title:
Testing the Cactus code on exact solutions of the Einstein field
  equations

abstract:
  The article presents a series of numerical simulations of exact solutions of
the Einstein equations performed using the Cactus code, a complete
3-dimensional machinery for numerical relativity. We describe an application
(``thorn'') for the Cactus code that can be used for evolving a variety of
exact solutions, with and without matter, including solutions used in modern
cosmology for modeling the early stages of the universe. Our main purpose has
been to test the Cactus code on these well-known examples, focusing mainly on
the stability and convergence of the code.


title: Aichelburg-Sexl boost of an isolated source in general relativity

abstract:
  A study of the Aichelburg--Sexl boost of the Schwarzschild field is described
in which the emphasis is placed on the field (curvature tensor) with the metric
playing a secondary role. This is motivated by a description of the Coulomb
field of a charged particle viewed by an observer whose speed relative to the
charge approaches the speed of light. Our approach is exemplified by carrying
out an Aichelburg-- Sexl type boost on the Weyl vacuum gravitational field due
to an isolated axially symmetric source. Detailed calculations of the boosts
transverse and parallel to the symmetry axis are given and the results, which
differ significantly, are discussed.


title: Finitary Cech-de Rham Cohomology: much ado without smoothness

abstract:
  The present paper is a continuation of our work on curved finitary spacetime
sheaves of incidence algebras and treats the latter along Cech cohomological
lines. In particular, we entertain the possibility of constructing a
non-trivial de Rham complex on these finite dimensional algebra sheaves along
the lines of the first author's axiomatic approach to differential geometry via
the theory of vector and algebra sheaves. The upshot of this study is that
important `classical' differential geometric constructions and results usually
thought of as being intimately associated with smooth manifolds carry through,
virtually unaltered, to the finitary-algebraic regime with the help of some
quite universal, because abstract, ideas taken mainly from sheaf-cohomology as
developed in the first author's Abstract Differential Geometry theory. At the
end of the paper, and due to the fact that the incidence algebras involved have
been previously interpreted as quantum causal sets, we discuss how these ideas
may be used in certain aspects of current research on discrete Lorentzian
quantum gravity.


title: Introduction to Modern Canonical Quantum General Relativity

abstract:
  This is an introduction to the by now fifteen years old research field of
canonical quantum general relativity, sometimes called "loop quantum gravity".
The term "modern" in the title refers to the fact that the quantum theory is
based on formulating classical general relativity as a theory of connections
rather than metrics as compared to in original version due to Arnowitt, Deser
and Misner. Canonical quantum general relativity is an attempt to define a
mathematically rigorous, non-perturbative, background independent theory of
Lorentzian quantum gravity in four spacetime dimensions in the continuum. The
approach is minimal in that one simply analyzes the logical consequences of
combining the principles of general relativity with the principles of quantum
mechanics. The requirement to preserve background independence has lead to new,
fascinating mathematical structures which one does not see in perturbative
approaches, e.g. a fundamental discreteness of spacetime seems to be a
prediction of the theory providing a first substantial evidence for a theory in
which the gravitational field acts as a natural UV cut-off. An effort has been
made to provide a self-contained exposition of a restricted amount of material
at the appropriate level of rigour which at the same time is accessible to
graduate students with only basic knowledge of general relativity and quantum
field theory on Minkowski space.

categories: gr-qc hep-th math-ph math.FA math.MP math.OA math.SG quant-ph


title: Partial observables

abstract:
  We discuss the distinction between the notion of partial observable and the
notion of complete observable. Mixing up the two is frequently a source of
confusion. The distinction bears on several issues related to observability,
such as (i) whether time is an observable in quantum mechanics, (ii) what are
the observables in general relativity, (iii) whether physical observables
should or should not commute with the Wheeler-DeWitt operator in quantum
gravity. We argue that the extended configuration space has a direct physical
interpretation, as the space of the partial observables. This space plays a
central role in the structure of classical and quantum mechanics and the
clarification of its physical meaning sheds light on this structure,
particularly in context of general covariant physics.


title:
Towards the observation of Hawking radiation in Bose--Einstein
  condensates

abstract:
  Acoustic analogues of black holes (dumb holes) are generated when a
supersonic fluid flow entrains sound waves and forms a trapped region from
which sound cannot escape. The surface of no return, the acoustic horizon, is
qualitatively very similar to the event horizon of a general relativity black
hole. In particular Hawking radiation (a thermal bath of phonons with
temperature proportional to the ``surface gravity'') is expected to occur. In
this note we consider quasi-one-dimensional supersonic flow of a Bose--Einstein
condensate (BEC) in a Laval nozzle (converging-diverging nozzle), with a view
to finding which experimental settings could magnify this effect and provide an
observable signal. We identify an experimentally plausible configuration with a
Hawking temperature of order 70 n K; to be contrasted with a condensation
temperature of the order of 90 n K.


title: Can Martinez's conjecture be extended to string theory?

abstract:
  The universality of Martinez's conjecture, which states that the quasilocal
energy of a black hole at the outer horizon reduces to twice its irreducible
mass, or equivalently, to $\sqrt{A/4\pi}$ ($A$ is the area of the black hole),
is investigated by calculating Brown-York quasilocal energies for stationary
black holes in heterotic string theory, e. g., for the stationary Kaluza-Klein
black hole, the rotating Cveti$\check{c}$-Youm black hole, the stationary
axisymmetric Einstein-Maxwell-dilaton-axion black hole, and the Kerr-Sen black
hole. It is shown that Martinez's conjecture can be extended from general
relativity to heterotic string theory since the quasilocal energies of these
stationary black holes tend to their Arnowitt-Dener-Misner masses at spatial
infinity, and reduce to $\sqrt{A/4\pi}$ at the event horizons.


title:
Relativistic r-modes in Slowly Rotating Neutron Stars: Numerical
  Analysis in the Cowling Approximation

abstract:
  We investigate the properties of relativistic $r$-modes of slowly rotating
neutron stars by using a relativistic version of the Cowling approximation. In
our formalism, we take into account the influence of the Coriolis like force on
the stellar oscillations, but ignore the effects of the centrifugal like force.
For three neutron star models, we calculated the fundamental $r$-modes with
$l'=m=2$ and 3. We found that the oscillation frequency $\bar\sigma$ of the
fundamental $r$-mode is in a good approximation given by $\bar\sigma\approx
\kappa_0 \Omega$, where $\bar\sigma$ is defined in the corotating frame at the
spatial infinity, and $\Omega$ is the angular frequency of rotation of the
star. The proportional coefficient $\kappa_0$ is only weakly dependent on
$\Omega$, but it strongly depends on the relativistic parameter $GM/c^2R$,
where $M$ and $R$ are the mass and the radius of the star. All the fundamental
$r$-modes with $l'=m$ computed in this study are discrete modes with distinct
regular eigenfunctions, and they all fall in the continuous part of the
frequency spectrum associated with Kojima's equation (Kojima 1998). These
relativistic $r$-modes are obtained by including the effects of rotation higher
than the first order of $\Omega$ so that the buoyant force plays a role, the
situation of which is quite similar to that for the Newtonian $r$-modes.


title:
Addendum: Hawking Radiation of Photons in a Variable-mass Kerr Black
  Hole

abstract:
  Hawking evaporation of photons in a variable-mass Kerr space-time is
investigated by using a method of the generalized tortoise coordinate
transformation. The blackbody radiant spectrum of photons displays a new
spin-rotation coupling effect obviously dependent on different helicity states
of photons.


title: GR16: Quantum General Relativity

abstract:
  This is the report of the "Quantum General Relativity" session, at the 16th
International Conference on General Relativity & Gravitation, held on July 15th
to 21st 2001, in Durban, South Africa. The report will appear on the
Proceedings of the conference. Comments and criticisms are welcome: they will
be taken into account for revising the text before the publication.


title:
Computational cost for detecting inspiraling binaries using a network of
  laser interferometric detectors

abstract:
  We extend a coherent network data-analysis strategy developed earlier for
detecting Newtonian waveforms to the case of post-Newtonian (PN) waveforms.
Since the PN waveform depends on the individual masses of the inspiraling
binary, the parameter-space dimension increases by one from that of the
Newtonian case. We obtain the number of templates and estimate the
computational costs for PN waveforms: For a lower mass limit of a solar mass,
for LIGO-I noise, and with 3% maximum mismatch, the online computational speed
requirement for single detector is a few Gflops; for a two-detector network it
is hundreds of Gflops and for a three-detector network it is tens of Tflops.
Apart from idealistic networks, we obtain results for realistic networks
comprising of LIGO and VIRGO. Finally, we compare costs incurred in a
coincidence detection strategy with those incurred in the coherent strategy
detailed above.


title:
Ricci and Matter inheritance collineations of Robertson-Walker
  space-times

abstract:
  It is well known that every Killing vector is a Ricci and Matter
collineation. Therefore the metric, the Ricci tensor and the energy-momentum
tensor are all members of a large family of second order symmetric tensors
which are invariant under a common group of symmetries. This family is
described by a generic metric which is defined from the symmetry group of the
space-time metric. The proper Ricci and Matter (inheritance) collineations are
the (conformal) Killing vectors of the generic metric which are not (conformal)
Killing vectors of the space-time metric. Using this observation we compute the
Ricci and Matter inheritance collineations of the Robertson-Walker space-times
and we determine the Ricci and Matter collineations without any further
calculations. It is shown that these higher order symmetries can be used as
supplementary conditions to produce an equation of state which is compatible
with the Geometry and the Physics of the Robertson-Walker space-times.


title: Cosmological anomalies and exotic smoothness structures

abstract:
  It seems to be generally accepted that apparently anomalous cosmological
observations, such as accelerating expansion, etc., necessarily are
inconsistent with standard general relativity and standard matter sources.
Following the suggestions of S{\l}adkowski, we point out that in addition to
exotic theories and exotic matter there is another possibility. We refer to
exotic differential structures on ${\mathbb R}^4$ which could be the source of
the observed anomalies without changing the Einstein equations or introducing
strange forms of matter.


title: Positivity of Spin Foam Amplitudes

abstract:
  The amplitude for a spin foam in the Barrett-Crane model of Riemannian
quantum gravity is given as a product over its vertices, edges and faces, with
one factor of the Riemannian 10j symbols appearing for each vertex, and simpler
factors for the edges and faces. We prove that these amplitudes are always
nonnegative for closed spin foams. As a corollary, all open spin foams going
between a fixed pair of spin networks have real amplitudes of the same sign.
This means one can use the Metropolis algorithm to compute expectation values
of observables in the Riemannian Barrett-Crane model, as in statistical
mechanics, even though this theory is based on a real-time (e^{iS}) rather than
imaginary-time (e^{-S}) path integral. Our proof uses the fact that when the
Riemannian 10j symbols are nonzero, their sign is positive or negative
depending on whether the sum of the ten spins is an integer or half-integer.
For the product of 10j symbols appearing in the amplitude for a closed spin
foam, these signs cancel. We conclude with some numerical evidence suggesting
that the Lorentzian 10j symbols are always nonnegative, which would imply
similar results for the Lorentzian Barrett-Crane model.


title: An efficient algorithm for the Riemannian 10j symbols

abstract:
  The 10j symbol is a spin network that appears in the partition function for
the Barrett-Crane model of Riemannian quantum gravity. Elementary methods of
calculating the 10j symbol require order(j^9) or more operations and order(j^2)
or more space, where j is the average spin. We present an algorithm that
computes the 10j symbol using order(j^5) operations and order(j^2) space, and a
variant that uses order(j^6) operations and a constant amount of space. An
implementation has been made available on the web.


title:
Combining general relativity and quantum theory: points of conflict and
  contact

abstract:
  The issues related to bringing together the principles of general relativity
and quantum theory are discussed. After briefly summarising the points of
conflict between the two formalisms I focus on four specific themes in which
some contact has been established in the past between GR and quantum field
theory: (i) The role of planck length in the microstructure of spacetime (ii)
The role of quantum effects in cosmology and origin of the universe (iii) The
thermodynamics of spacetimes with horizons and especially the concept of
entropy related to spacetime geometry (iv) The problem of the cosmological
constant.


title:
Three-dimensional general relativistic hydrodynamics II: long-term
  dynamics of single relativistic stars

abstract:
  This is the second in a series of papers on the construction and validation
of a three-dimensional code for the solution of the coupled system of the
Einstein equations and of the general relativistic hydrodynamic equations, and
on the application of this code to problems in general relativistic
astrophysics. In particular, we report on the accuracy of our code in the
long-term dynamical evolution of relativistic stars and on some new physics
results obtained in the process of code testing. The tests involve single
non-rotating stars in stable equilibrium, non-rotating stars undergoing radial
and quadrupolar oscillations, non-rotating stars on the unstable branch of the
equilibrium configurations migrating to the stable branch, non-rotating stars
undergoing gravitational collapse to a black hole, and rapidly rotating stars
in stable equilibrium and undergoing quasi-radial oscillations. The numerical
evolutions have been carried out in full general relativity using different
types of polytropic equations of state using either the rest-mass density only,
or the rest-mass density and the internal energy as independent variables. New
variants of the spacetime evolution and new high resolution shock capturing
(HRSC) treatments based on Riemann solvers and slope limiters have been
implemented and the results compared with those obtained from previous methods.
Finally, we have obtained the first eigenfrequencies of rotating stars in full
general relativity and rapid rotation. A long standing problem, such
frequencies have not been obtained by other methods. Overall, and to the best
of our knowledge, the results presented in this paper represent the most
accurate long-term three-dimensional evolutions of relativistic stars available
to date.


title: Radiation from Violently Accelerated Bodies

abstract:
  A determination is made of the radiation emitted by a linearly uniformly
accelerated uncharged dipole transmitter. It is found that, first of all, the
radiation rate is given by the familiar Larmor formula, but it is augmented by
an amount which becomes dominant for sufficiently high acceleration. For an
accelerated dipole oscillator, the criterion is that the center of mass motion
become relativistic within one oscillation period. The augmented formula and
the measurements which it summarizes presuppose an expanding inertial
observation frame. A static inertial reference frame will not do. Secondly, it
is found that the radiation measured in the expanding inertial frame is
received with 100% fidelity. There is no blueshift or redshift due to the
accelerative motion of the transmitter. Finally, it is found that a pair of
coherently radiating oscillators accelerating (into opposite directions) in
their respective causally disjoint Rindler-coordinatized sectors produces an
interference pattern in the expanding inertial frame. Like the pattern of a
Young double slit interferometer, this Rindler interferometer pattern has a
fringe spacing which is inversely proportional to the proper separation and the
proper frequency of the accelerated sources. The interferometer, as well as the
augmented Larmor formula, provide a unifying perspective. It joins adjacent
Rindler-coordinatized neighborhoods into a single spacetime arena for
scattering and radiation from accelerated bodies.


title:
Critical gravitational collapse with angular momentum: from critical
  exponents to universal scaling functions

abstract:
  We investigate the threshold of gravitational collapse with angular momentum,
under the assumption that the critical solution is spherical and self-similar
and has two growing modes, namely one spherical mode and one axial dipole mode
(threefold degenerate). This assumption holds for perfect fluid matter with the
equation of state p=kappa rho if the constant kappa is in the range
0<kappa<1/9. There is a region in the space of initial data where the mass and
angular momentum of the black hole created in the collapse are given in terms
of the initial data by two universal critical exponents and two universal
functions of one argument. These expressions are similar to those for the
correlation length and the magnetization in a ferromagnet near its critical
point, as a function of the temperature and the external magnetic field. We
discuss qualitative features of the scaling functions, and hence of critical
collapse with high angular momentum.


title: Positivity and conservation of superenergy tensors

abstract:
  Two essential properties of energy-momentum tensors T_{\mu\nu} are their
positivity and conservation. This is mathematically formalized by,
respectively, an energy condition, as the dominant energy condition, and the
vanishing of their divergence \nabla^\mu T_{\mu\nu}=0. The classical Bel and
Bel-Robinson superenergy tensors, generated from the Riemann and Weyl tensors,
respectively, are rank-4 tensors. But they share these two properties with
energy momentum tensors: the Dominant Property (DP) and the divergence-free
property in the absence of sources (vacuum). Senovilla defined a universal
algebraic construction which generates a basic superenergy tensor T{A} from any
arbitrary tensor A. In this construction the seed tensor A is structured as an
r-fold multivector, which can always be done. The most important feature of the
basic superenergy tensors is that they satisfy automatically the DP,
independently of the generating tensor A. In a previous paper we presented a
more compact definition of T{A} using the r-fold Clifford algebra. This form
for the superenergy tensors allowed to obtain an easy proof of the DP valid for
any dimension. In this paper we include this proof. We explain which new
elements appear when we consider the tensor T{A} generated by a
non-degree-defined r-fold multivector A and how orthogonal Lorentz
transformations and bilinear observables of spinor fields are included as
particular cases of superenergy tensors. We find some sufficient conditions for
the seed tensor A, which guarantee that the generated tensor T{A} is
divergence-free. These sufficient conditions are satisfied by some physical
fields, which are presented as examples.


title:
Conformal Collineations and Ricci Inheritance Symmetry in String Cloud
  and String Fluids

abstract:
  Conformal collineations (a generalization of conformal motion) and Ricci
inheritance collineations, defined by $\pounds_\xi R_{ab}=2\alpha R_{ab}$, for
string cloud and string fluids in general relativity are studied. By
investigating the kinematical and dynamical properties of such fluids and using
the field equations, some recent studies on the restrictions imposed by
conformal collineations are extended, and new results are found.


title:
Spatially Filtering the Binary Confusion Noise for Space Gravitational
  Wave Detectors

abstract:
  For the last fifteen years, the limiting noise source at the low frequency
end of the sensitivity window for space gravitational wave detectors has been
expected to be the confusion background of overlapping galactic binary stars.
Here, we present results of a study that investigates the correlation between
binary star signals and conclude that there is a spatial filter in the
position-dependent Doppler shift of each binary that sharply reduces the
contribution of the galactic binary confusion to the noise in the detector when
a monochromatic source is being detected. The sensitivity is thus determined by
the instrument alone, and the confusion noise may effectively be ignored.


title: Network sensitivity to geographical configuration

abstract:
  Gravitational wave astronomy will require the coordinated analysis of data
from the global network of gravitational wave observatories. Questions of how
to optimally configure the global network arise in this context. We have
elsewhere proposed a formalism which is employed here to compare different
configurations of the network, using both the coincident network analysis
method and the coherent network analysis method. We have constructed a network
model to compute a figure-of-merit based on the detection rate for a population
of standard-candle binary inspirals. We find that this measure of network
quality is very sensitive to the geographic location of component detectors
under a coincident network analysis, but comparatively insensitive under a
coherent network analysis.


title: Lorentz violations in canonical quantum gravity

abstract:
  This is a summary of a talk given at the CP01 meeting on possible Lorentz
anomalies in canonical quantum gravity. It briefly reviews some initial
explorations on the subject that have taken place recently, and should be only
be seen as a short pointer to the literature on the subject, mostly for
outsiders.


title: On the gravitomagnetic clock effect

abstract:
  General relativity predicts that two freely counter-revolving test particles
in the exterior field of a central rotating mass take different periods of time
to complete the same full orbit; this time difference leads to the
gravitomagnetic clock effect. The effect has been derived for circular
equatorial orbits; moreover, it has been extended via azimuthal closure to
spherical orbits around a slowly rotating mass. In this work, a general formula
is derived for the main gravitomagnetic clock effect in the case of slow motion
along an arbitrary {\it elliptical} orbit in the exterior field of a slowly
rotating mass. Some of the implications of this result are briefly discussed.


title: Quantum systems in weak gravitational fields

abstract:
  Fully covariant wave equations predict the existence of a class of
inertial-gravitational effects that can be tested experimentally. In these
equations inertia and gravity appear as external classical fields, but, by
conforming to general relativity, provide very valuable information on how
Einstein's views carry through in the world of the quantum.


title: Global Embeddings of Two-dimensional Dilatonic Black Holes

abstract:
  We obtain minimal (2+1) and (2+2) dimensional global flat embeddings of
uncharged and charged dilatonic black holes in (1+1) dimensions, respectively.
Moreover, we obtain the Hawking temperatures and the black hole ones of these
dilatonic black holes. However, even though the minimal flat embedding
structures are mathematically meaningful, through this minimal embeddings the
proper entropies are shown to be unattainable, contrast to the cases of other
black holes in (2+1) or much higher dimensions.


title: The Energy of the Gamma Metric in the M{\o}ller Prescription

abstract:
  We obtain the energy distribution of the gamma metric using the
energy-momentum complex of M{\o}ller. The result is the same as obtained by
Virbhadra in the Weinberg prescription.


title: The Cotton, Simon-Mars and Cotton-York Tensors in Stationary Spacetimes

abstract:
  The Cotton-York and Simon-Mars tensors in stationary vacuum spacetimes are
studied in the language of the congruence approach pioneered by Hawking and
Ellis. Their relationships with the Papapetrou field defined by the stationary
Killing congruence and with a recent characterization of the Kerr spacetime in
terms of the alignment between of the principal null directions of the Weyl
tensor with those of the Papapetrou field are also investigated in this more
transparent language.


title: A New Approach to the Geometrization of Matter

abstract:
  We show that the sum over geometries in the Lorentzian 4-D state sum model
for quantum GR in [1] includes terms which correspond to geometries on
manifolds with conical singularities. Natural approximations suggest that they
can be interpreted as gauge bosond for the standard model, plus fermions plus
matter.


title:
Relativistic Dynamics of Vector Bosons in the Field of Gravitational
  Radiation

abstract:
  We consider a model of the state evolution of relativistic vector bosons,
which includes both the dynamical equations for the particle four-velocity and
the equations for the polarization four-vector evolution in the field of a
nonlinear plane gravitational wave. In addition to the gravitational minimal
coupling, tidal forces linear in curvature tensor are suggested to drive the
particle state evolution. The exact solutions of the evolutionary equations are
obtained. Birefringence and tidal deviations from the geodesic motion are
discussed.


title:
Cosmological Evolution with \Lambda-Term and Any Linear equation of
  State

abstract:
  Recent observational indications of an accelerating universe enhance the
interest in studying models with a cosmological constant. We investigate
cosmological expansion (FRW metric) with $\Lambda>0$ for a general linear
equation of state $p=w\rho$, $w>-1$, so that the interplay between cosmological
vacuum and quintessence is allowed, as well.
  Four closed-form solutions (flat universe with any $w$, and $w=1/3$, $-1/3,
-2/3$) are given, of which the last one appears to be new. For the open
universe a simple relation between solutions with different parameters is
established: it turns out that a solution with some $w$ and (properly scaled)
$\Lambda$ is expressed algebraically via another solution with special
different values of these parameters.
  The expansion becomes exponential at large times, and the amplitude at the
exponent depends on the parameters. We study this dependence in detail,
deriving various representations for the amplitude in terms of integrals and
series. The closed-form solutions serve as benchmarks, and the solution
transformation property noted above serves as a useful tool. Among the results
obtained, one is that for the open universe with relatively small cosmological
constant the amplitude is independent of the equation of state. Also, estimates
of the cosmic age through the observable ratio $\Omega_\Lambda/\Omega_M$ and
parameter $w$ are given; when inverted, they provide an estimate of $w$, i. e.,
the state equation, through the known age of the universe.


title: Does The Force From an Extra Dimension Contradict Physics in 4D?

abstract:
  We examine the question of whether violation of 4D physics is an inevitable
consequence of existence of an extra non-compactified dimension. Recent
investigations in membrane and Kaluza-Klein theory indicate that when the
metric of the spacetime is allowed to depend on the extra coordinate, i.e., the
cilindricity condition is dropped, the equation describing the trajectory of a
particle in one lower dimension has an extra force with some abnormal
properties. Among them, a force term parallel to the four-velocity of the
particle and, what is perhaps more surprising,
  $u_{\mu}f^{\mu} \neq u^{\mu}f_{\mu}$. These properties violate basic concepts
in 4D physics. In this paper we argue that these abnormal properties are {\em
not} consequence of the extra dimension, but result from the formalism used. We
propose a new definition for the force, from the extra dimension, which is free
of any contradictions and consistent with usual 4D physics. We show, using
warp-products metrics, that this new definition is also more consistent with
our physical intuition. The effects of this force could be detected observing
objects moving with high speed, near black holes and/or in cosmological
situations.


title: Presheaves, Sheaves and their Topoi in Quantum Gravity and Quantum Logic

abstract:
  A brief synopsis of recent conceptions and results, the current status and
future outlook of our research program of applying sheaf and topos-theoretic
ideas to quantum gravity and quantum logic is presented.


title: Local Time and the Unification of Physics Part I. Local Time

abstract:
  The notions of time in the theories of Newton and Einstein are reviewed so
that the difficulty which impedes the unification of quantum mechanics (QM) and
general relativity (GR) is clarified. It is seen that GR by itself contains an
intrinsic difficulty relating to the definition of local clocks, as well as
that GR still requires a kind of absolute that can serve as an objective
reference standard. We present a new understanding of time, which gives a
consistent definition of a local time associated with each local system in a
quantum mechanical way, so that it serves the requirements of both GR as well
as QM. As a consequence, QM and GR are reconciled while preserving the current
mathematical formulations of both theories.


title: Local Time and the Unification of Physics Part II. Local System

abstract:
  As a continuation of Part I [8], a more precise formulation of local time and
local system is given. The observation process is reflected in order to give a
relation between the classical physics for centers of mass of local systems and
the quantum mechanics inside a local system. The relation will give a
unification of quantum mechanics and general relativity in some cases. The
existence of local time and local motion is proved so that the stationary
nature of the universe is shown to be consistent with the local motion.


title:
Motion of a Vector Particle in a Curved Spacetime. I. Lagrangian
  Approach

abstract:
  From the simple Lagrangian the equations of motion for the particle with spin
are derived. The spin is shown to be conserved on the particle world-line. In
the absence of a spin the equation coincides with that of a geodesic. The
equations of motion are valid for massless particles as well, since mass does
not enter the equations explicitely.


title: Note on flat foliations of spherically symmetric spacetimes

abstract:
  It is known that spherically symmetric spacetimes admit flat spacelike
foliations. We point out a simple method of seeing this result via the
Hamiltonian constraints of general relativity. The method yields explicit
formulas for the extrinsic curvatures of the slicings.


title: Bianchi Type I Universes with Causal Bulk Viscous Cosmological Fluid

abstract:
  We consider the dynamics of a causal bulk viscous cosmological fluid filled
constantly decelerating Bianchi type I space-time. The matter component of the
Universe is assumed to satisfy a linear barotropic equation of state and the
state equation of the small temperature Boltzmann gas. The resulting
cosmological models satisfy the condition of smallness of the viscous stress.
The time evolution of the relaxation time, temperature, bulk viscosity
coefficient and comoving entropy of the dissipative fluid is also obtained.


title: Hawking radiation from decoherence

abstract:
  It is argued that the thermal nature of Hawking radiation arises solely due
to decoherence. Thereby any information-loss paradox is avoided because for
closed systems pure states remain pure. The discussion is performed for a
massless scalar field in the background of a Schwarzschild black hole, but the
arguments should hold in general. The result is also compared to and contrasted
with the situation in inflationary cosmology.


title:
A decoupled system of hyperbolic equations for linearized cosmological
  perturbations

abstract:
  A decoupled system of hyperbolic partial differential equations for linear
perturbations around any spatially flat FRW universe is obtained for a wide
class of perturbations. The considered perturbing energy momentum-tensors can
be expressed as the sum of the perturbation of a minimally coupled scalar field
plus an arbitrary (weak) energy-momentum tensor which is covariantly conserved
with respect to the background. The key ingredient in obtaining the decoupling
of the equations is the introduction of a new covariant gauge which plays a
similar role as harmonic gauge does for perturbations around Minkowski
space-time. The case of universes satysfying a linear equation of state is
discussed in particular, and closed analytic expressions for the retarded
Green's functions solving the de Sitter, dust and radiation dominated cases are
given.


title: A note on the uniqueness of D=4 N=1 Supergravity

abstract:
  We investigate in 4 spacetime dimensions, all the consistent deformations of
the lagrangian ${\cal L}_2+{\cal L}_{{3/2}}$, which is the sum of the
Pauli-Fierz lagrangian ${\cal L}_2$ for a free massless spin 2 field and the
Rarita-Schwinger lagrangian ${\cal L}_{{3/2}}$ for a free massless spin 3/2
field. Using BRST cohomogical techniques, we show, under the assumptions of
locality, Poincar\'e invariance, conservation of the number of gauge symmetries
and the number of derivatives on each fields, that N=1 D=4 supergravity is the
only consistent interaction between a massless spin 2 and a massless spin 3/2
field. We do not assume general covariance. This follows automatically, as does
supersymmetry invariance. Various cohomologies related to conservations laws
are also given.


title: Schwarzschild horizon and the gravitational redshift formula

abstract:
  The gravitational redshift formula is usually derived in the geometric optics
approximation. In this note we consider an exact formulation of the problem in
the Schwarzschild space-time, with the intention to clarify under what
conditions this redshift law is valid. It is shown that in the case of shocks
the radial component of the Poynting vector can scale according to the redshift
formula, under a suitable condition. If that condition is not satisfied, then
the effect of the backscattering can lead to significant modifications. The
obtained results imply that the energy flux of the short wavelength radiation
obeys the standard gravitational redshift formula while the energy flux of long
waves can scale differently, with redshifts being dependent on the frequency.


title: Space Time Foam: a ground state candidate for Quantum Gravity

abstract:
  A model of space-time foam, made by $N$ wormholes is considered. The Casimir
energy leading to such a model is computed by means of the phase shift method
which is in agreement with the variational approach used in Refs.[9-14]. The
collection of Schwarzschild and Reissner-Nordstr\"{o}m wormholes are separately
considered to represent the foam. The Casimir energy shows that the
Reissner-Nordstr\"{o}m wormholes cannot be used to represent the foam.


title: The Schwarzschild solution in a Kaluza-Klein theory with two times

abstract:
  A new spherically-symmetric solution is determined in a noncompactified
Kaluza-Klein theory in which a time character is ascribed to the fifth
coordinate. This solution contains two independent parameters which are related
with mass and electric charge. The solution exhibits a Schwarzschild radius and
represents a generalization of the Schwarzschild solution in four dimensions.
The parameter of the solution connected with the electric charge depends on the
derivative of the fifth (second time) coordinate with respect to the ordinary
time coordinate. It is shown that the perihelic motion in four-dimensional
relativity has a counterpart in five dimensions in the perinucleic motion of a
negatively-charged particle. If the quantization conditions of the older
quantum theory are applied to that motion, an analogue of the fine-structure
formula of atomic spectra is obtained.


title:
Universal regular short distance behavior from an interaction with a
  scale invariant gravity

abstract:
  We assume that the fourdimensional quantum gravity is scale invariant at
short distances. We show through a simple scaling argument that correlation
functions of quantum fields interacting with gravity have a universal (more
regular) short distance behavior.


title: A Geometric Renormalisation Group in Discrete Quantum Space-Time

abstract:
  We model quantum space-time on the Planck scale as dynamical networks of
elementary relations or time dependent random graphs, the time dependence being
an effect of the underlying dynamical network laws. We formulate a kind of
geometric renormalisation group on these (random) networks leading to a
hierarchy of increasingly coarse-grained networks of overlapping lumps. We
provide arguments that this process may generate a fixed limit phase,
representing our continuous space-time on a mesoscopic or macroscopic scale,
provided that the underlying discrete geometry is critical in a specific sense
(geometric long range order). Our point of view is corroborated by a series of
analytic and numerical results, which allow to keep track of the geometric
changes, taking place on the various scales of the resolution of space-time. Of
particular conceptual importance are the notions of dimension of such random
systems on the various scales and the notion of geometric criticality.

categories: gr-qc cond-mat.dis-nn hep-th math-ph math.MP


title: Real null coframes in general relativity and GPS type coordinates

abstract:
  Based on work of Derrick, Coll, and Morales, we define a `symmetric' null
coframe with {\it four real null covectors}. We show that this coframe is
closely related to the GPS type coordinates recently introduced by Rovelli.


title: Lorentz violation and Hawking radiation

abstract:
  Since the event horizon of a black hole is a surface of infinite redshift, it
might be thought that Hawking radiation would be highly sensitive to Lorentz
violation at high energies. In fact, the opposite is true for subluminal
dispersion. For superluminal dispersion, however, the outgoing black hole modes
emanate from the singularity in a state determined by unknown quantum gravity
processes.


title:
Dilatonic wormholes: construction, operation, maintenance and collapse
  to black holes

abstract:
  The CGHS two-dimensional dilaton gravity model is generalized to include a
ghost Klein-Gordon field, i.e. with negative gravitational coupling. This
exotic radiation supports the existence of static traversible wormhole
solutions, analogous to Morris-Thorne wormholes. Since the field equations are
explicitly integrable, concrete examples can be given of various dynamic
wormhole processes, as follows. (i) Static wormholes are constructed by
irradiating an initially static black hole with the ghost field. (ii) The
operation of a wormhole to transport matter or radiation between the two
universes is described, including the back-reaction on the wormhole, which is
found to exhibit a type of neutral stability. (iii) It is shown how to maintain
an operating wormhole in a static state, or return it to its original state, by
turning up the ghost field. (iv) If the ghost field is turned off, either
instantaneously or gradually, the wormhole collapses into a black hole.


title:
Global Prescribed Mean Curvature foliations in cosmological spacetimes
  with matter, Part I

abstract:
  This work investigates some global questions about cosmological spacetimes
with two dimensional spherical, plane and hyperbolic symmetry containing
matter. The result is, that these spacetimes admit a global foliation by
prescribed mean curvature surfaces, which extends at least towards a crushing
singularity. The time function of the foliation is geometrically defined and
unique up to the choice of an initial Cauchy surface.
  This work generalizes a similar analysis on constant mean curvature
foliations and avoids the topological obstructions arising from the existence
problem.


title:
Global Prescribed Mean Curvature foliations in cosmological spacetimes
  with matter, Part II

abstract:
  This second part is devoted to the investigation of global properties of
Prescribed Mean Curvature (PMC) foliations in cosmological spacetimes with
local $U(1) \times U(1)$ symmetry and matter described by the Vlasov equation.
It turns out, that these spacetimes admit a global foliation by PMC surfaces,
as well, but the techniques to achieve this goal are more complex than in the
cases considered in part I.


title: Spherical Gravitating Systems of Arbitrary Dimension

abstract:
  We study spherically symmetric solutions to the Einstein field equations
under the assumption that the space-time may possess an arbitrary number of
spatial dimensions. The general solution of Synge is extended to describe
systems of any dimension. Arbitrary dimension analogues of known four
dimensional solutions are also presented, derived using the above scheme.
Finally, we discuss the requirements for the existence of Birkhoff's theorems
in space-times of arbitrary dimension with or without matter fields present.
Cases are discussed where the assumptions of the theorem are considerably
weakened yet the theorem still holds. We also discuss where the weakening of
certain conditions may cause the theorem to fail.


title: Horizon Properties of Einstein-Yang-Mills Black Hole

abstract:
  We consider static axially symmetric Einstein-Yang-Mills black holes in the
isolated horizon formalism. The mass of these hairy black holes is related to
the mass of the corresponding particle-like solutions by the horizon mass. The
hairy black holes violate the ``quasi-local uniqueness conjecture'', based on
the horizon charges.


title:
Close limit evolution of Kerr-Schild type initial data for binary black
  holes

abstract:
  We evolve the binary black hole initial data family proposed by Bishop {\em
et al.} in the limit in which the black holes are close to each other. We
present an exact solution of the linearized initial value problem based on
their proposal and make use of a recently introduced generalized formalism for
studying perturbations of Schwarzschild black holes in arbitrary coordinates to
perform the evolution. We clarify the meaning of the free parameters of the
initial data family through the results for the radiated energy and waveforms
from the black hole collision.


title: Warp Drive With Zero Expansion

abstract:
  It is commonly believed that Alcubierre's warp drive works by contracting
space in front of the warp bubble and expanding space behind it. We show that
this expansion/contraction is but a marginal consequence of the choice made by
Alcubierre, and explicitly construct a similar spacetime where no
contraction/expansion occurs. Global and optical properties of warp drive
spacetimes are also discussed.


title: What Casimir Energy can suggest about Space Time Foam?

abstract:
  In the context of a model of space-time foam, made by $N$ wormholes we
discuss the possibility of having a foam formed by different configurations. An
equivalence between Schwarzschild and Schwarzschild-Anti-de Sitter wormholes in
terms of Casimir energy is shown. An argument to discriminate which
configuration could represent a foamy vacuum coming from Schwarzschild black
hole transition frequencies is used. The case of a positive cosmological
constant is also discussed. Finally, a discussion involving charged wormholes
leads to the conclusion that they cannot be used to represent a ground state of
the foamy type.


title:
Parametric amplification of waves during gravitational collapse: a first
  investigation

abstract:
  We study the dynamical evolution of perturbations in the gravitational field
of a collapsing fluid star. Specifically, we consider the initial value problem
for a massless scalar field in a spacetime similar to the Oppenheimer-Snyder
collapse model, and numerically evolve in time the relevant wave equation. Our
main objective is to examine whether the phenomenon of parametric
amplification, known to be responsible for the strong amplification of
primordial perturbations in the expanding Universe, can efficiently operate
during gravitational collapse. Although the time-varying gravitational field
inside the star can, in principle, support such a process, we nevertheless find
that the perturbing field escapes from the star too early for amplification to
become significant. To put an upper limit in the efficiency of the
amplification mechanism (for a scalar field) we furthermore consider the case
of perturbations trapped inside the star for the entire duration of the
collapse. In this extreme case, the field energy is typically amplified at the
level ~ 1% when the star is about to cross its Schwarszchild radius.
Significant amplification is observed at later stages when the star has even
smaller radius. Therefore, the conclusion emerging from our simple model is
that parametric amplification is unlikely to be of significance during
gravitational collapse. Further work, based on more realistic collapse models,
is required in order to fully assess the astrophysical importance of parametric
amplification.


title: On the Einstein-Vlasov system with hyperbolic symmetry

abstract:
  It is shown that a spacetime with collisionless matter evolving from data on
a compact Cauchy surface with hyperbolic symmetry can be globally covered by
compact hypersurfaces on which the mean curvature is constant and by compact
hypersurfaces on which the area radius is constant. Results for the related
cases of spherical and plane symmetry are reviewed and extended. The prospects
of using the global time coordinates obtained in this way to investigate the
global geometry of the spacetimes concerned are discussed.


title: Classification of String-like Solutions in Dilaton Gravity

abstract:
  The static string-like solutions of the Abelian Higgs model coupled to
dilaton gravity are analyzed and compared to the non-dilatonic case. Except for
a special coupling between the Higgs Lagrangian and the dilaton, the solutions
are flux tubes that generate a non-asymptotically flat geometry. Any point in
parameter space corresponds to two branches of solutions with two different
asymptotic behaviors. Unlike the non-dilatonic case, where one branch is always
asymptotically conic, in the present case the asymptotic behavior changes
continuously along each branch.


title: On the tidally induced gravitational collapse of a particle cluster

abstract:
  An important issue in the dynamics of neutron star binaries is whether tidal
interaction can cause the individual stars to collapse into black holes during
inspiral. To understand this issue better, we study the dynamics of a cluster
of collisionless particles orbiting a non-rotating black hole, which is part of
a widely separated circular binary. The companion body's electric- and
magnetic-type tidal fields distort the black hole and perturb the cluster,
eventually causing the cluster to collapse into the hole as the companion
spirals in under the influence of gravitational radiation reaction. We find
that magnetic-type tidal forces do not significantly influence the evolution of
the cluster as a whole. However, individual orbits can be strongly affected by
these forces. For example, some orbits are destabilized due to the addition of
magnetic-type tidal forces. We find that the most stable orbits are close to
the companion's orbital plane and retrograde with respect to the companion's
orbit.


title: Recent Advances in Stochastic Gravity: Theory and Issues

abstract:
  Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the Einstein-
Langevin equation, which has in addition sources due to the noise kernel. The
noise kernel is the expectation value of the stress-energy bi-tensor which
describes the fluctuations of quantum matter fields. Here, we first give an
axiomatic derivation of the Einstein-Langevin equations and then show how they
can also be derived by the original method based on the influence functional.
As a first application we solve these equations and compute the two- point
correlation functions for the linearized Einstein tensor and for the metric
perturbations in a Minkowski background. We then turn to the important issue of
the validity of semiclassical gravity by examining the criteria based on the
ratio of the variance of fluctuations of the stress-energy tensor of a quantum
field to its mean. We show a calculation of these quantities for a massless
scalar field in the Minkowski and the Casimir vacua as a function of an
intrinsic scale defined by introducing a smeared field or by point separation.
Contrary to prior claims, the ratio of variance to mean-squared being of the
order unity does not necessarily imply the failure of semiclassical gravity.
Next we use the point-separation method to derive a general expression for a
regularized noise-kernel for quantum fields in general curved spacetimes. Using
these expressions one could investigate a host of important problems in early
universe and black hole physics.


title:
Generation of heterotic string theory solutions from the stationary
  Einstein-Maxwell fields

abstract:
  A new formalism for construction of the stationary solutions is developed for
the four-dimensional gravity coupled to the dilaton, Kalb-Ramond and two
Maxwell fields in a low-energy heterotic string theory form. The result of
generation is automatically invariant in respect to subgroup of the stationary
charging symmetry transformations; the generation can be started from the
stationary Einstein-Maxwell fields. The formalism is given both in real and new
compact complex form, the result of maximal symmetry extension of the
stationary Einstein-Maxwell theory to discussing string gravity model is
explicitly written down.


title: New progress in the stationary D=N=4 supergravity

abstract:
  A bosonic sector of the four-dimensional low-energy heterotic string theory
with two Abelian gauge fields is considered in the stationary case. A new 4X4
unitary null-curvature matrix representation of the theory is derived and the
corresponding formulation based on the use of a new 2X2 Ernst type matrix
complex potential is developed. The group of hidden symmetries is described and
classified in the matrix-valued quasi General Relativity form. A subgroup of
charging symmetries is constructed and representation which transforms linearly
under the action of this symmetry subgroup is established. Also the solution
generation procedure based on the application of the total charging symmetry
subgroup to the stationary Einstein-Maxwell theory is analyzed.


title:
Generation of the bosonic string theory solutions from the stationary
  Einstein fields via projection symmetry

abstract:
  A new formalism for generation of solutions in the consistently truncated
low-energy bosonic string theory is developed. This formalism gives a
correspondence of the projection type between the theories toroidally
compactified from the diverse to three dimensions. Taking the stationary
Einstein-dilaton gravity as the theory with a lower dimension, we generate its
bosonic string theory extension and calculate the bosonic string theory
solution corresponding to the Kerr-NUT one modified by the presence of the
charged dilaton field.


title: Kalb--Ramond dipole solution in low-energy bosonic string theory

abstract:
  We construct a new solution subspace for the bosonic string theory toroidally
compactified to 3 dimensions. This subspace corresponds to the complex harmonic
scalar field coupled to the effective 3--dimensional gravity. We calculate a
class of the asymptotically flat and free of the Dirac string peculiarity
solutions which describes a Kalb--Ramond dipole source with the generally
nontrivial dilaton charge.


title: Supergravity and the Poincare group

abstract:
  An action for 3+1-dimensional supergravity genuinely invariant under the
Poincare supergroup is proposed. The construction of the action is carried out
considering a bosonic lagrangian invariant under both local Lorentz rotations
and local Poincare translations as well as under diffeomorphism, and therefore
the Poincare algebra closes off-shell. Since the lagrangian is invariant under
the Poincare supergroup, the supersymmetry algebra closes off shell without the
need of auxiliary fields.


title: The Affine Quantum Gravity Program

abstract:
  The central principle of affine quantum gravity is securing and maintaining
the strict positivity of the matrix $\{\hg_{ab}(x)\}$ composed of the spatial
components of the local metric operator. On spectral grounds, canonical
commutation relations are incompatible with this principle, and they must be
replaced by noncanonical, affine commutation relations. Due to the partial
second-class nature of the quantum gravitational constraints, it is
advantageous to use the recently developed projection operator method, which
treats all quantum constraints on an equal footing. Using this method,
enforcement of regularized versions of the gravitational operator constraints
is formulated quite naturally by means of a novel and relatively well-defined
functional integral involving only the same set of variables that appears in
the usual classical formulation. It is anticipated that skills and insight to
study this formulation can be developed by studying special, reduced-variable
models that still retain some basic characteristics of gravity, specifically a
partial second-class constraint operator structure. Although perturbatively
nonrenormalizable, gravity may possibly be understood nonperturbatively from a
hard-core perspective that has proved valuable for specialized models. Finally,
developing a procedure to pass to the genuine physical Hilbert space involves
several interconnected steps that require careful coordination.


title: Cosmological model with movement in fifth dimension

abstract:
  Presented cosmological model is 3D brane world sheet moved in extra dimension
with variable scale factor. Analysis of the geodesic motion of the test
particle gives settle explanation of the Pioneer effect. It is found that for
considered metric the solution of the semi-classical Einstein equations with
various parameters conforms to isotropic expanded and anisotropic stationary
universe.


title:
Second post-Newtonian gravitational wave polarizations for compact
  binaries in elliptical orbits

abstract:
  The second post-Newtonian (2PN) contribution to the `plus' and `cross'
gravitational wave polarizations associated with gravitational radiation from
non-spinning, compact binaries moving in elliptic orbits is computed. The
computation starts from our earlier results on 2PN generation, crucially
employs the 2PN accurate generalized quasi-Keplerian parametrization of
elliptic orbits by Damour, Sch\"afer and Wex and provides 2PN accurate
expressions modulo the tail terms for gravitational wave polarizations
incorporating effects of eccentricity and periastron precession.


title:
Gravitomagnetic Effects in the Propagation of Electromagnetic Waves in
  Variable Gravitational Fields of Arbitrary-Moving and Spinning Bodies

abstract:
  Propagation of light in the gravitational field of self-gravitating spinning
bodies moving with arbitrary velocities is discussed. The gravitational field
is assumed to be "weak" everywhere. Equations of motion of a light ray are
solved in the first post-Minkowskian approximation that is linear with respect
to the universal gravitational constant $G$. We do not restrict ourselves with
the approximation of gravitational lens so that the solution of light geodesics
is applicable for arbitrary locations of source of light and observer. This
formalism is applied for studying corrections to the Shapiro time delay in
binary pulsars caused by the rotation of pulsar and its companion. We also
derive the correction to the light deflection angle caused by rotation of
gravitating bodies in the solar system (Sun, planets) or a gravitational lens.
The gravitational shift of frequency due to the combined translational and
rotational motions of light-ray-deflecting bodies is analyzed as well. We give
a general derivation of the formula describing the relativistic rotation of the
plane of polarization of electromagnetic waves (Skrotskii effect). This formula
is valid for arbitrary translational and rotational motion of gravitating
bodies and greatly extends the results of previous researchers. Finally, we
discuss the Skrotskii effect for gravitational waves emitted by localized
sources such as a binary system. The theoretical results of this paper can be
applied for studying various relativistic effects in microarcsecond space
astrometry and developing corresponding algorithms for data processing in space
astrometric missions such as FAME, SIM, and GAIA.


title: Galactic Collapse of Scalar Field Dark Matter

abstract:
  We present a scenario for galaxy formation based on the hypothesis of scalar
field dark matter. We interpret galaxy formation through the collapse of a
scalar field fluctuation. We find that a cosh potential for the
self-interaction of the scalar field provides a reasonable scenario for
galactic formation, which is in agreement with cosmological observations and
phenomenological studies in galaxies.


title: Anisotropic Stars in General Relativity

abstract:
  We present a class of exact solutions of Einstein's gravitational field
equations describing spherically symmetric and static anisotropic stellar type
configurations. The solutions are obtained by assuming a particular form of the
anisotropy factor. The energy density and both radial and tangential pressures
are finite and positive inside the anisotropic star. Numerical results show
that the basic physical parameters (mass and radius) of the model can describe
realistic astrophysical objects like neutron stars.


title: The First Law of Isolated Horizons via Noether Theorem

abstract:
  A general recipe proposed elsewhere to define, via Noether theorem, the
variation of energy for a natural field theory is applied to Einstein-Maxwell
theory. The electromagnetic field is analysed in the geometric framework of
natural bundles. Einstein-Maxwell theory turns then out to be natural rather
than gauge-natural. As a consequence of this assumption a correction term \a la
Regge-Teitelboim is needed to define the variation of energy, also for the pure
electromagnetic part of the Einstein-Maxwell Lagrangian. Integrability
conditions for the variational equation which defines the variation of energy
are analysed in relation with boundary conditions on physical data. As an
application the first law of thermodynamics for rigidly rotating horizons is
obtained.


title: On the Cardy-Verlinde Entropy Formula in Viscous Cosmology

abstract:
  The results of the paper of Verlinde [hep-th/0008140], discussing the
holographic principle in a radiation dominated universe, are extended when
allowing the cosmic fluid to possess a bulk viscosity. This corresponds to a
non-conformally invariant theory. The generalization of the Cardy-Verlinde
entropy formula to the case of a viscous universe seems from a formal point of
view to be possible, although we question on physical grounds some elements in
this kind of theory, especially the manner in which the Casimir energy is
evaluated. Our discussion suggests that for non-conformally invariant theories
the holographic definition of Casimir energy should be modified.


title:
On the role of conformal three-geometries in the dynamics of General
  Relativity

abstract:
  It is shown that the Chern-Simons functional, built in the spinor
representation from the initial data on spacelike hypersurfaces, is invariant
with respect to infinitesimal conformal rescalings if and only if the vacuum
Einstein equations are satisfied. As a consequence, we show that in the phase
space the Hamiltonian constraint of vacuum general relativity is the Poisson
bracket of the imaginary part of this Chern-Simons functional and Misner's time
(essentially the 3-volume). Hence the vacuum Hamiltonian constraint is the
condition on the canonical variables that the imaginary part of the Chern-
Simons functional be constant along the volume flow. The vacuum momentum
constraint can also be reformulated in a similar way as a (more complicated)
condition on the change of the imaginary part of the Chern-Simons functional
along the flow of York's time.


title: Space-time defects and teleparallelism

abstract:
  We consider the class of space-time defects investigated by Puntigam and
Soleng. These defects describe space-time dislocations and disclinations
(cosmic strings), and are in close correspondence to the actual defects that
arise in crystals and metals. It is known that in such materials dislocations
and disclinations require a small and large amount of energy, respectively, to
be created. The present analysis is carried out in the context of the
teleparallel equivalent of general relativity (TEGR). We evaluate the
gravitational energy of these space-time defects in the framework of the TEGR
and find that there is an analogy between defects in space-time and in
continuum material systems: the total gravitational energy of space-time
dislocations and disclinations (considered as idealized defects) is zero and
infinit, respectively.


title: Red-shifts near black holes

abstract:
  A simple ordinary differential equation is derived governing the red-shifts
of wave-fronts propagating through a non-stationary spherically symmetric
space-time. Approach to an event horizon corresponds to approach to a fixed
point; in general, the phase portrait of the equation illuminates the
qualitative features of the geometry. In particular, the asymptotics of the
red-shift as a horizon is approached, a critical ingredient of Hawking's
prediction of radiation from black holes, are easily brought out. This
asympotic behavior has elements in common with the universal behavior near
phase transitions in statistical physics. The validity of the Unruh vacuum for
the Hawking process can be understood in terms of this universality. The
concept of surface gravity is extended to to non-stationary spherically
symmetric black holes. Finally, it is shown that in the non-stationary case,
Hawking's predicted flux of radiation from a black hole would be modified.


title: Acceleration-induced nonlocality: kinetic memory versus dynamic memory

abstract:
  The characteristics of the memory of accelerated motion in Minkowski
spacetime are discussed within the framework of the nonlocal theory of
accelerated observers. Two types of memory are distinguished: kinetic and
dynamic. We show that only kinetic memory is acceptable, since dynamic memory
leads to divergences for nonuniform accelerated motion.


title: Probing the Brans-Dicke Gravitational Field by Cerenkov Radiation

abstract:
  The possibility that a charged particle propagating in a gravitational field
described by Brans-Dicke theory of gravity could emit Cerenkov radiation is
explored. This process is kinematically allowed depending on parameters
occurring in the theory. The Cerenkov effect disappears as the BD parameter
omega tends to inftinity, i.e. in the limit in which the Einstein theory is
recovered, giving a signature to probe the validity of the Brans-Dicke theory.


title: Elliptic-hyperbolic systems and the Einstein equations

abstract:
  The Einstein evolution equations are studied in a gauge given by a
combination of the constant mean curvature and spatial harmonic coordinate
conditions. This leads to a coupled quasilinear elliptic--hyperbolic system of
evolution equations. We prove that the Cauchy problem is locally strongly
well--posed and that a continuation principle holds. For initial data
satisfying the Einstein constraint and gauge conditions, the solutions to the
elliptic-hyperbolic system defined by the gauge fixed Einstein evolution
equations are shown to give vacuum spacetimes.


title: Probing neutron star superfluidity with gravitational-wave data

abstract:
  We discuss the possibility that future gravitational-wave detectors may be
able to detect various modes of oscillation of old, cold neutron stars. We
argue that such detections would provide unique insights into the superfluid
nature of neutron star cores, and could also lead to a much improved
understanding of pulsar glitches. Our estimates are based on a detector
configuration with several narrowbanded (cryogenic) interferometers operating
as a "xylophone" which could lead to high sensitivity at high frequencies. We
also draw on recent advances in our understanding of the dynamics of pulsating
superfluid neutron star cores.


title: The Gravitational Cherenkov Radiation

abstract:
  An example of discontinuity of the energy-momentum tensor moving at
superluminal velocity is discussed. It is shown that the gravitational Mach
cone is formed. The power spectrum of the corresponding Cherenkov radiation is
evaluated.


title: A State Sum Model for (2+1) Lorentzian Quantum Gravity

abstract:
  A state sum model based on the group SU(1,1) is defined. Investigations of
its geometry and asymptotics suggest it is a good candidate for modelling (2+1)
Lorentzian quantum gravity.


title:
A Gravitomagnetic Effect on the Orbit of a Test Body due to the Earth's
  Variable Angular Momentum

abstract:
  The well known general relativistic Lense-Thirring drag of the orbit of a
test particle in the stationary field of a central slowly rotating body is
generated, in the weak-field and slow-motion approximation of General
Relativity, by a gravitomagnetic Lorentz-like acceleration in the equations of
motion of the test particle. In it the gravitomagnetic field is due to the
central body's angular momentum supposed to be constant. In the context of the
gravitational analogue of the Larmor theorem, such acceleration looks like a
Coriolis inertial term in an accelerated frame. In this paper the effect of the
variation in time of the central body's angular momentum on the orbit of a test
mass is considered. It can be shown that it is analogue to the inertial
acceleration due to the time derivative of the angular velocity vector of an
accelerated frame. The possibility of detecting such effect in the
gravitational field of the Earth with LAGEOS-like satellites is investigated.
It turns out that the orbital effects are far too small to be measured.


title:
Is it possible to test directly General Relativity in the gravitational
  field of the Moon?

abstract:
  In this paper the possibility of measuring some general relativistic effects
in the gravitational field of the Moon via selenodetic missions, with
particular emphasis to the future Japanese SELENE mission, is investigated. For
a typical selenodetic orbital configuration the post-Newtonian Lense-Thirring
gravitomagnetic and the Einstein's gravitoelectric effects on the satellites
orbits are calculated and compared to the present-day orbit accuracy of lunar
missions. It turns out that for SELENE's Main Orbiter, at present, the
gravitoelectric periselenium shift, which is the largest general relativistic
effect, is 1 or 2 orders of magnitude smaller than the experimental
sensitivity. The systematic error induced by the mismodelled classical
periselenium precession due to the first even zonal harmonic J2 of the Moon's
non-spherical gravitational potential is 3 orders of magnitude larger than the
general relativistic gravitoelectric precession. The estimates of this work
could be used for future lunar missions having as their goals relativistic
measurements as well.


title: Process Physics: Inertia, Gravity and the Quantum

abstract:
  Process Physics models reality as self-organising relational or semantic
information using a self-referentially limited neural network model. This
generalises the traditional non-process syntactical modelling of reality by
taking account of the limitations and characteristics of self-referential
syntactical information systems, discovered by Goedel and Chaitin, and the
analogies with the standard quantum formalism and its limitations. In process
physics space and quantum physics are emergent and unified, and time is a
distinct non-geometric process. Quantum phenomena are caused by fractal
topological defects embedded in and forming a growing three-dimensional fractal
process-space. Various features of the emergent physics are briefly discussed
including: quantum gravity, quantum field theory, limited causality and the
Born quantum measurement metarule, inertia, time-dilation effects, gravity and
the equivalence principle, a growing universe with a cosmological constant,
black holes and event horizons, and the emergence of classicality.


title: Constancy of the Constants of Nature

abstract:
  The current observational and experimental bounds on time variation of the
constants of Nature are briefly reviewed.


title:
Causal Bulk Viscous Dissipative Isotropic Cosmologies with Variable
  Gravitational and Cosmological Constants

abstract:
  We consider the evolution of a flat Friedmann-Robertson-Walker Universe,
filled with a causal bulk viscous cosmological fluid, in the presence of
variable gravitational and cosmological constants. The basic equation for the
Hubble parameter, generalizing the evolution equation in the case of constant
gravitational coupling and cosmological term, is derived, under the
supplementary assumption that the total energy of the Universe is conserved. By
assuming that the cosmological constant is proportional to the square of the
Hubble parameter and a power law dependence of the bulk viscosity coefficient,
temperature and relaxation time on the energy density of the cosmological
fluid, two classes of exact solutions of the field equations are obtained. In
the first class of solutions the Universe ends in an inflationary era, while in
the second class of solutions the expansion of the Universe is non-inflationary
for all times. In both models the cosmological "constant" is a decreasing
function of time, while the gravitational "constant" increases in the early
period of evolution of the Universe, tending in the large time limit to a
constant value.


title: From Big Crunch to Big Bang: A Quantum String Cosmology Perspective

abstract:
  The scenario that the Universe contracts towards a big crunch and then
undergoes a transition to expanding Universe in envisaged in the quantum string
cosmology approach. The Wheeler-De Witt equation is solved exactly for an
exponential dilaton potential. S-duality invariant cosmological effective
action, for type IIB theory, is considered to derive classical solutions and
solve WDW equations.


title: BRST Quantisation of Histories Electrodynamics

abstract:
  This paper is a continuation of earlier work where a classical history theory
of pure electrodynamics was developed in which the the history fields have
\emph{five} components. The extra component is associated with an extra
constraint, thus enlarging the gauge group of histories electrodynamics. In
this paper we quantise the classical theory developed previously by two
methods. Firstly we quantise the reduced classical history space, to obtain a
reduced quantum history theory. Secondly we quantise the classical
BRST-extended history space, and use the BRST charge to define a
`cohomological' quantum history theory. Finally we show that the reduced
history theory is isomorphic, as a history theory, to the cohomological history
theory.


title: Ultimate Internets

abstract:
  In a previous paper (gr-qc/0103002), the inflationary universe was described
as a quantum growing network (QGN). Here, we propose our view of the QGN as the
"ultimate Internet", as it saturates the quantum limits to computation. Also,
we enlight some features of the QGN which are related to: i) the problem of
causality at the Planck scale, ii) the quantum computational aspects of
spacetime foam and decoherence, iii) the cosmological constant problem, iv) the
"information loss" puzzle. The resulting picture is a self-organizing system of
ultimate Internet-universes.


title: Thermodynamical properties of metric fluctuations during inflation

abstract:
  I study a thermodynamical approach to scalar metric perturbations during the
inflationary stage. In the power-law expanding universe here studied, I find a
negative heat capacity as a manifestation of superexponential growing for the
number of states in super Hubble scales. The power spectrum depends on the
Gibbons-Hawking and Hagedorn temperatures.


title:
Density perturbations in the Ekpyrotic Universe and string-inspired
  generalizations

abstract:
  We study density perturbations in several cosmological models motivated by
string theory. The evolution and the spectra of curvature perturbations ${\cal
R}$ are analyzed in the Ekpyrotic scenario and nonsingular string cosmologies.
We find that these string-inspired models generally exhibit blue spectra in
contrast to standard slow-roll inflationary scenarios. We also clarify the
parameter range where ${\cal R}$ is enhanced on superhorizon scales.


title:
Scalar vacuum structure in general relativity and alternative theories.
  Conformal continuations

abstract:
  We discuss the global properties of static, spherically symmetric
configurations of a self-gravitating real scalar field $\phi$ in general
relativity (GR), scalar-tensor theories (STT) and high-order gravity ($L=f(R)$)
in various dimensions. In GR, for fields with arbitrary potentials $V(\phi)$,
not necessarily positive-definite, it is shown that the list of all possible
types of space-time causal structure in the models under consideration is the
same as the one for $\phi = const$. In particular, there are no regular black
holes with any asymptotics. These features are extended to STT and $f(R)$
theories, connected with GR by conformal mappings, unless there is a conformal
continuation, i.e., a case when a singularity in a solution of GR maps to a
regular surface in an alternative theory, and the solution is continued through
such a surface. This effect is exemplified by exact solutions in GR with a
massless conformal scalar field, considered as a special STT. Necessary
conditions for the existence of a conformal continuation are found; they only
hold for special choices of STT and high-order theories of gravity.


title: Impulsive waves in the Nariai universe

abstract:
  A new class of exact solutions is presented which describes impulsive waves
propagating in the Nariai universe. It is constructed using a six-dimensional
embedding formalism adapted to the background. Due to the topology of the
latter, the wave front consists of two non-expanding spheres. Special
sub-classes representing pure gravitational waves (generated by null particles
with an arbitrary multipole structure) or shells of null dust are analyzed in
detail. Smooth isometries of the metrics are briefly discussed. Furthermore, it
is shown that the considered solutions are impulsive members of a more general
family of radiative Kundt spacetimes of type-II. A straightforward
generalization to impulsive waves in the anti-Nariai and Bertotti-Robinson
backgrounds is described. For a vanishing cosmological constant and
electromagnetic field, results for well known impulsive pp-waves are recovered.


title: Calculating the gravitational self force in Schwarzschild spacetime

abstract:
  We present a practical method for calculating the local gravitational
self-force (often called ``radiation-reaction force'') for a pointlike particle
orbiting a Schwarzschild black hole. This is an implementation of the method of
{\it mode-sum regularization}, in which one first calculates the (finite)
contribution to the force due to each individual multipole mode of the
perturbation, and then applies a certain regularization procedure to the mode
sum. Here we give the values of all the ``regularization parameters'' required
for implementing this regularization procedure, for any geodesic orbit in
Schwarzschild spacetime.


title:
Korteweg-de Vries adiabatic index solitons in barotropic open FRW
  cosmologies

abstract:
  Applying standard mathematical methods, it is explicitly shown how the
Riccati equation for the Hubble parameter H(\eta) of barotropic open FRW
cosmologies is connected with a Korteweg-de Vries equation for adiabatic index
solitons. It is also shown how one can embed a discrete sequence of adiabatic
indices of the type n^2({3/2}\gamma -1)^2 (\gamma \neq 2/3) in the sech FRW
adiabatic index soliton


title: Constraint-preserving boundary conditions in numerical relativity

abstract:
  This is the first paper in a series aimed to implement boundary conditions
consistent with the constraints' propagation in 3D numerical relativity. Here
we consider spherically symmetric black hole spacetimes in vacuum or with a
minimally coupled scalar field, within the Einstein-Christoffel symmetric
hyperbolic formulation of Einstein's equations. By exploiting the
characteristic propagation of the main variables and constraints, we are able
to single out the only free modes at the outer boundary for these problems. In
the vacuum case a single free modes exists which corresponds to a gauge
freedom, while in the matter case an extra mode exists which is associated with
the scalar field. We make use of the fact that the EC formulation has no
superluminal characteristic speeds to excise the singularity. We present a
second-order, finite difference discretization to treat these scenarios, where
we implement these constraint-preserving boundary conditions, and are able to
evolve the system for essentially unlimited times. As a test of the robustness
of our approach, we allow large pulses of gauge and scalar field enter the
domain through the outer boundary. We reproduce expected results, such as
trivial (in the physical sense) evolution in the vacuum case (even in gauge-
dynamical simulations), and the tail decay for the scalar field.


title: Null Cones in Lorentz-Covariant General Relativity

abstract:
  The oft-neglected issue of the causal structure in the flat spacetime
approach to Einstein's theory of gravity is considered. Consistency requires
that the flat metric's null cone be respected, but this does not happen
automatically. After reviewing the history of this problem, we introduce a
generalized eigenvector formalism to give a kinematic description of the
relation between the two null cones, based on the Segre' classification of
symmetric rank 2 tensors with respect to a Lorentzian metric. Then we propose a
method to enforce special relativistic causality by using the naive gauge
freedom to restrict the configuration space suitably. A set of new variables
just covers this smaller configuration space and respects the flat metric's
null cone automatically. In this smaller space, gauge transformations do not
form a group, but only a groupoid. Respecting the flat metric's null cone
ensures that the spacetime is globally hyperbolic, indicating that the Hawking
black hole information loss paradox does not arise.


title:
Conformally Flat Smoothed Particle Hydrodynamics: Application to Neutron
  Star Mergers

abstract:
  We present a new 3D SPH code which solves the general relativistic field +
hydrodynamics equations in the conformally flat approximation. Several test
cases are considered to test different aspects of the code. We finally apply
then the code to the coalescence of a neutron star binary system. The neutron
stars are modeled by a polytropic equation of state (EoS) with adiabatic
indices $\Gamma=2.0$, $\Gamma=2.6$ and $\Gamma=3.0$. We calculate the
gravitational wave signals, luminosities and frequency spectra by employing the
quadrupole approximation for emission and back reaction in the slow motion
limit. In addition, we consider the amount of ejected mass.


title:
Linear constraints from generally covariant systems with quadratic
  constraints

abstract:
  How to make compatible both boundary and gauge conditions for generally
covariant theories using the gauge symmetry generated by first class
constraints is studied. This approach employs finite gauge transformations in
contrast with previous works which use infinitesimal ones. Two kinds of
variational principles are taken into account; the first one features
non-gauge-invariant actions whereas the second includes fully gauge-invariant
actions. Furthermore, it is shown that it is possible to rewrite fully
gauge-invariant actions featuring first class constraints quadratic in the
momenta into first class constraints linear in the momenta (and homogeneous in
some cases) due to the full gauge invariance of their actions. This shows that
the gauge symmetry present in generally covariant theories having first class
constraints quadratic in the momenta is not of a different kind with respect to
the one of theories with first class constraints linear in the momenta if fully
gauge-invariant actions are taken into account for the former theories. These
ideas are implemented for the parametrized relativistic free particle,
parametrized harmonic oscillator, and the SL(2,R) model.


title: Gravitational waves from rotating neutron stars

abstract:
  In this review we examine the dynamics and gravitational wave detectability
of rotating strained neutron stars. The discussion is divided into two halves:
triaxial stars, and precessing stars. We summarise recent work on how crustal
strains and magnetic fields can sustain triaxiality, and suggest that Magnus
forces connected with pinned superfluid vortices might contribute to
deformation also. The conclusions that could be drawn following the successful
gravitational wave detection of a triaxial star are discussed, and areas
requiring further study identified. The latest ideas regarding free precession
are then outlined, and the recent suggestion of Middleditch et al (2000a,b)
that the remnant of SN1987A contains a freely precessing star, spinning-down by
gravitational wave energy loss, is examined critically. We describe what we
would learn about neutron stars should the gravitational wave detectors prove
this hypothesis to be correct.


title:
An Online Interactive Geometric Database: Including Exact Solutions of
  Einstein's Field Equations

abstract:
  We describe a new interactive database (GRDB) of geometric objects in the
general area of differential geometry. Database objects include, but are not
restricted to, exact solutions of Einstein's field equations. GRDB is designed
for researchers (and teachers) in applied mathematics, physics and related
fields. The flexible search environment allows the database to be useful over a
wide spectrum of interests, for example, from practical considerations of
neutron star models in astrophysics to abstract space-time classification
schemes. The database is built using a modular and object-oriented design and
uses several Java technologies (e.g. Applets, Servlets, JDBC). These are
platform-independent and well adapted for applications developed to run over
the World Wide Web. GRDB is accompanied by a virtual calculator (GRTensorJ), a
graphical user interface to the computer algebra system GRTensorII used to
perform online coordinate, tetrad or basis calculations. The highly interactive
nature of GRDB allows for systematic internal self-checking and a minimization
of the required internal records.


title: Torsion Gravity Effects on Charged-Particles and Neutron Interferometers

abstract:
  Torsion gravitational effects in the quantum interference of charged
particles are investigated. The influence of axial torsion in the
Schiff-Banhill effect (SB) inside a metallic shell is given. The effect of
torsion on the surface of the earth on (SB) experiment is estimated. Torsion
gravity effects on the Sagnac phase-shift of neutron interferometry are also
computed.


title: Adiabatic models of the cosmological radiative era

abstract:
  We consider a generalization of the Lemaitre-Tolman-Bondi (LTB) solutions by
keeping the LTB metric but replacing its dust matter source by an imperfect
fluid with anisotropic pressure $\Pi_{ab} $. Assuming that total matter-energy
density $\rho$ is the sum of a rest mass term, $\rhom$, plus a radiation
$\rhor=3p$ density where $p$ is the isotropic pressure, Einstein's equations
are fully integrated without having to place any previous assumption on the
form of $\Pi_{ab} $. Three particular cases of interest are contained: the
usual LTB dust solutions (the dust limit), a class of FLRW cosmologies (the
homogeneous limit) and of the Vaydia solution (the vacuum limit). Initial
conditions are provided in terms of suitable averages and contrast functions of
the initial densities of $\rhom, \rhor$ and the 3-dimensional Ricci scalar
along an arbitrary initial surface $t=t_i$. We consider the source of the
models as an interactive radiation-matter mixture in local thermal equilibrium
that must be consistent with causal Extended Irreversible Thermodynamics (hence
$\Pi_{ab} $ is shear viscosity). Assuming near equilibrium conditions
associated with small initial density and curvature contrasts, the evolution of
the models is qualitatively similar to that of adiabatic perturbations on a
matter plus radiation FLRW background. We show that initial conditions exist
that lead to thermodynamically consistent models, but only for the full
transport equation of Extended Irreversible Thermodynamics. These interactive
mixtures provide a reasonable approximation to a dissipative 'tight coupling'
characteristic of radiation-matter mixtures in the radiative pre-decoupling
era.


title: Equivalence Between Space-Time-Matter and Brane-World Theories

abstract:
  We study the relationship between space-time-matter (STM) and brane theories.
These two theories look very different at first sight, and have different
motivation for the introduction of a large extra dimension. However, we show
that they are equivalent to each other. First we demonstrate that STM predicts
local and non-local high-energy corrections to general relativity in 4D, which
are identical to those predicted by brane-world models. Secondly, we notice
that in brane models the usual matter in 4D is a consequence of the dependence
of five-dimensional metrics on the extra coordinate. If the 5D bulk metric is
independent of the extra dimension, then the brane is void of matter. Thus, in
brane theory matter and geometry are unified, which is exactly the paradigm
proposed in STM. Consequently, these two 5D theories share the same concepts
and predict the same physics. This is important not only from a theoretical
point of view, but also in practice. We propose to use a combination of both
methods to alleviate the difficult task of finding solutions on the brane. We
show an explicit example that illustrate the feasibility of our proposal.


title: Symmetries of spacetime and their relation to initial value problems

abstract:
  We consider covariant metric theories of coupled gravity-matter systems
satisfying the following two conditions: First, it is assumed that, by a
hyperbolic reduction process, a system of first order symmetric hyperbolic
partial differential equations can be deduced from the matter field equations.
Second, gravity is supposed to be coupled to the matter fields by requiring
that the Ricci tensor is a smooth function of the basic matter field variables
and the metric. It is shown then that the ``time'' evolution of these type of
gravity-matter systems preserves the symmetries of initial data specifications.


title:
Conservation laws for collisions of branes (or shells) in general
  relativity

abstract:
  We consider the collision of self-gravitating n-branes in a (n+2)-dimensional
spacetime. We show that there is a geometrical constraint which can be
expressed as a simple sum rule for angles characterizing Lorentz boosts between
branes and the intervening spacetime regions. This constraint can then be
re-interpreted as either energy or momentum conservation at the collision.


title:
A cosmological model in Weyl-Cartan spacetime: I. Field equations and
  solutions

abstract:
  In this first article of a series on alternative cosmological models we
present an extended version of a cosmological model in Weyl-Cartan spacetime.
The new model can be viewed as a generalization of a model developed earlier
jointly with Tresguerres. Within this model the non-Riemannian quantities, i.e.
torsion $T^{\alpha}$ and nonmetricity $Q_{\alpha \beta}$, are proportional to
the Weyl 1-form. The hypermomentum $\Delta_{\alpha \beta}$ depends on our
ansatz for the nonmetricity and vice versa. We derive the explicit form of the
field equations for different cases and provide solutions for a broad class of
parameters. We demonstrate that it is possible to construct models in which the
non-Riemannian quantities die out with time. We show how our model fits into
the more general framework of metric-affine gravity (MAG).


title:
On a description of Gravitational Interaction between moving objects in
  terms of Spatial Gravitational Fields

abstract:
  Analyzing two simple experimental situations we show that from Newton's law
of gravitation and Special Relativity it follows that the motion of particle in
an external gravitational field can be described in terms of effective spatial
fields which satisfy Maxwell-like system of equations and propagate with the
speed of light. The description is adequate in a linear approximation in
gravitational field and in a first order in v^2/c^2.


title: Spacetime states and covariant quantum theory

abstract:
  In it's usual presentation, classical mechanics appears to give time a very
special role. But it is well known that mechanics can be formulated so as to
treat the time variable on the same footing as the other variables in the
extended configuration space. Such covariant formulations are natural for
relativistic gravitational systems, where general covariance conflicts with the
notion of a preferred physical-time variable. The standard presentation of
quantum mechanics, in turns, gives again time a very special role, raising well
known difficulties for quantum gravity. Is there a covariant form of
(canonical) quantum mechanics? We observe that the preferred role of time in
quantum theory is the consequence of an idealization: that measurements are
instantaneous. Canonical quantum theory can be given a covariant form by
dropping this idealization. States prepared by non-instantaneous measurements
are described by "spacetime smeared states". The theory can be formulated in
terms of these states, without making any reference to a special time variable.
The quantum dynamics is expressed in terms of the propagator, an object
covariantly defined on the extended configuration space.


title: Non strict and strict hyperbolic systems for the Einstein equations

abstract:
  The integration of the Einstein equations split into the solution of
constraints on an initial space like 3 - manifold, an essentially elliptic
system, and a system which will describe the dynamical evolution, modulo a
choice of gauge. We prove in this paper that the simplest gauge choice leads to
a system which is causal, but hyperbolic non strict in the sense of Leray -
Ohya. We review some strictly hyperbolic systems obtained recently.


title:
Fundamental Disagreements of Experiments with Theories on Gravitation
  and Cosmology

abstract:
  From the equivalence principle (EP) and experiments on gravitational (G) time
dilation (GTD) it is proved that the standards of observers located in
different ``distances'' from the earth are physically different with respect to
each other. Thus the current mathematical relationships between their
measurements are physically inhomogeneous. This has caused fundamental errors
in gravitation, cosmology and astrophysics. The true transformations between
basic parameters of bodies located in different field positions, derived from
just experimental facts, are used to test fundamental hypotheses in current
literature. G fields do not exchange energy with bodies and radiation, but just
momentum. The G energy comes the bodies. The average relative distances and
cosmological redshifts in the universe cannot change after universe expansion
because the average increase of distances (G potentials) would change the sizes
of particles in identical proportion. Locally, atoms must be evolving,
indefinitely, in closed cycles between states of gas and linear black hole. The
last ones, after recovering energy, must explode thus regenerating gas. Most of
the universe must be in state of black galaxy cooled down by linear black
holes. They must account for the CMBR. The new scenarios are also explained by
using particle models consistent with the EP.


title: Non-relativistic quantum systems on topological defects space-times

abstract:
  We study the behavior of non-relativistic quantum particles interacting with
different potentials in the space-times generated by a cosmic string and a
global monopole. We find the energy spectra in the presence of these
topological defects and show how they differ from their free space-time values.


title: Decoherent histories on graphs

abstract:
  The consistent histories approach to quantum mechanics is traditionally based
on linearly ordered sequences of events. We extend the histories formalism to
sets of events whose causal ordering is described by directed acyclic graphs.
The need for a global time is eliminated and our construction reflects the
causal structure faithfully.


title:
Naked Singularity, Black-hole and the Mass Loss in a Spherically
  Symmetric Gravitational Collapse

abstract:
  A distinguishable physical property between a naked singularity and a
black-hole, formed during a gravitational collapse has important implications
for both experimental and theoretical relativity. We examine the energy
radiated during the spherically symmetric Gravitational collapse in this
context within the framework of general relativity. It is shown that total
energy radiated(Mass Loss) during the collapse ending in a naked singularity
scenario cannot be more than the case when a collapse scenario ends in a
black-hole. In cases of interest(for example stars having same mass, size, and
internal composition) considered here the total energy released in the collapse
ending in a black hole can be considerably more than the case otherwise.


title: Spin Foam Diagrammatics and Topological Invariance

abstract:
  We provide a simple proof of the topological invariance of the Turaev-Viro
model (corresponding to simplicial 3d pure Euclidean gravity with cosmological
constant) by means of a novel diagrammatic formulation of the state sum models
for quantum BF-theories. Moreover, we prove the invariance under more general
conditions allowing the state sum to be defined on arbitrary cellular
decompositions of the underlying manifold. Invariance is governed by a set of
identities corresponding to local gluing and rearrangement of cells in the
complex. Due to the fully algebraic nature of these identities our results
extend to a vast class of quantum groups. The techniques introduced here could
be relevant for investigating the scaling properties of non-topological state
sums, being proposed as models of quantum gravity in 4d, under refinement of
the cellular decomposition.


title: Gauge stability of 3+1 formulations of General Relativity

abstract:
  We present a general approach to the analysis of gauge stability of 3+1
formulations of General Relativity (GR). Evolution of coordinate perturbations
and the corresponding perturbations of lapse and shift can be described by a
system of eight quasi-linear partial differential equations. Stability with
respect to gauge perturbations depends on a choice of gauge and a background
metric, but it does not depend on a particular form of a 3+1 system if its
constrained solutions are equivalent to those of the Einstein equations.
Stability of a number of known gauges is investigated in the limit of
short-wavelength perturbations. All fixed gauges except a synchronous gauge are
found to be ill-posed. A maximal slicing gauge and its parabolic extension are
shown to be ill-posed as well. A necessary condition is derived for
well-posedness of metric-dependent algebraic gauges. Well-posed
metric-dependent gauges are found, however, to be generally unstable. Both
instability and ill-posedness are associated with perturbations of physical
accelerations of reference frames.


title: Gravitational radiation from the r-mode instability

abstract:
  The instability in the r-modes of rotating neutron stars can (in principle)
emit substantial amounts of gravitational radiation (GR) which might be
detectable by LIGO and similar detectors. Estimates are given here of the
detectability of this GR based the non-linear simulations of the r-mode
instability by Lindblom, Tohline and Vallisneri. The burst of GR produced by
the instability in the rapidly rotating 1.4 solar mass neutron star in this
simulation is fairly monochromatic with frequency near 960 Hz and duration
about 100 s. A simple analytical expression is derived here for the optimal S/N
for detecting the GR from this type of source. For an object located at a
distance of 20 Mpc we estimate the optimal S/N to be in the range 1.2 to about
12.0 depending on the LIGO II configuration.