title: Gravitation as Field and Curvature

abstract:
  We argue that space-time properties are not absolute with respect to the used
frame of reference as is to be expected according to ideas of relativity of
space and time properties by Berkley - Leibnitz - Mach- Poincar\'{e}. From this
point of view gravitation may manifests itself both as a field in Minkowski
space-time and as space-time curvature. If the motion of test particles is
described by the Thirring Lagrangian, then in the inertial frames of reference,
where space-time is pseudo-Euclidean, gravitation manifests itself as a field.
In reference frames, whose reference body is formed by point masses moving
under the effect of the field, it appears as Riemannian curvature which in
these frames is other than zero. For realization of this idea the author
bimetric gravitation equations are considered. The spherically - symmetric
solution of the equations in Minkowski space-time does not lead to the physical
singularity in the center. The energy of the gravitational field of a point
mass is finite. It follows from the properties of the gravitational force that
there can exist stable compact supermassive configurations of Fermi-gas without
an events horizon.


title:
General-covariant constraint-free evolution system for Numerical
  Relativity

abstract:
  A general covariant extension of Einstein's field equations is considered
with a view to Numerical Relativity applications. The basic variables are taken
to be the metric tensor and an additional four-vector. The extended field
equations, when supplemented by suitable coordinate conditions, determine the
time evolution of all these variables without any constraint. Einstein's
solutions are recovered when the additional four-vector vanishes, so that the
energy and momentum constraints hold true. The extended system is well posed
when using the natural extension of either harmonic coordinates or the harmonic
slicing condition in normal coordinates


title: Massless DKP fields in Riemann-Cartan space-times

abstract:
  We study massless Duffin-Kemmer-Petiau (DKP) fields in the context of
Einstein-Cartan gravitation theory, interacting via minimal coupling procedure.
In the case of an identically vanishing torsion (Riemannian space-times) we
show that there exists local gauge symmetries which reproduce the usual gauge
symmetries for the massless scalar and electromagnetic fields. On the other
hand, similarly to what happens with the Maxwell theory, a non-vanishing
torsion, in general, breaks the usual U(1) local gauge symmetry of the
electromagnetic field or, in a different point of view, impose conditions on
the torsion.


title: Black hole singularities: a new critical phenomenon

abstract:
  The singularitiy inside a spherical charged black hole, coupled to a
spherical, massless scalar field is studied numerically. The profile of the
characteristic scalar field was taken to be a power of advanced time with an
exponent $\alpha>0$. A critical exponent $\alpha_{\rm crit}$ exists. For
exponents below the critical one ($\alpha<\alpha_{\rm crit}$) the singularity
is a union of spacelike and null sectors, as is also the case for data with
compact support. For exponents greater than the critical one
($\alpha>\alpha_{\rm crit}$) an all-encompassing, spacelike singularity
evolves, which completely blocks the ``tunnel'' inside the black hole,
preventing the use of the black hole as a portal for hyperspace travel.


title:
Matters of Gravity, the newsletter of the APS Topical Group on
  Gravitation, Fall 2002

abstract:
  Contents:
  * Community news:
  Einstein prize update, by Clifford Will
  World year of physics, by Richard H. Price
  We hear that... by Jorge Pullin
  * Research briefs:
  R-mode epitaph? by John Friedman and Nils Andersson
  Gravitational waves from bumpy neutron stars, by Ben Owen
  LIGO science operations begin!, by Gary Sanders
  * Conference reports:
  Fourth international LISA symposium, by Peter Bender
  Initial Data for Binary Systems, by Gregory Cook
  Report on Joint LSC/Source Modeling Meeting, by Patrick Brady
  Greek Relativity Conference, NEB-X by Kostas Kokkotas and Nick Stergioulas
  Gravity, Astrophysics, and Strings @ the Black Sea, by Plamen Fiziev
  Quantum field theory at ESI, by Robert Wald
  School on quantum gravity in Chile, by Don Marolf
  Apples with apples workshop, by Miguel Alcubierre
  Radiation reaction focus session and 5th Capra meeting, by Eanna Flanagan
  Numerical Relativity Workshop at IMA, by Manuel Tiglio


title: Holographic View of Cosmological Perturbations

abstract:
  The cosmological perturbation theory is revisited from the holographic point
of view. In the case of the single brane model, it turns out that the AdS/CFT
correspondence plays an important role. In the case of the two-brane model, it
is shown that the effective equations of motion becomes the quasi-scalar-tensor
gravity. It is also demonstrated that the radion anisotropy gives the CMB
fluctuations through the Sachs-Wolfe effect.


title: Braneworld Effective Action at Low Energies

abstract:
  The low energy effective theory for the Randall-Sundrum two-brane system is
investigated with an emphasis on the role of the non-linear radion in the brane
world. It is shown that the gravity on the brane world is described by a
quasi-scalar-tensor theory with a specific coupling function.


title: Gravity from Spacetime Thermodynamics

abstract:
  The Einstein-Hilbert action (and thus the dynamics of gravity) can be
obtained by combining the principle of equivalence, special relativity and
quantum theory in the Rindler frame and postulating that the horizon area must
be proportional to the entropy. This approach uses the local Rindler frame as a
natural extension of the local inertial frame, and leads to the interpretation
that the gravitational action represents the free energy of the spacetime
geometry. As an aside, one obtains an insight into the peculiar structure of
Einstein-Hilbert action and a natural explanation to the questions:(i) Why does
the covariant action for gravity contain second derivatives of the metric
tensor? (ii) Why is the gravitational coupling constant is positive ? Some
geometrical features of gravitational action are clarified.


title:
Third post-Newtonian dynamics of compact binaries: Equations of motion
  in the center-of-mass frame

abstract:
  The equations of motion of compact binary systems and their associated
Lagrangian formulation have been derived in previous works at the third
post-Newtonian (3PN) approximation of general relativity in harmonic
coordinates. In the present work we investigate the binary's relative dynamics
in the center-of-mass frame (center of mass located at the origin of the
coordinates). We obtain the 3PN-accurate expressions of the center-of-mass
positions and equations of the relative binary motion. We show that the
equations derive from a Lagrangian (neglecting the radiation reaction), from
which we deduce the conserved center-of-mass energy and angular momentum at the
3PN order. The harmonic-coordinates center-of-mass Lagrangian is equivalent,
{\it via} a contact transformation of the particles' variables, to the
center-of-mass Hamiltonian in ADM coordinates that is known from the
post-Newtonian ADM-Hamiltonian formalism. As an application we investigate the
dynamical stability of circular binary orbits at the 3PN order.


title:
Bound states and decay times of fermions in a Schwarzschild black hole
  background

abstract:
  We compute the spectrum of normalizable fermion bound states in a
Schwarzschild black hole background. The eigenstates have complex energies. The
real part of the energies, for small couplings, closely follow a hydrogen-like
spectrum. The imaginary parts give decay times for the various states, due to
the absorption properties of the hole, with states closer to the hole having
shorter half-lives. As the coupling increases, the spectrum departs from that
of the hydrogen atom, as states close to the horizon become unfavourable.
Beyond a certain coupling the 1S1/2 state is no longer the ground state, which
shifts to the 2P3/2 state, and then to states of successively greater angular
momentum. For each positive energy state a negative energy counterpart exists,
with opposite sign of its real energy, and the same decay factor. It follows
that the Dirac sea of negative energy states is decaying, which may provide a
physical contribution to Hawking radiation.


title: Foliated Cobordism and Motion

abstract:
  The mathematical notion of foliated cobordism is presented, and its
relationship to both the motion of extended particles and wave motion is
detailed. The fact that wave motion, when represented in such a manner on a
four-dimensional spacetime, leads to a reduction of the bundle of linear frames
to an SO(2)-principle bundle is demonstrated. Invariants of foliated cobordism
are discussed as they relate to the aforementioned cases of motion.


title:
Spacetime structure of self-similar spherically symmetric perfect fluid
  solutions

abstract:
  We classify all spherically symmetric and homothetic spacetimes that are
allowed kinematically by constructing them from a small number of building
blocks. We then restrict attention to a particular dynamics, namely perfect
fluid matter with the scale-free barotropic equation of state p = alpha mu
where 0<alpha<1 is a constant. We assign conformal diagrams to all solutions in
the complete classification of Carr and Coley, and so establish which of the
kinematic possibilities are realized for these dynamics. We pay particular
attention to those solutions which arise as critical solutions during
gravitational collapse.


title: Nano-constraints on the spatial anisotropy of the Gravitational Constant

abstract:
  We present constraints from various experimental data that limit any spatial
anisotropy of the Gravitational constant to less than a part per billion or
even smaller. This rules out with a wide margin the recently reported claim of
a spatial anisotropy of G with a diurnal temporal signature.


title: Einstein-Maxwell fields generated from the gamma-metric and their limits

abstract:
  Two solutions of the coupled Einstein-Maxwell field equations are found by
means of the Horsky-Mitskievitch generating conjecture. The vacuum limit of
those obtained classes of spacetimes is the seed gamma-metric and each of the
generated solutions is connected with one Killing vector of the seed spacetime.
Some of the limiting cases of our solutions are identified with already known
metrics, the relations among various limits are illustrated through a limiting
diagram. We also verify our calculation through the Ernst potentials. The
existence of circular geodesics is briefly discussed in the Appendix.


title: Five-dimensional Gravity and the Pioneer Effect

abstract:
  In induced gravity theory the solution of the dynamics equations for the test
particle on null path leads to additional force in four-dimensional space-time.
We find such force from five-dimensional geodesic line equations and try to
apply this approach to analysis of additional acceleration of Pioneer 10/11,
using properties of the asymmetrically warped space-time.


title:
Gravity, torsion, Dirac field and computer algebra using MAPLE and
  REDUCE

abstract:
  The article presents computer algebra procedures and routines applied to the
study of the Dirac field on curved spacetimes. The main part of the procedures
is devoted to the construction of Pauli and Dirac matrices algebra on an
anholonomic orthonormal reference frame. Then these procedures are used to
compute the Dirac equation on curved spacetimes in a sequence of special
dedicated routines. A comparative review of such procedures obtained for two
computer algebra platforms (REDUCE + EXCALC and MAPLE + GRTensorII) is carried
out. Applications for the calculus of Dirac equation on specific examples of
spacetimes with or without torsion are pointed out.


title: Intrinsic angular momentum in general relativity

abstract:
  There are several definitions of the notion of angular momentum in general
relativity. However non of them can be said to capture the physical notion of
intrinsic angular momentum of the sources in the presence of gravitational
radiation. We present a definition which is appropriate for the description of
intrinsic angular momentum in radiative spacetimes. This notion is required in
calculations involving radiation of angular momentum, as for example is
expected in binary coalescence of black holes.


title: The stability of abstract boundary essential singularities

abstract:
  The abstract boundary has, in recent years, proved a general and flexible way
to define the singularities of space-time. In this approach an essential
singularity is a non-regular boundary point of an embedding which is accessible
by a chosen family of curves within finite parameter distance. Ashley and Scott
proved the first theorem relating essential singularities in strongly causal
space-times to causal geodesic incompleteness. Linking this with the work of
Beem on the $C^{r}$-stability of geodesic incompleteness allows proof of the
stability of these singularities. Here I present this result stating the
conditions under which essential singularities are $C^{1}$-stable against
perturbations of the metric.


title:
Precession of a particle with anomalous magnetic moment in
  electromagnetic and gravitational pp-wave fields

abstract:
  We present an exact solution of the Bargmann-Michel-Telegdi (BMT) equations
for the dynamics of a spin particle in external electromagnetic and
gravitational pp-wave fields. We demonstrate that an anomalous magnetic moment
gives rise to an additional spin rotation which is modulated both by the
electromagnetic and by the gravitational wave periodicities.


title: Spinor Formulations for Gravitational Energy-Momentum

abstract:
  We first describe a class of spinor-curvature identities (SCI) which have
gravitational applications. Then we sketch the topic of gravitational
energy-momentum, its connection with Hamiltonian boundary terms and the issues
of positivity and (quasi)localization. Using certain SCIs several spinor
expressions for the Hamiltonian have been constructed. One SCI leads to the
celebrated Witten positive energy proof and the Dougan-Mason quasilocalization.
We found two other SCIs which give alternate positive energy proofs and
quasilocalizations. In each case the spinor field has a different role. These
neat expressions for gravitational energy-momentum have much appeal. However it
seems that such spinor formulations just have no room for angular momentum;
which leads us to doubt that spinor formulations can really correctly capture
the elusive gravitational energy-momentum.


title: LISA, binary stars, and the mass of the graviton

abstract:
  We extend and improve earlier estimates of the ability of the proposed LISA
(Laser Interferometer Space Antenna) gravitational wave detector to place upper
bounds on the graviton mass, m_g, by comparing the arrival times of
gravitational and electromagnetic signals from binary star systems. We show
that the best possible limit on m_g obtainable this way is ~ 50 times better
than the current limit set by Solar System measurements. Among currently known,
well-understood binaries, 4U1820-30 is the best for this purpose; LISA
observations of 4U1820-30 should yield a limit ~ 3-4 times better than the
present Solar System bound. AM CVn-type binaries offer the prospect of
improving the limit by a factor of 10, if such systems can be better understood
by the time of the LISA mission. We briefly discuss the likelihood that radio
and optical searches during the next decade will yield binaries that more
closely approach the best possible case.


title: Hydrodynamic Simulations in 3+1 General Relativity

abstract:
  We solve Einstein's field equations coupled to relativistic hydrodynamics in
full 3+1 general relativity to evolve astrophysical systems characterized by
strong gravitational fields. We model rotating, collapsing and binary stars by
idealized polytropic equations of state, with neutron stars as the main
application. Our scheme is based on the BSSN formulation of the field
equations. We assume adiabatic flow, but allow for the formation of shocks. We
determine the appearance of black holes by means of an apparent horizon finder.
We introduce several new techniques for integrating the coupled
Einstein-hydrodynamics system. For example, we choose our fluid variables so
that they can be evolved without employing an artificial atmosphere. We also
demonstrate the utility of working in a rotating coordinate system for some
problems. We use rotating stars to experiment with several gauge choices for
the lapse function and shift vector, and find some choices to be superior to
others. We demonstrate the ability of our code to follow a rotating star that
collapses from large radius to a black hole. Finally, we exploit rotating
coordinates to evolve a corotating binary neutron star system in a
quasi-equilibrium circular orbit for more than two orbital periods.


title: Casimir energy in multiply connected static hyperbolic Universes

abstract:
  We generalize a previously obtained result, for the case of a few other
static hyperbolic universes with manifolds of nontrivial topology as spatial
sections.


title:
All static spherically symmetric perfect fluid solutions of Einstein's
  Equations

abstract:
  An algorithm based on the choice of a single monotone function (subject to
boundary conditions) is presented which generates all regular static
spherically symmetric perfect fluid solutions of Einstein's equations. For
physically relevant solutions the generating functions must be restricted by
non-trivial integral-differential inequalities. Nonetheless, the algorithm is
demonstrated here by the construction of an infinite number of previously
unknown physically interesting exact solutions.


title: SU(2) Loop Quantum Gravity seen from Covariant Theory

abstract:
  Covariant loop gravity comes out of the canonical analysis of the Palatini
action and the use of the Dirac brackets arising from dealing with the second
class constraints (``simplicity'' constraints). Within this framework, we
underline a quantization ambiguity due to the existence of a family of possible
Lorentz connections. We show the existence of a Lorentz connection generalizing
the Ashtekar-Barbero connection and we loop-quantize the theory showing that it
leads to the usual SU(2) Loop Quantum Gravity and to the area spectrum given by
the SU(2) Casimir. This covariant point of view allows to analyze closely the
drawbacks of the SU(2) formalism: the quantization based on the (generalized)
Ashtekar-Barbero connection breaks time diffeomorphisms and physical outputs
depend non-trivially on the embedding of the canonical hypersurface into the
space-time manifold. On the other hand, there exists a true space-time
connection, transforming properly under all diffeomorphisms. We argue that it
is this connection that should be used in the definition of loop variables.
However, we are still not able to complete the quantization program for this
connection giving a full solution of the second class constraints at the
Hilbert space level. Nevertheless, we show how a canonical quantization of the
Dirac brackets at a finite number of points leads to the kinematical setting of
the Barrett-Crane model, with simple spin networks and an area spectrum given
by the SL(2,C) Casimir.


title: Diagonalizability of Constraint Propagation Matrices

abstract:
  In order to obtain stable and accurate general relativistic simulations,
re-formulations of the Einstein equations are necessary. In a series of our
works, we have proposed using eigenvalue analysis of constraint propagation
equations for evaluating violation behavior of constraints. In this article, we
classify asymptotical behaviors of constraint-violation into three types
(asymptotically constrained, asymptotically bounded, and diverge), and give
their necessary and sufficient conditions. We find that degeneracy of
eigenvalues sometimes leads constraint evolution to diverge (even if its
real-part is not positive), and conclude that it is quite useful to check the
diagonalizability of constraint propagation matrices. The discussion is general
and can be applied to any numerical treatments of constrained dynamics.


title: Radiative falloff in the background of rotating black hole

abstract:
  We study numerically the late-time tails of linearized fields with any spin
$s$ in the background of a spinning black hole. Our code is based on the
ingoing Kerr coordinates, which allow us to penetrate through the event
horizon. The late time tails are dominated by the mode with the least multipole
moment $\ell$ which is consistent with the equatorial symmetry of the initial
data and is equal to or greater than the least radiative mode with $s$ and the
azimuthal number $m$.


title: Impact of Low-Energy Constraints on Lorentz Violation

abstract:
  We extend previous analyses of the violation of Lorentz invariance induced in
a non-critical string model of quantum space-time foam, discussing the
propagation of low-energy particles through a distribution of non-relativistic
D-particles.We argue that nuclear and atomic physics experiments do not
constitute sensitive probes of this approach to quantum gravity due to a
difference in the dispersion relations for massive probes as compared to those
for massless ones, predicted by the model.


title: Test of general relativity: 1995-2002 measurement of frame-dragging

abstract:
  After an introduction on phenomena due to spin and mass-energy currents on
clocks and photons, we review the 1995-2001 measurements of the gravitomagnetic
field of Earth and Lense-Thirring effect obtained by analyzing the orbits of
the two laser-ranged satellites LAGEOS and LAGEOS II; this method has provided
a direct measurement of Earth's gravitomagnetism with accuracy of the order of
20 %. A future accurate measurement of the Lense-Thirring effect, at the level
of 1 % accuracy, may include the LARES experiment that will also provide other
basic tests of general relativity and gravitation. Finally, we report the
latest measurement of the Lense-Thirring effect, obtained in 2002 with the
LAGEOS satellites over nearly 8 years of data. This 2002 result fully confirms
and improves our previous measurements of the Earth frame-dragging: the
Lense-Thirring effect exists and its experimental value is within ~ 20 % of
what is predicted by Einstein's theory of general relativity.


title: The Extended Equivalence Principle and the Kramers-Kronig Relations

abstract:
  A seemingly obvious extension of the weak equivalence principle, in which all
matter must respond to Post-Newtonian gravitational fields, such as
Lense-Thirring and radiation fields, in a composition-independent way, is
considered in light of the Kramers-Kronig dispersion relations for the linear
response of any material medium to these fields. It is argued that known
observational facts lead to violations of this extended form of the equivalence
principle. (PACS numbers: 04.80.Cc, 04.80.Nn, 03.65.Ud, 67.40.Bz)


title:
Re-formulating the Einstein equations for stable numerical simulations:
  Formulation Problem in Numerical Relativity

abstract:
  We review recent efforts to re-formulate the Einstein equations for fully
relativistic numerical simulations. The so-called numerical relativity
(computational simulations in general relativity) is a promising research field
matching with ongoing astrophysical observations such as gravitational wave
astronomy. Many trials for longterm stable and accurate simulations of binary
compact objects have revealed that mathematically equivalent sets of evolution
equations show different numerical stability in free evolution schemes. In this
article, we first review the efforts of the community, categorizing them into
the following three directions: (1) modifications of the standard
Arnowitt-Deser-Misner equations initiated by the Kyoto group, (2) rewriting of
the evolution equations in hyperbolic form, and (3) construction of an
"asymptotically constrained" system. We next introduce our idea for explaining
these evolution behaviors in a unified way using eigenvalue analysis of the
constraint propagation equations. The modifications of (or adjustments to) the
evolution equations change the character of constraint propagation, and several
particular adjustments using constraints are expected to diminish the
constraint-violating modes. We propose several new adjusted evolution
equations, and include some numerical demonstrations. We conclude by discussing
some directions for future research.


title:
A Class of Collisions of Plane Impulsive Light--Like Signals in General
  Relativity

abstract:
  We present a systematic study of collisions of homogeneous, plane--fronted,
impulsive light--like signals which do not interact after head--on collision.
For the head--on collision of two such signals, six real parameters are
involved, three from each of the incoming signals. We find two necessary
conditions to be satisfied by these six parameters for the signals to be
non--interacting after collision. We then solve the collision problem in
general when these necessary conditions hold. After collision the two signals
focus each other at Weyl curvature singularities on each others signal front.
Our family of solutions contains some known collision solutions as special
cases.


title: Squeezed States in Black-Hole Evaporation by Analytic Continuation

abstract:
  We compute the semi-classical quantum amplitude to go from an initial
spherically symmetric bosonic matter and gravitational field configuration to a
final radiation configuration, corresponding to the relic Hawking radiation
from a non-rotating, chargeless black hole which evaporates completely. This is
obtained via the classical action integral which is solely a boundary term. On
discretising the classical action, the quantum amplitude can be expressed in
terms of generalised coherent states of the harmonic oscillator. A
squeezed-state representation is obtained by complexifying the proper time
separation T at spatial infinity between the initial and final space-like
hypersurfaces. Such a procedure is deemed necessary as the two-surface problem
for Dirichlet boundary data and wave-like perturbations is not well posed. We
find that infinitesimal rotation into the lower complex T plane is equivalent
to a highly-squeezed final state for the relic radiation, similar to the relic
gravitational-wave background in cosmology. This final state is a pure state,
and so the unpredictability associated with the final momentarily-naked
singularity is avoided. The cosmological analogy is the tunnelling from an
initial smooth Euclidean or timeless state to a classical universe. The
high-squeezing limit corresponds to a final state of the Hawking flux which is
indistinguishable from a stochastic collection of standing waves. The phases
conjugate to the field amplitudes are squeezed to discrete values. We also
discuss the entropy of the final radiation in the high-squeezing limit.


title:
Energy-Momentum and Angular Momentum Carried by Gravitational Waves in
  Extended New General Relativity

abstract:
  In an extended, new form of general relativity, which is a teleparallel
theory of gravity, we examine the energy-momentum and angular momentum carried
by gravitational wave radiated from Newtonian point masses in a weak-field
approximation. The resulting wave form is identical to the corresponding wave
form in general relativity, which is consistent with previous results in
teleparallel theory. The expression for the dynamical energy-momentum density
is identical to that for the canonical energy-momentum density in general
relativity up to leading order terms on the boundary of a large sphere
including the gravitational source, and the loss of dynamical energy-momentum,
which is the generator of \emph{internal} translations, is the same as that of
the canonical energy-momentum in general relativity. Under certain asymptotic
conditions for a non-dynamical Higgs-type field $\psi^{k}$, the loss of
``spin'' angular momentum, which is the generator of \emph{internal} $SL(2,C)$
transformations, is the same as that of angular momentum in general relativity,
and the losses of canonical energy-momentum and orbital angular momentum, which
constitute the generator of Poincar\'{e} \emph{coordinate} transformations, are
vanishing. The results indicate that our definitions of the dynamical
energy-momentum and angular momentum densities in this extended new general
relativity work well for gravitational wave radiations, and the extended new
general relativity accounts for the Hulse-Taylor measurement of the pulsar
PSR1913+16.


title: Toward stable 3D numerical evolutions of black-hole spacetimes

abstract:
  Three dimensional (3D) numerical evolutions of static black holes with
excision are presented. These evolutions extend to about 8000M, where M is the
mass of the black hole. This degree of stability is achieved by using
growth-rate estimates to guide the fine tuning of the parameters in a
multi-parameter family of symmetric hyperbolic representations of the Einstein
evolution equations. These evolutions were performed using a fixed gauge in
order to separate the intrinsic stability of the evolution equations from the
effects of stability-enhancing gauge choices.


title: Improving the "No-Hair" Theorem for the Proca Field

abstract:
  This paper reconsider the problem of a Proca field in the exterior of a
static black hole. The original Bekenstein's demonstration on the vanishing of
this field, based on an integral identity, is improved by using more natural
arguments at the event horizon. In particular, the use of the so-called
standard integration measure in the horizon is fully justified. Accordingly,
the horizon contribution to the Bekenstein integral identity is more involved
and its vanishing can be only established using the related Einstein equations.
With the new reasoning the ``no-hair'' theorem for the Proca field now rest on
better founded grounds.


title: Extended diffeomorphism algebras in (quantum) gravitational physics

abstract:
  We construct an explicit representation of the algebra of local
diffeomorphisms of a manifold with realistic dimensions. This is achieved in
the setting of a general approach to the (quantum) dynamics of a physical
system which is characterized by the fundamental role assigned to a basic
underlying symmetry. The developed mathematical formalism makes contact with
the relevant gravitational notions by means of the addition of some extra
structure. The specific manners in which this is accomplished, together with
their corresponding physical interpretation, lead to different gravitational
models. Distinct strategies are in fact briefly outlined, showing the
versatility of the present conceptual framework.


title: Circular holonomy in the Taub-NUT spacetime

abstract:
  Parallel transport around closed circular orbits in the equatorial plane of
the Taub-NUT spacetime is analyzed to reveal the effect of the gravitomagnetic
monopole parameter on circular holonomy transformations. Investigating the
boost/rotation decomposition of the connection 1-form matrix evaluated along
these orbits, one finds a situation that reflects the behavior of the general
orthogonally transitive stationary axisymmetric case and indeed along Killing
trajectories in general.


title: Time Dependence of Brans-Dicke Parameter w for an Expanding Universe

abstract:
  We have studied the time dependence of w for an expanding universe in the
generalised B-D theory and have obtained its explicit dependence on the nature
of matter contained in the universe,in different era.Lastly we discuss how the
observed accelerated expansion of the present universe can be accomodated in
the formalism.


title: The Theory Of Gravity

abstract:
  In the framework of the special theory of relativity, the relativistic theory
of gravitation (RTG) is constructed. The energy-momentum tensor density of all
the matter fields (including gravitational one) is treated as a source of the
gravitational field. The energy-momentum and the angular momentum conservation
laws are fulfilled in this theory. Such an approach permits us to unambiguously
construct the gravitional field theory as a gauge theory. According to the RTG,
the homogeneous and isotropic Universe is to be ``flat''. It evolves cyclewise
from some maximal density to the minimal one, etc.
  The book is designed for scientific workers, post-graduates and upper-year
students majoring in theoretical physics.


title: Doing numerical cosmology with the Cactus code

abstract:
  The article presents some aspects concerning the construction of a new thorn
for the Cactus code, a complete 3-dimensional machinery for numerical
relativity. This thorn is completely dedicated to numerical simulations in
cosmology, that means it can provide evolutions of different cosmological
models, mainly based on Friedman-Robertson-Walker metric. Some numerical
results are presented, testing the convergence, stability and the applicability
of the code.


title: Geodesics in spacetimes with expanding impulsive gravitational waves

abstract:
  We study geodesic motion in expanding spherical impulsive gravitational waves
propagating in a Minkowski background. Employing the continuous form of the
metric we find and examine a large family of geometrically preferred geodesics.
For the special class of axially symmetric spacetimes with the spherical
impulse generated by a snapping cosmic string we give a detailed physical
interpretation of the motion of test particles.


title: A learning approach to the detection of gravitational wave transients

abstract:
  We investigate the class of quadratic detectors (i.e., the statistic is a
bilinear function of the data) for the detection of poorly modeled
gravitational transients of short duration. We point out that all such
detection methods are equivalent to passing the signal through a filter bank
and linearly combine the output energy. Existing methods for the choice of the
filter bank and of the weight parameters rely essentially on the two following
ideas: (i) the use of the likelihood function based on a (possibly
non-informative) statistical model of the signal and the noise, (ii) the use of
Monte-Carlo simulations for the tuning of parametric filters to get the best
detection probability keeping fixed the false alarm rate. We propose a third
approach according to which the filter bank is "learned" from a set of training
data. By-products of this viewpoint are that, contrarily to previous methods,
(i) there is no requirement of an explicit description of the probability
density function of the data when the signal is present and (ii) the filters we
use are non-parametric. The learning procedure may be described as a two step
process: first, estimate the mean and covariance of the signal with the
training data; second, find the filters which maximize a contrast criterion
referred to as deflection between the "noise only" and "signal+noise"
hypothesis. The deflection is homogeneous to the signal-to-noise ratio and it
uses the quantities estimated at the first step. We apply this original method
to the problem of the detection of supernovae core collapses. We use the
catalog of waveforms provided recently by Dimmelmeier et al. to train our
algorithm. We expect such detector to have better performances on this
particular problem provided that the reference signals are reliable.


title: Fresh inflation from five-dimensional vacuum state

abstract:
  I study fresh inflation from a five-dimensional vacuum state, where the fifth
dimension is constant. In this framework, the universe can be seen as inflating
in a four-dimensional FRW metric embedding in a five-dimensional metric.
Finally, the experimental data $n_s=1$ are consistent with $(p+\rho_t)/\rho_t
\simeq1/3$ in the fresh inflationary scenario.


title:
Complex Structure of Kerr Geometry and Rotating `Photon Rocket'
  Solutions

abstract:
  In the frame of the Kerr-Schild approach, we obtain a generalization of the
Kerr solution to a nonstationary case corresponding to a rotating source moving
with arbitrary acceleration. Similar to the Kerr solution, the solutions
obtained have the geodesic and shear free principal null congruence. The
current parameters of the solutions are determined by a complex retarded-time
construction via a given complex worldline of source. The real part of the
complex worldline defines the values of the boost and acceleration while the
imaginary part controls the rotation. The acceleration of the source is
accompanied by a lightlike radiation along the principal null congruence. The
solutions obtained generalize to the rotating case the known Kinnersley class
of the "photon rocket" solutions.


title: Anti-deSitter gravitational collapse

abstract:
  We describe a formalism for studying spherically symmetric collapse of the
massless scalar field in any spacetime dimension, and for any value of the
cosmological constant $\Lambda$. The formalism is used for numerical
simulations of gravitational collapse in four spacetime dimensions with
negative $\Lambda$. We observe critical behaviour at the onset of black hole
formation, and find that the critical exponent is independent of $\Lambda$.


title: Effects of Differential Rotation on the Maximum Mass of Neutron Stars

abstract:
  The merger of binary neutron stars is likely to lead to differentially
rotating remnants. In this paper we numerically construct models of
differentially rotating neutron stars in general relativity and determine their
maximum allowed mass. We model the stars adopting a polytropic equation of
state and tabulate maximum allowed masses as a function of differential
rotation and stiffness of the equation of state. We also provide a crude
argument that yields a qualitative estimate of the effect of stiffness and
differential rotation on the maximum allowed mass.


title: Black Hole Fluctuations and Backreaction in Stochastic Gravity

abstract:
  We present a framework for analyzing black hole backreaction from the point
of view of quantum open systems using influence functional formalism. We focus
on the model of a black hole described by a radially perturbed quasi-static
metric and Hawking radiation by a conformally coupled massless quantum scalar
field. It is shown that the closed-time-path (CTP) effective action yields a
non-local dissipation term as well as a stochastic noise term in the equation
of motion, the Einstein-Langevin equation. Once the thermal Green's function in
a Schwarzschild background becomes available to the required accuracy the
strategy described here can be applied to obtain concrete results on
backreaction. We also present an alternative derivation of the CTP effective
action in terms of the Bogolyubov coefficients, thus making a connection with
the interpretation of the noise term as measuring the difference in particle
production in alternative histories.


title: Improving the Sensitivity of LISA

abstract:
  It has been shown in the past, that the six Doppler data streams obtained
LISA configuration can be combined by appropriately delaying the data streams
for cancelling the laser frequency noise. Raw laser noise is several orders of
magnitude above the other noises and thus it is essential to bring it down to
the level of shot, acceleration noises. A rigorous and systematic formalism
using the techniques of computational commutative algebra was developed which
generates all the data combinations cancelling the laser frequency noise. The
relevant data combinations form a first module of syzygies. In this paper we
use this formalism for optimisation of the LISA sensitivity by analysing the
noise and signal covariance matrices. The signal covariance matrix, averaged
over polarisations and directions, is calculated for binaries whose frequency
changes at most adiabatically. We then present the extremal SNR curves for all
the data combinations in the module. They correspond to the eigenvectors of the
noise and signal covariance matrices. We construct LISA `network' SNR by
combining the outputs of the eigenvectors which improves the LISA sensitivity
substantially. The maximum SNR curve can yield an improvement upto 70 % over
the Michelson, mainly at high frequencies, while the improvement using the
network SNR ranges from 40 % to over 100 %. Finally, we describe a simple toy
model, in which LISA rotates in a plane. In this analysis, we estimate the
improvement in the LISA sensitivity, if one switches from one data combination
to another as it rotates. Here the improvement in sensitivity, if one switches
optimally over three cyclic data combinations of the eigenvector is about 55 %
on an average over the LISA band-width. The corresponding SNR improvement is 60
%, if one maximises over the module.


title: On the Gravitomagnetic Time Delay

abstract:
  We study the gravitational time delay in ray propagation due to rotating
masses in the linear approximation of general relativity. Simple expressions
are given for the gravitomagnetic time delay that occurs when rays of radiation
cross a slowly rotating shell and propagate in the field of a distant rotating
source. Moreover, we calculate the local gravitational time delay in the Goedel
universe. The observational consequences of these results in the case of weak
gravitational lensing are discussed.


title: Limits on the Detectability of Cosmic Topology in Hyperbolic Universes

abstract:
  We reexamine the possibility of the detection of the cosmic topology in
nearly flat hyperbolic Friedmann-Lemaitre-Robertson-Walker (FLRW) universes by
using patterns repetition. We update and extend our recent results in two
important ways: by employing recent observational constraints on the
cosmological density parameters as well as the recent mathematical results
concerning small hyperbolic 3-manifolds. This produces new bounds with
consequences for the detectability of the cosmic topology. In addition to
obtaining new bounds, we also give a concrete example of the sensitive
dependence of detectability of cosmic topology on the uncertainties in the
observational values of the density parameters.


title:
Angular and Linear Momentum in General Relativity: Their Geometric
  Structure and Interrelation

abstract:
  Generalized definitions for angular and linear momentum are given and shown
to reduce to the ADM (at spatial infinity) definitions and the definitions at
null infinity in the appropriate limit. These definitions are used to express
angular momentum in terms of linear momentum. The formalism allows one to see
the connection with the classical and special relativitistic notions of
momenta. Further, the techniques elucidate, for the first time, the geometric
nature of these conserved quantities. The boosted Schwarzschild solution is
used to illustrate some aspects. The definitions are useful and give insight in
the region far from all masses where gravity waves are detected.


title:
Quasi-periodic accretion and gravitational waves from oscillating
  "toroidal neutron stars" around a Schwarzschild black hole

abstract:
  We present general relativistic hydrodynamics simulations of constant
specific angular momentum tori orbiting a Schwarzschild black hole. These tori
are expected to form as a result of stellar gravitational collapse, binary
neutron star merger or disruption, can reach very high rest-mass densities and
behave effectively as neutron stars but with a toroidal topology (i.e.
``toroidal neutron stars''). Our attention is here focussed on the dynamical
response of these objects to axisymmetric perturbations. We show that, upon the
introduction of perturbations, these systems either become unstable to the
runaway instability or exhibit a regular oscillatory behaviour resulting in a
quasi-periodic variation of the accretion rate as well as of the mass
quadrupole. The latter, in particular, is responsible for the emission of
intense gravitational radiation whose signal-to-noise ratio at the detector is
comparable or larger than the typical one expected in stellar-core collapse,
making these new sources of gravitational waves potentially detectable. We
discuss a systematic investigation of the parameter space both in the linear
and nonlinear regimes, providing estimates of how the gravitational radiation
emitted depends on the mass of the torus and on the strength of the
perturbation.


title: A Radiation Scalar for Numerical Relativity

abstract:
  This letter describes a scalar curvature invariant for general relativity
with a certain, distinctive feature. While many such invariants exist, this one
vanishes in regions of space-time which can be said unambiguously to contain no
gravitational radiation. In more general regions which incontrovertibly support
non-trivial radiation fields, it can be used to extract local,
coordinate-independent information partially characterizing that radiation.
While a clear, physical interpretation is possible only in such radiation
zones, a simple algorithm can be given to extend the definition smoothly to
generic regions of space-time.


title: Black Hole Thermodynamics in Carath\'eodory's Approach

abstract:
  We show that, in the framework of Carath\'eodory's approach to
thermodynamics, one can implement black hole thermodynamics by realizing that
there exixts a quasi-homogeneity symmetry of the Pfaffian form $\deq$
representing the infinitesimal heat exchanged reversibly by a Kerr-Newman black
hole; this allow us to calculate readily an integrating factor, and, as a
consequence, a foliation of the thermodynamic manifold can be recovered.


title: Quasi-Homogeneous Thermodynamics and Black Holes

abstract:
  We propose a generalized thermodynamics in which quasi-homogeneity of the
thermodynamic potentials plays a fundamental role. This thermodynamic formalism
arises from a generalization of the approach presented in paper [1], and it is
based on the requirement that quasi-homogeneity is a non-trivial symmetry for
the Pfaffian form $\delta Q_{rev}$. It is shown that quasi-homogeneous
thermodynamics fits the thermodynamic features of at least some
self-gravitating systems. We analyze how quasi-homogeneous thermodynamics is
suggested by black hole thermodynamics. Then, some existing results involving
self-gravitating systems are also shortly discussed in the light of this
thermodynamic framework. The consequences of the lack of extensivity are also
recalled. We show that generalized Gibbs-Duhem equations arise as a consequence
of quasi-homogeneity of the thermodynamic potentials. An heuristic link between
this generalized thermodynamic formalism and the thermodynamic limit is also
discussed.


title: On future asymptotics of polarized Gowdy T^3-models

abstract:
  Gowdy's model of cosmological spacetimes is a much investigated subject in
classical and quantum gravity. Depending on spatial topology recollapsing as
well as expanding models are known. Several analytic tools were used in order
to clarify singular behaviour in this class of spacetimes. Here we investigate
the structure of a certain subclass, the polarized Gowdy models with spatial
T^3-topology, in the large. The asymptotics for general solutions of the
dynamical equation for one of the gravitational degrees of freedom plays a key
role while the asymptotic behaviour of the remaining metric function is a
result of solving the Hamiltonian constraint equation. Using both we are able
to prove (future) geodesic completeness in all spacetimes of this type.


title: Quantization and spacetime topology

abstract:
  We consider classical and quantum dynamics of a free particle in de Sitter's
space-times with different topologies to see what happens to space-time
singularities of removable type in quantum theory. We find analytic solution of
the classical dynamics. The quantum dynamics is solved by finding an
essentially self-adjoint representation of the algebra of observables
integrable to the unitary representations of the symmetry group of each
considered gravitational system. The dynamics of a massless particle is
obtained in the zero-mass limit of the massive case. Our results indicate that
taking account of global properties of space-time enables quantization of
particle dynamics in all considered cases.


title:
Algebraic approach to quantum black holes: logarithmic corrections to
  black hole entropy

abstract:
  The algebraic approach to black hole quantization requires the horizon area
eigenvalues to be equally spaced. As shown previously, for a neutral
non-rotating black hole, such eigenvalues must be $2^{n}$-fold degenerate if
one constructs the black hole stationary states by means of a pair of creation
operators subject to a specific algebra. We show that the algebra of these two
building blocks exhibits $U(2)\equiv U(1)\times SU(2)$ symmetry, where the area
operator generates the U(1) symmetry. The three generators of the SU(2)
symmetry represent a {\it global} quantum number (hyperspin) of the black hole,
and we show that this hyperspin must be zero. As a result, the degeneracy of
the $n$-th area eigenvalue is reduced to $2^{n}/n^{3/2}$ for large $n$, and
therefore, the logarithmic correction term $-3/2\log A$ should be added to the
Bekenstein-Hawking entropy. We also provide a heuristic approach explaining
this result, and an evidence for the existence of {\it two} building blocks.


title:
On "time-periodic" black-hole solutions to certain spherically symmetric
  Einstein-matter systems

abstract:
  This paper explores ``black hole'' solutions of various Einstein-wave matter
systems admitting an isometry of their domain of outer communications taking
every point to its future. In the first two parts, it is shown that such
solutions, assuming in addition that they are spherically symmetric and the
matter has a certain structure, must be Schwarzschild or Reissner-Nordstrom.
Non-trivial examples of matter for which the result applies are a wave map and
a massive charged scalar field interacting with an electromagnetic field. The
results thus generalize work of Bekenstein [1] and Heusler [12] from the static
to the periodic case. In the third part, which is independent of the first two,
it is shown that Dirac fields preserved by an isometry of a spherically
symmetric domain of outer communications of the type described above must
vanish. It can be applied in particular to the Einstein-Dirac-Maxwell equations
or the Einstein-Dirac-Yang/Mills equations, generalizing work of Finster,
Smoller, and Yau [9], [7], [8], and also [6].


title: Black Holes and the Third Law of Thermodynamics

abstract:
  We discuss in the framework of black hole thermodynamics some aspects
relative to the third law in the case of black holes of the Kerr-Newman family.
In the light of the standard proof of the equivalence between the
unattainability of the zero temperature and the entropic version of the third
law it is remarked that the unattainability has a special character in black
hole thermodynamics. Also the zero temperature limit which obtained in the case
of very massive black holes is discussed and it is shown that a violation of
the entropic version in the charged case occurs. The violation of the
Bekenstein-Hawking law in favour of zero entropy S_E=0 in the case of extremal
black holes is suggested as a natural solution for a possible violation of the
second law of thermodynamics. Thermostatic arguments in support of the
unattainability are explored, and $S_E=0$ for extremal black holes is shown to
be again a viable solution. The third law of black hole dynamics by W.Israel is
then interpreted as a further strong corroboration to the picture of a
discontinuity between extremal states and non-extremal ones.


title: Criticality and convergence in Newtonian collapse

abstract:
  We study through numerical simulation the spherical collapse of isothermal
gas in Newtonian gravity. We observe a critical behavior which occurs at the
threshold of gravitational instability leading to core formation. For a given
initial density profile, we find a critical temperature, which is of the same
order as the virial temperature of the initial configuration. For the exact
critical temperature, the collapse converges to a self-similar form, the first
member in Hunter's family of self-similar solutions. For a temperature close to
the critical value, the collapse first approaches this critical solution. Later
on, in the supercritical case, the collapse converges to another self-similar
solution, which is called the Larson-Penston solution. In the subcritical case,
the gas bounces and disperses to infinity. We find two scaling laws: one for
the collapsed mass in the supercritical case and the other for the maximum
density reached before dispersal in the subcritical case. The value of the
critical exponent is measured to be $\simeq 0.11$ in the supercritical case,
which agrees well with the predicted value $\simeq 0.10567$. These critical
properties are quite similar to those observed in the collapse of a radiation
fluid in general relativity. We study the response of the system to temperature
fluctuation and discuss astrophysical implications for the insterstellar medium
structure and for the star formation process. Newtonian critical behavior is
important not only because it provides a simple model for general relativity
but also because it is relevant for astrophysical systems such as molecular
clouds.


title:
Effects of finite arm-length of LISA on analysis of gravitational waves
  from MBH binaries

abstract:
  Response of an interferometer becomes complicated for gravitational wave
shorter than the arm-length of the detector, as nature of wave appears
strongly. We have studied how parameter estimation for merging massive black
hole binaries are affected by this complicated effect in the case of LISA. It
is shown that three dimensional positions of some binaries might be determined
much better than the past estimations that use the long wave approximation. For
equal mass binaries this improvement is most prominent at $\sim 10^5\sol$.


title: The photothermal effect in interferometers

abstract:
  We have measured the photothermal effect in a single cross-polarized
interferometer at audio frequencies (5 Hz - 4 kHz). In a Fabry-Perot
interferometer, light in one polarization is chopped to periodically heat the
interferometer mirrors, while light in the orthogonal polarization measures the
mirror length changes. Tests of a polished solid metal mirror show good
agreement with relevant proposed theories by Braginsky et al. ["Thermodynamical
fluctuations and photo-thermal shot noise in gravitational wave antennae,"
Physics Letters A 264, 1-10 (1999)] and Cerdonio et al. ["Thermoelastic effects
at low temperatures and quantum limits in displacement measurements," Physical
Review D 63 082003 (2001)] describing uncoated optics.


title:
On the Possibility of Measuring the Gravitomagnetic Clock Effect in an
  Earth Space-Based Experiment

abstract:
  In this paper the effect of the post-Newtonian gravitomagnetic force on the
mean longitudes $l$ of a pair of counter-rotating Earth artificial satellites
following almost identical circular equatorial orbits is investigated. The
possibility of measuring it is examined. The observable is the difference of
the times required to $l$ in passing from 0 to 2$\pi$ for both senses of
motion. Such gravitomagnetic time shift, which is independent of the orbital
parameters of the satellites, amounts to 5$\times 10^{-7}$ s for Earth; it is
cumulative and should be measured after a sufficiently high number of
revolutions. The major limiting factors are the unavoidable imperfect
cancellation of the Keplerian periods, which yields a constraint of 10$^{-2}$
cm in knowing the difference between the semimajor axes $a$ of the satellites,
and the difference $I$ of the inclinations $i$ of the orbital planes which, for
$i\sim 0.01^\circ$, should be less than $0.006^\circ$. A pair of spacecrafts
endowed with a sophisticated intersatellite tracking apparatus and drag-free
control down to 10$^{-9}$ cm s$^{-2}$ Hz$^{-{1/2}}$ level might allow to meet
the stringent requirements posed by such a mission.


title: Late-time asymptotic dynamics of Bianchi VIII cosmologies

abstract:
  In this paper we give, for the first time, a complete description of the
late-time evolution of non-tilted spatially homogeneous cosmologies of Bianchi
type VIII. The source is assumed to be a perfect fluid with equation of state
$p = (\gamma - 1)\mu$, where $\gamma$ is a constant which satisfies $1 \leq
\gamma \leq 2$. Using the orthonormal frame formalism and Hubble-normalized
variables, we rigorously establish the limiting behaviour of the models at late
times, and give asymptotic expansions for the key physical variables. The main
result is that asymptotic self-similarity breaking occurs, and is accompanied
by the phenomenon of `Weyl curvature dominance', characterized by the
divergence of the Hubble-normalized Weyl curvature at late times.


title:
Detection methods for non-Gaussian gravitational wave stochastic
  backgrounds

abstract:
  We address the issue of finding an optimal detection method for a
discontinuous or intermittent gravitational wave stochastic background. Such a
signal might sound something like popcorn popping. We derive an appropriate
version of the maximum likelihood detection statistic, and compare its
performance to that of the standard cross-correlation statistic both
analytically and with Monte Carlo simulations. The maximum likelihood statistic
performs better than the cross-correlation statistic when the background is
sufficiently non-Gaussian. For both ground and space based detectors, this
results in a gain factor, ranging roughly from 1 to 3, in the minimum
gravitational-wave energy density necessary for detection, depending on the
duty cycle of the background. Our analysis is exploratory, as we assume that
the time structure of the events cannot be resolved, and we assume white,
Gaussian noise in two collocated, aligned detectors. Before this detection
method can be used in practice with real detector data, further work is
required to generalize our analysis to accommodate separated, misaligned
detectors with realistic, colored, non-Gaussian noise.


title:
Vortex in a weakly relativistic Bose gas at zero temperature and
  relativistic fluid approximation

abstract:
  The Bogoliubov procedure in quantum field theory is used to describe a
relativistic almost ideal Bose gas at zero temperature. Special attention is
given to the study of a vortex. The radius of the vortex in the field
description is compared to that obtained in the relativistic fluid
approximation. The Kelvin waves are studied and, for long wavelengths, the
dispersion relation is obtained by an asymptotic matching method and compared
with the non relativistic result.


title: Particle creation in a tunneling universe

abstract:
  An expanding closed universe filled with radiation can either recollapse or
tunnel to the regime of unbounded expansion, if the cosmological constant is
nonzero. We re-examine the question of particle creation during tunneling, with
the purpose of resolving a long-standing controversy. Using a perturbative
superspace model with a conformally coupled massless scalar field, which is
known to give no particle production, we explicitly show that the breakdown of
the semiclassical approximation and the ``catastrophic particle production''
claimed earlier in the literature are due to an inappropriate choice of the
initial quantum state prior to the tunneling.


title: Cauchy horizon stability in self-similar collapse: scalar radiation

abstract:
  The stability of the Cauchy horizon in spherically symmetric self-similar
collapse is studied by determining the flux of scalar radiation impinging on
the horizon. This flux is found to be finite.


title: Linking, Legendrian linking and causality

abstract:
  The set N of all null geodesics of a globally hyperbolic (d+1)-dimensional
spacetime (M,g) is naturally a smooth (2d-1)-dimensional contact manifold. The
sky of an event is the subset of N defined by all null geodesics through that
event, and is an embedded Legendrian submanifold of N diffeomorphic to a
(d-1)-dimensional sphere. It was conjectured by Low that for d=2 two events are
causally related iff their skies are linked (in an appropriate sense). We use
the contact structure and knot polynomial calculations to prove this conjecture
in certain particular cases, and suggest that for d=3 smooth linking should be
replaced with Legendrian linking.


title: Regional averaging and scaling in relativistic cosmology

abstract:
  Averaged inhomogeneous cosmologies lie at the forefront of interest, since
cosmological parameters like the rate of expansion or the mass density are to
be considered as volume-averaged quantities and only these can be compared with
observations. For this reason the relevant parameters are intrinsically
scale-dependent and one wishes to control this dependence without restricting
the cosmological model by unphysical assumptions. In the latter respect we
contrast our way to approach the averaging problem in relativistic cosmology
with shortcomings of averaged Newtonian models. Explicitly, we investigate the
scale-dependence of Eulerian volume averages of scalar functions on Riemannian
three-manifolds. We propose a complementary view of a Lagrangian smoothing of
(tensorial) variables as opposed to their Eulerian averaging on spatial
domains. This program is realized with the help of a global Ricci deformation
flow for the metric. We explain rigorously the origin of the Ricci flow which,
on heuristic grounds, has already been suggested as a possible candidate for
smoothing the initial data set for cosmological spacetimes. The smoothing of
geometry implies a renormalization of averaged spatial variables. We discuss
the results in terms of effective cosmological parameters that would be
assigned to the smoothed cosmological spacetime.


title: Maximal acceleration or maximal accelerations?

abstract:
  We review the arguments supporting the existence of a maximal acceleration
for a massive particle and show that different values of this upper limit can
be predicted in different physical situations.


title: Supergravity Vacua Today

abstract:
  We review the definition of (maximally supersymmetric) vacuum in supergravity
theories, the currently known vacua in arbitrary dimensions and how the
associated supersymmetry algebras can be found. (Invited talk at the Spanish
Relativity Meeting (``EREs'') 2002, Mao, Menorca, September 21-23 2002.)


title: Quintessence and black holes

abstract:
  We present new static spherically-symmetric exact solutions of Einstein
equations with the quintessential matter surrounding a black hole charged or
not as well as for the case without the black hole. A condition of additivity
and linearity in the energy-momentum tensor is introduced, which allows one to
get correct limits to the known solutions for the electromagnetic static field
implying the relativistic relation between the energy density and pressure, as
well as for the extraordinary case of cosmological constant, i.e. de Sitter
space. We classify the horizons, which evidently reveal themselves in the
static coordinates, and derive the Gibbons-Hawking temperatures. An example of
quintessence with the state parameter w=-2/3 is discussed in detail.


title:
Waves in Schwarzschild spacetimes: How strong can imprints of the
  spacetime curvature be

abstract: Misprints corrected, two references added. To appear in the Phys. Rev. D.


title: Dynamics of spinning test particles in Kerr spacetime

abstract:
  We investigate the dynamics of relativistic spinning test particles in the
spacetime of a rotating black hole using the Papapetrou equations. We use the
method of Lyapunov exponents to determine whether the orbits exhibit sensitive
dependence on initial conditions, a signature of chaos. In the case of
maximally spinning equal-mass binaries (a limiting case that violates the
test-particle approximation) we find unambiguous positive Lyapunov exponents
that come in pairs +/- lambda, a characteristic of Hamiltonian dynamical
systems. We find no evidence for nonvanishing Lyapunov exponents for physically
realistic spin parameters, which suggests that chaos may not manifest itself in
the gravitational radiation of extreme mass-ratio binary black-hole inspirals
(as detectable, for example, by LISA, the Laser Interferometer Space Antenna).


title: Time-frequency detection algorithm for gravitational wave bursts

abstract:
  An efficient algorithm is presented for the identification of short bursts of
gravitational radiation in the data from broad-band interferometric detectors.
The algorithm consists of three steps: pixels of the time-frequency
representation of the data that have power above a fixed threshold are first
identified. Clusters of such pixels that conform to a set of rules on their
size and their proximity to other clusters are formed, and a final threshold is
applied on the power integrated over all pixels in such clusters. Formal
arguments are given to support the conjecture that this algorithm is very
efficient for a wide class of signals. A precise model for the false alarm rate
of this algorithm is presented, and it is shown using a number of
representative numerical simulations to be accurate at the 1% level for most
values of the parameters, with maximal error around 10%.


title: Cosmic Strings Coupled With a Massless Scalar Field

abstract:
  A scalar field generalization of Xanthopoulos's cylindrically symmetric
solutions of the vacuum Einstein equation is obtained. The obtained solution
preserves the properties of the Xanthopoulos solution, which are regular on the
axis, asymptotically flat and free from the curvature singularities. The
solution describes stable, infinite length of rotating cosmic string
interacting with gravitational and scalar waves.


title: Cosmological parameters are dressed

abstract:
  In the context of the averaging problem in relativistic cosmology, we provide
a key to the interpretation of cosmological parameters by taking into account
the actual inhomogeneous geometry of the Universe. We discuss the relation
between `bare' cosmological parameters determining the cosmological model, and
the parameters interpreted by observers with a ``Friedmannian bias'', which are
`dressed' by the smoothed-out geometrical inhomogeneities of the surveyed
spatial region.


title: Failure of a Stability Conjecture in General Relativity

abstract:
  By employing an exact back-reaction geometry, Helliwell-Konkowski stability
conjecture is shown to fail. This happens when a test null dust is inserted to
the interaction region of cross-polarized Bell-Szekeres spacetime.


title: Chaos and Brane-worlds

abstract:
  The early time behaviour of brane-world models is analysed in the presence of
anisotropic stresses. It is shown that that the initial singularity cannot be
isotropic, unless there is also an isotropic fluid stiffer than radiation
present. Also, a magnetic Bianchi type I brane-world is analysed in detail. It
is known that the Einstein equations for the magnetic Bianchi type I models are
in general oscillatory and are believed to be chaotic, but in the brane-world
model this chaotic behaviour does not seem to be possible.


title: Constructing Time Machines

abstract:
  The existence of time machines, understood as spacetime constructions
exhibiting physically realised closed timelike curves (CTCs), would raise
fundamental problems with causality and challenge our current understanding of
classical and quantum theories of gravity. In this paper, we investigate three
proposals for time machines which share some common features: cosmic strings in
relative motion, where the conical spacetime appears to allow CTCs; colliding
gravitational shock waves, which in Aichelburg-Sexl coordinates imply
discontinuous geodesics; and the superluminal propagation of light in
gravitational radiation metrics in a modified electrodynamics featuring
violations of the strong equivalence principle. While we show that ultimately
none of these constructions creates a working time machine, their study
illustrates the subtle levels at which causal self-consistency imposes itself,
and we consider what intuition can be drawn from these examples for future
theories.


title: Tests of relativity using a microwave resonator

abstract:
  The frequencies of a cryogenic sapphire oscillator and a hydrogen maser are
compared to set new constraints on a possible violation of Lorentz invariance.
We determine the variation of the oscillator frequency as a function of its
orientation (Michelson-Morley test) and of its velocity (Kennedy-Thorndike
test) with respect to a preferred frame candidate. We constrain the
corresponding parameters of the Mansouri and Sexl test theory to $\delta -
\beta + 1/2 = (1.5\pm 4.2) \times 10^{-9}$ and $\beta - \alpha - 1 = (-3.1\pm
6.9) \times 10^{-7}$ which is equivalent to the best previous result for the
former and represents a 30 fold improvement for the latter.


title: Hyperbolic slicings of spacetime: singularity avoidance and gauge shocks

abstract:
  I study the Bona-Masso family of hyperbolic slicing conditions, considering
in particular its properties when approaching two different types of
singularities: focusing singularities and gauge shocks. For focusing
singularities, I extend the original analysis of Bona et. al and show that both
marginal and strong singularity avoidance can be obtained for certain types of
behavior of the slicing condition as the lapse approaches zero. For the case of
gauge shocks, I re-derive a condition found previously that eliminates them.
Unfortunately, such a condition limits considerably the type of slicings
allowed. However, useful slicing conditions can still be found if one asks for
this condition to be satisfied only approximately. Such less restrictive
conditions include a particular member of the 1+log family, which in the past
has been found empirically to be extremely robust for both Brill wave and black
hole simulations.


title: Spin Foam Models of Yang-Mills Theory Coupled to Gravity

abstract:
  We construct a spin foam model of Yang-Mills theory coupled to gravity by
using a discretized path integral of the BF theory with polynomial interactions
and the Barret-Crane ansatz. In the Euclidian gravity case we obtain a vertex
amplitude which is determined by a vertex operator acting on a simple spin
network function. The Euclidian gravity results can be straightforwardly
extended to the Lorentzian case, so that we propose a Lorentzian spin foam
model of Yang-Mills theory coupled to gravity.


title: Comparing Criteria for Circular Orbits in General Relativity

abstract:
  We study a simple analytic solution to Einstein's field equations describing
a thin spherical shell consisting of collisionless particles in circular orbit.
We then apply two independent criteria for the identification of circular
orbits, which have recently been used in the numerical construction of binary
black hole solutions, and find that both yield equivalent results. Our
calculation illustrates these two criteria in a particularly transparent
framework and provides further evidence that the deviations found in those
numerical binary black hole solutions are not caused by the different criteria
for circular orbits.


title:
Study of the coincidences between the gravitational wave detectors
  EXPLORER and NAUTILUS in 2001

abstract:
  We report the result from a search for bursts of gravitational waves using
data collected by the cryogenic resonant detectors EXPLORER and NAUTILUS during
the year 2001, for a total measuring time of 90 days. With these data we
repeated the coincidence search performed on the 1998 data (which showed a
small coincidence excess) applying data analysis algorithms based on known
physical characteristics of the detectors. With the 2001 data a new interesting
coincidence excess is found when the detectors are favorably oriented with
respect to the Galactic Disk.


title: A Quantum Cosmological Model With Static and Dynamic Wormholes

abstract:
  Quantization is performed of a Friedmann-Robertson-Walker universe filled
with a conformally invariant scalar field and a perfect fluid with equation of
state $p=\alpha \rho$. A well-known discrete set of static quantum wormholes is
shown to exist for radiation ($\alpha =1/3$), and a novel continuous set is
found for cosmic strings ($\alpha = -1/3$), the latter states having throat
radii of any size. In both cases wave-packet solutions to the Wheeler-DeWitt
equation are obtained with all the properties of evolving quantum wormholes. In
the case of a radiation fluid, a detailed analysis of the quantum dynamics is
made in the context of the Bohm-de Broglie interpretation. It is shown that a
repulsive quantum force inversely proportional to the cube of the scale factor
prevents singularities in the quantum domain. For the states considered, there
are no particle horizons either.


title: Near Horizon Geometry of Extreme Black Holes and Colliding Waves

abstract:
  The correspondence between black holes and colliding waves extends to cover
the near horizon geometry of rotating black holes and colliding waves with
cross polarization. Extreme Kerr and Kerr-Newman geometries are given as
examples.


title: A Discrete Time Presentation of Quantum Dynamics

abstract:
  Inspired by the discrete evolution implied by the recent work on loop quantum
cosmology, we obtain a discrete time description of usual quantum mechanics
viewing it as a constrained system. This description, obtained without any
approximation or explicit discretization, mimics features of the discrete time
evolution of loop quantum cosmology. We discuss the continuum limit, physical
inner product and matrix elements of physical observables to bring out various
issues regarding viability of a discrete evolution. We also point out how a
continuous time could emerge without appealing to any continuum limit.


title: A New Look to Massive Neutron Cores

abstract:
  We reconsider the problem of modelling static spherically symmetric perfect
fluid configurations with an equation of state from a point of view of that
requires the use of the concept of principal transform of a 3-dimensional
Riemannian metric. We discuss from this new point of view the meaning of those
familiar quantities that we call density, pressure and geometry in a
relativistic context. This is not simple semantics. To prove it we apply the
new ideas to recalculate the maximum mass that a massive neutron core can have.
This limit is found to be of the order of 3.8 $M_\odot$ substantially larger
than the Oppenheimer and Volkoff limit.


title: The mass of the graviton and the cosmological constant

abstract:
  We show that the graviton acquires a mass in a de Sitter background given by
$m_{g}^{2}=-{2/3}\Lambda.$ This is precisely the fine-tuning value required for
the perturbed gravitational field to mantain its two degrees of freedom.


title: String theory, cosmology and varying constants

abstract:
  In string theory the coupling ``constants'' appearing in the low-energy
effective Lagrangian are determined by the vacuum expectation values of some (a
priori) massless scalar fields (dilaton, moduli). This naturally leads one to
expect a correlated variation of all the coupling constants, and an associated
violation of the equivalence principle. We review some string-inspired
theoretical models which incorporate such a spacetime variation of coupling
constants while remaining naturally compatible both with phenomenological
constraints coming from geochemical data (Oklo; Rhenium decay) and with present
equivalence principle tests. Barring a very unnatural fine-tuning of
parameters, a variation of the fine-structure constant as large as that
recently ``observed'' by Webb et al. in quasar absorption spectra appears to be
incompatible with these phenomenological constraints. Independently of any
model, it is emphasized that the best experimental probe of varying constants
are high-precision tests of the universality of free fall, such as MICROSCOPE
and STEP.


title: Clifford Algebra, Geometry and Physics

abstract:
  The geometric calculus based on Clifford algebra is a very useful tool for
geometry and physics. It describes a geometric structure which is much richer
than the ordinary geometry of spacetime. A Clifford manifold (C-space) consists
not only of points, but also of 1-loops, 2-loops, etc.. They are associated
with multivectors which are the wedge product of the basis vectors, the
generators of Clifford algebra. Within C-space we can perform the so called
polydimensional rotations which reshuffle the multivectors, e.g., a bivector
into a vector, etc.. A consequence of such a polydimensional rotation is that
the signature can change: it is relative to a chosen set of basis vectors.
Another important consequence is that the well known unconstrained Stueckelberg
theory is embedded within the constrained theory based on C-space. The essence
of the Stueckelberg theory is the existence of an evolution parameter which is
invariant under the Lorentz transformations. The latter parameter is
interpreted as being the true time - associated with our perception of the
passage of time.


title: "Observables" in causal set cosmology

abstract:
  The ``generic'' family of classical sequential growth dynamics for causal
sets provides cosmological models of causal sets which are a testing ground for
ideas about the, as yet unknown, quantum theory. In particular we can
investigate how general covariance manifests itself and address the problem of
identifying and interpreting covariant ``observables'' in quantum gravity. The
problem becomes, in this setting, that of identifying measurable covariant
collections of causal sets, to each of which corresponds the question: ``Does
the causal set that occurs belong to this collection?'' It has for answer the
probability measure of the collection. Answerable covariant questions, then,
correspond to measurable collections of causal sets which are independent of
the labelings of the causal sets. However, what the transition probabilities of
the classical sequential growth dynamics provide directly is a measure on the
space of {\it labeled} causal sets and the physical interpretation of the
covariant measurable collections is consequently obscured. We show that there
is a physically meaningful characterisation of the class of measurable
covariant sets as unions and differences of ``stem sets''.


title:
Kovalevski exponents and integrability properties in class A homogeneous
  cosmological models

abstract:
  Qualitative approach to homogeneous anisotropic Bianchi class A models in
terms of dynamical systems reveals a hierarchy of invariant manifolds. By
calculating the Kovalevski Exponents according to Adler - van Moerbecke method
we discuss how algebraic integrability property is distributed in this class of
models. In particular we find that algebraic nonintegrability of vacuum Bianchi
VII_0 model is inherited by more general Bianchi VIII and Bianchi IX vacuum
types. Matter terms (cosmological constant, dust and radiation) in the Einstein
equations typically generate irrational or complex Kovalevski exponents in
class A homogeneous models thus introducing an element of nonintegrability even
though the respective vacuum models are integrable.


title: Doubly-Special Relativity: First Results and Key Open Problems

abstract:
  I examine the results obtained so far in exploring the recent proposal of
theories of the relativistic transformations between inertial observers that
involve both an observer-independent velocity scale and an observer-independent
length/momentum scale. I also discuss what appear to be the key open issues for
this research line.


title: Implementing causality in the spin foam quantum geometry

abstract:
  We analyse the classical and quantum geometry of the Barrett-Crane spin foam
model for four dimensional quantum gravity, explaining why it has to be
considering as a covariant realization of the projector operator onto physical
quantum gravity states. We discuss how causality requirements can be
consistently implemented in this framework, and construct causal transiton
amplitudes between quantum gravity states, i.e. realising in the spin foam
context the Feynman propagator between states. The resulting causal spin foam
model can be seen as a path integral quantization of Lorentzian first order
Regge calculus, and represents a link between several approaches to quantum
gravity as canonical loop quantum gravity, sum-over-histories formulations,
dynamical triangulations and causal sets. In particular, we show how the
resulting model can be rephrased within the framework of quantum causal sets
(or histories).


title:
Testing General Relativity with Satellite Laser Ranging: Recent
  Developments

abstract:
  In this paper the most recent developments in testing General Relativity in
the gravitational field of the Earth with the technique of Satellite Laser
Ranging are presented. In particular, we concentrate our attention on some
gravitoelectric and gravitomagnetic post--Newtonian orbital effects on the
motion of a test body in the external field of a central mass.


title: The speed of light need not be constant

abstract:
  Recent observations of the fine structure of spectral lines in the early
universe have been interpreted as a variation of the fine structure constant.
  From the assumed validity of Maxwell equations in general relativity and well
known experimental facts, it is proved that $e$ and $\hbar$ are absolute
constants. On the other hand, the speed of light need not be constant.


title:
4-D homogeneous isotropic cosmological models generated by the 5-D
  vacuum

abstract:
  4-dimensional homogeneous isotropic cosmological models obtained from
solutions of vacuum 5-dimensional Einstein equations are considered. It is
assumed, that the G(55)-component of the 5-d metric simulates matter in the
comoving frame of reference. Observable 4-d metric is defined up to conformal
transformations of the metric of 4-d section \tilde{g}, with a conformal factor
as a function of the component G(55). It is demonstrated, that the form of this
function determines the matter equation of state. Possible equations of state
are analyzed separately for flat, open and close models.


title:
Study of a formalism modeling massive particles at the speed of light on
  a Machian basis

abstract:
  In this paper we develop a formalism which models all massive particles as
travelling at the speed of light(c). This is done by completing the 3-velocity
v of a test particle to the speed of light by adding an auxiliary 3-velocity
component z for the particle. According to the observations and laws of physics
defined in our spacetime these vectors are generalized to domains and then two
methods are developed to define c domain in terms of our spacetime(v domain).
By using these methods the formalism is applied on relativistic quantum theory
and general theory of relativity. From these, the relation between the
formalism and Mach's principle is investigated. The ideas and formulae
developed from application of the formalism on general relativity are compared
with the characteristics of anomalous accelerations detected on Pioneer 10/11,
Ulysses and Galileo spacecrafts and an explanation according to the formalism
is suggested. Possible relationships between Mach's principle and the
nondeterministic nature of the universe are also explored. In this study Mach's
principle, on which current debate still continues, is expressed from an
unconventional point of view, as a naturally arising consequence of the
formalism, and the approaches are simplified accordingly.


title:
On the singularities of gravity in the presence of non-minimally coupled
  scalar fields

abstract:
  We investigate the robustness of some recent results obtained for homogeneous
and isotropic cosmological models with conformally coupled scalar fields. For
this purpose, we investigate anisotropic homogeneous solutions of the models
described by the action $$ S=\int d^4x \sqrt{-g}\left\{F(\phi)R -
\partial_a\phi\partial^a\phi -2V(\phi) \right\}, $$ with general $F(\phi)$ and
$V(\phi)$. We show that such a class of models leads generically to geometrical
singularities if for some value of $\phi$, $F(\phi)=0$, rendering previous
cosmological results obtained for the conformal coupling case highly unstable.
We show that stable models can be obtained for suitable choices of $F(\phi)$
and $V(\phi)$. Implications for other recent results are also discussed.


title:
Quantized massive scalar fields in the spacetime of a charged dilatonic
  black hole

abstract:
  Employing the approximate effective action constructed from the coincidence
limit of the Hadamard-Minakshisundaram-DeWitt (HaMiDeW) coefficient a_{3}, the
renormalized stress-energy tensor of the quantized massive scalar field in the
spacetime of a static and electrically charged dilatonic black hole is
calculated. Special attention is paid to the minimally and conformally coupled
fields propagating in geometries with a=1, and to the power expansion of the
general stress-energy tensor for small values of charge. A compact expression
for the trace of the stress-energy tensor is presented.


title: New first-order formulation for the Einstein equations

abstract:
  We derive a new first-order formulation for Einstein's equations which
involves fewer unknowns than other first-order formulations that have been
proposed. The new formulation is based on the 3+1 decomposition with arbitrary
lapse and shift. In the reduction to first order form only 8 particular
combinations of the 18 first derivatives of the spatial metric are introduced.
In the case of linearization about Minkowski space, the new formulation
consists of symmetric hyperbolic system in 14 unknowns, namely the components
of the extrinsic curvature perturbation and the 8 new variables, from whose
solution the metric perturbation can be computed by integration.


title:
The theory of Multiverse, multiplicity of physical objects and physical
  constants

abstract:
  The Multiverse is collection of parallel universes. In this article a formal
theory and a topos-theoretic models of the multiverse are given. For this the
Lawvere-Kock Synthetic Differential Geometry and topos models for smooth
infinitesimal analysis are used. Physical properties of multi-variant and
many-dimensional parallel universes are discussed. The source of multiplicity
of physical objects is set of physical constants.


title: Bianchi type I universes with dilaton and magnetic fields

abstract:
  We consider the dynamics of a Bianchi type I spacetime in the presence of
dilaton and magnetic fields. The general solution of the Einstein-Maxwell
dilaton field equations can be obtained in an exact parametric form. Depending
on the numerical values of the parameters of the model there are three distinct
classes of solutions. The time evolution of the mean anisotropy, shear and
deceleration parameter is considered in detail and it is shown that a
magnetic-dilaton anisotropic Bianchi type I geometry does not isotropize, the
initial anisotropy being present in the universe for all times.


title: Rotating Yang-Mills dyons in anti-de Sitter spacetime

abstract:
  We construct new axially symmetric solutions of SU(2) Yang-Mills theory in a
four dimensional anti-de Sitter spacetime. Possessing nonvanishing nonabelian
charges, these regular configurations have also a nonzero angular momentum.
Numerical arguments are also presented for the existence of rotating solutions
of Einstein-Yang-Mills equations in an asymptotically anti-de Sitter spacetime.


title: What `shape' is space-time?

abstract:
  Some examples from the mathematics of shape are presented that question some
of the almost hidden assumptions behind results on limiting behaviour of
finitary approximations to space-time. These are presented so as to focus
attention on the observational problem of refinement and suggest the necessity
for an alternative theory of `fractafolds' against which the limiting theory of
$C^\infty$-differential manifolds usually underlying (quantum) general
relativity can be measured.


title: Retardation of Gravity in Binary Pulsars

abstract:
  We study the effect of retardation of gravity in binary pulsars. It appears
in pulsar timing formula as a periodic excess time delay to the Shapiro effect.
The retardation of gravity effect can be large enough for observation in binary
pulsars with the nearly edgewise orbits and relatively large ratio of the
projected semimajor axis to the orbital period of the pulsar. If one succeeds
in measuring the retardation of gravity it will give further experimental
evidence in favor of General Relativity.


title: On the Vacuum Energy in Expanding Space-Times

abstract:
  If there is a shortest length in nature, for example at the Planck scale of
10^-35m, then the cosmic expansion should continually create new comoving
modes. A priori, each of the new modes comes with its own vacuum energy, which
could contribute to the cosmological constant. I discuss possible mathematical
models for a shortest length and an explicit model for a corresponding
mode-generating mechanism.


title: Perturbations of Spatially Closed Bianchi III Spacetimes

abstract:
  Motivated by the recent interest in dynamical properties of topologically
nontrivial spacetimes, we study linear perturbations of spatially closed
Bianchi III vacuum spacetimes, whose spatial topology is the direct product of
a higher genus surface and the circle. We first develop necessary mode
functions, vectors, and tensors, and then perform separations of (perturbation)
variables. The perturbation equations decouple in a way that is similar to but
a generalization of those of the Regge--Wheeler spherically symmetric case. We
further achieve a decoupling of each set of perturbation equations into
gauge-dependent and independent parts, by which we obtain wave equations for
the gauge-invariant variables. We then discuss choices of gauge and stability
properties. Details of the compactification of Bianchi III manifolds and
spacetimes are presented in an appendix. In the other appendices we study
scalar field and electromagnetic equations on the same background to compare
asymptotic properties.


title: Dark energy, dark matter and the Chaplygin gas

abstract:
  The possibility that the dark energy may be described by the Chaplygin gas is
discussed. Some observational constraints are established. These observational
constraints indicate that a unified model for dark energy and dark matter
through the employement of the Chaplygin gas is favored.


title:
Vacuum Plane Waves in 4+1 D and Exact solutions to Einstein's Equations
  in 3+1 D

abstract:
  In this paper we derive homogeneous vacuum plane-wave solutions to Einstein's
field equations in 4+1 dimensions. The solutions come in five different types
of which three generalise the vacuum plane-wave solutions in 3+1 dimensions to
the 4+1 dimensional case. By doing a Kaluza-Klein reduction we obtain solutions
to the Einstein-Maxwell equations in 3+1 dimensions. The solutions generalise
the vacuum plane-wave spacetimes of Bianchi class B to the non-vacuum case and
describe spatially homogeneous spacetimes containing an extremely tilted fluid.
Also, using a similar reduction we obtain 3+1 dimensional solutions to the
Einstein equations with a scalar field.


title: Horizons in 1+1 Dimensional Dilaton Gravity Coupled to Matter

abstract:
  We study the local behaviour of static solutions of a general 1+1 dimensional
dilaton gravity theory coupled to scalar fields and Abelian gauge fields near
horizons. This type of model includes in particular reductions of higher
dimensional theories invariant under a sufficiently large isometry group. The
solution near the horizon can in general be obtained solving a system of
integral equations or in favourable cases in the form of a convergent series in
the dilaton field.


title:
The Weyl tensor and equilibrium configurations of self-gravitating
  fluids

abstract:
  It is shown that (except for two well defined cases), the necessary and
sufficient condition for any spherically symmetric distribution of fluid to
leave the state of equilibrium (or quasi-equilibrium), is that the Weyl tensor
changes with respect to its value in the state of equilibrium (or
quasi-equilibrium).


title:
Adaptive spectral identification techniques in presence of undetected
  non linearities

abstract:
  The standard procedure for detection of gravitational wave coalescing
binaries signals is based on Wiener filtering with an appropriate bank of
template filters. This is the optimal procedure in the hypothesis of addictive
Gaussian and stationary noise. We study the possibility of improving the
detection efficiency with a class of adaptive spectral identification
techniques, analyzing their effect in presence of non stationarities and
undetected non linearities in the noise


title: Towards a theory of thin self-gravitating crossing shells

abstract:
  In the case of crossing thin dust shells the momentum conservation law is
found. For two crossing isotropic shells it coincides with the 't Hooft-Dray
formula. The system of one isotropic and one time-like shell is considered. In
this case we found a very simple formula which relate velocities of dust shell
before and after crossing.


title:
A Dirac Sea for a General Non-Inertial Observer In Flat 1+1 Dimensional
  Spacetime

abstract:
  A coordinate system is set up for a general accelerating observer and is used
to determine the particle content of the Dirac vacuum for that observer.
Equations are obtained for the spatial distribution and total number of
particles for massless fermions as seen by this observer, generalising previous
work.


title: Planck-scale models of the Universe

abstract:
  Suppose the usual description of spacetime as a 4-dimensional manifold with a
Lorentzian metric breaks down at Planck energies. Can we still construct
sensible theoretical models of the universe? Are they testable? Do they lead to
a consistent quantum cosmology? Is this cosmology different than the standard
one? The answer is yes, to all these questions, assuming that quantum theory is
still valid at this scale. I describe the basic features of such models, mainly
quantum causal histories and spin foams. They are given by a partition
function, similar to spin systems and lattice gauge theories. This suggests
that we should treat this approach to quantum gravity as a problem in
statistical physics, but with significant complications: there is no background
and, in particular, no external time. Gravity and the familiar 3+1 manifold
spacetime are to be derived as the low-energy continuum approximation to these
models.


title: Brans-Dicke cosmology with a scalar field potential

abstract:
  Three solutions of the Brans-Dicke theory with a self-interacting quartic
potential and perfect fluid distribution are presented for a spatially flat
geometry. The physical behavior is consistent with the recent cosmological
scenario favored by type Ia supernova observations, indicating an accelerated
expansion of the Universe.


title:
On the existence of global solutions for $T^{3}$-Gowdy spacetimes with
  stringy matter

abstract:
  We show a global existence theorem for Einstein-matter equations of
$T^{3}$-Gowdy symmetric spacetimes with stringy matter. The areal time
coordinate is used. It is shown that this spacetime has a crushing singularity
into the past. From these results we can show that the spacetime is foliated by
compact hypersurfaces of constant mean curvature.


title: Cosmological bulk viscosity, the Burnett regime, and the BGK equation

abstract:
  Einstein's field equations in FRW space-times are coupled to the BGK equation
in order to derive the stress energy tensor including dissipative effects up to
second order in the thermodynamical forces. The space-time is assumed to be
matter-dominated, but in a low density regime for which a second order
(Burnett) coefficient becomes relevant. Cosmological implications of the
solutions, as well as the physical meaning of transport coefficients in an
isotropic homogeneous universe are discussed.


title: Superinflation and quintessence in the presence of an extra field

abstract:
  We discuss the implication of the introduction of an extra field to the
dynamics of a scalar field conformally coupled to gravitation in a homogeneous
isotropic spatially flat universe. We show that for some reasonable parameter
values the dynamical effects are similar to those of our previous model with a
single scalar field. Nevertheless for other parameter values new dynamical
effects are obtained.


title: Storage rings as detectors for relic gravitational-wave background ?

abstract:
  We argue that storage rings can be used for the detection of low-frequency
gravitational-wave background. We explain the systematic shrinkage of the
machine circumference of the storage ring of the SPring-8, observed by Takao
and Shimada (Proceedings of EPAC 2000, Vienna, 2000, p.1572), by the influence
of the relic gravitational-wave background. We show that the systematic
shrinkage of the machine circumference can be explained by a relic
gravitational-wave background even if it is treated as a stochastic system
incoming on the plane of the machine circumference from all quarters of the
Universe. We show that the rate of the systematic shrinkage of the machine
circumference does not depend on the radius of the storage ring and it should
be universal for storage rings with any radius.

categories: gr-qc astro-ph hep-ex hep-ph hep-th physics.acc-ph


title: On solutions of the vacuum Einstein equation in the radiation regime

abstract:
  We review recent results by the author, in collaboration with Erwann Delay,
Olivier Lengard, and Rafe Mazzeo, on existence and properties of space-times
with controlled asymptotic behavior at null infinity.


title:
Resonance widening in spherical GW detectors: model descriptions of the
  dissipation processes

abstract:
  Internal friction effects are responsible for line widening of the resonance
frequencies in mechanical oscillators and result in damped oscillations of its
eigenmodes with a decay time Q/\omega. We study the solutions to the equations
of motion for the case of spherical oscillators, to be used as next generation
of acoustic gravitational wave detectors, based on various different
assumptions about the material's constituent equations. Quality factor
dependence on mode frequency is determined in each case, and a discussion of
its applicability to actual gravitational wave detectors is made on the basis
of available experimental evidence.