title:
Data analysis of gravitational-wave signals from spinning neutron stars.
  I. The signal and its detection

abstract:
  We present a theoretical background for the data analysis of the
gravitational-wave signals from spinning neutron stars for Earth-based laser
interferometric detectors. We introduce a detailed model of the signal
including both the frequency and the amplitude modulations. We include the
effects of the intrinsic frequency changes and the modulation of the frequency
at the detector due to the Earth motion. We estimate the effects of the star's
proper motion and of relativistic corrections. Moreover we consider a signal
consisting of two components corresponding to a frequency $f$ and twice that
frequency. From the maximum likelihood principle we derive the detection
statistics for the signal and we calculate the probability density function of
the statistics. We obtain the data analysis procedure to detect the signal and
to estimate its parameters. We show that for optimal detection of the amplitude
modulated signal we need four linear filters instead of one linear filter
needed for a constant amplitude signal. Searching for the doubled frequency
signal increases further the number of linear filters by a factor of two. We
indicate how the fast Fourier transform algorithm and resampling methods
commonly proposed in the analysis of periodic signals can be used to calculate
the detection statistics for our signal. We find that the probability density
function of the detection statistics is determined by one parameter: the
optimal signal-to-noise ratio. We study the signal-to-noise ratio by means of
the Monte Carlo method for all long-arm interferometers that are currently
under construction. We show how our analysis can be extended to perform a joint
search for periodic signals by a network of detectors and we perform Monte
Carlo study of the signal-to-noise ratio for a network of detectors.


title:
Gravitational Waves around a Naked Singularity -- Odd-Parity
  Perturbation of Lemaitre-Tolman-Bondi Space-Time --

abstract:
  The motion of a spherical dust cloud is described by the
Lemaitre-Tolman-Bondi solution and is completely specified by initial values of
distributions of the rest mass density and specific energy of the dust fluid.
  From generic initial conditions of this spherically symmetric collapse, there
appears a naked singularity at the symmetric center in the course of the
gravitational collapse of the dust cloud. So this might be a counter example to
the cosmic censorship hypothesis. To investigate the genericity of this
example, we examine the stability of the `nakedness' of this singularity
against odd-parity modes of non-spherical linear perturbations for the metric,
i.e., linear gravitational waves. We find that the perturbations do not diverge
but are well-behaved even in the neighborhood of the central naked singularity.
This means that the naked singularity formation process is marginally stable
against the odd-parity modes of linear gravitational waves.


title: About model of the Universe with accelerated movement of the time

abstract:
  Cosmological model based on metric of Fridmann-Robertson-Walker with
permanent size and acceleration of time is considered. The problem of the dark
matter is analyzed within this model .


title: Photon propagation in Einstein and Higher Derivative Gravity

abstract:
  In the wave equation obeyed by electromagnetic fields in curved spacetime
there are Riemann and Ricci curvature coupling terms to the photon
polarisation, which result in a polarisation dependent deviation of the photon
trajectories from null geodesics. Photons are found to have an effective mass
in an external gravitational field and their velocity in an inertial frame is
in general less than $c$. A consequence of this is that the curvature
corrections to the propagation of electromagnetic radiation keep the velocities
subluminal provided the strong energy condition is satisfied. We further show
that the claims of superluminal velocities in higher derivative gravity
theories are erroneous and arise due to the neglect of Riemann and Ricci
coupling terms in the wave equation, which exists in Einstein's gravity itself.


title:
Multidimensional perfect fluid cosmology with stable compactified
  internal dimensions

abstract:
  Multidimensional cosmological models in the presence of a bare cosmological
constant and a perfect fluid are investigated under dimensional reduction to
4-dimensional effective models. Stable compactification of the internal spaces
is achieved for a special class of perfect fluids. The external space behaves
in accordance with the standard Friedmann model. Necessary restrictions on the
parameters of the models are found to ensure dynamical behavior of the external
(our) universe in agreement with observations.


title: On the dynamics of Einstein's equations in the Ashtekar formulation

abstract:
  We study the dynamics of Einstein's equations in Ashtekar's variables from
the point of view of the theory of hyperbolic systems of evolution equations.
We extend previous results and show that by a suitable modification of the
Hamiltonian vector flow outside the sub-manifold of real and constrained
solutions, a symmetric hyperbolic system is obtained for any fixed choice of
lapse-shift pair, without assuming the solution to be a priori real. We notice
that the evolution system is block diagonal in the pair $(\sigma^a,A_b)$, and
provide explicit and very simple formulae for the eigenvector-eigenvalue pairs
in terms of an orthonormal tetrad with one of its components pointing along the
propagation direction. We also analyze the constraint equations and find that
when viewed as functions of the extended phase space they form a symmetric
hyperbolic system on their own. We also provide simple formulae for its
eigenvectors-eigenvalues pairs.


title: Thermal Hair of Quantum Black Hole

abstract:
  We investigate the possibility of statistical explanation of the black hole
entropy by counting quasi-bounded modes of thermal fluctuation in two
dimensional black hole spacetime. The black hole concerned is quantum in the
sense that it is in thermal equilibrium with its Hawking radiation. It is shown
that the fluctuation around such a black hole obeys a wave equation with a
potential whose peaks are located near the black hole and which is caused by
quantum effect. We can construct models in which the potential in the above
sense has several positive peaks and there are quai-bounded modes confined
between these peaks. This suggests that these modes contribute to the black
hole entropy. However it is shown that the entropy associated with these modes
dose not obey the ordinary area law. Therefore we can call these modes as an
additional thermal hair of the quantum black hole.


title: Hamiltonian formalism for the Oppenheimer-Snyder model

abstract:
  A family of effective actions in Hamiltonian form is derived for a
self-gravitating sphere of isotropic homogeneous dust. Starting from the
Einstein-Hilbert action for barotropic perfect fluids and making use of the
symmetry and equation of state of the matter distribution we obtain reduced
actions for two canonical variables, namely the radius of the sphere and its
ADM energy, the latter being conserved along trajectories of the former. These
actions differ by the value of the (conserved) geodesic energy of the radius of
the sphere which defines (disconnected) classes of solutions in correspondence
to the inner geometry and proper volume of the sphere. Each class is thus
treated as one constrained dynamical system and the union of all classes covers
the full phase space of the model. Generalization to the (inhomogeneous) Tolman
model is shown to be straightforward. Quantization is also discussed.


title: Covariant cosmological perturbation dynamics in the large-scale limit

abstract:
  Using the existence of a covariant conserved quantity on large perturbation
scales in a spatially flat perfect fluid or scalar field universe, we present a
general formula for gauge-invariantly defined comoving energy density
perturbations which encodes the entire linear perturbation dynamics in a closed
time integral. On this basis we discuss perturbation modes in different
cosmological epochs.


title: Regular spherical dust spacetimes

abstract:
  Physical (and weak) regularity conditions are used to determine and classify
all the possible types of spherically symmetric dust spacetimes in general
relativity. This work unifies and completes various earlier results. The
junction conditions are described for general non-comoving (and non-null)
surfaces, and the limits of kinematical quantities are given on all comoving
surfaces where there is Darmois matching. We show that an inhomogeneous
generalisation of the Kantowski-Sachs metric may be joined to the
Lemaitre-Tolman-Bondi metric. All the possible spacetimes are explicitly
divided into four groups according to topology, including a group in which the
spatial sections have the topology of a 3-torus. The recollapse conjecture (for
these spacetimes) follows naturally in this approach.


title: Sources of gravitational radiation and prospects for their detection

abstract:
  In the coming decade, the LIGO/VIRGO/GEO network of ground-based
kilometer-scale laser interferometer gravitational wave detectors will open up
a new astronomical window on the Universe: gravitational waves in the frequency
band 10 to 10^4 Hz. In addition, if the proposed, 5 million kilometer long,
space based interferometer LISA flies, another window will be opened in the
frequency band 10^(-4) to 1 Hz. I review the various possible sources that
might be detected in these frequency bands, and the information that might be
obtainable from observed sources. This article closely parallels my talk at the
GR15 conference, but is updated to include discussions of some recent
developments.


title: Exact isotropic cosmologies with local fractal number counts

abstract:
  We construct an exact relativistic cosmology in which an inhomogeneous but
isotropic local region has fractal number counts and matches to a homogeneous
background at a scale of the order of $10^2$ Mpc. We show that Einstein's
equations and the matching conditions imply either a nonlinear Hubble law or a
very low large-scale density.


title:
All-sky search algorithms for monochromatic signals in resonant bar GW
  detector data

abstract:
  In this paper we design and develop several filtering strategies for the
analysis of data generated by a resonant bar Gravitational Wave (GW) antenna,
with the goal to assess the presence (or absence) in them of long duration
monochromatic GW signals, as well as their eventual amplitude and frequency,
within the sensitivity band of the detector. Such signals are most likely
generated in the fast rotation of slightly asymmetric spinning stars. We shall
develop the practical procedures, together with the study of their statistical
properties, which will provide us with useful information on each technique's
performance. The selection of candidate events will then be established
according to threshold-crossing probabilities, based on the Neyman-Pearson
criterion. In particular, it will be shown that our approach, based on phase
estimation, presents better signal-to-noise ratio than the most common one of
pure spectral analysis.


title: Godel metric as a squashed anti-de Sitter geometry

abstract:
  We show that the non flat factor of the Godel metric belongs to a one
parameter family of 2+1 dimensional geometries that also includes the anti-de
Sitter metric. The elements of this family allow a generalization a la
Kaluza-Klein of the usual 3+1 dimensional Godel metric. Their lightcones can be
viewed as deformations of the anti-de Sitter ones, involving tilting and
squashing. This provides a simple geometric picture of the causal structure of
these space-times, anti-de Sitter geometry appearing as the boundary between
causally safe and causally pathological spaces. Furthermore, we construct a
global algebraic isometric embedding of these metrics in 4+3 or 3+4 dimensional
flat spaces, thereby illustrating in another way the occurrence of the closed
timelike curves.


title:
Generalized Alignment of Gravitational Intencities and Electromagnetic
  Strengths in Kerr-Newman Space-Time

abstract:
  It is shown that, in the case of Kerr-Newman space-time, the complex
electromagnetic strength E + i H and analogous complex intensity of the
gravitational field share the common complexified spatial direction.


title:
Currents and Superpotentials in classical gauge invariant theories I.
  Local results with applications to Perfect Fluids and General Relativity

abstract:
  E. Noether's general analysis of conservation laws has to be completed in a
Lagrangian theory with local gauge invariance. Bulk charges are replaced by
fluxes of superpotentials. Gauge invariant bulk charges may subsist when
distinguished one-dimensional subgroups are present. As a first illustration we
propose a new {\it Affine action} that reduces to General Relativity upon gauge
fixing the dilatation (Weyl 1918 like) part of the connection and elimination
of auxiliary fields. It allows a comparison of most gravity superpotentials and
we discuss their selection by the choice of boundary conditions. A second and
independent application is a geometrical reinterpretation of the convection of
vorticity in barotropic nonviscous fluids. We identify the one-dimensional
subgroups responsible for the bulk charges and thus propose an impulsive
forcing for creating or destroying selectively helicity. This is an example of
a new and general Forcing Rule.


title: Quantization of the Reissner-Nordstr\"{o}m Black Hole

abstract:
  The Reissner--Nordstr\"{o}m family of solutions can be understood to arise
from the spherically symmetric sector of a nonlinear SO(2,1)/SO(1,1) sigma
model coupled to three dimensional Euclidean gravity. In this context a group
theoretical quantization is performed. We identify the observables of the
theory and calculate their spectra.


title: Causality, Time Arrow and Half Cycling Universe

abstract:
  If one introduces causality into quantum cosmology, then the prescription for
the no-boundary universe should be revised. We show that the thermodymanic
arrow of time associated with the perturbation modes should be reversed at the
maximum expansion for the oscillating Hawking model. To an observer equipped
with the time arrow, the universe will terminate its evolution after an half
cycle.


title: $<\phi^2>$ in the Spacetime of a Cylindrical Black Hole

abstract:
  The renormalised value of $<\phi^2>$ is calculated for a massless,
conformally coupled scalar field in the Hartle-Hawking vacuum state. This
calculation is a first step towards the calculation of the gravitational back
reaction of the field in a black cosmic string spacetime which is
asymptotically anti-DeSitter and possesses a non constant dilaton field. It is
found that the field is divergence free throughout the spacetime and attains
its maximum value near the horizon.


title: It's all in GR: spinors, time, and gauge symmetry

abstract:
  This paper shows how to obtain the spinor field and dynamics from the
vielbein and geometry of General Relativity. The spinor field is physically
realized as an orthogonal transformation of the vielbein, and the spinor action
enters as the requirement that the unit time form be the gradient of a scalar
time field.


title: Cosmological Constraints from Primordial Black Holes

abstract:
  Primordial black holes may form in the early Universe, for example from the
collapse of large amplitude density perturbations predicted in some
inflationary models. Light black holes undergo Hawking evaporation, the energy
injection from which is constrained both at the epoch of nucleosynthesis and at
the present. The failure as yet to unambiguously detect primordial black holes
places important constraints. In this article, we are particularly concerned
with the dependence of these constraints on the model for the complete
cosmological history, from the time of formation to the present. Black holes
presently give the strongest constraint on the spectral index $n$ of density
perturbations, though this constraint does require $n$ to be constant over a
very wide range of scales.


title: Thermal conduction before relaxation in slowly rotating fluids

abstract:
  For slowly rotating fluids, we establish the existence of a critical point
similar to the one found for non-rotating systems. As the fluid approaches the
critical point, the effective inertial mass of any fluid element decreases,
vanishing at that point and changing of sign beyond it. This result implies
that first order perturbative method is not always reliable to study
dissipative processes ocurring before relaxation. Physical consequences that
might follow from this effect are commented.


title: The structure of singularities in inhomogeneous cosmological models

abstract:
  Recent progress in understanding the structure of cosmological singularities
is reviewed. The well-known picture due to Belinskii, Khalatnikov and Lifschitz
(BKL) is summarized briefly and it is discussed what existing analytical and
numerical results have to tell us about the validity of this picture. If the
BKL description is correct then most cosmological singularities are
complicated. However there are some cases where it predicts simple
singularities. These cases should be particularly amenable to mathematical
investigation and the results in this direction which have been achieved so far
are described.


title: World Spinors - Construction and Some Applications

abstract:
  The existence of a topological double-covering for the $GL(n,R)$ and
diffeomorphism groups is reviewed. These groups do not have finite-dimensional
faithful representations. An explicit construction and the classification of
all $\bar{SL}(n,R)$, $n=3,4$ unitary irreducible representations is presented.
Infinite-component spinorial and tensorial $\bar{SL}(4,R)$ fields,
"manifields", are introduced. Particle content of the ladder manifields, as
given by the $\bar{SL}(3,R)$ "little" group is determined. The manifields are
lifted to the corresponding world spinorial and tensorial manifields by making
use of generalized infinite-component frame fields. World manifields transform
w.r.t. corresponding $\bar{Diff}(4,R)$ representations, that are constructed
explicitly.


title: World Spinors Revisited

abstract:
  World spinors are objects that transform w.r.t. double covering group
$\bar{Diff}(4,R)$ of the Group of General Coordinate Transformations. The basic
mathematical results and the corresponding physical interpretation concerning
these, infinite-dimensional, spinorial representations are reviewed. The role
of groups $Diff(4,R)$, $GA(4,R)$, $GL(4,R)$, $SL(4,R)$, $SO(3,1)$ and the
corresponding covering groups is pointed out. New results on the infinite
dimensionality of spinorial representations, explicit construction of the
$\bar{SL}(4,R)$ representations in the basis of finite-dimensional non-unitary
$SL(2,C)$ representations, $SL(4,R)$ representation regrouping of tensorial and
spinorial fields of an arbitrary spin lagrangian field theory, as well as its
$SL(5,R)$ generalization in the case of infinite-component world spinor and
tensor field theories are presented.


title: Quantum Geometry and Black Holes

abstract:
  Non-perturbative quantum general relativity provides a possible framework to
analyze issues related to black hole thermodynamics from a fundamental
perspective. A pedagogical account of the recent developments in this area is
given. The emphasis is on the conceptual and structural issues rather than
technical subtleties. The article is addressed to post-graduate students and
beginning researchers.


title: Critical collapse of collisionless matter - a numerical investigation

abstract:
  In recent years the threshold of black hole formation in spherically
symmetric gravitational collapse has been studied for a variety of matter
models. In this paper the corresponding issue is investigated for a matter
model significantly different from those considered so far in this context. We
study the transition from dispersion to black hole formation in the collapse of
collisionless matter when the initial data is scaled. This is done by means of
a numerical code similar to those commonly used in plasma physics. The result
is that for the initial data for which the solutions were computed, most of the
matter falls into the black hole whenever a black hole is formed. This results
in a discontinuity in the mass of the black hole at the onset of black hole
formation.


title:
Cosmological constraints on primordial black holes produced in the
  near-critical gravitational collapse

abstract:
  The mass function of primordial black holes created through the near-critical
gravitational collapse is calculated in a manner fairly independent of the
statistical distribution of underlying density fluctuation, assuming that it
has a sharp peak on a specific scale. Comparing it with various cosmological
constraints on their mass spectrum, some newly excluded range is found in the
volume fraction of the region collapsing into black holes as a function of the
horizon mass.


title:
Multidimensional Cosmology for Intersecting p-Branes with Static
  Internal Spaces

abstract:
  Multidimensional cosmological model with static internal spaces describing
the evolution of an Einstein space of non-zero curvature and n internal spaces
is considered. The action contains several dilatonic scalar fields and
antisymmetric forms and lambda-term. When forms are chosen to be proportional
to volume forms of p-brane submanifolds of internal space manifold, the
Toda-like Lagrange representation arises. Exact solutions for the model are
obtained, when scale factors of internal spaces are constant. It is shown that
they are de Sitter or anti-de Sitter. Behaviour of cosmological constant and
its generation by p-branes is demonstrated.


title: Split structures in general relativity and the Kaluza-Klein theories

abstract:
  We construct a general approach to decomposition of the tangent bundle of
pseudo-Riemannian manifolds into direct sums of subbundles, and the associated
decomposition of geometric objects. An invariant structure {\cal H}^r defined
as a set of r projection operators is used to induce decomposition of the
geometric objects into those of the corresponding subbundles. We define the
main geometric objects characterizing decomposition. Invariant non-holonomic
generalizations of the Gauss-Codazzi-Ricci's relations have been obtained. All
the known types of decomposition (used in the theory of frames of reference, in
the Hamiltonian formulation for gravity, in the Cauchy problem, in the theory
of stationary spaces, and so on) follow from the present work as special cases
when fixing a basis and dimensions of subbundles, and parameterization of a
basis of decomposition. Various methods of decomposition have been applied here
for the Unified Multidimensional Kaluza-Klein Theory and for relativistic
configurations of a perfect fluid. Discussing an invariant form of the
equations of motion we have found the invariant equilibrium conditions and
their 3+1 decomposed form. The formulation of the conservation law for the curl
has been obtained in the invariant form.


title: Gravitational waves from hot young rapidly rotating neutron stars

abstract:
  Gravitational radiation drives an instability in the r-modes of young rapidly
rotating neutron stars. This instability is expected to carry away most of the
angular momentum of the star by gravitational radiation emission, leaving a
star rotating at about 100 Hz. In this paper we model in a simple way the
development of the instability and evolution of the neutron star during the
year-long spindown phase. This allows us to predict the general features of the
resulting gravitational waveform. We show that a neutron star formed in the
Virgo cluster could be detected by the LIGO and VIRGO gravitational wave
detectors when they reach their ``enhanced'' level of sensitivity, with an
amplitude signal-to-noise ratio that could be as large as about 8 if
near-optimal data analysis techniques are developed. We also analyze the
stochastic background of gravitational waves produced by the r-mode radiation
from neutron-star formation throughout the universe. Assuming a substantial
fraction of neutron stars are born with spin frequencies near their maximum
values, this stochastic background is shown to have an energy density of about
10^-9 of the cosmological closure density, in the range 20 Hz to 1 kHz. This
radiation should be detectable by ``advanced'' LIGO as well.


title:
Perturbations in the Kerr-Newman Dilatonic Black Hole Background:
  Maxwell Waves, the Dilaton Background and Gravitational Lensing

abstract:
  In this paper we continue the analysis of our previous papers and study the
affect of the existence of a non-trivial dilaton background on the propagation
of electromagnetic waves in the Kerr-Newman dilatonic black hole space-time.
For this purpose we again employ the double expansion in both the background
electric charge and the wave parameters of the relevant quantities in the
Newman-Penrose formalism and then identify the first order at which the dilaton
background enters the Maxwell equations. We then assume that gravitational and
dilatonic waves are negligible (at that order in the charge parameter) with
respect to electromagnetic waves and argue that this condition is consistent
with the solutions already found in the previous paper. Explicit expressions
are given for the asymptotic behavior of scattered waves, and a simple physical
model is proposed in order to test the effects. An expression for the relative
intensity is obtained for Reissner-Nordstrom dilaton black holes using
geometrical optics. A comparison with the approximation of geometrical optics
for Kerr-Newman dilaton black holes shows that at the order to which the
calculations are carried out gravitational lensing of optical images cannot
probe the dilaton background.


title: Gauge-Invariant Variables on Cosmological Hypersurfaces

abstract:
  We show how gauge-invariant cosmological perturbations may be constructed by
an unambiguous choice of hypersurface-orthogonal time-like vector field (i.e.,
time-slicing). This may be defined either in terms of the metric quantities
such as curvature or shear, or using some matter field. As an example, we show
how linear perturbations in the covariant fluid-flow approach can then be
presented in an explicitly gauge-invariant form in the coordinate based
formalism.


title:
Using strictly isospectral unbroken nonrelativistic supersymmetry as a
  toy model

abstract:
  I employ heuristically the strictly isospectral double Darboux method based
on the general superpotential of unbroken nonrelativistic supersymmetry
suggesting a few small steps of principle for extending its range of
applications toward relativistic (gauge) physics. The application of the method
to minisuperspace quantum cosmology is also briefly presented


title:
Construction of Highly Accurate Models of Rotating Neutron Stars -
  Comparison of Three Different Numerical Schemes

abstract:
  We conduct a direct comparison of three different representative numerical
codes for constructing models of rapidly rotating neutron stars in general
relativity. Our aim is to evaluate the accuracy of the codes and to investigate
how the accuracy is affected by the choice of interpolation, domain of
integration and equation of state. In all three codes, the same physical
parameters, equations of state and interpolation method are used. We construct
25 selected models for polytropic equations of state and 22 models with
realistic neutron star matter equations of state. The three codes agree well
with each other (typical agreement is better than 0.1% to 0.01%) for most
models, except for the extreme assumption of uniform density stars. We conclude
that the codes can be used for the construction of highly accurate initial data
configurations for polytropes of index N > 0.5 (which typically correspond to
realistic neutron stars), when the domain of integration includes all space and
for realistic equations with no phase transitions. With the exception of the
uniform density case, the obtained values of physical parameters for the models
considered in this paper can be regarded as ``standard'' and we display them in
detail for all models.


title: Conservation Laws for Cosmological Perurbations

abstract:
  We briefly recall the problem of defining conserved quantities such as energy
in general relativity, and the solution by introducing a symmetric background.
We apply the general formalism to perturbed Robertson-Walker spacetimes with de
Sitter geometry as background. We relate the obtained conserved quantities to
Traschen's integral constraints and mention a few applications in cosmology.


title: The gravitating $\sigma$ model in 2+1 dimensions: black hole solutions

abstract:
  We derive and discuss black-hole solutions to the gravitating O(3) $\sigma$
model in (2+1) dimensions. Three different kinds of static black holes are
found. One of these resembles the static BTZ black hole, another is completely
free of singularities, and the last type has the same Penrose diagram as the
(3+1)-dimensional Schwarzschild black hole. We also construct static and
dynamical multi-black hole systems.


title: The wave function discord

abstract:
  Linde's proposal of a Euclidean path integral with the ``wrong'' sign of
Euclidean action is often identified with the tunneling proposal for the wave
function of the universe. However, the two proposals are in fact quite
different. I illustrate the difference and point out that recent criticism by
Hawking and Turok does not apply to the tunneling proposal.


title: Three Dimensional Numerical Relativity with a Hyperbolic Formulation

abstract:
  We discuss a successful three-dimensional cartesian implementation of the
Bona-Mass\'o hyperbolic formulation of the 3+1 Einstein evolution equations in
numerical relativity. The numerical code, which we call ``Cactus,'' provides a
general framework for 3D numerical relativity, and can include various
formulations of the evolution equations, initial data sets, and analysis
modules. We show important code tests, including dynamically sliced flat space,
wave spacetimes, and black hole spacetimes. We discuss the numerical
convergence of each spacetime, and also compare results with previously tested
codes based on other formalisms, including the traditional ADM formalism. This
is the first time that a hyperbolic reformulation of Einstein's equations has
been shown appropriate for three-dimensional numerical relativity in a wide
variety of spacetimes.


title:
Recovering the effective cosmological constant in extended gravity
  theories

abstract:
  In the framework of extended gravity theories, we discuss the meaning of a
time dependent "cosmological constant" and give a set of conditions to recover
asymptotic de Sitter behaviour for a class of cosmological models independently
of initial data. To this purpose we introduce a time-dependent (effective)
quantity which asymptotically becomes the true cosmological constant. We will
deal with scalar-tensor, fourth and higher than fourth-order theories.


title:
Relations between three formalisms for irrotational binary neutron stars
  in general relativity

abstract:
  Various formalisms proposed recently for irrotational binary systems in
general relativity are compared and explicit relations between them are
exhibited. It is notably shown that the formalisms of (i) Teukolsky, (ii)
Shibata and (iii) Bonazzola et al. (as corrected by Asada) are equivalent, i.e.
yield exactly the same solution, although the former two are simpler than the
latter one.


title: On the Construction of Time-Symmetric Black Hole Initial Data

abstract:
  We review in a pedagogical fashion the 3+1-split which serves to put
Einstein's equations into the form of a dynamical system with constraints. We
then discuss the constraint equations under the simplifying assumption of
time-symmetry. Multi-Black-Hole data are presented and more explicitly
described in the case of two holes. The effect of different topologies is
emphasized.


title: On the quadratic action of the Hawking-Turok instanton

abstract:
  Positive definiteness of the quadratic part of the action of the
Hawking-Turok instanton is investigated. The Euclidean quadratic action for
scalar perturbations is expressed in terms of a single gauge invariant quantity
$q$. The mode functions satisfy a Schr\"odinger type equation with a potential
$U$. It is shown that the potential $U$ tends to a positive constant at the
regular end of the instanton. The detailed shape of $U$ depends on the initial
data of the instanton, on parameters of the background scalar field potential
$V$ and on a positive integer, $p$, labeling different spherical harmonics. For
certain well behaved scalar field potentials it is proven analytically that for
$p>1$ quadratic action is non-negative. For the lowest $p=1$ (homogeneous)
harmonic numerical solution of the Schr\"odinger equation for different scalar
field potentials $V$ and different initial data show that in some cases the
potential $U$ is negative in the intermediate region. We investigated the
monotonously growing potentials and a potential with a false vacuum. For the
monotonous potentials no negative modes are found about the Hawking-Turok
instanton. For a potential with the false vacuum the HT instanton is shown to
have a negative mode for certain initial data.


title: Computing the merger of black-hole binaries: the IBBH problem

abstract:
  Gravitational radiation arising from the inspiral and merger of binary black
holes (BBH's) is a promising candidate for detection by kilometer-scale
interferometric gravitational wave observatories. This paper discusses a
serious obstacle to searches for such radiation and to the interpretation of
any observed waves: the inability of current computational techniques to evolve
a BBH through its last ~10 orbits of inspiral (~100 radians of
gravitational-wave phase). A new set of numerical-relativity techniques is
proposed for solving this ``Intermediate Binary Black Hole'' (IBBH) problem:
(i) numerical evolutions performed in coordinates co-rotating with the BBH, in
which the metric coefficients evolve on the long timescale of inspiral, and
(ii) techniques for mathematically freezing out gravitational degrees of
freedom that are not excited by the waves.


title: Quantum corrections to critical phenomena in gravitational collapse

abstract:
  We investigate conformally coupled quantum matter fields on spherically
symmetric, continuously self-similar backgrounds. By exploiting the symmetry
associated with the self-similarity the general structure of the renormalized
quantum stress-energy tensor can be derived. As an immediate application we
consider a combination of classical, and quantum perturbations about exactly
critical collapse. Generalizing the standard argument which explains the
scaling law for black hole mass, $M \propto |\eta-\eta^*|^\beta$, we
demonstrate the existence of a quantum mass gap when the classical critical
exponent satisfies $\beta \geq 0.5$. When $\beta < 0.5$ our argument is
inconclusive; the semi-classical approximation breaks down in the spacetime
region of interest.


title: Event Horizons in Numerical Relativity II: Analyzing the Horizon

abstract:
  We present techniques and methods for analyzing the dynamics of event
horizons in numerically constructed spacetimes. There are three classes of
analytical tools we have investigated. The first class consists of proper
geometrical measures of the horizon which allow us comparison with perturbation
theory and powerful global theorems. The second class involves the location and
study of horizon generators. The third class includes the induced horizon
2-metric in the generator comoving coordinates and a set of membrane-paradigm
like quantities. Applications to several distorted, rotating, and colliding
black hole spacetimes are provided as examples of these techniques.


title: Kerr-like initial data for the collision of two locally Kerr holes

abstract:
  A technical flaw was found in this paper, and it has been withdrawn. It is
intended that a modified version will be submitted soon.


title:
Classical Euclidean general relativity from ``left-handed area =
  right-handed area"

abstract:
  A classical continuum theory corresponding to Barrett and Crane's model of
Euclidean quantum gravity is presented. The fields in this classical theory are
those of SO(4) BF theory, a simple topological theory of an so(4) valued 2-form
field, $B^{IJ}_{\m\n}$, and an so(4) connection. The left handed (self-dual)
and right handed (anti-self-dual) components of $B$ define a left handed and a,
generally distinct, right handed area for each spacetime 2-surface. The theory
being presented is obtained by adding to the BF action a Lagrange multiplier
term that enforces the constraint that the left handed and the right handed
areas be equal. It is shown that Euclidean general relativity (GR) forms a
sector of the resulting theory. The remaining three sectors of the theory are
also characterized and it is shown that, except in special cases, GR canonical
initial data is sufficient to specify the GR sector as well as a specific
solution within this sector.
  Finally, the path integral quantization of the theory is discussed at a
formal level and a hueristic argument is given suggesting that in the
semiclassical limit the path integral is dominated by solutions in one of the
non-GR sectors, which would mean that the theory quantized in this way is not a
quantization of GR.


title: Multipole solutions in metric--affine gravity

abstract:
  Above Planck energies, the spacetime might become non--Riemannian, as it is
known fron string theory and inflation. Then geometries arise in which
nonmetricity and torsion appear as field strengths, side by side with
curvature. By gauging the affine group, a metric affine gauge theory emerges as
dynamical framework. Here, by using the harmonic map ansatz, a new class of
multipole like solutions in the metric affine gravity theory (MAG) is obtained.


title:
On the area operators of the Husain-Kuchar-Rovelli model and
  Canonical/Loop Quantum Gravity

abstract:
  I investigate the relation between an operative definition of the area of a
surface specified by matter fields and the area operators recently introduced
in the canonical/loop approach to Quantum Gravity and in Rovelli's variant of
the Husain-Kuchar Quantum-Gravity toy model. The results suggest that the
discreteness of the spectra of the area operators might not be observable.


title: Cold Scalar-Tensor Black Holes: Causal Structure, Geodesics, Stability

abstract:
  We study the structure and stability of spherically symmetric Brans-Dicke
black-hole type solutions with an infinite horizon area and zero Hawking
temperature, existing for negative values of the coupling constant $\omega$.
These solutions split into two classes, depending on finite (B1) or infinite
(B2) proper time needed for an infalling particle to reach the horizon. Class
B1 metrics can be extended through the horizon only for discrete values of mass
and scalar charge, depending on two integers m and n. For even m-n, the
space-time is globally regular; for odd m, the metric changes its signature on
the horizon but remains Lorentzian. Geodesics are smoothly continued across the
horizon, but for odd m timelike geodesics become spacelike and vice versa.
Causality problems, arising in some cases, are discussed. Tidal forces are
shown to grow infinitely near type B1 horizons. All vacuum static, spherically
symmetric solutions of the Brans-Dicke theory with $\omega<-3/2$ are found to
be linearly stable against spherical perturbations. This result extends to the
generic case of the Bergmann-Wagoner class of scalar-tensor theories with the
coupling function $\omega(\phi) < -3/2$.


title:
How to Avoid Artificial Boundaries in the Numerical Calculation of Black
  Hole Spacetimes

abstract:
  This is the first of a series of papers describing a numerical implementation
of the conformally rescaled Einstein equation, an implementation designed to
calculate asymptotically flat spacetimes, especially spacetimes containing
black holes.
  In the present paper we derive the new first order time evolution equations
to be used in the scheme. These time evolution equations can either be written
in symmetric hyperbolic or in flux-conservative form. Since the conformally
rescaled Einstein equation, also called the conformal field equations, formally
allow us to place the grid boundaries outside the physical spacetime, we can
modify the equations near the grid boundaries and get a consistent and stable
discretisation. Even if we calculate spacetimes containing black holes, there
is no need for introducing artifical boundaries in the physical spacetime,
which then would complicate, influence, or even exclude the computation of
certain spacetime regions.


title: Interaction of Hawking radiation and a static electric charge

abstract:
  We investigate whether the equality found for the response of static scalar
sources interacting (i) with {\em Hawking radiation in Schwarzschild spacetime}
and (ii) with the Fulling-Davies-Unruh thermal bath in the Rindler wedge is
maintained in the case of electric charges. We find a finite result in the
Schwarzschild case, which is computed exactly, in contrast with the divergent
result associated with the infrared catastrophe in the Rindler case, i.e. in
the case of uniformly accelerated charges in Minkowski spacetime. Thus, the
equality found for scalar sources does not hold for electric charges.


title:
Boson Stars with Large Self-interaction in (2+1) dimensions: an Exact
  Solution

abstract:
  An exact solution for a nonrotating boson star in (2+1) dimensional gravity
with a negative cosmological constant is found. The relations among mass,
particle number, and radius of the (2+1) dimensional boson star are shown.


title: Computer algebra in gravity: Programs for (non-)Riemannian spacetimes. I

abstract:
  Computer algebra programs are presented for application in general
relativity, in electrodynamics, and in gauge theories of gravity. The
mathematical formalism used is the calculus of exterior differential forms, the
computer algebra system applied Hearn's Reduce with Schruefer's exterior form
package Excalc. As a non-trivial example we discuss a metric of Plebanski &
Demianski (of Petrov type D) together with an electromagnetic potential and a
triplet of post-Riemannian one-forms. This whole geometrical construct
represents an exact solution of a metric-affine gauge theory of gravity. We
describe a sample session and verify by computer that this exact solution
fulfills the appropriate field equations.-- Computer programs are described for
the irreducible decomposition of (non-Riemannian) curvature, torsion, and
nonmetricity.


title:
A possibility of emission of high frequency gravitational radiation from
  d-wave to s-wave type superconductor junctions

abstract:
  Recent measurements on a class of high-Tc superconductors (HTSC) have shown
that Cooper-pairs binding may be associated to a d-wave, while in another
class, d and s waves may coexist. When d-wave Cooper-pairs are injected in a
superconductor that can sustain s-wave binding, d-wave pairs decay to s-wave
pairs and energy is irradiated by means of gravitons. We show that in s-wave to
d-wave type superconductor (SDS) junctions in an equilibrium condition no net
gravitational wave energy is emitted, on the other hand under non equilibrium
conditions a net gravitational wave energy is emitted by the junction.
Experiments which show a gravitational interaction between inomogeneus high-Tc
superconductors, under non equilibrium conditions, and test objects may be
understood by accepting a possibility of emission of gravitational radiation
from SDS junctions.


title: A Non-Singular Black Hole

abstract:
  We present a completely integrable deformation of the CGHS dilaton gravity
model in two dimensions. The solution is a singularity free black hole that at
large distances asymptoticaly joins to the CGHS solution.


title: so(4) Plebanski Action and Relativistic Spin Foam Model

abstract:
  In this note we study the correspondence between the ``relativistic spin
foam'' model introduced by Barrett, Crane and Baez and the so(4) Plebanski
action. We argue that the $so(4)$ Plebanski model is the continuum analog of
the relativistic spin foam model. We prove that the Plebanski action possess
four phases, one of which is gravity and outline the discrepancy between this
model and the model of Euclidean gravity. We also show that the Plebanski model
possess another natural dicretisation and can be associate with another, new,
spin foam model that appear to be the $so(4)$ counterpart of the spin foam
model describing the self dual formulation of gravity.


title:
Approaches to the Monopole-Dynamic Dipole Vacuum Solution Concerning the
  Structure of its Ernst's Potential on the Symmetry Axis

abstract:
  The FHP algorithm allows to obtain the relativistic multipole moments of a
vacuum stationary axisymmetric solution in terms of coefficients which appear
in the expansion of its Ernst's potential on the symmetry axis. First of all,
we will use this result in order to determine, at a certain approximation
degree, the Ernst's potential on the symmetry axis of the metric whose only
multipole moments are mass and angular momentum.
  By using Sibgatullin's method we analyse a series of exacts solutions with
the afore mentioned multipole characteristic. Besides, we present an
approximate solution whose Ernst's potential is introduced as a power series of
a dimensionless parameter. The calculation of its multipole moments allows us
to understand the existing differences between both approximations to the
proposed pure multipole solution.


title: GRAIL, an omni-directional gravitational wave detector

abstract:
  A cryogenic spherical and omni-directional resonant-mass detector proposed by
the GRAIL collaboration is described.


title:
Cylindrically symmetric spinning Brans-Dicke spacetimes with closed
  timelike curves

abstract:
  We present here three new solutions of Brans-Dicke theory for a stationary
geometry with cylindrical symmetry in the presence of matter in rigid rotation
with $T^\mu_\mu\neq 0$. All the solutions have eternal closed timelike curves
in some region of the spacetime, the size of which depends on $\omega$.
Moreover, two of them do not go over a solution of general relativity in the
limit $\omega \to \infty$.


title:
Initial data and the end state of spherically symmetric gravitational
  collapse

abstract:
  Generalizing earlier results on the initial data and the final fate of dust
collapse, we study here the relevance of the initial state of a spherically
symmetric matter cloud towards determining its end state in the course of a
continuing gravitational collapse. It is shown that given an arbitrary regular
distribution of matter at the initial epoch, there always exists an evolution
from this initial data which would result either in a black hole or a naked
singularity depending on the allowed choice of free functions available in the
solution. It follows that given any initial density and pressure profiles for
the cloud, there is a non-zero measure set of configurations leading either to
black holes or naked singularities, subject to the usual energy conditions
ensuring the positivity of energy density. We also characterize here wide new
families of black hole solutions resulting from spherically symmetric collapse
without requiring the cosmic censorship assumption.


title: Seeds of large-scale anisotropy in string cosmology

abstract:
  Pre-big bang cosmology predicts tiny first-order dilaton and metric
perturbations at very large scales. Here we discuss the possibility that other
-- more copiously generated -- perturbations may act, at second order, as
scalar seeds of large-scale structure and CMB anisotropies. We study, in
particular, the cases of electromagnetic and axionic seeds. We compute the
stochastic fluctuations of their energy-momentum tensor and determine the
resulting contributions to the multipole expansion of the temperature
anisotropy. In the axion case it is possible to obtain a flat or slightly
tilted blue spectrum that fits present data consistently, both for massless and
for massive (but very light) axions.


title: Supersymmetric Vacuum Configurations in String Cosmology

abstract:
  We examine in a cosmological context the conditions for unbroken
supersymmetry in N=1 supergravity in D=10 dimensions. We show that the
cosmological solutions of the equations of motion obtained considering only the
bosonic sector correspond to vacuum states with spontaneous supersymmetry
breaking. With a non vanishing gravitino-dilatino condensate we find a solution
of the equations of motion that satisfies necessary conditions for unbroken
supersymmetry and that smoothly interpolates between Minkowski space and
DeSitter space with a linearly growing dilaton, thus providing a possible
example of a supersymmetric and non-singular pre-big-bang cosmology.


title: Cosmic Gravitational Wave Background in String Cosmology

abstract:
  String cosmology models predict a stochastic cosmic background of
gravitational waves with a characteristic spectrum. I describe the background,
present astrophysical and cosmological bounds on it, and outline how it may be
possible to detect it with gravitational wave detectors.


title:
Gravitational field and equations of motion of compact binaries to 5/2
  post-Newtonian order

abstract:
  We derive the gravitational field and equations of motion of compact binary
systems up to the 5/2 post-Newtonian approximation of general relativity (where
radiation-reaction effects first appear). The approximate post-Newtonian
gravitational field might be used in the problem of initial conditions for the
numerical evolution of binary black-hole space-times. On the other hand we
recover the Damour-Deruelle 2.5PN equations of motion of compact binary
systems. Our method is based on an expression of the post-Newtonian metric
valid for general (continuous) fluids. We substitute into the fluid metric the
standard stress-energy tensor appropriate for a system of two point-like
particles. We remove systematically the infinite self-field of each particle by
means of the Hadamard partie finie regularization.


title:
Toda chains with type A_m Lie algebra for multidimensional m-component
  perfect fluid cosmology

abstract:
  We consider a D-dimensional cosmological model describing an evolution of
Ricci-flat factor spaces, M_1,...M_n (n > 2), in the presence of an m-component
perfect fluid source (n > m > 1). We find characteristic vectors, related to
the matter constants in the barotropic equations of state for fluid components
of all factor spaces.
  We show that, in the case where we can interpret these vectors as the root
vectors of a Lie algebra of Cartan type A_m=sl(m+1,C), the model reduces to the
classical open m-body Toda chain.
  Using an elegant technique by Anderson (J. Math. Phys. 37 (1996) 1349) for
solving this system, we integrate the Einstein equations for the model and
present the metric in a Kasner-like form.


title: Extended Inflation with an Exponential Potential

abstract:
  In this paper we investigate extended inflation with an exponential potential
$V(\sigma)= V_0 e^{-\kappa\sigma}$, which provides a simple cosmological
scenario where the distribution of the constants of Nature is mostly determined
by $\kappa$. In particular, we show that this theory predicts a uniform
distribution for the Planck mass at the end of inflation, for the entire
ensemble of universes that undergo stochastic inflation. Eternal inflation
takes place in this scenario for a broad family of initial conditions, all of
which lead up to the same value of the Planck mass at the end of inflation. The
predicted value of the Planck mass is consistent with the observed value within
a comfortable range of values of the parameters involved.


title:
On a Possibility of Phase Transitions in the Geometric Structure of
  Space-Time

abstract:
  It is shown that space-time may be not only in a state which is described by
Riemann geometry but also in states which are described by Finsler geometry.
Transitions between various metric states of space-time have the meaning of
phase transitions in its geometric structure. These transitions together with
the evolution of each of the possible metric states make up the general picture
of space-time manifold dynamics.


title: One-loop corrections for Schwarzschild black hole via 2D dilaton gravity

abstract:
  We study quantum corrections for the Schwarzshild black hole by considering
it as a vacuum solution of a 2D dilaton gravity theory obtained by spherical
reduction of 4D gravity coupled with matter. We find perturbatively the vacuum
solution for the standard one-loop effective action in the case of null-dust
matter and in the case of minimally coupled scalar field. The corresponding
state is in both cases 2D Hartle-Hawking vacuum, and we evaluate the
corresponding quantum corrections for the thermodynamical parameters of the
black hole. We also find that the standard effective action does not allow
boundary conditions corresponding to a 4D Hartle-Hawking vacuum state.


title: The "phenomenological" spacetime and quantization

abstract:
  In order to obtain a well defined quantum gravity we define the spacetime in
relation to the "phenomenology" of the physical interactions; however we shall
to speculate with this "in General". Besides, we comment the reasons that give
to the gravitational field a privilegied situation over the others.


title:
Evidence for an oscillatory singularity in generic U(1) symmetric
  cosmologies on $T^3 \times R$

abstract:
  A longstanding conjecture by Belinskii, Lifshitz, and Khalatnikov that the
singularity in generic gravitational collapse is locally oscillatory is tested
numerically in vacuum, U(1) symmetric cosmological spacetimes on $T^3 \times
R$. If the velocity term dominated (VTD) solution to Einstein's equations is
substituted into the Hamiltonian for the full Einstein evolution equations, one
term is found to grow exponentially. This generates a prediction that
oscillatory behavior involving this term and another (which the VTD solution
causes to decay exponentially) should be observed in the approach to the
singularity. Numerical simulations strongly support this prediction.


title: Inflation and Nonsingular Spacetimes of Cosmic Strings

abstract:
  Inflation of cosmic gauge and global strings is investigated by numerically
solving the combined Einstein and field equations. Above some critical
symmetry-breaking scales, the strings undergo inflation along the radial
direction as well as the axial direction at the core. The nonsingular nature of
the spacetimes around supercritical gauge and global strings is discussed and
contrasted to the singular static solutions that have been discussed in the
literature.


title:
Scattering of Straight Cosmic Strings by Black Holes: Weak Field
  Approximation

abstract:
  The scattering of a straight, infinitely long string moving with velocity $v$
by a black hole is considered. We analyze the weak-field case, where the impact
parameter ($b_{imp}$) is large, and obtain exact solutions to the equations of
motion. As a result of scattering, the string is displaced in the direction
perpendicular to the velocity by an amount $\Delta b\sim -2\pi GMv\gamma/c^3
-\pi (GM)^2/ (4c^3 v b_{imp})$, where $\gamma=(1-(v/c)^2)^{-1/2}$. The second
term dominates at low velocities $v/c<(GM/b_{imp})^{1/2}$ . The late-time
solution is represented by a kink and anti-kink, propagating in opposite
directions at the speed of light, and leaving behind them the string in a new
``phase''. The solutions are applied to the problem of string capture, and are
compared to numerical results.


title:
Gravitational Capture and Scattering of Straight Test Strings with Large
  Impact Parameters

abstract:
  The capture or scattering of an initially straight infinite test cosmic
string by a Kerr-Newman black hole, or by any other small source of an
electrovac gravitational field, is analyzed analytically when the string moves
with initial velocity v and large impact parameter b >> M so that the string
stays very nearly straight (except during the final capture process, if that
occurs, or except far behind the gravitating object, if b is not much greater
than the energy of the object in the frame of the string). The critical impact
parameter for capture at low velocities is shown to be
[(pi/2)(M^2-Q^2)/v]^{1/2}. For all larger b, the displacement of the string
from the plane of the gravitating object after the scattering approaches the
final value [b^2 - (pi/2)(M^2-Q^2)/v]^{1/2} - 2 pi M v/(1-v^2)^{1/2}, for any
v, so long as b >> M.


title:
Construction of N = 2 Chiral Supergravity Compatible with the Reality
  Condition

abstract:
  We construct N = 2 chiral supergravity (SUGRA) which leads to Ashtekar's
canonical formulation. The supersymmetry (SUSY) transformation parameters are
not constrained at all and auxiliary fields are not required in contrast with
the method of the two-form gravity. We also show that our formulation is
compatible with the reality condition, and that its real section is reduced to
the usual N = 2 SUGRA up to an imaginary boundary term.


title: Sources for the Majumdar-Papapetrou Space-Times

abstract:
  Einstein's field equations are solved exactly for static charged dust
distributions. These solutions generalize the Majumdar Papapetrou metrics.
Maxwell's equations lead to the equality of charge and mass densities of the
dust distribution. Einstein's equatins reduce to a nonlinear version of
Poisson's equation.


title: Superluminal travel requires negative energies

abstract:
  I investigate the relationship between faster-than-light travel and
weak-energy-condition violation, i.e., negative energy densities. In a general
spacetime it is difficult to define faster-than-light travel, and I give an
example of a metric which appears to allow superluminal travel, but in fact is
just flat space. To avoid such difficulties, I propose a definition of
superluminal travel which requires that the path to be traveled reach a
destination surface at an earlier time than any neighboring path. With this
definition (and assuming the generic condition) I prove that superluminal
travel requires weak-energy-condition violation.


title:
Thermodynamics of Reissner-Nordstrom-anti-de Sitter black holes in the
  grand canonical ensemble

abstract:
  The thermodynamical properties of the Reissner-Nordstr\"om-anti-de Sitter
black hole in the grand canonical ensemble are investigated using York's
formalism. The black hole is enclosed in a cavity with finite radius where the
temperature and electrostatic potential are fixed. The boundary conditions
allow us to compute the relevant thermodynamical quantities, e.g. thermal
energy, entropy and charge. The stability conditions imply that there are
thermodynamically stable black hole solutions, under certain conditions.
Instantons with negative heat capacity are also found.


title: Plane torsion waves in quadratic gravitational theories

abstract:
  The definition of the Riemann-Cartan space of the plane wave type is given.
The condition under which the torsion plane waves exist is found. It is
expressed in the form of the restriction imposed on the coupling constants of
the 10-parametric quadratic gravitational Lagrangian. In the mathematical
appendix the formula for commutator of the variation operator and Hodge
operator is proved. This formula is applied for the variational procedure when
the gravitational field equations are obtained in terms of the exterior
differential forms.


title: Gravitational instantons admit hyper-K\"ahler structure

abstract:
  We construct the explicit form of three almost complex structures that a
Riemannian manifold with self-dual curvature admits and show that their
Nijenhuis tensors vanish so that they are integrable. This proves that
gravitational instantons with self-dual curvature admit hyper-K\"{a}hler
structure. In order to arrive at the three vector valued 1-forms defining
almost complex structure, we give a spinor description of real 4-dimensional
Riemanian manifolds with Euclidean signature in terms of two independent sets
of 2-component spinors. This is a version of the original Newman-Penrose
formalism that is appropriate to the discussion of the mathematical, as well as
physical properties of gravitational instantons. We shall build on the work of
Goldblatt who first developed an NP formalism for gravitational instantons but
we shall adopt it to differential forms in the NP basis to make the formalism
much more compact. We shall show that the spin coefficients, connection 1-form,
curvature 2-form, Ricci and Bianchi identities, as well as the Maxwell
equations naturally split up into their self-dual and anti-self-dual parts
corresponding to the two independent spin frames. We shall give the complex
dyad as well as the spinor formulation of the almost complex structures and
show that they reappear under the guise of a triad basis for the Petrov
classification of gravitational instantons. Completing the work of Salamon on
hyper-K\"ahler structure, we show that the vanishing of the Nijenhuis tensor
for all three almost complex structures depends on the choice of a self-dual
gauge for the connection which is guaranteed by virtue of the fact that the
curvature 2-form is self-dual for gravitational instantons.


title: Isotropic cosmology in metric-affine gauge theory of gravity

abstract:
  Geometrical structure of homogeneous isotropic models in the frame of the
metric-affine gauge theory of gravity (MAGT) is analyzed. By using general form
of gravitational Lagrangian including both a scalar curvature and various
invariants quadratic in the curvature, torsion and nonmetricity tensors,
gravitational equations of MAGT for homogeneous isotropic models are deduced.
It is shown, that obtained gravitational equations lead to generalized
cosmological Friedmann equation for the metrics by certain restrictions on
indefinite parameters of gravitational Lagrangian. Isotropic models in the
Weyl-cartan space-time are discussed.


title: The late-time singularity inside non-spherical black holes

abstract:
  It was long believed that the singularity inside a realistic, rotating black
hole must be spacelike. However, studies of the internal geometry of black
holes indicate a more complicated structure is typical. While it seems likely
that an observer falling into a black hole with the collapsing star encounters
a crushing spacelike singularity, an observer falling in at late times
generally reaches a null singularity which is vastly different in character to
the standard Belinsky, Khalatnikov and Lifschitz (BKL) spacelike singularity.
In the spirit of the classic work of BKL we present an asymptotic analysis of
the null singularity inside a realistic black hole. Motivated by current
understanding of spherical models, we argue that the Einstein equations reduce
to a simple form in the neighborhood of the null singularity. The main results
arising from this approach are demonstrated using an almost plane symmetric
model. The analysis shows that the null singularity results from the blueshift
of the late-time gravitational wave tail; the amplitude of these gravitational
waves is taken to decay as an inverse power of advanced time as suggested by
perturbation theory. The divergence of the Weyl curvature at the null
singularity is dominated by the propagating modes of the gravitational field.
The null singularity is weak in the sense that tidal distortion remains bounded
along timelike geodesics crossing the Cauchy horizon. These results are in
agreement with previous analyses of black hole interiors. We briefly discuss
some outstanding problems which must be resolved before the picture of the
generic black hole interior is complete.


title: Quantum Tests of the Foundations of General Relativity

abstract:
  The role of the equivalence principle in the context of non-relativistic
quantum mechanics and matter wave interferometry, especially atom beam
interferometry, will be discussed. A generalised form of the weak equivalence
principle which is capable of covering quantum phenomena too, will be proposed.
It is shown that this generalised equivalence principle is valid for matter
wave interferometry and for the dynamics of expectation values. In addition,
the use of this equivalence principle makes it possible to determine the
structure of the interaction of quantum systems with gravitational and inertial
fields. It is also shown that the path of the mean value of the position
operator in the case of gravitational interaction does fulfill this generalised
equivalence principle.


title: Thiemann transform for gravity with matter fields

abstract:
  The generalised Wick transform discovered by Thiemann provides a
well-established relation between the Euclidean and Lorentzian theories of
general relativity. We extend this Thiemann transform to the Ashtekar
formulation for gravity coupled with spin-1/2 fermions, a non-Abelian
Yang-Mills field, and a scalar field. It is proved that, on functions of the
gravitational and matter phase space variables, the Thiemann transform is
equivalent to the composition of an inverse Wick rotation and a constant
complex scale transformation of all fields. This result holds as well for
functions that depend on the shift vector, the lapse function, and the Lagrange
multipliers of the Yang-Mills and gravitational Gauss constraints, provided
that the Wick rotation is implemented by means of an analytic continuation of
the lapse. In this way, the Thiemann transform is furnished with a geometric
interpretation. Finally, we confirm the expectation that the generator of the
Thiemann transform can be determined just from the spin of the fields and give
a simple explanation for this fact.


title: Black Holes with Scalar Hair in (2+1) dimensions

abstract:
  Nonrotating and rotating black hole soltuions in (2+1) dimensions are studied
in a model including a real scalar field with a simple potential coupled to
gravity.


title: Rotating Stars in Relativity

abstract:
  Rotating relativistic stars are receiving significant attention in recent
years, because of the information they can yield about the equation of state of
matter at extremely high densities and because they are one of the more
possible sources of detectable gravitational waves. We review the latest
theoretical and numerical methods for modeling rotating relativistic stars,
including stars with a strong magnetic field and hot proto-neutron stars. We
also review nonaxisymmetric oscillations and instabilities in rotating stars
and summarize the latest developments regarding the gravitational wave-driven
(CFS) instability in both polar and axial quasi-normal modes.


title: Quantum Inflaton Dynamics

abstract:
  We show that the quantum dynamics of a real scalar field for a large class of
potentials in the symmetric Gaussian state, where the nonperturbative quantum
contributions are taken into account, can be described equivalently by a
two-dimensional nonlinear dynamical system with a definite angular momentum
(U(1) charge of a complex theory). It is found that the Gaussian state with a
nearly minimal uncertainty and a large quantum fluctuation, as an initial
condition, naturally explains the most of the essential features of the early
stage of the inflationary Universe.


title:
Emergence of classicality for primordial fluctuations: Concepts and
  analogies

abstract:
  We clarify the way in which cosmological perturbations of quantum origin,
produced during inflation, assume classical properties. Two features play an
important role in this process: First, the dynamics of fluctuations which are
presently on large cosmological scales leads to a very peculiar state (highly
squeezed) that is indistinguishable, in a precise sense, from a classical
stochastic process. This holds for almost all initial quantum states. Second,
the process of decoherence by interaction with the environment distinguishes
the field amplitude basis as a robust pointer basis. We discuss in detail the
interplay between these features and use simple analogies such as the free
quantum particle to illustrate the main conceptual issues.


title: Black Hole Solution of Quantum Gravity

abstract:
  We present a spherically symmetric and static exact solution of Quantum
Einstein Equations. This solution is asymptotically (for large $r$) identical
with the black hole solution on the anti--De Sitter background and, for some
range of values of the mass possesses two horizons. We investigate
thermodynamical properties of this solution.


title:
Damping of Gravitational Waves and Density Perturbations in the Early
  Universe

abstract:
  Since the discovery of the large angular scale anisotropies in the microwave
background radiation, the behaviour of cosmological perturbations (especially,
density perturbations and gravitational waves) has been of great interest. In
this study, after a detailed and rigorous treatment of the behaviour of
gravitational waves in viscous cosmic media, we conclude that the damping of
cosmological gravitational waves of long wavelengths is negligible for most
cases of physical interest. A preliminary analysis suggests that similar
results hold for density perturbations in the long wavelength limit. Therefore,
long wavelength cosmological perturbations have not been practically affected
by viscous processes,and are good probes of the very early Universe.


title: Axistationary perfect fluids -- a tetrad approach

abstract:
  Stationary axisymmetric perfect fluid space-times are investigated using the
curvature description of geometries. Attention is focused on space-times with a
vanishing electric part of the Weyl tensor. It is shown that the only
incompressible axistationary magnetic perfect fluid is the interior
Schwarzschild solution. The existence of a rigidly rotating perfect fluid,
generalizing the interior Schwarzschild metric is proven. Theorems are stated
on Petrov types and electric/magnetic Weyl tensors.


title: Kaluza-Klein Gravity

abstract:
  We review higher-dimensional unified theories from the general relativity,
rather than the particle physics side. Three distinct approaches to the subject
are identified and contrasted: compactified, projective and noncompactified. We
discuss the cosmological and astrophysical implications of extra dimensions,
and conclude that none of the three approaches can be ruled out on
observational grounds at the present time.


title: Dynamic wormholes

abstract:
  A new framework is proposed for general dynamic wormholes, unifying them with
black holes. Both are generically defined locally by outer trapping horizons,
temporal for wormholes and spatial or null for black and white holes. Thus
wormhole horizons are two-way traversible, while black-hole and white-hole
horizons are only one-way traversible. It follows from the Einstein equation
that the null energy condition is violated everywhere on a generic wormhole
horizon. It is suggested that quantum inequalities constraining negative energy
break down at such horizons. Wormhole dynamics can be developed as for
black-hole dynamics, including a reversed second law and a first law involving
a definition of wormhole surface gravity. Since the causal nature of a horizon
can change, being spatial under positive energy and temporal under sufficient
negative energy, black holes and wormholes are interconvertible. In particular,
if a wormhole's negative-energy source fails, it may collapse into a black
hole. Conversely, irradiating a black-hole horizon with negative energy could
convert it into a wormhole horizon. This also suggests a possible final state
of black-hole evaporation: a stationary wormhole. The new framework allows a
fully dynamical description of the operation of a wormhole for practical
transport, including the back-reaction of the transported matter on the
wormhole. As an example of a matter model, a Klein-Gordon field with negative
gravitational coupling is a source for a static wormhole of Morris & Thorne.


title: Conservation Laws and Cosmological Perturbations in Curved Universes

abstract:
  When working in synchronous gauges, pseudo-tensor conservation laws are often
used to set the initial conditions for cosmological scalar perturbations, when
those are generated by topological defects which suddenly appear in an up to
then perfectly homogeneous and isotropic universe. However those conservation
laws are restricted to spatially flat (K=0) Friedmann-Lema\^\i tre spacetimes.
In this paper, we first show that in fact they implement a matching condition
between the pre- and post- transition eras and, in doing so, we are able to
generalize them and set the initial conditions for all $K$. Finally, in the
long wavelength limit, we encode them into a vector conservation law having a
well-defined geometrical meaning.


title: A note on the iterative approach to twisting type N solutions

abstract:
  We present the results of the computation of a twisting type N solution to
vacuum Einstein equations following an iterative approach. Our results show
that the higher order terms fail to provide a full exact solution with
non-vanishing twist. Nevertheless, our fourth-order solution still represents a
regular and twisting type N solution.


title: Comments on the Non-Commutative Description of Classical Gravity

abstract:
  We find a one-parameter family of Lagrangian descriptions for classical
general relativity in terms of tetrads which are not c-numbers. Rather, they
obey exotic commutation relations. These noncommutative properties drop out in
the metric sector of the theory, where the Christoffel symbols and the Riemann
tensor are ordinary commuting objects and they are given by the usual
expression in terms of the metric tensor. Although the metric tensor is not a
c-number, we argue that all measurements one can make in this theory are
associated with c-numbers, and thus that the common invariant sector of our
one--parameter family of deformed gauge theories (for the case of zero torsion)
is physically equivalent to Einstein's general relativity.


title: Initial Data and Coordinates for Multiple Black Hole Systems

abstract:
  We present here an alternative approach to data setting for spacetimes with
multiple moving black holes generalizing the Kerr-Schild form for rotating or
non-rotating single black holes to multiple moving holes. Because this scheme
preserves the Kerr-Schild form near the holes, it selects out the behaviour of
null rays near the holes, may simplify horizon tracking, and may prove useful
in computational applications. For computational evolution, a discussion of
coordinates (lapse function and shift vector) is given which preserves some of
the properties of the single-hole Kerr-Schild form.


title: Bounds on negative energy densities in flat spacetime

abstract:
  We generalise results of Ford and Roman which place lower bounds -- known as
quantum inequalities -- on the renormalised energy density of a quantum field
averaged against a choice of sampling function. Ford and Roman derived their
results for a specific non-compactly supported sampling function; here we use a
different argument to obtain quantum inequalities for a class of smooth, even
and non-negative sampling functions which are either compactly supported or
decay rapidly at infinity. Our results hold in $d$-dimensional Minkowski space
($d\ge 2$) for the free real scalar field of mass $m\ge 0$. We discuss various
features of our bounds in 2 and 4 dimensions. In particular, for massless field
theory in 2-dimensional Minkowski space, we show that our quantum inequality is
weaker than Flanagan's optimal bound by a factor of 3/2.


title: Gravitational light bending in Euclidean space

abstract:
  Both the non-homogeneous slowness of electromagnetic waves in gravitational
fields and the frequency red shift contribute to the gravitational light
bending. This twofold contribution explains the measured deflection of light
rays by the Sun under Euclidean geometry of space.


title:
Stability of coalescing binary stars against gravitational collapse:
  hydrodynamical simulations

abstract:
  We perform simulations of relativistic binary stars in post-Newtonian gravity
to investigate their dynamical stability prior to merger against gravitational
collapse in a tidal field. In general, our equations are only strictly accurate
to first post-Newtonian order, but they recover full general relativity for
spherical, static stars. We study both corotational and irrotational binary
configurations of identical stars in circular orbits. We adopt a soft,
adiabatic equation of state with $\Gamma = 1.4$, for which the onset of
instability occurs at a sufficiently small value of the compaction $M/R$ that a
post-Newtonian approximation is quite accurate. For such a soft equation of
state there is no innermost stable circular orbit, so that we can study
arbitrarily close binaries. This choice still allows us to study all the
qualitative features exhibited by any adiabatic equation of state regarding
stability against gravitational collapse. We demonstrate that, independent of
the internal stellar velocity profile, the tidal field from a binary companion
stabilizes a star against gravitational collapse.


title: Null Surfaces and Legendre Submanifolds

abstract:
  It is shown that the main variable Z of the Null Surface Formulation of GR is
the generating function of a constrained Lagrange submanifold that lives on the
energy surface H=0 and that its level surfaces Z=const. are Legendre
submanifolds on that energy surface.
  The behaviour of the variable Z at the caustic points is analysed and a
genralization of this variable is discussed.


title: A Spinning Anti-de Sitter Wormhole

abstract:
  We construct a 2+1 dimensional spacetime of constant curvature whose spatial
topology is that of a torus with one asymptotic region attached. It is also a
black hole whose event horizon spins with respect to infinity. An observer
entering the hole necessarily ends up at a "singularity"; there are no inner
horizons.
  In the construction we take the quotient of 2+1 dimensional anti-de Sitter
space by a discrete group Gamma. A key part of the analysis proceeds by
studying the action of Gamma on the boundary of the spacetime.


title: Photon redshift and the appearance of a naked singularity

abstract:
  In this paper we analyze the redshift as observed by an external observer
receiving photons which terminate in the past at the naked singularity formed
in a Tolman-Bondi dust collapse. Within the context of models considered here
it is shown that photons emitted from a weak curvature naked singularity are
always finitely redshifted to an external observer. Certain cases of strong
curvature naked singularities, including the self-similar one, where the
photons are infinitely redshifted are also pointed out.


title:
Dimensional reduction and gauge group reduction in Bianchi-Type
  cosmology

abstract:
  In this paper we examine in detail the implementation, with its associated
difficulties, of the Killing conditions and gauge fixing into the variational
principle formulation of Bianchi-Type cosmologies. We address problems raised
in the literature concerning the Lagrangian and the Hamiltonian formulations:
We prove their equivalence, make clear the role of the Homogeneity Preserving
Diffeomorphisms in the phase space approach, and show that the number of
physical degrees of freedom is the same in the Hamiltonian and Lagrangian
formulations. Residual gauge transformations play an important role in our
approach, and we suggest that Poincar\'e transformations for special
relativistic systems can be understood as residual gauge transformations. In
Appendices, we give the general computation of the equations of motion and the
Lagrangian for any Bianchi-Type vacuum metric and for spatially homogeneous
Maxwell fields in a nondynamical background (with zero currents). We also
illustrate our counting of degrees of freedom in an Appendix.


title:
How to reduce the suspension thermal noise in LIGO without improving the
  Q's of the pendulum and violin modes

abstract:
  The suspension noise in interferometric gravitational wave detectors is
caused by losses at the top and the bottom attachments of each suspension
fiber. We use the Fluctuation-Dissipation theorem to argue that by careful
positioning of the laser beam spot on the mirror face it is possible to reduce
the contribution of the bottom attachment point to the suspension noise by
several orders of magnitude. For example, for the initial and enhanced LIGO
design parameters (i.e. mirror masses and sizes, and suspension fibers' lengths
and diameters) we predict a reduction of $\sim 100$ in the "bottom" spectral
density throughout the band $35-100\hbox{Hz}$ of serious thermal noise. We then
propose a readout scheme which suppresses the suspension noise contribution of
the top attachment point. The idea is to monitor an averaged horizontal
displacement of the fiber of length $ l$; this allows one to record the
contribution of the top attachment point to the suspension noise, and later
subtract it it from the interferometer readout. For enhanced LIGO this would
allow a suppression factor about 100 in spectral density of suspension thermal
noise.


title: Exact inflationary solutions

abstract:
  We present a new class of exact inflationary solutions for the evolution of a
universe with spatial curvature, filled with a perfect fluid, a scalar field
with potential $V_{\pm}(\phi)=\lambda(\phi^2\pm\delta^2)^2$ and a cosmological
constant $\Lambda$. With the $V_+(\phi)$ potential and a negative cosmological
constant, the scale factor experiments a graceful exit.
  We give a brief discussion about the physical meaning of the solutions.


title: Schwarzschild Space-Time in Gauge Theories of Gravity

abstract:
  In Poincar\'e gauge theory of gravity and in $\overline{\mbox{Poincar\'e}}$
gauge theory of gravity, we give the necessary and sufficient condition in
order that the Schwarzschild space-time expressed in terms of the Schwarzschild
coordinates is obtainable as a torsionless exact solution of gravitational
field equations with a spinless point-like source having the energy-momentum
density $\widetilde{\mbox{\boldmath $T$}}_\mu^{~\nu}(x) = - Mc^2
\delta_\mu^{~0} \delta_0^{~\nu} \delta^{(3)}(\mbox{\boldmath $x$})$. Further,
for the case when this condition is satisfied, the energy-momentum and the
angular momentum of the Schwarzschild space-time are examined in their
relations to the asymptotic forms of vierbein fields. We show, among other
things, that asymptotic forms of vierbeins are restricted by requiring the
equality of the active gravitational mass and the inertial mass. Conversely
speaking, this equality is violated for a class of vierbeins giving the
Schwarzschild metric.


title: From topological to parametrized field theory

abstract:
  It has been proposed to study the theory resulting from setting the
gravitational constant to zero in the first order formalism for general
relativity. In this letter we investigate this theory in the presence of matter
fields, establish its equivalence with parametrized field theory on a flat
background, and relate it to previous results in topological field theory (BF
theory).


title: A dynamical symmetry breaking model in Weyl space

abstract:
  The dynamical process following the breaking of Weyl geometry to Riemannian
geometry is considered by studying the motion of de Sitter bubbles in a Weyl
vacuum. The bubbles are given in terms of an exact, spherically symmetric thin
shell solution to the Einstein equations in a Weyl-Dirac theory with a
time-dependent scalar field of the form beta = f(t)/r. The dynamical solutions
obtained lead to a number of possible applications. An important feature of the
thin shell model is the manner in which beta provides a connection between the
interior and exterior geometries since information about the exterior geometry
is contained in the boundary conditions for beta.


title: Dissipative Collapse Through the Critical Point

abstract:
  A relativistic model of a heat conducting collapsing star, which includes
thermal pre-relaxation processes, is presented. Particular attention is paid to
the influence of a given parameter defined in terms of thermodynamic variables,
on the outcome of evolution. Evaluation of the system when passing through a
critical value of the aforesaid parameter, does not yield evidence of anomalous
behaviour.


title:
Quantum Inequality Restrictions on Negative Energy Densities in Curved
  Spacetimes

abstract:
  In quantum field theory, there exist states in which the expectation value of
the energy density for a quantized field is negative. These negative energy
densities lead to many problems. Although quantum field theory introduces
negative energies, it also provides constraints in the form of quantum
inequalities (QI's). These uncertainty principle-type relations limit the
magnitude and duration of any negative energy. We derive a general form of the
QI on the energy density for both the quantized scalar and electromagnetic
fields in static curved spacetimes. In the case of the scalar field, the QI can
be written as the Euclidean wave operator acting on the Euclidean Green's
function. Additionally, a small distance expansion on the Green's function is
used to derive the QI in the short sampling time limit. It is found that the QI
in this limit reduces to the flat space form with subdominant correction terms
which depend on the spacetime geometry.
  Several example spacetimes are studied in which exact forms of the QI's can
be found. These include the three- and four-dimensional static Robertson-Walker
spacetimes, flat space with perfectly reflecting mirrors, Rindler and static de
Sitter space, and the spacetime outside a black hole. Finally, the application
of the quantum inequalities to the Alcubierre warp drive spacetime leads to
strict constraints on the thickness of the negative energy region needed to
maintain the warp drive. Under these constraints, we discover that the total
negative energy required exceeds the total mass of the visible universe by a
hundred billion times.


title: An Ever Expanding Universe?

abstract:
  It is pointed out that very recent results based on supernovae observations
that the universe will accelerate and expand for ever with ever decreasing
density have been predicted in a recent cosmological model which also deduces
hitherto purely empirical features like the mysterious relation between the
pion mass and the Hubble Constant.


title: Comments on entanglement entropy

abstract:
  A new interpretation of entanglement entropy is proposed: entanglement
entropy of a pure state with respect to a division of a Hilbert space into two
subspaces 1 and 2 is an amount of information, which can be transmitted through
1 and 2 from a system interacting with 1 to another system interacting with 2.
The transmission medium is quantum entanglement between 1 and 2. In order to
support the interpretation, suggestive arguments are given: variational
principles in entanglement thermodynamics and quantum teleportation. It is
shown that a quantum state having maximal entanglement entropy plays an
important role in quantum teleportation. Hence, the entanglement entropy is, in
some sense, an index of efficiency of quantum teleportation. Finally,
implications for the information loss problem and Hawking radiation are
discussed.


title: Two-boundary problems in Euclidean quantum gravity

abstract:
  Recent work in the literature has studied a new set of local boundary
conditions for the quantized gravitational field, where the spatial components
of metric perturbations, and ghost modes, are subject to Robin boundary
conditions, whereas normal components of metric perturbations obey Dirichlet
boundary conditions. Such boundary conditions are here applied to evaluate the
one-loop divergence on a portion of flat Euclidean four-space bounded by two
concentric three-spheres.


title:
A point mass in an isotropic universe. Existence, uniqueness and basic
  properties

abstract:
  Criteria which a space-time must satisfy to represent a point mass embedded
in an open Robertson--Walker (RW) universe are given. It is shown that
McVittie's solution in the case $k=0$ satisfies these criteria, but does not in
the case $k=-1$. Existence of a solution for the case $k=-1$ is proven and its
representation in terms of an elliptic integral is given. The following
properties of this and McVittie's $k=0$ solution are studied; uniqueness,
behaviour at future null infinity, recovery of the RW and Schwarzschild limits,
compliance with energy conditions and the occurence of singularities. Existence
of solutions representing more general spherical objects embedded in a RW
universe is also proven.


title: Symmetric hyperbolic systems for Bianchi equations

abstract:
  We obtain a family of first-order symmetric hyperbolic systems for the
Bianchi equations. They have only physical characteristics: the light cone and
timelike hypersurfaces. In the proof of the hyperbolicity, new positivity
properties of the Bel tensor are used.


title: Multiply Unstable Black Hole Critical Solutions

abstract:
  The gravitational collapse of a complex scalar field in the harmonic map is
modeled in spherical symmetry. Previous work has shown that a change of
stability of the attracting critical solution occurs in parameter space from
the discretely self-similarity critical (DSS) solution originally found by
Choptuik to the continuously self-similar (CSS) solution found by Hirschmann
and Eardley. In the region of parameter space in which the DSS is the
attractor, a family of initial data is found which finds the CSS as its
critical solution despite the fact that it has more than one unstable mode. An
explanation of this is proposed in analogy to families that find the DSS in the
region where the CSS is the attractor.


title: Scalar-Tensor Cosmologies and their Late Time Evolution

abstract:
  We study the asymptotic behavior at late times of Friedmann-Robertson-Walker
(uniform density) cosmological models within scalar-tensor theories of gravity.
Particularly, we analyze the late time behavior in the present (matter
dominated) epoch of the universe. The result of Damour and Nordtvedt that for a
massless scalar in a flat cosmology the Universe evolves towards a state
indistinguishable from general relativity is generalized. We first study a
massless scalar field in an open universe. It is found that, while the universe
tends to approach a state with less scalar contribution to gravity, the
attractor mechanism is not effective enough to drive the theory towards a final
state indistinguishable from general relativity. For the self-interacting case
it is found that the scalar field potential dominates the late time behavior.
In most cases this makes the attractor mechanism effective, thus resulting in a
theory of gravity with vanishingly small scalar contribution even for the open
Universe.


title: Disturbing the Black Hole

abstract:
  I describe some examples in support of the conjecture that the horizon area
of a near equilibrium black hole is an adiabatic invariant. These include a
Schwarzschild black hole perturbed by quasistatic scalar fields (which may be
minimally or nonminimally coupled to curvature), a Kerr black under the
influence of scalar radiation at the superradiance treshold, and a
Reissner--Nordstr\"om black hole absorbing a charge marginally. These clarify
somewhat the conditions under which the conjecture would be true. The desired
``adiabatic theorem'' provides an important motivation for a scheme for black
hole quantization.


title: String Dilaton Fluid Cosmology

abstract:
  We investigate $(n+1)$-dimensional string-dilaton cosmology with effective
dilaton potential in presence of perfect-fluid matter.We get exact solutions
parametrized by the constant $\gam$ of the state equation $p=(\gam-1)\rho$, the
spatial dimension number $n$, the bulk of matter, and the spatial curvature
constant $k$. Several interesting cosmological behaviours are selected. Finally
we discuss the recovering of ordinary Einstein gravity starting from string
dominated regime and a sort of asymptotic freedom due to string effective
coupling.


title: Evidence for the adiabatic invariance of the black hole horizon area

abstract:
  Some examples in support of the conjecture that the horizon area of a near
equilibrium black hole is an adiabatic invariant are described. These clarify
somewhat the conditions under which the conjecture would be true.


title: Cosmological perturbations in singularity-free, deflationary models

abstract:
  We consider scalar perturbations of energy-density for a class of
cosmological models where an early phase of accelerated expansion evolves,
without any fine-tuning for graceful exit, towards the standard Friedman eras
of observed universe. The quantum geometric procedure which generates such
models agrees with results for string cosmology since it works if dynamics is
dominated by a primordial fluid of extended massive objects. The main result is
that characteristic scales of cosmological interest, connected with the
extension of such early objects, are selected.


title: Discrete approaches to quantum gravity in four dimensions

abstract:
  The construction of a consistent theory of quantum gravity is a problem in
theoretical physics that has so far defied all attempts at resolution. One
ansatz to try to obtain a non-trivial quantum theory proceeds via a
discretization of space-time and the Einstein action. I review here three major
areas of research: gauge-theoretic approaches, both in a path-integral and a
Hamiltonian formulation, quantum Regge calculus, and the method of dynamical
triangulations, confining attention to work that is strictly four-dimensional,
strictly discrete, and strictly quantum in nature.


title:
Non-Existence of Time-Periodic Solutions of the Dirac Equation in a
  Reissner-Nordstrom Black Hole Background

abstract:
  It is shown analytically that the Dirac equation has no normalizable,
time-periodic solutions in a Reissner-Nordstrom black hole background; in
particular, there are no static solutions of the Dirac equation in such a
background field. The physical interpretation is that Dirac particles can
either disappear into the black hole or escape to infinity, but they cannot
stay on a periodic orbit around the black hole.


title: Trapped surfaces in prolate collapse in the Gibbons-Penrose construction

abstract:
  We investigate existence and properties of trapped surfaces in two models of
collapsing null dust shells within the Gibbons-Penrose construction. In the
first model, the shell is initially a prolate spheroid, and the resulting
singularity forms at the ends first (relative to a natural time slicing by flat
hyperplanes), in analogy with behavior found in certain prolate collapse
examples considered by Shapiro and Teukolsky. We give an explicit example in
which trapped surfaces are present on the shell, but none exist prior to the
last flat slice, thereby explicitly showing that the absence of trapped
surfaces on a particular, natural slicing does not imply an absence of trapped
surfaces in the spacetime. We then examine a model considered by Barrabes,
Israel and Letelier (BIL) of a cylindrical shell of mass M and length L, with
hemispherical endcaps of mass m. We obtain a "phase diagram" for the presence
of trapped surfaces on the shell with respect to essential parameters $\lambda
\equiv M/L$ and $\mu \equiv m/M$. It is found that no trapped surfaces are
present on the shell when $\lambda$ or $\mu$ are sufficiently small. (We are
able only to search for trapped surfaces lying on the shell itself.) In the
limit $\lambda \to 0$, the existence or nonexistence of trapped surfaces lying
within the shell is seen to be in remarkably good accord with the hoop
conjecture.


title: About Duality and Killing tensors

abstract:
  In this paper the isometries of the dual space were investigated. The dual
structural equations of a Killing tensor of order two were found . The flat
space case was analyzed in details.


title: Symmetries of the dual metrics

abstract:
  In this paper the symmetries of the dual manifold were investigated. We found
the conditions when the manifold and its dual admit the same Killing vectors
and Killing-Yano tensors. In the case of an Einstein's metric $g_{\mu\nu}$ the
corresponding equations for its dual were found. The examples of Kerr-Newman
geometry and the separable coordinates in 1+1 dimensions were analyzed in
details.


title: Killing-Yano tensors and Nambu tensors

abstract:
  We present the conditions when a Killing-Yano tensor becomes a Nambu tensor.
We have shown that in the flat space case all Killing-Yano tensors are Nambu
tensors.
  In the case of Taub-NUT metric and Kerr-Newmann metric we found that a
Killing-Yano tensor of order two generate a Nambu tensor of order three.


title: Physical nature of the central singularity in spherical collapse

abstract:
  We examine here the nature of the central singularity forming in the
spherically symmetric collapse of a dust cloud and it is shown that this is
always a strong curvature singularity where gravitational tidal forces diverge
powerfully. An important consequence is that the nature of the naked
singularity forming in the dust collapse turns out to be stable against the
perturbations in the initial data from which the collapse commences.


title:
Gravitational Wave Radiation from Compact Binary Systems in the
  Jordan-Brans-Dicke Theory

abstract:
  In this paper we analyze the signal emitted by a compact binary system in the
Jordan-Brans-Dicke theory. We compute the scalar and tensor components of the
power radiated by the source and study the scalar waveform. Eventually we
consider the detectability of the scalar component of the radiation by
interferometers and resonant-mass detectors.


title: The omega-tends-to-infinity limit of Brans-Dicke theory

abstract:
  The standard tenet that Brans-Dicke theory reduces to general relativity in
the omega-tends-to-infinity limit has been shown to be false when the trace of
the matter energy-momentum tensor vanishes. The issue is clarified in a new
approach and the asymptotic behaviour of the Brans-Dicke scalar is rigorously
derived.


title:
Aspects and applications of quantum field theory on spaces with conical
  singularities

abstract:
  One-loop effects for quantum fields living on manifolds containing conical
singularities are investigated in the context of cosmic string background and
of finite-temperature theory in the Rindler wedge or outside the horizon of a
Schwarzschild black-hole.