title: Axi-Dilaton Gravity in D \geq 4 Dimensional Space-Times with Torsion

abstract:
  We study models of axi-dilaton gravity in space-time geometries with torsion.
We discuss conformal re-scaling rules in both Riemannian and non-Riemannian
formulations. We give static, spherically symmetric solutions and examine their
singularity structure.


title: Bianchi Cosmologies: A Tale of Two Tilted fluids

abstract:
  We use a dynamical systems approach to study Bianchi type VI$_0$ cosmological
models containing two tilted $\gamma$-law perfect fluids. The full state space
is 11-dimensional, but the existence of a monotonic function simplifies the
analysis considerably. We restrict attention to a particular, physically
interesting, invariant subspace and find all equilibrium points that are future
stable in the full 11-dimensional state space; these are consequently local
attractors and serve as late-time asymptotes for an open set of tilted type
VI$_0$ models containing two tilted fluids. We find that if one of the fluids
has an equation of state parameter $\gamma<6/5$, the stiffest fluid will be
dynamically insignificant at late times. For the value $\gamma=6/5$ there is a
2-dimensional bifurcation set, and if both fluids are stiffer than $\gamma=6/5$
both fluids will have extreme tilt asymptotically. We investigate the case in
which one fluid is extremely tilting in detail. We also consider the case with
one stiff fluid ($\gamma=2$) close to the initial singularity, and find that
the chaotic behaviour which occurs in general Bianchi models with $\gamma<2$ is
suppressed.


title: Theory of Quantum Space-Time

abstract:
  A generalised equivalence principle is put forward according to which
space-time symmetries and internal quantum symmetries are indistinguishable
before symmetry breaking. Based on this principle, a higher-dimensional
extension of Minkowski space is proposed and its properties examined. In this
scheme the structure of space-time is intrinsically quantum mechanical. It is
shown that the causal geometry of such a quantum space-time possesses a rich
hierarchical structure. The natural extension of the Poincare group to quantum
space-time is investigated. In particular, we prove that the symmetry group of
a quantum space-time is generated in general by a system of irreducible Killing
tensors. When the symmetries of a quantum space-time are spontaneously broken,
then the points of the quantum space-time can be interpreted as space-time
valued operators. The generic point of a quantum space-time in the broken
symmetry phase thus becomes a Minkowski space-time valued operator. Classical
space-time emerges as a map from quantum space-time to Minkowski space. It is
shown that the general such map satisfying appropriate causality-preserving
conditions ensuring linearity and Poincare invariance is necessarily a density
matrix.


title: Local thermal equilibrium and ideal gas Stephani universes

abstract:
  The Stephani universes that can be interpreted as an ideal gas evolving in
local thermal equilibrium are determined. Five classes of thermodynamic schemes
are admissible, which give rise to five classes of regular models and three
classes of singular models. No Stephani universes exist representing an exact
solution to a classical ideal gas (one for which the internal energy is
proportional to the temperature). But some Stephani universes may approximate a
classical ideal gas at first order in the temperature: all of them are
obtained. Finally, some features about the physical behavior of the models are
pointed out.


title: Status of VIRGO

abstract:
  The French-Italian interferometric gravitational wave detector VIRGO is
currently being commissioned. Its principal instrument is a Michelson
interferometer with 3 km long optical cavities in the arms and a
power-recycling mirror. This paper gives an overview of the present status of
the system. We report on the presently attained sensitivity and the system's
performance during the recent commissioning runs.
  After a sequence of intermediate stages, the interferometer is now being used
in the so-called recombined configuration. The input laser beam is spatially
filtered by a 144 m long input mode-cleaner before being injected to the main
interferometer. The main optics are suspended from so-called \sa s, which
provide an excellent seismic isolation. The two 3 km long Fabry-Perot arm
cavities are kept in resonance with the laser light, and the Michelson
interferometer is held on the dark fringe. An automatic mirror alignment system
based on the Anderson technique has been implemented for the arm cavities. The
light leaving the dark port contains the gravitational wave signal; this light
is filtered by an output mode-cleaner before being detected by a photo
detector. This setup is the last step on the way to the final configuration,
which will include power recycling.


title: The Hough transform search for continuous gravitational waves

abstract:
  This paper describes an incoherent method to search for continuous
gravitational waves based on the Hough transform, a well known technique used
for detecting patterns in digital images. We apply the Hough transform to
detect patterns in the time-frequency plane of the data produced by an
earth-based gravitational wave detector. Two different flavors of searches will
be considered, depending on the type of input to the Hough transform: either
Fourier transforms of the detector data or the output of a coherent
matched-filtering type search. We present the technical details for
implementing the Hough transform algorithm for both kinds of searches, their
statistical properties, and their sensitivities.


title:
Proofs of existence of local potentials for trace-free symmetric 2-forms
  using dimensionally dependent identities

abstract:
  We exploit four-dimensional tensor identities to give a very simple proof of
the existence of a Lanczos potential for a Weyl tensor in four dimensions with
any signature, and to show that the potential satisfies a simple linear second
order differential equation, e.g., a wave equation in Lorentz signature.
Furthermore, we exploit higher dimensional tensor identities to obtain the
analogous results for (m,m)-forms in 2m dimensions.


title: Probing Universality Of Gravity

abstract:
  I wish to expound a novel perspective of probing universal character of
gravity. To begin with, inclusion of zero mass particle in mechanics leads to
special relativity while its interaction with a universal force shared by all
particles leads to general relativity. The universal nature of force further
suggests that it is intrinsically attractive, self interactive and higher
dimensional. I argue that the principle of universality could serve as a good
guide for future directions.


title:
Geometric transport along circular orbits in stationary axisymmetric
  spacetimes

abstract:
  Parallel transport along circular orbits in orthogonally transitive
stationary axisymmetric spacetimes is described explicitly relative to Lie
transport in terms of the electric and magnetic parts of the induced
connection. The influence of both the gravitoelectromagnetic fields associated
with the zero angular momentum observers and of the Frenet-Serret parameters of
these orbits as a function of their angular velocity is seen on the behavior of
parallel transport through its representation as a parameter-dependent Lorentz
transformation between these two inner-product preserving transports which is
generated by the induced connection. This extends the analysis of parallel
transport in the equatorial plane of the Kerr spacetime to the entire spacetime
outside the black hole horizon, and helps give an intuitive picture of how
competing "central attraction forces" and centripetal accelerations contribute
with gravitomagnetic effects to explain the behavior of the 4-acceleration of
circular orbits in that spacetime.


title: CPT Violation and Decoherence in Quantum Gravity

abstract:
  In these lectures I review, in as much pedagogical way as possible, various
theoretical ideas and motivation for violation of CPT invariance in some models
of Quantum Gravity, and discuss the relevant phenomenology. Since the subject
is vast, I pay particular emphasis on the CPT Violating decoherence scenario
for quantum gravity, due to space-time foam. In my opinion this seems to be the
most likely scenario to be realised in Nature, should quantum gravity be
responsible for the violation of this symmetry. In this context, I also discuss
how the CPT Violating decoherence scenario can explain experimental
``anomalies'' in neutrino data, such as LSND results, in agreement with the
rest of the presently available data, without enlarging the neutrino sector.


title: Energy conditions outside a dielectric ball

abstract:
  We show analytically that the vacuum electromagnetic stress-energy tensor
outside a ball with constant dielectric constant and permeability always obeys
the weak, null, dominant, and strong energy conditions. There are still no
known examples in quantum field theory in which the averaged null energy
condition in flat spacetime is violated.


title:
The Physical Role of Gravitational and Gauge Degrees of Freedom in
  General Relativity - II: Dirac versus Bergmann observables and the
  Objectivity of Space-Time

abstract:
  (abridged)The achievements of the present work include: a) A clarification of
the multiple definition given by Bergmann of the concept of {\it (Bergmann)
observable. This clarification leads to the proposal of a {\it main conjecture}
asserting the existence of i) special Dirac's observables which are also
Bergmann's observables, ii) gauge variables that are coordinate independent
(namely they behave like the tetradic scalar fields of the Newman-Penrose
formalism). b) The analysis of the so-called {\it Hole} phenomenology in strict
connection with the Hamiltonian treatment of the initial value problem in
metric gravity for the class of Christoudoulou -Klainermann space-times, in
which the temporal evolution is ruled by the {\it weak} ADM energy. It is
crucial the re-interpretation of {\it active} diffeomorphisms as {\it passive
and metric-dependent} dynamical symmetries of Einstein's equations, a
re-interpretation which enables to disclose their (nearly unknown) connection
to gauge transformations on-shell; this is expounded in the first paper
(gr-qc/0403081). The use of the Bergmann-Komar {\it intrinsic
pseudo-coordinates} allows to construct a {\it physical atlas} of 4-coordinate
systems for the 4-dimensional {\it mathematical} manifold, in terms of the
highly non-local degrees of freedom of the gravitational field (its four
independent {\it Dirac observables}), and to realize the {\it physical
individuation} of the points of space-time as {\it point-events} as a
gauge-fixing problem, also associating a non-commutative structure to each
4-coordinate system.


title:
Optimal statistic for detecting gravitational wave signals from binary
  inspirals with LISA

abstract:
  A binary compact object early in its inspiral phase will be picked up by its
nearly monochromatic gravitational radiation by LISA. But even this innocuous
appearing candidate poses interesting detection challenges. The data that will
be scanned for such sources will be a set of three functions of LISA's twelve
data streams obtained through time-delay interferometry, which is necessary to
cancel the noise contributions from laser-frequency fluctuations and
optical-bench motions to these data streams. We call these three functions
pseudo-detectors. The sensitivity of any pseudo-detector to a given sky
position is a function of LISA's orbital position. Moreover, at a given point
in LISA's orbit, each pseudo-detector has a different sensitivity to the same
sky position. In this work, we obtain the optimal statistic for detecting
gravitational wave signals, such as from compact binaries early in their
inspiral stage, in LISA data. We also present how the sensitivity of LISA,
defined by this optimal statistic, varies as a function of sky position and
LISA's orbital location. Finally, we show how a real-time search for inspiral
signals can be implemented on the LISA data by constructing a bank of templates
in the sky positions.


title: Massive quasi-normal mode

abstract:
  This paper purposes to study quasi-normal modes due to massive scalar fields.
We, in particular, investigate the dependence of QNM frequencies on the field
mass. By this research, we find that there are quasi-normal modes with
arbitrarily long life when the field mass has special values. It is also found
that QNM can disappear when the field mass exceed these values.


title: No Swiss-cheese universe on the brane

abstract:
  We study the possibility of brane-world generalization of the Einstein-Straus
Swiss-cheese cosmological model. We find the modifications induced by the
brane-world scenario. At a first glance only the motion of the boundary is
modified and the fluid in the exterior region is allowed to have pressure. The
general relativistic Einstein-Straus model emerges in the low density limit. By
imposing that the brane is static, a combination of the junction conditions and
modified cosmological evolution leads to the conclusion that the brane is flat.
Thus no static Swiss-cheese universe can exist on the brane. The conclusion is
not altered by the introduction of a cosmological constant in the FLRW regions.
This result mimics a similar general relativistic result: static
Einstein-Straus universes do not exist.


title: Optimal Constraint Projection for Hyperbolic Evolution Systems

abstract:
  Techniques are developed for projecting the solutions of symmetric hyperbolic
evolution systems onto the constraint submanifold (the constraint-satisfying
subset of the dynamical field space). These optimal projections map a field
configuration to the ``nearest'' configuration in the constraint submanifold,
where distances between configurations are measured with the natural metric on
the space of dynamical fields. The construction and use of these projections is
illustrated for a new representation of the scalar field equation that exhibits
both bulk and boundary generated constraint violations. Numerical simulations
on a black-hole background show that bulk constraint violations cannot be
controlled by constraint-preserving boundary conditions alone, but are
effectively controlled by constraint projection. Simulations also show that
constraint violations entering through boundaries cannot be controlled by
constraint projection alone, but are controlled by constraint-preserving
boundary conditions. Numerical solutions to the pathological scalar field
system are shown to converge to solutions of a standard representation of the
scalar field equation when constraint projection and constraint-preserving
boundary conditions are used together.


title:
Towards a novel wave-extraction method for numerical relativity. I.
  Foundations and initial-value formulation

abstract:
  The Teukolsky formalism of black hole perturbation theory describes weak
gravitational radiation generated by a mildly dynamical hole near equilibrium.
A particular null tetrad of the background Kerr geometry, due to Kinnersley,
plays a singularly important role within this formalism. In order to apply the
rich physical intuition of Teukolsky's approach to the results of fully
non-linear numerical simulations, one must approximate this Kinnersley tetrad
using raw numerical data, with no a priori knowledge of a background. This
paper addresses this issue by identifying the directions of the tetrad fields
in a quasi-Kinnersley frame. This frame provides a unique, analytic extension
of Kinnersley's definition for the Kerr geometry to a much broader class of
space-times including not only arbitrary perturbations, but also many examples
which differ non-perturbatively from Kerr. This paper establishes concrete
limits delineating this class and outlines a scheme to calculate the
quasi-Kinnersley frame in numerical codes based on the initial-value
formulation of geometrodynamics.


title:
Towards wave extraction in numerical relativity: the quasi-Kinnersley
  frame

abstract:
  The Newman-Penrose formalism may be used in numerical relativity to extract
coordinate-invariant information about gravitational radiation emitted in
strong-field dynamical scenarios. The main challenge in doing so is to identify
a null tetrad appropriately adapted to the simulated geometry such that
Newman-Penrose quantities computed relative to it have an invariant physical
meaning. In black hole perturbation theory, the Teukolsky formalism uses such
adapted tetrads, those which differ only perturbatively from the background
Kinnersley tetrad. At late times, numerical simulations of astrophysical
processes producing isolated black holes ought to admit descriptions in the
Teukolsky formalism. However, adapted tetrads in this context must be
identified using only the numerically computed metric, since no background Kerr
geometry is known a priori. To do this, this paper introduces the notion of a
quasi-Kinnersley frame. This frame, when space-time is perturbatively close to
Kerr, approximates the background Kinnersley frame. However, it remains
calculable much more generally, in space-times non-perturbatively different
from Kerr. We give an explicit solution for the tetrad transformation which is
required in order to find this frame in a general space-time.


title:
Conformal Null Infinity Does Not Exist for Radiating Solutions in Odd
  Spacetime Dimensions

abstract:
  We show that for general relativity in odd spacetime dimensions greater than
4, all components of the unphysical Weyl tensor for arbitrary smooth, compact
spatial support perturbations of Minkowski spacetime fail to be smooth at null
infinity at leading nonvanishing order. This implies that for nearly flat
radiating spacetimes, the non-smoothness of the unphysical metric at null
infinity manifests itself at the same order as it describes deviations from
flatness of the physical metric. Therefore, in odd spacetime dimensions, it
does not appear that conformal null infinity can be in any way useful for
describing radiation.


title:
Semiclassical limit for Dirac particles interacting with a gravitational
  field

abstract:
  The behavior of spin-1/2 particle in a weak static gravitational field is
considered. The Dirac Hamiltonian is diagonalized by the Foldy-Wouthuysen
transformation providing also the simple form for the momentum and spin
polarization operators. The operator equations of momentum and spin motion are
derived for a first time. Their semiclassical limit is analyzed. The dipole
spin-gravity coupling in the previously found (another) Hamiltonian does not
lead to any observable effects. The general agreement between the quantum and
classical analysis is established, contrary to several recent claims. The
expression for gravitational Stern-Gerlach force is derived. The helicity
evolution in the gravitational field and corresponding accelerated frame
coincides, being the manifestation of the equivalence principle.


title:
An approximate global stationary axisymmetric solution of the Einstein
  equations

abstract:
  In this work we present an approximate solution of the Einstein equations
describing a global model for the gravitational field generated by a bounded,
self-gravitating stationary and axisymmetric body rotating rigidly with
constant angular velocity.


title: Spherically Symmetric Quantum Geometry: States and Basic Operators

abstract:
  The kinematical setting of spherically symmetric quantum geometry, derived
from the full theory of loop quantum gravity, is developed. This extends
previous studies of homogeneous models to inhomogeneous ones where interesting
field theory aspects arise. A comparison between a reduced quantization and a
derivation of the model from the full theory is presented in detail, with an
emphasis on the resulting quantum representation. Similar concepts for
Einstein-Rosen waves are discussed briefly.


title: The Volume Operator in Spherically Symmetric Quantum Geometry

abstract:
  The spherically symmetric volume operator is discussed and all its
eigenstates and eigenvalues are computed. Even though the operator is more
complicated than its homogeneous analog, the spectra are related in the sense
that the larger spherically symmetric volume spectrum adds fine structure to
the homogeneous spectrum. The formulas of this paper complete the derivation of
an explicit calculus for spherically symmetric models which is needed for
future physical investigations.


title: Modelling Disk-Planet Interactions in N-Body Numerical Simulations

abstract:
  We have withdrawn this paper because we have presented the basic results in
Brunini & Cionco (2005), Icarus 177, 264


title: On the viability of local criteria for chaos

abstract:
  We consider here a recently proposed geometrical criterion for local
instability based on the geodesic deviation equation. Although such a criterion
can be useful in some cases, we show here that, in general, it is neither
necessary nor sufficient for the occurrence of chaos. To this purpose, we
introduce a class of chaotic two-dimensional systems with Gaussian curvature
everywhere positive and, hence, locally stable. We show explicitly that chaotic
behavior arises from some trajectories that reach certain non convex parts of
the boundary of the effective Riemannian manifold. Our result questions, once
more, the viability of local, curvature-based criteria to predict chaotic
behavior.


title: de Sitter attractors in generalized gravity

abstract:
  We obtain conditions for the existence and stability of de Sitter attractors
in the phase space of spatially homogeneous and isotropic cosmology in
generalized theories of gravity (including non-linear and scalar-tensor
theories). These conditions are valid for any form of the coupling functions of
the theory. Stability with respect to inhomogeneous perturbations is analyzed
using a covariant and gauge-invariant formalism. The relevance for inflationary
scenarios of the early universe and for quintessence models of the present era
is discussed.


title: Spacetime and Euclidean Geometry

abstract:
  Using only the principle of relativity and Euclidean geometry we show in this
pedagogical article that the square of proper time or length in a
two-dimensional spacetime diagram is proportional to the Euclidean area of the
corresponding causal domain. We use this relation to derive the Minkowski line
element by two geometric proofs of the "spacetime Pythagoras theorem".


title: General relativity and quintessence explain the Pioneer anomaly

abstract:
  The anomalous time depending blueshift, the so-called "Pioneer anomaly", that
was detected in the radio-metric data from Pioneer 10/11, Ulysses and Galileo
spacecraft may not result from a real change of velocity. Rather, the Pioneer
anomaly may be understood within the framework of general relativity as a time
depending gravitational frequency shift accounting for the time dependence of
the density of the dark energy when the latter is identified with quintessence.
Thus, instead of being in conflict with Einstein equivalence principle, the
main Pioneer anomaly appears merely as a new validation of general relativity
in the weak field and low velocity limit.


title:
A toy model of the five-dimensional universe with the cosmological
  constant

abstract:
  A value of the cosmological constant in a toy model of the five-dimensional
universe is calculated in such a manner that it remains in agreement with both
astronomical observations and the quantum field theory concerning the
zero-point fluctuations of the vacuum. The (negative) cosmological constant is
equal to the inverse of the Planck length squared, which means that in the toy
model the vanishing of the observed value of the cosmological constant is a
consequence of the existence of an energy cutoff exactly at the level of the
Planck scale. In turn, a model for both a virtual and a real
particle-antiparticle pair is proposed which describes properly some energetic
properties of both the vacuum fluctuations and created particles, as well as it
allows one to calculate the discrete "bare" values of an elementary-particle
mass, electric charge and intrinsic angular momentum (spin) at the energy
cutoff. The relationships between the discussed model and some phenomena such
as the Zitterbewegung and the Unruh-Davies effect are briefly analyzed, too.
The proposed model also allows one to derive the Lorentz transformation and the
Maxwell equations while considering the properties of the vacuum filled with
the sea of virtual particles and their antiparticles. Finally, the existence of
a finite value of the vacuum-energy density resulting from the toy model leads
us to the formulation of dimensionless Einstein field equations which can be
derived from the Lagrangian with a dimensionless (naively renormalized)
coupling constant.


title: Performance of the WaveBurst algorithm on LIGO data

abstract:
  In this paper we describe the performance of the WaveBurst algorithm which
was designed for detection of gravitational wave bursts in interferometric
data. The performance of the algorithm was evaluated on the test data set
collected during the second LIGO Scientific run. We have measured the false
alarm rate of the algorithm as a function of the threshold and estimated its
detection efficiency for simulated burst waveforms.


title: Energy of Kerr-Newman Black-Holes and Gravitomagnetism

abstract:
  New formulae are obtained for the energy of K.N. b.h.'s that point out a
gravitomagnetic energy effect. The results are valid for slowly or rapidly
rotating black-holes. The expression of the energy density of Kerr-Newman
back-holes in the slow rotation case, is obtained afterwards, and shown to be
essentially positive. Subsequently,we show how to attain a "repulsive"
gravitation (antigravitation) state identified with negative energy
distribution contents in a limited region of space, without violating the
Positive Energy Theorem.


title: Inflation in Oscillating Universe

abstract:
  We make use of possible high energy correction to the Friedmann equation to
implement the bounce and study the behavior of massive scalar field before and
after bounce semianalytically and numerically. We find that the slow-roll
inflation can be preceded by the kinetic dominated contraction. During this
process, the field initially in the bottom of its potential can be driven by
the anti-frictional force resulted from the contraction and roll up its
potential hill, and when it rolls down after the bounce, it can driven a period
inflation. The required e-folds number during the inflation limits the energy
scale of bounce. Further unlike that expected, the field during the contraction
can not be driven to arbitrary large value, even though the bounce occurs at
Planck scale.


title:
On the existence of turning points in D-dimensionsal Schwarzschild-de
  Sitter and anti-de Sitter spacetimes

abstract:
  We investigate the motion of a test particle in a d-dimensional, spherically
symmetric and static space-time supported by a mass $M$ plus a $\Lambda$-term.
The motion is strongly dependent on the sign of $\Lambda$. In Schwarzschild-de
Sitter (SdS) space-time ($\Lambda > 0$), besides the physical singularity at
$r=0$ there are cases with two horizons and two turning points, one horizon and
one turning point and the complete absence of horizon and turning points. For
Schwarzschild-Anti de Sitter (SAdS) space-time ($\Lambda < 0$) the horizon
coordinate is associated to a unique turning point.


title:
Transition from accelerated to decelerated regimes in JT and CGHS
  cosmologies

abstract:
  In this work we discuss the possibility of positive-acceleration regimes, and
their transition to decelerated regimes, in two-dimensional (2D) cosmological
models. We use general relativity and the thermodynamics in a 2D space-time,
where the gas is seen as the sources of the gravitational field. An
early-Universe model is analyzed where the state equation of van der Waals is
used, replacing the usual barotropic equation. We show that this substitution
permits the simulation of a period of inflation, followed by a
negative-acceleration era. The dynamical behavior of the system follows from
the solution of the Jackiw-Teitelboim equations (JT equations) and the
energy-momentum conservation laws. In a second stage we focus the
Callan-Giddings-Harvey-Strominger model (CGHS model); here the transition from
the inflationary period to the decelerated period is also present between the
solutions, although this result depend strongly on the initial conditions used
for the dilaton field. The temporal evolution of the cosmic scale function, its
acceleration, the energy density and the hydrostatic pressure are the physical
quantities obtained in through the analysis.


title:
Particle Motion and Scalar Field Propagation in Myers-Perry Black Hole
  Spacetimes in All Dimensions

abstract:
  We study separability of the Hamilton-Jacobi and massive Klein-Gordon
equations in the general Myers-Perry black hole background in all dimensions.
Complete separation of both equations is carried out in cases when there are
two sets of equal black hole rotation parameters, which significantly enlarges
the rotational symmetry group. We explicitly construct a nontrivial irreducible
Killing tensor associated with the enlarged symmetry group which permits
separation. We also derive first-order equations of motion for particles in
these backgrounds and examine some of their properties.


title: Coherent Waveform Consistency Test for LIGO Burst Candidates

abstract:
  The burst search in LIGO relies on the coincident detection of transient
signals in multiple interferometers. As only minimal assumptions are made about
the event waveform or duration, the analysis pipeline requires loose
coincidence in time, frequency and amplitude. Confidence in the resulting
events and their waveform consistency is established through a time-domain
coherent analysis: the r-statistic test.
  This paper presents a performance study of the r-statistic test for triple
coincidence events in the second LIGO Science Run (S2), with emphasis on its
ability to suppress the background false rate and its efficiency at detecting
simulated bursts of different waveforms close to the S2 sensitivity curve.


title:
Semiclassical and Quantum Field Theoretic Bounds for Traversable
  Lorentzian Stringy Wormholes

abstract:
  A lower bound on the size of a Lorentzian wormhole can be obtained by
semiclassically introducing the Planck cut-off on the magnitude of tidal forces
(Horowitz-Ross constraint). Also, an upper bound is provided by the quantum
field theoretic constraint in the form of the Ford-Roman Quantum Inequality for
massless minimally coupled scalar fields. To date, however, exact static
solutions belonging to this scalar field theory have not been worked out to
verify these bounds. To fill this gap, we examine the wormhole features of two
examples from the Einstein frame description of the vacuum low energy string
theory in four dimensions which is the same as the minimally coupled scalar
field theory. Analyses in this paper support the conclusion of Ford and Roman
that wormholes in this theory can have sizes that are indeed only a few order
of magnitudes larger than the Planck scale. It is shown that the two types of
bounds are also compatible. In the process, we point out a "wormhole" analog of
naked black holes.


title: Superfluid Picture for Rotating Space-Times

abstract:
  A new prescription, in the framework of condensate models for space-times,
for physical stationary gravitational fields is presented. We show that the
spinning cosmic string metric describes the gravitational field associated with
the single vortex in a superfluid condensate model for space-time outside the
vortex core. This metric differs significantly from the usual acoustic metric
for the Onsager-Feynman vortex. We also consider the question of what happens
when many vortices are present, and show that on large scales a G\"odel-like
metric emerges. In both the single and multiple vortex cases the failure of
general relativity exemplified by the presence of closed time-like curves is
attributed to the breakdown of superfluid rigidity.


title: Border of Spacetime

abstract:
  It is still uncertain whether the cosmic censorship conjecture is true or
not. To get a new insight into this issue, we propose the concept of the border
of spacetime as a generalization of the spacetime singularity and discuss its
visibility. The visible border, corresponding to the naked singularity, is not
only relevant to mathematical completeness of general relativity but also a
window into new physics in strongly curved spacetimes, which is in principle
observable.


title: Axially symmetric SU(3) Gravitating Skyrmions

abstract:
  Axially symmetric gravitating multi-skyrmion configurations are obtained
using the harmonic map ansatz introduced in [1]. In particular, the effect of
gravity on the energy and baryon densities of the SU(3) non-gravitating
multi-skyrmion configurations is studied in detail.


title:
Deriving formulations for numerical computation of binary neutron stars
  in quasicircular orbits

abstract:
  Two relations, the virial relation $M_{\rm ADM}=M_{\rm K}$ and the first law
in the form $\delta M_{\rm ADM}=\Omega \delta J$, should be satisfied by a
solution and a sequence of solutions describing binary compact objects in
quasiequilibrium circular orbits. Here, $M_{\rm ADM}$, $M_{\rm K}$, $J$, and
$\Omega$ are the ADM mass, Komar mass, angular momentum, and orbital angular
velocity, respectively. $\delta$ denotes an Eulerian variation. These two
conditions restrict the allowed formulations that we may adopt. First, we
derive relations between $M_{\rm ADM}$ and $M_{\rm K}$ and between $\delta
M_{\rm ADM}$ and $\Omega \delta J$ for general asymptotically flat spacetimes.
Then, to obtain solutions that satisfy the virial relation and sequences of
solutions that satisfy the first law at least approximately, we propose a
formulation for computation of quasiequilibrium binary neutron stars in general
relativity. In contrast to previous approaches in which a part of the Einstein
equation is solved, in the new formulation, the full Einstein equation is
solved with maximal slicing and in a transverse gauge for the conformal
three-metric. Helical symmetry is imposed in the near zone, while in the
distant zone, a waveless condition is assumed. We expect the solutions obtained
in this formulation to be excellent quasiequilibria as well as initial data for
numerical simulations of binary neutron star mergers.


title:
Non-spherical sources of static gravitational fields: investigating the
  boundaries of the no-hair theorem

abstract:
  A new, globally regular model describing a static, non spherical gravitating
object in General Relativity is presented. The model is composed by a vacuum
Weyl--Levi-Civita special field - the so called gamma metric - generated by a
regular static distribution of mass-energy. Standard requirements of physical
reasonableness such as, energy, matching and regularity conditions are
satisfied. The model is used as a toy in investigating various issues related
to the directional behavior of naked singularities in static spacetimes and the
blackhole (Schwarschild) limit.


title: Vacuum Instability

abstract:
  Following fresh attempts to resolve the problem of the energy density of the
vacuum, we reconsider the case where the cosmological constant is derived from
a higher-dimensional version of general relativity, and interpret the
gauge-dependence of $\Lambda $ as a dynamical effect. This leads to a relation
between the change in $\Lambda $ and the line element (action) which is
independent of gauge choices and fundamental constants: $d\Lambda ds^{2}=-3$.
This implies that the (classical) vacuum is unstable, with implications for
particle production.


title: Sensitivity of a small matter-wave interferometer to gravitational waves

abstract:
  We study the possibility of using matter wave interferometry techniques to
build a gravitational wave detector. We derive the response function and find
that it contains a term proportional to the derivative of the gravitational
wave, a point which has been disputed recently. We then study in detail the
sensitivity that can be reached by such a detector and find that, if it is
operated near resonance, it can reach potentially interesting values in the
high frequency regime. The correlation between two or more of such devices can
further improve the sensitivity for a stochastic signal.


title:
Equilibrium points of the tilted perfect fluid Bianchi VI$_h$ state
  space

abstract:
  We present the full set of evolution equations for the spatially homogeneous
cosmologies of type VI$_h$ filled with a tilted perfect fluid and we provide
the corresponding equilibrium points of the resulting dynamical state space. It
is found that only when the group parameter satisfies $h>-1$ a self-similar
solution exists. In particular we show that for $h>-{\frac 19}$ there exists a
self-similar equilibrium point provided that $\gamma \in
({\frac{2(3+\sqrt{-h})}{5+3\sqrt{-h}}},{\frac 32}) $ whereas for $h<-{\frac
19}$ the state parameter belongs to the interval $\gamma \in
(1,{\frac{2(3+\sqrt{-h})}{5+3\sqrt{-h}}}) $. This family of new exact
self-similar solutions belongs to the subclass $n_\alpha ^\alpha =0$ having
non-zero vorticity. In both cases the equilibrium points have a five
dimensional stable manifold and may act as future attractors at least for the
models satisfying $n_\alpha ^\alpha =0$. Also we give the exact form of the
self-similar metrics in terms of the state and group parameters. As an
illustrative example we provide the explicit form of the corresponding
self-similar radiation model ($\gamma={\frac 43}$), parametrised by the group
parameter $h$. Finally we show that there are no tilted self-similar models of
type III and irrotational models of type VI$_h$.


title: On isotropic cylindrically symmetric stellar models

abstract:
  We attempt to match the most general cylindrically symmetric vacuum
space-time with a Robertson-Walker interior. The matching conditions show that
the interior must be dust filled and that the boundary must be comoving.
Further, we show that the vacuum region must be polarized. Imposing the
condition that there are no trapped cylinders on an initial time slice, we can
apply a result of Thorne's and show that trapped cylinders never evolve. This
results in a simplified line element which we prove to be incompatible with the
dust interior. This result demonstrates the impossibility of the existence of
an isotropic cylindrically symmetric star (or even a star which has a
cylindrically symmetric portion). We investigate the problem from a different
perspective by looking at the expansion scalars of invariant null geodesic
congruences and, applying to the cylindrical case, the result that the product
of the signs of the expansion scalars must be continuous across the boundary.
The result may also be understood in relation to recent results about the
impossibility of the static axially symmetric analogue of the Einstein-Straus
model.


title: Isolated and dynamical horizons and their applications

abstract:
  Over the past three decades, black holes have played an important role in
quantum gravity, mathematical physics, numerical relativity and gravitational
wave phenomenology. However, conceptual settings and mathematical models used
to discuss them have varied considerably from one area to another. Over the
last five years a new, quasi-local framework was introduced to analyze diverse
facets of black holes in a unified manner. In this framework, evolving black
holes are modeled by dynamical horizons and black holes in equilibrium by
isolated horizons. We review basic properties of these horizons and summarize
applications to mathematical physics, numerical relativity and quantum gravity.
This paradigm has led to significant generalizations of several results in
black hole physics. Specifically, it has introduced a more physical setting for
black hole thermodynamics and for black hole entropy calculations in quantum
gravity; suggested a phenomenological model for hairy black holes; provided
novel techniques to extract physics from numerical simulations; and led to new
laws governing the dynamics of black holes in exact general relativity.


title:
How can exact and approximate solutions of Einstein's field equations be
  compared?

abstract:
  The problem of comparison of the stationary axisymmetric vacuum solutions
obtained within the framework of exact and approximate approaches for the
description of the same general relativistic systems is considered. We suggest
two ways of carrying out such comparison: (i) through the calculation of the
Ernst complex potential associated with the approximate solution whose form on
the symmetry axis is subsequently used for the identification of the exact
solution possessing the same multipole structure, and (ii) the generation of
approximate solutions from exact ones by expanding the latter in series of
powers of a small parameter. The central result of our paper is the derivation
of the correct approximate analogues of the double-Kerr solution possessing the
physically meaningful equilibrium configurations. We also show that the
interpretation of an approximate solution originally attributed to it on the
basis of some general physical suppositions may not coincide with its true
nature established with the aid of a more accurate technique.


title: Comment on "The double-Kerr solution" by W.B. Bonnor and B.R. Steadman

abstract:
  Erroneous statements on the mathematical and physical properties of the
double-Kerr solution made in a recent paper (Class. Quantum Grav. 21 (2004)
2723) are commented.


title: Reissner-Nordstr\"om Metric in the Friedman-Robertson-Walker Universe

abstract:
  The metric for a Reissner-Nordstr$\ddot{o}$m black hole in the background of
the Friedman-Robertson-Walker universe is obtained. Then we verified it and
discussed the influence of the evolution of the universe on the size of the
black hole. To study the problem of the orbits of a planet in the expanding
universe, we rewrote the metric in the Schwarzschild coordinates system and
deduced the equation of motion for a planet.


title: Cosmological solutions in the brane-bulk system

abstract:
  In this work we find cosmological solutions in the brane-bulk system starting
from a 5-D line element which is a simple extension, for cosmological
applications, of the pioneering Randall-Sundrum line element. From the
knowledge of the bulk metric, assumed to have the form of plane waves
propagating in the fifth dimension, we solve the corresponding 4-D Einstein
equations on the brane with a well defined energy-momentum tensor.


title:
Is it possible to measure the Lense-Thirring effect on the orbits of the
  planets in the gravitational field of the Sun?

abstract:
  Here we explore a novel approach in order to try to measure the
post-Newtonian 1/c^2 Lense-Thirring secular effect induced by the
gravitomagnetic field of the Sun on the planetary orbital motion. Due to the
relative smallness of the solar angular momentum J and the large values of the
planetary semimajor axes a, the gravitomagnetic precessions, which affect the
nodes Omega and the perihelia omega and are proportional to J/a^3, are of the
order of 10^-3 arcseconds per century only for, e.g., Mercury. This value lies
just at the edge of the present-day observational sensitivity in reconstructing
the planetary orbits, although future missions to Mercury like Messenger and
BepiColombo could allow to increase it. The major problems come from the main
sources of systematic errors. They are the aliasing classical precessions
induced by the multipolar expansion of the Sun's gravitational potential and
the classical secular N-body precessions which are of the same order of
magnitude or much larger than the Lense-Thirring precessions of interest. This
definitely rules out the possibility of analyzing only one orbital element of,
e.g., Mercury. In order to circumvent these problems, we propose a suitable
linear combination of the orbital residuals of the nodes of Mercury, Venus and
Mars which is, by construction, independent of such classical secular
precessions. A 1-sigma reasonable estimate of the obtainable accuracy yields a
36% error. Since the major role in the proposed combination is played by the
Mercury's node, it could happen that the new, more accurate ephemerides
available in future thanks to the Messenger and BepiColombo missions will offer
an opportunity to improve the present unfavorable situation.


title: Strong gravitational force induced by static electromagnetic fields

abstract:
  It is argued that static electric or magnetic fields induce
Weyl-Majumdar-Papapetrou solutions for the metric of spacetime. Their
gravitational acceleration includes a term many orders of magnitude stronger
than usual perturbative terms. It gives rise to a number of effects, which can
be detected experimentally. Four electrostatic and four magnetostatic examples
of physical set-ups with simple symmetries are proposed. The different ways in
which mass sources enter and complicate the pure electromagnetic picture are
described.


title:
Third post-Newtonian accurate generalized quasi-Keplerian
  parametrization for compact binaries in eccentric orbits

abstract:
  We present Keplerian-type parametrization for the solution of third
post-Newtonian (3PN) accurate equations of motion for two non-spinning compact
objects moving in an eccentric orbit. The orbital elements of the
parametrization are explicitly given in terms of the 3PN accurate conserved
orbital energy and angular momentum in both Arnowitt, Deser, and Misner-type
and harmonic coordinates. Our representation will be required to construct
post-Newtonian accurate `ready to use' search templates for the detection of
gravitational waves from compact binaries in inspiralling eccentric orbits. Due
to the presence of certain 3PN accurate gauge invariant orbital elements, the
parametrization should be useful to analyze the compatibility of general
relativistic numerical simulations involving compact binaries with the
corresponding post-Newtonian descriptions. If required, the present
parametrization will also be needed to compute post-Newtonian corrections to
the currently employed `timing formula' for the radio observations of
relativistic binary pulsars.


title:
Absorption of mass and angular momentum by a black hole: Time-domain
  formalisms for gravitational perturbations, and the small-hole/slow-motion
  approximation

abstract:
  The first objective of this work is to obtain practical prescriptions to
calculate the absorption of mass and angular momentum by a black hole when
external processes produce gravitational radiation. These prescriptions are
formulated in the time domain within the framework of black-hole perturbation
theory. Two such prescriptions are presented. The first is based on the
Teukolsky equation and it applies to general (rotating) black holes. The second
is based on the Regge-Wheeler and Zerilli equations and it applies to
nonrotating black holes. The second objective of this work is to apply the
time-domain absorption formalisms to situations in which the black hole is
either small or slowly moving. In the context of this small-hole/slow-motion
approximation, the equations of black-hole perturbation theory can be solved
analytically, and explicit expressions can be obtained for the absorption of
mass and angular momentum. The changes in the black-hole parameters can then be
understood in terms of an interaction between the tidal gravitational fields
supplied by the external universe and the hole's tidally-induced mass and
current quadrupole moments. For a nonrotating black hole the quadrupole moments
are proportional to the rate of change of the tidal fields on the hole's world
line. For a rotating black hole they are proportional to the tidal fields
themselves.


title: Black hole entropy from Quantum Geometry

abstract:
  Quantum Geometry (the modern Loop Quantum Gravity using graphs and
spin-networks instead of the loops) provides microscopic degrees of freedom
that account for the black-hole entropy. However, the procedure for state
counting used in the literature contains an error and the number of the
relevant horizon states is underestimated. In our paper a correct method of
counting is presented. Our results lead to a revision of the literature of the
subject. It turns out that the contribution of spins greater then 1/2 to the
entropy is not negligible. Hence, the value of the Barbero-Immirzi parameter
involved in the spectra of all the geometric and physical operators in this
theory is different than previously derived. Also, the conjectured relation
between Quantum Geometry and the black hole quasi-normal modes should be
understood again.


title: Black hole entropy in Loop Quantum Gravity

abstract:
  We calculate the black hole entropy in Loop Quantum Gravity as a function of
the horizon area and provide the exact formula for the leading and sub-leading
terms. By comparison with the Bekenstein-Hawking formula we uniquely fix the
value of the 'quantum of area' in the theory.


title: Gravitoelectromagnetism and other decompositions of the Riemann tensor

abstract:
  We present gravitoelectromagnetism and other decompositions of the Riemann
tensor from the differential-geometrical point of view.


title: Evolving spherical boson stars on a 3D cartesian grid

abstract:
  A code to evolve boson stars in 3D is presented as the starting point for the
evolution of scalar field systems with arbitrary symmetries. It was possible to
reproduce the known results related to perturbations discovered with 1D
numerical codes in the past, which include evolution of stable and unstable
equilibrium configurations. In addition, the apparent and event horizons masses
of a collapsing boson star are shown for the first time. The present code is
expected to be useful at evolving possible sources of gravitational waves
related to scalar field objects and to handle toy models of systems perturbed
with scalar fields in 3D.


title: State reduction and energy extraction from black holes

abstract:
  I show that attempts to detect Hawking quanta would reduce the quantum state
to one containing ultra-energetic incoming particles; couplings of these to
other systems would extract ultra-high energies from the gravitational
collapse. As the collapse proceeds, these energies grow exponentially, rapidly
become trans-Planckian, and quantum-gravitational effects must enter.


title: Massive Gauge Fields and the Planck Scale

abstract:
  The present work is devoted to massive gauge fields in special relativity
with two fundamental constants-the velocity of light, and the Planck length, so
called doubly special relativity (DSR). The two invariant scales are accounted
for by properly modified boost parameters. Within above framework we construct
the vector potential as the (1/2,0)x(0,1/2) direct product, build the
associated field strength tensor together with the Dirac spinors and use them
to calculate various observables as functions of the Planck length.


title:
Search algorithm for a gravitational wave signal in association with
  Gamma Ray Burst GRB030329 using the LIGO detectors

abstract:
  One of the brightest Gamma Ray Burst ever recorded, GRB030329, occurred
during the second science run of the LIGO detectors. At that time, both
interferometers at the Hanford, WA LIGO site were in lock and acquiring data.
The data collected from the two Hanford detectors was analyzed for the presence
of a gravitational wave signal associated with this GRB. This paper presents a
detailed description of the search algorithm implemented in the current
analysis.


title: On Black-Brane Instability In an Arbitrary Dimension

abstract:
  The black-hole black-string system is known to exhibit critical dimensions
and therefore it is interesting to vary the spacetime dimension $D$, treating
it as a parameter of the system. We derive the large $D$ asymptotics of the
critical, i.e. marginally stable, string following an earlier numerical
analysis. For a background with an arbitrary compactification manifold we give
an expression for the critical mass of a corresponding black brane. This
expression is completely explicit for ${\bf T}^n$, the $n$ dimensional torus of
an arbitrary shape. An indication is given that by employing a higher
dimensional torus, rather than a single compact dimension, the total critical
dimension above which the nature of the black-brane black-hole phase transition
changes from sudden to smooth could be as low as $D\leq 11$.


title: Comments on Cahill's Quantum Foam Inflow Theory of Gravity

abstract:
  We reveal an underlying flaw in Reginald T. Cahill's recently promoted
quantum foam inflow theory of gravity. It appears to arise from a confusion of
the idea of the Galilean invariance of the acceleration of an individual flow
with what is obtained as an acceleration when a homogeneous flow is superposed
with an inhomogeneous flow. We also point out that the General Relativistic
covering theory he creates by substituting a generalized Painleve-Gullstrand
metric into Einstein's field equations leads to absurd results.


title:
Gravity as Nonmetricity: General Relativity in Metric-Affine Space
  (Ln,g)

abstract:
  A new geometric interpretation for General Relativity (GR) is proposed. We
show that in the presence of an arbitrary affine connection, the gravitational
field is described as nonmetricity of the affine connection. An affine
connection can be interpreted as induced by a frame of reference (FR). Although
the gravitational field equations are identical to Einstein's equations of GR,
this formulation leads to a covariant tensor (instead of the pseudotensor) of
energy-momentum of the gravitational field and covariant conservation laws. We
further develop a geometric representation of FR as a metric-affine space, with
transition between FR represented as affine deformation of the connection.
Geodesic and autoparallel worldlines are considered. We show that the affine
connection of a NIFR has curvature and may have torsion. We calculate the
curvature for the uniformly accelerated FR. Finally, we show that GR is
inadequate to describe the gravitational field in a NIFR. We propose a
generalization of GR, which describes gravity as nonmetricity of the affine
connection induced in a FR. This generalization contains GR as a special case
of the inertial FR.


title: Curvature in causal BD-type inflationary cosmology

abstract:
  We study a closed model of the universe filled with viscous fluid and
quintessence matter components in a Brans-Dicke type cosmological model. The
dynamical equations imply that the universe may look like an accelerated flat
Friedmann-Robertson-Walker universe at low redshift. We consider here
dissipative processes which follow a causal thermodynamics. The theory is
applied to viscous fluid inflation, where accepted values for the total entropy
in the observable universe is obtained.


title:
Structure of solutions near the initial singularity for the
  surface-symmetric Einstein-Vlasov system

abstract:
  Results on the behaviour in the past time direction of cosmological models
with collisionless matter and a cosmological constant $\Lambda$ are presented.
It is shown that under the assumption of non-positive $\Lambda$ and spherical
or plane symmetry the area radius goes to zero at the initial singularity.
Under a smallness assumption on the initial data, these properties hold in the
case of hyperbolic symmetry and negative $\Lambda$ as well as in the positive
$\Lambda$ case. Furthermore in the latter cases past global existence of
spatially homogeneous solutions is proved for generic initial data. The
early-time asymptotics is shown to be Kasner-like for small data.


title:
Inner boundary conditions for black hole Initial Data derived from
  Isolated Horizons

abstract:
  We present a set of boundary conditions for solving the elliptic equations in
the Initial Data Problem for space-times containing a black hole, together with
a number of constraints to be satisfied by the otherwise freely specifiable
standard parameters of the Conformal Thin Sandwich formulation. These
conditions altogether are sufficient for the construction of a horizon that is
instantaneously in equilibrium in the sense of the Isolated Horizons formalism.
We then investigate the application of these conditions to the Initial Data
Problem of binary black holes and discuss the relation of our analysis with
other proposals that exist in the literature.


title:
Corrections and Comments on the Multipole Moments of Axisymmetric
  Electrovacuum Spacetimes

abstract:
  Following the method of Hoenselaers and Perj\'{e}s we present a new corrected
and dimensionally consistent set of multipole gravitational and electromagnetic
moments for stationary axisymmetric spacetimes. Furthermore, we use our results
to compute the multipole moments, both gravitational and electromagnetic, of a
Kerr-Newman black hole.


title:
Gauge fields and particle-like formations associated with shear-free
  null congruences

abstract:
  We consider Maxwell fields associated with any shear-free null geodesic
congruence on Minkowski or Riemannian background space-time. Bounded singular
loci of these fields are treated as particle-like formations, possess
"self-quantized" electric charge and undergo self-consistent time dynamics.
Complicated singular solutions of Maxwell (as well as related complex
Yang-Mills) equations can be obtained in a purely algebraic way using the Kerr
theorem.


title: The Relativistically Spinning Charged Sphere

abstract:
  When the equatorial spin velocity, $v$, of a charged conducting sphere
approaches $c$, the Lorentz force causes a remarkable rearrangement of the
total charge $q$.
  Charge of that sign is confined to a narrow equatorial belt at latitudes $b
\leqslant \sqrt{3} (1 - v^2/c^2)^{{1/2}}$ while charge of the opposite sign
occupies most of the sphere's surface. The change in field structure is shown
to be a growing contribution of the `magic' electromagnetic field of the
charged Kerr-Newman black hole with Newton's G set to zero. The total charge
within the narrow equatorial belt grows as $(1-v^2/c^2)^{-{1/4}}$ and tends to
infinity as $v$ approaches $c$. The electromagnetic field, Poynting vector,
field angular momentum and field energy are calculated for these
configurations.
  Gyromagnetic ratio, g-factor and electromagnetic mass are illustrated in
terms of a 19th Century electron model. Classical models with no spin had the
small classical electron radius $e^2/mc^2\sim$ a hundredth of the Compton
wavelength, but models with spin take that larger size but are so
relativistically concentrated to the equator that most of their mass is
electromagnetic.
  The method of images at inverse points of the sphere is shown to extend to
charges at points with imaginary co-ordinates.


title: Modified Einstein equations

abstract:
  Standard general relativity fails to take into account the changes in
coordinates induced by the variation of metric in the Hilbert action principle.
We propose to include such changes by introducing a fundamental compensating
tensor field and modifying the usual variational procedure.


title: Perfect Fluid FRW with Time-varying Constants Revisited

abstract:
  In this paper we revise a perfect fluid FRW model with time-varying constants
\textquotedblleft but\textquotedblright taking into account the effects of a
\textquotedblleft$c$-variable\textquotedblright into the curvature tensor. We
study the model under the following assumptions, $div(T)=0$ and $div(T)\neq0,$
and in each case the flat and the non-flat cases are studied. Once we have
outlined the new field equations, it is showed in the flat case i.e. K=0, that
there is a non-trivial homothetic vector field i.e. that this case is
self-similar. In this way, we find that there is only one symmetry, the scaling
one, which induces the same solution that the obtained one in our previous
model. At the same time we find that \textquotedblleft constants" $G$ and $c$
must verify, as integration condition of the field equations, the relationship
$G/c^{2}=const.$ in spite of that both \textquotedblleft constants" vary. We
also find that there is a narrow relationship between the equation of state and
the behavior of the time functions $G,c$ and the sign of $\Lambda$ in such a
way that these functions may be growing as well as decreasing functions on time
$t,$ while $\Lambda$ may be a positive or negative decreasing function on time
$t.$ In the non-flat case it will be showed that there is not any symmetry. For
the case $div(T)\neq0,$ it will be studied again the flat and the non-flat
cases. In order to solve this case it is necessary to make some assumptions on
the behavior of the time functions $G,c$ and $\Lambda.$ We also find the flat
case with $div(T)=0,$ is a particular solution of the general case
$div(T)\neq0.$


title: Genericness of inflation in isotropic loop quantum cosmology

abstract:
  Non-perturbative corrections from loop quantum cosmology (LQC) to the scalar
matter sector is already known to imply inflation. We prove that the LQC
modified scalar field generates exponential inflation in the small scale factor
regime, for all positive definite potentials, independent of initial conditions
and independent of ambiguity parameters. For positive semi-definite potentials
it is always possible to choose, without fine tuning, a value of one of the
ambiguity parameters such that exponential inflation results, provided zeros of
the potential are approached at most as a power law in the scale factor. In
conjunction with generic occurrence of bounce at small volumes, particle
horizon is absent thus eliminating the horizon problem of the standard Big Bang
model.


title:
Searching for a solution to the cosmological constant problem - a toy
  model

abstract:
  This paper concerns the so-called cosmological constant problem. In order to
solve it, we propose a toy model providing an extension of the dimensionality
of spacetime, with an additional spatial dimension which is macroscopically
unobservable. The toy model introduces no corrections to most predictions of
the "standard" general relativity regarding, among others, the so-called "five
tests of general relativity". However, it seems that the toy model could
provide an explanation to the flatness of circular velocity curves of spiral
galaxies without introducing any dark matter. The proposed model has quite
important cosmological consequences. By introducing certain corrections to
Friedmann's currently accepted model(s), the toy model allows one to solve
problems related to the present density of matter in the Universe and, finally,
it does not contain the initial singularity.


title:
Gauge Invariant Wave Equations in Curved Space-Times and Primordial
  Magnetic Fields

abstract:
  The inflationary production of magnetic field seeds for galaxies is
discussed. The analysis is carried out by writing the wave equation of the
electromagnetic field in curved spacetimes. The conformal invariance is broken
by taking into account of the interaction of the electromagnetic field with the
curvature tensor of the form $\lambda R_{\alpha\beta\gamma\delta}
F^{\alpha\beta} F^{\gamma\delta}$. Such a term induces an amplification of the
magnetic field during the reheating phase of the universe, but no growth of the
magnetic field occurs in the de Sitter epoch. The resulting primordial magnetic
field turns out to have strengths of astrophysical interest.


title:
Regular electrically charged structures in Nonlinear Electrodynamics
  coupled to General Relativity

abstract:
  We address the question of existence of regular spherically symmetric
electrically charged solutions in Nonlinear Electrodynamics coupled to General
Relativity. Stress-energy tensor of the electromagnetic field has the algebraic
structure $T_0^0=T_1^1$. In this case the Weak Energy Condition leads to the de
Sitter asymptotic at approaching a regular center. In de Sitter center of an
electrically charged NED structure, electric field, geometry and stress-energy
tensor are regular without Maxwell limit which is replaced by de Sitter limit:
energy density of a field is maximal and gives an effective cut-off on
self-energy density, produced by NED coupled to gravity and related to
cosmological constant $\Lambda$. Regular electric solutions satisfying WEC,
suffer from one cusp in the Lagrangian ${\cal L}(F)$, which creates the problem
in an effective geometry whose geodesics are world lines of NED photons. We
investigate propagation of photons and show that their world lines never
terminate which suggests absence of singularities in the effective geometry. To
illustrate these results we present the new exact analytic spherically
symmetric electric solution with the de Sitter center.


title: Effective Hamiltonian for Isotropic Loop Quantum Cosmology

abstract:
  For a class of solutions of the fundamental difference equation of isotropic
loop quantum cosmology, the difference equation can be replaced by a
differential equation valid for {\em all} values of the triad variable. The
differential equation admits a `unique' non-singular continuation through
vanishing triad. A WKB approximation for the solutions leads to an effective
continuum Hamiltonian. The effective dynamics is also non-singular (no big bang
singularity) and approximates the classical dynamics for large volumes. The
effective evolution is thus a more reliable model for further phenomenological
implications of the small volume effects.


title: Genericness of Big Bounce in isotropic loop quantum cosmology

abstract:
  The absence of isotropic singularity in loop quantum cosmology can be
understood in an effective classical description as the universe exhibiting a
Big Bounce. We show that with scalar matter field, the big bounce is generic in
the sense that it is independent of quantization ambiguities and details of
scalar field dynamics. The volume of the universe at the bounce point is
parametrized by a single parameter. It provides a minimum length scale which
serves as a cut-off for computations of density perturbations thereby
influencing their amplitudes.


title: Gravitational Vacuum Condensate Stars

abstract:
  A new final state of gravitational collapse is proposed. By extending the
concept of Bose-Einstein condensation to gravitational systems, a cold, dark,
compact object with an interior de Sitter condensate $p_{_V} = -\rho_{_V}$ and
an exterior Schwarzschild geometry of arbitrary total mass $M$ is constructed.
These are separated by a shell with a small but finite proper thickness $\ell$
of fluid with equation of state $p=+\rho$, replacing both the Schwarzschild and
de Sitter classical horizons. The new solution has no singularities, no event
horizons, and a global time. Its entropy is maximized under small fluctuations
and is given by the standard hydrodynamic entropy of the thin shell, which is
of order $k_{_B}\ell Mc/\hbar$, instead of the Bekenstein-Hawking entropy
formula, $S_{_{BH}}= 4\pi k_{_B} G M^2/\hbar c$. Hence unlike black holes, the
new solution is thermodynamically stable and has no information paradox.


title:
Classical Gravity as an Eikonal Approximation to a Manifestly Lorentz
  Covariant Quantum Theory with Brownian Interpretation

abstract:
  We discuss in this Chapter a series of theoretical developments which
motivate the introduction of a quantum evolution equation for which the eikonal
approximation results in the geodesics of a four dimensional manifold. This
geodesic motion can be put into correspondence with general relativity. One
obtains in this way a quantum theory on a flat spacetime, obeying the rules of
the standard quantum theory in Lorentz covariant form, with a spacetime
dependent Lorentz tensor $g_{\mu\nu}$, somewhat analogous to a gauge field,
coupling to the kinetic terms. Since the geodesics predicted by the eikonal
approximation, with appropriate choice of $g_{\mu\nu}$, can be those of general
relativity, this theory provides a quantum theory which could be underlying to
classical gravitation, and coincides with it in this classical ray
approximation. In order to understand the possible origin of the structure of
this equation, we appeal to the approach of Nelson in constructing a
Schroedinger equation from the properties of Brownian motion. Extending the
notion of Browninan motion to spacetime in a covariant way, we show that such
an equation follows from correlations between spacetime dimensions in the
stochastic process.


title:
Reflection on event horizon and escape of particles from confinement
  inside black holes

abstract:
  Several recently found properties of the event horizon of black holes are
discussed. One of them is the reflection of the incoming particles on the
horizon. A particle approaching the black hole can bounce on the horizon back,
into the outside world, which drastically reduces the absorption cross section
in the infrared region. Another, though related phenomenon takes place for
particles inside the horizon. A locked inside particle has, in fact, an
opportunity to escape into the outside world. Thus, the confinement inside the
horizon is not absolute. The escape from within the interior region of the
horizon allows the transfer of information from this region into the outside
world. This result may help resolve the information paradox for black holes.
Both the reflection and escape phenomena happen due to pure quantum reasons,
being impossible in the classical approximation.


title: Excision boundary conditions for black hole initial data

abstract:
  We define and extensively test a set of boundary conditions that can be
applied at black hole excision surfaces when the Hamiltonian and momentum
constraints of general relativity are solved within the conformal thin-sandwich
formalism. These boundary conditions have been designed to result in black
holes that are in quasiequilibrium and are completely general in the sense that
they can be applied with any conformal three-geometry and slicing condition.
Furthermore, we show that they retain precisely the freedom to specify an
arbitrary spin on each black hole. Interestingly, we have been unable to find a
boundary condition on the lapse that can be derived from a quasiequilibrium
condition. Rather, we find evidence that the lapse boundary condition is part
of the initial temporal gauge choice. To test these boundary conditions, we
have extensively explored the case of a single black hole and the case of a
binary system of equal-mass black holes, including the computation of
quasi-circular orbits and the determination of the inner-most stable circular
orbit. Our tests show that the boundary conditions work well.


title: Volume Integral Theorem for Exotic Matter

abstract:
  We answer an important question in general relativity about the volume
integral theorem for exotic matter by suggesting an exact integral quantifier
for matter violating Averaged Null Energy Condition (ANEC). It is checked
against some well known static, spherically symmetric traversable wormhole
solutions of general relativity with a sign reversed kinetic term minimally
coupled scalar field. The improved quantifier is consistent with the principle
that traversable wormholes can be supported by arbitrarily small quantities of
exotic matter.


title: Electromagnetic fields in curved spacetimes

abstract:
  We consider the evolution of electromagnetic fields in curved spacetimes and
calculate the exact wave equations of the associated electric and magnetic
components. Our analysis is fully covariant, applies to a general spacetime and
isolates all the sources that affect the propagation of these waves. Among
others, we explicitly show how the different parts of the gravitational field
act as driving sources of electromagnetic disturbances. When applied to
perturbed FRW cosmologies, our results argue for a superadiabatic-type
amplification of large-scale cosmological magnetic fields in Friedmann models
with open spatial curvature.


title:
Inertial modes in stratified rotating neutron stars : An evolutionary
  description

abstract:
  With (non-barotropic) equations of state valid even when the neutron, proton
and electron content of neutron star cores is not in beta equilibrium, we study
inertial and composition gravity modes of relativistic rotating neutron stars.
We solve the relativistic Euler equations in the time domain with a three
dimensional numerical code based on spectral methods, in the slow rotation,
relativistic Cowling and anelastic approximations. Principally, after a short
description of the gravity modes due to smooth composition gradients, we focus
our analysis on the question of how the inertial modes are affected by
non-barotropicity of the nuclear matter. In our study, the deviation with
respect to barotropicity results from the frozen composition of non-superfluid
matter composed of neutrons, protons and electrons, when beta equilibrium is
broken by millisecond oscillations. We show that already for moderatly fast
rotating stars the increasing coupling between polar and axial modes makes
those two cases less different than for very slowly rotating stars. In
addition, as we directly solve the Euler equations, without coupling only a few
number of spherical harmonics, we always found, for the models that we use, a
discrete spectrum for the $l = m = 2$ inertial mode. Finally, we find that, for
non-barotropic stars, the frequency of this mode, which is our main focus,
decreases in a non-negligible way, whereas the time dependence of the energy
transfer between polar and axial modes is substantially different due to the
existence of low-frequencies gravity modes.


title: The Carmeli metric correctly describes spiral galaxy rotation curves

abstract:
  The metric by Carmeli accurately produces the Tully-Fisher type relation in
spiral galaxies, a relation showing the fourth power of the rotation speed
proportional to the mass of the galaxy. And therefore it is claimed that it is
also no longer necessary to invoke dark matter to explain the anomalous
dynamics in the arms of spiral galaxies. An analysis is presented here showing
Carmeli?s 5 dimensional space-time-velocity metric can also indeed describe the
rotation curves of spiral galaxies based on the properties of the metric alone.


title: Carmeli's accelerating universe is spatially flat without dark matter

abstract:
  Carmeli's 5D brane cosmology has been applied to the expanding accelerating
universe and it has been found that the distance redshift relation will fit the
data of the high-z supernova teams without the need for dark matter. Also the
vacuum energy contribution to gravity indicates that the universe is
asymptotically expanding towards a spatially flat state, where the total
mass/energy density tends to unity.


title:
A new relativity principle for bodies in different gravitational
  potentials

abstract:
  A general principle of non-equivalence for bodies and observers in different
G potentials (GP) was derived from correspondence of the Einstein's equivalence
principle either with optical physics or with gravitational experiments in
which bodies and observers are in different GP. According to it some relative
physical changes occur to any well defined part of an object after a change of
GP.. Such changes cannot be measured by observers travelling with the object
because his instruments change in identical proportions. The same principle was
derived from a new gravitational theory based on a particle model made up of
photons in stationary states. Such model accounts for the inertial and
gravitational properties of matter. This principle is not consistent with both,
the classical hypotheses on the relative invariability of the bodies after a
change of GP and with the G field energy hypothesis. The two kinds of errors
are of the same magnitude and opposite signs. Such errors are cancelled but
only when the two hypotheses are used. This accounts for the good predictions
of general relativity for the classical gravitational tests. The new properties
of the universe derived from the new principle, are radically different from
the classical ones. They are more clearly consistent with the astronomical
observations.


title:
Massless scalar fields and infrared divergences in the inflationary
  brane world

abstract:
  We study the quantum effects induced by bulk scalar fields in a model with a
de Sitter (dS) brane in a flat bulk (the Vilenkin-Ipser-Sikivie model) in more
than four dimensions. In ordinary dS space, it is well known that the stress
tensor in the dS invariant vacuum for an effectively massless scalar
($m_\eff^2=m^2+\xi {\cal R}=0$ with ${\cal R}$ the Ricci scalar) is infrared
divergent except for the minimally coupled case. The usual procedure to tame
this divergence is to replace the dS invariant vacuum by the Allen Follaci (AF)
vacuum. The resulting stress tensor breaks dS symmetry but is regular.
Similarly, in the brane world context, we find that the dS invariant vacuum
generates $\tmn$ divergent everywhere when the lowest lying mode becomes
massless except for massless minimal coupling case. A simple extension of the
AF vacuum to the present case avoids this global divergence, but $\tmn$ remains
to be divergent along a timelike axis in the bulk. In this case, singularities
also appear along the light cone emanating from the origin in the bulk,
although they are so mild that $\tmn$ stays finite except for non-minimal
coupling cases in four or six dimensions. We discuss implications of these
results for bulk inflaton models. We also study the evolution of the field
perturbations in dS brane world. We find that perturbations grow linearly with
time on the brane, as in the case of ordinary dS space. In the bulk, they are
asymptotically bounded.


title: Scalar--Field Amplitudes in Black--Hole Evaporation

abstract:
  We study the quantum-mechanical decay of a Schwarzschild-like black hole into
almost-flat space and weak radiation at a very late time, evaluating quantum
amplitudes (not just probabilities) for transitions from initial to final
states. No information is lost. The model contains gravity and a massless
scalar field. The quantum amplitude to go from given initial to final bosonic
data in a slightly complexified time-interval $T={\tau}{\exp}(-i{\theta})$ at
infinity is approximately $\exp(-I)$, where $I$ is the (complex) Euclidean
action of the classical solution filling in between the boundary data. And in a
locally supersymmetric (supergravity) theory, the amplitude const. exp(-I) is
exact. Dirichlet boundary data for gravity and the scalar field are posed on an
initial spacelike hypersurface extending to spatial infinity, just prior to
collapse, and on a corresponding final spacelike surface, sufficiently far to
the future of the initial surface to catch all the Hawking radiation. In an
averaged sense this radiation has an approximately spherically-symmetric
distribution. If the time-interval $T$ were exactly real, the resulting
`hyperbolic Dirichlet boundary-value problem' would not be well posed. If
instead (`Euclidean strategy'), one takes $T$ complex, as above
($0<\theta{\leq}{\pi}/2$), the field equations become strongly elliptic, with a
unique solution to the classical boundary-value problem. Expanding the bosonic
part of the action to quadratic order in perturbations about the classical
solution gives the quantum amplitude for weak-field final configurations, up to
normalization. Such amplitudes are calculated for weak final scalar fields.


title: Tidal gravitational effects in a satellite

abstract:
  Atomic wave interferometers are tied to a telescope pointing towards a
faraway star in a nearly free falling satellite. Such a device is sensitive to
the acceleration and the rotation relatively to the local inertial frame and to
the tidal gravitational effects too. We calculate the rotation of the telescope
due to the aberration and the deflection of the light in the gravitational
field of a central mass (the Earth and Jupiter). Within the framework of a
general parametrized description of the problem, we discuss the contributions
which must be taken into account in order to observe the Lense-Thirring effect.
Using a geometrical model, we consider some perturbations to the idealized
device and we calculate the corresponding effect on the periodic components of
the signal. Some improvements in the knowledge of the gravitational field are
still necessary as well as an increase of the experimental capabilities;
however our conclusions support a reasonable optimism for the future. Finally
we put forward the necessity of a more complete, realistic and powerful model
in order to obtain a definitive conclusion on the feasibility of the experiment
as far as the observation of the Lense-Thirring effect is involved.


title: On the Solutions of Einstein Equations with Massive Point Source

abstract:
  We show that Einstein equations are compatible with the presence of massive
point particles and find corresponding two parameter family of their solutions
which depends on the bare mechanical mass $M_0>0$ and the Keplerian mass
$M<M_0$ of the point source of gravity. The global analytical properties of
these solutions in the complex plane define a unique preferable radial variable
of the problem.


title: Brany Liouville Inflation

abstract:
  We present a specific model for cosmological inflation driven by the
Liouville field in a non-critical supersymmetric string framework, in which the
departure from criticality is due to open strings stretched between the two
moving Type-II 5-branes. We use WMAP and other data on fluctuations in the
cosmic microwave background to fix parameters of the model, such as the
relative separation and velocity of the 5-branes, respecting also the
constraints imposed by data on light propagation from distant gamma-ray
bursters. The model also suggests a small, relaxing component in the present
vacuum energy that may accommodate the breaking of supersymmetry.


title: Classical information transmission capacity of quantum black holes

abstract:
  The fate of classical information incident on a quantum black hole has been
the subject of an ongoing controversy in theoretical physics, because a
calculation within the framework of semi-classical curved-space quantum field
theory appears to show that the incident information is irretrievably lost, in
contradiction to time-honored principles such as time-reversibility and
unitarity. Here, we show within this framework embedded in quantum
communication theory that signaling from past to future infinity in the
presence of a Schwarzschild black hole can occur with arbitrary accuracy, and
thus that classical information is not lost in black hole dynamics. The
calculation relies on a treatment that is manifestly unitary from the outset,
where probability conservation is guaranteed because black holes stimulate the
emission of radiation in response to infalling matter. This stimulated
radiation is non-thermal, and contains all of the information about the
infalling matter, while Hawking radiation contains none of it.


title:
The dynamics of precessing binary black holes using the post-Newtonian
  approximation

abstract:
  We investigate the (conservative) dynamics of binary black holes using the
Hamiltonian formulation of the post-Newtonian (PN) equations of motion. The
Hamiltonian we use includes spin-orbit coupling, spin-spin coupling, and mass
monopole/spin-induced quadrupole interaction terms. In the case of both
quasi-circular and eccentric orbits, we search for the presence of chaos (using
the method of Lyapunov exponents) for a large variety of initial conditions.
For quasi-circular orbits, we find no chaotic behavior for black holes with
total mass 10 - 40 solar masses when initially at a separation corresponding to
a Newtonian gravitational-wave frequency less than 150 Hz. Only for rather
small initial radial distances, for which spin-spin induced oscillations in the
radial separation are rather important, do we find chaotic solutions, and even
then they are rare. Moreover, these chaotic quasi-circular orbits are of
questionable astrophysical significance, since they originate from direct
parametrization of the equations of motion rather than from widely separated
binaries evolving to small separations under gravitational radiation reaction.
In the case of highly eccentric orbits, which for ground-based interferometers
are not astrophysically favored, we again find chaotic solutions, but only at
pericenters so small that higher order PN corrections, especially higher spin
PN corrections, should also be taken into account.


title: Tidal Force in Metric-Affine Gravity

abstract:
  Generalization of an idea may lead to very interesting result. Learning how
torsion influences on tidal force reveals similarity between tidal equation for
geodesic and the Killing equation of second type.
  The relationship between tidal acceleration, curvature and torsion gives an
opportunity to measure torsion.


title: Spacetime topology from causality

abstract:
  We prove that a globally hyperbolic spacetime with its causality relation is
a bicontinuous poset whose interval topology is the manifold topology. This
provides an abstract mathematical setting in which one can study causality
independent of geometry and differentiable structure.


title: A domain of spacetime intervals in general relativity

abstract:
  Beginning from only a countable dense set of events and the causality
relation, it is possible to reconstruct a globally hyperbolic spacetime in a
purely order theoretic manner. The ultimate reason for this is that globally
hyperbolic spacetimes belong to a category that is equivalent to a special
category of domains called interval domains.


title: Lectures on Mathematical Cosmology

abstract:
  We present mathematical details of several cosmological models, whereby the
topological and the geometrical background will be emphasized.


title: A cosmology with variable c

abstract:
  A new varying-$c$ cosmological model constructed using two additional
assumptions, which was introduced in our previous work, is briefly reviewed and
the dynamic equation of the model is derived distinctly from a semi-Newtonian
approach. The results of this model, using a $\Lambda$ term and an extra
energy-momentum tensor, are considered separately. It is shown that the
Universe began from a hot Big Bang and expands forever with a constant
deceleration parameter regardless of its curvature. Finally, the age, the
radius, and the energy content of the Universe are estimated and some
discussion about the type of the geometry of the Universe is provided.


title: Disappearance of Black Hole Singularity in Quantum Gravity

abstract:
  We apply techniques recently introduced in quantum cosmology to the
Schwarzschild metric inside the horizon and near the black hole singularity at
r = 0. In particular, we use the quantization introduced by Husain and Winkler,
which is suggested by Loop Quantum Gravity and is based on an alternative to
the Schrodinger representation introduced by Halvorson. Using this quantization
procedure, we show that the black hole singularity disappears and spacetime can
be dynamically extended beyond the classical singularity.


title:
Special Relativity as a non commutative geometry: Lessons for Deformed
  Special Relativity

abstract:
  Deformed Special Relativity (DSR) is obtained by imposing a maximal energy to
Special Relativity and deforming the Lorentz symmetry (more exactly the
Poincar\'e symmetry) to accommodate this requirement. One can apply the same
procedure deforming the Galilean symmetry in order to impose a maximal speed
(the speed of light). This leads to a non-commutative space structure, to the
expected deformations of composition of speed and conservation of
energy-momentum. In doing so, one runs into most of the ambiguities that one
stumbles onto in the DSR context. However, this time, Special Relativity is
there to tell us what is the underlying physics, in such a way that we can
understand and interpret these ambiguities. We use these insights to comment on
the physics of DSR.


title:
The asymptotic conformal invariance in Chern-Simons theory with matter
  in curved space-time

abstract:
  We consider the asymptoic behaviour of the Chern - Simons (CS) theory with
matter in curved space - time. The asymptotics of effective couplings are
discussed.


title: Cosmological Spacetimes with $\Lambda > 0$

abstract:
  We discuss some global properties of cosmological spacetimes of de Sitter
type, based on results with Lars Andersson obtained in hep-th/0202161. We
relate the geometry and topology of conformal infinity to the occurrence of
singularities in such spacetimes, thereby extending to a much broader context
certain properties of some well-known cosmological models.


title:
Adiabatic regularization of the graviton stress-energy tensor in de
  Sitter space-time

abstract:
  We study the renormalized energy-momentum tensor of gravitons in a de Sitter
space-time. After canonically quantizing only the physical degrees of freedom,
we adopt the standard adiabatic subtraction used for massless minimally coupled
scalar fields as a regularization procedure and find that the energy density of
gravitons in the E(3) invariant vacuum is proportional to H^4, where H is the
Hubble parameter, but with a positive sign. According to this result the scalar
expansion rate, which is gauge invariant in de Sitter space-time, is increased
by the fluctuations. This implies that gravitons may then add to conformally
coupled matter in driving the Starobinsky model of inflation.


title: Synthetic LISA: Simulating Time Delay Interferometry in a Model LISA

abstract:
  We report on three numerical experiments on the implementation of Time-Delay
Interferometry (TDI) for LISA, performed with Synthetic LISA, a C++/Python
package that we developed to simulate the LISA science process at the level of
scientific and technical requirements. Specifically, we study the laser-noise
residuals left by first-generation TDI when the LISA armlengths have a
realistic time dependence; we characterize the armlength-measurements
accuracies that are needed to have effective laser-noise cancellation in both
first- and second-generation TDI; and we estimate the quantization and
telemetry bitdepth needed for the phase measurements. Synthetic LISA generates
synthetic time series of the LISA fundamental noises, as filtered through all
the TDI observables; it also provides a streamlined module to compute the TDI
responses to gravitational waves according to a full model of TDI, including
the motion of the LISA array and the temporal and directional dependence of the
armlengths. We discuss the theoretical model that underlies the simulation, its
implementation, and its use in future investigations on system characterization
and data-analysis prototyping for LISA.


title: Minisuperspace Approach of Generalized Gravitational Models

abstract:
  Motivated by the dark energy issue, the minisuperspace approach for general
relativistic cosmological theories is outlined.


title: The physical gravitational degrees of freedom

abstract:
  When constructing general relativity (GR), Einstein required 4D general
covariance. In contrast, we derive GR (in the compact, without boundary case)
as a theory of evolving 3-dimensional conformal Riemannian geometries obtained
by imposing two general principles: 1) time is derived from change; 2) motion
and size are relative. We write down an explicit action based on them. We
obtain not only GR in the CMC gauge, in its Hamiltonian 3 + 1 reformulation but
also all the equations used in York's conformal technique for solving the
initial-value problem. This shows that the independent gravitational degrees of
freedom obtained by York do not arise from a gauge fixing but from hitherto
unrecognized fundamental symmetry principles. They can therefore be identified
as the long-sought Hamiltonian physical gravitational degrees of freedom.


title: A Structure Theorem for Stationary Perfect Fluids

abstract:
  It is proven that, under mild physical assumptions, an isolated stationary
relativistic perfect fluid consists of a finite number of cells fibred by
invariant annuli or invariant tori. For axially symmetric circular flows it is
shown that the fluid consists of cells fibred by rigidly rotating annuli or
tori.


title: Exactly Soluble Quantum Wormhole in Two Dimensions

abstract:
  We are presenting a quantum traversable wormhole in an exactly soluble
two-dimensional model. This is different from previous works since the exotic
negative energy that supports the wormhole is generated from the quantization
of classical energy-momentum tensors. This explicit illustration shows the
quantum-mechanical energy can be used as a candidate for the exotic source. As
for the traversability, after a particle travels through the wormhole, the
static initial wormhole geometry gets a back reaction which spoils the wormhole
structure. However, it may still maintain the initial structure along with the
appropriate boundary condition.


title: Sudden future singularities in FLRW cosmologies

abstract:
  The standard energy conditions of classical general relativity are applied to
FLRW cosmologies containing sudden future singularities. Here we show, in a
model independent way, that although such cosmologies can satisfy the null,
weak and strong energy conditions, they always fail to satisfy the dominant
energy condition. They require a divergent spacelike energy flux in all but the
comoving frame.


title:
Coupling of radial and non-radial oscillations of relativistic stars:
  gauge-invariant formalism

abstract:
  Linear perturbation theory is appropriate to describe small oscillations of
stars, while a mild non-linearity is still tractable perturbatively but
requires to consider mode coupling. It is natural to start to look at this
problem by considering the coupling between linear radial and non-radial modes.
Radial pulsations of a spherical compact objects do not per se emit
gravitational waves but, if the coupling is efficient in driving and possibly
amplifying the non-radial modes, gravitational radiation could then be produced
to a significant level.
  In this paper we develop the relativistic formalism to study the coupling of
radial and non-radial first order perturbations of a compact spherical star.
  From a mathematical point of view, it is convenient to treat the two sets of
perturbations as separately parametrized, using a 2-parameter perturbative
expansion of the metric, the energy-momentum tensor and Einstein equations in
which $\lambda$ is associated with the radial modes, $\epsilon$ with the
non-radial perturbations, and the $\lambda\epsilon$ terms describe the
coupling. This approach provides a well-defined framework to consider the gauge
dependence of perturbations, allowing us to use $\epsilon$ order
gauge-invariant non-radial variables on the static background and to define new
second order $\lambda\epsilon$ gauge-invariant variables describing the
non-linear coupling. We present the evolution and constraint equations for our
variables outlining the setup for numerical computations, and briefly discuss
the surface boundary conditions in terms of the second order $\lambda\epsilon$
Lagrangian pressure perturbation.


title: Gravitational collapse and naked singularities

abstract:
  Gravitational collapse is one of the most striking phenomena in gravitational
physics. The cosmic censorship conjecture has provided strong motivation for
researches in this field. In the absence of general proof for the censorship,
many examples have been proposed, in which naked singularity is the outcome of
gravitational collapse. Recent development has revealed that there are examples
of naked singularity formation in the collapse of physically reasonable matter
fields, although the stability of these examples is still uncertain. We propose
the concept of ``effective naked singularities'', which will be quite helpful
because general relativity has the limitation of its application for
high-energy end. The appearance of naked singularities is not detestable but
can open a window for new physics of strongly curved spacetimes.


title: Numerical Relativity Using a Generalized Harmonic Decomposition

abstract:
  A new numerical scheme to solve the Einstein field equations based upon the
generalized harmonic decomposition of the Ricci tensor is introduced. The
source functions driving the wave equations that define generalized harmonic
coordinates are treated as independent functions, and encode the coordinate
freedom of solutions. Techniques are discussed to impose particular gauge
conditions through a specification of the source functions. A 3D, free
evolution, finite difference code implementing this system of equations with a
scalar field matter source is described. The second-order-in-space-and-time
partial differential equations are discretized directly without the use first
order auxiliary terms, limiting the number of independent functions to
fifteen--ten metric quantities, four source functions and the scalar field.
This also limits the number of constraint equations, which can only be enforced
to within truncation error in a numerical free evolution, to four. The
coordinate system is compactified to spatial infinity in order to impose
physically motivated, constraint-preserving outer boundary conditions. A
variant of the Cartoon method for efficiently simulating axisymmetric
spacetimes with a Cartesian code is described that does not use interpolation,
and is easier to incorporate into existing adaptive mesh refinement packages.
Preliminary test simulations of vacuum black hole evolution and black hole
formation via scalar field collapse are described, suggesting that this method
may be useful for studying many spacetimes of interest.


title: Quasinormal modes, quantized black holes, and correspondence principle

abstract:
  Contrary to the wide-spread belief, the correspondence principle does not
dictate any relation between the asymptotics of quasinormal modes and the
spectrum of quantized black holes. Moreover, this belief is in conflict with
simple physical arguments.