title:
Classical and Quantum Thermodynamics of horizons in spherically
  symmetric spacetimes

abstract:
  A general formalism for understanding the thermodynamics of horizons in
spherically symmetric spacetimes is developed. The formalism reproduces known
results in the case of black hole spacetimes. But its power lies in being able
to handle more general situations like: (i) spacetimes which are not
asymptotically flat (like the de Sitter spacetime) and (ii) spacetimes with
multiple horizons having different temperatures (like the Schwarzschild-de
Sitter spacetime) and provide a consistent interpretation for temperature,
entropy and energy. I show that it is possible to write Einstein's equations
for a spherically symmetric spacetime in the form $TdS-dE=PdV$ near {\it any}
horizon of radius $a$ with $S=(1/4)(4\pi a^2), |E| = (a/2)$ and the temperature
$T$ determined from the surface gravity at the horizon. The pressure $P$ is
provided by the source of the Einstein's equations and $dV$ is the change in
the volume when the horizon is displaced infinitesimally. The same results can
be obtained by evaluating the quantum mechanical partition function {\it
without using Einstein's equations or WKB approximation for the action}. Both
the classical and quantum analysis provide a simple and consistent
interpretation of entropy and energy for de Sitter spacetime as well as for
$(1+2)$ dimensional gravity. For the Rindler spacetime the entropy per unit
transverse area turns out to be $(1/4)$ while the energy is zero. The approach
also shows that the de Sitter horizon -- like the Schwarzschild horizon -- is
effectively one dimensional as far as the flow of information is concerned,
while the Schwarzschild-de Sitter, Reissner-Nordstrom horizons are not. The
implications for spacetimes with multiple horizons are discussed.


title: Why do we observe a small but non zero cosmological constant ?

abstract:
  The current observations seem to suggest that the universe has a positive
cosmological constant of the order of $H_0^2$ while the most natural value for
the cosmological constant will be $L_P^{-2}$ where $L_P = (G\hbar/c^3)^{1/2}$
is the Planck length. This reduction of the cosmological constant from
$L_P^{-2} $ to $L_P^{-2}(L_PH_0)^2$ may be interpreted as due to the ability of
quantum micro structure of spacetime to readjust itself and absorb bulk vacuum
energy densities. Being a quantum mechanical process, such a cancellation
cannot be exact and the residual quantum fluctuations appear as the ``small''
cosmological constant. I describe the features of a toy model for the spacetime
micro structure which could allow for the bulk vacuum energy densities to be
canceled leaving behind a small residual value of the the correct magnitude.
Some other models (like the ones based on canonical ensemble for the four
volume or quantum fluctuations of the horizon size) lead to an insignificantly
small value of $H_0^2(L_PH_0)^n$ with $n=0.5-1$ showing that obtaining the
correct order of magnitude for the residual fluctuations in the cosmological
constant is a nontrivial task, becaue of the existence of the small
dimensionless number $H_0L_P$ .


title:
Cardy-Verlinde Entropy Formula in the Presence of a General Cosmological
  State Equation

abstract:
  As recently found by Youm [hep-th/0201268], the entropy of the universe will
no longer be expressible in the conventional Cardy-Verlinde form if one relaxes
the radiation dominance state equation and instead assumes a more general
equation of the form p=(\gamma-1)\rho, with \gamma a constant. We show that
Youm's generalized entropy formula remains valid when the cosmic fluid is no
longer ideal, but is allowed to possess a constant bulk viscosity \zeta. We
supply our analysis with some numerical estimates, thus calculating the scale
factor a(t) for a k=0 universe, and also calculate via a perturbative expansion
in \zeta the magnitude of the viscosity-induced correction to the scale factor
if the universe is radiation dominated.


title: The quantum physics of chronology protection

abstract:
  This is a brief survey of the current status of Stephen Hawking's
``chronology protection conjecture''. That is: ``Why does nature abhor a time
machine?'' I'll discuss a few examples of spacetimes containing ``time
machines'' (closed causal curves), the sorts of peculiarities that arise, and
the reactions of the physics community. While pointing out other possibilities,
this article concentrates on the possibility of ``chronology protection''. As
Stephen puts it:
  ``It seems that there is a Chronology Protection Agency which prevents the
appearance of closed timelike curves and so makes the universe safe for
historians.''


title: Cosmic Force

abstract:
  The determination of the geodesics in Scharzschild black hole and de Sitter
space.


title: Generalized Chiral Membrane Dynamics

abstract:
  We develop the dynamics of the chiral superconducting membranes(with null
current) in an alternative geometrical approach. Besides of this, we show the
equivalence of the resulting description to the one known Dirac-Nambu-Goto
(DNG) case. Integrability for chiral string model is obtained using a proposed
light-cone gauge. In a similar way, domain walls are integrated by means of a
simple ansatz.


title: Towards gravitating discs around stationary black holes

abstract:
  This article outlines the search for an exact general relativistic
description of the exterior (vacuum) gravitational field of a rotating
spheroidal black hole surrounded by a realistic axially symmetric disc of
matter. The problem of multi-body stationary spacetimes is first exposed from
the perspective of the relativity theory and astrophysics, listing the basic
methods employed and results obtained. Then, basic formulas for stationary
axisymmetric solutions are summarized. Remaining sections review what we have
learnt with Miroslav Zacek and Tomas Zellerin about certain static and
stationary situations recently. Although the survey part is quite general, the
list of references cannot be complete. Our main desideratum was the informative
value rather than originality -- novelties have been preferred, mainly reviews
and those with detailed introductions.


title:
Taking Kaluza seriously leads to a non-gauge-invariant electromagnetic
  theory in a curved space-time

abstract:
  Kaluza's mertic with the cylinder condition is considered without the weak
gravitational field approximation. It is shown that these hypoteses lead to a
non-gauge-invariant electromagnetic theory in a curved space-time. The problem
of electro-gravitational unification is considered from this point of view.


title: New Observational Bounds to Quantum Gravity Signals

abstract:
  We consider a new set of effects arising from the quantum gravity corrections
to the propagation of fields, associated with fluctuations of the spacetime
geometry. Using already existing experimental data, we can put bounds on these
effects that are more stringent by several orders of magnitude than those
expected to be obtained in astrophysical observations. In fact these results
can be already interpreted as questioning the whole scenario of linear (in
$l_P$) corrections to the dispersion relations for free fields in Lorentz
violating theories.


title: Fresnel analysis of the wave propagation in nonlinear electrodynamics

abstract:
  We study the wave propagation in nonlinear electrodynamical models.
Particular attention is paid to the derivation and the analysis of the Fresnel
equation for the wave covectors. For the class of general nonlinear Lagrangian
models, we demonstrate how the originally quartic Fresnel equation factorizes,
yielding the generic birefringence effect. We show that the closure of the
effective constitutive (or jump) tensor is necessary and sufficient for the
absence of birefringence, i.e., for the existence of a unique light cone
structure. As another application of the Fresnel approach, we analyze the light
propagation in a moving isotropic nonlinear medium. The corresponding effective
constitutive tensor contains non-trivial skewon and axion pieces. For
nonmagnetic matter, we find that birefringence is induced by the nonlinearity,
and derive the corresponding optical metrics.


title: Black hole entropy without brick walls

abstract:
  The properties of the thermal radiation are discussed by using the new
equation of state density motivated by the generalized uncertainty relation in
the quantum gravity. There is no burst at the last stage of the emission of a
Schwarzshild black hole. When the new equation of state density is utilized to
investigate the entropy of a scalar field outside the horizon of a static black
hole, the divergence appearing in the brick wall model is removed, without any
cutoff. The entropy proportional to the horizon area is derived from the
contribution of the vicinity of the horizon.


title:
New solutions of Einstein equations in spherical symmetry: the Cosmic
  Censor to the court

abstract:
  A new class of solutions of the Einstein field equations in spherical
symmetry is found. The new solutions are mathematically described as the
metrics admitting separation of variables in area-radius coordinates.
Physically, they describe the gravitational collapse of a class of anisotropic
elastic materials. Standard requirements of physical acceptability are
satisfied, in particular, existence of an equation of state in closed form,
weak energy condition, and existence of a regular Cauchy surface at which the
collapse begins. The matter properties are generic in the sense that both the
radial and the tangential stresses are non vanishing, and the kinematical
properties are generic as well, since shear, expansion, and acceleration are
also non-vanishing. As a test-bed for cosmic censorship, the nature of the
future singularity forming at the center is analyzed as an existence problem
for o.d.e. at a singular point using techniques based on comparison theorems,
and the spectrum of endstates - blackholes or naked singularities - is found in
full generality. Consequences of these results on the Cosmic Censorship
conjecture are discussed.


title: Vacuum polarization in the spacetime of charged nonlinear black hole

abstract:
  Building on general formulas obtained from the approximate renormalized
effective action, the approximate stress-energy tensor of the quantized massive
scalar field with arbitrary curvature coupling in the spacetime of charged
black hole being a solution of coupled equations of nonlinear electrodynamics
and general relativity is constructed and analysed. It is shown that in a few
limiting cases, the analytical expressions relating obtained tensor to the
general renormalized stress-energy tensor evaluated in the geometry of the
Reissner-Nordstr\"{o}m black hole could be derived. A detailed numerical
analysis with special emphasis put on the minimal coupling is presented and the
results are compared with those obtained earlier for the conformally coupled
field. Some novel features of the renormalized stress-energy tensor are
discussed.


title:
The Dynamics of Test Particles and Pointlike Gyroscopes in the Brane
  World and Other 5D Models

abstract:
  We study the dynamics of test particles and pointlike gyroscopes in 5D
manifolds like those used in the Randall-Sundrum brane world and non-compact
Kaluza-Klein models. Our analysis is based on a covariant foliation of the
manifold using 3+1 dimensional spacetime slices orthogonal to the extra
dimension, and is hence similar to the ADM 3+1 split in ordinary general
relativity. We derive gauge invariant equations of motion for freely-falling
test particles in the 5D and 4D affine parameterizations and contrast these
results with previous work concerning the so-called ``fifth force''. Motivated
by the conjectured localization of matter fields on a 3-brane, we derive the
form of the classical non-gravitational force required to confine particles to
a 4D hypersurface and show that the resulting trajectories are geometrically
identical to the spacetime geodesics of Einstein's theory. We then discuss the
issue of determining the 5D dynamics of a torque-free spinning body in the
point-dipole approximation, and then perform a covariant (3+1)+1 decomposition
of the relevant formulae (i.e. the 5D Fermi-Walker transport equation) for the
cases of freely-falling and hypersurface-confined point gyroscopes. In both
cases, the 4D spin tensor is seen to be subject to an anomalous torque. We
solve the spin equations for a gyroscope confined to a single spacetime section
in a simple 5D cosmological model and observe a cosmological variation of the
magnitude and orientation of the 4D spin.


title: On the origin of the cosmological constant

abstract:
  Under the assumption that the cosmological constant vanishes in the true
ground state with lowest possible energy density, we argue that the observed
small but finite vacuum-like energy density can be explained if we consider a
theory with two or more degenerate perturbative vacua, which are unstable due
to quantum tunneling, and if we still live in one of such states. An example is
given making use of the topological vacua in non-Abelian gauge theories.


title: (In)finiteness of Spherically Symmetric Static Perfect Fluids

abstract:
  This work is concerned with the finiteness problem for static, spherically
symmetric perfect fluids in both Newtonian Gravity and General Relativity. We
derive criteria on the barotropic equation of state guaranteeing that the
corresponding perfect fluid solutions possess finite/infinite extent. In the
Newtonian case, for the large class of monotonic equations of state, and in
General Relativity we improve earlier results.


title:
Energy and angular momentum of the gravitational field in the
  teleparallel geometry

abstract:
  The Hamiltonian formulation of the teleparallel equivalent of general
relativity is considered. Definitions of energy, momentum and angular momentum
of the gravitational field arise from the integral form of the constraint
equations of the theory. In particular, the gravitational energy-momentum is
given by the integral of scalar densities over a three-dimensional spacelike
hypersurface. The definition for the gravitational energy is investigated in
the context of the Kerr black hole. In the evaluation of the energy contained
within the external event horizon of the Kerr black hole we obtain a value
strikingly close to the irreducible mass of the latter. The gravitational
angular momentum is evaluated for the gravitational field of a thin, slowly
rotating mass shell.


title:
Naked singularities in cylindrical collapse of counter-rotating dust
  shells

abstract:
  Solutions describing the gravitational collapse of asymptotically flat
cylindrical and prolate shells of (null) dust are shown to admit globally naked
singularities.


title: Criteria for (in)finite extent of static perfect fluids

abstract:
  In Newton's and in Einstein's theory we give criteria on the equation of
state of a barotropic perfect fluid which guarantee that the corresponding
one-parameter family of static, spherically symmetric solutions has finite
extent. These criteria are closely related to ones which are known to ensure
finite or infinite extent of the fluid region if the assumption of spherical
symmetry is replaced by certain asymptotic falloff conditions on the solutions.
We improve this result by relaxing the asymptotic asumptions. Our conditions on
the equation of state are also related to (but less restrictive than) ones
under which it has been shown in Relativity that static, asymptotically flat
fluid solutions are spherically symmetric. We present all these results in a
unified way.


title: Weak energy condition violation and superluminal travel

abstract:
  Recent solutions to the Einstein Field Equations involving negative energy
densities, i.e., matter violating the weak-energy-condition, have been
obtained, namely traversable wormholes, the Alcubierre warp drive and the
Krasnikov tube. These solutions are related to superluminal travel, although
locally the speed of light is not surpassed. It is difficult to define
faster-than-light travel in generic space-times, and one can construct metrics
which apparently allow superluminal travel, but are in fact flat Minkowski
space-times. Therefore, to avoid these difficulties it is important to provide
an appropriate definition of superluminal travel.


title:
No Go Theorem for Kinematic Self-Similarity with A Polytropic Equation
  of State

abstract:
  We have investigated spherically symmetric spacetimes which contain a perfect
fluid obeying the polytropic equation of state and admit a kinematic
self-similar vector of the second kind which is neither parallel nor orthogonal
to the fluid flow. We have assumed two kinds of polytropic equations of state
and shown in general relativity that such spacetimes must be vacuum.


title: New Cosmological Singularities in Braneworld Models

abstract:
  Higher-dimensional braneworld models which contain both bulk and brane
curvature terms in the action admit cosmological singularities of rather
unusual form and nature. These `quiescent' singularities, which can occur both
during the contracting as well as the expanding phase, are characterised by the
fact that while the matter density and Hubble parameter remain finite, all
higher derivatives of the scale factor ($\stackrel{..}{a}$, $\stackrel{...}{a}$
etc.) diverge as the cosmological singularity is approached. The singularities
are the result of the embedding of the (3+1)-dimensional brane in the bulk and
can exist even in an empty homogeneous and isotropic (FRW) universe. The
possibility that the present universe may expand into a singular state is
discussed.


title: Higher Dimensional Metrics of Colliding Gravitational Plane Waves

abstract:
  We give a higher even dimensional extension of vacuum colliding gravitational
plane waves with the combinations of collinear and non-collinear polarized
four-dimensional metric. The singularity structure of space-time depends on the
parameters of the solution.


title:
Vacuum branes in D-dimensional static spacetimes with spatial symmetry
  IO(D-2), O(D-1) or O+(D-2,1)

abstract:
  In this paper, we give a complete classification of vacuum branes, i.e.,
everywhere umbilical time-like hypersurfaces whose extrinsic curvature is a
constant multiple of the induced metric, K_mn=k g_mn, in D-dimensional static
spacetimes with spatial symmetry G(D-2,K), where G(n,K) is the isometry group
of an n-dimensional space with constant sectional curvature K. D>=4 is assumed.
It is shown that all possible configurations of a brane are invariant under an
isometry subgroup G(D-3,K') for some K'>= K. In particular, configurations of a
brane with non-zero k are always G(D-2,K) invariant, except for those in five
special one-parameter families of spacetimes. Further, such G(D-2,K)-invariant
configurations are allowed only in spacetimes whose Ricci tensors are isotropic
in the two planes orthogonal to each G(D-2,K)-orbit, or for special values of
k, which do not exist in generic cases. On the basis of these results, we prove
the non-existence of a vacuum brane with black hole geometry in static bulk
spacetimes with spatial symmetry G(D-2,K). We also discuss mathematical
implications of these results.


title:
Problems and Successes in the Numerical Approach to the Conformal Field
  Equations

abstract:
  This talk reports on the status of an approach to the numerical study of
isolated systems with the conformal field equations. We first describe the
algorithms used in a code which has been developed at AEI in the last years,
and discuss a milestone result obtained by Huebner. Then we present more recent
results as examples to sketch the problems we face in the conformal approach to
numerical relativity and outline a possible roadmap toward making this approach
a practical tool.


title: On Gowdy vacuum spacetimes

abstract:
  By Fuchsian techniques, a large family of Gowdy vacuum spacetimes have been
constructed for which one has detailed control over the asymptotic behaviour.
In this paper we formulate a condition on initial data yielding the same form
of asymptotics.


title:
Comment about quasi-isotropic solution of Einstein equations near
  cosmological singularity

abstract:
  We generalize for the case of arbitrary hydrodynamical matter the
quasi-isotropic solution of Einstein equations near cosmological singularity,
found by Lifshitz and Khalatnikov in 1960 for the case of radiation-dominated
universe. It is shown that this solution always exists, but dependence of terms
in the quasi-isotropic expansion acquires a more complicated form.


title: Supergravity Inflation on the Brane

abstract:
  We study N=1 Supergravity inflation in the context of the braneworld
scenario. Particular attention is paid to the problem of the onset of inflation
at sub-Planckian field values and the ensued inflationary observables. We find
that the so-called $\eta$-problem encountered in supergravity inspired
inflationary models can be solved in the context of the braneworld scenario,
for some range of the parameters involved. Furthermore, we obtain an upper
bound on the scale of the fifth dimension, $M_5 \lsim 10^{-3} M_P$, in case the
inflationary potential is quadratic in the inflaton field, $\phi$. If the
inflationary potential is cubic in $\phi$, consistency with observational data
requires that $M_5 \simeq 9.2 \times 10^{-4} M_P$.


title: Radion and Large Scale Anisotropy on the Brane

abstract:
  We investigate the effect of the radion on cosmological perturbations in the
brane world. The S^1/Z_2 compactified 5D Anti-de Sitter spacetime bounded by
positive and negative tension branes is considered. The radion is the relative
displacement of the branes in this model. We find two different kinds of the
radion at the linear perturbation order for a cosmological brane. One describes
a "fluctuation" of the brane which does not couple to matter on the brane. The
other describes a "bend" of the brane which couples to the matter. The bend
determines the curvature perturbation on the brane. At large scales, the radion
interacts with anisotopic perturbations in the bulk. By solving the bulk
anisotropic perturbations, large scale metric perturbations and anisotropies of
the Cosmic Microwave Background (CMB) on the positive tension brane are
calculated. We find an interesting fact that the radion contributes to the CMB
anisotropies. The observational consequences of these effects are discussed.


title:
Classical and quantized aspects of dynamics in five dimensional
  relativity

abstract:
  A null path in 5D can appear as a timelike path in 4D, and for a certain
gauge in 5D the motion of a massive particle in 4D obeys the usual quantization
rule with an uncertainty-type relation. Generalizations of this result are
discussed in regard to induced-matter and membrane theory.


title:
Quantum theory of gravitational collapse (lecture notes on quantum
  conchology)

abstract:
  Preliminary version No.~2 of the lecture notes for the talk ``Quantum theory
of gravitational collapse'' given at the 271. WE-Heraeus-Seminar ``Aspects of
Quantum Gravity'' at Bad Honnef, 25 February--1 March 2002


title: Gravitational Wave Propagation in Isotropic Cosmologies

abstract:
  We study the propagation of gravitational waves carrying arbitrary
information through isotropic cosmologies. The waves are modelled as small
perturbations of the background Robertson-Walker geometry. The perfect fluid
matter distribution of the isotropic background is, in general, modified by
small anisotropic stresses. For pure gravity waves, in which the perturbed Weyl
tensor is radiative (i.e. type N in the Petrov classification), we construct
explicit examples for which the presence of the anisotropic stress is shown to
be essential and the histories of the wave-fronts in the background
Robertson-Walker geometry are shear-free null hypersurfaces. The examples
derived in this case are analogous to the Bateman waves of electromagnetic
theory.


title: Quantum-Gravity Phenomenology: Status and Prospects

abstract:
  Over the last few years part of the quantum-gravity community has adopted a
more optimistic attitude toward the possibility of finding experimental
contexts providing insight on non-classical properties of spacetime. I review
those quantum-gravity phenomenology proposals which were instrumental in
bringing about this change of attitude, and I discuss the prospects for the
short-term future of quantum-gravity phenomenology.


title: Adjoint operators and perturbation theory of black holes

abstract:
  We present a new approach for finding conservation laws in the perturbation
theory of black holes which applies for the more general cases of non-Hermitian
equations governing the perturbations. The approach is based on a general
result which establishes that a covariantly conserved current can be obtained
from a solution of any system of homogeneous linear differential equations and
a solution of the adjoint system. It is shown that the results obtained from
the present approach become essentially the same (with some diferences) to
those obtained by means of the traditional methods in the simplest black hole
geometry corresponding to the Schwarzschild space-time. The future applications
of our approach for studying the perturbations of black hole space-time in
string theory is discussed.


title: On Unitary Time Evolution in Gowdy $T^3$ Cosmologies

abstract:
  A non-perturbative canonical quantization of Gowdy $T^3$ polarized models
carried out recently is considered. This approach profits from the equivalence
between the symmetry reduced model and 2+1 gravity coupled to a massless real
scalar field. The system is partially gauge fixed and a choice of internal time
is performed, for which the true degrees of freedom of the model reduce to a
massless free scalar field propagating on a 2-dimensional expanding torus. It
is shown that the symplectic transformation that determines the classical
dynamics cannot be unitarily implemented on the corresponding Hilbert space of
quantum states. The implications of this result for both quantization of fields
on curved manifolds and physically relevant questions regarding the initial
singularity are discussed.


title:
Local continuity laws on the phase space of Einstein equations with
  sources

abstract:
  Local continuity equations involving background fields and variantions of the
fields, are obtained for a restricted class of solutions of the
Einstein-Maxwell and Einstein-Weyl theories using a new approach based on the
concept of the adjoint of a differential operator. Such covariant conservation
laws are generated by means of decoupled equations and their adjoints in such a
way that the corresponding covariantly conserved currents possess some
gauge-invariant properties and are expressed in terms of Debye potentials.
These continuity laws lead to both a covariant description of bilinear forms on
the phase space and the existence of conserved quantities. Differences and
similarities with other approaches and extensions of our results are discussed.


title:
Summary Talk on Cosmology and Gravitation: XXII Brazilian National
  Meeting on Particles and Fields

abstract:
  The plenary lectures, parallel talks, oral presentations and panel
contributions on Cosmology and Gravitation presented during the XXII Brazilian
National Meeting on Particles and Fields are briefly reviewed. Some remarks on
the area are also presented.


title: Bremsstrahlung in the gravitational field of a global monopole

abstract:
  We investigate the radiation emitted by a uniformly moving charged scalar
particle in the space-time of a point-like global monopole. We calculate the
total energy radiated by the particle and the corresponding spectrum, for small
solid angle deficit. We show that the radiated energy is proportional to the
cube of the velocity of the particle and to the cube of the Lorenz factor, in
the non-relativistic and ultra-relativistic cases, respectively.


title: Numerical relativity with the conformal field equations

abstract:
  I discuss the conformal approach to the numerical simulation of radiating
isolated systems in general relativity. The method is based on conformal
compactification and a reformulation of the Einstein equations in terms of
rescaled variables, the so-called ``conformal field equations'' developed by
Friedrich. These equations allow to include ``infinity'' on a finite grid,
solving regular equations, whose solutions give rise to solutions of the
Einstein equations of (vacuum) general relativity. The conformal approach
promises certain advantages, in particular with respect to the treatment of
radiation extraction and boundary conditions. I will discuss the essential
features of the analytical approach to the problem, previous work on the
problem - in particular a code for simulations in 3+1 dimensions, some new
results, open problems and strategies for future work.


title: Spin-2 field theory in curved spacetime

abstract:
  We present a new formulation to deal with the consistency problem of a
massive spin-2 field in a curved spacetime. Using Fierz variables to represent
the spin-2 field, we show how to avoid the arbitrariness and inconsistency that
exists in the standard formulation of spin-2 field coupled to gravity. The
superiority of the Fierz frame appears explicitly in the combined set of
equations for spin-2 field and gravity: it preserves the standard Einstein
equations of motion.


title:
Braneworld Dynamics of Inflationary Cosmologies with Exponential
  Potentials

abstract:
  In this work we consider Randall-Sundrum braneworld type scenarios, in which
the spacetime is described by a five-dimensional manifold with matter fields
confined in a domain wall or three-brane. We present the results of a
systematic analysis, using dynamical systems techniques, of the qualitative
behaviour of Friedmann-Lemaitre-Robertson-Walker type models, whose matter is
described by a scalar field with an exponential potential. We construct the
state spaces for these models and discuss how their structure changes with
respect to the general-relativistic case, in particular, what new critical
points appear and their nature and the occurrence of bifurcation.


title: High-energy head-on collisions of particles and hoop conjecture

abstract:
  We investigate the apparent horizon formation for high-energy head-on
collisions of particles in multi-dimensional spacetime. The apparent horizons
formed before the instance of particle collision are obtained analytically.
Using these solutions, we discuss the feature of the apparent horizon formation
in the multi-dimensional spacetime from the viewpoint of the hoop conjecture.


title: Quantum cosmology and eternal inflation

abstract:
  This contribution consists of two parts. In the first part, I review the
tunneling approach to quantum cosmology and comment on the alternative
approaches. In the second part, I discuss the relation between quantum
cosmology and eternal inflation. In particular, I discuss whether or not we
need quantum cosmology in the light of eternal inflation, and whether or not
quantum cosmology makes any testable predictions.


title: Sakharov's induced gravity: a modern perspective

abstract:
  Sakharov's 1967 notion of ``induced gravity'' is currently enjoying a
significant resurgence. The basic idea, originally presented in a very brief
3-page paper with a total of 4 formulas, is that gravity is not ``fundamental''
in the sense of particle physics. Instead it was argued that gravity (general
relativity) emerges from quantum field theory in roughly the same sense that
hydrodynamics or continuum elasticity theory emerges from molecular physics. In
this article I will translate the key ideas into modern language, and explain
the various versions of Sakharov's idea currently on the market.


title: Simultaneity and generalized connections in general relativity

abstract:
  Stationary extended frames in general relativity are considered. The
requirement of stationarity allows to treat the spacetime as a principal fiber
bundle over the one-dimensional group of time translations. Over this bundle a
connection form establishes the simultaneity between neighboring events
accordingly with the Einstein synchronization convention. The mathematics
involved is that of gauge theories where a gauge choice is interpreted as a
global simultaneity convention. Then simultaneity in non-stationary frames is
investigated: it turns to be described by a gauge theory in a fiber bundle
without structure group, the curvature being given by the Fr\"olicher-Nijenhuis
bracket of the connection. The Bianchi identity of this gauge theory is a
differential relation between the vorticity field and the acceleration field.
In order for the simultaneity connection to be principal, a necessary and
sufficient condition on the 4-velocity of the observers is given.


title:
Possible evidence from laboratory measurements for a latitude and
  longitude dependence of G

abstract:
  Stability arguments suggest that the Kaluza-Klein (KK) internal scalar field,
$\Phi$, should be coupled to some external fields. An external bulk real scalar
field, $\psi$, minimally coupled to gravity is proved to be satisfactory. At
low temperature, the coupling of $\psi$ to the electromagnetic (EM) field
allows $\Phi$ to be much stronger coupled to the EM field than in the genuine
five dimensional KK theory. It is shown that the coupling of $\Phi$ to the
geomagnetic field may explain the observed dispersion in laboratory
measurements of the (effective) gravitational constant. The analysis takes into
account the spatial variations of the geomagnetic field. Except the high PTB
value, the predictions are found in good agreement with all of the experimental
data.


title: Super-Acceleration from Massless, Minimally Coupled $\phi^4$

abstract:
  We derive a simple form for the propagator of a massless, minimally coupled
scalar in a locally de Sitter geometry of arbitrary spacetime dimension. We
then employ it to compute the fully renormalized stress tensor at one and two
loop orders for a massless, minimally coupled phi^4 theory which is released in
Bunch-Davies vacuum at t=0 in co-moving coordinates. In this system the
uncertainty principle elevates the scalar above the minimum of its potential,
resulting in a phase of super-acceleration. With the non-derivative
self-interaction the scalar's breaking of de Sitter invariance becomes
observable. It is also worth noting that the weak energy condition is violated
on cosmological scales. An interesting subsidiary result is that canceling
overlapping divergences in the stress tensor requires a conformal counterterm
which has no effect on purely scalar diagrams.


title: A Kerr Metric Solution in Tetrad Theory of Gravitation

abstract:
  Using an axial parallel vector field we obtain two exact solutions of a
vacuum gravitational field equations. One of the exact solutions gives the
Schwarzschild metric while the other gives the Kerr metric. The parallel vector
field of the Kerr solution have an axial symmetry. The exact solution of the
Kerr metric contains two constants of integration, one being the gravitational
mass of the source and the other constant $h$ is related to the angular
momentum of the rotating source, when the spin density ${S_{i j}}^\mu$ of the
gravitational source satisfies $\partial_\mu {S_{i j}}^\mu=0$. The singularity
of the Kerr solution is studied.


title: Gravitons from a loop representation of linearised gravity

abstract:
  Loop quantum gravity is based on a classical formulation of 3+1 gravity in
terms of a real SU(2) connection. Linearization of this classical formulation
about a flat background yields a description of linearised gravity in terms of
a {\em real} $U(1)\times U(1)\times U(1)$ connection. A `loop' representation,
in which holonomies of this connection are unitary operators, can be
constructed. These holonomies are not well defined operators in the standard
graviton Fock representation. We generalise our recent work on photons and U(1)
holonomies to show that Fock space gravitons are associated with distributional
states in the $U(1)\times U(1)\times U(1)$ loop representation. Our results may
illuminate certain aspects of the much deeper (and as yet unkown,) relation
between gravitons and states in nonperturbative loop quantum gravity.
  This work leans heavily on earlier seminal work by Ashtekar, Rovelli and
Smolin (ARS) on the loop representation of linearised gravity using {\em
complex} connections. In the last part of this work, we show that the loop
representation based on the {\em real} $U(1)\times U(1)\times U(1)$ connection
also provides a useful kinematic arena in which it is possible to express the
ARS complex connection- based results in the mathematically precise language
currently used in the field.


title:
First-order symmetrizable hyperbolic formulations of Einstein's
  equations including lapse and shift as dynamical fields

abstract:
  First-order hyperbolic systems are promising as a basis for numerical
integration of Einstein's equations. In previous work, the lapse and shift have
typically not been considered part of the hyperbolic system and have been
prescribed independently. This can be expensive computationally, especially if
the prescription involves solving elliptic equations. Therefore, including the
lapse and shift in the hyperbolic system could be advantageous for numerical
work. In this paper, two first-order symmetrizable hyperbolic systems are
presented that include the lapse and shift as dynamical fields and have only
physical characteristic speeds.


title: A Kinetic Theory Approach to Quantum Gravity

abstract:
  We describe a kinetic theory approach to quantum gravity -- by which we mean
a theory of the microscopic structure of spacetime, not a theory obtained by
quantizing general relativity. A figurative conception of this program is like
building a ladder with two knotted poles: quantum matter field on the right and
spacetime on the left. Each rung connecting the corresponding knots represent a
distinct level of structure. The lowest rung is hydrodynamics and general
relativity; the next rung is semiclassical gravity, with the expectation value
of quantum fields acting as source in the semiclassical Einstein equation. We
recall how ideas from the statistical mechanics of interacting quantum fields
helped us identify the existence of noise in the matter field and its effect on
metric fluctuations, leading to the establishment of the third rung: stochastic
gravity, described by the Einstein-Langevin equation. Our pathway from
stochastic to quantum gravity is via the correlation hierarchy of noise and
induced metric fluctuations. Three essential tasks beckon: 1) Deduce the
correlations of metric fluctuations from correlation noise in the matter field;
2) Reconstituting quantum coherence -- this is the reverse of decoherence --
from these correlation functions 3) Use the Boltzmann-Langevin equations to
identify distinct collective variables depicting recognizable metastable
structures in the kinetic and hydrodynamic regimes of quantum matter fields and
how they demand of their corresponding spacetime counterparts. This will give
us a hierarchy of generalized stochastic equations -- call them the
Boltzmann-Einstein hierarchy of quantum gravity -- for each level of spacetime
structure, from the macroscopic (general relativity) through the mesoscopic
(stochastic gravity) to the microscopic (quantum gravity).


title:
Finslerian Anisotropic Relativistic Metric Function Obtainable under
  Breakdown of Rotational Symmetry

abstract:
  We undertake to show how the relativistic Finslerian Metric Function (FMF)
should arise under uni-directional violation of spatial isotropy, keeping the
condition that the indicatrix (mass-shell) is a space of constant negative
curvature. By evaluating respective Finslerian tetrads, and treating them
consistently as the bases of inertial reference frame (RF), the generalized
Finslerian kinematic transformations follow in a convenient explicit form. The
concomitant Finslerian relativistic relations generalize their Lorentzian
prototypes through the presence of one characteristic parameter g, so that the
constraints on the parameter may be found in future high-precision
post-Lorentzian experiments. As the associated Finslerian Hamiltonian function
is also obtainable in a clear explicit form, convenient prospects for the
Finslerian extension of particle dynamics are also opened. Additionally, the
Finslerian extension of the general-relativistic Schwarzschild metric can
unambiguously be proposed.


title: Asymptotic states in brane cosmology with a nonlocal anisotropic stress

abstract:
  We investigate the dynamics of a Bianchi I brane Universe in the presence of
a nonlocal anisotropic stress ${\cal P}_{\mu\nu}$ proportional to a "dark
energy" ${\cal U}$. Using this ansatz for the case ${\cal U} > 0$ we prove that
if a matter on a brane satisfies the equation of state $p=(\gamma-1)\rho$ with
$\gamma \le 4/3$ then all such models isotropize. For $\gamma > 4/3$
anisotropic future asymptotic states are found. We also describe the past
asymptotic regimes for this model.


title: Quantum behavior of FRW radiation-filled universes

abstract:
  We study the quantum vacuum fluctuations around closed
Friedmann-Robertson-Walker (FRW) radiation-filled universes with nonvanishing
cosmological constant. These vacuum fluctuations are represented by a
conformally coupled massive scalar field and are treated in the lowest order of
perturbation theory. In the semiclassical approximation, the perturbations are
governed by differential equations which, properly linearized, become
generalized Lame equations. The wave function thus obtained must satisfy
appropriate regularity conditions which ensure its finiteness for every field
configuration. We apply these results to asymptotically anti de-Sitter
Euclidean wormhole spacetimes and show that there is no catastrophic particle
creation in the Euclidean region, which would lead to divergences of the wave
function.


title:
New Relativistic Effects in the Dynamics of Nonlinear Hydrodynamical
  Waves

abstract:
  In Newtonian and relativistic hydrodynamics the Riemann problem consists of
calculating the evolution of a fluid which is initially characterized by two
states having different values of uniform rest-mass density, pressure and
velocity. When the fluid is allowed to relax, one of three possible
wave-patterns is produced, corresponding to the propagation in opposite
directions of two nonlinear hydrodynamical waves. New effects emerge in a
special relativistic Riemann problem when velocities tangential to the initial
discontinuity surface are present. We show that a smooth transition from one
wave-pattern to another can be produced by varying the initial tangential
velocities while otherwise maintaining the initial states unmodified. These
special relativistic effects are produced by the coupling through the
relativistic Lorentz factors and do not have a Newtonian counterpart.


title: Back Reaction And Local Cosmological Expansion Rate

abstract:
  We calculate the back reaction of cosmological perturbations on a general
relativistic variable which measures the local expansion rate of the Universe.
Specifically, we consider a cosmological model in which matter is described by
a single field. We analyze back reaction both in a matter dominated Universe
and in a phase of scalar field-driven chaotic inflation. In both cases, we find
that the leading infrared terms contributing to the back reaction vanish when
the local expansion rate is measured at a fixed value of the matter field which
is used as a clock, whereas they do not appear to vanish if the expansion rate
is evaluated at a fixed value of the background time. We discuss possible
implications for more realistic models with a more complicated matter sector.


title: Entropy of a nonuniformly rectilinearly accelerating black hole

abstract:
  Adopting thin film brick-wall model, we calculate the entropy of a
nonuniformly rectilinearly accelerating non-stationary black hole expressed by
Kinnersley metric. Because the black hole is accelerated, the event horizon is
axisymmetric. The different points of horizon surface may have different
temperature. We calculate the temperature and the entropy density at every
point of the horizon at first, then we obtain the total entropy through
integration, which is proportional to the aera of event horizon as the same as
the stationary black holes. It is shown that the black hole entropy may be
regarded as the entropy of quantum fields just on the surface of event horizon.


title: Anisotropic generalizations of de Sitter spacetime

abstract:
  It is known that de Sitter spacetime can be seen as the solution of field
equation for completely isotropic matter. In the present paper a new class of
exact solutions in spherical symmetry is found and discussed, such that the
energy--momentum tensor has two 2--dimensional distinct isotropic subspaces.


title:
Anomalous particle-production thresholds through systematic and
  non-systematic quantum-gravity effects

abstract:
  A growing number of studies is being devoted to the identification of
plausible quantum properties of spacetime which might give rise to observably
large effects. The literature on this subject is now relatively large,
including studies in string theory, loop quantum gravity and noncommutative
geometry. It is useful to divide the various proposals into proposals involving
a systematic quantum-gravity effect (an effect that would shift the
main/average prediction for a given observable quantity) and proposals
involving a non-systematic quantum-gravity effect (an effect that would
introduce new fundamental uncertanties in some observable quantity). The case
of quantum-gravity-induced particle-production-threshold anomalies, a much
studied example of potentially observable quantum-gravity effect, is here used
as an example to illustrate the differences to be expected between systematic
and non-systematic effects.


title:
Generalized uncertainty relations in a quantum theory and thermodynamics
  from the uniform point of view

abstract:
  A generalization of the thermodynamic uncertainty relations is proposed. It
is done by introducing of an additional term proportional to the interior
energy into the standard thermodynamic uncertainty relation that leads to
existence of the lower limit of inverse temperature.


title: What did we learn from studying acoustic black holes ?

abstract:
  The study of acoustic black holes has been undertaken to provide new insights
about the role of high frequencies in black hole evaporation. Because of the
infinite gravitational redshift from the event horizon, Hawking quanta emerge
from configurations which possessed ultra high (trans-Planckian) frequencies.
Therefore Hawking radiation cannot be derived within the framework of a low
energy effective theory; and in all derivations there are some assumptions
concerning Planck scale physics. The analogy with condensed matter physics was
thus introduced to see if the asymptotic properties of the Hawking phonons
emitted by an acoustic black hole, namely stationarity and thermality, are
sensitive to the high frequency physics which stems from the granular character
of matter and which is governed by a non-linear dispersion relation. In 1995
Unruh showed that they are not sensitive in this respect, in spite of the fact
that phonon propagation near the (acoustic) horizon drastically differs from
that of photons. In 2000 the same analogy was used to establish the robustness
of the spectrum of primordial density fluctuations in inflationary models. This
analogy is currently stimulating research for experimenting Hawking radiation.
Finally it could also be a useful guide for going beyond the semi-classical
description of black hole evaporation.


title: Evidence for the Black Hole Event Horizon

abstract:
  Roughly a dozen X-ray binaries are presently known in which the compact
accreting primary stars are too massive to be neutron stars. These primaries
are identified as black holes, though there is as yet no definite proof that
any of the candidate black holes actually possesses an event horizon. We
discuss how Type I X-ray bursts may be used to verify the presence of the event
horizon in these objects. Type I bursts are caused by thermonuclear explosions
when gas accretes onto a compact star. The bursts are commonly seen in many
neutron star X-ray binaries, but they have never been seen in any black hole
X-ray binary. Our model calculations indicate that black hole candidates ought
to burst frequently if they have surfaces. Based on this, we argue that the
lack of bursts constitutes strong evidence for the presence of event horizons
in these objects.


title: Black Holes, Mergers, and the Entropy Budget of the Universe

abstract:
  Vast amounts of entropy are produced in black hole formation, and the amount
of entropy stored in supermassive black holes at the centers of galaxies is now
much greater than the entropy free in the rest of the universe. Either mergers
involved in forming supermassive black holes are rare,or the holes must be very
efficient at capturing nearly all the entropy generated in the process.
  We argue that this information can be used to constrain supermassive black
hole production, and may eventually provide a check on numerical results for
mergers involving black holes.


title: Isoperimetric inequality for higher-dimensional black holes

abstract:
  The initial data sets for the five-dimensional Einstein equation have been
examined. The system is designed such that the black hole ($\simeq S^3$) or the
black ring ($\simeq S^2\times S^1$) can be found. We have found that the
typical length of the horizon can become arbitrarily large but the area of
characteristic closed two-dimensional submanifold of the horizon is bounded
above by the typical mass scale. We conjecture that the isoperimetric
inequality for black holes in $n$-dimensional space is given by $V_{n-2}
\lesssim GM$, where $V_{n-2}$ denotes the volume of typical closed
$(n-2)$-section of the horizon and $M$ is typical mass scale, rather than
$C\lesssim (GM)^{1/(n-2)}$ in terms of the hoop length $C$, which holds only
when $n=3$.


title:
Linear Response and the Validity of the Semi-Classical Approximation in
  Gravity

abstract:
  We propose a quantitative test for the validity of the semi-classical
approximation in gravity, namely that the solutions to the semi-classical
equations should be stable to linearized perturbations, in the sense that no
gauge invariant perturbation should become unbounded in time. We show that a
self-consistent linear response analysis of these perturbations based upon an
invariant effective action principle involves metric fluctuations about the
mean semi-classical geometry and brings in the two-point correlation function
of the quantum energy-momentum tensor in a natural way. The properties of this
correlation function are discussed and it is shown on general grounds that it
contains no state-dependent divergences and requires no new renormalization
counterterms beyond those required in the leading order semi-classical
approximation.


title: Ghost neutrinos and radiative Kerr metric in Einstein-Cartan gravity

abstract:
  Ghost neutrinos in radiative Kerr spacetime endowed with totally
skew-symmetric Cartan contortion is presented. The computations are made by
using the Newman-Penrose (NP) calculus. The model discussed here maybe useful
in several astrophysical applications specially in black hole astrophysics.


title: A disintegrating cosmic string

abstract:
  We present a simple sandwich gravitational wave of the Robinson-Trautman
family. This is interpreted as representing a shock wave with a spherical
wavefront which propagates into a Minkowski background minus a wedge. (i.e. the
background contains a cosmic string.) The deficit angle (the tension) of the
string decreases through the gravitational wave, which then ceases. This leaves
an expanding spherical region of Minkowski space behind it. The decay of the
cosmic string over a finite interval of retarded time may be considered to
generate the gravitational wave.


title:
Schwarzschild black holes and propagation of electromagnetic and
  gravitational waves

abstract:
  Disturbing of a spacetime geometry may result in the appearance of an
oscillating and damped radiation - the so-called quasinormal modes. Their
periods of oscillations and damping coefficients carry unique information about
the mass and the angular momentum, that would allow one to identify the source
of the gravitational field. In this talk we present recent bounds on the
diffused energy, applicable to the Schwarzschild spacetime, that give also
rough estimates of the energy of excited quasinormal modes.


title: Report on GR16, Session A3: Mathematical Studies of the Field Equations

abstract:
  In this report, which is an extended version of that appearing in the
Proceedings of GR16, I will give a summary of the main topics covered in
Session A.3. on mathematical relativity at GR16, Durban. The summary is mainly
based on extended abstracts submitted by the speakers. I would like to thank
all participants for their contributions and help with this summary.


title: Hawking Radiation in the Swiss Cheese Universe

abstract:
  The Hawking radiation forms the essential basis of the black hole
thermodynamics. The black hole thermodynamics denotes a nice correspondence
between black hole kinematics and the laws of ordinary thermodynamics, but has
been so far considered only in an asymptotically flat case. Does such the
correspondence rely strongly on the feature of the gravity vanishing at the
infinity? In order to resolve this question, it should be considered for the
first to extend the Hawking radiation to a case with a dynamical boundary
condition like an expanding universe. Therefore the Hawking radiation in an
expanding universe is discussed in this paper. As a concrete model of a black
hole in an expanding universe, we use the swiss cheese universe which is the
spacetime including a Schwarzschild black hole in the
Friedmann-Robertson-Walker universe. Further for simplicity, our calculation is
performed in two dimension. The resultant spectrum of the Hawking radiation
measured by a comoving observer is generally different from a thermal one. We
find that the qualitative behavior of the non-thermal spectrum is of dumping
oscillation as a function of the frequency measured by the observer, and that
the intensity of the Hawking radiation is enhanced by the presence of a
cosmological expansion. It is appropriate to say that it a black hole with an
asymptotically flat boundary condition stays in a lowest energy thermal
equilibrium state, and that, once a black hole is put into an expanding
universe, it is excited to a non-equilibrium state and emits its mass energy
with stronger intensity than a thermal one.


title:
Inflation in Bianchi models and the cosmic no hair theorem in brane
  world

abstract:
  In this paper, the cosmic no hair theorem for anisotropic Bianchi models
which admit inflation with a scalar field is studied in the framework of Brane
world. It is found that all Bianchi models except Bianchi type IX, transit to
an inflationary regime with vanishing anisotropy. In the Brane world,
anisotropic universe approaches the inflationary era much faster than that in
the general theory of relativity. The form of the potential does not affect the
evolution in the inflationary epoch. However, the late time behaviour is
controlled by a constant additive factor in the potential for the inflaton
field.


title: An Overview of Gravitational-Wave Sources

abstract:
  We review current best estimates of the strength and detectability of the
gravitational waves from a variety of sources, for both ground-based and
space-based detectors, and we describe the information carried by the waves.


title:
Gravitational collapse of Type II fluid in higher dimensional
  space-times

abstract:
  We find the general solution of the Einstein equation for spherically
symmetric collapse of Type II fluid (null strange quark fluid) in higher
dimensions. It turns out that the nakedness and curvature strength of the shell
focusing singularities carry over to higher dimensions. However, there is
shrinkage of the initial data space for a naked singularity of the Vaidya
collapse due to the presence of strange quark matter.


title: Possible astrophysical probes of quantum gravity

abstract:
  A satisfactory theory of quantum gravity will very likely require
modification of our classical perception of space-time, perhaps by giving it a
'foamy' structure at scales of order the Planck length. This is expected to
modify the propagation of photons and other relativistic particles such as
neutrinos, such that they will experience a non-trivial refractive index even
in vacuo. The implied spontaneous violation of Lorentz invariance may also
result in alterations of kinematical thresholds for key astrophysical processes
involving high energy cosmic radiation. We discuss experimental probes of these
possible manifestations of the fundamental quantum nature of space-time using
observations of distant astrophysical sources such as gamma-ray bursts and
active galactic nuclei.


title:
Regularization parameters for the self force in Schwarzschild spacetime:
  I. scalar case

abstract:
  We derive the explicit values of all regularization parameters (RP) for a
scalar particle in an arbitrary geodesic orbit around a Schwarzschild black
hole. These RP are required within the previously introduced mode-sum method,
for calculating the local self-force acting on the particle. In this method one
first calculates the (finite) contribution to the self-force due to each
individual multipole mode of the particle's field, and then applies a certain
regularization procedure to the mode sum, involving the RP. The explicit values
of the RP were presented in a recent Letter [Phys. Rev. Lett. {\bf 88}, 091101
(2002)]. Here we give full details of the RP derivation in the scalar case. The
calculation of the RP in the electromagnetic and gravitational cases will be
discussed in an accompanying paper.


title: Runaway dilaton and equivalence principle violations

abstract:
  In a recently proposed scenario, where the dilaton decouples while
cosmologically attracted towards infinite bare string coupling, its residual
interactions can be related to the amplitude of density fluctuations generated
during inflation, and are large enough to be detectable through a modest
improvement on present tests of free-fall universality. Provided it has
significant couplings to either dark matter or dark energy, a runaway dilaton
can also induce time-variations of the natural "constants" within the reach of
near-future experiments.


title: Discrete Quantum Mechanics I: Quantum Covariance

abstract:
  Classically general covariance is found from the idea that a vector is a
physical quantity which exists independently of choice of coordinate system and
is unchanged by a change of coordinate system. It is often assumed that there
exists some form of absolute mathematical space or space-time, and that in a
flat space approximation vectors can be imagined between defined points in this
space-time, much as we can imagine an arrowed line drawn on a piece of paper.
However, while classical vector quantities can be represented on paper, in the
quantum domain physical quantities do not in general exist with precise values
except in measurement; a change of apparatus, for example by rotating it, may
affect the outcome of the measurement, so the condition for general covariance
does not apply. The purpose of this paper is to re-examine covariance within
the context of an orthodox, Dirac-Von Neumann interpretation of quantum
mechanics, to replace it with a new condition, here called quantum covariance,
and to show that quantum covariance is the required condition for the
unification of general relativity with quantum mechanics for non-interacting
particles.


title: Rigidity theorems in the braneworld model

abstract:
  In the present paper, we give some theorems representing ridigity of a vacuum
brane in static bulk spacetimes. As an application, we show that a static bulk
spacetime with dimension D>3 and spatial symmetry IO(D-2), O(D-1) or O_+(D-2,1)
does not allow a vacuum brane with a black hole on it. We also show that if a
static bulk spacetime with dimension D>4 satsifying the vacuum Einstein
equations can be foliated by a continuous family of vacuum branes with
asymptotically constant curvature, it is a black string solution.


title: Universal Self Force from an Extended-Object Approach

abstract:
  We present a consistent extended-object approach for determining the self
force acting on an accelerating charged particle. In this approach one
considers an extended charged object of finite size $\epsilon $, and calculates
the overall contribution of the mutual electromagnetic forces. Previous
implementations of this approach yielded divergent terms $\propto 1/\epsilon $
that could not be cured by mass-renormalization. Here we explain the origin of
this problem and fix it. We obtain a consistent, universal, expression for the
extended-object self force, which conforms with Dirac's well known formula.


title: Gravitational and cosmological properties of a brane-universe

abstract:
  The aim of this contribution is to provide a short introduction to recently
investigated models in which our accessible universe is a four-dimensional
submanifold, or brane, embedded in a higher dimensional spacetime and ordinary
matter is trapped in the brane. I focus here on the gravitational and
cosmological aspects of such models with a single extra-dimension.


title: Cosmic Censorship, Area Theorem, and Self-Energy of Particles

abstract:
  The (zeroth-order) energy of a particle in the background of a black hole is
given by Carter's integrals. However, exact calculations of a particle's {\it
self-energy} (first-order corrections) are still beyond our present reach in
many situations. In this paper we use Hawking's area theorem in order to derive
bounds on the self-energy of a particle in the vicinity of a black hole.
Furthermore, we show that self-energy corrections {\it must} be taken into
account in order to guarantee the validity of Penrose cosmic censorship
conjecture.


title:
An explicit time variable for cosmology and the matter-vacuum energy
  coincidence

abstract:
  By allowing for non zero vacuum expectation values for some of the fields
that appear in the Hamiltonian constraint of canonical general relativity a
time variable, with usual properties, can be identified; the constraint plays
the role of the ordinary Hamiltonian. The energy eigenvalues contribute to the
variation of the scale parameter similarly to the way matter density does. For
a universe described by a superposition of eigenstates or by a thermodynamic
ensemble the dominant contribution comes from energy, or equivalently effective
matter density, of the same order as the vacuum energy (cosmological constant).
This may explain the observed ``coincidence'' of these two values.


title: Quasinormal modes of stringy black holes

abstract:
  String theory is a promising candidate for a fundamental quantum theory of
all interactions including Einstein gravity. Some solutions in string theory
can be interpreted as black holes. Using the semi-analytic method and WKB
method,the quasinormal modes(QNMs)of 1+1 dimensional black hole in string
theory are studied.The QNMs of 1+3 dimensional black hole in string theory are
also calculated through numerical approach.The numerical investigation has
shown that the late time gravitational oscillation of the black hole under an
external perturbaton is dominated by certain QNMs.


title: Last orbits of binary black holes

abstract:
  Binary black hole systems in the pre-coalescence stage are numerically
constructed by demanding that the associated spacetime admits a helical Killing
vector. Comparison with third order post-Newtonian calculations indicates a
rather good agreement until the innermost stable circular orbit.


title:
Eigenmodes of 3-dimensional spherical spaces and their application to
  cosmology

abstract:
  This article investigates the computation of the eigenmodes of the Laplacian
operator in multi-connected three-dimensional spherical spaces. General
mathematical results and analytical solutions for lens and prism spaces are
presented. Three complementary numerical methods are developed and compared
with our analytic results and previous investigations. The cosmological
applications of these results are discussed, focusing on the cosmic microwave
background (CMB) anisotropies. In particular, whereas in the Euclidean case too
small universes are excluded by present CMB data, in the spherical case there
will always exist candidate topologies even if the total energy density
parameter of the universe is very close to unity.


title: Energy dominance and the Hawking Ellis vacuum conservation theorem

abstract:
  At a time when uninhibited speculation about negative tension -- and by
implication negative mass density -- world branes has become commonplace, it
seems worthwhile to call attention to the risk involved in sacrificing
traditional energy positivity postulates such as are required for the classical
vacuum stability theorem of Hawking and Ellis. As well as recapitulating the
technical content of this reassuring (when applicable) theorem, the present
article provides a new, rather more economical proof.


title: Generalisation of the Einstein-Straus model to anisotropic settings

abstract:
  We study the possibility of generalising the Einstein--Straus model to
anisotropic settings, by considering the matching of locally cylindrically
symmetric static regions to the set of $G_4$ on $S_3$ locally rotationally
symmetric (LRS) spacetimes. We show that such matchings preserving the symmetry
are only possible for a restricted subset of the LRS models in which there is
no evolution in one spacelike direction. These results are applied to spatially
homogeneous (Bianchi) exteriors where the static part represents a finite
bounded interior region without holes. We find that it is impossible to embed
finite static strings or other locally cylindrically symmetric static objects
(such as bottle or coin-shaped objects) in reasonable Bianchi cosmological
models, irrespective of the matter content. Furthermore, we find that if the
exterior spacetime is assumed to have a perfect fluid source satisfying the
dominant energy condition, then only a very particular family of LRS stiff
fluid solutions are compatible with this model.
  Finally, given the interior/exterior duality in the matching procedure, our
results have the interesting consequence that the Oppenheimer-Snyder model of
collapse cannot be generalised to such anisotropic cases.


title:
Navigation of Spacetime Ships in Unified Gravitational and
  Electromagnetic Waves

abstract:
  On the basis of a "local" principle of equivalence of general relativity, we
consider a navigation in a kind of "4D-ocean" involving measurements of
conformally invariant physical properties only. Then, applying the Pfaff theory
for PDE to a particular conformally equivariant system of differential
equations, we show the dependency of any kind of function describing "spacetime
waves", with respect to 20 parametrizing functions. These latter, appearing in
a linear differential Spencer sequence and determining gauge fields of
deformations relatively to "ship-metrics" or to "flat spacetime ocean metrics",
may be ascribed to unified electromagnetic and gravitational waves. The present
model is based neither on a classical gauge theory of gravitation or a
gravitation theory with torsion, nor on any Kaluza-Klein or Weyl type
unifications, but rather on a post-Newtonian approach of gravitation in a four
dimensional conformal Cosserat spacetime.


title: The logic of causally closed spacetime subsets

abstract:
  The causal structure of space-time offers a natural notion of an opposite or
orthogonal in the logical sense, where the opposite of a set is formed by all
points non time-like related with it. We show that for a general space-time the
algebra of subsets that arises from this negation operation is a complete
orthomodular lattice, and thus has several of the properties characterizing the
algebra physical propositions in quantum mechanics. We think this fact could be
used to investigate causal structure in an algebraic context. As a first step
in this direction we show that the causal lattice is in addition atomic, find
its atoms, and give necesary and sufficient conditions for ireducibility.


title: Fermat Potentials for Non-Perturbative Gravitational Lensing

abstract:
  The images of many distant galaxies are displaced, distorted and often
multiplied by the presence of foreground massive galaxies near the line of
sight; the foreground galaxies act as gravitational lenses. Commonly, the lens
equation, which relates the placement and distortion of the images to the real
source position in the thin-lens scenario, is obtained by extremizing the time
of arrival among all the null paths from the source to the observer (Fermat's
principle). We show that the construction of envelopes of certain families of
null surfaces consitutes an alternative variational principle or version of
Fermat's principle that leads naturally to a lens equation in a generic
spacetime with any given metric. We illustrate the construction by deriving the
lens equation for static asymptotically flat thin lens spacetimes. As an
application of the approach, we find the bending angle for moving thin lenses
in terms of the bending angle for the same deflector at rest. Finally we apply
this construction to cosmological spacetimes (FRW) by using the fact they are
all conformally related to Minkowski space.


title:
Robust statistics for deterministic and stochastic gravitational waves
  in non-Gaussian noise. II: Bayesian analyses

abstract:
  In a previous paper (gr-qc/0105100) we derived a set of near-optimal signal
detection techniques for gravitational wave detectors whose noise probability
distributions contain non-Gaussian tails. The methods modify standard methods
by truncating sample values which lie in those non-Gaussian tails. The methods
were derived, in the frequentist framework, by minimizing false alarm
probabilities at fixed false detection probability in weak signal limit. For
stochastic signals, the resulting statistic consisted of a sum of an
auto-correlation term and a cross-correlation term; it was necessary to discard
by hand the auto-correlation term to obtain the correct, generalized
cross-correlation statistic. In the present paper, we present an alternative
Bayesian derivation of the same signal detection techniques. We compute the
probability that a signal is present in the data, in the limit where the
signal-to-noise ratio squared per frequency bin is small, where the integrated
signal-to-noise ratio is large compared to one, and where the total probability
in the non-Gaussian tail part of the noise distribution is small. We show that,
for each model considered, the resulting probability is to a good approximation
a monotonic function of the detection statistic derived in the previous paper.
Moreover, for stochastic signals, the new Bayesian derivation automatically
eliminates the problematic auto-correlation term.


title: Dynamics of Fermat potentials in non-perturbative gravitational lensing

abstract:
  We present a framework, based on the null-surface formulation of general
relativity, for discussing the dynamics of Fermat potentials for gravitational
lensing in a generic situation without approximations of any kind.
Additionally, we derive two lens equations: one for the case of thick compact
lenses and the other one for lensing by gravitational waves. These equations in
principle generalize the astrophysical scheme for lensing by removing the
thin-lens approximation while retaining the weak fields.


title: Cosmological Constant and Zeta-Function

abstract:
  The relativistic invariant zeta-function approach to computation of the
vacuum energy contribution to cosmological constant is discussed. It is shown
that this value is determined by the fourth power of the quantized field mass,
while the dependence from the large mass scale is only logarithmic. This value
is compared to the result obtained in the dimensional regularization scheme
which also satisfies the relativistic invariance condition, and found to be the
same up to irrelevant finite terms. The consequences of the renormalization
group invariance are also briefly discussed.


title:
Radiative falloff of a scalar field in a weakly curved spacetime without
  symmetries

abstract:
  We consider a massless scalar field propagating in a weakly curved spacetime
whose metric is a solution to the linearized Einstein field equations. The
spacetime is assumed to be stationary and asymptotically flat, but no other
symmetries are imposed -- the spacetime can rotate and deviate strongly from
spherical symmetry. We prove that the late-time behavior of the scalar field is
identical to what it would be in a spherically-symmetric spacetime: it decays
in time according to an inverse power-law, with a power determined by the
angular profile of the initial wave packet (Price falloff theorem). The field's
late-time dynamics is insensitive to the nonspherical aspects of the metric,
and it is governed entirely by the spacetime's total gravitational mass; other
multipole moments, and in particular the spacetime's total angular momentum, do
not enter in the description of the field's late-time behavior. This extended
formulation of Price's falloff theorem appears to be at odds with previous
studies of radiative decay in the spacetime of a Kerr black hole. We show,
however, that the contradiction is only apparent, and that it is largely an
artifact of the Boyer-Lindquist coordinates adopted in these studies.


title: Higher-order geodesic deviations applied to the Kerr metric

abstract:
  Starting with an exact and simple geodesic, we generate approximate geodesics
by summing up higher-order geodesic deviations within a General Relativistic
setting, without using Newtonian and post-Newtonian approximations. We apply
this method to the problem of closed orbital motion of test particles in the
Kerr metric space-time. With a simple circular orbit in the equatorial plane
taken as the initial geodesic we obtain finite eccentricity orbits in the form
of Taylor series with the eccentricity playing the role of small parameter. The
explicit expressions of these higher-order geodesic deviations are derived
using successive systems of linear equations with constant coefficients, whose
solutions are of harmonic oscillator type. This scheme gives best results when
applied to the orbits with low eccentricities, but with arbitrary values of
$(GM/Rc^2)$, smaller than 1/6 in the Schwarzschild limit.


title: Asymmetrically warped R=0 braneworlds

abstract:
  A new family of five dimensional, R=0 braneworlds with asymmetric warp
factors is proposed. Beginning with the invariance of the Ricci scalar for the
general class of asymmetrically warped spacetimes we, subsequently specialise
to the R=0 case. Solutions are obtained by choosing a particular relation
(involving a parameter $\nu$) between the warp factors. Symmetric warping
arises as a special case (particular value of $\nu$). Over a range of values of
$\nu$ the energy conditions for the matter stress energy are found to hold
good. It turns out that the energy density and pressures required to support
these spacetimes decay as the inverse square of the fifth (extra) coordinate.
The projection of this bulk stress--energy (for symmetric warping) on the
3--brane yields an effective cosmological constant. We conclude with brief
comments on spacetimes with constant Ricci scalar and the extension of our
results to diverse dimensions.


title: Immirzi Ambiguity in the Kinematics of Quantum General Relativity

abstract:
  The Immirzi ambiguity arises in loop quantum gravity when geometric operators
are represented in terms of different connections that are related by means of
an extended Wick transform. We analyze the action of this transform in gravity
coupled with matter fields and discuss its analogy with the Wick rotation on
which the Thiemann transform between Euclidean and Lorentzian gravity is based.
In addition, we prove that the effect of this extended Wick transform is
equivalent to a constant scale transformation as far as the symplectic
structure and kinematical constraints are concerned. This equivalence is broken
in the dynamical evolution. Our results are applied to the discussion of the
black hole entropy in the limit of large horizon areas. We first argue that,
since the entropy calculation is performed for horizons of fixed constant area,
one might in principle choose an Immirzi parameter that depends on this
quantity. This would spoil the linearity with the area in the entropy formula.
We then show that the Immirzi parameter appears as a constant scaling in all
the steps where dynamical information plays a relevant role in the entropy
calculation. This fact, together with the kinematical equivalence of the
Immirzi ambiguity with a change of scale, is used to preclude the potential
non-linearity of the entropy on physical grounds.


title:
On the stability of gravity in the presence of a non-minimally coupled
  scalar field

abstract:
  We show that Einstein's gravity coupled to a non-minimally coupled scalar
field is stable even for high values of the scalar field, when the sign of the
Einstein-Hilbert action is reversed. We also discuss inflationary solutions and
a possible new mechanism of reheating.


title: On rigidly rotating perfect fluid cylinders

abstract:
  The gravitational field of a rigidly rotating perfect fluid cylinder with
gamma- law equation of state is found analytically. The solution has two
parameters and is physically realistic for gamma in the interval (1.41,2].
Closed timelike curves always appear at large distances.


title:
Constraints on Non-Singular Cosmological Models with Quadratic
  Lagrangians

abstract:
  We consider the generalized set of theories of gravitation whose Lagrangians
contain the term $R^{2}$ : $L=\sqrt{-g}(R+\beta R^{2})$. Inserting the RW
metric with an imposed non-singular and inflationary behaviour of the scale
factor $a(t)$, and using a arbitrary perfect fluid, we study the properties of
$\rho $ and $p$ in this context. By requiring the positivity of the energy
density, as well as real and finite velocity of sound, we can obtain the range
of values of $\beta $ that ensure the inflationary behaviour and absence of
singularity.


title: Lagrangian fluid mechanics

abstract:
  The method of Lagrangians with covariant derivative (MLCD) is applied to a
special type of Lagrangian density depending on scalar and vector fields as
well as on their first covariant derivatives. The corresponding
Euler-Lagrange's equations and energy-momentum tensors are found on the basis
of the covariant Noether's identities.


title: The Weyl Curvature Conjecture

abstract:
  In this paper we review Penrose's Weyl curvature conjecture which states that
the concept of gravitational entropy and the Weyl tensor is somehow linked, at
least in a cosmological setting. We give a description of a certain entity
constructed from the Weyl tensor, from the very early history of our universe
until the present day. Inflation is an important mechanism in our early
universe for homogenisation and isotropisation, and thus it must cause large
effects upon the evolution of the gravitational entropy. Therefore the effects
from inflationary fluids and a cosmological constant are studied in detail.


title:
Energy and angular momentum radiated for non head-on binary black hole
  collisions

abstract:
  We investigate the possible total radiated energy produced by a binary black
hole system containing non-vanishing total angular momentum. For the scenearios
considered we find that the total radiated energy does not exceed 1%.
Additionally we explore the gravitational radiation field and the variation of
angular momentum in the process.


title:
Gravitational Energy of Kerr and Kerr Anti-de Sitter Space-times in the
  Teleparallel Geometry

abstract:
  In the context of the Hamiltonian formulation of the teleparallel equivalent
of general relativity we compute the gravitational energy of Kerr and Kerr
Anti-de Sitter (Kerr-AdS) space-times. The present calculation is carried out
by means of an expression for the energy of the gravitational field that
naturally arises from the integral form of the constraint equations of the
formalism. In each case, the energy is exactly computed for finite and
arbitrary spacelike two-spheres, without any restriction on the metric
parameters. In particular, we evaluate the energy at the outer event horizon of
the black holes.


title: Space-Time--Time

abstract:
  Space-time--time is a natural hybrid of Kaluza's five-dimensional geometry
and Weyl's conformal space-time geometry. Translations along the secondary time
dimension produce the electromagnetic gauge transformations of Kaluza--Klein
theory and the metric gauge transformations of Weyl theory, related as Weyl
postulated. Geometrically, this phenomenon resides in an exponential-expansion
producing ``conformality constraint'', which replaces Kaluza's ``cylinder
condition''. The curvature tensors exhibit a wealth of ``interactions'' among
geometrical entities with physical interpretations. Unique to the conformally
constrained geometry is a sectionally isotropic, ultralocally determined
``residual curvature'', useful in construction of an action density for field
equations. A space-time--time geodesic describes a test particle whose rest
mass m and electric charge q evolve according to definite laws. The particls's
motion is governed by four apparent forces: the Einstein gravitational force,
the Lorentz electromagnetic force, a force proportional to the electromagnetic
potential, and a force proportional to a gradient d(ln phi), where the scalar
field phi is essentially the space-time--time residual radius of curvature. The
particle appears suddenly at an event E1 with q = -phi(E1) and vanishes at an
event E2 with q = phi(E2). At E1 and E2 the phi-force infinitely dominates the
others, causing E1 and E2 to tend to occur near where phi has an extreme value;
application to the modeling of orbital transitions of atomic electrons suggests
itself. The equivalence of a test particle's inertial mass and its passive
gravitational mass follows from the gravitational force's proportionality to m.
No connection is apparent between m and active gravitational mass or between q
and active electric charge, nor does the theory seem to require any.


title: ADDENDUM on the mass neutrino oscillation in a gravitational field

abstract:
  In the article {\it Gen. Rel. Grav.} {\bf 32}, 1633 (2000), by J. G. Pereira
and C. M. Zhang, the special relativity energy-momentum tensor was used to
discuss the neutrino phase-splitting in a weak gravitational field. However, it
would be more appropriate to use the general relativity energy-momentum tensor.
When we do that, as we are going to see, some results change, but the basic
conclusion remains the same.


title:
Does a Nambu-Goto wall emit gravitational waves? -- Cylindrical
  Nambu-Goto wall as an example of gravitating non-spherical walls --

abstract:
  Gravitational field of a cylindrical Nambu-Goto wall in the vacuum spacetime
is considered in order to clarify the interaction between Nambu-Goto membranes
and gravitational waves. If one neglects the emission of gravitational waves by
the wall motion, the spacetime becomes singular. It is also shown that the
emission of gravitational waves does occur by the motion of the cylindrical
wall if the initial data is singularity free. The energy loss rate due to
radiation of gravitational waves agrees with that estimated from the test wall
motion and the quadrupole formula for the gravitational wave emission. This is
quite different from the oscillatory behavior of gravitating Nambu-Goto
membranes: the presence of gravity induces the wall to lose its dynamical
degree of freedom.


title: Canonical Quantization of Gravity without "Frozen Formalism"

abstract:
  We write down a quantum gravity equation which generalizes the Wheeler-DeWitt
one in view of including a time dependence in the wave functional. The obtained
equation provides a consistent canonical quantization of the 3-geometries
resulting from a ``gauge-fixing'' (3 + 1)-slicing of the space-time.
  Our leading idea relies on a criticism to the possibility that, in a quantum
space-time, the notion of a (3 + 1)-slicing formalism (underlying the
Wheeler-DeWitt approach) has yet a precise physical meaning. As solution to
this problem we propose of adding to the gravity-matter action the so-called
{\em kinematical action} (indeed in its reduced form, as implemented in the
quantum regime), and then we impose the new quantum constraints.
  As consequence of this revised approach, the quantization procedure of the
3-geometries takes place in a fixed reference frame and the wave functional
acquires a time evolution along a one-parameter family of spatial hypersurfaces
filling the space-time. We show how the states of the new quantum dynamics can
be arranged into an Hilbert space, whose associated inner product induces a
conserved probability notion for the 3-geometries.
  Finally, since the constraints we quantize violate the classical symmetries
(i. e. the vanishing nature of the super-Hamiltonian), then a key result is to
find a (non-physical) restriction on the initial wave functional phase,
ensuring that general relativity outcomes when taking the appropriate classical
limit. However we propose a physical interpretation of the kinematical
variables which, based on the analogy with the so-called {\em Gaussian
reference fluid}, makes allowance even for such classical symmetry violation.


title: Approximating the inspiral of test bodies into Kerr black holes

abstract:
  We present a new approximate method for constructing gravitational radiation
driven inspirals of test-bodies orbiting Kerr black holes. Such orbits can be
fully described by a semi-latus rectum $p$, an eccentricity $e$, and an
inclination angle $\iota$; or, by an energy $E$, an angular momentum component
$L_z$, and a third constant $Q$. Our scheme uses expressions that are exact
(within an adiabatic approximation) for the rates of change ($\dot{p}$,
$\dot{e}$, $\dot{\iota}$) as linear combinations of the fluxes ($\dot{E}$,
$\dot{L_z}$, $\dot{Q}$), but uses quadrupole-order formulae for these fluxes.
This scheme thus encodes the exact orbital dynamics, augmenting it with
approximate radiation reaction. Comparing inspiral trajectories, we find that
this approximation agrees well with numerical results for the special cases of
eccentric equatorial and circular inclined orbits, far more accurate than
corresponding weak-field formulae for ($\dot{p}$, $\dot{e}$, $\dot{\iota}$). We
use this technique to study the inspiral of a test-body in inclined, eccentric
Kerr orbits. Our results should be useful tools for constructing approximate
waveforms that can be used to study data analysis problems for the future LISA
gravitational-wave observatory, in lieu of waveforms from more rigorous
techniques that are currently under development.


title: An Improved Exact Riemann Solver for Multidimensional Relativistic Flows

abstract:
  We extend our approach for the exact solution of the Riemann problem in
relativistic hydrodynamics to the case in which the fluid velocity has
components tangential to the initial discontinuity. As in one-dimensional
flows, we here show that the wave-pattern produced in a multidimensional
relativistic Riemann problem can be predicted entirely by examining the initial
conditions. Our method is logically very simple and allows for a numerical
implementation of an exact Riemann solver which is both straightforward and
computationally efficient. The simplicity of the approach is also important for
revealing special relativistic effects responsible for a smooth transition from
one wave-pattern to another when the tangential velocities in the initial
states are suitably varied. While the content of this paper is focussed on a
flat spacetime, the local Lorentz invariance allows its use also in fully
general relativistic calculations.


title:
A generalization of the Lorentz ether to gravity with
  general-relativistic limit

abstract:
  Does relativistic gravity provide arguments against the existence of a
preferred frame? Our answer is negative. We define a viable theory of gravity
with preferred frame. In this theory, the EEP holds exactly, and the Einstein
equations of GR limit are obtained in a natural limit. Despite some remarkable
differences (stable "frozen stars" instead of black holes, a "big bounce"
instead of the big bang, exclusion of nontrivial topologies and closed causal
loops, and a preference for a flat universe) the theory is viable.
  The equations of the theory are derived from simple axioms about some
fundamental condensed matter (the generalized Lorentz ether), so that, in
particular, the EEP is not postulated but derived.
  The theory is compatible with the condensed matter interpretation for the
fermions and gauge fields of the standard model.


title: Introduction to branes and M-theory for relativists and cosmologists

abstract:
  We review the recent developments in superstrings. We start with a brief
summary of various consistent superstring theories and discuss T-duality which
necessarily leads to the presence of D-branes. The properties of D-branes are
summarized and we discuss how these suggest the existence of 11-dimensional
quantum theory, M-theory, which is believed to give rise to various
superstrings as perturbative expansions around particular backgrounds in the
theory. We also discuss the interpretation of brane solutions as black holes in
string theories and statistical explanation of Bekenstein-Hawking entropy. The
idea behind this interpretation is that there is a fundamental duality between
closed (gravity) and open (gauge theory) string degrees of freedom, one of
whose manifestation is what is kown as AdS/CFT correspondence. The idea is used
to discuss the greybody factors for BTZ black holes. Finally the entropy of
various balck holes are discussed in connection with Cardy-Verlinde formula.


title: Continuum spin foam model for 3d gravity

abstract:
  An example illustrating a continuum spin foam framework is presented. This
covariant framework induces the kinematics of canonical loop quantization, and
its dynamics is generated by a {\em renormalized} sum over colored polyhedra.
  Physically the example corresponds to 3d gravity with cosmological constant.
Starting from a kinematical structure that accommodates local degrees of
freedom and does not involve the choice of any background structure (e. g.
triangulation), the dynamics reduces the field theory to have only global
degrees of freedom. The result is {\em projectively} equivalent to the
Turaev-Viro model.


title: Gravitational Waves from a Fissioning White Hole

abstract:
  We present a fully nonlinear calculation of the waveform of the gravitational
radiation emitted in the fission of a vacuum white hole. At early times, the
waveforms agree with close-approximation perturbative calculations but they
reveal dramatic time and angular dependence in the nonlinear regime. The
results pave the way for a subsequent computation of the radiation emitted
after a binary black hole merger.


title:
Spacetime, General Covariance, Dirac-Bergmann Observables and
  Non-Inertial Frames

abstract:
  Talk at the 25th Johns Hopkins Workshop "2001: A Relativistic Spacetime
Odyssey", Firenze September 2001.


title: How loud can Schwarzschild black holes ring?

abstract:
  A numerical procedure is described for the maximization of the energy
diffusion due to the backscattering of the gravitational radiation. The
obtained maxima are solutions dominated by low frequency waves. They give rise
to robust gravitational ringing, with amplitudes of the order of the original
signal.


title:
Fate of the first traversible wormhole: black-hole collapse or
  inflationary expansion

abstract:
  We study numerically the stability of Morris & Thorne's first traversible
wormhole, shown previously by Ellis to be a solution for a massless ghost
Klein-Gordon field. Our code uses a dual-null formulation for spherically
symmetric space-time integration, and the numerical range covers both universes
connected by the wormhole. We observe that the wormhole is unstable against
Gaussian pulses in either exotic or normal massless Klein-Gordon fields. The
wormhole throat suffers a bifurcation of horizons and either explodes to form
an inflationary universe or collapses to a black hole, if the total input
energy is respectively negative or positive. As the perturbations become small
in total energy, there is evidence for critical solutions with a certain
black-hole mass or Hubble constant. The collapse time is related to the initial
energy with an apparently universal critical exponent. For normal matter, such
as a traveller traversing the wormhole, collapse to a black hole always
results. However, carefully balanced additional ghost radiation can maintain
the wormhole for a limited time. The black-hole formation from a traversible
wormhole confirms the recently proposed duality between them. The inflationary
case provides a mechanism for inflating, to macroscopic size, a Planck-sized
wormhole formed in space-time foam.


title: A Local Effective Action for Photon-Gravity Interactions

abstract:
  Quantum phenomena such as vacuum polarisation in curved spacetime induce
interactions between photons and gravity with quite striking consequences,
including the violation of the strong equivalence principle and the apparent
prediction of `superluminal' photon propagation. These quantum interactions can
be encoded in an effective action. In this paper, we extend previous results on
the effective action for QED in curved spacetime due to Barvinsky, Vilkovisky
and others and present a new, local effective action valid to all orders in a
derivative expansion, as required for a full analysis of the quantum theory of
high-frequency photon propagation in gravitational fields.


title:
Computing the gravitational self-force on a compact object plunging into
  a Schwarzschild black hole

abstract:
  We compute the gravitational self-force (or ``radiation reaction'' force)
acting on a particle falling radially into a Schwarzschild black hole. Our
calculation is based on the ``mode-sum'' method, in which one first calculates
the individual $\ell$-multipole contributions to the self-force (by numerically
integrating the decoupled perturbation equations) and then regularizes the sum
over modes by applying a certain analytic procedure. We demonstrate the
equivalence of this method with the $\zeta-$function scheme. The convergence
rate of the mode-sum series is considerably improved here (thus notably
reducing computational requirements) by employing an analytic approximation at
large $\ell$.


title: Well-Posed Initial-Boundary Evolution in General Relativity

abstract:
  Maximally dissipative boundary conditions are applied to the initial-boundary
value problem for Einstein's equations in harmonic coordinates to show that it
is well-posed for homogeneous boundary data and for boundary data that is small
in a linearized sense. The method is implemented as a nonlinear evolution code
which satisfies convergence tests in the nonlinear regime and is robustly
stable in the weak field regime. A linearized version has been stably matched
to a characteristic code to compute the gravitational waveform radiated to
infinity.


title: Gamma-Ray Bursts - a Primer For Relativists

abstract:
  Gamma-Ray Bursts (GRBs) - short bursts of 100-1MeV photons arriving from
random directions in the sky are probably the most relativistic objects
discovered so far. Still, somehow they did not attract the attention of the
relativistic community. In this short review I discuss briefly GRB observations
and show that they lead us to the fireball model - GRBs involve macroscopic
relativistic motion with Lorentz factors of a few hundred or more. I show that
GRB sources involve, most likely, new born black holes, and their progenitors
are Supernovae or neutron star mergers. I show that both GRB progenitors and
the process of GRB itself produce gravitational radiation and I consider the
possibility of detecting this emission. Finally I show that GRBs could serve as
cosmological indicators that could teach us about the high redshift ($z \approx
5-15$) dark ages of the universe.


title: Gravity beyond linear perturbations in the braneworld

abstract:
  In the present paper, we briefly review recent studies of second-order
gravitational perturbations in braneworld models. After we consider the
possibility of pathological behavior of gravity at higher orders of
perturbation, second-order perturbations are discussed in Randall-Sundrum
braneworld models. Because the mass spectra in these braneworld models are
different, we analyze them using different approaches. In the respective
models, it is discussed that 4D Einstein gravity is approximately recovered at
level of second-order perturbation, although there are some exceptional cases
in the two-brane model.


title: Nonlinear gravitational waves and their polarization

abstract:
  Vacuum gravitational fields invariant for a non Abelian Lie algebra generated
by two Killing fields whose commutator is light-like are analyzed. It is shown
that they represent nonlinear gravitational waves obeying to two nonlinear
superposition laws. The energy and the polarization of this family of waves are
explicitely evaluated.


title: Thermodynamic properties of massive dilaton black holes II

abstract:
  We numerically reanalyze static and spherically symmetric black hole
solutions in an Einstein-Maxwell-dilaton system with a dilaton potential
$m_{d}^{2}\phi^{2}$. We investigate thermodynamic properties for various
dilaton coupling constants and find that thermodynamic properties change at a
critical dilaton mass $m_{d,crit}$. For $m_{d}\geq m_{d,crit}$, the black hole
becomes an extreme solution for a nonzero horizon radius $r_{h,ex}$ as the
Reissner-Nordstr\"om black hole. However, if $m_{d}$ is nearly equal to
$m_{d,crit}$, there appears a solution of smaller horizon radius than
$r_{h,ex}$. For $m_{d}<m_{d,crit}$, a solution continues to exist until the
horizon approaches zero. The Hawking temperature in the zero horizon limit
resembles that of a massless dilaton black hole for arbitrary dilaton coupling
constant.