title: Conformal symmetry and deflationary gas universe

abstract:
  We describe the ``deflationary'' evolution from an initial de Sitter phase to
a subsequent Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) period as a
specific non-equilibrium configuration of a self-interacting gas. The
transition dynamics corresponds to a conformal, timelike symmetry of an
``optical'' metric, characterized by a refraction index of the cosmic medium
which continously decreases from a very large initial value to unity in the
FLRW phase.


title: Introduction to relativistic astrophysics and cosmology through Maple

abstract:
  The basics of the relativistic astrophysics including the celestial mechanics
in weak field, black holes and cosmological models are illustrated and analyzed
by means of Maple 6


title: Constraining Post_Newtonian Parameters With Gravitational Waves

abstract:
  We re-express gravitational wave results in terms of post-Newtonian
parameters. Using these expressions, and some simplifying assumptions, we
compute that in a favorable case, i.e. a ten solar-mass black hole spiraling in
to a 10^6 solar-mass black hole, LISA observations will be able to constrain at
the 10% level or better a single combination of post-post-Newtonian parameters
one order higher than those already constrained by solar system evidence. This
significant constraint will be possible even if the signal-to-noise level is so
low that the signal can only be found by matched filtering, and hence only
deviations between alternate signal interpretations of order one half cycle or
more can be detected.


title: On a scalar theory of gravitation

abstract:
  That preferred-frame theory accounts for special relativity and reduces to it
if the gravitation field cancels. Starting from an interpretation of gravity as
a pressure force, it is based on just one scalar field. This scalar gives the
relation between the flat "background" metric and the curved "physical" metric,
due to an equivalence principle between the absolute effects of motion and
gravitation. The scalar is also a potential for the gravity acceleration
vector. Motion is governed by an extension of the special-relativistic form of
Newton's second law. This provides a new equation for continuum dynamics, that
gives the gravitational modification of Maxwell's equations, consistent with
photon dynamics. The same effects on light rays as in GR are predicted at the
post-Newtonian approximation (PNA). An asymptotic PNA is being studied, in
order to build a consistent celestial mechanics in the theory. The cosmic
acceleration is predicted and nonsingular cosmological models are obtained.


title: Mechanics of Rotating Isolated Horizons

abstract:
  Black hole mechanics was recently extended by replacing the more commonly
used event horizons in stationary space-times with isolated horizons in more
general space-times (which may admit radiation arbitrarily close to black
holes). However, so far the detailed analysis has been restricted to
non-rotating black holes (although it incorporated arbitrary distortion, as
well as electromagnetic, Yang-Mills and dilatonic charges). We now fill this
gap by first introducing the notion of isolated horizon angular momentum and
then extending the first law to the rotating case.


title:
Coincidence of the Universe Description Stemming from D-branes Theory
  and ENU Model

abstract:
  The contribution provides a comparison of consequences stemming from D-brane
theories and Expansive Nondecelerative Universe model, and calls attention to
coincidence of the results arising from the mentioned approaches to a
description of the Universe. It follows from the comparison that the effects of
quantum gravitation should appear at the energy near to 2 TeV.


title: Information, Expansive Nondecelerative Universe, and Superstring Theory

abstract:
  Stemming from relationships between a number of information describing a
system and entropy content of the system it is possible to determine maximal
cosmological time. The contribution manifests a compatibility of the
superstring theory and the model of Expansive Nondecelerative Universe.


title: Is torsion needed in theory of gravity?

abstract:
  It is known that General Relativity ({\bf GR}) uses Lorentzian Manifold
$(M_4;g)$ as a geometrical model of the physical space-time. $M_4$ means here a
four-dimensional differentiable manifold endowed with Lorentzian metric $g$.
The metric $g$ satisfies Einstein equations. Since the 1970s many authors have
tried to generalize this geometrical model of the physical space-time by
introducing torsion and even more general metric-affine geometry. In this paper
we discuss status of torsion in the theory of gravity. At first, we emphasize
that up to now we have no experimental evidence for the existence of torsion in
Nature. Contrary, the all experiments performed in weak gravitational field
(Solar System) or in strong regime (binary pulsars) and tests of the Einstein
Equivalence Principle ({\bf EEP}) confirmed {\bf GR} and Lorentzian manifold
$(M_4;g)$ as correct geometrical model of the physical space-time. Then, we
give theoretical arguments against introducing of torsion into geometrical
model of the physical space-time. At last, we conclude that the
general-relativistic model of the physical space-time is sufficient and it
seems to be the most satisfactory.


title:
Initial data for a head on collision of two Kerr-like black holes with
  close limit

abstract:
  We prove the existence of a family of initial data for the Einstein vacuum
equation which can be interpreted as the data for two Kerr-like black holes in
arbitrary location and with spin in arbitrary direction. This family of initial
data has the following properties: (i) When the mass parameter of one of them
is zero or when the distance between them goes to infinity, it reduces exactly
to the Kerr initial data. (ii) When the distance between them is zero, we
obtain exactly a Kerr initial data with mass and angular momentum equal to the
sum of the mass and angular momentum parameters of each of them. The initial
data depends smoothly on the distance, the mass and the angular momentum
parameters.


title:
Will hyperbolic formulations help numerical relativity? - Experiments
  using Ashtekar's connection variables

abstract:
  In order to perform accurate and stable long-term numerical integration of
the Einstein equations, several hyperbolic systems have been proposed. We here
report our numerical comparisons between weakly hyperbolic, strongly
hyperbolic, and symmetric hyperbolic systems based on Ashtekar's connection
variables. The primary advantage for using this connection formulation is that
we can keep using the same dynamical variables for all levels of hyperbolicity.
  We also study asymptotically constrained systems, "$\lambda$-system" and
"adjusted system", for numerical integration of the Einstein equations, which
are intended to be robust against perturbative errors for the free evolution of
the initial data. These systems are tested in the Maxwell system and in the
Ashtekar's system. This mechanism affects more than the system's symmetric
hyperbolicity.
  (This workshop contribution is the summary of our gr-qc/0005003 [CQG 17
(2000) 4799] and gr-qc/0007034 [CQG 18 (2001) 441].)


title: Constraint propagation in the family of ADM systems

abstract:
  The current important issue in numerical relativity is to determine which
formulation of the Einstein equations provides us with stable and accurate
simulations. Based on our previous work on "asymptotically constrained"
systems, we here present constraint propagation equations and their eigenvalues
for the Arnowitt-Deser-Misner (ADM) evolution equations with additional
constraint terms (adjusted terms) on the right hand side. We conjecture that
the system is robust against violation of constraints if the amplification
factors (eigenvalues of Fourier-component of the constraint propagation
equations) are negative or pure-imaginary. We show such a system can be
obtained by choosing multipliers of adjusted terms. Our discussion covers
Detweiler's proposal (1987) and Frittelli's analysis (1997), and we also
mention the so-called conformal-traceless ADM systems.


title: Can Schwarzschildean gravitational fields suppress gravitational waves?

abstract:
  Gravitational waves in the linear approximation propagate in the
Schwarzschild spacetime similarly as electromagnetic waves. A fraction of the
radiation scatters off the curvature of the geometry. The energy of the
backscattered part of an initially outgoing pulse of the quadrupole
gravitational radiation is estimated by compact formulas depending on the
initial energy, the Schwarzschild radius, and the location and width of the
pulse. The backscatter becomes negligible in the short wavelength regime.


title: Behaviour of spin-half particles in curved space-time

abstract:
  We study the behaviour of spin-half particles in curved space-time. Since
Dirac equation gives the dynamics of spin-half particles, we mainly study the
Dirac equation in Schwarzschild, Kerr, Reissner-Nordstr\"om geometry. Due to
the consideration of existence of black hole in space-time (the curved
space-time), particles are influenced and equation will be modified. As a
result the solution will be changed from that due to flat space.


title: Detecting a relic background of scalar waves with LIGO

abstract:
  We discuss the possible detection of a stochastic background of massive,
non-relativistic scalar particles, through the cross correlation of the two
LIGO interferometers in the initial, enhanced and advanced configuration. If
the frequency corresponding to the mass of the scalar field lies in the
detector sensitivity band, and the non-relativistic branch of the spectrum
gives a significant contribution to energy density required to close the
Universe, we find that the scalar background can induce a non-negligible
signal, in competition with a possible signal produced by a stochastic
background of gravitational radiation.


title: The Confrontation between General Relativity and Experiment

abstract:
  The status of experimental tests of general relativity and of theoretical
frameworks for analysing them are reviewed. Einstein's equivalence principle
(EEP) is well supported by experiments such as the E\"otv\"os experiment, tests
of special relativity, and the gravitational redshift experiment. Future tests
of EEP and of the inverse square law will search for new interactions arising
from unification or quantum gravity. Tests of general relativity at the
post-Newtonian level have reached high precision, including the light
deflection, the Shapiro time delay, the perihelion advance of Mercury, and the
Nordtvedt effect in lunar motion. Gravitational wave damping has been detected
to half a percent using the binary pulsar, and new binary pulsar systems may
yield further improvements. When direct observation of gravitational radiation
from astrophysical sources begins, new tests of general relativity will be
possible.


title:
Master equation for quantum Brownian motion derived by stochastic
  methods

abstract:
  The master equation for a linear open quantum system in a general environment
is derived using a stochastic approach. This is an alternative derivation to
that of Hu, Paz and Zhang, which was based on the direct computation of path
integrals, or to that of Halliwell and Yu, based on the evolution of the Wigner
function for a linear closed quantum system. We first show by using the
influence functinal formalism that the reduced Wigner function for the open
system coincides with a distribution function resulting from averaging both
over the initial conditions and the stochastic source of a formal Langevin
equation. The master equation for the reduced Wigner function can then be
deduced as a Fokker-Planck equation obtained from the formal Langevin equation.


title: Varying c and Particle Horizons

abstract:
  We explore what restrictions may impose the second law of thermodynamics on
varying speed of light theories. We find that the attractor scenario solving
the flatness problem is consistent with the generalized second law at late
time.


title: Matter as Curvature and Torsion of General Metric-Affine Space

abstract:
  General equations of the unified field theory, based on the curved and
torsional space-time, are presented. They contain only independent geometrical
parameters (metric and connections) of metric-affine space, and describe the
distribution and motion of matter, which represents the curvature and torsion
of the space-time. Equations, describing spherically symmetric fields, are
derived for various cases: a gravitational field in vacuum, arbitrary material
field, and massless fluid with spin. Equations with and without cosmological
terms, are derived for the closed and open uniform isotropic models of the
Universe for the gravitational field in vacuum, arbitrary material field, and
massless fluid with spin. The respective solutions without singularities were
obtained for all these cases. It is shown that there is no necessity to
quantize the obtained general equations. Applying the concept of the unified
geometrical field, the hypothetical qualitative explanations of some
non-conventional phenomena are proposed.


title:
Mesoscopic Casimir forces from effects of discrete particle number in
  the quantum vacuum

abstract:
  Traditionally it is assumed that the Casimir vacuum pressure does not depend
on the ultraviolet cut-off. There are, however, some arguments that the effect
actually depends on the regularization procedure and thus on the
trans-Planckian physics. We provide the condensed matter example where the
Casimir forces do explicitly depend on the microscopic (correspondingly
trans-Planckian) physics due to the mesoscopic finite-N effects, where N is the
number of bare particles in condensed matter (or correspondingly the number of
the elements comprising the quantum vacuum). The finite-N effects lead to
mesoscopic fluctuations of the vacuum pressure. The amplitude of the mesoscopic
flustuations of the Casimir force in a system with linear dimension L is larger
by the factor N^{1/3}\sim L/a than the traditional value of the Casimir force
given by effective theory, where a is the interatomic distance which plays the
role of the Planck length.


title:
Quasiequilibrium sequences of synchronized and irrotational binary
  neutron stars in general relativity. II. Newtonian limits

abstract:
  We study equilibrium sequences of close binary systems composed of identical
polytropic stars in Newtonian gravity. The solving method is a multi-domain
spectral method which we have recently developed. An improvement is introduced
here for accurate computations of binary systems with stiff equation of state
($\gamma > 2$). The computations are performed for both cases of synchronized
and irrotational binary systems with adiabatic indices $\gamma=3,~2.5,~2.25,~2$
and 1.8. It is found that the turning points of total energy along a
constant-mass sequence appear only for $\gamma \ge 1.8$ for synchronized binary
systems and $\gamma \ge 2.3$ for irrotational ones. In the synchronized case,
the equilibrium sequences terminate by the contact between the two stars. On
the other hand, for irrotational binaries, it is found that the sequences
terminate at a mass shedding limit which corresponds to a detached
configuration.


title: Perturbations with angular momentum of Robinson-Trautman spacetimes

abstract:
  We study the possible asymptotically flat perturbations of Robinson-Trautman
spacetimes. We differentiate between algebraically special perturbations and
general perturbations. The equations that determine physically realistic
spacetimes with angular momentum are presented.


title: 1-Loop Divergences of Quantum Gravity Using Conformal Parametrization

abstract:
  We calculate the one-loop divergences for quantum gravity with cosmological
constant, using new parametrization of quantum metric. The conformal factor of
the metric is treated as an independent variable. As a result the theory
possesses an additional degeneracy and one needs an extra conformal gauge
fixing. We verify the on shell independence of the divergences from the
parameter of the conformal gauge fixing, and find a special conformal gauge in
which the divergences coincide with the ones obtained by t'Hooft and Veltman
(1974). Using conformal invariance of the counterterms one can restore the
divergences for the conformal metric-scalar gravity.


title: The Meaning of Einstein's Equation

abstract:
  This is a brief introduction to general relativity, designed for both
students and teachers of the subject. While there are many excellent
expositions of general relativity, few adequately explain the geometrical
meaning of the basic equation of the theory: Einstein's equation. Here we give
a simple formulation of this equation in terms of the motion of freely falling
test particles. We also sketch some of its consequences, and explain how the
formulation given here is equivalent to the usual one in terms of tensors.
Finally, we include an annotated bibliography of books, articles and websites
suitable for the student of relativity.


title:
Inflation from a massive scalar field and scalar perturbations of the
  metric

abstract:
  In the framework of chaotic inflation we study the case of a massive scalar
field using a semiclassical approach. We derive the energy density fluctuations
in the infrared sector generated by matter field and gauge-invariant metric
fluctuations by means of a different method. We find that the super Hubble
density fluctuations increase during inflation for a massive scalar field, in
agreement with the result obtained in a previous work in which a power-law
expanding universe was considered.


title: Neutrino Oscillations Induced by Space-Time Torsion

abstract:
  The gravitational neutrino oscillation problem is studied by considering the
Dirac Hamiltonian in a Riemann-Cartan space-time and calculating the dynamical
phase. Torsion contributions which depend on the spin direction of the mass
eigenstates are found. These effects are of the order of Planck scales.


title:
A Characterisation of the Weylian Structure of Space-Time by Means of
  Low Velocity Tests

abstract:
  The compatibility axiom in Ehlers, Pirani and Schild's (EPS) constructive
axiomatics of the space-time geometry that uses light rays and freely falling
particles with high velocity, is replaced by several constructions with low
velocity particles only. For that purpose we describe in a space-time with a
conformal structure and an arbitrary path structure the radial acceleration, a
Coriolis acceleration and the zig-zag construction. Each of these quantities
give effects whose requirement to vanish can be taken as alternative version of
the compatibility axiom of EPS. The procedural advantage lies in the fact, that
one can make null-experiments and that one only needs low velocity particles to
test the compatibility axiom. We show in addition that Perlick's standard clock
can exist in a Weyl space only.


title:
Generalized Hamiltonian Dynamics of Friedmann Cosmology with Scalar and
  Spinor Matter Source Fields

abstract:
  The classical and quantum dynamics of the Friedmann-Robertson-Walker Universe
with massless scalar and massive fermion matter field as a source is discussed
in the framework of the Dirac generalized Hamiltonian formalism. The
Hamiltonian reduction of this constrained system is realized for two cases of
minimal and conformal coupling between gravity and matter. It is shown that in
both cases for all values of curvature, of maximally symmetric space there
exists a time independent reduced local Hamiltonian which describes the
dynamics of the cosmic scale factor. The relevance of conformal time-like
Killing vector fields in FRW space-time to the existence of time independent
Hamiltonian and the corresponding notion of conserved energy is discussed. The
extended quantization with the Wheeler-deWitt equation is compared with the
canonical quantization of unconstrained system. It is shown that quantum
observables treated as expectation values of the Dirac observables properly
describe the original classical theory.


title: On the Conventionality of Simultaneity

abstract:
  Starting from the experimental fact that light propagates over a closed path
at speed c (L/c law), we show to what extent the isotropy of the speed of light
can be considered a matter of convention. We prove the consistence of
anisotropic and inhomogenous conventions, limiting the allowed possibilities.
All conventions lead to the same physical theory even if its formulation can
change in form. The mathematics involved is that of gauge theories and the
choice of a simultaneity convention is interpreted as a choice of the gauge.
Moreover we prove that a Euclidean space where the L/c law holds, gives rise to
a spacetime with Minkowskian causal structure, and we exploit the consequences
for the causal approach to the conventionality of simultaneity.


title: Tensor-Scalar Torsion

abstract:
  A theory of gravity with torsion is examined in which the torsion tensor is
constructed from the exterior derivative of an antisymmetric rank two potential
plus the dual of the gradient of a scalar field. Field equations for the theory
are derived by demanding that the action be stationary under variations with
respect to the metric, the antisymmetric potential, and the scalar field. A
material action is introduced and the equations of motion are derived. The
correct conservation law for rotational angular momentum plus spin is observed
to hold in this theory.


title:
The Indeterministic Einstein Equation: Quantum Jumps, Spacetime
  Structure, and Dark Pseudomatter

abstract:
  Current physics is faced with the fundamental problem of unifying quantum
theory and general relativity, which would have resulted in quantum gravity.
The main effort to construct the latter has been bent on quantizing spacetime
structure, in particular metric. Meanwhile, taking account of the
indeterministic aspect of the quantum description of matter, which manifests
itself in quantum jumps, essentially affects classical spacetime structure and
the Einstein equation. Quantum jumps give rise to a family of sets of
simultaneous events, which implies the existence of universal cosmological
time. In view of the jumps, the requirement for metric and its time derivative
to be continuous implies that the Einstein equation should involve pseudomatter
along with matter. Pseudomatter manifests itself only in gravitational effects,
being thereby an absolutely dark ``matter''.


title: On the interpretation of Michelson-Morley experiments

abstract:
  Recent proposals for improved optical tests of Special Relativity have
renewed interest in the interpretation of such tests. In this paper we discuss
the interpretation of modern realizations of the Michelson-Morley experiment in
the context of a new model of electrodynamics featuring a vector-valued photon
mass. This model is gauge invariant, unlike massive-photon theories based on
the Proca equation, and it predicts anisotropy of both the speed of light and
the electric field of a point charge. The latter leads to an orientation
dependence of the length of solid bodies which must be accounted for when
interpreting the results of a Michelson-Morley experiment. Using a simple model
of ionic solids we show that, in principle, the effect of orientation dependent
length can conspire to cancel the effect of an anisotropic speed of light in a
Michelson-Morley experiment, thus, complicating the interpretation of the
results.


title: Relaxation versus collision times in the cosmological radiative era

abstract:
  We consider the Lema\^{\i}tre-Tolman-Bondi metric with an inhomogeneous
viscous fluid source satisfying the equation of state of an interactive mixture
of radiation and matter. Assuming conditions prior to the decoupling era, we
apply Extended Irreversible Thermodynamcs (EIT) to this mixture. Using the full
transport equation of EIT we show that the relaxation time of shear viscosity
can be several orders of magnitude larger than the Thomson collision time
between photons and electrons. A comparison with the ``truncated'' transport
equation for these models reveals that the latter cannot describe properly the
decoupling of matter and radiation

categories: gr-qc astro-ph cond-mat.stat-mech physics.plasm-ph


title: Late-time dynamics of rapidly rotating black holes

abstract:
  We study the late-time behaviour of a dynamically perturbed rapidly rotating
black hole. Considering an extreme Kerr black hole, we show that the large
number of virtually undamped quasinormal modes (that exist for nonzero values
of the azimuthal eigenvalue m) combine in such a way that the field (as
observed at infinity) oscillates with an amplitude that decays as 1/t at late
times. This is in clear contrast with the standard late time power-law fall-off
familiar from studies of non-rotating black holes. This long-lived oscillating
``tail'' will, however, not be present for non-extreme (presumably more
astrophysically relevant) black holes, for which we find that many quasinormal
modes (individually excited to a very small amplitude) combine to give rise to
an exponentially decaying field. This result could have implications for the
detection of gravitational-wave signals from rapidly spinning black holes,
since the required theoretical templates need to be constructed from linear
combinations of many modes. Our main results are obtained analytically, but we
support the conclusions with numerical time-evolutions of the Teukolsky
equation. These time-evolutions provide an interesting insight into the notion
that the quasinormal modes can be viewed as waves trapped in the spacetime
region outside the horizon. They also suggest that a plausible mechanism for
the behaviour we observe for extreme black holes is the presence of a
``superradiance resonance cavity'' immediately outside the black hole.


title: The End of Time?

abstract:
  I discuss J. Barbour's Machian theories of dynamics, and his proposal that a
Machian perspective enables one to solve the problem of time in quantum
geometrodynamics (by saying that there is no time). I concentrate on his recent
book 'The End of Time' (1999).


title:
Application of linear hyperbolic PDE to linear quantum fields in curved
  spacetimes: especially black holes, time machines and a new semi-local vacuum
  concept

abstract:
  Several situations of physical importance may be modelled by linear quantum
fields propagating in fixed spacetime-dependent classical background fields.
For example, the quantum Dirac field in a strong and/or time-dependent external
electromagnetic field accounts for the creation of electron-positron pairs out
of the vacuum. Also, the theory of linear quantum fields propagating on a given
background curved spacetime is the appropriate framework for the derivation of
black-hole evaporation (Hawking effect) and for studying the question whether
or not it is possible in principle to manufacture a time-machine. It is a
well-established metatheorem that any question concerning such a linear quantum
field may be reduced to a definite question concerning the corresponding
classical field theory (i.e. linear hyperbolic PDE with non-constant
coefficients describing the background in question) -- albeit not necessarily a
question which would have arisen naturally in a purely classical context. The
focus in this talk will be on the covariant Klein-Gordon equation in a fixed
curved background, although we shall draw on analogies with other background
field problems and with the time-dependent harmonic oscillator. The aim is to
give a sketch-impression of the whole subject of Quantum Field Theory in Curved
Spacetime, focussing on work with which the author has been personally
involved, and also to mention some ideas and work-in-progress by the author and
collaborators towards a new "semi-local" vacuum construction for this subject.
A further aim is to introduce, and set into context, some recent advances in
our understanding of the general structure of quantum fields in curved
spacetimes which rely on classical results from microlocal analysis.


title: The bending of light and the cosmological constant

abstract:
  The bending of light in Kottler space (the Schwarzschild vacuum with
cosmological constant) is examined. Unlike the advance of the perihelion, the
cosmological constant produces no change in the bending of light. In this note
we examine the conditions under which this statement holds.


title: Locally Anisotropic Wormholes and Flux Tubes in 5D Gravity

abstract:
  In this article we examine a class of wormhole and flux tube like solutions
to 5D vacuum Einstein equations. These solutions possess generic local
anisotropy, and their local isotropic limit is shown to be conformally
equivalent to the spherically symmetric 5D solutions of gr-qc/9807086. The
anisotropic solutions investigated here have two physically distinct
signatures: First, they can give rise to angular-dependent, anisotropic
``electromagnetic'' interactions. Second, they can result in a gravitational
running of the ``electric'' and ``magnetic'' charges of the solutions. This
gravitational running of the electromagnetic charges is linear rather than
logarithmic, and could thus serve as an indirect signal for the presence of
higher dimensions. The local anisotropy of these solutions is modeled using the
technique of anholonomic frames with respect to which the metrics are
diagonalized. If holonomic coordinates frames were used then such metrics would
have off-diagonal components.


title: A Proof of the Generalized Second Law for Two-Dimensional Black Holes

abstract:
  We investigate the generalized second law for two-dimensional black holes in
equilibrium (Hartle-Hawking) and nonequilibrium (Unruh) with the heat bath
surrounding the black holes. We obtain a simple expression for the change of
total entropy in terms of covariant thermodynamic variables, which is valid not
only for the Hartle-Hawking state but also for the Unruh state up to leading
order, without assuming a quasi-stationary evolution of the black holes. Using
this expression, it is shown that the rate of local entropy production is
non-negative in the two-dimensional black hole systems.


title: Stability analysis of cosmological models through Liapunov's method

abstract:
  We investigate the general asymptotic behaviour of Friedman-Robertson-Walker
(FRW) models with an inflaton field, scalar-tensor FRW cosmological models and
diagonal Bianchi-IX models by means of Liapunov's method. This method provides
information not only about the asymptotic stability of a given equilibrium
point but also about its basin of attraction. This cannot be obtained by the
usual methods found in the literature, such as linear stability analysis or
first order perturbation techniques. Moreover, Liapunov's method is also
applicable to non-autonomous systems. We use this advantadge to investigate the
mechanism of reheating for the inflaton field in FRW models.


title: Notes on moving mirrors

abstract:
  The Davies-Fulling (DF) model describes the scattering of a massless field by
a non-inertial mirror in two dimensions. In this paper, we generalize this
model in two different ways. First, we consider partially reflecting mirrors.
We show that the Bogoliubov coefficients relating inertial modes can be
expressed in terms of the frequency dependent reflection factor which is
specified in the rest frame of the mirror and the transformation from the
inertial modes to the modes at rest with respect to the mirror. In this
perspective, the DF model is simply the limiting case when this factor is unity
for all frequencies. In the second part, we introduce an alternative model
which is based on self-interactions described by an action principle. When the
coupling is constant, this model can be solved exactly and gives rise to a
partially reflecting mirror. The usefulness of this dynamical model lies in the
possibility of switching off the coupling between the mirror and the field.
This allows to obtain regularized expressions for the fluxes in situations
where they are singular when using the DF model. Two examples are considered.
The first concerns the flux induced by the disappearance of the reflection
condition, a situation which bears some analogies with the end of the
evaporation of a black hole. The second case concerns the flux emitted by a
uniformly accelerated mirror.


title: Trapped and excited w modes of stars with a phase transition and R>=5M

abstract:
  The trapped $w$-modes of stars with a first order phase transition (a density
discontinuity) are computed and the excitation of some of the modes of these
stars by a perturbing shell is investigated. Attention is restricted to odd
parity (``axial'') $w$-modes. With $R$ the radius of the star, $M$ its mass,
$R_{i}$ the radius of the inner core and $M_{i}$ the mass of such core, it is
shown that stars with $R/M\geq 5$ can have several trapped $w$-modes, as long
as $R_{i}/M_{i}<2.6$. Excitation of the least damped $w$-mode is confirmed for
a few models. All of these stars can only exist however, for values of the
ratio between the densities of the two phases, greater than $\sim 46$. We also
show that stars with a phase transition and a given value of $R/M$ can have far
more trapped modes than a homogeneous single density star with the same value
of $R/M$, provided both $R/M$ and $R_{i}/M_{i}$ are smaller than 3. If the
phase transition is very fast, most of the stars with trapped modes are
unstable to radial oscillations. We compute the time of instability, and find
it to be comparable to the damping of the $w$-mode excited in most cases where
$w$-mode excitation is likely. If on the other hand the phase transition is
slow, all the stars are stable to radial oscillations.


title:
A family of regular quantum interiors for non-rotating black holes I:
  The GRNSS spacetimes

abstract:
  A seemingly natural mechanism is proposed, that could stop the gravitational
collapse of a very massive body. Without needing to change the concept of the
collapsing process itself, that is, without invoking thin layers nor resorting
to asymptoticity (as has been usually done in the literature), it is proven
that a model can be built in which the quantum vacuum is able to produce a
negative stress that may stop the collapse of the black hole, reaching a final
state of the spacetime structure that is a static de Sitter model. The solution
is found by looking into a generic family of spacetimes: that of maximal
spherically symmetric ones expanded by a geodesic radial null one-form from
flat spacetime. They are called here GRNSS spaces, and are proven to constitute
a distinguished family of Kerr-Schild metrics. The models considered previously
in the literature are easily recovered in this approach, which yields,
moreover, an infinite set of possible candidates for the interior of the black
hole. First steps towards their semiclassical quantization are undertaken. It
is shown that the quantization protocol may be here more easily carried out
than within conformal field theory.


title:
A family of quantum interiors for ordinary and stringy black holes II:
  Geometric considerations and global properties

abstract:
  A family of spacetimes suitable for describing the matter conditions of a
static, spherically symmetric quantum vacuum is studied, as well as its
reliability for describing a regular model for the interior of a semiclassical,
static black hole ---without ever invoking a mass shell for the final object.
In paper I, this condition was seen to limit the search to only one,
distinguished family, that was investigated in detail. Here it will be proven
that, aside from being mathematically generic (in its uniqueness), this family
exhibits beautiful physical properties, that one would reasonably demand from a
collapse process, including the remarkable result that isotropization may take
place conveniently far from the (unavoidable) regularization scale. The
analysis is also extended in order to include the possibility of a stringy
core, always within the limits imposed by the semiclassical approach to
gravitation. This constitutes a first approximation to the final goal of trying
to characterize a regular, self-gravitating, stringy black hole.


title: Spacetime geometry of static fluid spheres

abstract:
  We exhibit a simple and explicit formula for the metric of an arbitrary
static spherically symmetric perfect fluid spacetime. This class of metrics
depends on one freely specifiable monotone non-increasing generating function.
We also investigate various regularity conditions, and the constraints they
impose. Because we never make any assumptions as to the nature (or even the
existence) of an equation of state, this technique is useful in situations
where the equation of state is for whatever reason uncertain or unknown.
  To illustrate the power of the method we exhibit a new form of the
``Goldman--I'' exact solution and calculate its total mass. This is a
three-parameter closed-form exact solution given in terms of algebraic
combinations of quadratics. It interpolates between (and thereby unifies) at
least six other reasonably well-known exact solutions.


title:
A Connection of Apparent Horizon and Naked Singularities in Spherical
  Dust Collapse

abstract:
  We show that the behaviour of outgoing radial null geodesic congruence on the
apparent horizon is related to the property of nakedness in spherical dust
collapse justifying the difference between the Penrose diagrams in the naked
and covered dust collapse scenarios. We provide arguments suggesting that the
relationship could be generally valid.


title:
Principles of Equivalence: Their Role in Gravitation Physics and
  Experiments that Test Them

abstract:
  Modern formulations of equivalence principles provide the foundation for an
efficient approach to understanding and organizing the structural features of
gravitation field theories. Since theories' predictions reflect differences in
their structures, principles of equivalence also support an efficient
experimental strategy for testing gravitation theories and for exploring the
range of conceivable gravitation physics. These principles focus attention
squarely on empirical consequences of the fundamental structural differences
that distinguish one gravitation theory from another. Interestingly, the
variety of such consequences makes it possible to design and perform
experiments that test equivalence principles stringently but do so in markedly
different ways than the most familiar experimental tests.


title: A Classical Interpretation of Maxwell's Equations in Curved Spacetime

abstract:
  We present an integral formulation of observer-dependent Maxwell's equations
in curved spacetime and give a classical interpretation of them.


title: Scaling Behavior in the Einstein-Yang-Mills Monopoles and Dyons

abstract:
  Scaling behavior in the moduli space of monopole and dyon solutions in the
Einstein-Yang-Mills theory in the asymptotically anti-de Sitter space is
derived. The mass of monopoles and dyons scales with respect to their magnetic
and electric charges, independent of the values of the cosmological constant
and gauge coupling constant. The stable monopole and dyon solutions are
approximated by solutions in the fixed anti-de Sitter spacetime. Unstable
solutions can be viewed as the Bartnik-McKinnon solutions dressed with monopole
and dyon solutions in the fixed anti-de Sitter space.


title: Shortcuts in the fifth dimension

abstract:
  If our Universe is a three-brane embedded in a five-dimensional anti-deSitter
spacetime, in which matter is confined to the brane and gravity inhabits an
infinite bulk space, then the causal propagation of luminous and gravitational
signals is in general different. A gravitational signal traveling between two
points on the brane can take a ``shortcut'' through the bulk, and appear
quicker than a photon traveling between the same two points along a geodesic on
the brane. Similarly, in a given time interval, a gravitational signal can
propagate farther than a luminous signal. We quantify this effect, and analyze
the impact of these shortcuts through the fifth dimension on cosmology.


title: Quantum Radiation of Uniformly Accelerated Spherical Mirrors

abstract:
  We study quantum radiation generated by a uniformly accelerated motion of
small spherical mirrors. To obtain Green's function for a scalar massless field
we use Wick's rotation. In the Euclidean domain the problem is reduced to
finding an electric potential in 4D flat space in the presence of a metallic
toroidal boundary. The latter problem is solved by a separation of variables.
After performing an inverse Wick's rotation we obtain the Hadamard function in
the wave-zone regime and use it to calculate the vacuum fluctuations and the
vacuum expectation for the energy density flux in the wave zone.


title:
Symmetry properties of the metric energy-momentum tensor in classical
  field theories and gravity

abstract:
  We derive a generic identity which holds for the metric (i.e. variational)
energy-momentum tensor under any field transformation in any generally
covariant classical Lagrangian field theory. The identity determines the
conditions under which a symmetry of the Lagrangian is also a symmetry of the
energy-momentum tensor. It turns out that the stress tensor acquires the
symmetry if the Lagrangian has the symmetry in a generic curved spacetime. In
this sense a field theory in flat spacetime is not self-contained. When the
identity is applied to the gauge invariant spin-two field in Minkowski space,
we obtain an alternative and direct derivation of a known no-go theorem: a
linear gauge invariant spin-2 field, which is dynamically equivalent to
linearized General Relativity, cannot have a gauge invariant metric
energy-momentum tensor. This implies that attempts to define the notion of
gravitational energy density in terms of the metric energy--momentum tensor in
a field-theoretical formulation of gravity must fail.


title:
How strings solve the apparent contradiction between black holes and
  quantum coherence

abstract:
  Computations in the calculable small coupling regime of string theories and
the general consensus that no new physics has to be invoked in continuing to
the large coupling (black hole) regime, suggest the following picture. Quantum
states are not black holes even if energy distributions would suggest so. Black
holes appear as macrostates, i.e. with the same procedure that blures the
quantum coherence of microstates. It is also discussed how a spacetime
description - and thus geometry, causal properties and event horizons - may
stem from decoherence in the pregeometric approach represented by string
theories.


title:
Local Wick Polynomials and Time Ordered Products of Quantum Fields in
  Curved Spacetime

abstract:
  In order to have well defined rules for the perturbative calculation of
quantities of interest in an interacting quantum field theory in curved
spacetime, it is necessary to construct Wick polynomials and their time ordered
products for the noninteracting theory. A construction of these quantities has
recently been given by Brunetti, Fredenhagen, and Kohler, and by Brunetti and
Fredenhagen, but they did not impose any ``locality'' or ``covariance''
condition in their constructions. As a consequence, their construction of time
ordered products contained ambiguities involving arbitrary functions of
spacetime point rather than arbitrary parameters. In this paper, we construct
an ``extended Wick polynomial algebra''-large enough to contain the Wick
polynomials and their time ordered products. We then define the notion of a
{\it local, covariant quantum field}, and seek a definition of {\it local} Wick
polynomials and their time ordered products as local, covariant quantum fields.
We impose scaling requirements on our local Wick polynomials and their time
ordered products as well as certain additional requirements-such as commutation
relations with the free field and appropriate continuity properties under
variations of the spacetime metric. For a given polynomial order in powers of
the field, we prove that these conditions uniquely determine the local Wick
polynomials and their time ordered products up to a finite number of
parameters. (These parameters correspond to the usual renormalization
ambiguities occurring in Minkowski spacetime together with additional
parameters corresponding to the coupling of the field to curvature.) We also
prove existence of local Wick polynomials. However, the issue of existence of
local time ordered products is deferred to a future investigation


title: Space missions to detect the cosmic gravitational-wave background

abstract:
  It is thought that a stochastic background of gravitational waves was
produced during the formation of the universe. A great deal could be learned by
measuring this Cosmic Gravitational-wave Background (CGB), but detecting the
CGB presents a significant technological challenge. The signal strength is
expected to be extremely weak, and there will be competition from unresolved
astrophysical foregrounds such as white dwarf binaries. Our goal is to identify
the most promising approach to detect the CGB. We study the sensitivities that
can be reached using both individual, and cross-correlated pairs of space based
interferometers. Our main result is a general, coordinate free formalism for
calculating the detector response that applies to arbitrary detector
configurations. We use this general formalism to identify some promising
designs for a GrAvitational Background Interferometer (GABI) mission. Our
conclusion is that detecting the CGB is not out of reach.


title:
New Perspectives on the Relativistically Rotating Disk and
  Non-time-orthogonal Reference Frames

abstract:
  The rotating disk problem is analyzed on the premise that proper
interpretation of experimental evidence leads to the conclusion that the
postulates upon which relativity theory is based, particularly the invariance
of the speed of light, are not applicable to rotating frames. Different
postulates based on the Sagnac experiment are proposed, and from these
postulates a new relativistic theory of rotating frames is developed following
steps similar to those initially followed by Einstein for rectilinear motion.
The resulting theory agrees with all experiments, resolves problems with the
traditional approach to the rotating disk, and exhibits both traditionally
relativistic and non-relativistic characteristics. Of particular note, no
Lorentz contraction exists on the rotating disk circumference, and the disk
surface, contrary to the assertions of Einstein and others, is found to be
Riemann flat. The variable speed of light found in the Sagnac experiment is
then shown to be characteristic of non-time-orthogonal reference frames, of
which the rotating frame is one. In addition, the widely accepted postulate for
the equivalence of inertial and non-inertial standard rods with zero relative
velocity, used liberally in prior rotating disk analyses, is shown to be
invalid for such frames. Further, the new theory stands alone in correctly
predicting what was heretofore considered a "spurious" non-null effect on the
order of 10^-13 found by Brillet and Hall in the most accurate Michelson-Morley
type test to date. The presentation is simple and pedagogic in order to make it
accessible to the non-specialist.
  Key words: relativistic, rotating disk, Sagnac, rotating frame,
non-time-orthogonal frame.


title: Dimension in a Radiative Stellar Atmosphere

abstract:
  Dimensional scales are examined in an extended 3+1 Vaidya atmosphere
surrounding a Schwarzschild source. At one scale, the Vaidya null fluid
vanishes and the spacetime contains only a single spherical 2-surface. Both of
these behaviors can be addressed by including higher dimensions in the
spacetime metric.


title: Two Black Hole Holography, Lensing and Intensity

abstract:
  We numerically verify the analysis of the "expanding horizon" theory of
Susskind in relation to the 't Hooft holographic conjecture. By using a
numerical simulation to work out the image formed by two black holes upon a
screen very far away, it is seen that it is impossible for a horizon to hide
behind another. We also compute the intensity distribution of such an
arrangement.


title: On Observational Predictions from Multidimensional Gravity

abstract:
  We discuss possible observational manifestations of static, spherically
symmetric solutions of a class of multidimensional theories of gravity, which
includes the low energy limits of supergravities and superstring theories as
special cases. We discuss the choice of a physical conformal frame to be used
for the description of observations. General expressions are given for (i) the
Eddington parameters $\beta$ and $\gamma$, characterizing the post-Newtonian
gravitational field of a central body, (ii) p-brane black hole temperatures in
different conformal frames and (iii) the Coulomb law modified by extra
dimensions. It is concluded, in particular, that $\beta$ and $\gamma$ depend on
the integration constants and can be therefore different for different central
bodies. If, however, the Einstein frame is adopted for describing observations,
we always obtain $\gamma=1$. The modified Coulomb law is shown to be
independent of the choice of a 4-dimensional conformal frame. We also argue the
possible existence of specific multidimensional objects, T-holes, potentially
observable as bodies with mirror surfaces.


title: Cauchy horizon stability and cosmic censorship

abstract:
  Some interesting consequences of spacelike matter shells are presented, in
particular the possibility of travelling through Cauchy horizons and violating
the strong cosmic censorship principle. These show that the weak energy
condition does not guarentee cosmic censorship.


title: Immirzi parameter in the Barrett-Crane model?

abstract:
  We study the generalised constrained BF theory described in gr-qc/0102073 in
order to introduce the Immirzi parameter in spin foam models. We show that the
resulting spin foam model is still based on simple representations and that the
generalised BF action is simply a deformation of the Barrett-Crane model. The
Immirzi parameter doesn't change the representations used in the spin foam
model, so it doesn't affect the geometry of the model. However we show how it
may still appear as a factor in the area spectrum.


title:
Evolution of the Density Contrast with Particle Production in a
  Gauge-Invariant Formalism

abstract:
  In this work we derive the evolution equation for the density contrast
considering open system cosmology, where the influence of adiabatic particle
production process on the dynamic of a homogeneous and isotropic is
investigated within a manifestly covariant formulation. As application we
derive the solution for two sources, one of them is a generalization of
Prigogine's model. Then we establish the condition for the reach of the
non-linear regime for the density contrast which turns out to be a necessary
condition for the structure formation.


title: Density perturbations in an Universe dominated by the Chaplygin gas

abstract:
  We study the fate of density perturbations in an Universe dominate by the
Chaplygin gas, which exhibit negative pressure. We show that it is possible to
obtain the value for the density contrast observed in large scale structure of
the Universe by fixing a free parameter in the equation of state of this gas.
The negative character of pressure must be significant only very recently.


title: The Phase Space Structure of Multi Particle Models in 2+1 Gravity

abstract:
  What can we learn about quantum gravity from a simple toy model, without
actually quantizing it? The toy model consists of a finite number of point
particles, coupled to three dimensional Einstein gravity. It has finitely many
physical degrees of freedom. These are basically the relative positions of the
particles in spacetime and the conjugate momenta. The resulting reduced phase
space is derived from Einstein gravity as a topological field theory. The
crucial point is thereby that we do not make any a priori assumptions about
this phase space, except that the dynamics of the gravitational field is
defined by the Einstein Hilbert action. This already leads to some interesting
features of the reduced phase space, such as a non-commutative structure of
spacetime when the model is quantized.


title: The 2+1 Kepler Problem and Its Quantization

abstract:
  We study a system of two pointlike particles coupled to three dimensional
Einstein gravity. The reduced phase space can be considered as a deformed
version of the phase space of two special-relativistic point particles in the
centre of mass frame. When the system is quantized, we find some possibly
general effects of quantum gravity, such as a minimal distances and a foaminess
of the spacetime at the order of the Planck length. We also obtain a
quantization of geometry, which restricts the possible asymptotic geometries of
the universe.


title: Asymptotic tails of massive scalar fields in Schwarzschild background

abstract:
  We investigate the asymptotic tail behavior of massive scalar fields in
Schwarzschild background. It is shown that the oscillatory tail of the scalar
field has the decay rate of $t^{-5/6}$ at asymptotically late times, and the
oscillation with the period $2\pi/m$ for the field mass $m$ is modulated by the
long-term phase shift. These behaviors are qualitatively similar to those found
in nearly extreme Reissner-Nordstr\"{o}m background, which are discussed in
terms of a resonant backscattering due to the space-time curvature.


title: Bounce behaviour in Kantowski-Sachs and Bianchi Cosmologies

abstract:
  Many cosmological scenarios envisage either a bounce of the universe at early
times, or collapse of matter locally to form a black hole which re-expands into
a new expanding universe region. Energy conditions preclude this happening for
ordinary matter in general relativistic universes, but scalar or dilatonic
fields can violate some of these conditions, and so could possibly provide
bounce behaviour. In this paper we show that such bounces cannot occur in
Kantowski-Sachs models without violating the {\it reality condition}
$\dot{\phi}^2\geq 0$. This also holds true for other isotropic spatially
homogenous Bianchi models, with the exception of closed
Friedmann-Robertson-Walker and Bianchi IX models; bounce behaviour violates the
{\em weak energy condition} $\rho\geq 0$ and $\rho+p\geq 0$. We turn to the
Randall-Sundrum type braneworld scenario for a possible resolution of this
problem.


title: Measuring the relativistic perigee advance with Satellite Laser Ranging

abstract:
  One of the most famous classical tests of General Relativity is the
gravitoelectric secular advance of the pericenter of a test body in the
gravitational field of a central mass. In this paper we explore the possibility
of performing a measurement of the gravitoelectric pericenter advance in the
gravitational field of the Earth by analyzing the laser-ranged data to some
existing, or proposed, laser-ranged geodetic satellites. At the present level
of knowledge of various error sources, the relative precision obtainable with
the data from LAGEOS and LAGEOS II, suitably combined, is of the order of
$10^{\rm -3}$. Nevertheless, these accuracies could sensibly be improved in the
near future when the new data on the terrestrial gravitational field from the
CHAMP and GRACE missions will be available. The use of the perigee of LARES
(LAser RElativity Satellite), in the context of a suitable combination of
orbital residuals including also LAGEOS II, should further raise the precision
of the measurement. As a secondary outcome of the proposed experiment, with the
so obtained value of $\ppn$ and with $\et=4\beta-\gamma-3$ from Lunar Laser
Ranging it could be possible to obtain an estimate of the PPN parameters
$\gamma$ and $\beta$ at the $10^{-2}-10^{-3}$ level.


title: On Perturbations in Warm Inflation

abstract:
  Warm inflation is an interesting possibility of describing the early
universe, whose basic feature is the absence, at least in principle, of a
preheating or reheating phase. Here we analyze the dynamics of warm inflation
generalizing the usual slow-roll parameters that are useful for characterizing
the inflationary phase. We study the evolution of entropy and adiabatic
perturbations, where the main result is that for a very small amount of
dissipation the entropy perturbations can be neglected and the purely adiabatic
perturbations will be responsible for the primordial spectrum of
inhomogeneities. Taking into account the COBE-DMR data of the cosmic microwave
background anisotropy as well as the fact that the interval of inflation for
which the scales of astrophysical interest cross outside the Hubble radius is
about 50 e-folds before the end of inflation, we could estimate the magnitude
of the dissipation term. It was also possible to show that at the end of
inflation the universe is hot enough to provide a smooth transition to the
radiation era.


title: Formation and Evaporation of Charged Black Holes

abstract:
  We investigate the dynamical formation and evaporation of a spherically
symmetric charged black hole. We study the self-consistent one loop order
semiclassical back-reaction problem. To this end the mass-evaporation is
modeled by an expectation value of the stress-energy tensor of a neutral
massless scalar field, while the charge is not radiated away. We observe the
formation of an initially non extremal black hole which tends toward the
extremal black hole $M=Q$, emitting Hawking radiation. If also the discharge
due to the instability of vacuum to pair creation in strong electric fields
occurs, then the black hole discharges and evaporates simultaneously and decays
regularly until the scale where the semiclassical approximation breaks down. We
calculate the rates of the mass and the charge loss and estimate the life-time
of the decaying black holes.


title: Coriolis Force and Sagnac Effect

abstract:
  We consider the optical Sagnac effect, when the fictitious gravitational
field simulates the reflections from the mirrors. It is shown that no
contradiction exists between the conclusions of the laboratory and rotated
observers. Because of acting of gravity-like Coriolis force the trajectories of
co- and anti-rotating photons have different radii in the rotating reference
frame, while in the case of the equal radius the effective gravitational
potentials for the photons have to be different.


title: Null Singularities in Colliding Waves

abstract:
  Colliding Einstein-Maxwell-Scalar fields need not necessarily doomed to
become in a spacelike singularity. Examples are given in which null
singularities emerge as intermediate stages between a spacelike singularity and
a regular horizon.


abstract:
  The shear dynamics in Bianchi I cosmological model on the brane with a
perfect fluid (the equation of state is $p=(\gamma-1) \mu$) is studied. It is
shown that for $1 < \gamma < 2$ the shear parameter has maximum at some moment
during a transition period from nonstandard to standard cosmology. An exact
formula for the matter density $\mu$ in the epoch of maximum shear parameter as
a function of the equation of state is obtained.


title:
Algebraically special, expanding and twisting gravitational fields with
  vanishing NUT parameter

abstract:
  The system of Einstein-Maxwell equations for fields mentioned in the title is
simplified. Known pure radiation solutions are systematized and new solutions
are given by separating the variables.


title:
Pure-radiation gravitational fields with a simple twist and a Killing
  vector

abstract:
  Pure-radiation solutions are found, exploiting the analogy with the Euler-
Darboux equation for aligned colliding plane waves and the Euler-Tricomi
equation in hydrodynamics of two-dimensional flow. They do not depend on one of
the spacelike coordinates and comprise the Hauser solution as a special
subcase.


title:
Expanding, axisymmetric pure-radiation gravitational fields with a
  simple twist

abstract:
  New expanding, axisymmetric pure-radiation solutions are found, exploiting
the analogy with the Euler-Darboux equation for aligned colliding plane waves.


title: Who is the Inflaton?

abstract:
  In the context of the two-fluid model introduced to tame the transplanckian
problem of black hole physics, the inflaton field of the chaotic inflation
scenario is identified with the fluctuation of the density of modes. Its mass
comes about from the exchange of degrees of freedom between the two fluids.


title: Cosmology in scalar tensor theory and asymptotically de-Sitter Universe

abstract:
  We have investigated the cosmological scenarios with a four dimensional
effective action which is connected with multidimensional, supergravity and
string theories. The solution for the scale factor is such that initially
universe undergoes a decelerated expansion but in late times it enters into the
accelerated expansion phase. Infact, it asymptotically becomes a de-Sitter
universe. The dilaton field in our model is a decreasing function of time and
it becomes a constant in late time resulting the exit from the scalar tensor
theory to the standard Einstein's gravity. Also the dilaton field results the
existence of a positive cosmological constant in late times.


title:
A cure for unstable numerical evolutions of single black holes:
  adjusting the standard ADM equations

abstract:
  Numerical codes based on a direct implementation of the standard ADM
formulation of Einstein's equations have generally failed to provide long-term
stable and convergent evolutions of black hole spacetimes when excision is used
to remove the singularities. We show that, for the case of a single black hole
in spherical symmetry, it is possible to circumvent these problems by adding to
the evolution equations terms involving the constraints, thus adjusting the
standard ADM system. We investigate the effect that the choice of the lapse and
shift has on the stability properties of numerical simulations and thus on the
form of the added constraint term. To facilitate this task, we introduce the
concept of quasi well-posedness, a version of well-posedness suitable for
ADM-like systems involving second-order spatial derivatives.


title:
Liouville Lost, Liouville Regained: Central Charge in a Dynamical
  Background

abstract:
  Several recent approaches to black hole entropy obtain the density of states
from the central charge of a Liouville theory. If Liouville theory is coupled
to a dynamical spacetime background, however, the classical central charge
vanishes. I show that the central charge can be restored by introducing
appropriate constraints, which may be interpreted as fall-off conditions at a
boundary such as a black hole horizon.


title: Inflation Induced by Vacuum Energy and Graceful Exit from it

abstract:
  Motivated by brane cosmology we solve the Einstein equations with a time
dependent cosmological constant. Assuming that at an early epoch the vacuum
energy scales as $1/logt $, we show that the universe passes from a fast
growing phase (inflation) to an expanding phase in a natural way.


title: Manufacture of Gowdy spacetimes with spikes

abstract:
  In numerical studies of Gowdy spacetimes evidence has been found for the
development of localized features (spikes) involving large gradients near the
singularity. The rigorous mathematical results available up to now did not
cover this kind of situation. In this work we show the existence of large
classes of Gowdy spacetimes exhibiting features of the kind discovered
numerically. These spacetimes are constructed by applying certain
transformations to previously known spacetimes without spikes. It is possible
to control the behaviour of the Kretschmann scalar near the singularity in
detail. This curvature invariant is found to blow up in a way which is
non-uniform near the spike in some cases. When this happens it demonstrates
that the spike is a geometrically invariant feature and not an artefact of the
choice of variables used to parametrize the metric. We also identify another
class of spikes which are artefacts. The spikes produced by our method are
compared with the results of numerical and heuristic analyses of the same
situation.


title: General Birkhoff's Theorem

abstract:
  Space-time is spherically symmetric if it admits the group of SO(3) as a
group of isometries,with the group orbits spacelike two-surfaces. These orbits
are necessarily two-surface of constant positive curavture. One commonly
chooses coordinate ${t,r,\theta,\phi}$ so that the group orbits become surfaces
${t,r = const.}$ and the radial coordinate r is defined by the requirment that
$4\pi r^2$ is the area of these spacelike two-surfaces with the range of zero
to infinity. According to the Birkhoff's theorem upon the above assumptions,
the Schwarzschild metric is the only soltion of the vacuum Einstein field
equations. Our aim is to reconsider the solution of the sperically symmetric
vacuum Einstein field equtions by regarding a weaker requirement. We admit the
evident fact that in the completely empty sace the radial coordinate r may be
defined so that $4\pi r^2$ becomes the area of spacelike two-surfaces ${t,r =
const.}$ with the range of zero to infinity. This is not necessarily to be true
in the presence of a material point mass M. It turns out that inspite of
imposing asymptotically flatness and staticness as initial conditions the
equations have general classes of solutions which the Schwarzschild metric is
the only member of them which has an intrinsic singularity at the location of
the point mass M. The area of ${t,r = const.}$ is $4\pi(r +\alpha M)^2$ in one
class and $4\pi(r^2 + a_1 Mr +a_2 M^2)$ in the other class while the center of
symmetry is at r = 0.


title:
Invariant dynamics of scalar perturbations of inflanton and
  gravitational fields

abstract:
  A gauge-independent, invariant theory of linear scalar perturbations of
inflation and gravitational fields has been created. This invariant theory
allows one to compare gauges used in the work of other researchers and to find
the unambiguous criteria to separate the physical and coordinate effects. It is
shown, in particular, that the so-called longitudial gauge, commonly used when
considering inflation instability, leads to a fundamental overestimation of the
effect because of non-physical perturbations of the proper time in the frame of
reference specified by this gauge. Back reaction theories employing this sort
of gauge [1] also involve coordinate effects. The invariant theory created here
shows that the classical Lifshitz (1946) [2] gauge does not lead to
non-physical perturbations of the proper time and can be used to analyze the
inflation regime and the back reaction of perturbations on this regime
properly. The first theory of back reaction on background of all types of
perturbations (scalar, vector and tensor) based on this gauge was published in
1975 [3] and has been applied recently to the inflation [4]. The investigation
of long-length perturbations, which characterize the stability of the
inflationary process, and quantum fluctuations, which form the
Harrison-Zel'dovich spectrum at the end of inflation, is performed in the
invariant form. The invariant theory proposed allows one to examine the effect
of quantum fluctuations on the inflationary stage when the periodic regime
changes to an aperiodic one. That only the invariant theory must be used to
analyze space experiments is one of the conclusions of the present work.


title: Area spectrum in Lorentz covariant loop gravity

abstract:
  We use the manifestly Lorentz covariant canonical formalism to evaluate
eigenvalues of the area operator acting on Wilson lines. To this end we modify
the standard definition of the loop states to make it applicable to the present
case of non-commutative connections. The area operator is diagonalized by using
the usual shift ambiguity in definition of the connection. The eigenvalues are
then expressed through quadratic Casimir operators. No dependence on the
Immirzi parameter appears.


title: Tension Perturbations of Black Brane Spacetimes

abstract:
  We consider black-brane spacetimes that have at least one spatial translation
Killing field that is tangent to the brane. A new parameter, the tension of a
spacetime, is defined. The tension parameter is associated with spatial
translations in much the same way that the ADM mass is associated with the time
translation Killing field. In this work, we explore the implications of the
spatial translation symmetry for small perturbations around a background black
brane. For static charged black branes we derive a law which relates the
tension perturbation to the surface gravity times the change in the the horizon
area, plus terms that involve variations in the charges and currents. We find
that as a black brane evaporates the tension decreases. We also give a simple
derivation of a first law for black brane spacetimes. These constructions hold
when the background stress-energy is governed by a Hamiltonian, and the results
include arbitrary perturbative stress-energy sources.


title: The future asymptotics of Bianchi VIII vacuum solutions

abstract:
  Bianchi VIII vacuum solutions to Einstein's equations are causally
geodesically complete to the future, given an appropriate time orientation, and
the objective of this article is to analyze the asymptotic behaviour of
solutions in this time direction. For the Bianchi class A spacetimes, there is
a formulation of the field equations that was presented in an article by
Wainwright and Hsu, and we analyze the asymptotic behaviour of solutions in
these variables. We also try to give the analytic results a geometric
interpretation by analyzing how a normalized version of the Riemannian metric
on the spatial hypersurfaces of homogeneity evolves.


title: Inappropriateness of the Rindler quantization

abstract:
  It is argued that the Rindler quantization is not a correct approach to study
the effects of acceleration on quantum fields. First, the "particle"-detector
approach based on the Minkowski quantization is not equivalent to the approach
based on the Rindler quantization. Second, the event horizon, which plays the
essential role in the Rindler quantization, cannot play any physical role for a
local noninertial observer.


title:
Local and global properties of conformally flat initial data for black
  hole collisions

abstract:
  We study physical properties of conformal initial value data for single and
binary black hole configurations obtained using conformal-imaging and
conformal-puncture methods. We investigate how the total mass M_tot of a
dataset with two black holes depends on the configuration of linear or angular
momentum and separation of the holes. The asymptotic behavior of M_tot with
increasing separation allows us to make conclusions about an unphysical
``junk'' gravitation field introduced in the solutions by the conformal
approaches. We also calculate the spatial distribution of scalar invariants of
the Riemann tensor which determine the gravitational tidal forces. For single
black hole configurations, these are compared to known analytical solutions.
Spatial distribution of the invariants allows us to make certain conclusions
about the local distribution of the additional field in the numerical datasets.


title: Energy Associated with a Charged Regular Black Hole

abstract:
  We show that using an adequate coordinate transformation the charged regular
black hole solution given by Ayon-Beato and Garcia can be put in the
Kerr-Schild form. Then we use this metric in Kerr-Schild Cartesian coordinates
with a result given by Virbhadra and obtain the energy distribution associated
with this.


title:
Torsion Degrees of Freedom in the Regge Calculus as Dislocations on the
  Simplicial Lattice

abstract:
  Using the notion of a general conical defect, the Regge Calculus is
generalized by allowing for dislocations on the simplicial lattice in addition
to the usual disclinations. Since disclinations and dislocations correspond to
curvature and torsion singularities, respectively, the method we propose
provides a natural way of discretizing gravitational theories with torsion
degrees of freedom like the Einstein-Cartan theory. A discrete version of the
Einstein-Cartan action is given and field equations are derived, demanding
stationarity of the action with respect to the discrete variables of the
theory.


title: On Hyperbolic "Theories" of Relativistic Dissipative Fluids

abstract:
  It is argued that, at least for the case of Navier-Stokes fluids, the
so-called hyperbolic theories of dissipation are not viable.


title: The Big Bang in $T^3$ Gowdy Cosmological Models

abstract:
  We establish a formal relationship between stationary axisymmetric spacetimes
and $T^3$ Gowdy cosmological models which allows us to derive several
preliminary results about the generation of exact cosmological solutions and
their possible behavior near the initial singularity. In particular, we argue
that it is possible to generate a Gowdy model from its values at the
singularity and that this could be used to construct cosmological solutions
with any desired spatial behavior at the Big Bang.


title: Cosmological Creation of Vector Bosons and Fermions

abstract:
  The cosmological creation of primordial vector bosons and fermions is
described in the Standard Model of strong and electro-weak interactions given
in a space-time with the relative standard of measurement of geometric
intervals. Using the reparametrization - invariant perturbation theory and the
holomorphic representation of quantized fields we derive equations for the
Bogoliubov coefficients and distribution functions of created particles. The
main result is the intensive cosmological creation of longitudinal Z and W
bosons (due to their mass singularity) by the universe in the rigid state. We
introduce the hypothesis that the decay of the primordially created vector
bosons is the origin of the Cosmic Microwave Background radiation.


title: Analog gravity from field theory normal modes?

abstract:
  We demonstrate that the emergence of a curved spacetime ``effective
Lorentzian geometry'' is a common and generic result of linearizing a field
theory around some non-trivial background. This investigation is motivated by
considering the large number of ``analog models'' of general relativity that
have recently been developed based on condensed matter physics, and asking
whether there is something more fundamental going on. Indeed, linearization of
a classical field theory (a field theoretic ``normal mode analysis'') results
in fluctuations whose propagation is governed by a Lorentzian-signature curved
spacetime ``effective metric''. For a single scalar field, this procedure
results in a unique effective metric, which is quite sufficient for simulating
kinematic aspects of general relativity (up to and including Hawking
radiation). Quantizing the linearized fluctuations, the one-loop effective
action contains a term proportional to the Einstein--Hilbert action, suggesting
that while classical physics is responsible for generating an ``effective
geometry'', quantum physics can be argued to induce an ``effective dynamics''.
The situation is strongly reminiscent of Sakharov's ``induced gravity''
scenario, and suggests that Einstein gravity is an emergent low-energy
long-distance phenomenon that is insensitive to the details of the high-energy
short-distance physics. (We mean this in the same sense that hydrodynamics is a
long-distance emergent phenomenon, many of whose predictions are insensitive to
the short-distance cutoff implicit in molecular dynamics.)


title: The Tolman VII solution, trapped null orbits and w - modes

abstract:
  The Tolman VII solution is an exact static spherically symmetric perfect
fluid solution of Einstein's equations that exhibits a surprisingly good
approximation to a neutron star. We show that this solution exhibits trapped
null orbits in a causal region even for a tenuity (total radius to mass ratio)
$> 3$. In this region the dynamical part of the potential for axial w - modes
dominates over the centrifugal part.


title: Noncommutative regime of fundamental physics

abstract:
  We further develop a model unifying general relativity with quantum mechanics
proposed in our earlier papers (J. Math. Phys. 38, 5840 (1998); 41, 5168
(2000)). The model is based on a noncommutative algebra $A$ defined on a
groupoid $\Gamma = E \times G$ where $E$ is the total space of a fibre bundle
over space-time and $G$ a Lie group acting on $E$. In this paper, the algebra
$A$ is defined in such a way that the model works also if $G$ is a noncompact
group. Differential algebra based on derivations of this algebra is elaborated
which allows us to construct a "noncommutative general relativity". The left
regular representation of the algebra $A$ in a Hilbert space leads to the
quantum sector of our model. Its position and momentum representations are
discussed in some detail. It is shown that the model has correct correspondence
with the standard theories: with general relativity, by restricting the algebra
$A$ to a subset of its center; with quantum mechanics, by changing from the
groupoid $\Gamma $ to its algebroid; with classical mechanics, by changing from
the groupoid $\Gamma $ to its tangent groupoid. We also construct a
noncommutative Fock space based on the proposed model.


title: On the Bergmann Energy-Momentum Complex of a Charged Regular Black Hole

abstract:
  We use the Bergmann energy-momentum complex to calculate the energy of a
charged regular black hole. The energy distribution is the same as we obtained
in the Einstein prescription. Also, we get the expression of the energy in the
Bergmann prescription for a general spherically symmetric space-time of the
Kerr-Schild class.


title: Phenomenological description of space-time foam

abstract:
  The expectation that it should not be possible to gain experimental insight
on the structure of space-time at Planckian distance scales has been recently
challenged by several studies. With respect to space-time fluctuations, one of
the conjectured features of quantum-gravity foam, the experiments that have the
best sensitivity are the ones which were originally devised for searches of the
classical-physics phenomenon of gravity waves. In experiments searching for
classical gravity waves the presence of space-time fluctuations would introduce
a source of noise just like the ordinary (non-gravitational) quantum properties
of the photons composing the laser beam used in interferometry introduce a
source of noise. Earlier studies of the noise induced by quantum properties of
space-time have shown that certain simple pictures of fluctuations of
space-time occuring genuinely at the Planck scale would lead to an observably
large effect. Experimentalists would benefit from the guidance of detailed
description of this noise, but quantum-gravity theories are not yet developed
to the point of allowing such detailed analysis of physical processes. I
propose a new phenomenological approach to the description of foam-induced
noise.


title: Watching Gravitational Waves

abstract:
  In the vicinity of merging neutron strar binaries or supernova remnants,
gravitational waves can interact with the prevailing strong magnetic fields.
The resulting partial conversion of gravitational waves into electromagnetic
(radio) waves might prove to be an indirect way of detecting gravitational
waves from such sources.
  Another interesting interaction considered in this article is the excitation
of magnetosonic plasma waves by a gravitational wave passing through the
surrounding plasma. The transfer of gravitational wave energy into the plasma
might help to fuel the `fireball' of electromagnetic radiation observed in
gamma ray bursts. In the last section of the article, a dispersion relation is
derived for such magnetosonic plasma waves driven by a gravitational wave.


title: Gravity on branes

abstract:
  We consider the four dimensional discontinuity generated by two identical
pieces of a five dimensional space pasted along their edge (that is a "brane"
in a "$Z_2$-symmetric" "bulk"). Using a four plus one decomposition of the
Riemann tensor we write the equations for gravity on the brane and recover in a
simple manner a number of known "brane world" scenarios. We study under which
conditions these equations reduce, exactly or approximately, to the four
dimensional Einstein equations. We conclude that if the bulk is imposed to be
only an Einstein space near the brane, Einstein's equations can be recovered
approximately on the brane, but if it is imposed to be strictly anti-de Sitter
space then the Einstein equations cannot hold, even approximately, on a
quasi-Minkowskian brane, unless matter obeys a very contrived equation of
state.


title: Equations of Motion in Kaluza-Klein Gravity Revisited

abstract:
  We discuss the equations of motion of test particles for a version of
Kaluza-Klein theory where the cylinder condition is not imposed. The metric
tensor of the five-dimensional manifold is allowed to depend on the fifth
coordinate. This is the usual working scenario in brane-world, induced-matter
theory and other Kaluza-Klein theories with large extra dimensions. We present
a new version for the fully covariant splitting of the 5D equations. We show
how to change the usual definition of various physical quantities in order to
make physics in 4D invariant under transformations in 5D. These include the
redefinition of the electromagnetic tensor, force and Christoffel symbols. With
our definitions, each of the force terms in the equation of motion is gauge
invariant and orthogonal to the four-velocity of the particle. The "hidden"
parameter associated with the rate of motion along the extra dimension is
identified with the electric charge, regardless of whether there is an
electromagnetic field or not. In addition, for charged particles, the
charge-to-mass ratio should vary. Therefore, the motion of a charged particle
should differ from the motion of a neutral particle, with the same initial mass
and energy, even in the absence of electromagnetic field. These predictions
have important implications and could in principle be experimentally detected.


title: Radiating sources in higher-dimensional gravity

abstract:
  We study a time-dependent 5D metric which contains a static 4D sub-metric
whose 3D part is spherically symmetric. An expansion in the metric coefficient
allow us to obtain close-to Schwarzschild approximation to a class of
spherically-symmetric solutions. Using Campbell's embedding theorem and the
induced-matter formalism we obtain two 4D solutions. One describes a source
with the stiff equation of state believed to be applicable to dense
astrophysical objects, and the other describes a spherical source with a radial
heat flow.


title:
Logarithmic correction to the Bekenstein-Hawking entropy of the BTZ
  black hole

abstract:
  We derive an exact expression for the partition function of the Euclidean BTZ
black hole. Using this, we show that for a black hole with large horizon area,
the correction to the Bekenstein-Hawking entropy is $-3/2 log(Area)$, in
agreement with that for the Schwarzschild black hole obtained in the canonical
gravity formalism and also in a Lorentzian computation of BTZ black hole
entropy. We find that the right expression for the logarithmic correction in
the context of the BTZ black hole comes from the modular invariance associated
with the toral boundary of the black hole.


title: Geometrization of the Lax Pair Tensors

abstract:
  The tensorial form of the Lax pair equations are given in a compact and
geometrically transparent form in the presence of Cartan's torsion tensor.
Three-dimensional spacetimes admitting Lax tensors are analyzed in detail.
Solutions to Lax tensor equations include interesting examples as separable
coordinates and the Toda lattice.


title:
Self-Force on a Scalar Charge in Circular Orbit around a Schwarzschild
  Black Hole

abstract:
  In an accompanying paper, we have formulated two types of regulariz_ation
methods to calculate the scalar self-force on a particle of charge $q$ moving
around a black hole of mass $M$, one of which is called the ``power expansion
regularization''. In this paper, we analytically evaluate the self-force (which
we also call the reaction force) to the third post-Newtonian (3PN) order on the
scalar particle in circular orbit around a Schwarzschild black hole by using
the power expansion regularization. It is found that the $r$-component of the
self-force arises at the 3PN order, whereas the $t$- and $\phi$-components,
which are due to the radiation reaction, appear at the 2PN and 1.5PN orders,
respectively.


title:
Derivation of the Einstein Equivalence Principle in a Class of Condensed
  Matter Theories

abstract:
  We consider a class of condensed matter theories in a Newtonian framework
with a Lagrange formalism related in a natural way with the classical
conservation laws \partial_t \rho + \partial_i (\rho v^i) = 0 \partial_t (\rho
v^j) + \partial_i (\rho v^i v^j + p^{ij}) = 0 We show that for an algebraically
defined ``effective Lorentz metric'' g_{\mu\nu} and ``effective matter fields''
\phi these theories are equivalent to material models of a metric theory of
gravity with Lagrangian L = L_{GR} + L_{matter}
  - (8\pi G)^{-1}(\Upsilon g^{00}-\Xi (g^{11}+g^{22}+g^{33}))\sqrt{-g} which
fulfils the Einstein equivalence principle and leads to the Einstein equations
in the limit \Xi,\Upsilon\to 0.


title: Gravitating Fluxbranes

abstract:
  We consider the effect that gravity has when one tries to set up a constant
background form field. We find that in analogy with the Melvin solution, where
magnetic field lines self-gravitate to form a flux-tube, the self-gravity of
the form field creates fluxbranes. Several exact solutions are found
corresponding to different transverse spaces and world-volumes, a dilaton
coupling is also considered.


title: Interaction between gravitational waves and domain walls

abstract:
  We study the gravitational perturbations of thick domain walls. The
refraction index and spin properties of the solutions interior to the wall are
analyzed in detail. It is shown that the gravitational waves suffer a
refraction process by domain walls. The reflection and transmission
coefficients are derived in the thin wall limit. In relation to the spin
content, it is shown that the ``$\times$'' helicity 2 gravitational wave mode
maintains in the domain wall the same polarization state as in vacuum. On the
contrary, the ``+'' mode, of pure helicity 2 in vacuum, is contaminated inside
the wall with a spin 0 state, as well as with spin 2, helicity 0 and 1 states.


title: Does brane cosmology have realistic principles?

abstract:
  The maximal symmetry, or Perfect Cosmological Principle(PCP), that prevents
AdS type spaces from degenerating into anti-inflationary collapse is argued to
be unphysical. For example, the simple requirement that brane-bulk models
should be the result of having evolved from even more energetic string
phenomena picks out a preferred time direction.
  We question whether quantum cosmological reasoning can be applied in any
meaningful way to obtain, what are essentially, classical constructs . An
alternative scheme is to more readily accept the PCP and allow the branes to
also become eternal. A perpetually expanding and contracting brane model could
be driven by the presence of charged black holes in the AdS bulk, that
effectively violates the weak-energy condition as singularities are approached.
This can be contrasted with the so-called Ekpyrotic universe which also closely
accepts the PCP. This being broken only by occasional collisions between
branes, that can then simulate a big bang cosmology.


title: Quantum Black Holes from Quantum Collapse

abstract:
  The Schwarzschild black hole can be viewed as the special case of the
marginally bound Lema\^\i tre-Tolman-Bondi models of dust collapse which
corresponds to a constant mass function. We have presented a midi-superspace
quantization of this model for an arbitrary mass-function in a separate
publication. In this communication we show that our solution leads both to
Bekenstein's area spectrum for black holes as well as to the black hole
entropy, which, in this context, is naturally interpreted as the loss of
information of the original matter distribution within the collapsing dust
cloud.


title: Levi-Civita Solutions Coupled with Electromagnetic Fields

abstract:
  The local and global properties of the Levi-Civita (LC) solutions coupled
with an electromagnetic field are studied and some limits to the vacuum LC
solutions are given. By doing such limits, the physical and geometrical
interpretations of the free parameters involved in the solutions are made
clear. Sources for both the LC vacuum solutions and the LC solutions coupled
with an electromagnetic field are studied, and in particular it is found that
all the LC vacuum solutions with $\sigma \ge 0$ can be produced by
cylindrically symmetric thin shells that satisfy all the energy conditions,
weak, dominant, and strong. When the electromagnetic field is present, the
situation changes dramatically. In the case of a purely magnetic field, all the
solutions with $\sigma \ge 1/\sqrt{8}$ or $\sigma \le - 1/\sqrt{8}$ can be
produced by physically acceptable cylindrical thin shells, while in the case of
a purely electric field, no such shells are found for any value of $\sigma$.


title: Comparing metrics at large: harmonic vs quo-harmonic coordinates

abstract:
  To compare two space-times on large domains, and in particular the global
structure of their manifolds, requires using identical frames of reference and
associated coordinate conditions. In this paper we use and compare two classes
of time-like congruences and corresponding adapted coordinates: the harmonic
and quo-harmonic classes. Besides the intrinsic definition and some of their
intrinsic properties and differences we consider with some detail their
differences at the level of the linearized approximation of the field
equations. The hard part of this paper is an explicit and general determination
of the harmonic and quo-harmonic coordinates adapted to the stationary
character of three well-know metrics, Schwarzschild's, Curzon's and Kerr's, to
order five of their asymptotic expansions. It also contains some relevant
remarks on such problems as defining the multipoles of vacuum solutions or
matching interior and exterior solutions.


title: Black Hole Excision for Dynamic Black Holes

abstract:
  We extend previous work on 3D black hole excision to the case of distorted
black holes, with a variety of dynamic gauge conditions that are able to
respond naturally to the spacetime dynamics. We show that the combination of
excision and gauge conditions we use is able to drive highly distorted,
rotating black holes to an almost static state at late times, with well behaved
metric functions, without the need for any special initial conditions or
analytically prescribed gauge functions. Further, we show for the first time
that one can extract accurate waveforms from these simulations, with the full
machinery of excision and dynamic gauge conditions. The evolutions can be
carried out for long times, far exceeding the longevity and accuracy of even
better resolved 2D codes. While traditional 2D codes show errors in quantities
such as apparent horizon mass of over 100% by t = 100M, and crash by t = 150M,
with our new techniques the same systems can be evolved for hundreds of M's in
full 3D with errors of only a few percent.


title:
Observables, gauge invariance, and the role of the observers in the
  limit from general relativity to special relativity

abstract:
  Some conceptual issues concerning general invariant theories, with special
emphasis on general relativity, are analyzed. The common assertion that
observables must be required to be gauge invariant is examined in the light of
the role played by a system of observers. Some features of the reduction of the
gauge group are discussed, including the fact that in the process of a partial
gauge fixing the reduction at the level of the gauge group and the reduction at
the level of the variational principle do not commute. Distinctions between the
mathematical and the physical concept of gauge symmetry are discussed and
illustrated with examples. The limit from general relativity to special
relativity is considered as an example of a gauge group reduction that is
allowed in some specific physical circumstances. Whether and when the
Poincar\'e group must be considered as a residual gauge group will come out as
a result of our analysis, that applies, in particular, to asymptotically flat
spaces.


title: Testing gravitomagnetism on the Earth

abstract:
  The paper contains a proposed experiment for testing the gravitomagnetic
effect on the propagation of light around a rotating mass. The idea is to use a
rotating spherical laboratory-scale shell, around which two mutually orthogonal
lightguides are wound acting as the arms of an interferometer. Numerical
estimates show that time of flight differences between the equatorial and polar
guides could be in the order of $\sim 10^{-20}$ s, actually detectable with
sensitivity perfectly comparable with those expected in gravitational wave
detection experiments.


title: Non-Turing computations via Malament-Hogarth space-times

abstract:
  We investigate the Church-Kalm\'ar-Kreisel-Turing Theses concerning
theoretical (necessary) limitations of future computers and of deductive
sciences, in view of recent results of classical general relativity theory.
  We argue that (i) there are several distinguished Church-Turing-type Theses
(not only one) and (ii) validity of some of these theses depend on the
background physical theory we choose to use. In particular, if we choose
classical general relativity theory as our background theory, then the above
mentioned limitations (predicted by these Theses) become no more necessary,
hence certain forms of the Church-Turing Thesis cease to be valid (in general
relativity). (For other choices of the background theory the answer might be
different.)
  We also look at various ``obstacles'' to computing a non-recursive function
(by relying on relativistic phenomena) published in the literature and show
that they can be avoided (by improving the ``design'' of our future computer).
We also ask ourselves, how all this reflects on the arithmetical hierarchy and
the analytical hierarchy of uncomputable functions.


title:
Non-linear harmonic generation in finite amplitude black hole
  oscillations

abstract:
  The non-linear generation of harmonics in gravitational perturbations of
black holes is explored using numerical relativity based on an in-going
light-cone framework. Localised, finite, perturbations of an isolated black
hole are parametrised by amplitude and angular harmonic form. The response of
the black hole spacetime is monitored and its harmonic content analysed to
identify the strength of the non-linear generation of harmonics as a function
of the initial data amplitude. It is found that overwhelmingly the black hole
responds at the harmonic mode perturbed, even for spacetimes with 10% of the
black hole mass radiated. The relative efficiencies of down and up-scattering
in harmonic space are computed for a range of couplings. Down-scattering,
leading to smoothing out of angular structure is found to be equally or more
efficient than the up-scatterings that would lead to increased rippling. The
details of this non-linear balance may form the quantitative mechanism by which
black holes avoid fission even for arbitrary strong distortions.


title: Pulsars and Redshifts

abstract:
  Gravitational redshifts of neutron stars have a theoretical upper limit of
z=0.62. Also, it is generally believed that neutron stars have magnetic fields
on the order of ten to the twelfth to ten to the thirteenth G. A previously
predicted electromagnetic time dilation effect has been shown to correctly
predict decay lifetimes of muons bound to atomic nuclei. In this paper it is
shown that the electromagnetic time dilation effect, along with the
gravitational time dilation effect, can produce total neutron star redshifts
that are substantially larger than 0.62. For instance, the redshift can cutoff
radiation for B on the order of ten to the thirteenth G. Consequently, we can
have a neutron star that is unobservable except for localized surface regions
where the cutoff condition does not hold. Assuming coherent radiation, a
surface region of this kind that does not include the star's rotation axis will
emit a lighthouse type beam. Since the magnetic field in these regions will
usually be strong enough to cause significant redshifts, and there is no reason
to expect these regions to always be of constant size, shape, or field
strength, this model explains the predominance of radio waves, and the
existence of pulse variations, (e.g. nulling and drifting) in pulsars.


title: Gravitational wave bursts from cusps and kinks on cosmic strings

abstract:
  The strong beams of high-frequency gravitational waves (GW) emitted by cusps
and kinks of cosmic strings are studied in detail. As a consequence of these
beams, the stochastic ensemble of GW's generated by a cosmological network of
oscillating loops is strongly non Gaussian, and includes occasional sharp
bursts that stand above the ``confusion'' GW noise made of many smaller
overlapping bursts. Even if only 10% of all string loops have cusps these
bursts might be detectable by the planned GW detectors LIGO/VIRGO and LISA for
string tensions as small as $G \mu \sim 10^{-13}$. In the implausible case
where the average cusp number per loop oscillation is extremely small, the
smaller bursts emitted by the ubiquitous kinks will be detectable by LISA for
string tensions as small as $G \mu \sim 10^{-12}$. We show that the strongly
non Gaussian nature of the stochastic GW's generated by strings modifies the
usual derivation of constraints on $G \mu$ from pulsar timing experiments. In
particular the usually considered ``rms GW background'' is, when $G \mu \gaq
10^{-7}$, an overestimate of the more relevant confusion GW noise because it
includes rare, intense bursts. The consideration of the confusion GW noise
suggests that a Grand Unified Theory (GUT) value $ G \mu \sim 10^{-6}$ is
compatible with existing pulsar data, and that a modest improvement in pulsar
timing accuracy could detect the confusion noise coming from a network of cuspy
string loops down to $ G \mu \sim 10^{-11}$. The GW bursts discussed here might
be accompanied by Gamma Ray Bursts.