accurate calculation of the radiation generated from
colliding black holes that form from matter collapse.


title:
Response of finite-time particle detectors in non-inertial frames and
  curved spacetime

abstract:
  The response of the Unruh-DeWitt type monopole detectors which were coupled
to the quantum field only for a finite proper time interval is studied for
inertial and accelerated trajectories, in the Minkowski vacuum in (3+1)
dimensions. Such a detector will respond even while on an inertial trajctory
due to the transient effects. Further the response will also depend on the
manner in which the detector is switched on and off. We consider the response
in the case of smooth as well as abrupt switching of the detector. The former
case is achieved with the aid of smooth window functions whose width, $T$,
determines the effective time scale for which the detector is coupled to the
field. We obtain a general formula for the response of the detector when a
window function is specified, and work out the response in detail for the case
of gaussian and exponential window functions. A detailed discussion of both $T
\rightarrow 0$ and $T \rightarrow \infty$ limits are given and several
subtlities in the limiting procedure are clarified. The analysis is extended
for detector responses in Schwarzschild and de-Sitter spacetimes in (1+1)
dimensions.


title: Characterisation of orthogonal perfect fluid cosmological spacetimes

abstract:
  We consider the general orthogonal metric separable in space and time
variables in comoving coordinates. We then characterise perfect fluid models
admitted by such a metric. It turns out that the homogeneous models can only be
either FLRW or Bianchi I while the inhomogeneous ones can only admit $G_2 $
(two mutually as well as hypersurface orthogonal spacelike Killing vectors)
isometry. The latter can possess singularities of various kinds or none. The
non-singular family is however unique and cylindrically symmetric.


title: On the remote sensing of Hawking grey pulses

abstract:
  This is a short note on the black hole remote-sensing problem, i.e., finding
out `surface' temperature distributions of various types of small
(micron-sized) black holes from the spectral measurements of their Hawking grey
pulses. Chen's modified Moebius inverse transform is illustrated in this
context


title: General form of thermodynamical entropy for black hole

abstract:
  The entropy of a black hole can be different from a quarter of the area even
at the semiclassical level.


title: The Head-On Collision of Two Equal Mass Black Holes Peter Anninos

abstract:
  We study the head-on collision of two equal mass, nonrotating black holes.
Various initial configurations are investigated, including holes which are
initially surrounded by a common apparent horizon to holes that are separated
by about $20M$, where $M$ is the mass of a single black hole. We have extracted
both $\ell = 2$ and $\ell=4$ gravitational waveforms resulting from the
collision. The normal modes of the final black hole dominate the spectrum in
all cases studied. The total energy radiated is computed using several
independent methods, and is typically less than $0.002 M$. We also discuss an
analytic approach to estimate the total gravitational radiation emitted in the
collision by generalizing point particle dynamics to account for the finite
size and internal dynamics of the two black holes. The effects of the tidal
deformations of the horizons are analysed using the membrane paradigm of black
holes. We find excellent agreement between the numerical results and the
analytic estimates.


title: The Head-On Collision of Two Equal Mass Black Holes: Numerical Methods

abstract:
  The head-on collision of two nonrotating axisymmetric equal mass black holes
is treated numerically. We take as initial data the single parameter family of
time-symmetric solutions discovered by Misner which consists of two
Einstein-Rosen bridges that can be placed arbitrarily distant from one another.
A number of problems associated with previous attempts to evolve these data
sets have been overcome. In this article, we discuss our choices for coordinate
systems, gauges, and the numerical algorithms that we have developed to evolve
this system.


title:
Quasi-Normal Mode Expansion for Linearized Waves in Gravitational
  Systems

abstract:
  The quasinormal modes (QNM's) of gravitational systems modeled by the
Klein-Gordon equation with effective potentials are studied in analogy to the
QNM's of optical cavities. Conditions are given for the QNM's to form a
complete set, i.e., for the Green's function to be expressible as a sum over
QNM's, answering a conjecture by Price and Husain [Phys. Rev. Lett. {\bf 68},
1973 (1992)]. In the cases where the QNM sum is divergent, procedures for
regularization are given. The crucial condition for completeness is the
existence of spatial discontinuities in the system, e.g., the discontinuity at
the stellar surface in the model of Price and Husain.


title: A Schrodinger Equation for Quantum Universes

abstract:
  We discuss how to fix the gauge in the canonical treatment of Lagrangians,
with finite number of degrees of freedom, endowed with time reparametrization
invariance. The motion can then be described by an effective Hamiltonian acting
on the gauge shell canonical space. The system is then suited for quantization.
We apply this treatment to the case of a Robertson--Walker metric interacting
with zero modes of bosonic fields and write a \S equation for the on--shell
wave function (Presented at the International Workshop ``Birth of the Universe
and Fundamental Physics'', Rome, May 18-21, 1994).


title: Dust as a Standard of Space and Time in Canonical Quantum Gravity

abstract:
  The coupling of the metric to an incoherent dust introduces into spacetime a
privileged dynamical reference frame and time foliation. The comoving
coordinates of the dust particles and the proper time along the dust worldlines
become canonical coordinates in the phase space of the system. The Hamiltonian
constraint can be resolved with respect to the momentum that is canonically
conjugate to the dust time. Imposition of the resolved constraint as an
operator restriction on the quantum states yields a functional Schr\"{o}dinger
equation. The ensuing Hamiltonian density has an extraordinary feature: it
depends only on the geometric variables, not on the dust coordinates or time.
This has three important consequences. First, the functional Schr\"{o}dinger
equation can be solved by separating the dust time from the geometric
variables. Second, the Hamiltonian densities strongly commute and therefore can
be simultaneously defined by spectral analysis. Third, the standard constraint
system of vacuum gravity is cast into a form in which it generates a true Lie
algebra. The particles of dust introduce into space a privileged system of
coordinates that allows the supermomentum constraint to be solved explicitly.
The Schr\"{o}dinger equation yields a conserved inner product that can be
written in terms of either the instantaneous state functionals or the solutions
of constraints. Examples of gravitational observables are given, though neither
the intrinsic metric nor the extrinsic curvature are observables. Disregarding
factor--ordering difficulties, the introduction of dust provides a satisfactory
phenomenological approach to the problem of time in canonical quantum gravity.


title:
Qualitative Analysis of Brans-Dicke Universes with a Cosmological
  Constant

abstract:
  Solutions to flat space Friedmann-Robertson-Walker cosmologies in Brans-Dicke
theory with a cosmological constant are investigated. The matter is modelled as
a $\gamma$-law perfect fluid. The field equations are reduced from fourth order
to second order through a change of variables, and the resulting
two-dimensional system is analyzed using dynamical system theory. When the
Brans-Dicke coupling constant is positive $(\omega > 0)$, all initially
expanding models approach exponential expansion at late times, regardless of
the type of matter present. If $\omega < 0$, then a wide variety of
qualitatively distinct models are present, including nonsingular ``bounce''
universes, ``vacillating'' universes and, in the special case of $\omega = -1$,
models which approach stable Minkowski spacetime with an exponentially
increasing scalar field at late times. Since power-law solutions do not exist,
none of the models appear to offer any advantage over the standard deSitter
solution of general relativity in achieving a graceful exit from inflation.


title: Nonlinearity in Quantum Theory and Closed Timelike Curves

abstract:
  We examine consequences of the density matrix approach to quantum theory in
the context of a model spacetime containing closed timelike curves and find
that in general, an initially pure state will evolve in a nonlinear way to a
mixed quantum state. CPT invariance and the implications of this nonlinearity
for the statistical interpretation of quantum theory are discussed.


title: MATTERS OF GRAVITY, a newsletter for the gravity community, Number 4

abstract:
  Table of contents
  Editorial.
  Gravity News:
  Report on the APS topical group in gravitation, Beverly Berger.
  Research briefs:
  Gravitational microlensing and the search for dark matter, Bohdan Paczynski.
  Laboratory gravity: the G mystery, Riley Newman.
  LIGO project update, Stan Whitcomb.
  Conference Reports
  PASCOS '94, Peter Saulson.
  The Vienna Meeting, P. Aichelburg, R. Beig.
  The Pitt binary black hole grand challenge meeting, Jeff Winicour.
  International symposium on experimental gravitation at Pakistan,Munawar Karim
  10th Pacific coast gravity meeting, Jim Isenberg.


title: Spacetime Information

abstract:
  In usual quantum theory, the information available about a quantum system is
defined in terms of the density matrix describing it on a spacelike surface.
This definition must be generalized for extensions of quantum theory which do
not have a notion of state on a spacelike surface. It must be generalized for
the generalized quantum theories appropriate when spacetime geometry fluctuates
quantum mechanically or when geometry is fixed but not foliable by spacelike
surfaces. This paper introduces a four-dimensional notion of the information
available about a quantum system's boundary conditions in the various sets of
decohering histories it may display. The idea of spacetime information is
applied in several contexts: When spacetime geometry is fixed the information
available through alternatives restricted to a spacetime region is defined. The
information available through histories of alternatives of general operators is
compared to that obtained from the more limited coarse- grainings of
sum-over-histories quantum mechanics. The definition of information is
considered in generalized quantum theories. We consider as specific examples
time-neutral quantum mechanics with initial and final conditions, quantum
theories with non-unitary evolution, and the generalized quantum frameworks
appropriate for quantum spacetime. In such theories complete information about
a quantum system is not necessarily available on any spacelike surface but must
be searched for throughout spacetime. The information loss commonly associated
with the ``evolution of pure states into mixed states'' in black hole
evaporation is thus not in conflict with the principles of generalized quantum
mechanics.


title: Topology Changes by Quantum Tunneling in Four Dimensions

abstract:
  We investigate topology-changing processes in 4-dimensional quantum gravity
with a negative cosmological constant. By playing the ``gluing-polytope game"
in hyperbolic geometry, we explicitly construct an instanton-like solution
without singularity. Because of cusps, this solution is non-compact but has a
finite volume. Then we evaluate a topology change amplitude in the WKB
approximation in terms of the volume of this solution.


title: Essay on mesoscopic and quantum brain

abstract:
  In the pure essay style (no mathematical formulas), I present a number of
speculative reflections and suggestions on possible applications of mesoscopic
methods and of quantum mechanical concepts to as such a complex system as the
human brain. As an initial guide for this essay I used ``The Emperor's New
Mind" of Roger Penrose


title: Dumb Holes and the Effects of High Frequencies on Black Hole Evaporation

abstract:
  The naive calculation of black hole evaporation makes the thermal emission
depend on the arbitrary high frequency behaviour of the theory where the theory
is certainly wrong. Using the sonic analog to black holes-- dumb holes-- I show
numerically that a change in the dispersion relation at high frequencies does
not seem to alter the evaporation process, lending weight to the reality of the
black hole evaporation process. I also suggest a reason for the insensitivity
of the process to high frequency regime.


title:
Global Properties of Locally Spatially Homogeneous Cosmological Models
  with Matter

abstract:
  The existence and nature of singularities in locally spatially homogeneous
solutions of the Einstein equations coupled to various phenomenological matter
models is investigated. It is shown that, under certain reasonable assumptions
on the matter, there are no singularities in an expanding phase of the
evolution and that unless the spacetime is empty a contracting phase always
ends in a singularity where at least one scalar invariant of the curvature
diverges uniformly. The class of matter models treated includes perfect fluids,
mixtures of non-interacting perfect fluids and collisionless matter.


title: Indeterministic Quantum Gravity

abstract:
  A theory which claims to describe all the universe is advanced. It unifies
general relativity, quantum field theory, and indeterministic conception. Basic
entities are: classical metric tensor $g$, cosmic reference frame (including
cosmic time $t$), operator $T$ of energy-momentum tensor, Hamiltonian $H_t$,
and state vector $\Psi$. Dynamical equations are: the Einstein equation
$G[g]=(\Psi,T\Psi)$ ($G$ is the Einstein tensor), the Heisenberg equation
$dT/dt=i[H_t,T]$, and the condition $H_t\Psi_t=\epsilon_t \Psi_t$ arising from
the cosmic energy determinacy principle advanced in the theory. The last
equation describes quantum jump dynamics. Quantum jumps lead to the
instantaneous transferring of action and information, which, however, neither
violates the causality principle, nor contradicts quantum field theory and
general relativity. The cosmic energy determinacy principle implies the eternal
universe, i.e., the cyclic one without beginning and ending, the minimal energy
in every cycle being finite.


title: Lifetimes of spherically symmetric closed universes

abstract:
  It is proven that any spherically symmetric spacetime that possesses a
compact Cauchy surface $\Sigma$ and that satisfies the dominant-energy and
non-negative-pressures conditions must have a finite lifetime in the sense that
all timelike curves in such a spacetime must have a length no greater than $10
\max_\Sigma(2m)$, where $m$ is the mass associated with the spheres of
symmetry. This result gives a complete resolution, in the spherically symmetric
case, of one version of the closed-universe recollapse conjecture (though it is
likely that a slightly better bound can be established). This bound has the
desirable properties of being computable from the (spherically symmetric)
initial data for the spacetime and having a very simple form. In fact, its form
is the same as was established, using a different method, for the spherically
symmetric massless scalar field spacetimes, thereby proving a conjecture
offered in that work. Prospects for generalizing these results beyond the
spherically symmetric case are discussed.


title:
The No-Boundary Wave Function and the Duration of the Inflationary
  Period

abstract:
  For the simplest minisuperspace model based on a homogeneous, isotropic
metric and a minimally coupled scalar field we derive analytic expressions for
the caustic which separates Euklidean and Minkowskian region and its breakdown
value $\p_*$. This value represents the prediction of the no-boundary wave
function for the scalar field at the beginning of inflation. We use our results
to search for inflationary models which can render the no-boundary wave
function consistent with the requirement of a sufficiently long inflationary
period.


title: Entropy, Area, and Black Hole Pairs

abstract:
  We clarify the relation between gravitational entropy and the area of
horizons. We first show that the entropy of an extreme Reissner-Nordstr\"om
black hole is $zero$, despite the fact that its horizon has nonzero area. Next,
we consider the pair creation of extremal and nonextremal black holes. It is
shown that the action which governs the rate of this pair creation is directly
related to the area of the acceleration horizon and (in the nonextremal case)
the area of the black hole event horizon. This provides a simple explanation of
the result that the rate of pair creation of non-extreme black holes is
enhanced by precisely the black hole entropy. Finally, we discuss black hole
$annihilation$, and argue that Planck scale remnants are not sufficient to
preserve unitarity in quantum gravity.


title: Consistency of Semiclassical Gravity

abstract:
  We discuss some subtleties which arise in the semiclassical approximation to
quantum gravity. We show that integrability conditions prevent the existence of
Tomonaga-Schwinger time functions on the space of three-metrics but admit them
on superspace. The concept of semiclassical time is carefully examined. We
point out that central charges in the matter sector spoil the consistency of
the semiclassical approximation unless the full quantum theory of gravity and
matter is anomaly-free. We finally discuss consequences of these considerations
for quantum field theory in flat spacetime, but with arbitrary foliations.


title: Do We Understand Black Hole Entropy ?

abstract:
  I review various proposals for the nature of black hole entropy and for the
mechanism behind the operation of the generalized second law. I stress the
merits of entanglement entropy {\tenit qua\/} black hole entropy, and point out
that, from an operational viewpoint, entanglement entropy is perfectly finite.
Problems with this identification such as the multispecies problem and the
trivialization of the information puzzle are mentioned. This last leads me to
associate black hole entropy rather with the multiplicity of density operators
which describe a black hole according to exterior observers. I relate this
identification to Sorkin's proof of the generalized second law. I discuss in
some depth Frolov and Page's proof of the same law, finding it relevant only
for scattering of microsystems by a black hole. Assuming that the law is
generally valid I make evident the existence of the universal bound on entropy
regardless of issues of acceleration buoyancy, and discuss the question of why
macroscopic objects cannot emerge in the Hawking radiance.


title: Comments on the multi-dimensional Wheeler-DeWitt equation

abstract:
  It is argued heuristically -- using an ${\bf S}^3 \times {\bf S}^6$
minisuperspace model -- that there might be a fundamental quantum gravity
effect stabilizing internal spaces with non-vanishing Ricci curvature.


title: Critique of weakly decohering and linearly positive histories

abstract:
  The usual composition rule of independent systems, as applied to decoherent
histories or to linearly positive histories, requires (at least) medium
decoherence and, consequently, a second constraint for the linearly positive
histories of Goldstein and Page. Other plausible classical features, valid for
medium decoherence, seem anyhow to be missed by linear positive histories.


title: How Objective is Black Hole Entropy?

abstract:
  The objectivity of black hole entropy is discussed in the particular case of
a Schwarzchild black hole. Using Jaynes' maximum entropy formalism and
Euclidean path integral evaluation of partition function, it is argued that in
the semiclassical limit when the fluctutation of metric is neglected, the black
hole entropy of a Schwarzchild black hole is equal to the maximal information
entropy of an observer whose sole knowledge of the black hole is its mass.
Black hole entropy becomes a measure of number of its internal mass eigenstates
in accordance with the Boltzmann principle only in the limit of negligible
relative mass fluctutation. {}From the information theoretic perspective, the
example of a Schwarzchild black hole seems to suggest that black hole entropy
is no different from ordinary thermodynamic entropy. It is a property of the
experimental data of a black hole, rather than being an intrinsic physical
property of a black hole itself independent of any observer. However, it is
still weakly objective in the sense that different observers given the same set
of data of a black hole will measure the same maximal information entropy.


title: Anomaly In 2d-Gravity Coupled To Matter Fields

abstract:
  In 1+1 dimensions, the Wheeler-DeWitt equation cannot be imposed owing to an
anomaly in its commutator with the diffeomorphism constraint. A similar
obstruction prevents also a semiclassical definition of a WKB local time. (Talk
given at the 7th Marcel Grossman Meeting on General Relativity, Stanford, July
28, 1994.)


title:
Exact Einstein-scalar field solutions for formation of black holes in a
  cosmological setting

abstract:
  We consider self-interacting scalar fields coupled to gravity. Two classes of
exact solutions to Einstein's equations are obtained: the first class
corresponds to the minimal coupling, the second one to the conformal coupling.
One of the solutions is shown to describe a formation of a black hole in a
cosmological setting. Some properties of this solution are described. There are
two kinds of event horizons: a black hole horizon and cosmological horizons.
The cosmological horizons are not smooth. There is a mild curvature
singularity, which affects extended bodies but allows geodesics to be extended.
It is also shown that there is a critical value for a parameter on which the
solution depends. Above the critical point, the black hole singularity is
hidden within a global black hole event horizon. Below the critical point, the
singularity appears to be naked. The relevance to cosmic censorship is
discussed.


title: What Cavities?

abstract:
  Most spherical thin shells, enclosing black body radiation satisfy the
dominant energy condition if they have at least $\simeq 30\%$ of the total
mass-energy. Containers with less mass energy, able to sustain high pressures,
contain mostly unstable radiation. If they have negligible mass energy they are
unable to sustain the pressures and the radiation is unstable to gravitational
collapse. Containers with black holes and radiation in thermal equilibrium,
considered in the literature, are often unrealistic.


title: Spinors and the reference point of quasilocal energy

abstract:
  This paper investigates the relationship between the quasilocal energy of
Brown and York and certain spinorial expressions for gravitational energy
constructed from the Witten-Nester integral. A key feature of the Brown-York
method for defining quasilocal energy is that it allows for the freedom to
assign the reference point of the energy. When possible, it is perhaps most
natural to reference the energy against flat space, i.e. assign flat-space the
zero value of energy. It is demonstrated that the Witten-Nester integral when
evaluated on solution spinors to the Sen-Witten equation (obeying appropriate
boundary conditions) is essentially the Brown-York quasilocal energy with a
reference point determined by the Sen-Witten spinors. For the case of round
spheres in the Schwarzschild geometry, these spinors determine the flat-space
reference point. A similar viewpoint is proposed for the Schwarzschild-case
quasilocal energy of Dougan and Mason.


title: Evaporating Black Holes And An Entropic Scale-Hierarchy

abstract:
  It is argued that a characteristic length may be associated with the entropic
state of a spherically symmetric black hole in the cosmological context. This
length is much smaller than the Schwarzschild-radius of a black hole and may
act as a regulator of arbitrarily high frequencies apparently entering the
usual derivation of Hawking's radiation.


title:
The Role of the Apparent Horizon in the Evolution of Robinson-Trautman
  %%EINSTEIN-Maxwell Spacetimes

abstract:
  The `runaway solutions' of the Lorentz-Dirac equation of a charged particle
interacting with its own field in classical electrodynamics are well-known.
This type of self accelerated phenomena also exists in the solutions of the
Einstein-Maxwell equations in general relativity. In particular, runaway
solutions occur in a class of simple models known as the `Asymptotically Flat
Robinson-Trautman Einstein-Maxwell' (AFRTEM) spacetimes. Consequently these
spacetimes cannot evolve to their unique regular steady state, viz. a charged
non-rotating black hole. This seems to contradict the established results that
charged non-rotating black holes are stable under first order perturbations. We
show that if an AFRTEM spacetime also possesses an apparent horizon, then it
has a Lyapunov functional. This suggests that the evolution equations with
additional constraints arising from the apparent horizon would evolve stably to
a charged non-rotating black hole. We also demonstrate that the linearised
equations of these restricted spacetimes are stable and the exponentially
growing dipole modes, which give rise to self accelerated motions in classical
electrodynamics are also eliminated.


title: The Bianchi Ix (MIXMASTER) Cosmological Model is Not Integrable

abstract:
  The perturbation of an exact solution exhibits a movable transcendental
essential singularity, thus proving the nonintegrability. Then, all possible
exact particular solutions which may be written in closed form are isolated
with the perturbative Painlev\'e test; this proves the inexistence of any
vacuum solution other than the three known ones.


title:
Quantum Fluctuations of Planck Mass as Mutation Mechanism in a Theory of
  Evolution of the Universe

abstract:
  Contributed talk at the Seventh Marcel Grossman Meeting on Gravity, June
24-30. A theory of evolution of the universe requires both a mutation mechanism
and a selection mechanism. We believe that both can be encountered in the
stochastic approach to quantum cosmology. In Brans-Dicke chaotic inflation, the
quantum fluctuations of Planck mass behave as mutations, such that new
inflationary domains may contain values of Planck mass that differ slightly
from their parent's. The selection mechanism establishes that the value of
Planck mass should be such as to increase the proper volume of the inflationary
domain, which will then generate more offsprings. This mechanism predicts that
the effective Planck scale at the end of inflation should be much larger than
any given scale in the model.


title: Gravitational Analog of the Aharonov-Casher Effect

abstract:
  The gravitational interaction between a massive particle and a spinning
particle in the weak-field limit is studied. We show that a system of a
spinning point-like particle and a massive rod exhibit a topological effect
analogous to the electromagnetic Aharonov-Casher effect. We discuss the effect
also for systems with a cosmic string instead of a massive rod and in the
context of 2+1-dimensional gravity.


title: On maximum number of decoherent histories

abstract:
  It is shown that $N^2$ is the upper limit for the number of histories in a
decohering family of $N$-state quantum system. Simple criterion is found for a
family of $N^2$ fine grained decohering histories of Gell-Mann and Hartle to be
identical with a family of Griffiths' consistent quantum trajectories.


title:
A Method for Calculating the Structure of (Singular) Spacetimes in the
  Large

abstract:
  A formalism and its numerical implementation is presented which allows to
calculate quantities determining the spacetime structure in the large directly.
This is achieved by conformal techniques by which future null infinity
($\Scri{}^+$) and future timelike infinity ($i^+$) are mapped to grid points on
the numerical grid. The determination of the causal structure of singularities,
the localization of event horizons, the extraction of radiation, and the
avoidance of unphysical reflections at the outer boundary of the grid, are
demonstrated with calculations of spherically symmetric models with a scalar
field as matter and radiation model.


title: Black holes cannot support conformal scalar hair

abstract:
  It is shown that the only static asymptotically flat non-extrema black hole
solution of the Einstein-conformally invariant scalar field equations having
the scalar field bounded on the horizon, is the Schwarzschild one. Thus black
holes cannot be endowed with conformal scalar hair of finite length.


title:
Bibliography of Publications related to Classical and Quantum Gravity in
  terms of the Ashtekar Variables

abstract:
  This bibliography attempts to give a comprehensive overview of all the
literature related to the Ashtekar variables. The original version was compiled
by Peter H\"ubner in 1989, and it has been subsequently updated by Gabriela
Gonzalez, Bernd Br\"ugmann, and Troy Schilling. Information about additional
literature, new preprints, and especially corrections are always welcome.


title: Extrinsic Time in Quantum Cosmology

abstract:
  An extrinsic time is identified in most isotropic and homogeneous
cosmological models by matching them with the ideal clock - a parametrized
system whose only "degree of freedom" is time -. Once this matching is
established, the cosmological models are quantized in the same way as the ideal
clock. The space of solutions of the Wheeler-DeWitt equation is turned out into
a Hilbert space by inserting a time dependent operator in the inner product,
yielding a unitary theory equivalent to the phase space reduced theory.


title: Massive Superstrings are Black Holes

abstract:
  We argue that it is inconsistent to ignore the gravitational backreaction for
on-shell superstring states at the Planck mass and beyond, and that these
quantum states become Kerr black holes in the classical limit. Consequences are
discussed.


title: Inside a Spinning Black String

abstract:
  We show that mass inflation occurs inside spinning black cosmic string, which
is a solution of a low-energy effective string theory in $(3+1)$-dimensions.
This confirms Poisson and Israel's conjecture that the inner mass parameter
diverges even if spacetime is not spherically symmetric.


title:
Self-Similar Scalar Field Collapse: Naked Singularities and Critical
  Behaviour

abstract:
  Homothetic scalar field collapse is considered in this article. By making a
suitable choice of variables the equations are reduced to an autonomous system.
Then using a combination of numerical and analytic techniques it is shown that
there are two classes of solutions. The first consists of solutions with a
non-singular origin in which the scalar field collapses and disperses again.
There is a singularity at one point of these solutions, however it is not
visible to observers at finite radius. The second class of solutions includes
both black holes and naked singularities with a critical evolution (which is
neither) interpolating between these two extremes. The properties of these
solutions are discussed in detail. The paper also contains some speculation
about the significance of self-similarity in recent numerical studies.


title: The Spectral Analysis Inner Product for Quantum Gravity

abstract:
  This submission to the Proceedings of the Seventh Marcel-Grossman Conference
is an advertisement for the use of the ``spectral analysis inner product" for
minisuperspace models in quantum gravity.


title: Properties of Consistent Histories

abstract:
  We describe some properties of consistent sets of histories in the
Gell-Mann--Hartle formalism, and give an example to illustrate that one cannot
recover the standard predictions, retrodictions and inferences of
quasiclassical physics using the criterion of consistency alone.


title:
The Gravitational and Electrostatic Fields Far from an Isolated
  Einstein-Maxwell Source

abstract:
  The exterior solution for an arbitrary charged, massive source, is studied as
a static deviation from the Reissner-Nordstr\o m metric. This is reduced to two
coupled ordinary differential equations for the gravitational and electrostatic
potential functions. The homogeneous equations are explicitly solved in the
particular case $q^2=m^2$, obtaining a multipole expansion with radial
hypergeometric dependence for both potentials. In the limiting case of a
neutral source, the equations are shown to coincide with recent results by
Bondi and Rindler.


title:
Spherically Symmetric Solutions in M\o ller's Tetrad Theory of
  Gravitation

abstract:
  The general solution of M\o ller's field equations in case of spherical
symmetry is derived. The previously obtained solutions are verified as special
cases of the general solution.


title: A remark on kinks and time machines

abstract:
  We describe an elementary proof that a manifold with the topology of the
Politzer time machine does not admit a nonsingular, asymptotically flat Lorentz
metric.


title:
Cosmological solutions of the Vlasov-Einstein system with spherical,
  plane, and hyperbolic symmetry

abstract:
  The Vlasov-Einstein system describes a self-gravitating, collisionless gas
within the framework of general relativity. We investigate the initial value
problem in a cosmological setting with spherical, plane, or hyperbolic symmetry
and prove that for small initial data solutions exist up to a spacetime
singularity which is a curvature and a crushing singularity. An important tool
in the analysis is a local existence result with a continuation criterion
saying that solutions can be extended as long as the momenta in the support of
the phase-space distribution of the matter remain bounded.


title:
Decoherence of Homogeneous and Isotropic Metrics in the Presence of
  Massive Vector Fields

abstract:
  Retrieval of classical behaviour in quantum cosmology is usually discussed in
the framework of minisuperspace models in the presence of scalar fields
together with the inhomogeneous modes either of the gravitational or of the
scalar fields. In this work we propose alternatively a model where the scalar
field is replaced by a massive vector field with global $U(1)$ or SO(3)
symmetries.


title: Scale anomalies imply violation of the averaged null energy condition

abstract:
  Considerable interest has recently been expressed regarding the issue of
whether or not quantum field theory on a fixed but curved background spacetime
satisfies the averaged null energy condition (ANEC). A comment by Wald and
Yurtsever [Phys. Rev. D43, 403 (1991)] indicates that in general the answer is
no. In this note I explore this issue in more detail, and succeed in
characterizing a broad class of spacetimes in which the ANEC is guaranteed to
be violated. Finally, I add some comments regarding ANEC violation in
Schwarzschild spacetime.


title:
Is there a connection between no-hair behavior and universality in
  gravitational collapse?

abstract:
  We apply linear perturbation theory to the study of the universality and
criticality first observed by Choptuik in gravitational collapse. Since these
are essentially nonlinear phenomena our attempt is only a rough approximation.
In spite of this, universal behavior of the final black hole mass is observed
with an exponent of 1/2, slightly higher than the observed value of 0.367. The
universal behavior is rooted in the universal form that in-falling
perturbations on black holes have at the horizon.


title: An Analytic Model with Critical Behavior in Black Hole Formation

abstract:
  A simple analytic model is presented which exhibits a critical behavior in
black hole formation, namely, collapse of a thin shell coupled with outgoing
null fluid. It is seen that the critical behavior is caused by the
gravitational nonlinearity near the event horizon. We calculate the value of
the critical exponent analytically and find that it is very dependent on the
coupling constants of the system.


title: The General Self-dual solution of the Einstein Equations

abstract:
  We obtain the most general explicit (anti)self-dual solution of the Einstein
equations. We find that any (anti)self-dual solution can be characterised by
three free functions of which one is harmonic. Any stationary (anti)self-dual
solution can be characterised by a harmonic function. It turns out that the
form of the Gibbons and Hawking multi-center metrics is the most general
stationary (anti)self-dual solution. We further note that the stationary
(anti)self-dual Einstein equations can be reinterpreted as the (anti)self-dual
Maxwell equations on the Euclidean background metric.


title: Embedding initial data for black hole collisions

abstract:
  We discuss isometric embedding diagrams for the visualization of initial data
for the problem of the head-on collision of two black holes. The problem of
constructing the embedding diagrams is explicitly presented for the best
studied initial data, the Misner geometry. We present a partial solution of the
embedding diagrams and discuss issues related to completing the solution.


title: Infinite Blueshift of Charged Null Particles

abstract:
  We demonstrate that charged null particles can be infinitely blue\-shifted in
a Kerr-Newman spacetime. The surface of infinite blueshift can be outside of
the ergosphere in a Kerr-Newman spacetime, and outside of the outer event
horizon for a Reissner-Nordstrom spacetime. Implications for extensions of the
standard model which incorporate charged neutrinos are discussed.


title: Observables and a Hilbert Space for Bianchi IX

abstract:
  We consider a quantization of the Bianchi IX cosmological model based on
taking the constraint to be a self-adjoint operator in an auxiliary Hilbert
space. Using a WKB-style self-consistent approximation, the constraint chosen
is shown to have only continuous spectrum at zero. Nevertheless, the auxiliary
space induces an inner product on the zero-eigenvalue generalized eigenstates
such that the resulting physical Hilbert space has countably infinite
dimension. In addition, a complete set of gauge-invariant operators on the
physical space is constructed by integrating differential forms over the
spacetime. The behavior of these operators indicates that this quantization
preserves Wald's classical result that the Bianchi IX spacetimes expand to a
maximum volume and then recollapse.


title:
Higgs Mechanism and the Structure of the Energy-Momentum Tensor in
  Einstein Gravity and Conformal Gravity

abstract:
  In the standard treatment of the Einstein gravitational theory the
energy-momentum tensor has always been taken to be composed of perfect fluid
aggregates of kinematic Newtonian point test particles with fundamental
mechanical masses. Moreover, this standard prescription was not revised after
the discovery of the mass-generating Higgs mechanism which is known to be
present in the elementary particle physics of these self-same sources and which
is also required in the conformal invariant gravitational alternative being
considered by Mannheim and Kazanas. In this short contribution we show that
despite the presence of the Higgs mechanism, the standard geodesic motion and
Euler hydrodynamics still obtain in the one-particle sector of the theory even
while the overall energy-momentum tensor differs substantially from the
conventional kinematic one.


title:
A Three-Dimensional Hydrodynamics Code for Modeling Sources of
  Gravitational Radiation

abstract:
  We have developed a 3-D Eulerian hydrodynamics code to model sources of
gravitational radiation. The code is written in cylindrical coordinates
$(\varpi,z,\varphi)$ and has moving grids in the $\varpi$ and $z$-directions.
We use Newtonian gravity and calculate the gravitational radiation in the
quadrupole approximation. This code has been tested on a variety of problems to
verify its accuracy and stability, and the results of these tests are reported
here.


title: The Statistical Mechanics of the (2+1)-Dimensional Black Hole

abstract:
  The presence of a horizon breaks the gauge invariance of (2+1)-dimensional
general relativity, leading to the appearance of new physical states at the
horizon. I show that the entropy of the (2+1)-dimensional black hole can be
obtained as the logarithm of the number of these microscopic states.


title:
Nonequilibrium Quantum Fields in Cosmology: Comments on Selected Current
  Topics

abstract:
  Concepts of quantum open systems and ideas of correlation dynamics in
nonequilibrium statistical mechanics, as well as methods of closed-time-path
effective action and influence functional in quantum field theory can be
usefully applied for the analysis of quantum statistical processes in
gravitation and cosmology. We raise a few conceptual questions and suggest some
new directions of research on selected currrent topics on the physics of the
early universe, such as entropy generation in cosmological particle creation,
quantum theory of galaxy formation, and phase transition in inflationary
cosmology.


title:
Gravitational Waves from a Particle Orbiting Around a Rotating Black
  Holes: Post-Newtonian Expansion

abstract:
  Using the Teukolsky and Sasaki-Nakamura equations for the gravitational
perturbation of the Kerr spacetime, we calculate the post-Newtonian expansion
of the energy and angular momentum luminosities of gravitational waves from a
test particle orbiting around a rotating black hole up through ${\rm P^{5/2}N}$
order beyond the quadrupole formula. We apply a method recently developed by
Sasaki to the case of a rotating black hole. We take into account a small
inclination of the orbital plane to the lowest order of the Carter constant.
The result to $ P^{3/2}N}$ order is in agreement with a similar calculation by
Poisson as well as with the standard post-Newtonian calculation by Kidder et
al. Using our result, we calculate the integrated phase of gravitational waves
from a neutron star-neutron star binary and a black hole-neutron star binary
during their inspiral stage. We find that, in both cases, spin-dependent terms
in the P$^2$N and P$^{5/2}$N corrections are important to construct effective
template waveforms which will be used for future laser-interferometric
gravitational wave detectors.


title:
Effects of weak self-interactions in a relativistic plasma on
  cosmological perturbations

abstract:
  The exact solutions for linear cosmological perturbations which have been
obtained for collisionless relativistic matter within thermal field theory are
extended to a self-interacting case. The two-loop contributions of scalar
$\lambda\phi^4$ theory to the thermal graviton self-energy are evaluated, which
give the $O(\lambda)$ corrections in the perturbation equations. The changes
are found to be perturbative on scales comparable to or larger than the Hubble
horizon, but the determination of the large-time damping behavior of subhorizon
perturbations requires a resummation of thermally induced masses.


title: Conditions for inflation in an initially inhomogeneous universe

abstract:
  Using a long wavelength iteration scheme to solve Einstein's equations near
the Big-Bang singularity of a universe driven by a massive scalar field, we
find how big initial quasi-isotropic inhomogeneities can be before they can
prevent inflation to set in.


title:
Gravitational Radiation from Nonaxisymmetric Instability in a Rotating
  Star

abstract:
  We present the first calculations of the gravitational radiation produced by
nonaxisymmetric dynamical instability in a rapidly rotating compact star. The
star deforms into a bar shape, shedding $\sim 4\%$ of its mass and $\sim 17\%$
of its angular momentum. The gravitational radiation is calculated in the
quadrupole approximation. For a mass $M \sim 1.4$ M$_{\odot}$ and radius $R
\sim 10$ km, the gravitational waves have frequency $\sim 4$ kHz and amplitude
$h \sim 2 \times 10^{-22}$ at the distance of the Virgo Cluster. They carry off
energy $\Delta E/M \sim 0.1\%$ and radiate angular momentum $\Delta J/J \sim
0.7\%$.


title: A Quantum-Driven Time (QDT) Quantization of the Taub Cosmology

abstract:
  We present here an application of a new quantization scheme. We quantize the
Taub cosmology by quantizing only the anisotropy parameter $\beta$ and imposing
the super-Hamiltonian constraint as an expectation-value equation to recover
the relationship between the scale factor $\Omega$ and the time $t$. This
approach appears to avoid the problem of time. (Paper to appear in the Seventh
Marcel Grossmann Conference Proceedings.)


title: On a possible solution for the Polonyi problem in string cosmology

abstract:
  We establish the main features of homogeneous and isotropic dilaton, metric
and Yang-Mills configurations in a cosmological framework. We identify a new
energy exchange term between the dilaton and the Yang-Mills field which may
lead to a possible solution to the Polonyi problem in 4-dimensional string
models.


title:
Modification of the Coulomb potential from a Kaluza-Klein model with a
  Gauss-Bonnet term in the action

abstract:
  In four dimensions a Gauss-Bonnet term in the action corre- sponds to a total
derivative, and it does not contribute to the classical equations of motion.
For higher-dimensional geometries this term has the interesting property
(shared with other dimensionally continued Euler densities) that when the
action is varied with respect to the metric, it gives rise to a symmetric,
covariantly conserved tensor of rank two which is a function of the metric and
its first and second order derivatives. Here we review the unification of
General Relativity and electromagnetism in the classical five-dimen- sional,
restricted (with g_55 = 1) Kaluza-Klein model. Then we discuss the
modifications of the Einstein-Maxwell theory that results from adding the
Gauss-Bonnet term in the action. The resulting four-dimensional theory
describes a non-linear U(1) gauge theory non-minimally coupled to gravity. For
a point charge at rest, we find a perturbative solution for large distances
which gives a mass-dependent correction to the Coulomb potential. Near the
source we find a power-law solution which seems to cure the short-distance
divergency of the Coulomb potential. Possible ways to obtain an experimen- tal
upper limit to the coupling of the hypothetical Gauss- Bonnet term are also
considered.


title: What is the reference frame of an accelerated observer?

abstract:
  The general construction of extended refrence frames for noninertial
observers in flat space is studied. It is shown that, if the observer moves
inertially before and after an arbitrary acceleration and rotation, the region
where reference frames can coincide with an inertial system is bounded for
final velocities exceeding 0.6 c.


title: The Final Fate of Spherical Inhomogeneous Dust Collapse

abstract:
  We examine the role of the initial density and velocity distribution in the
gravitational collapse of a spherical inhomogeneous dust cloud. Such a collapse
is described by the Tolman-Bondi metric which has two free functions: the
`mass-function' and the `energy function', which are determined by the initial
density and velocity profile of the cloud. The collapse can end in a black-hole
or a naked singularity, depending on the initial parameters characterizing
these profiles. In the marginally bound case, we find that the collapse ends in
a naked singularity if the leading non-vanishing derivative of the density at
the center is either the first one or the second one. If the first two
derivatives are zero, and the third derivative non-zero, the singularity could
either be naked or covered, depending on a quantity determined by the third
derivative and the central density. If the first three derivatives are zero,
the collapse ends in a black hole. In particular, the classic result of
Oppenheimer and Snyder, that homogeneous dust collapse leads to a black hole,
is recovered as a special case. Analogous results are found when the cloud is
not marginally bound, and also for the case of a cloud starting from rest. We
also show how the strength of the naked singularity depends on the density and
velocity distribution. Our analysis generalizes and simplifies the earlier work
of Christodoulou and Newman [4,5] by dropping the assumption of evenness of
density functions. It turns out that relaxing this assumption allows for a
smooth transition from the naked singularity phase to the black-hole phase, and
also allows for the occurrence of strong curvature naked singularities.


title: Fermi-Walker coordinates in 2+1 dimensional gravity

abstract:
  It is shown that in 2+1 dimensions the Fermi-Walker gauge allows the general
solution of the problem of determining the metric from the sources in terms of
simple quadratures. This technique is used to solve the problem of the
occurrence of closed time like curves (CTC's) in stationary solutions. In fact
the Fermi-Walker gauge, due to its physical nature, allows to exploit the weak
energy condition and in this connection it is proved that, both for open and
closed universes with axial symmetry, the energy condition imply the total
absence of closed time like curves. The extension of this theorem to the
general stationary problem, in absence of axial symmetry is considered and at
present the proof of such generalization is subject to some assumptions on the
behavior of the determinant of the dreibeins in this gauge.


title: Can Galactic Observations Be Explained by a Relativistic Gravity Theory?

abstract:
  We consider the possibility of an alternative gravity theory explaining the
dynamics of galactic systems without dark matter. From very general assumptions
about the structure of a relativistic gravity theory we derive a general
expression for the metric to order $(v/c)^2$. This allows us to compare the
predictions of the theory with various experimental data: the Newtonian limit,
light deflection and retardation, rotation of galaxies and gravitational
lensing. Our general conclusion is that the possibility for any gravity theory
to explain the behaviour of galaxies without dark matter is rather improbable.


title: Initial Data for General Relativity with Toroidal Conformal Symmetry

abstract:
  A new class of time-symmetric solutions to the initial value constraints of
vacuum General Relativity is introduced. These data are globally regular,
asymptotically flat (with possibly several asymptotic ends) and in general have
no isometries, but a $U(1)\times U(1)$ group of conformal isometries. After
decomposing the Lichnerowicz conformal factor in a double Fourier series on the
group orbits, the solutions are given in terms of a countable family of
uncoupled ODEs on the orbit space.


title: On the topology of stationary black holes

abstract:
  We prove that the domain of outer communication of a stationary, globally
hyperbolic spacetime satisfying the null energy condition must be simply
connected. Under suitable additional hypotheses, this implies, in particular,
that each connected component of a cross-section of the event horizon of a
stationary black hole must have spherical topology.


title: Bosonic physical states in N=1 supergravity?

abstract:
  It is argued that states in $N=1$ supergravity that solve all of the
constraint equations cannot be bosonic in the sense of being independent of the
fermionic degrees of freedom. (Based on a talk given by Miguel Ortiz at the 7th
Marcel Grossmann Meeting.)


title: Conservation Laws in the Quantum Mechanics of Closed Systems

abstract:
  We investigate conservation laws in the quantum mechanics of closed systems.
We review an argument showing that exact decoherence implies the exact
conservation of quantities that commute with the Hamiltonian including the
total energy and total electric charge. However, we also show that decoherence
severely limits the alternatives which can be included in sets of histories
which assess the conservation of these quantities when they are not coupled to
a long-range field arising from a fundamental symmetry principle. We then
examine the realistic cases of electric charge coupled to the electromagnetic
field and mass coupled to spacetime curvature and show that when alternative
values of charge and mass decohere, they always decohere exactly and are
exactly conserved as a consequence of their couplings to long-range fields.
Further, while decohering histories that describe fluctuations in total charge
and mass are also subject to the limitations mentioned above, we show that
these do not, in fact, restrict {\it physical} alternatives and are therefore
not really limitations at all.


title:
Decoherence on Quantum Tunneling in the Multi-Dimensional Wave Function
  Approach

abstract:
  We consider fundamental problems on the understanding of the tunneling
phenomena in the context of the multi-dimensional wave function. In this paper,
we reconsider the quantum state after tunneling and extend our previous
formalism to the case when the quantum state before tunneling is in a squeezed
state. Through considering this problem, we reveal that the quantum decoherence
plays a crucial role to allow us of the concise description of the quantum
state after tunneling.


title:
Quantum Complex Scalar Field in the Two-Dimensional Spacetime with
  Closed Timelike Curves and a Time-Machine Problem

abstract:
  It is considered the quantum complex scalar field which obeys the
authomorphic condition in the two-dimensional spacetime with closed timelike
curves and the chronology horizon. The renormalized stress-energy tensor is
obtained. It is shown that the value of the stress-energy tensor is regular at
the chronology horizon for specific authomorphic parameters. Thus the
particular example of field configuration is given for which the Hawking's
chronology protection conjecture is violated.


title:
Short-distance Cut-off from the Quantum Dynamics of Fields in a Black
  Hole Spacetime

abstract:
  We stress the strongly observer-dependent nature of the fine-grained black
hole entropy as computed by usual methods. From the point of view of a fiducial
observer, the 1-loop renormalized energy-momentum tensor diverges at the
horizon. Considering, more consistently, the computation of the black hole
entropy in the back-reaction-corrected curved background an ultraviolet
cut-off, needed to regularize the entropy, naturally araises. Examples of the
classical dynamics of test particles, strings and relativistic membranes in the
corrected metric are also given to complete the picture.


title:
A reformulation of the Ponzano-Regge quantum gravity model in terms of
  surfaces

abstract:
  We reformulate the Ponzano-Regge quantum gravity model in terms of surfaces
on a 3-dimensional simplex lattice. This formulation (1) has a clear relation
to the loop representation of the canonical quantum general relativity in
3-dimensions, (2) may have a 4-dimensional analogue, in contrast to the 6-j
symbolic formalism of the Ponzano-Regge model, and (3) is purely a theory of
surfaces, in the sense that it does not include any field variables; hence it
is coordinate-free on the surface and background-free in spacetime. We discuss
implications and applications of this formulation.


title:
A tensorial Lax pair equation and integrable systems in relativity and
  classical mechanics

abstract:
  It is shown that the Lax pair equation dL/dt = [L,A] can be given a neat
tensorial interpretation for finite-dimensional quadratic Hamiltonians. The Lax
matrices L and A are shown to arise from third rank tensors on the
configuration space. The second Lax matrix A is related to a connection which
characterizes the Hamiltonian system. The Toda lattice system is used to
motivate the definition of the Lax pair tensors. The possible existence of
solutions to the Einstein equations having the Lax pair property is discussed.


title: Signature change induces compactification

abstract:
  It is shown -- using a FRW model with ${\bf S}^3 \times {\bf S}^6$ as spatial
sections and a positive cosmological constant -- that classical signature
change implies a new compactification mechanism. The internal scale factor is
of the order $\Lambda^{-1/2}$, and the solutions are stable against small
perturbatons. In the case of compactified ${\bf S}^6$, it is shown that the
effective four-dimensional space-time metric has Lorentzian signature,
undergoes exponential inflation in ${\bf S}^3$ and is unique. Speculations
concerning relations to quantum cosmology and conceivable modifications are
added.


title: Reality Conditions and Ashtekar Variables: a Different Perspective

abstract:
  We give in this paper a modified self-dual action that leads to the
$SO(3)$-ADM formalism without having to face the difficult second class
constraints present in other approaches (for example if one starts from the
Hilbert-Palatini action). We use the new action principle to gain some new
insights into the problem of the reality conditions that must be imposed in
order to get real formulations from complex general relativity. We derive also
a real formulation for Lorentzian general relativity in the Ashtekar phase
space by using the modified action presented in the paper.


title: Real Ashtekar Variables for Lorentzian Signature Space-times

abstract:
  I suggest in this letter a new strategy to attack the problem of the reality
conditions in the Ashtekar approach to classical and quantum general
relativity. By writing a modified Hamiltonian constraint in the usual $SO(3)$
Yang-Mills phase space I show that it is possible to describe space-times with
Lorentzian signature without the introduction of complex variables. All the
features of the Ashtekar formalism related to the geometrical nature of the new
variables are retained; in particular, it is still possible, in principle, to
use the loop variables approach in the passage to the quantum theory. The key
issue in the new formulation is how to deal with the more complicated
Hamiltonian constraint that must be used in order to avoid the introduction of
complex fields.


title: Thermodynamics of a black hole in a cavity

abstract:
  We present a unified thermodynamical description of the configurations
consisting on self-gravitating radiation with or without a black hole. We
compute the thermal fluctuations and evaluate where will they induce a
transition from metastable configurations towards stable ones. We show that the
probability of finding such a transition is exponentially small. This indicates
that, in a sequence of quasi equilibrium configurations, the system will remain
in the metastable states till it approaches very closely the critical point
beyond which no metastable configuration exists. Near that point, we relate the
divergence of the local temperature fluctuations to the approach of the
instability of the whole system, thereby generalizing the usual fluctuations
analysis in the cases where long range forces are present. When angular
momentum is added to the cavity, the above picture is slightly modified.
Nevertheless, at high angular momentum, the black hole loses most of its mass
before it reaches the critical point at which it evaporates completely.


title: Non-Abelian Black Holes and Catastrophe Theory II: Charged Type

abstract:
  We reanalyze the gravitating monopole and its black hole solutions in the
Einstein-Yang-Mills-Higgs system and we discuss their stabilities from the
point of view of catastrophe theory. Although these non-trivial solutions
exhibit fine and complicated structures, we find that stability is
systematically understood via a swallow tail catastrophe. The
Reissner-Nordstr\"{o}m trivial solution becomes unstable from the point where
the non-trivial monopole black hole appears. We also find that, within a very
small parameter range, the specific heat of a monopole black hole changes its
sign .


title: The inequivalence of thermodynamic ensembles

abstract:
  The inequivalence of thermodynamical ensembles related by a Legendre
transformation is manifest in self-gravitating systems and in black hole
thermodynamics. Using the Poincare's method of the linear series, we describe
the mathematical reasons which lead to this inequivalence which in turn induces
a hierarchy of ensembles: the most stable ensemble describes the most isolated
system. Moreover, we prove that one can obtain the degree of stability of all
equilibrium configurations in any ensensemble related by Legendre
transformations to the most stable if one knows the degree of stability in the
most stable ensemble.


title: Knots and Quantum Gravity: Progress and Prospects

abstract:
  Recent work on the loop representation of quantum gravity has revealed
previously unsuspected connections between knot theory and quantum gravity, or
more generally, 3-dimensional topology and 4-dimensional generally covariant
physics. We review how some of these relationships arise from a `ladder of
field theories' including quantum gravity and BF theory in 4 dimensions,
Chern-Simons theory in 3 dimensions, and the G/G gauged WZW model in 2
dimensions. We also describe the relation between link (or multiloop)
invariants and generalized measures on the space of connections. In addition,
we pose some research problems and describe some new results, including a proof
(due to Sawin) that the Chern-Simons path integral is not given by a
generalized measure.


title: The Dipole Coupling of Atoms and Light in Gravitational Fields

abstract:
  The dipole coupling term between a system of N particles with total charge
zero and the electromagnetic field is derived in the presence of a weak
gravitational field. It is shown that the form of the coupling remains the same
as in flat space-time if it is written with respect to the proper time of the
observer and to the measurable field components. Some remarks concerning the
connection between the minimal and the dipole coupling are given.


title: Particle Creation If a Cosmic String Snaps

abstract:
  We calculate the Bogolubov coefficients for a metric which describes the
snapping of a cosmic string. If we insist on a matching condition for all times
{\it and} a particle interpretation, we find no particle creation.


title: Black Holes in Three Dimensional Topological Gravity

abstract:
  We investigate the black hole solution to (2+1)-dimensional gravity coupled
to topological matter, with a vanishing cosmological constant. We calculate the
total energy, angular momentum and entropy of the black hole in this model and
compare with results obtained in Einstein gravity. We find that the theory with
topological matter reverses the identification of energy and angular momentum
with the parameters in the metric, compared with general relativity, and that
the entropy is determined by the circumference of the inner rather than the
outer horizon. We speculate that this results from the contribution of the
topological matter fields to the conserved currents. We also briefly discuss
two new possible (2+1)-dimensional black holes.


title: Do Cosmic Strings Give Rise to Vacuum Fluctuations ?

abstract:
  The role of the identification of the vacuum and non-vacuum space-times in
the computation of vacuum fluctuations in the presence of a cosmic string is
discussed and an alternative interpretation of the renormalization is proposed.
This procedure does not give rise to vacuum fluctuations.


title: Topology of Event Horizons and Topological Censorship

abstract:
  We prove that, under certain conditions, the topology of the event horizon of
a four dimensional asymptotically flat black hole spacetime must be a 2-sphere.
No stationarity assumption is made. However, in order for the theorem to apply,
the horizon topology must be unchanging for long enough to admit a certain kind
of cross section. We expect this condition is generically satisfied if the
topology is unchanging for much longer than the light-crossing time of the
black hole. More precisely, let $M$ be a four dimensional asymptotically flat
spacetime satisfying the averaged null energy condition, and suppose that the
domain of outer communication $\C_K$ to the future of a cut $K$ of $\Sm$ is
globally hyperbolic. Suppose further that a Cauchy surface $\Sigma$ for $\C_K$
is a topological 3-manifold with compact boundary $\partial\S$ in $M$, and
$\S'$ is a compact submanifold of $\bS$ with spherical boundary in $\S$ (and
possibly other boundary components in $M/\S$). Then we prove that the homology
group $H_1(\Sigma',Z)$ must be finite. This implies that either $\partial\S'$
consists of a disjoint union of 2-spheres, or $\S'$ is nonorientable and
$\partial\S'$ contains a projective plane. Further,
$\partial\S=\partial\Ip[K]\cap\partial\Im[\Sp]$, and $\partial \Sigma$ will be
a cross section of the horizon as long as no generator of $\partial\Ip[K]$
becomes a generator of $\partial\Im[\Sp]$. In this case, if $\S$ is orientable,
the horizon cross section must consist of a disjoint union of 2-spheres.}


title: A New Formula for the Mass of a Stationary Axisymmetric Configuration

abstract:
  It has been shown by one of the authors$^1$ that in isotropic spherical
coordinates there is a relation between the mass of a static spherical
gravitating body and the pressure distribution inside it. In this paper the
result is generalized for the case of stationary axisymmetric configurations.


title:
Cosmological Perturbations of Quantum Mechanical Origin and Anisotropy
  of the Microwave Background Radiation

abstract:
  A theory of quantum-mechanical generation of cosmological perturbations is
considered. The conclusion of this study is that if the large-angular-scale
anisotropy in the cosmic microwave background radiation is caused by the
long-wavelength cosmological perturbations of quantum mechanical origin, they
are, most likely, gravitational waves, rather than density perturbations or
rotational perturbations. Some disagreements with previous publications are
clarified. This contribution to the Proceedings is based on Reference~[34].


title:
Quantum Effects Associated with an Electrically Neutral Massless
  Particle in a Bertotti-Robinson Universe

abstract:
  In the present article we obtain, by separation of variables, an exact
solution to the Dirac equation with anomalous momentum for an electrically
neutral massless particle in a Bertotti-Robinson universe. We discuss the
phenomenom of particle creation in this model.


title:
Decoherence of Friedmann-Robertson-Walker Geometries in the Presence of
  Massive Vector Fields with U(1) or SO(3) Global Symmetries

abstract:
  Retrieval of classical behaviour in quantum cosmology is usually discussed in
the framework of {\em midi}superspace models in the presence of scalar fields
and the inhomogeneous modes corresponding either to gravitational or scalar
fields. In this work, we propose an alternative model to study the decoherence
of homogeneous and isotropic geometries where the scalar field is replaced by a
massive vector field with a global internal symmetry. We study here the cases
with $U(1)$ and $SO(3)$ global internal symmetries. The presence of a mass term
breaks the conformal invariance and allows for the longitudinal modes of the
spin-1 field to be present in the Wheeler-DeWitt equation. In the case of the
U(1) global internal symmetry, we have only one single ``classical'' degree of
freedom while in the case of the SO(3) global symmetry, we are led to consider
a simple two-dimensional minisuperspace model. These minisuperspaces are shown
to be equivalent to a set of coupled harmonic oscillators where the kinetic
term of the longitudinal modes has a coefficient proportional to the inverse of
the scale factor. The conditions for a suitable decoherence process and
correlations between coordinate and momenta are established. The validity of
the semi-classical Einstein equations when massive vector fields (Abelian and
non-Abelian) are present is also discussed.


title: Thermodynamic Stability of a Multi-Bubble Cosmological Model

abstract:
  Multibubble solutions for a cosmological model which lead to thermal
inflationary states due to a semi-classical tunneling of gravity are
calculated.


title: Symmetries, superselection rules, and decoherence

abstract:
  We discuss the applicability of the programme of decoherence -- emergence of
approximate classical behaviour through interaction with the environment -- to
cases where it was suggested that the presence of symmetries would lead to
exact superselection rules. For this discussion it is useful to make a
distinction between pure symmetries and redundancies, which results from an
investigation into the constraint equations of the corresponding theories. We
discuss, in particular, superpositions of states with different charges, as
well as with different masses, and suggest how the corresponding interference
terms, although they exist in principle, become inaccessible through
decoherence.


title: Stability Properties of the Starobinsky Cosmological Model

abstract:
  We discuss the instabilities appearing in the cosmological model with a quasi
de Sitter phase following from a fourth-order gravity theory. Both the
classical equation as well as the quantization in form of a Wheeler - De Witt
equation are conformally related to the analogous model with Einstein's theory
of gravity with a minimally coupled scalar field. Results are: 1. In the
non-tachyonic case, classical fourth-order gravity is not more unstable than
Einstein's theory itself. 2. The well-known classically valid conformal
relation is also (at least for some typical cases) valid on the level of the
corresponding Wheeler - De Witt equations, which turns out to be a non-trivial
statement. (to appear in: Proc. Sem. Relativistic Astrophysics Potsdam 1994, Ed
.: J. M\"ucket)


title: Inconsistent Physics in the Presence of Time Machines

abstract:
  We present a scenario in $1 + 1$ and $3 + 1$ dimensional space time which is
paradoxical in the presence of a time machine. We show that the paradox cannot
be resolved and the scenario has {\em no} consistent classical solution. Since
the system is macroscopic, quantisation is unlikely to resolve the paradox.
Moreover, in the absence of a consistent classical solution to a macroscopic
system, it is not obvious how to carry out the path integral quantisation.
Ruling out, by fiat, the troublesome initial conditions will resolve the
paradox, by not giving rise to it in the first place. However this implies that
time machines have an influence on events, extending indefinitely into the
past, and also tachyonic communication between physical events in an era when
no time machine existed. If no resolution to the paradox can be found, the
logical conclusion is that time machines of a certain, probably large, class
cannot exist in $3 + 1$ and $1 + 1$ dimensional space time, maintaining the
consistency of known physical laws.


title:
Exotic black hole solution in teleparallel theory of (2+1)-dimensional
  gravity

abstract:
  A black hole solution in a teleparallel theory of (2+1)-dimensional gravity,
given in a previous paper, is examined. This solution is also a solution of the
three-dimensional vacuum Einstein equation with a vanishing cosmological
constant. Remarkable is the fact that this solution gives a black hole in a \lq
\lq flat-land" in the Einstein theory and a Newtonian limit. Coordinate
transformations to \lq \lq Minkowskian" coordinates, however, are singular not
only at the origin, but also on the event horizon. {\em In the
three-dimensional Einstein theory, vacuum regions of space-times can be locally
non-trivial}.


title:
An Exact Calculation of the Energy Density of Cosmological Gravitational
  Waves

abstract:
  In this paper we calculate the Bogoliubov coefficients and the energy density
of the stochastic gravitational wave background for a universe that undergoes
inflation followed by radiation domination and matter domination, using a
formalism that gives the Bogoliubov coefficients as continous functions of
time. By making a reasonable assumption for the equation of state during
reheating, we obtain in a natural way the expected high frequency cutoff in the
spectral energy density.


title:
Free Energy and Entropy for Semi-classical Black Holes in the Canonical
  Ensemble

abstract:
  We consider the thermodynamics of a black hole coupled to thermal radiation
in a spatially finite (spherical) region. Thermodynamic state functions are
derived in the canonical ensemble, defined by elements of radius $r_o$ and
boundary temperature $T(r_o)$. Using recent solutions of the semi-classical
back reaction problem, we compute the $O(\hbar)$ corrections to the mass of the
black hole, thermal energy, the entropy and free energy due to the presence of
hot conformal scalars, massless spinors and U(1) gauge quantum fields in the
vicinity of the hole. The free energy is particularly important for assessing
under what conditions the nucleation of black holes from hot flat space is
likely to occur.


title:
Magnetic Black Hole Pair-Production: One-Loop Tunneling Rate in the Weak
  Field Limit

abstract:
  Pair-production of magnetic Reissner-Nordstr\"{o}m black holes (of charges
$\pm q$) is studied in the next-to-leading WKB approximation. We consider
generic quantum fluctuations in the corresponding instanton geometry, a
detailed study of which suggests that, for sufficiently weak field $B$, the
problem can be reduced to that of quantum fluctuations around a single
truncated near-extremal Euclidean black hole in thermal equilibrium. Typical
one-loop contributions are such that the leading WKB exponent is corrected by a
small fraction $\sim \hbar /q^2$. We show that this effect is merely due to a
semiclassical shift of the black hole mass-to-charge ratio that persists even
in the extremal limit. We close with a few final comments.


title: Quantum Particle As Seen In Light Scattering

abstract:
  A possible mathematical model has been proposed for motion of illuminated
quantum particles seen by eyes or similar devices mapping the scattered light.


title: Continuously Diagonalized Density Operatorof Open Systems

abstract:
  We showed several years ago that the density operator of Markovian open
systems can be diagonalized continuously in time. The resulting pure state jump
processes correspond to quantum trajectories proposed in recent quantum optics
calculations or, at fundamental level, to exact consistent histories.


title: Vacuum Weyl Cosmologies in D Dimensions

abstract:
  Vacuum cosmological models are considered in the context of a
multidimensional theory of gravity with integrable Weyl geometry. A family of
exact solutions with a chain of internal spaces is obtained. Models with one
internal space are considered in more detail; nonsingular models are selected.


title: Dynamical Systems Treatment of Scalar Field Cosmologies

abstract:
  The conformal equivalence of some cosmological models in Brans-Dicke theory
to general relativistic cosmologies with a scalar field is discussed. In the
case of radiation-dominated universes, it is shown that the presence of the
scalar field has a negligible impact upon the evolution of the models in the
Einstein frame. It is also shown that power-law inflation in general
relativity, which is conformal to ``extended'' power-law inflation in
Brans-Dicke theory, is not a unique attractor for expanding closed universes,
but rather that the occurence of inflation depends upon the initial kinetic
energy of the scalar field.


title:
Strong cosmic censorship in vacuum space--times with compact, locally
  homogeneous Cauchy surfaces

abstract:
  We consider the question of strong cosmic censorship in spatially compact,
spatially locally homogeneous vacuum models. We show in particular that strong
cosmic censorship holds in Bianchi IX vacuum space--times with spherical
spatial topology.


title: Scattering Processes at the Planck Scale

abstract:
  The ultrarelativistic limit of the Kerr - Newman geometry is studied in
detail. We find the gravitational shock wave background associated with this
limit. We study the scattering of scalar fields in the gravitational shock wave
geometries and compare this with the scattering by ultrarelativistic extended
sources and with the scattering of fundamental strings.
  We also study planckian energy string collisions in flat spacetime as the
scattering of a string in the effective curved background produced by the
others as the impact parameter $b$ decreases. We find the effective energy
density distribution generated by these collisions. The effective metric
generated by these collisions is a gravitational shock wave with profile
$f(\rho)\sim p\rho^{4-D}$, for large impact parameter $b$. For intermediate
$b$, $f(\rho)\sim q\rho^2$, corresponding to an extended source of momentum
$q$.
  We finally study the emergence of string instabilities in $D$ - dimensional
black hole spacetimes and De Sitter space. We solve the first order string
fluctuations around the center of mass motion at spatial infinity, near the
horizon and at the spacetime singularity. We find that the time components are
always well behaved in the three regions and in the three backgrounds. The
radial components are unstable: imaginary frequencies develop in the
oscillatory modes near the horizon, and the evolution is like $(\tau-\tau_0)^
{-P}$, $(P>0)$, near the spacetime singularity, $r\to0$, where the world -
sheet time $(\tau-\tau_0)\to0$, and the proper string length grows infinitely.


title: Asymptotic Symmetry Groups of Long-Ranged Gauge Configurations

abstract:
  We make some general remarks on long-ranged configurations in gauge or
diffeomorphism invariant theories where the fields are allowed to assume some
non vanishing values at spatial infinity. In this case the Gauss constraint
only eliminates those gauge degrees of freedom which lie in the connected
component of asymptotically trivial gauge transformations. This implies that
proper physical symmetries arise either from gauge transformations that reach
to infinity or those that are asymptotically trivial but do not lie in the
connected component of transformations within that class. The latter
transformations form a discrete subgroup of all symmetries whose position in
the ambient group has proven to have interesting implications. We explain this
for the dyon configuration in the $SO(3)$ Yang-Mills-Higgs theory, where we
prove that the asymptotic symmetry group is $Z_{|m|}\times \Re$ where $m$ is
the monopole number. We also discuss the application of the general setting to
general relativity and show that here the only implication of discrete
symmetries for the continuous part is a possible extension of the rotation
group $SO(3)$ to $SU(2)$.