title: The Post-Newtonian Treatment of the VLBI Experiment on September 8, 2002

abstract:
  Gravitational physics of VLBI experiment conducted on September 8, 2002 and
dedicated to measure the speed of gravity (a fundamental constant in the
Einstein equations) is treated in the first post-Newtonian approximation.
Explicit speed-of-gravity parameterization is introduced to the Einstein
equations to single out the retardation effect caused by the finite speed of
gravity in the relativistic time delay of light, passing through the variable
gravitational field of the solar system. The speed-of-gravity 1.5
post-Newtonian correction to the Shapiro time delay is derived and compared
with our previous result obtained by making use of the post-Minkowskian
approximation. We confirm that the 1.5 post-Newtonian correction to the Shapiro
delay depends on the speed of gravity $c_g$ that is a directly measurable
parameter in the VLBI experiment.


title: An Optical Analog of a Black Holes

abstract:
  Using media with extremely low group velocities one can create an optical
analog of a curved space-time. Leonhardt and Piwnicki have proposed that a
vortex flow will act as an optical black hole. We show that although the
Leonhardt - Piwnicki flow has an orbit of no return and an infinite red-shift
surface, it is not a true black hole since it lacks a null hypersurface.
However a radial flow will produce a true optical black hole that has a Hawking
temperature and obeys the first law of black hole mechanics. By combining the
Leonhardt - Piwnicki flow with a radial flow we obtain the analog of the Kerr
black hole.


title: Back reaction of the neutrino field in an Einstein universe

abstract:
  The back reaction effect of the neutrino field at finite temperature in the
background of the static Einstein universe is investigated. A relationship
between the temperature of the universe and its radius is found. As in the
previously studied cases of the massless scalar field and the photon field,
this relation exhibit a minimum radius below which no self-consistent solution
for the Einstein field equation can be found. A maximum temperature marks the
transition from a vacuum dominated state to the radiation dominated state
universe. In the light of the results obtained for the scalar, neutrino and
photon fields the role of the back reaction of quantum fields in controling the
value of the cosmological constant is briefly discussed.


title: Relativistic shells: Dynamics, horizons, and shell crossing

abstract:
  We consider the dynamics of timelike spherical thin matter shells in vacuum.
A general formalism for thin shells matching two arbitrary spherical spacetimes
is derived, and subsequently specialized to the vacuum case. We first examine
the relative motion of two dust shells by focusing on the dynamics of the
exterior shell, whereby the problem is reduced to that of a single shell with
different active Schwarzschild masses on each side. We then examine the
dynamics of shells with non-vanishing tangential pressure $p$, and show that
there are no stable--stationary, or otherwise--solutions for configurations
with a strictly linear barotropic equation of state, $p=\alpha\sigma$, where
$\sigma$ is the proper surface energy density and $\alpha\in(-1,1)$. For {\em
arbitrary} equations of state, we show that, provided the weak energy condition
holds, the strong energy condition is necessary and sufficient for stability.
We examine in detail the formation of trapped surfaces, and show explicitly
that a thin boundary layer causes the apparent horizon to evolve
discontinuously. Finally, we derive an analytical (necessary and sufficient)
condition for neighboring shells to cross, and compare the discrete shell model
with the well-known continuous Lema\^{\i}tre-Tolman-Bondi dust case.


title:
Unpolarized radiative cylindrical spacetimes: Trapped surfaces and
  quasilocal energy

abstract:
  We consider the most general vacuum cylindrical spacetimes, which are defined
by two global, spacelike, commuting, non-hypersurface-orthogonal Killing vector
fields. The cylindrical waves in such spacetimes contain both + and $\times$
polarizations, and are thus said to be unpolarized. We show that there are no
trapped cylinders in the spacetime, and present a formal derivation of Thorne's
C-energy, based on a Hamiltonian reduction approach. Using the Brown-York
quasilocal energy prescription, we compute the actual physical energy (per unit
Killing length) of the system, which corresponds to the value of the
Hamiltonian that generates unit proper-time translations orthogonal to a given
fixed spatial boundary. The C-energy turns out to be a monotonic non-polynomial
function of the Brown-York quasilocal energy. Finally, we show that the
Brown-York energy at spatial infinity is related to an asymptotic deficit angle
in exactly the same manner as the specific mass of a straight cosmic string is
to the former.


title: On Quasinormal Modes, Black Hole Entropy, and Quantum Geometry

abstract:
  Loop quantum gravity can account for the Bekenstein-Hawking entropy of a
black hole provided a free parameter is chosen appropriately. Recently, it was
proposed that a new choice of the Immirzi parameter could predict both black
hole entropy and the frequencies of quasinormal modes in the large $n$ limit,
but at the price of changing the gauge group of the theory. In this note we use
a simple physical argument within loop quantum gravity to arrive at the same
value of the parameter. The argument uses strongly the necessity of having
fermions satisfying basic symmetry and conservation principles, and therefore
supports SU(2) as the relevant gauge group of the theory.


title: Dirac Monopoles and Hawking radiation in Kottler spacetime

abstract:
  The natural extension of Schwarzschild metric to the case of nonzero
cosmological constant $\Lambda$ known as the Kottler metric is considered and
it is discussed under what circumstances the given metric could describe the
Schwarzschild black hole immersed in a medium with nonzero energy density.
Under the latter situation such an object might carry topologically
inequivalent configurations of various fields. The given possibility is
analysed for complex scalar field and it is shown that the mentioned
configurations might be tied with natural presence of Dirac monopoles on black
hole under consideration. In turn, this could markedly modify the Hawking
radiation process.


title:
Operational indistinguishabilty of doubly special relativities from
  special relativity

abstract:
  We argue that existing doubly special relativities may not be operationally
distinguishable from the special relativity. In the process we point out that
some of the phenomenologically motivated modifications of dispersion relations,
and arrived conclusions, must be reconsidered. Finally, we reflect on the
possible conceptual issues that arise in quest for a theory of spacetime with
two invariant scales.


title: Matching of the continuous gravitational wave in an all sky search

abstract:
  We investigate the matching of continuous gravitational wave (CGW) signals in
an all sky search with reference to Earth based laser interferometric
detectors. We consider the source location as the parameters of the signal
manifold and templates corresponding to different source locations. It has been
found that the matching of signals from locations in the sky that differ in
their co-latitude and longitude by $\pi$ radians decreases with source
frequency. We have also made an analysis with the other parameters affecting
the symmetries. We observe that it may not be relevant to take care of the
symmetries in the sky locations for the search of CGW from the output of
LIGO-I, GEO600 and TAMA detectors.


title:
Action principle formulation for motion of extended bodies in General
  Relativity

abstract:
  We present an action principle formulation for the study of motion of an
extended body in General Relativity in the limit of weak gravitational field.
This gives the classical equations of motion for multipole moments of arbitrary
order coupling to the gravitational field. In particular, a new force due to
the octupole moment is obtained. The action also yields the gravitationally
induced phase shifts in quantum interference experiments due to the coupling of
all multipole moments.


title: The Challenging Cosmic Singularity

abstract:
  In this talk we discuss the cosmic singularity. We motivate the need to
correct general relativity in the study of singularities, and mention the kind
of corrections provided by string theory. We review how string theory resolves
time-like singularities with two examples. Then, a simple toy model with
lightlike singularities is presented, and studied in classical string theory.
It turns out that classical string theory cannot resolve these singularities,
and therefore better understanding of the full quantum theory is needed. The
implications of this result for the Ekpyrotic/Cyclic Model are discussed. We
end by mentioning the known suggestions for explaining the cosmological
singularity.


title: Black holes as a source of neutrino asymmetry in Universe

abstract:
  Propagation of fermions in curved space-time generates gravitational
interaction due to coupling of its spin with space-time curvature connection.
This gravitational interaction, which is an axial-four-vector multiplied by a
four gravitational vector potential, appears as CPT violating term in the
Lagrangian which generates an opposite sign and thus asymmetry between the
left-handed and right handed partners under CPT transformation. In the case of
neutrinos this property can generate neutrino asymmetry in the Universe. If the
background metric is of rotating black hole, Kerr geometry, this interaction
for neutrino is non-zero. Therefore the dispersion energy relation for neutrino
and its anti-neutrino are different which gives rise to the difference in their
number densities and neutrino asymmetry in the Universe in addition to the
known relic asymmetry.


title: Traversable wormholes with arbitrarily small energy condition violations

abstract:
  Traversable wormholes necessarily require violations of the averaged null
energy condition; this being the definition of ``exotic matter''. However, the
theorems which guarantee the energy condition violation are remarkably silent
when it comes to making quantitative statements regarding the ``total amount''
of energy condition violating matter in the spacetime. We develop a suitable
measure for quantifying this notion, and demonstrate the existence of spacetime
geometries containing traversable wormholes that are supported by arbitrarily
small quantities of ``exotic matter''.


title:
Duality of Quasilocal Gravitational Energy and Charges with
  Non-orthogonal Boundaries

abstract:
  We study the duality of quasilocal energy and charges with non-orthogonal
boundaries in the (2+1)-dimensional low-energy string theory. Quasilocal
quantities shown in the previous work and some new variables arisen from
considering the non-orthogonal boundaries as well are presented, and the boost
relations between those quantities are discussed. Moreover, we show that the
dual properties of quasilocal variables such as quasilocal energy density,
momentum densities, surface stress densities, dilaton pressure densities, and
Neuve-Schwarz(NS) charge density, are still valid in the moving observer's
frame.


title:
Characteristic initial value problems for integrable hyperbolic
  reductions of Einstein's equations

abstract:
  A unified general approach is presented for construction of solutions of the
characteristic initial value problems for various integrable hyperbolic
reductions of Einstein's equations for space-times with two commuting
isometries in General Relativity and in some string theory induced gravity
models. In all cases the associated linear systems of similar structures are
used, and their fundamental solutions admit an alternative representations by
two ``scattering'' matrices of a simple analytical structures on the spectral
plane. The condition of equivalence of these representations leads to the
linear ``integral evolution equations'' whose scalar kernels and right hand
sides are determined completely by the initial data for the fields specified on
the two initial characteristics. If the initial data for the fields are given,
all field components of the corresponding solution can be expressed in
quadratures in terms of a unique solution of these quasi - Fredholm integral
evolution equations.


title: An Axisymmetric Gravitational Collapse Code

abstract:
  We present a new numerical code designed to solve the Einstein field
equations for axisymmetric spacetimes. The long term goal of this project is to
construct a code that will be capable of studying many problems of interest in
axisymmetry, including gravitational collapse, critical phenomena,
investigations of cosmic censorship, and head-on black hole collisions. Our
objective here is to detail the (2+1)+1 formalism we use to arrive at the
corresponding system of equations and the numerical methods we use to solve
them. We are able to obtain stable evolution, despite the singular nature of
the coordinate system on the axis, by enforcing appropriate regularity
conditions on all variables and by adding numerical dissipation to hyperbolic
equations.


title:
Classical Boundary-value Problem in Riemannian Quantum Gravity and
  Taub-Bolt-anti-de Sitter Geometries

abstract:
  For an $SU(2)\times U(1)$-invariant $S^3$ boundary the classical Dirichlet
problem of Riemannian quantum gravity is studied for positive-definite regular
solutions of the Einstein equations with a negative cosmological constant
within biaxial Bianchi-IX metrics containing bolts, i.e., within the family of
Taub-Bolt-anti-de Sitter (Taub-Bolt-AdS) metrics. Such metrics are obtained
from the two-parameter Taub-NUT-anti-de Sitter family. The condition of
regularity requires them to have only one free parameter ($L$) and constrains
$L$ to take values within a narrow range; the other parameter is determined as
a double-valued function of $L$ and hence there is a bifurcation within the
family. We found that {\it{any}} axially symmetric $S^3$-boundary can be filled
in with at least one solution coming from each of these two branches despite
the severe limit on the permissible values of $L$. The number of infilling
solutions can be one, three or five and they appear or disappear
catastrophically in pairs as the values of the two radii of $S^3$ are varied.
The solutions occur simultaneously in both branches and hence the total number
of independent infillings is two, six or ten. We further showed that when the
two radii are of the same order and large the number of solutions is two. In
the isotropic limit this holds for small radii as well. These results are to be
contrasted with the one-parameter self-dual Taub-NUT-AdS infilling solutions of
the same boundary-value problem studied previously.


title: Stability of the Vacuum Non Singular Black Hole

abstract:
  The singularity of the black hole solutions obtained before in M{\o}ller's
theory are studied. It is found that although the two solutions reproduce the
same associated metric the asymptotic behavior of the scalars of torsion tensor
and basic vector are quite different. The stability of the associated metric of
those solutions which is spherically symmetric non singular black hole is
studied using the equations of geodesic deviation. The condition for the
stability is obtained. From this condition the stability of the Schwarzschild
solution and di Sitter solution can be obtained.


title:
Finite-temperature scalar fields and the cosmological constant in an
  Einstein universe

abstract:
  We study the back reaction effect of massless minimally coupled scalar field
at finite temperatures in the background of Einstein universe. Substituting for
the vacuum expectation value of the components of the energy-momentum tensor on
the RHS of the Einstein equation, we deduce a relationship between the radius
of the universe and its temperature. This relationship exhibit a maximum
temperature, below the Planck scale, at which the system changes its behaviour
drastically. The results are compared with the case of a conformally coupled
field. An investigation into the values of the cosmological constant exhibit a
remarkable difference between the conformally coupled case and the minimally
coupled one.


title: Generation of dark radiation in the bulk inflaton model

abstract:
  We investigate the dynamics of a bulk scalar field with various decay
channels in the Randall-Sundrum infinite braneworld scenario. A bulk scalar
field in this scenario has a quasi-localized mode which dominates the late-time
behavior near the brane. As for this mode, an interesting point is the presence
of dissipation caused by the escape of the energy in the direction away from
the brane, even if the bulk scalar field does not have the interaction with the
other bulk fields in the bulk and fields on the brane. We can interpret that
this lost energy is transfered to the dark radiation. We show that such an
effective 4-dimensional description for a bulk scalar field is valid including
the various processes of energy dissipation.


title: Head-on/Near Head-on Collisions of Neutron Stars With a Realistic EOS

abstract:
  It has been conjectured that in head-on collisions of neutron stars (NSs),
the merged object would not collapse promptly even if the total mass is higher
than the maximum stable mass of a cold NS. In this paper, we show that the
reverse is true: even if the total mass is {\it less} than the maximum stable
mass, the merged object can collapse promptly. We demonstrate this for the case
of NSs with a realistic equation of state (the Lattimer-Swesty EOS) in head-on
{\it and} near head-on collisions. We propose a ``Prompt Collapse Conjecture''
for a generic NS EOS for head on and near head-on collisions.


title: Extrinsic Curvature Embedding Diagrams

abstract:
  Embedding diagrams have been used extensively to visualize the properties of
curved space in Relativity. We introduce a new kind of embedding diagram based
on the {\it extrinsic} curvature (instead of the intrinsic curvature). Such an
extrinsic curvature embedding diagram, when used together with the usual kind
of intrinsic curvature embedding diagram, carries the information of how a
surface is {\it embedded} in the higher dimensional curved space. Simple
examples are given to illustrate the idea.


title: On non-Riemannian geometry of superfluids

abstract:
  The Gross-Pitaevski (GP) equation describing helium superfluids is extended
to non-Riemannian spacetime background where torsion is shown to induce the
splitting in the potential energy of the flow. A cylindrically symmetric
solution for Minkowski background with constant torsion is obtained which shows
that torsion induces a damping on the superfluid flow velocity. The Sagnac
phase shift is computed from the superfluid flow velocity obtained from the
solution of GP equations.


title: Nonlinear r-Modes in Neutron Stars: Instability of an unstable mode

abstract:
  We study the dynamical evolution of a large amplitude r-mode by numerical
simulations. R-modes in neutron stars are unstable growing modes, driven by
gravitational radiation reaction. In these simulations, r-modes of amplitude
unity or above are destroyed by a catastrophic decay: A large amplitude r-mode
gradually leaks energy into other fluid modes, which in turn act nonlinearly
with the r-mode, leading to the onset of the rapid decay. As a result the
r-mode suddenly breaks down into a differentially rotating configuration. The
catastrophic decay does not appear to be related to shock waves at the star's
surface. The limit it imposes on the r-mode amplitude is significantly smaller
than that suggested by previous fully nonlinear numerical simulations.


title: Atomic equivalence violations via a massive and massless dilaton

abstract:
  This paper has being withdrawn by the authors due to an error in the
conclusion.


title:
Particle and light motion in a space-time of a five-dimensional rotating
  black hole

abstract:
  We study motion of particles and light in a space-time of a 5-dimensional
rotating black hole. We demonstrate that the Myers-Perry metric describing such
a black hole in addition to three Killing vectors possesses also a Killing
tensor. As a result, the Hamilton-Jacobi equations of motion allow a separation
of variables. Using first integrals we present the equations of motion in the
first-order form. We describe different types of motion of particles and light
and study some interesting special cases. We proved that there are no stable
circular orbits in equatorial planes in the background of this metric.


title: A More Sensitive Lorentzian State Sum

abstract:
  We give the construction modulo normalization of a new state sum model for
lorentzian quantum general relativity, using the construction of Dirac's
expansors to include quantum operators corresponding to edge lengths as well as
the quantum bivectors of the Barrett-Crane model, and discuss the problem of
its normalization. The new model gives rise to a new picture of quantum
geometry in which lengths come in a discrete spectrum, while areas have a
continuum of values.


title: Unconventional Gravitational Excitation of a Schwarzschild Black Hole

abstract:
  Besides the well-known quasinormal modes, the gravitational spectrum of a
Schwarzschild black hole also has a continuum part on the negative imaginary
frequency axis. The latter is studied numerically for quadrupole waves. The
results show unexpected striking behavior near the algebraically special
frequency $\Omega=-4i$. This reveals a pair of unconventional damped modes very
near $\Omega$, confirmed analytically.


title:
Ellipsoidal shapes in general relativity: general definitions and an
  application

abstract:
  A generalization of the notion of ellipsoids to curved Riemannian spaces is
given and the possibility to use it in describing the shapes of rotating bodies
in general relativity is examined. As an illustrative example, stationary,
axisymmetric perfect-fluid spacetimes with a so-called confocal inside
ellipsoidal symmetry are investigated in detail under the assumption that the
4-velocity of the fluid is parallel to a time-like Killing vector field. A
class of perfect-fluid metrics representing interior NUT-spacetimes is obtained
along with a vacuum solution with a non-zero cosmological constant.


title:
Vacuum Einstein metrics with bidimensional Killing leaves. I-Local
  aspects

abstract:
  The solutions of vacuum Einstein's field equations, for the class of
Riemannian metrics admitting a non Abelian bidimensional Lie algebra of Killing
fields, are explicitly described. They are parametrized either by solutions of
a transcendental equation (the tortoise equation), or by solutions of a linear
second order differential equation in two independent variables. Metrics,
corresponding to solutions of the tortoise equation, are characterized as those
that admit a 3-dimensional Lie algebra of Killing fields with bidimensional
leaves.


title:
Vacuum Einstein metrics with bidimensional Killing leaves.II-Global
  aspects

abstract:
  A formalism (zeta-complex analysis), allowing one to construct global
Einstein metrics by matching together local ones described in the papers Phys.
Lett. B 513(2001)142-146; Diff. Geom. Appl. 16(2002)95-120, is developed. With
this formalism the singularities of the obtained metrics are described
naturally as well.


title: A generalisation of the Heckmann - Schucking cosmological solution

abstract:
  An exact solution of the Einstein equations for a Bianchi -I universe in the
presence of dust, stiff matter and cosmological constant, generalising the
well-known Heckmann-Schucking solution is presented. PACS: 04.20-q; 04.20.Dw
Keywords: Exact cosmological solutions


title: Remarks on the Entropy of Non-Stationary Black Holes

abstract:
  The definition of entropy obtained for stationary black holes is extended in
this paper to the case of non-stationary black holes. Entropy is defined as a
macroscopical thermodynamical quantity which satisfies the first principle of
thermodynamics. In the non-stationary case a volume term appears since the
solution does not admit a Killing vector.


title: Lunar Laser Ranging - a comprehensive probe of post-Newtonian gravity

abstract:
  More than 30 years of lunar laser ranging has produced several key tests of
gravitational theory, including confirmation that bodies fall in external
gravity at rates independent of their internal gravitational binding energy,
and that Newton's G is constant to a part in 10^12 per year precision. The
fitting of LLR data depends on the entire scope of 1/c^2 order features of the
gravitational equation of motion, including non-linearity, gravitomagnetism,
and inductive inertial forces.


title:
A faster implementation of the hierarchical search algorithm for
  detection of gravitational waves from inspiraling compact binaries

abstract:
  The first scientific runs of kilometer scale laser interferometric detectors
like LIGO are underway. Data from these detectors will be used to look for
signatures of gravitational waves (GW) from astrophysical objects like
inspiraling neutron star/blackhole binaries using matched filtering. The
computational resources required for online flat-search implementation of the
matched filtering are large if searches are carried out for small total mass.
In this paper we report an improved implementation of the hierarchical search,
wherein we extend the domain of hierarchy to an extra dimension - namely the
time of arrival of the signal in the bandwidth of the interferometer. This is
accomplished by lowering the Nyquist sampling rate of the signal in the trigger
stage. We show that this leads to further improvement in the efficiency of data
analysis and speeds up the online computation by a factor of $\sim 65 - 70$
over the flat search. We also take into account and discuss issues related to
template placement, trigger thresholds and other peculiar problems that do not
arise in earlier implementation schemes of the hierarchical search. We present
simulation results for 2PN waveforms embedded in the noise expected for initial
LIGO detectors.


title: Viscous Cosmology and the Cardy-Verlinde Formula

abstract:
  The holographic principle in a radiation dominated universe is extended to
incorporate the case of a bulk-viscous cosmic fluid. This corresponds to a
nonconformally invariant theory. Generalization of the Cardy-Verlinde entropy
formula to the viscous case appears to be possible from a formal point of view,
although we question on physical grounds the manner in which the Casimir energy
is evaluated in this case. Also, we consider an observation recently made by
Youm, namely that the entropy of the universe is no longer expressible in the
conventional Cardy-Verlinde form if one relaxes the radiation dominance
equation of state and instead merely assumes that the pressure is proportional
to the energy density. We show that Youm's generalized entropy formula remains
valid when the cosmic fluid is no longer ideal, but endowed with a constant
bulk viscosity.


title: Brane and Hot Big Bang Cosmology

abstract: See hep-ph/0304045


title: Dynamical extensions for shell-crossing singularities

abstract:
  We derive global weak solutions of Einstein's equations for spherically
symmetric dust-filled space-times which admit shell-crossing singularities. In
the marginally bound case, the solutions are weak solutions of a conservation
law. In the non-marginally bound case, the equations are solved in a
generalized sense involving metric functions of bounded variation. The
solutions are not unique to the future of the shell-crossing singularity, which
is replaced by a shock wave in the present treatment; the metric is bounded but
not continuous.


title:
The problem of singularities of higher order curvature corrections in
  four dimensional string gravity

abstract:
  The influence of higher order (stringly inspired) curvature corrections to
the classical General Relativity spherically symmetric solution is studied. In
string gravity these curvature corrections have a special form and can provide
a singular contribution to the field equations because they generate higher
derivatives of metric functions multiplied by a small parameter. Analytically
and numerically it is shown that sometimes in 4D string gravity the
Schwarzschild solution is not recovered when the string coupling constant
vanishes and limited number of higher order curvature corrections is
considered.


title: Gravitational Lensing and Gravitomagnetic Time Delay

abstract:
  We derive the delay in travel time of photons due to the spin of a body both
inside a rotating shell and outside a rotating body. We then show that this
time delay by the spin of an astrophysical object might be detected in
different images of the same source by gravitational lensing; it might be
relevant in the determination of the Hubble constant using accurate
measurements of the time delay between the images of some gravitational lens
systems. The measurement of the spin-time-delay might also provide a further
observable to estimate the dark matter content in galaxies, clusters, or
super-clusters of galaxies.


title:
Motion of the mass centers in a scalar theory of gravitation: the point
  particle limit

abstract:
  We investigate the point-particle limit of the equations of motion valid for
a system of extended bodies in a scalar alternative theory of gravitation: the
size of one of the bodies being a small parameter xi, we calculate the limit,
as xi tends towards 0, of the post-Newtonian (PN) acceleration of this small
body. We use the asymptotic scheme of PN approximation, that expands all
fields. We find that the PN acceleration A of the small body keeps a
structure-dependent part at this limit. In particular, if the only massive body
is static and spherical, then A differs from the PN acceleration of a test
particle in a Schwarzschild field only by this structure-dependent part. The
presence of the latter is due to the fact that the PN metric depends on the
first spatial derivatives of the Newtonian potential. Since just the same form
of PN metric is valid for the standard form of Schwarzschild's solution, the
acceleration of a small body might keep a structure-dependent part at the point
limit in general relativity also, depending on the gauge. The magnitude of the
structure-dependent acceleration is already challenging on Earth. For the
Pioneer spacecrafts, it is likely to discard the current version of the scalar
theory. A modified version has been outlined in a quoted reference.
  Keywords: Weak equivalence principle violation, Asymptotic post-Newtonian
scheme


title: Phenomenology of the relic dilaton background

abstract:
  We discuss the expected amplitude of a cosmic background of massive,
non-relativistic dilatons, and we report recent results about its possible
detection. This paper is a contracted version of a talk given at the 15th
SIGRAV Conference on "General Relativity and Gravitational Physics" (Villa
Mondragone, Roma, September 2002).


title: Solar System test for the existence of gravitational waves

abstract:
  Starting from a static spherically symmetric solution of the Einstein's field
equations in the second approximation in the perfect fluid scheme, in the
exterior of the source, the corresponding metrics depend on a dimensionless
parameter $\alpha $ . Taking the Sun for the source, the influence of $\alpha $
on the classical Solar System tests of the general relativity is estimated, the
only $\alpha $-dependent one being the Shapiro radar-echo delay experiment.
Performing such a test in given conditions within a precision better than
$10^{-2}$, it is possible to obtain an experimental value for $\alpha $ . If
$\alpha =1$, the Einstein's equations in the weak field approximation take the
D'Alembert form, which attests the existence of gravitational waves


title:
Cover Schemes, Frame-Valued Sets and Their Potential Uses in Spacetime
  Physics

abstract:
  The immensely fruitful concept of Grothendieck topology or covering issued
from the efforts of algebraic geometers to study "sheaf-like" objects defined
on categories more general than the lattice of open sets on a topological
space. In the present paper the covering concept - here called a cover scheme
-is presented and developed in the simple case when the underlying category is
a preordered set. The relationship between cover schemes, frames (complete
Heyting algebras), Kripke models, and frame-valued set theory is discussed.
Finally cover schemes and frame-valued set theory are applied in the context of
Markopoulou's 1999 account of discrete spacetime as sets "evolving" over a
causal set.


title:
Cosmological perturbations of an expanding brane in an anti-de Sitter
  bulk: a short review

abstract:
  Since the Randall-Sundrum 1999 papers, braneworlds have been a favourite
playground to test string inspired cosmological models. The subject has
developped into two main directions : elaborating more complex models in order
to strenghten the connection with string theories, and trying to confront them
with observations, in particular the Cosmic Microwave Background anisotropies.
We review here the latter and see that, even in the simple, "paradigmatic",
case of a single expanding brane in a 5D anti-de Sitter bulk, there is still a
missing link between the "view from the brane" and the "view from the bulk"
which prevents definite predictions.


title:
Linearized gravity on branes: from Newton's law to cosmological
  perturbations

abstract:
  We review here how Newton's law can be approximately recovered in the simple,
"paradigmatic", case of a flat $Z_2$-symmetric brane in a 5-D anti-de Sitter
bulk. We then comment on the difficulties encountered so far in extending this
analysis to cosmological perturbations on a Robertson-Walker brane.


title: Elementary particles under the lens of the black holes

abstract:
  After a brief review of the historical development and CLASSICAL properties
of the BLACK HOLES, we discuss how our present knowledge of some of their
QUANTUM properties shed light on the very concept of ELEMENTARY PARTICLE. As an
illustration, we discuss in this context the decay of accelerated protons,
which may be also relevant to astrophysics.


title: Black hole entropy from classical Liouville theory

abstract:
  In this article we compute the black hole entropy by finding a classical
central charge of the Virasoro algebra of a Liouville theory using the Cardy
formula. This is done by performing a dimensional reduction of the Einstein
Hilbert action with the ansatz of spherical symmetry and writing the metric in
conformally flat form. We obtain two coupled field equations. Using the near
horizon approximation the field equation for the conformal factor decouples.
The one concerning the conformal factor is a Liouville equation, it posses the
symmetry induced by a Virasoro algebra. We argue that it describes the
microstates of the black hole, namely the generators of this symmetry do not
change the thermodynamical properties of the black hole.


title: Eternal expansion of closed universe

abstract:
  The expansion of the closed two-component universe has been considered. The
potential barrier of the expansion has been investigated and its overcoming
condition has been obtained. The restrictions on the Friedmann integrals,
cosmological constant and density parameters have been analyzed. The
phase-space has been considered and the phase curves of eternally expanding
closed universes have been plotted. A questionable coincidence of the our
Universe Friedmann integrals has been discussed.


title:
General Covariance and the Objectivity of Space-Time Point-Events: The
  Physical Role of Gravitational and Gauge Degrees of Freedom in General
  Relativity

abstract:
  (Abridged Abstract) This paper deals with a number of technical achievements
that are instrumental for a dis-solution of the so-called {\it Hole Argument}
in general relativity. The work is carried through in metric gravity for the
class of Christoudoulou-Klainermann space-times, in which the temporal
evolution is ruled by the {\it weak} ADM energy. The main results of the
investigation are the following: 1) A re-interpretation of {\it active}
diffeomorphisms as {\it passive and metric-dependent} dynamical symmetries of
Einstein's equations, a re-interpretation that discloses their (up to now
unknown) connection to gauge transformations on-shell; understanding such
connection also enlightens the real content of the {\it Hole Argument}. 2) The
utilization of Bergmann-Komar {\it intrinsic coordinates} for a peculiar
gauge-fixing to the superhamiltonian and supermomentum constraints which
embodies on shell a {\it physical individuation} of the mathematical points of
$M^4$ as point-events in terms of the non-local intrinsic degrees of freedom of
the gravitational field (Dirac observables). 3) A clarification of the notion
of {\it Bergmann observable} that leads to a main conjecture asserting the
existence of i) special Dirac observables which are also Bergmann observables;
ii) tensorial (scalar) gauge variables.


title:
Classification of Spherically Symmetric Static Spacetimes according to
  their Matter Collineations

abstract:
  The spherically symmetric static spacetimes are classified according to their
matter collineations. These are studied when the energy-momentum tensor is
degenerate and also when it is non-degenerate. We have found a case where the
energy-momentum tensor is degenerate but the group of matter collineations is
finite. For the non-degenerate case, we obtain either {\it four}, {\it five},
{\it six} or {\it ten} independent matter collineations in which four are
isometries and the rest are proper. We conclude that the matter collineations
coincide with the Ricci collineations but the constraint equations are
different which on solving can provide physically interesting cosmological
solutions.


title: Inflation without a beginning: a null boundary proposal

abstract:
  We develop our recent suggestion that inflation may be made past eternal, so
that there is no initial cosmological singularity or "beginning of time".
Inflation with multiple vacua generically approaches a steady-state statistical
distribution of regions at these vacua, and our model follows directly from
making this distribution hold at all times. We find that this corresponds (at
the semi-classical level) to particularly simple cosmological boundary
conditions on an infinite null surface near which the spacetime looks de
Sitter. The model admits an interesting arrow of time that is well-defined and
consistent for all physical observers that can communicate, even while the
statistical description of the entire universe admits a symmetry that includes
time-reversal. Our model suggests, but does not require, the identification of
antipodal points on the manifold. The resulting "elliptic" de Sitter spacetime
has interesting classical and quantum properties. The proposal may be
generalized to other inflationary potentials, or to boundary conditions that
give semi-eternal but non-singular cosmologies.


title:
What can the quantum liquid say on the brane black hole, the entropy of
  extremal black hole and the vacuum energy?

abstract:
  Using quantum liquids one can simulate the behavior of the quantum vacuum in
the presence of the event horizon. The condensed matter analogs demonstrate
that in most cases the quantum vacuum resists to formation of the horizon, and
even if the horizon is formed different types of the vacuum instability
develop, which are faster than the process of Hawking radiation. Nevertheless,
it is possible to create the horizon on the quantum-liquid analog of the brane,
where the vacuum life-time is long enough to consider the horizon as the
quasistationary object. Using this analogy we calculate the Bekenstein entropy
of the nearly extremal and extremal black holes, which comes from the fermionic
microstates in the region of the horizon -- the fermion zero modes. We also
discuss how the cancellation of the large cosmological constant follows from
the thermodynamics of the vacuum.


title:
Space-time correlations within pairs produced during inflation, a
  wave-packet analysis

abstract:
  In homogeneous universes the propagation of quantum fields gives rise to pair
creation of quanta with opposite momenta. When computing expectation values of
operators, the correlations between these quanta are averaged out and no
space-time structure is obtained. In this article, by an appropriate use of
wave packets, we reveal the space-time structure of these correlations. We show
that every pair emerges from vacuum configurations which are torn apart so as
to give rise to two semi-classical currents: that carried by the particle and
that of its `partner'. The partner's current lives behind the Hubble horizon
centered around the particle. Hence any measurement performed within a Hubble
patch would correspond to an uncorrelated density matrix, as for Hawking
radiation. However, when inflation stops, the Hubble radius grows and
eventually encompasses the partner. When this is realized the coherence is
recovered within a patch. Our analysis applies to rare pair creation events as
well as to cases leading to arbitrary high occupation numbers. Hence it might
be applied to primordial gravitational waves which evolve into highly squeezed
states.


title: The Classical Singularity Theorems and their Quantum Loopholes

abstract:
  The singularity theorems of classical general relativity are briefly
reviewed. The extent to which their conclusions might still apply when quantum
theory is taken into account is discussed. There are two distinct quantum
loopholes: quantum violation of the classical energy conditions, and the
presence of quantum fluctuations of the spacetime geometry. The possible
significance of each is discussed.


title: $\Delta-$string - a hybrid between wormhole and string

abstract:
  The flux tube solutions in 5D Kaluza-Klein theory can be considered as a
string-like object - $\Delta-$string. The initial 5D metric can be reduced to
some inner degrees of freedom living on the $\Delta-$string. The propagation of
electromagnetic waves through the $\Delta-$string is considered. It is shown
that the difference between $\Delta$ and ordinary strings are connected with
the fact that for the $\Delta-$string such limitations as critical dimensions
are missing.


title: Quantum Gravity Vacuum and Invariants of Embedded Spin Networks

abstract:
  We show that the path integral for the three-dimensional SU(2) BF theory with
a Wilson loop or a spin network function inserted can be understood as the
Rovelli-Smolin loop transform of a wavefunction in the Ashtekar connection
representation, where the wavefunction satisfies the constraints of quantum
general relativity with zero cosmological constant. This wavefunction is given
as a product of the delta functions of the SU(2) field strength and therefore
it can be naturally associated to a flat connection spacetime. The loop
transform can be defined rigorously via the quantum SU(2) group, as a spin foam
state sum model, so that one obtains invariants of spin networks embedded in a
three-manifold. These invariants define a flat connection vacuum state in the
q-deformed spin network basis. We then propose a modification of this
construction in order to obtain a vacuum state corresponding to the flat metric
spacetime.


title:
Thermal and back-action noises in dual-sphere gravitational-waves
  detectors

abstract:
  We study the sensitivity limits of a broadband gravitational-waves detector
based on dual resonators such as nested spheres. We determine both the thermal
and back-action noises when the resonators displacements are read-out with an
optomechanical sensor. We analyze the contributions of all mechanical modes,
using a new method to deal with the force-displacement transfer functions in
the intermediate frequency domain between the two gravitational-waves sensitive
modes associated with each resonator. This method gives an accurate estimate of
the mechanical response, together with an evaluation of the estimate error. We
show that very high sensitivities can be reached on a wide frequency band for
realistic parameters in the case of a dual-sphere detector.


title:
Templates for stellar mass black holes falling into supermassive black
  holes

abstract:
  The spin modulated gravitational wave signals, which we shall call smirches,
emitted by stellar mass black holes tumbling and inspiralling into massive
black holes have extremely complicated shapes. Tracking these signals with the
aid of pattern matching techniques, such as Wiener filtering, is likely to be
computationally an impossible exercise. In this article we propose using a
mixture of optimal and non-optimal methods to create a search hierarchy to ease
the computational burden. Furthermore, by employing the method of principal
components (also known as singular value decomposition) we explicitly
demonstrate that the effective dimensionality of the search parameter space of
smirches is likely to be just three or four, much smaller than what has
hitherto been thought to be about nine or ten. This result, based on a limited
study of the parameter space, should be confirmed by a more exhaustive study
over the parameter space as well as Monte-Carlo simulations to test the
predictions made in this paper.


title: Lorentz contraction and accelerated systems

abstract:
  The paper discusses the problem of the Lorentz contraction in accelerated
systems, in the context of the special theory of relativity. Equal proper
accelerations along different world lines are considered, showing the
differences arising when the world lines correspond to physically connected or
disconnected objects. In all cases the special theory of relativity proves to
be completely self-consistent


title: Asymptotic symmetries in 3d gravity with torsion

abstract:
  We study the nature of asymptotic symmetries in topological 3d gravity with
torsion. After introducing the concept of asymptotically anti-de Sitter
configuration, we find that the canonical realization of the asymptotic
symmetry is characterized by the Virasoro algebra with classical central
charge, the value of which is the same as in general relativity: c=3l/2G.


title:
Quasinormal modes of Reissner-Nordstr\"om-anti-de Sitter black holes:
  scalar, electromagnetic and gravitational perturbations

abstract:
  We study scalar, electromagnetic and gravitational perturbations of a
Reissner-Nordstr\"om-anti-de Sitter (RN-AdS) spacetime, and compute its
quasinormal modes (QNM's). We confirm and extend results previously found for
Schwarzschild-anti-de Sitter (S-AdS) black holes. For ``large'' black holes,
whose horizon is much larger than the AdS radius, different classes of
perturbations are almost exactly {\it isospectral}; this isospectrality is
broken when the black hole's horizon radius is comparable to the AdS radius. We
provide very accurate fitting formulas for the QNM's, which are valid for black
holes of any size and charge $Q<Q_{ext}/3$.
  Electromagnetic and axial perturbations of large black holes are
characterized by the existence of pure-imaginary (purely damped) modes. The
damping of these modes tends to infinity as the black hole charge approaches
the extremal value; if the corresponding mode amplitude does not tend to zero
in the same limit, this implies that {\it extremally charged RN-AdS black holes
are marginally unstable}. This result is relevant in view of the AdS/CFT
conjecture, since, according to it, the AdS QNM's give the timescales for
approach to equilibrium in the corresponding conformal field theory.


title: Newtonian limit of String-Dilaton Gravity

abstract:
  We study the weak-field limit of string-dilaton gravity and derive
corrections to the Newtonian potential which strength directly depends on the
self interaction potential and the nonminimal coupling of the dilaton scalar
field. We discuss also possible astrophysical applications of the results, in
particular the flat rotation curves of spiral galaxies.


title: A solution of the cosmological constant problem

abstract:
  General relativity can be cast as a gauge theory by introducing a tetrad
field and a spin-connection. This formalism was extended by replacing the
tetrad field with a mixed tensor field independent of the metric tensor in
order to develop a mechanism of adjustment of the vacuum energy density that
takes advantage of Weinberg's no-go theorem. With no anthropic considerations,
it was shown that the vacuum energy density is bounded and the gravitational
and cosmological constants are proportional to a tiny dimensionless parameter
determined by the coupling constants of the model.


title: Gravitomagnetism in the Kerr-Newman-Taub-NUT spacetime

abstract:
  We study the motion of test particles and electromagnetic waves in the
Kerr-Newman-Taub-NUT spacetime in order to elucidate some of the effects
associated with the gravitomagnetic monopole moment of the source. In
particular, we determine in the linear approximation the contribution of this
monopole to the gravitational time delay and the rotation of the plane of the
polarization of electromagnetic waves. Moreover, we consider "spherical" orbits
of uncharged test particles in the Kerr-Taub-NUT spacetime and discuss the
modification of the Wilkins orbits due to the presence of the gravitomagnetic
monopole.


title:
Gyroscope deviation from geodesic motion: quasiresonant oscillations on
  a circular orbit

abstract:
  General relativistic spin-orbit interaction leads to the quasiresonant
oscillation of the gyroscope mass center along the orbital normal. The beating
amplitude does not include the speed of light and equals the ratio of the
intrinsic momentum of the gyroscope to its orbital momentum. The modulation
frequency equals the angular velocity of the geodetic precession that prevents
the oscillation from resonance. The oscillation represents the precession of
the gyroscope orbital momentum. Within an acceptable time the oscillation
amplitude reaches the values that are amenable to being analyzed
experimentally. Taking into account the source oblateness decreases the beating
amplitude and increases the modulation frequency by the factor that is equal to
the ratio of the quadrupole precession velocity to the geodetic precession
velocity. The period of the quadrupole precession turns out to be a quite
sufficient time to form a measurable amplitude of the oscillation.


title: Standard Clocks, Orbital Precession and the Cosmological Constant

abstract:
  We discuss the influence of the cosmological constant on the gravitomagnetic
clock effect and the gravitational time delay of electromagnetic rays.
Moreover, we consider the relative motion of a binary system to linear order in
the cosmological constant $\Lambda$. The general expression for the effect of
$\Lambda$ on pericenter precession is given for arbitrary orbital eccentricity.


title: Cerenkov's Effect and Neutrino Oscillations in Loop Quantum Gravity

abstract:
  Bounds on the scale parameter {\cal L} arising in loop quantum gravity theory
are derived in the framework of Cerenkov's effect and neutrino oscillations.
Assuming that {\cal L} is an universal constant, we infer {\cal L}>
10^{-18}eV^{-1}, a bound compatible with ones inferred in different physical
context.


title:
Killing Tensors and Conformal Killing Tensors from Conformal Killing
  Vectors

abstract:
  Koutras has proposed some methods to construct reducible proper conformal
Killing tensors and Killing tensors (which are, in general, irreducible) when a
pair of orthogonal conformal Killing vectors exist in a given space. We give
the completely general result demonstrating that this severe restriction of
orthogonality is unnecessary. In addition we correct and extend some results
concerning Killing tensors constructed from a single conformal Killing vector.
A number of examples demonstrate how it is possible to construct a much larger
class of reducible proper conformal Killing tensors and Killing tensors than
permitted by the Koutras algorithms. In particular, by showing that all
conformal Killing tensors are reducible in conformally flat spaces, we have a
method of constructing all conformal Killing tensors (including all the Killing
tensors which will in general be irreducible) of conformally flat spaces using
their conformal Killing vectors.


title:
A numerical relativistic model of a massive particle in orbit near a
  Schwarzschild black hole

abstract:
  We present a method for computing the evolution of a spacetime containing a
massive particle and a black hole. The essential idea is that the gravitational
field is evolved using full numerical relativity, with the particle generating
a non-zero source term in the Einstein equations. The matter fields are not
evolved by hydrodynamic equations. Instead the particle is treated as a rigid
body whose center follows a geodesic. The necessary theoretical framework is
developed and then implemented in a computer code that uses the null-cone, or
characteristic, formulation of numerical relativity. The performance of the
code is illustrated in test runs, including a complete orbit (near r=9M) of a
Schwarzschild black hole.


title: Towards a Statistical Geometrodynamics

abstract:
  Can the spatial distance between two identical particles be explained in
terms of the extent that one can be distinguished from the other? Is the
geometry of space a macroscopic manifestation of an underlying microscopic
statistical structure? Is geometrodynamics derivable from general principles of
inductive inference? Tentative answers are suggested by a model of
geometrodynamics based on the statistical concepts of entropy, information
geometry, and entropic dynamics.


title: Scalar hairy black holes and solitons in asymptotically flat spacetimes

abstract:
  A numerical analysis shows that a class of scalar-tensor theories of gravity
with a scalar field minimally and nonminimally coupled to the curvature allows
static and spherically symmetric black hole solutions with scalar-field hair in
asymptotically flat spacetimes. In the limit when the horizon radius of the
black hole tends to zero, regular scalar solitons are found. The asymptotically
flat solutions are obtained provided that the scalar potential $V(\phi)$ of the
theory is not positive semidefinite and such that its local minimum is also a
zero of the potential, the scalar field settling asymptotically at that
minimum. The configurations for the minimal coupling case, although unstable
under spherically symmetric linear perturbations, are regular and thus can
serve as counterexamples to the no-scalar-hair conjecture. For the nonminimal
coupling case, the stability will be analyzed in a forthcoming paper.


title:
Discrete model of spacetime in terms of inverse spectra of the $T_0$
  Alexandroff topological spaces

abstract:
  The theory of inverse spectra of $T_0$ Alexandroff topological spaces is used
to construct a model of $T_0$-discrete four-dimensional spacetime. The universe
evolution is interpreted in terms of a sequence of topology changes in the set
of $T_0$-discrete spaces realized as nerves of the canonical partitions of
three-dimensional compact manifolds. The cosmological time arrow arises being
connected with the refinement of the canonical partitions, and it is defined by
the action of homomorphisms in the proper inverse spectrum of three-dimensional
$T_0$-discrete spaces. A new causal order relation in this spectrum is
postulated having the basic properties of the causal order in the
pseudo-Riemannian spacetime however also bearing certain quasi-quantum
features. An attempt is made to describe topological changes between compact
manifolds in terms of bifurcations of proper inverse spectra; this led us to
the concept of bispectrum. As a generalization of this concept, inverse
multispectra and superspectrum are introduced. The last one enables us to
introduce the discrete superspace, a discrete counterpart of the
Wheeler--DeWitt superspace.


title: Holographic Principle bounds on Primordial Black Hole abundances

abstract:
  The generalized Second Law of thermodynamics and the Holographic Principle
are combined to obtain the maximum mass of black holes formed inside a static
spherical box of size $R$ filled with radiation at initial temperature $T_{i}$.
The final temperature after the formation of black holes is evaluated, and we
show that a critical threshold exists for the radiation to be fully consumed by
the process. We next argue that if some form of Holographic Principle holds,
upper bounds to the mass density of PBHs formed in the early universe may be
obtained. The limits are worked out for inflationary and non-inflationary
cosmological models. This method is independent of the known limits based on
the background fluxes (from cosmic rays, radiation and other forms of energy)
and applies to potentially important epochs of PBH formation, resulting in
quite strong constraints to $\Omega_{pbh}$.


title: Acceleration-Induced Nonlocality

abstract:
  The standard relativistic theory of accelerated reference frames in Minkowski
spacetime is described. The measurements of accelerated observers are
considered and the limitations of the standard theory, based on the hypothesis
of locality, are pointed out. The physical principles of the nonlocal theory of
accelerated observers are presented. The implications of the nonlocal theory
are briefly discussed.


title: The gauge condition in gravitation theory with a background metric

abstract:
  In gravitation theory with a background metric, a gravitational field is
described by a (1,1)-tensor field. The energy-momentum conservation law imposes
a gauge condition on this field.


title: Gravitational Analogues, Geometric Effects and Gravitomagnetic Charge

abstract:
  This essay discusses some geometric effects associated with gravitomagnetic
fields and gravitomagnetic charge as well as the gravity theory of the latter.
Gravitomagnetic charge is the duality of gravitoelectric charge (mass) and is
therefore also termed the dual mass which represents the topological property
of gravitation. The field equation of gravitomagnetic matter is suggested and a
static spherically symmetric solution of this equation is offered. A possible
explanation of the anomalous acceleration acting on Pioneer spacecrafts are
briefly proposed.


title: Back-action cancellation in interferometers by quantum locking

abstract:
  We show that back-action noise in interferometric measurements such as
gravitational-waves detectors can be completely suppressed by a local control
of mirrors motion. An optomechanical sensor with an optimized measurement
strategy is used to monitor mirror displacements. A feedback loop then
eliminates radiation-pressure effects without adding noise. This very efficient
technique leads to an increased sensitivity for the interferometric
measurement, which becomes only limited by phase noise. Back-action
cancellation is furthermore insensitive to losses in the interferometer.


title: Center of mass integral in canonical general relativity

abstract:
  For a two-surface B tending to an infinite--radius round sphere at spatial
infinity, we consider the Brown--York boundary integral H_B belonging to the
energy sector of the gravitational Hamiltonian. Assuming that the lapse
function behaves as N \sim 1 in the limit, we find agreement between H_B and
the total Arnowitt--Deser--Misner energy, an agreement first noted by Braden,
Brown, Whiting, and York. However, we argue that the Arnowitt--Deser--Misner
mass--aspect differs from a gauge invariant mass--aspect by a pure divergence
on the unit sphere. We also examine the boundary integral H_B corresponding to
the Hamiltonian generator of an asymptotic boost, in which case the lapse N
\sim x^k grows like one of the asymptotically Cartesian coordinate functions.
Such an integral defines the kth component of the center of mass for a Cauchy
surface \Sigma bounded by B. In the large--radius limit, we find agreement
between H_B and an integral introduced by Beig and O'Murchadha. Although both
H_B and the Beig--O'Murchadha integral are naively divergent, they are in fact
finite modulo the Hamiltonian constraint. Furthermore, we examine the
relationship between H_B and a certain two--surface integral linear in the
spacetime Riemann curvature tensor. Similar integrals featuring the curvature
appear in works by Ashtekar and Hansen, Penrose, Goldberg, and Hayward. Within
the canonical 3+1 formalism, we define gravitational energy and
center--of--mass as certain moments of Riemann curvature.


title: Chronology violation and the Cosmological Argument

abstract:
  The Kalam Cosmological Argument is perhaps the most solid and widly discussed
argument for a caused creation of the universe. The usual objections to the
argument mainly focus on the second premise. In this paper we discuss the
dependency of the first premise on the topological structure of the space-time
manifold adopted for the underlying cosmological model. It is shown that in
chronology-violating space-times the first premise is also violated. The
chronology-violation, in turn, requires a massive violation of the so-called
energy conditions which could have observational effects that are briefly
discussed here. Hence, astronomical observations could be relevant for the
validity of the metaphysical argument. In this sense, it is possible to talk of
"observational theology".


title: On the Asymptotics for the Vacuum Einstein Constraint Equations

abstract:
  Given asymptotically flat initial data on M^3 for the vacuum Einstein field
equation, and given a bounded domain in M, we construct solutions of the vacuum
constraint equations which agree with the original data inside the given
domain, and are identical to that of a suitable Kerr slice (or identical to a
member of some other admissible family of solutions) outside a large ball in a
given end. The data for which this construction works is shown to be dense in
an appropriate topology on the space of asymptotically flat solutions of the
vacuum constraints. This construction generalizes work of the first author,
where the time-symmetric case was studied.


title:
A hyperbolic tetrad formulation of the Einstein equations for numerical
  relativity

abstract:
  The tetrad-based equations for vacuum gravity published by Estabrook,
Robinson, and Wahlquist are simplified and adapted for numerical relativity. We
show that the evolution equations as partial differential equations for the
Ricci rotation coefficients constitute a rather simple first-order
symmetrizable hyperbolic system, not only for the Nester gauge condition on the
acceleration and angular velocity of the tetrad frames considered by Estabrook
et al., but also for the Lorentz gauge condition of van Putten and Eardley, and
for a fixed gauge condition. We introduce a lapse function and a shift vector
to allow general coordinate evolution relative to the timelike congruence
defined by the tetrad vector field.


title:
On mapping properties of the general relativistic constraints operator
  in weighted function spaces, with applications

abstract:
  Generalising an analysis of Corvino and Schoen, we study surjectivity
properties of the constraint map in general relativity in a large class of
weighted Sobolev spaces. As a corollary we prove several perturbation, gluing,
and extension results: we show existence of non-trivial, singularity-free,
vacuum space-times which are stationary in a neighborhood of $i^0$; for small
perturbations of parity-covariant initial data sufficiently close to those for
Minkowski space-time this leads to space-times with a smooth global Scri; we
prove existence of initial data for many black holes which are exactly Kerr --
or exactly Schwarzschild -- both near infinity and near each of the connected
components of the apparent horizon; under appropriate conditions we obtain
existence of vacuum extensions of vacuum initial data across compact
boundaries; we show that for generic metrics the deformations in the
Isenberg-Mazzeo-Pollack gluings can be localised, so that the initial data on
the connected sum manifold coincide with the original ones except for a small
neighborhood of the gluing region; we prove existence of asymptotically flat
solutions which are static or stationary up to $r^{-m}$ terms, for any fixed
$m$, and with multipole moments freely prescribable within certain ranges.


title: Non-Riemannian geometrical optics in QED

abstract:
  A non-minimal photon-torsion axial coupling in the quantum electrodynamics
(QED) framework is considered. The geometrical optics in Riemannian-Cartan
spacetime is considering and a plane wave expansion of the electromagnetic
vector potential is considered leading to a set of the equations for the ray
congruence. Since we are interested mainly on the torsion effects in this first
report we just consider the Riemann-flat case composed of the Minkowskian
spacetime with torsion. It is also shown that in torsionic de Sitter background
the vacuum polarisation does alter the propagation of individual photons, an
effect which is absent in Riemannian spaces.


title: Accelerating Universe and Dynamical Compactification of Extra Dimensions

abstract:
  We study a $(4+D)$-dimensional Kaluza-Klein cosmology with a Robertson-Walker
type metric having two scale factors $a$ and $R$, corresponding to
$D$-dimensional internal space and 4-dimensional universe, respectively. By
introducing an exotic matter as the space-time part of the higher dimensional
energy-momentum tensor, a 4-dimensional decaying cosmological term is appeared
as $\Lambda \sim R^{-2}$, playing the role of an evolving dark energy in the
universe. The resulting field equations yield the exponential solutions for the
scale factors. These exponential behaviors may account for the dynamical
compactification of extra dimensions and the accelerating expansion of the
4-dimensional universe in terms of the Hubble parameter. The acceleration of
universe may be explained by the negative pressure of the exotic matter. It is
shown that the rate of compactification of higher dimensions depends on the
dimension, $D$. We then obtain the Wheeler-DeWitt equation and find the general
exact solutions in $D$-dimensions. A good correspondence between the classical
solutions and the Wheeler-DeWitt solutions, in any dimension $D$, is obtained.


title: Computer Algebra Applications for Numerical Relativity

abstract:
  We discuss the application of computer algebra to problems commonly arising
in numerical relativity, such as the derivation of 3+1-splits, manipulation of
evolution equations and automatic code generation. Particular emphasis is put
on working with abstract index tensor quantities as much as possible.


title: Gravitation as Anholonomy

abstract:
  A gravitational field can be seen as the anholonomy of the tetrad fields.
This is more explicit in the teleparallel approach, in which the gravitational
field-strength is the torsion of the ensuing Weitzenboeck connection. In a
tetrad frame, that torsion is just the anholonomy of that frame. The infinitely
many tetrad fields taking the Lorentz metric into a given Riemannian metric
differ by point-dependent Lorentz transformations. Inertial frames constitute a
smaller infinity of them, differing by fixed-point Lorentz transformations.
Holonomic tetrads take the Lorentz metric into itself, and correspond to
Minkowski flat spacetime. An accelerated frame is necessarily anholonomic and
sees the electromagnetic field strength with an additional term.


title:
Quasinormal modes of the near extremal Schwarzschild-de Sitter black
  hole

abstract:
  We present an exact expression for the quasinormal modes of scalar,
electromagnetic and gravitational perturbations of a near extremal
Scwarzschild-de Sitter black hole and we show why a previous approximation
holds exactly in this near extremal regime. In particular, our results give the
asymptotic behavior of the quasinormal frequencies for highly damped modes,
which as recently attracted much attention due to the proposed identification
of its real part with the Barbero-Immirzi parameter.


title: Black Holes in 2+1 Teleparallel Theories of Gravity

abstract:
  We apply the Hamiltonian formulation of teleparallel theories of gravity in
2+1 dimensions to a circularly symmetric geometry. We find a family of
one-parameter black hole solutions. The BTZ solution fixes the unique free
parameter of the theory. The resulting field equations coincide with the
teleparallel equivalent of Einstein's three-dimensional equations. We calculate
the gravitational energy of the black holes by means of the simple expression
that arises in the Hamiltonian formulation and conclude that the resulting
value is identical to that calculated by means of the Brown-York method.


title:
Massless field perturbations and gravitomagnetism in the Kerr-Taub-NUT
  spacetime

abstract:
  A single master equation is given describing spin $s\le2$ test fields that
are gauge- and tetrad-invariant perturbations of the Kerr-Taub-NUT spacetime
representing a source with mass $M$, gravitomagnetic monopole moment $-\ell$
and gravitomagnetic dipole moment (angular momentum) per unit mass $a$. This
equation can be separated into its radial and angular parts. The behavior of
the radial functions at infinity and near the horizon is studied and used to
examine the influence of $\ell$ on the phenomenon of superradiance, while the
angular equation leads to spin-weighted spheroidal harmonic solutions
generalizing those of the Kerr spacetime. Finally the coupling between the spin
of the perturbing field and the gravitomagnetic monopole moment is discussed.


title:
Harmonic potential as an effective limit of a discrete classical
  interaction

abstract:
  Motivated by improving the understanding of the quantum-to-classical
transition we use a simple model of classical discrete interactions for
studying the discrete-to-continuous transition in the classical harmonic
oscillator. A parallel is traced with gravity for stressing the relevance of
such discrete interaction models.


title:
General Stationary, Spherically-Symmetric Solutions in the Gauge Theory
  of Gravity

abstract:
  This paper was withdrawn by the authors because it has been supplanted by
gr-qc/0311007 and gr-qc/0311038.


title: Spin-1 gravitational waves and their natural sources

abstract:
  Non-vacuum exact gravitational waves invariant for a non Abelian
two-dimensional Lie algebra generated by two Killing fields whose commutator is
of light type, are described. The polarization of these waves, already known
from previous works, is related to the sources. Non vacuum exact gravitational
waves admitting only one Killing field of light type are also discussed.


title: Multidimensional Global Monopole and Nonsingular Cosmology

abstract:
  We consider a spherically symmetric global monopole in general relativity in
$(D=d+2)$-dimensional spacetime. The monopole is shown to be asymptotically
flat up to a solid angle defect in case $\gamma < d-1$, where $\gamma$ is a
parameter characterizing the gravitational field strength. In the range $d-1<
\gamma < 2d(d+1)/(d+2)$ the monopole space-time contains a cosmological
horizon. Outside the horizon the metric corresponds to a cosmological model of
Kantowski-Sachs type, where spatial sections have the topology ${\R\times
\S}^d$. In the important case when the horizon is far from the monopole core,
the temporal evolution of the Kantowski-Sachs metric is described analytically.
The Kantowski-Sachs space-time contains a subspace with a $(d+1)$-dimensional
Friedmann-Robertson-Walker metric, and its possible cosmological application is
discussed. Some numerical estimations in case $d=3$ are made showing that this
class of nonsingular cosmologies can be viable. Other results, generalizing
those known in the 4-dimensional space-time, are derived, in particular, the
existence of a large class of singular solutions with multiple zeros of the
Higgs field magnitude.


title: Brane Cosmology and Higher Derivative Theory

abstract: See hep-ph/0304045


title: Brane and Wormhole in Baby Universe

abstract: See hep-ph/0304045


title: A New Type of Isotropic Cosmological Model

abstract: See hep-ph/0304045


title: Brane and Nonisotropic Bianchi Cosmology

abstract: See hep-ph/0304045


title:
Small localized black holes in a braneworld: Formulation and numerical
  method

abstract:
  No realistic black holes localized on a 3-brane in the Randall-Sundrum
infinite braneworld have been found so far. The problem of finding a static
black hole solution is reduced to a boundary value problem. We solve it by
means of a numerical method, and show numerical examples of a localized black
hole whose horizon radius is small compared to the bulk curvature scale. The
sequence of small localized black holes exhibits a smooth transition from a
five-dimensional Schwarzschild black hole, which is a solution in the limit of
small horizon radius. The localized black hole tends to flatten as its horizon
radius increases. However, it becomes difficult to find black hole solutions as
its horizon radius increases.


title: Temperature and entropy of Schwarzschild-de Sitter space-time

abstract:
  In the light of recent interest in quantum gravity in de Sitter space, we
investigate semi-classical aspects of 4-dimensional Schwarzschild-de Sitter
space-time using the method of complex paths. The standard semi-classical
techniques (such as Bogoliubov coefficients and Euclidean field theory) have
been useful to study quantum effects in space-times with single horizons;
however, none of these approaches seem to work for Schwarzschild-de Sitter or,
in general, for space-times with multiple horizons. We extend the method of
complex paths to space-times with multiple horizons and obtain the spectrum of
particles produced in these space-times. We show that the temperature of
radiation in these space-times is proportional to the effective surface gravity
-- inverse harmonic sum of surface gravity of each horizon. For the
Schwarzschild-de Sitter, we apply the method of complex paths to three
different coordinate systems -- spherically symmetric, Painleve and Lemaitre.
We show that the equilibrium temperature in Schwarzschild-de Sitter is the
harmonic mean of cosmological and event horizon temperatures. We obtain
Bogoliubov coefficients for space-times with multiple horizons by analyzing the
mode functions of the quantum fields near the horizons. We propose a new
definition of entropy for space-times with multiple horizons analogous to the
entropic definition for space-times with a single horizon. We define entropy
for these space-times to be inversely proportional to the square of the
effective surface gravity. We show that this definition of entropy for
Schwarzschild-de Sitter satisfies the D-bound conjecture.


title: Microscopic Black Holes and Cosmic Shells

abstract:
  In the first part of this thesis the relativistic viscous fluid equations
describing the outflow of high temperature matter created via Hawking radiation
from microscopic black holes are solved numerically for a realistic equation of
state. We focus on black holes with initial temperatures greater than 100 GeV
and lifetimes less than 6 days. The spectra of direct photons and photons from
neutral pion decay are calculated for energies greater than 1 GeV. We calculate
the diffuse gamma ray spectrum from black holes distributed in our galactic
halo. However, the most promising route for their observation is to search for
point sources emitting gamma rays of ever-increasing energy. We also calculate
the spectra of all three flavors of neutrinos arising from direct emission from
the fluid at the neutrino-sphere and from the decay of pions and muons from
their decoupling at much larger radii and smaller temperatures for neutrino
energies between 1 GeV and the Planck energy. The results for neutrino spectra
may be applicable for the last few hours and minutes of the lifetime of a
microscopic black hole. In the second part of this thesis the combined field
equations of gravity and a scalar field are studied. When a potential for a
scalar field has two local minima there arise spherical shell-type solutions of
the classical field equations due to gravitational attraction. We establish
such solutions numerically in a space which is asymptotically de Sitter. It
generically arises when the energy scale characterizing the scalar field
potential is much less than the Planck scale. It is shown that the mirror image
of the shell appears in the other half of the Penrose diagram. The
configuration is smooth everywhere with no physical singularity.


title: No statistical excess in Explorer/Nautilus observations in the year 2001

abstract:
  A recent report on gravitational wave detector data from the NAUTILUS and
EXPLORER detector groups claims a statistically significant excess of
coincident events when the detectors are oriented in a way that maximizes their
sensitivity to gravitational wave sources in the galactic plane. While not
claiming a detection of gravitational waves, they do strongly suggest that the
origin of the excess is of gravitational wave origin. In this note we show that
the statistical analysis that led them to the conclusion that there is a
statistical excess is flawed and that the reported observation is entirely
consistent with the normal Poisson statistics of the reported detector
background.


title: Brane in the Relativistic Theory of Gravitation

abstract: See hep-ph/0304045


title:
Spontaneous Symmetry Breaking and Higgs Field in Higher Derivative
  Gravity Theory

abstract: See hep-ph/0304045


title: Self-interacting gas in a gravitational wave field

abstract:
  We investigate a relativistic self-interacting gas in the field of an
external {\it pp} gravitational wave. Based on symmetry considerations we ask
for those forces which are able to compensate the imprint of the gravitational
wave on the macroscopic 4-acceleration of the gaseous fluid. We establish an
exactly solvable toy model according to which the stationary states which
characterize such a situation have negative entropy production and are
accompanied by instabilities of the microscopic particle motion. These features
are similar to those which one encounters in phenomena of self-organization in
many-particle systems.


title: The Normal Conformal Cartan Connection and the Bach Tensor

abstract:
  The goal of this paper is to express the Bach tensor of a four dimensional
conformal geometry of an arbitrary signature by the Cartan normal conformal
(CNC) connection. We show that the Bach tensor can be identified with the
Yang-Mills current of the connection. It follows from that result that a
conformal geometry whose CNC connection is reducible in an appropriate way has
a degenerate Bach tensor. As an example we study the case of a CNC connection
which admits a twisting covariantly constant twistor field. This class of
conformal geometries of this property is known as given by the Fefferman metric
tensors. We use our result to calculate the Bach tensor of an arbitrary
Fefferman metric and show it is proportional to the tensorial square of the
four-fold eigenvector of the Weyl tensor. Finally, we solve the Yang-Mills
equations imposed on the CNC connection for all the homogeneous Fefferman
metrics. The only solution is the Nurowski-Plebanski metric.


title: Some Remarks on Dirac's Contributions to General Relativity

abstract:
  I provide a very brief sketch of some of Dirac's interests and work in
gravity, particularly his Hamiltonian formulation of Einstein's theory and its
relation to his earlier research.


title:
Spherically-symmetric gravitational fields in the metric-affine gauge
  theory of gravitation

abstract:
  Geometric structure of spherically-symmetric space-time in metric-affine
gauge theory of gravity is studied. Restrictions on curvature tensor and
Bianchi identities are obtained. By using certain simple gravitational
Lagrangian the solution of gravitational equations for vacuum
spherically-symmetric gravitational field is obtained.


title: Chaos in an Exact Relativistic 3-body Self-Gravitating System

abstract:
  We consider the problem of three body motion for a relativistic
one-dimensional self-gravitating system. After describing the canonical
decomposition of the action, we find an exact expression for the 3-body
Hamiltonian, implicitly determined in terms of the four coordinate and momentum
degrees of freedom in the system. Non-relativistically these degrees of freedom
can be rewritten in terms of a single particle moving in a two-dimensional
hexagonal well. We find the exact relativistic generalization of this
potential, along with its post-Newtonian approximation. We then specialize to
the equal mass case and numerically solve the equations of motion that follow
from the Hamiltonian. Working in hexagonal-well coordinates, we obtaining
orbits in both the hexagonal and 3-body representations of the system, and plot
the Poincare sections as a function of the relativistic energy parameter $\eta
$. We find two broad categories of periodic and quasi-periodic motions that we
refer to as the annulus and pretzel patterns, as well as a set of chaotic
motions that appear in the region of phase-space between these two types.
Despite the high degree of non-linearity in the relativistic system, we find
that the the global structure of its phase space remains qualitatively the same
as its non-relativisitic counterpart for all values of $\eta $ that we could
study. However the relativistic system has a weaker symmetry and so its
Poincare section develops an asymmetric distortion that increases with
increasing $\eta $. For the post-Newtonian system we find that it experiences a
KAM breakdown for $\eta \simeq 0.26$: above which the near integrable regions
degenerate into chaos.


title: Dynamics of Gravitomagnetic Charge

abstract:
  The physically interesting gravitational analogue of magnetic monopole in
electrodynamics is considered in the present paper. The author investigates the
field equation of gravitomagnetic matter, and the exact static cylindrically
symmetric solution of field equation as well as the motion of gravitomagnetic
charge in gravitational fields. Use is made of the mechanism of gravitational
Meissner effect, a potential interpretation of anomalous, constant,
acceleration acting on the Pioneer 10/11, Galileo and Ulysses spacecrafts is
also suggested.


title: Future asymptotic expansions of Bianchi VIII vacuum metrics

abstract:
  Bianchi VIII vacuum solutions to Einstein's equations are causally
geodesically complete to the future, given an appropriate time orientation, and
the objective of this article is to analyze the asymptotic behaviour of
solutions in this time direction. For the Bianchi class A spacetimes, there is
a formulation of the field equations that was presented in an article by
Wainwright and Hsu, and in a previous article we analyzed the asymptotic
behaviour of solutions in these variables. One objective of this paper is to
give an asymptotic expansion for the metric. Furthermore, we relate this
expansion to the topology of the compactified spatial hypersurfaces of
homogeneity. The compactified spatial hypersurfaces have the topology of
Seifert fibred spaces and we prove that in the case of NUT Bianchi VIII
spacetimes, the length of a circle fibre converges to a positive constant but
that in the case of general Bianchi VIII solutions, the length tends to
infinity at a rate we determine.


title:
Equality of the Inertial and the Gravitational Masses for a Quantum
  Particle

abstract:
  We investigate the interaction of the gravitational field with a quantum
particle. We derive the wave equation in the curved galilean spacetime from the
very broad Quantum mechanical assumptions and from covariance under the Milne
group. The inertial and gravitational masses are equal in that equation. So, we
give the proof of the equality for the non-relativistic quantum particle,
without applying the equivalence principle to the Schr\"odinger equation and
witout imposing any relation to the classical equations of motion. This result
constitutes a substantial strengthening of the previous result obtained by
Herdegen and the author.


title:
Axisymmetric general relativistic hydrodynamics: Long-term evolution of
  neutron stars and stellar collapse to neutron stars and black holes

abstract:
  We report a new implementation for axisymmetric simulation in full general
relativity. In this implementation, the Einstein equations are solved using the
Nakamura-Shibata formulation with the so-called cartoon method to impose an
axisymmetric boundary condition, and the general relativistic hydrodynamic
equations are solved using a high-resolution shock-capturing scheme based on an
approximate Riemann solver. As tests, we performed the following simulations:
(i) long-term evolution of non-rotating and rapidly rotating neutron stars,
(ii) long-term evolution of neutron stars of a high-amplitude damping
oscillation accompanied with shock formation, (iii) collapse of unstable
neutron stars to black holes, and (iv) stellar collapses to neutron stars. The
tests (i)--(iii) were carried out with the $\Gamma$-law equation of state, and
the test (iv) with a more realistic parametric equation of state for
high-density matter. We found that this new implementation works very well: It
is possible to perform the simulations for stable neutron stars for more than
10 dynamical time scales, to capture strong shocks formed at stellar core
collapses, and to accurately compute the mass of black holes formed after the
collapse and subsequent accretion. In conclusion, this implementation is robust
enough to apply to astrophysical problems such as stellar core collapse of
massive stars to a neutron star and black hole, phase transition of a neutron
star to a high-density star, and accretion-induced collapse of a neutron star
to a black hole. The result for the first simulation of stellar core collapse
to a neutron star started from a realistic initial condition is also presented.


title: Scalar and Spinor Perturbation to the Kerr-NUT Spacetime

abstract:
  We study the scalar and spinor perturbation, namely the Klein-Gordan and
Dirac equations, in the Kerr-NUT space-time. The metric is invariant under the
duality transformation involving the exchange of mass and NUT parameters on one
hand and radial and angle coordinates on the other. We show that this
invariance is also shared by the scalar and spinor perturbation equations.
Further, by the duality transformation, one can go from the Kerr to the dual
Kerr solution, and vice versa, and the same applies to the perturbation
equations. In particular, it turns out that the potential barriers felt by the
incoming scalar and spinor fields are higher for the dual Kerr than that for
the Kerr. We also comment on existence of horizon and singularity.


title: Numerical studies of Phi^2-Oscillatons

abstract:
  We present an exhaustive analysis of the numerical evolution of the
Einstein-Klein-Gordon equations for the case of a real scalar field endowed
with a quadratic self-interaction potential. The self-gravitating equilibrium
configurations are called oscillatons and are close relatives of boson stars,
their complex counterparts. Unlike boson stars, for which the oscillations of
the two components of the complex scalar field are such that the spacetime
geometry remains static, oscillatons give rise to a geometry that is
time-dependent and oscillatory in nature. However, they can still be classified
into stable (S-branch) and unstable (U-branch) cases. We have found that
S-oscillatons are indeed stable configurations under small perturbations and
typically migrate to other S-profiles when perturbed strongly. On the other
hand, U-oscillatons are intrinsically unstable: they migrate to the S-branch if
their mass is decreased and collapse to black holes if their mass is increased
even by a small amount. The S-oscillatons can also be made to collapse to black
holes if enough mass is added to them, but such collapse can be efficiently
prevented by the gravitational cooling mechanism in the case of diluted
oscillatons.


title:
Scalar field in a minimally coupled brane world: no-hair and other no-go
  theorems

abstract:
  In the brane-world framework, we consider static, spherically symmetric
configurations of a scalar field with the Lagrangian $(\d\phi)^2/2 - V(\phi)$,
confined on the brane. We use the 4D Einstein equations on the brane obtained
by Shiromizu et al., containing the usual stress tensor $T\mN$, the tensor
$\Pi\mN$, quadratic in $T\mN$, and $E\mN$ describing interaction with the bulk.
For models under study, the tensor $\Pi\mN$ has zero divergence, so we can
consider a "minimally coupled" brane with $E\mN = 0$, whose 4D gravity is
decoupled from the bulk geometry. Assuming $E\mN =0$, we try to extend to brane
worlds some theorems valid for scalar fields in general relativity (GR). Thus,
the list of possible global causal structures in all models under consideration
is shown to be the same as is known for vacuum with a $Lambda$ term in GR:
Minkowski, Schwarzschild, (A)dS and Schwarzschild-(A)dS. A no-hair theorem,
saying that, given a potential $V\geq 0$, asymptotically flat black holes
cannot have nontrivial external scalar fields, is proved under certain
restrictions. Some objects, forbidden in GR, are allowed on the brane, e.g,
traversable wormholes supported by a scalar field, but only at the expense of
enormous matter densities in the strong field region.


title: Relativistically rotating dust

abstract:
  Dust configurations play an important role in astrophysics and are the
simplest models for rotating bodies. The physical properties of the
general--relativistic global solution for the rigidly rotating disk of dust,
which has been found recently as the solution of a boundary value problem, are
discussed.


title:
Poisson structure and symmetry in the Chern-Simons formulation of
  (2+1)-dimensional gravity

abstract:
  In the formulation of (2+1)-dimensional gravity as a Chern-Simons gauge
theory, the phase space is the moduli space of flat Poincar\'e group
connections. Using the combinatorial approach developed by Fock and Rosly, we
give an explicit description of the phase space and its Poisson structure for
the general case of a genus g oriented surface with punctures representing
particles and a boundary playing the role of spatial infinity. We give a
physical interpretation and explain how the degrees of freedom associated with
each handle and each particle can be decoupled. The symmetry group of the
theory combines an action of the mapping class group with asymptotic Poincar\'e
transformations in a non-trivial fashion. We derive the conserved quantities
associated to the latter and show that the mapping class group of the surface
acts on the phase space via Poisson isomorphisms.


title:
Emergent discrete time and quantization: relativistic particle with
  extradimensions

abstract:
  We study the reparametrization invariant system of a classical relativistic
particle moving in (5+1) dimensions, of which two internal ones are
compactified to form a torus. A discrete physical time is constructed based on
a quasi-local invariant observable. Due to ergodicity, it is simply related to
the proper time on average. The external motion in Minkowski space can then be
described as a unitary quantum mechanical evolution.


title: Radiation from accelerated black holes in de Sitter universe

abstract:
  Radiative properties of gravitational and electromagnetic fields generated by
uniformly accelerated charged black holes in asymptotically de Sitter spacetime
are studied by analyzing the C-metric exact solution of the Einstein-Maxwell
equations with a positive cosmological constant. Its global structure and
physical properties are thoroughly discussed. We explicitly find and describe
the specific pattern of radiation which exhibits the dependence of the fields
on a null direction along which the (spacelike) conformal infinity is
approached. This directional characteristic of radiation supplements the
peeling behavior of the fields near the infinity. The interpretation of the
solution is achieved by means of various coordinate systems, and suitable
tetrads. Relation to the Robinson-Trautman framework is also presented.


title: Moving black holes via singularity excision

abstract:
  We present a singularity excision algorithm appropriate for numerical
simulations of black holes moving throughout the computational domain. The
method is an extension of the excision procedure previously used to obtain
stable simulations of single, non-moving black holes. The excision procedure
also shares elements used in recent work to study the dynamics of a scalarfield
in the background of a single, boosted black hole. The robustness of our
excision method is tested with single black-hole evolutions using a coordinate
system in which the coordinate location of the black hole, and thus the
excision boundary, moves throughout the computational domain.


title: Towards a Realistic Neutron Star Binary Inspiral

abstract:
  An approach to general relativity based on conformal flatness and
quasiequilibrium (CFQE) assumptions has played an important role in the study
of the inspiral dynamics and in providing initial data for fully general
relativistic numerical simulations of coalescing compact binaries. However, the
regime of validity of the approach has never been established. To this end, we
develop an analysis that determines the violation of the CFQE approximation in
the evolution of the binary described by the full Einstein theory. With this
analysis, we show that the CFQE assumption is significantly violated even at
relatively large orbital separations in the case of corotational neutron star
binaries. We also demonstrate that the innermost stable circular orbit (ISCO)
determined in the CFQE approach for corotating neutron star binaries may have
no astrophysical significance.


title: Spin Foam Models for Quantum Gravity

abstract:
  In this article we review the present status of the spin foam formulation of
non-perturbative (background independent) quantum gravity. The article is
divided in two parts. In the first part we present a general introduction to
the main ideas emphasizing their motivations from various perspectives.
Riemannian 3-dimensional gravity is used as a simple example to illustrate
conceptual issues and the main goals of the approach. The main features of the
various existing models for 4-dimensional gravity are also presented here. We
conclude with a discussion of important questions to be addressed in four
dimensions (gauge invariance, discretization independence, etc.).
  In the second part we concentrate on the definition of the Barrett-Crane
model. We present the main results obtained in this framework from a critical
perspective. Finally we review the combinatorial formulation of spin foam
models based on the dual group field theory technology. We present the
Barrett-Crane model in this framework and review the finiteness results
obtained for both its Riemannian as well as its Lorentzian variants.


title:
Proposal for a New Test of the Time Independence Of The Fine Structure
  Constant, alpha, Using Orthogonally Polarised Whispering Gallery Modes in a
  Single Sapphire Resonator

abstract:
  A new experiment to test for the time independence of the fine structure
constant, alpha, is proposed. The experiment utilizes orthogonally polarized
Transverse Electric and Transverse Magnetic Whispering Gallery Modes in a
single sapphire resonator tuned to similar frequencies. When configured as a
dual mode sapphire clock, we show that the anisotropy of sapphire makes it is
possible to undertake a sensitive measurement from the beat frequency between
the two modes. At infrared frequencies this is possible due to the different
effect of the lowest phonon frequency on the two orthogonally polarized modes.
At microwave frequencies we show that the phonon effect is too small. We show
that the Electron Spin Resonance of paramagnetic impurities (such as Cr3+) in
the lattice effects only one polarization with an alpha^6 dependence. This
enables an enhancement of the sensitivity to temporal changes in a at microwave
frequencies.


title: Regge-Teitelboim Goedetic Brane Gravity and Effective Cosmology

abstract: See hep-ph/0304045


title: Spinning particles in the Perturbed Schwarzschild-de Sitter Space-Time

abstract: See hep-ph/0304045


title: The origin of causal set structure in the quantum universe

abstract:
  We discuss the origin of causal set structure and the emergence of classical
space and time in the universe. Given that the universe is a closed
self-referential quantum automaton with a quantum register consisting of a vast
number of elementary quantum subregisters, we find two distinct but intimately
related causal sets. One of these is associated with the factorization and
entanglement properties of states of the universe and encodes phenomena such as
quantum correlations and violations of Bell-type inqualities. The concepts of
separations and entanglements of states are used to show how state reduction
dynamics generates the familial relationships which gives this causal set
structure. The other causal set structure is generated by the factorization
properties of the observables (the Hermitian operators) over the quantum
register. The concept of skeleton sets of operators is used to show how the
factorization properties of these operators could generate the classical causal
set structures associated with Einstein locality.


title: Some properties of a $\Delta-$string

abstract:
  The properties of 5D gravitational flux tubes are considered. With the cross
section and 5th dimension in the Planck region such tubes can be considered as
string-like objects, namely $\Delta-$strings. A model of attachment of
$\Delta-$string to a spacetime is offered. It is shown that the attachment
point is a model of an electric charge for an observer living in the spacetime.
Magnetic charges are forbidden in this model.