title: On a spacetime duality in 2+1 gravity

abstract:
  We consider 2+1 dimensional gravity with a cosmological constant, and explore
a duality that exists between space-times that have the De Sitter group SO(3,1)
as its local isometry group. In particular, the Lorentzian theory with a
positive cosmological constant is dual to the Euclidean theory with a negative
cosmological constant. We use this duality to construct a mapping between
apparently unrelated space-times. More precisely, we exhibit a relation between
the Euclidean BTZ family and some $T^2$-cosmological solutions, and between
De-Sitter point particle space-times and the analytic continuations of Anti-De
Sitter point particles. We discuss some possible applications for BH and AdS
thermodynamics.


title: Gravitation as a Quantum Diffusion

abstract:
  Inhomogeneous Nelson's diffusion in flat spacetime with a tensor of diffusion
can be described as a homogeneous one in a Riemannian manifold with this tensor
of diffusion as a metric tensor. The influence of matter to the energy density
of the stochastic background (vacuum) is considered. It is shown that
gravitation can be represented as inhomogeneity of the quantum diffusion, the
Einstein equations for the metrics can be derived as the equations for the
corresponding tensor of diffusion.


title: Scale Symmetries of Spherical String Fluids

abstract:
  We consider homothetic maps in a family of spherical relativistic star
models. A generalization of Vaidya's radiating metric provides a fluid
atmosphere of radiation and strings. The similarity structure of the string
fluid is investigated.


title:
Schwinger's Result On Particle Production From Complex Paths WKB
  Approximation

abstract:
  This paper presents the derivation of Schwinger's gauge invariant result of
$Im \cal{L}_{eff}$ upto one loop approximation, for particle production in an
uniform electric field through the method of complex trajectory WKB
approximation (CWKB). The CWKB proposed by one of the author's \cite{bis:ijtp}
looks upon particle production as due to the motion of a particle in complex
space-time plane, thereby requiring tunneling paths both in space and time.
Recently \cite{srini:iucaa,srini1:iucaa} there have been some efforts to
calculate the reflection and transmission co-efficients for particle production
in uniform electric field that differ from our expressions for the same. In
this paper we clarify the confusion in this regard and establish the
correctness of CWKB.


title: Dirac Current in Expanding Spacetime

abstract:
  We study the behaviour of Dirac current in expanding spacetime with
Schr{\"o}dinger and de Sitter form for the evolution of the scale-factor. The
study is made to understand the particle-antiparticle rotation and the
evolution of quantum vacuum leading to particle production in such spacetime.


title: Inflationary cosmology with two-component fluid and thermodynamics

abstract:
  We present a simple and self-consistent cosmology with a phenomenological
model of quantum creation of radiation and matter due to decay of the
cosmological constant $\Lambda$. The decay drives a non-isentropic inflationary
epoch, which exits smoothly to the radiation-dominated era, without reheating,
and then evolves to the dust era. The initial vacuum for radiation and matter
is a regular Minkowski vacuum. The created radiation and matter obeys standard
thermodynamic laws, and the total entropy produced is consistent with the
accepted value. This paper is an extension of the model with the decaying
cosmological constant considered previously. We compare our model with the
quantum field theory approach to creation of particles in curved space.


title:
On the relation between a zero-point-field-induced inertial effect and
  the Einstein-de Broglie formula

abstract:
  It has been proposed that the scattering of electromagnetic zero-point
radiation by accelerating objects results in a reaction force that may account,
at least in part, for inertia [1,2,3]. This arises because of asymmetries in
the electromagnetic zero-point field (ZPF) or electromagnetic quantum vacuum as
perceived from an accelerating reference frame. In such a frame, the Poynting
vector and momentum flux of the ZPF become non-zero. If one assumes that
scattering of the ZPF radiation takes place at the level of quarks and
electrons constituting matter, then it is possible for both Newton's equation
of motion, ${\bf f}=m{\bf a}$, and its relativistic covariant generalization,
${\cal F}=d{\cal P}/d\tau$, to be obtained as a consequence of the non-zero ZPF
momentum flux. We now conjecture that this scattering must take place at the
Compton frequency of a particle, and that this interpretation of mass leads
directly to the de Broglie relation characterizing the wave nature of that
particle in motion, $\lambda_B=h/p$. This suggests a perspective on a
connection between electrodynamics and the quantum wave nature of matter.
Attempts to extend this perspective to other aspects of the vacuum are left for
future consideration.


title:
On dilaton solutions of de Sitter inflation and primordial spin-torsion
  density fluctuations

abstract:
  Four classes of exact solutions of Einstein-Cartan dilatonic inflationary de
Sitter cosmology are given.The first is obtained from the equation of state of
massless dilaton instead of an unpolarized fermion fluid used previously by
Gasperini.Repulsive gravity is found in the case where dilatons are constraint
by the presence of spin-torsion effects.The second and third solutions
represent respectively massive dilatons in the radiation era with the massive
potential and torsion kinks and finally the dust of spinning
particles.Primordial spin-density fluctuations are also computed based on
Primordial fluctuations of temperature obtained from COBE data.The temperature
fluctuation can also be computed from the nearly flat spectrum of the
gravitational waves produced during inflation and by the result that the
dilaton mass would be proportional to the Hubble constant.This result agrees
with the COBE data.This idea is also used to compute the spin-torsion density
in the inflation era.


title: On the (non) existence of several gravitomagnetic effects

abstract:
  Due to the resemblance between Maxwell and the gravitomagnetic equations
obtained in the weak field and slow motion limit of General Relativity, one can
ask if it is possible to amplify a seed intrinsic rotation or spin motion by a
gravitomagetic dynamo, in analogy with the well-known dynamo effect. Using the
Galilean limits of the gravitomagnetic equations, the answer to this question
is negative, due to the fact that a "magnetic" Galilean limit for the
gravitomagnetic equations is physically inconsistent. Also, we prove that, in
spite of some claims, a gravitational Meissner effect does not exists.


title: Super-energy tensors

abstract:
  A purely algebraic construction of super-energy tensors for arbitrary fields
is presented in any dimensions. These tensors have good mathematical and
physical properties, and they can be used in any theory having as basic arena
an n-dimensional manifold with a metric of Lorentzian signature. In general,
the completely timelike component of these s-e tensors has the mathematical
features of an energy density: they are positive definite and satisfy the
dominant property. Similarly, the super-momentum vectors have mathematical
properties of s-e flux vectors. The classical Bel-Robinson tensor is included
in our general definition. The energy-momentum and super-energy tensors of
physical fields are also obtained, and the procedure is illustrated by writing
down these tensors explicitly for the cases of scalar, electromagnetic, and
Proca fields. Moreover, `(super)$^k$-energy' tensors are defined and shown to
be meaningful and in agreement for the different physical fields. In flat
spacetimes, they provide infinitely many conserved quantities. In non-flat
spacetimes conserved s-e currents are found for any minimally coupled scalar
field whenever there is a Killing vector. Furthermore, the exchange of
gravitational and electromagnetic super-energy is also shown by studying the
propagation of discontinuities.


title: Irrotational binary neutron stars in quasiequilibrium

abstract:
  We report on numerical results from an independent formalism to describe the
quasi-equilibrium structure of nonsynchronous binary neutron stars in general
relativity. This is an important independent test of controversial numerical
hydrodynamic simulations which suggested that nonsynchronous neutron stars in a
close binary can experience compression prior to the last stable circular
orbit. We show that, for compact enough stars the interior density increases
slightly as irrotational binary neutron stars approach their last orbits. The
magnitude of the effect, however, is much smaller than that reported in
previous hydrodynamic simulations.


title: Interior Weyl-type Solutions of the Einstein-Maxwell Field Equations

abstract:
  Static solutions of the electro-gravitational field equations exhibiting a
functional relationship between the electric and gravitational potentials are
studied. General results for these metrics are presented which extend previous
work of Majumdar. In particular, it is shown that for any solution of the field
equations exhibiting such a Weyl-type relationship, there exists a relationship
between the matter density, the electric field density and the charge density.
It is also found that the Majumdar condition can hold for a bounded perfect
fluid only if the matter pressure vanishes (that is, charged dust). By
restricting to spherically symmetric distributions of charged matter, a number
of exact solutions are presented in closed form which generalise the
Schwarzschild interior solution. Some of these solutions exhibit functional
relations between the electric and gravitational potentials different to the
quadratic one of Weyl. All the non-dust solutions are well-behaved and, by
matching them to the Reissner-Nordstr\"{o}m solution, all of the constants of
integration are identified in terms of the total mass, total charge and radius
of the source. This is done in detail for a number of specific examples. These
are also shown to satisfy the weak and strong energy conditions and many other
regularity and energy conditions that may be required of any physically
reasonable matter distribution.


title:
The generalization of the Regge-Wheeler equation for self-gravitating
  matter fields

abstract:
  It is shown that the dynamical evolution of perturbations on a static
spacetime is governed by a standard pulsation equation for the extrinsic
curvature tensor. The centerpiece of the pulsation equation is a wave operator
whose spatial part is manifestly self-adjoint. In contrast to metric
formulations, the curvature-based approach to gravitational perturbation theory
generalizes in a natural way to self-gravitating matter fields. For a certain
relevant subspace of perturbations the pulsation operator is symmetric with
respect to a positive inner product and therefore allows spectral theory to be
applied. In particular, this is the case for odd-parity perturbations of
spherically symmetric background configurations. As an example, the pulsation
equations for self-gravitating, non-Abelian gauge fields are explicitly shown
to be symmetric in the gravitational, the Yang Mills, and the off-diagonal
sector.


title:
Stable monopole and dyon solutions in the Einstein-Yang-Mills theory in
  asymptotically anti-de Sitter Space

abstract:
  A continuum of new monopole and dyon solutions in the Einstein-Yang-Mills
theory in asymptotically anti-de Sitter space are found. They are regular
everywhere and specified with their mass, and non-Abelian electric and magnetic
charges. A class of monopole solutions which have no node in non-Abelian
magnetic fields are shown to be stable against spherically symmetric linear
perturbations.


title:
The binary black-hole problem at the third post-Newtonian approximation
  in the orbital motion: Static part

abstract:
  Post-Newtonian expansions of the Brill-Lindquist and Misner-Lindquist
solutions of the time-symmetric two-black-hole initial value problem are
derived. The static Hamiltonians related to the expanded solutions, after
identifying the bare masses in both solutions, are found to differ from each
other at the third post-Newtonian approximation. By shifting the position
variables of the black holes the post-Newtonian expansions of the three metrics
can be made to coincide up to the fifth post-Newtonian order resulting in
identical static Hamiltonians up the third post-Newtonian approximation. The
calculations shed light on previously performed binary point-mass calculations
at the third post-Newtonian approximation.


title: Geodesic multiplication and quantum kinematics in a Newtonian spacetime

abstract:
  We consider a quantum test particle in the background of a Newtonian
gravitational field in the framework of Cartan's formulation of nonrelativistic
spacetimes. We have proposed a novel quantization of a point particle which
amounts to introducing its position operators as multiplication with the
corresponding Riemann normal coordinates and momentum operators as
infinitesimal right translation operators determined by geodesic multiplication
of points of the spacetime. We present detailed calculations for the simplest
model of a two-dimensional Newtonian spacetime.


title:
A four-dimensional Hooke's law can encompass linear elasticity and
  inertia

abstract:
  The question is examined, whether the formally straightforward extension of
Hooke's time-honoured stress-strain relation to the four dimensions of special
and of general relativity can make physical sense. The four-dimensional Hooke's
law is found able to account for the inertia of matter; in the flat space, slow
motion approximation the field equations for the ``displacement'' four-vector
field can encompass both linear elasticity and inertia. In this limit one just
recovers the equations of motion of the classical theory of elasticity.


title: Einstein Constraints on Asymptotically Euclidean Manifolds

abstract:
  We consider the Einstein constraints on asymptotically euclidean manifolds
$M$ of dimension $n \geq 3$ with sources of both scaled and unscaled types. We
extend to asymptotically euclidean manifolds the constructive method of proof
of existence. We also treat discontinuous scaled sources. In the last section
we obtain new results in the case of non-constant mean curvature.


title: Bounded Area Theorems for Higher Genus Black Holes

abstract:
  By a simple modification of Hawking's well-known topology theorems for black
hole horizons, we find lower bounds for the areas of smooth apparent horizons
and smooth cross-sections of stationary black hole event horizons of genus
$g>1$ in four dimensions. For a negatively curved Einstein space, the bound is
${{4\pi (g-1)}\over {-\ell}}$ where $\ell$ is the cosmological constant of the
spacetime. This is complementary to the known upper bound on the area of $g=0$
black holes in de Sitter spacetime. It also emerges that $g>1$ quite generally
requires a mean negative energy density on the horizon. The bound is sharp; we
show that it is saturated by certain extreme, asymptotically locally anti-de
Sitter spacetimes. Our results generalize a recent result of Gibbons.


title:
Does the Isotropy of the CMB Imply a Homogeneous Universe? Some
  Generalised EGS Theorems

abstract:
  We demonstrate that the high isotropy of the Cosmic Microwave Background
(CMB), combined with the Copernican principle, is not sufficient to prove
homogeneity of the universe -- in contrast to previous results on this subject.
The crucial additional factor not included in earlier work is the acceleration
of the fundamental observers. We find the complete class of irrotational
perfect fluid spacetimes admitting an exactly isotropic radiation field for
every fundamental observer and show that are FLRW if and only if the
acceleration is zero. While inhomogeneous in general, these spacetimes all
possess three-dimensional symmetry groups, from which it follows that they also
admit a thermodynamic interpretation. In addition to perfect fluids models we
also consider multi-component fluids containing non-interacting radiation, dust
and a quintessential scalar field or cosmological constant in which the
radiation is isotropic for the geodesic (dust) observers. It is shown that the
non-acceleration of the fundamental observers forces these spacetimes to be
FLRW. While it is plausible that fundamental observers (galaxies) in the real
universe follow geodesics, it is strictly necessary to determine this from
local observations for the cosmological principle to be more than an
assumption. We discuss how observations may be used to test this.


title:
Detectability of the primordial origin of the gravitational wave
  background in the Universe

abstract:
  The appearance of peaks in various primordial fluctuation Fourier power
spectra is a generic prediction of the inflationary scenario. We investigate
whether future experiments, in particular the satellite experiment PLANCK, will
be able to detect the possible appearance of these peaks in the B-mode
polarization multipole power spectrum. This would yield a conclusive proof of
the presence of a primordial background of gravitational waves.


title: Gravitating monopole and its black hole solution in Brans-Dicke Theory

abstract:
  We find a self-gravitating monopole and its black hole solution in
Brans-Dicke theory. We mainly discuss the properties of these solutions in the
Einstein frame and compare the solutions with those in general relativity.


title: Conceptual issues in quantum cosmology

abstract:
  I give a review of the conceptual issues that arise in theories of quantum
cosmology. I start by emphasising some features of ordinary quantum theory that
also play a crucial role in understanding quantum cosmology. I then give
motivations why spacetime cannot be treated classically at the most fundamental
level. Two important issues in quantum cosmology -- the problem of time and the
role of boundary conditions -- are discussed at some length. Finally, I discuss
how classical spacetime can be recovered as an approximate notion. This
involves the application of a semiclassical approximation and the process of
decoherence. The latter is applied to both global degrees of freedom and
primordial fluctuations in an inflationary Universe.


title: Plane waves in quantum gravity: breakdown of the classical spacetime

abstract:
  Starting with the Hamiltonian formulation for spacetimes with two commuting
spacelike Killing vectors, we construct a midisuperspace model for linearly
polarized plane waves in vacuum gravity. This model has no constraints and its
degrees of freedom can be interpreted as an infinite and continuous set of
annihilation and creation like variables. We also consider a simplified version
of the model, in which the number of modes is restricted to a discrete set. In
both cases, the quantization is achieved by introducing a Fock representation.
We find regularized operators to represent the metric and discuss whether the
coherent states of the quantum theory are peaked around classical spacetimes.
It is shown that, although the expectation value of the metric on Killing
orbits coincides with a classical solution, its relative fluctuations become
significant when one approaches a region where null geodesics are focused. In
that region, the spacetimes described by coherent states fail to admit an
approximate classical description. This result applies as well to the vacuum of
the theory.


title:
A new numerical scheme to compute 3D configurations of quasiequilibrium
  compact stars in general relativity -- Application to synchronously rotating
  binary star systems --

abstract:
  We have developed a new numerical scheme to obtain quasiequilibrium
structures of nonaxisymmetric compact stars such as binary neutron star systems
as well as the spacetime around those systems in general relativity. Concerning
quasiequilibrium states of binary systems in general relativity, several
investigations have been already carried out by assuming conformal flatness of
the spatial part of the metric. However, the validity of the conformally flat
treatment has not been fully analyzed except for axisymmetric configurations.
Therefore it is desirable to solve quasiequilibrium states by developing
totally different methods from the conformally flat scheme. In this paper we
present a new numerical scheme to solve directly the Einstein equations for 3D
configurations without assuming conformal flatness, although we make use of the
simplified metric for the spacetime. This new formulation is the extension of
the scheme which has been successfully applied for structures of axisymmetric
rotating compact stars in general relativity. It is based on the integral
representation of the Einstein equations by taking the boundary conditions at
infinity into account. We have checked our numerical scheme by computing
equilibrium sequences of binary polytropic star systems in Newtonian gravity
and those of axisymmetric polytropic stars in general relativity. We have
applied this numerical code to binary star systems in general relativity and
have succeeded in obtaining several equilibrium sequences of synchronously
rotating binary polytropes with the polytropic indices N = 0.0, 0.5 and 1.0.


title: An Algebraic Quantization of Causal Sets

abstract:
  A scheme for an algebraic quantization of the causal sets of Sorkin et al. is
presented. The suggested scenario is along the lines of a similar
algebraization and quantum interpretation of finitary topological spaces due to
Zapatrin and this author. To be able to apply the latter procedure to causal
sets Sorkin's `semantic switch' from `partially ordered sets as finitary
topological spaces' to `partially ordered sets as locally finite causal sets'
is employed. The result is the definition of `quantum causal sets'. Such a
procedure and its resulting definition is physically justified by a property of
quantum causal sets that meets Finkelstein's requirement from `quantum
causality' to be an immediate, as well as an algebraically represented,
relation between events for discrete locality's sake. The quantum causal sets
introduced here are shown to have this property by direct use of a result from
the algebraization of finitary topological spaces due to Breslav, Parfionov and
Zapatrin.


title:
Schwarzschild Black Hole Quantum Statistics from Z(2) Orientation
  Degrees of Freedom and its Relations to Ising Droplet Nucleation

abstract:
  Generalizing previous quantum gravity results for Schwarzschild black holes
from 4 to D > 3 space-time dimensions yields an energy spectrum E_n = alpha
n^{(D-3)/(D-2)} E_P, n=1,2,..., alpha = O(1), where E_P is the Planck energy in
that space-time. This spectrum means that the quantized area A_{D-2}(n) of the
D-2 dimensional horizon has universally the form A_{D-2} = n a_{D-2}, where
a_{D-2} is essentially the (D-2)th power of the D-dimensional Planck length.
Assuming that the basic area quantum has a Z(2)-degeneracy according to its two
possible orientation degrees of freedom implies a degeneracy d_n = 2^n for the
n-th level. The energy spectrum with such a degeneracy leads to a quantum
canonical partition function which is the same as the classical grand canonical
partition function of a primitive Ising droplet nucleation model for 1st-order
phase transitions in D-2 spatial dimensions. The analogy to this model suggests
that E_n represents the surface energy of a "droplet" of n horizon quanta.
Exploiting the well-known properties of the so-called critical droplets of that
model immediately leads to the Hawking temperature and the Bekenstein-Hawking
entropy of Schwarzschild black holes. The values of temperature and entropy
appear closely related to the imaginary part of the partition function which
describes metastable states


title:
Group Theoretical Quantization of a Phase Space S^1 x R^+ and the Mass
  Spectrum of Schwarzschild Black Holes in D Space-Time Dimensions

abstract:
  The symplectic reduction of pure spherically symmetric (Schwarzschild)
classical gravity in D space-time dimensions yields a 2-dimensional phase space
of observables consisting of the Mass M (>0) and a canonically conjugate
(Killing) time variable T. Imposing (mass-dependent) periodic boundary
conditions in time on the associated quantum mechanical plane waves which
represent the Schwarzschild system in the period just before or during the
formation of a black hole, yields an energy spectrum of the hole which realizes
the old Bekenstein postulate that the quanta of the horizon A_{D-2} are
multiples of a basic area quantum. In the present paper it is shown that the
phase space of such a Schwarzschild black hole in D space-time dimensions is
symplectomorphic to a symplectic manifold
  S={(phi in R mod 2 pi, p = A_{D-2} >0)} with the symplectic form d phi wedge
d p. As the action of the group SO_+(1,2) on that manifold is transitive,
effective and Hamiltonian, it can be used for a group theoretical quantization
of the system. The area operator p for the horizon corresponds to the generator
of the compact subgroup SO(2) and becomes quantized accordingly: The positive
discrete series of the irreducible unitary representations of SO_+(1,2) yields
an (horizon) area spectrum proportional k+n, where k =1,2,... characterizes the
representation and n = 0,1,2,... the number of area quanta. If one employs the
unitary representations of the universal covering group of SO_+(1,2) the number
k can take any fixed positive real value (theta-parameter). The unitary
representations of the positive discrete series provide concrete Hilbert spaces
for quantum Schwarzschild black holes.


title:
Field Configurations and their Instability Induced by Higher Dimensions
  of Spacetime: An Example

abstract:
  We use the model of L. Randall et al to investigate the stability of allowed
quantum field configurations. Firstly, we find that due to the topology of this
5 dimensional model, there are 2 possible configurations of the scalar field,
untwisted and twisted. They give rise to two types of instability. Secondly,
when allowed to interact in the 3-brane, the untwisted field becomes unstable
even if it is at the true vacuum groundstate.This instability in the 4D
submanifold results results from one-loop corrections that arise from coupling
with the twisted field. On the other hand, the twisted field can make the two
3-branes distinguishable by causing an energy difference between them. That is
due to the antiperiodicity of the twisted fields, when rotating with $\pi$ to
go from one 3-brane to the other. The energy difference between the branes
renders the fifth dimension unstable.


title: Vortices and black holes in dilatonic gravity

abstract:
  We study analytically black holes pierced by a thin vortex in dilatonic
gravity for an arbitrary coupling of the vortex to the dilaton in an arbitrary
frame. We show that the horizon of the charged black hole supports the
long-range fields of the Nielsen-Olesen vortex that can be considered as black
hole hair for both massive and massless dilatons. We also prove that extremal
black holes exhibit a flux expulsion phenomenon for a sufficiently thick
vortex. We consider the gravitational back-reaction of the thin vortex on the
spacetime geometry and dilaton, and discuss under what circumstances the vortex
can be used to smooth out the singularities in the dilatonic C-metrics. The
effect of the vortex on the massless dilaton is to generate an additional
dilaton flux across the horizon.


title: Dual-Resonator Speed Meter for a Free Test Mass

abstract:
  A description and analysis are given of a ``speed meter'' for monitoring a
classical force that acts on a test mass. This speed meter is based on two
microwave resonators (``dual resonators''), one of which couples evanescently
to the position of the test mass. The sloshing of the resulting signal between
the resonators, and a wise choice of where to place the resonators' output
waveguide, produce a signal in the waveguide that (for sufficiently low
frequencies) is proportional to the test-mass velocity (speed) rather than its
position. This permits the speed meter to achieve force-measurement
sensitivities better than the standard quantum limit (SQL), both when operating
in a narrow-band mode and a wide-band mode. A scrutiny of experimental issues
shows that it is feasible, with current technology, to construct a
demonstration speed meter that beats the wide-band SQL by a factor 2. A concept
is sketched for an adaptation of this speed meter to optical frequencies; this
adaptation forms the basis for a possible LIGO-III interferometer that could
beat the gravitational-wave standard quantum limit h_SQL, but perhaps only by a
factor 1/xi = h_SQL/h ~ 3 (constrained by losses in the optics) and at the
price of a very high circulating optical power --- larger by 1/xi^2 than that
required to reach the SQL.


title:
New Supplementary Conditions for a Non-Linear Field Theory: General
  Relativity

abstract:
  The Einstein theory of general relativity provides a peculiar example of
classical field theory ruled by non-linear partial differential equations. A
number of supplementary conditions (more frequently called gauge conditions)
have also been considered in the literature. In the present paper, starting
from the de Donder gauge, which is not conformally invariant but is the
gravitational counterpart of the Lorenz gauge, we consider, led by geometric
structures on vector bundles, a new family of gauges in general relativity,
which involve fifth-order covariant derivatives of metric perturbations. A
review of recent results by the authors is presented: restrictions on the
general form of the metric on the vector bundle of symmetric rank-two tensor
fields over space-time; admissibility of such gauges in the case of linearized
theory about flat Euclidean space; generalization to a suitable class of curved
Riemannian backgrounds, by solving an integral equation. Eventually, the
applications to Euclidean quantum gravity are discussed.


title:
A Numerical Study of Boson Stars: Einstein Equations with a Matter
  Source

abstract:
  The study of the properties and dynamics of self-gravitating bosonic objects
in Einstein gravity was conducted. We studied self-coupled boson stars and
determined the quasinormal mode (QNM) frequencies of stable boson stars in
spherical symmetry. The study was carried out in the standard Einstein theory
of General Relativity and in Brans-Dicke theory. We also studied the formation
of these objects in Brans-Dicke theory showing that they can form from the
self-gravitation of bosonic matter. We also studied the studied the possibility
of a bosonic halo surrounding galaxies.
  After an extensive study in spherical symmetry we carried out numerical
studies of boson star dynamics in full 3+1 dimension. One focus of the 3D study
was on the validation of the numerical code constructed to solve Einstein
equations with matter sources. Boson Stars do not suffer from the surface
problems of neutron stars or the singularities of black holes. The code was
first tested with spherical perturbations and compared with the spherical
results. We determined the coordinate conditions needed to provide stable
evolutions. We then went on to study their behavior under non-spherical
perturbations. We reproduced the QNM frequencies of the stars, as determined by
perturbation studies carried out by other groups. The energy generated by the
perturbation was studied with different radiation indicators. We also observed
the collapse to black holes of unstable boson-star configurations. We simulated
the collision of two boson stars. This is of interest as the two body problem
is as yet unresolved in general relativity.


title:
Null infinity and an electromagnetic analog of the relativistic Kepler
  problem

abstract:
  This paper has been withdrawn by the author. It has come to my attention that
most of the results reported here have already been obtained by Ludvigsen.


title: Anderson et al. Reply (to the Comment by Katz on Pioneer 10/11)

abstract:
  We conclude that Katz's proposal (anisotropic heat reflection off of the back
of the spacecraft high-gain antennae, the heat coming from the RTGs) does not
provide enough power and so can not explain the Pioneer anomaly.


title: Anderson et al. Reply (to the Comment by Murphy on Pioneer 10/11)

abstract:
  We conclude that Murphy's proposal (radiation of the power of the main-bus
electrical systems from the rear of the craft) can not explain the anomalous
Pioneer acceleration.


title: The Entropy of Taub-Bolt Solution

abstract:
  A geometrical framework for the definition of entropy in General Relativity
via Noether theorem is briefly recalled and the entropy of Taub-Bolt Euclidean
solutions of Einstein equations is then obtained as an application. The
computed entropy agrees with previously known results, obtained by statistical
methods. It was generally believed that the entropy of Taub-Bolt solution could
not be computed via Noether theorem, due to the particular structure of
singularities of this solution. We show here that this is not true. The Misner
string singularity is, in fact, considered and its contribution to the entropy
is analyzed. As a result, in our framework entropy does not obey the
"one-quarter area law" and it is not directly related to horizons, as sometimes
erroneously suggested in current literature on the subject.


title: Large scale-small scale duality and cosmological constant

abstract:
  We study a model of quantum cosmology originating from a classical model of
gravitation where a self interacting scalar field is coupled to gravity with
the metric undergoing a signature transition. We show that there are dual
classical signature changing solutions, one at large scales and the other at
small scales. It is possible to fine-tune the physics in both scales with an
infinitesimal effective cosmological constant.


title: Quantum Mechanics of a Black Hole

abstract:
  Beginning with Bekenstein, many authors have considered a uniformly spaced
discrete quantum spectrum for black hole horizon area. It is also believed that
the huge degeneracy of these area levels corresponds to the notion of black
hole entropy. Starting from these two assumptions we here infer the algebra of
a Schwarzschild black hole's observables. This algebra then serves as
motivation for introducing in the system's Hamiltonian an interaction term. The
interaction contains the horizon area operator, which is a number operator, and
its canonical conjugate, the phase operator. The Hawking radiation from a
Schwarzschild black hole is seen to be a consequence of an area-phase
interaction. Using this interaction we have reproduced the semi-classical
result for the Hawking radiation power. Furthermore, we show that the initial
state of the black hole determines the nature of its development. Thus, a state
which is an area eigenstate describes a static eternal black hole, but a
coherent state describes a radiating black hole. Hence, it is the observer's
initial knowledge or uncertainty about the horizon area which determines the
evolution.


title: Cyclotron motion in a gravitational-wave background

abstract:
  We examine the motion of a relativistic charged particle in a constant
magnetic field perturbed by gravitational waves incident along the direction of
the magnetic field. We apply a generalized energy conservation law to compute
the variations of the kinetic energy of the particle during passage of the
waves. We also explicitly compute the change in the orbit due to a wave with
constant curvature.


title: Neutrinos of Universe and masses of particles

abstract:
  The detection and the research of the neutrinos background of
  Universe are the attractive problems. This problems do not seem the
unpromising one in the case of the high neutrinos density that is necessary for
the explanation of the nucleons-antinucleons asymmetry of Universe. It was
offered before to use the low energy neutrinos background of Universe for the
explanation of the gravitational phenomena with the quantum position attracting
the Casimir's effect for this. As a result it was connected the gravitational
constant with the parameters characterizing the electroweak interactions. If
now we shall be based on the results of the experements fixing the equality of
the gravitation mass and the inert one then it can consider that the spectrum
of the particle masses is defined by their interaction with the neutrinos
background of Universe.


title: Gravitational memory of natural wormholes

abstract:
  A traversable wormhole solution of general scalar-tensor field equations is
presented. We have shown, after a numerical analysis for the behavior of the
scalar field of Brans-Dicke theory, that the solution is completely
singularity--free. Furthermore, the analysis of more general scalar field
dependent coupling constants indicates that the gravitational memory phenomenon
may play an important role in the fate of natural wormholes.


title: Energy-Momentum Tensor of Particles Created in an Expanding Universe

abstract:
  We present a general formulation of the time-dependent initial value problem
for a quantum scalar field of arbitrary mass and curvature coupling in a FRW
cosmological model. We introduce an adiabatic number basis which has the virtue
that the divergent parts of the quantum expectation value of the
energy-momentum tensor <T_ab> are isolated in the vacuum piece of <T_ab>, and
may be removed using adiabatic subtraction. The resulting renormalized <T_ab>
is conserved, independent of the cutoff, and has a physically transparent,
quasiclassical form in terms of the average number of created adiabatic
`particles'. By analyzing the evolution of the adiabatic particle number in de
Sitter spacetime we exhibit the time structure of the particle creation
process, which can be understood in terms of the time at which different
momentum scales enter the horizon. A numerical scheme to compute <T_ab> as a
function of time with arbitrary adiabatic initial states (not necessarily de
Sitter invariant) is described. For minimally coupled, massless fields, at late
times the renormalized <T_ab> goes asymptotically to the de Sitter invariant
state previously found by Allen and Folacci, and not to the zero mass limit of
the Bunch-Davies vacuum. If the mass m and the curvature coupling xi differ
from zero, but satisfy m^2+xi R=0, the energy density and pressure of the
scalar field grow linearly in cosmic time demonstrating that, at least in this
case, backreaction effects become significant and cannot be neglected in de
Sitter spacetime.


title: Curvature invariants in type-III spacetimes

abstract:
  The results of paper [1] are generalized for vacuum type-III solutions with,
in general, a non-vanishing cosmological constant Lambda. It is shown that all
curvature invariants containing derivatives of the Weyl tensor vanish if a
type-III spacetime admits a non-expanding and non-twisting null geodesic
congruence.
  A non-vanishing curvature invariant containing first derivatives of the Weyl
tensor is found in the case of type-III spacetime with expansion or twist.


title:
Regular Bouncing Cosmological Solutions in Effective Actions in Four
  Dimensions

abstract:
  We study cosmological scenarios resulting from effective actions in four
dimensions which are, under some assumptions, connected with multidimensional,
supergravity and string theories. These effective actions are labeled by the
parameters $\omega$, the dilaton coupling constant, and $n$ which establishes
the coupling between the dilaton and a scalar field originated from the gauge
field existing in the original theories. There is a large class of bouncing as
well as Friedmann-like solutions. We investigate under which conditions
bouncing regular solutions can be obtained. In the case of the string effective
action, regularity is obtained through the inclusion of contributions from the
Ramond-Ramond sector of superstring.


title: Relativistic Celestial Mechanics with PPN Parameters

abstract:
  Starting from the global parametrized post-Newtonian (PPN) reference system
with two PPN parameters $\gamma$ and $\beta$ we consider a space-bounded
subsystem of matter and construct a local reference system for that subsystem
in which the influence of external masses reduces to tidal effects. Both the
metric tensor of the local PPN reference system in the first post-Newtonian
approximation as well as the coordinate transformations between the global PPN
reference system and the local one are constructed in explicit form. The terms
proportional to $\eta=4\beta-\gamma-3$ reflecting a violation of the
equivalence principle are discussed in detail. We suggest an empirical
definition of multipole moments which are intended to play the same role in PPN
celestial mechanics as the Blanchet-Damour moments in General Relativity.
Starting with the metric tensor in the local PPN reference system we derive
translational equations of motion of a test particle in that system. The
translational and rotational equations of motion for center of mass and spin of
each of $N$ extended massive bodies possessing arbitrary multipole structure
are derived. As an application of the general equations of motion a
monopole-spin dipole model is considered and the known PPN equations of motion
of mass monopoles with spins are rederived.


title:
Critical gravitational collapse of a perfect fluid: nonspherical
  perturbations

abstract:
  Continuously self-similar (CSS) solutions for the gravitational collapse of a
spherically symmetric perfect fluid, with the equation of state p=kappa rho,
with 0<kappa<1 a constant, are constructed numerically and their linear
perturbations, both spherical and nonspherical, are investigated. The l=1 axial
perturbations admit an analytical treatment. All others are studied
numerically. For intermediate equations of state, with 1/9<kappa<0.49, the CSS
solution has one spherical growing mode, but no nonspherical growing modes.
That suggests that it is a critical solution even in (slightly) nonspherical
collapse. For this range of kappa we predict the critical exponent for the
black hole angular momentum to be 5(1+3kappa)/3(1+kappa) times the critical
exponent for the black hole mass. For kappa=1/3 this gives an angular momentum
critical exponent of mu=0.898, correcting a previous result. For stiff
equations of state, 0.49<kappa<1, the CSS solution has one spherical and
several nonspherical growing modes. For soft equations of state, 0<kappa<1/9,
the CSS solution has 1+3 growing modes: a spherical one, and an l=1 axial mode
(with m=-1,0,1).


title: Inhomogeneous cosmologies with Q-matter and varying $\Lambda$

abstract:
  Starting from the inhomogeneous shear--free Nariai metric we show, by solving
the Einstein--Klein--Gordon field equations, how a self--interacting scalar
field plus a material fluid, a variable cosmological term and a heat flux can
drive the universe to its currently observed state of homogeneous accelerated
expansion. A quintessence scenario where power-law inflation takes place for a
string-motivated potential in the late--time dominated field regime is
proposed.


title: Entropy from Conformal Field Theory at Killing Horizons

abstract:
  On a manifold with boundary, the constraint algebra of general relativity may
acquire a central extension, which can be computed using covariant phase space
techniques. When the boundary is a (local) Killing horizon, a natural set of
boundary conditions leads to a Virasoro subalgebra with a calculable central
charge. Conformal field theory methods may then be used to determine the
density of states at the boundary. I consider a number of cases---black holes,
Rindler space, de Sitter space, Taub-NUT and Taub-Bolt spaces, and dilaton
gravity---and show that the resulting density of states yields the expected
Bekenstein-Hawking entropy. The statistical mechanics of black hole entropy may
thus be fixed by symmetry arguments, independent of details of quantum gravity.


title: Black-Hole Polarization and Cosmic Censorship

abstract:
  The destruction of the black-hole event horizon is ruled out by both cosmic
censorship and the generalized second law of thermodynamics. We test the
consistency of this prediction in a (more) `dangerous' version of the gedanken
experiment suggested by Bekenstein and Rosenzweig. A U(1)-charged particle is
lowered {\it slowly} into a near extremal black hole which is not endowed with
a U(1) gauge field. The energy delivered to the black hole can be {\it
red-shifted} by letting the assimilation point approach the black-hole horizon.
At first sight, therefore, the particle is not hindered from entering the black
hole and removing its horizon. However, we show that this dangerous situation
is excluded by a combination of {\it two} factors not considered in former
gedanken experiments: the effect of the spacetime curvature on the
electrostatic {\it self-interaction} of the charged system (the black-hole
polarization), and the {\it finite} size of the charged body.


title: The Conformal Group SO(4,2) and Robertson-Walker Spacetimes

abstract:
  The Robertson-Walker spacetimes are conformally flat and so are conformally
invariant under the action of the Lie group SO(4,2), the conformal group of
Minkowski spacetime. We find a local coordinate transformation allowing the
Robertson-Walker metric to be written in a manifestly conformally flat form for
all values of the curvature parameter k continuously and use this to obtain the
conformal Killing vectors of the Robertson-Walker spacetimes directly from
those of the Minkowski spacetime. The map between the Minkowski and
Robertson-Walker spacetimes preserves the structure of the Lie algebra so(4,2).
Thus the conformal Killing vector basis obtained does not depend upon k, but
has the disadvantage that it does not contain explicitly a basis for the
Killing vector subalgebra. We present an alternative set of bases that depend
(continuously) on k and contain the Killing vector basis as a sub-basis (these
are compared with a previously published basis). In particular, bases are
presented which include the Killing vectors for all Robertson-Walker spacetimes
with additional symmetry, including the Einstein static spacetimes and the de
Sitter family of spacetimes, where the basis depends on the Ricci scalar R.


title: Cosmological models with variable constants

abstract:
  The behavior of the constants, G,c,h,a,e,m and Lambda, considering them as
variable, in the framework of a flat cosmological model with FRW symmetries
described by a bulk viscous fluid and considering mechanisms of adiabatic
matter creation are investigated. Within two models; one with radiation
predominance and another of matter predominance, this behavior are studied.


title:
Refined Algebraic Quantization in the oscillator representation of
  SL(2,R)

abstract:
  We investigate Refined Algebraic Quantization (RAQ) with group averaging in a
constrained Hamiltonian system with unreduced phase space T^*R^4 and gauge
group SL(2,R). The reduced phase space M is connected and contains four
mutually disconnected `regular' sectors with topology R x S^1, but these
sectors are connected to each other through an exceptional set where M is not a
manifold and where M has non-Hausdorff topology. The RAQ physical Hilbert space
H_{phys} decomposes as H_{phys} = (direct sum of) H_i, where the four subspaces
H_i naturally correspond to the four regular sectors of M. The RAQ observable
algebra A_{obs}, represented on H_{phys}, contains natural subalgebras
represented on each H_i. The group averaging takes place in the oscillator
representation of SL(2,R) on L^2(R^{2,2}), and ensuring convergence requires a
subtle choice for the test state space: the classical analogue of this choice
is to excise from M the exceptional set while nevertheless retaining
information about the connections between the regular sectors. A quantum theory
with the Hilbert space H_{phys} and a finitely-generated observable subalgebra
of A_{obs} is recovered through both Ashtekar's Algebraic Quantization and
Isham's group theoretic quantization.


title:
A Linear-Nonlinear Formulation of Einstein Equations for the Two-Body
  Problem in General Relativity

abstract:
  A formulation of Einstein equations is presented that could yield advantages
in the study of collisions of binary compact objects during regimes between
linear-nonlinear transitions. The key idea behind this formulation is a
separation of the dynamical variables into i) a fixed conformal 3-geometry, ii)
a conformal factor possessing nonlinear dynamics and iii) transverse-traceless
perturbations of the conformal 3-geometry.


title:
Gravitational waves from eccentric compact binaries: Reduction in
  signal-to-noise ratio due to nonoptimal signal processing

abstract:
  Inspiraling compact binaries have been identified as one of the most
promising sources of gravitational waves for interferometric detectors. Most of
these binaries are expected to have circularized by the time their
gravitational waves enter the instrument's frequency band. However, the
possibility that some of the binaries might still possess a significant
eccentricity is not excluded. We imagine a situation in which eccentric signals
are received by the detector but not explicitly searched for in the data
analysis, which uses exclusively circular waveforms as matched filters. We
ascertain the likelihood that these filters, though not optimal, will
nevertheless be successful at capturing the eccentric signals. We do this by
computing the loss in signal-to-noise ratio incurred when searching for
eccentric signals with those nonoptimal filters. We show that for a binary
system of a given total mass, this loss increases with increasing eccentricity.
We show also that for a given eccentricity, the loss decreases as the total
mass is increased.


title: The Ori-Soen time machine

abstract:
  Ori and Soen have proposed a spacetime which has closed causal curves on the
boundary of a region of normal causality, all within a region where the weak
energy condition (positive energy density) is satisfied. I analyze the causal
structure of this spacetime in some simplified models, show that the Cauchy
horizon is compactly generated, and argue that any attempt to build such a
spacetime with normal matter might lead to singular behavior where the
causality violation would otherwise take place.


title:
Unified approach to study quantum properties of primordial black holes,
  wormholes and of quantum cosmology

abstract:
  We review the anomaly induced effective action for dilaton coupled spinors
and scalars in large N and s-wave approximation. It may be applied to study the
following fundamental problems: construction of quantum corrected black holes
(BHs), inducing of primordial wormholes in the early Universe (this effect is
confirmed) and the solution of initial singularity problem. The recently
discovered anti-evaporation of multiple horizon BHs is discussed. The existance
of such primordial BHs may be interpreted as SUSY manifestation. Quantum
corrections to BHs thermodynamics maybe also discussed within such scheme.


title: Who's Afraid of Naked Singularities?

abstract:
  To probe naked spacetime singularities with waves rather than with particles
we study the well-posedness of initial value problems for test scalar fields
with finite energy so that the natural function space of initial data is the
Sobolev space. In the case of static and conformally static spacetimes we
examine the essential self-adjointness of the time translation operator in the
wave equation defined in the Hilbert space. For some spacetimes the classical
singularity becomes regular if probed with waves while stronger classical
singularities remain singular. If the spacetime is regular when probed with
waves we may say that the spacetime is `globally hyperbolic.'


title:
Exact inhomogeneous cosmologies whose source is a radiation-matter
  mixture with consistent thermodynamics

abstract:
  We derive a new class of exact solutions of Einstein's equations providing a
physically plausible hydrodynamical description of cosmological matter in the
radiative era ($10^6 K > T > 10^3 K$), between nucleosynthesis and decoupling.
The solutions are characterized by the Lema\^{\i}tre-Tolman -Bondi metric with
a viscous fluid source, subjected to the following conditions: (a) the
equilibrium state variables satisfy the equation of state of a mixture of an
ultra-relativistic and a non-relativistic ideal gases, where the internal
energy of the latter has been neglected, (b) the particle numbers of the
mixture components are independently conserved, (c) the viscous stress is
consistent with the transport equation and entropy balance law of Extended
Irreversible Thermodynamics, with the coefficient of shear viscosity provided
by Kinetic Theory for the `radiative gas' model. The fulfilment of (a), (b) and
(c) restricts initial conditions in terms of an initial value function,
$\Delta_i^{(s)}$, related to the average of spatial gradients of the
fluctuations of photon entropy per baryon in the initial hypersurface.
Constraints on the observed anisotropy of the microwave cosmic radiation and
the condition that decoupling occurs at $T=T_{_D}\approx 4\times 10^3$ K yield
an estimated value: $|\Delta_i^{(s)}|\approx 10^{-8}$ which can be associated
with a bound on promordial entropy fluctuations. The Jeans mass at decoupling
is of the same order of magnitude as that of baryon dominated perturbation
models ($\approx 10^{16} M_\odot$)

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


title:
Entropies of the general nonextreme stationary axisymmetric black hole:
  statistical mechanics and thermodynamics

abstract:
  Starting from metric of the general nonextreme stationary axisymmetric black
hole in four-dimensional spacetime, both statistical-mechanical and
thermodynamical entropies are studied. First, by means of the "brick wall"
model in which the Dirichlet condition is replaced by a scattering ansatz for
the field functions at the horizon and with Pauli-Villars regularization
scheme, an expression for the statistical-mechanical entropy arising from the
nonminimally coupled scalar fields is obtained. Then, by using the conical
singularity method Mann and Solodukhin's result for the Kerr-Newman black hole
(Phys. Rev. D54, 3932(1996)) is extended to the general stationary black hole
and the nonminimally coupled scalar field. We last shown by comparing the two
results that the statistical-mechanical entropy and one-loop correction to the
thermodynamical entropy are equivalent for coupling $\xi\leq 0$. After
renormalization, a relation between the two entropies is given.


title: The traversable wormhole with classical scalar fields

abstract:
  We study the Lorentzian static traversable wormholes coupled to quadratic
scalar fields. We also obtain the solutions of the scalar fields and matters in
the wormhole background and find that the minimal size of the wormhole should
be quantized under the appropriate boundary conditions for the positive
non-minimal massive scalar field.


title: Wormhole as the end state of two-dimensional black hole evaporation

abstract:
  We present a specific two-dimensional dilaton gravity model in which a black
hole evaporates leaving a wormhole at the end state. As the black hole formed
by infalling matter in a initially static spacetime evaporates by emitting
Hawking radiation, the black hole singularity that is initially hidden behind a
timelike apparent horizon meets the shrinking horizon. At this intersection
point, we imposed boundary conditions which require disappearance of the black
hole singularity and generation of the exotic matter which is the source of the
wormhole as the end state of the black hole. These, of course, preserve energy
conservation and continuity of the metric.


title:
Curvature-induced phase transitions in the inflationary universe -
  Supersymmetric Nambu-Jona-Lasinio Model in de Sitter spacetime -

abstract:
  The phase structure associated with the chiral symmetry is thoroughly
investigated in de Sitter spacetime in the supersymmetric Nambu-Jona-Lasinio
model with supersymmetry breaking terms. The argument is given in the three and
four space-time dimensions in the leading order of the 1/N expansion and it is
shown that the phase characteristics of the chiral symmetry is determined by
the curvature of de Sitter spacetime. It is found that the symmetry breaking
takes place as the first order as well as second order phase transition
depending on the choice of the coupling constant and the parameter associated
with the supersymmetry breaking term. The critical curves expressing the phase
boundary are obtained. We also discuss the model in the context of the chaotic
inflation scenario where topological defects (cosmic strings) develop during
the inflation.


title: Cosmology with two compactification scales

abstract:
  We consider a (4+d)-dimensional spacetime broken up into a (4-n)-dimensional
Minkowski spacetime (where n goes from 1 to 3) and a compact (n+d)-dimensional
manifold. At the present time the n compactification radii are of the order of
the Universe size, while the other d compactification radii are of the order of
the Planck length.


title: Cosmological models with dynamical lambda in scalar-tensor theories

abstract:
  In the context of a family os scalar-tensor theories with a dynamical
$\Lambda$, that is a binomial on the scalar field, the cosmological equations
are considered. A general barotropic state equation $p=(\gamma-1)\rho$, for a
perfect fluid is used for the matter content of the Universe. Some Friedmann-
Robertson-Walker exact solutions are found, they have scale factor wich shows
exponential or power law dependence on time. For some models the singularity
can be avoided. Cosmological parameters as $\Omega_m$, $\Omega_{\Lambda}$,
$q_0$ and $t_0$ are obtained and compared with observational data.


title:
Generalised equilibrium of cosmological fluids in second-order
  thermodynamics

abstract:
  Combining the second-order entropy flow vector of the causal Israel-Stewart
theory with the conformal Killing-vector property of $u_{i}/T$, where $u_{i}$
is the four-velocity of the medium and T its equilibrium temperature, we
investigate generalized equilibrium states for cosmological fluids with
nonconserved particle number. We calculate the corresponding equilibrium
particle production rate and show that this quantity is reduced compared with
the results of the previously studied first-order theory. Generalized
equilibrium for massive particles turns out to be compatible with a dependence
$\rho \propto a ^{-2}$ of the fluid energy density $\rho$ on the scale factor a
of the Robertson-Walker metric and may be regarded as a realization of
so-called K-matter.


title: A point mass in an isotropic universe: III. The region $R\leq 2m$

abstract:
  McVittie's solution of Einstein's field equations, representing a point mass
embedded into an isotropic universe, possesses a scalar curvature singularity
at proper radius $R=2m$. The singularity is space-like and precedes, in the
expanding case, all other events in the space-time. It is shown here that this
singularity is gravitationally weak, and the possible structure of the region
$R\leq 2m$ is investigated. A characterization of this solution which does not
involve asymptotics is given.


title:
Null geodesics in the Alcubierre warp drive spacetime: the view from the
  bridge

abstract:
  The null geodesic equations in the Alcubierre warp drive spacetime are
numerically integrated to determine the angular deflection and redshift of
photons which propagate through the distortion of the ``warp drive'' bubble to
reach an observer at the origin of the warp effect. We find that for a starship
with an effective warp speed exceeding the speed of light, stars in the forward
hemisphere will appear closer to the direction of motion than they would to an
observer at rest. This aberration is qualitatively similar to that caused by
special relativity. Behind the starship, a conical region forms from within
which no signal can reach the starship, an effective ``horizon''. Conversely,
there is also an horizon-like structure in a conical region in front of the
starship, into which the starship cannot send a signal. These causal structures
are somewhat analogous to the Mach cones associated with supersonic fluid flow.
The existence of these structures suggests that the divergence of quantum
vacuum energy when the starship effectively exceeds the speed of light, first
discovered in two dimensions, will likely be present in four dimensions also,
and prevent any warp-drive starship from ever exceeding the effective speed of
light.


title: Semi-Teleparallel Theories of Gravitation

abstract:
  A class of theories of gravitation that naturally incorporates preferred
frames of reference is presented. The underlying space-time geometry consists
of a partial parallelization of space-time and has properties of Riemann-Cartan
as well as teleparallel geometry. Within this geometry, the kinematic
quantities of preferred frames are associated with torsion fields. Using a
variational method, it is shown in which way action functionals for this
geometry can be constructed. For a special action the field equations are
derived and the coupling to spinor fields is discussed.


title:
What happens in the vicinity of the Schwarzschild sphere when nonzero
  graviton rest mass is present

abstract:
  In this paper a solution for a static spherically symmetric body is
thoroughly considered in the framework of the Relativistic Theory of
Gravitation. By the comparison of this solution with the Schwarzschild solution
in General Relativity their substantial difference is established in the region
close to the Schwarzschild sphere. Just this difference excludes the
possibility of collapse to form ``black holes''.


title: Expanding impulsive gravitational waves

abstract:
  We explicitly demonstrate that the known solutions for expanding impulsive
spherical gravitational waves that have been obtained by a "cut and paste"
method may be considered to be impulsive limits of the Robinson-Trautman vacuum
type N solutions. We extend these results to all the generically distinct
subclasses of these solutions in Minkowski, de Sitter and anti-de Sitter
backgrounds. For these we express the solutions in terms of a continuous
metric. Finally, we also extend the class of spherical shock gravitational
waves to include a non-zero cosmological constant.


title: On a class of invariant coframe operators with application to gravity

abstract:
  Let a differential 4D-manifold with a smooth coframe field be given. Consider
the operators on it that are linear in the second order derivatives or
quadratic in the first order derivatives of the coframe, both with coefficients
that depend on the coframe variables. The paper exhibits the class of operators
that are invariant under a general change of coordinates, and, also, invariant
under the global SO(1,3)-transformation of the coframe. A general class of
field equations is constructed. We display two subclasses in it. The subclass
of field equations that are derivable from action principles by free variations
and the subclass of field equations for which spherical-symmetric solutions,
Minkowskian at infinity exist. Then, for the spherical-symmetric solutions, the
resulting metric is computed. Invoking the Geodesic Postulate, we find all the
equations that are experimentally (by the 3 classical tests) indistinguishable
from Einstein field equations. This family includes, of course, also Einstein
equations. Moreover, it is shown, explicitly, how to exhibit it. The basic tool
employed in the paper is an invariant formulation reminiscent of Cartan's
structural equations. The article sheds light on the possibilities and
limitations of the coframe gravity. It may also serve as a general procedure to
derive covariant field equations.


title: A gauge theoretical view of the charge concept in Einstein gravity

abstract:
  We will discuss some analogies between internal gauge theories and gravity in
order to better understand the charge concept in gravity. A dimensional
analysis of gauge theories in general and a strict definition of elementary,
monopole, and topological charges are applied to electromagnetism and to
teleparallelism, a gauge theoretical formulation of Einstein gravity.
  As a result we inevitably find that the gravitational coupling constant has
dimension $\hbar/l^2$, the mass parameter of a particle dimension $\hbar/l$,
and the Schwarzschild mass parameter dimension l (where l means length). These
dimensions confirm the meaning of mass as elementary and as monopole charge of
the translation group, respectively. In detail, we find that the Schwarzschild
mass parameter is a quasi-electric monopole charge of the time translation
whereas the NUT parameter is a quasi-magnetic monopole charge of the time
translation as well as a topological charge. The Kerr parameter and the
electric and magnetic charges are interpreted similarly. We conclude that each
elementary charge of a Casimir operator of the gauge group is the source of a
(quasi-electric) monopole charge of the respective Killing vector.


title:
On bare masses in time-symmetric initial-value solutions for two black
  holes

abstract:
  The Brill-Lindquist time-symmetric initial-value solution for two uncharged
black holes is rederived using the Hamiltonian constraint equation with Dirac
delta distributions as a source for the binary black-hole field. The bare
masses of the Brill-Lindquist black holes are introduced in a way which is
applied, after straightforward modification, to the Misner-Linquist binary
black-hole solution.


title: On the warp drive space-time

abstract:
  In this paper the problem of the quantum stability of the two-dimensional
warp drive spacetime moving with an apparent faster than light velocity is
considered. We regard as a maximum extension beyond the event horizon of that
spacetime its embedding in a three-dimensional Minkowskian space with the
topology of the corresponding Misner space. It is obtained that the interior of
the spaceship bubble becomes then a multiply connected nonchronal region with
closed timelike curves and that the most natural vacuum allows quantum
fluctuations which do not induce any divergent behaviour of the re-normalized
stress-energy tensor, even on the event (Cauchy) chronology horizon. In such a
case, the horizon encloses closed timelike curves only at scales close to the
Planck length, so that the warp drive satisfies the Ford's negative energy-time
inequality. Also found is a connection between the superluminal two-dimensional
warp drive space and two-dimensional gravitational kinks. This connection
allows us to generalize the considered Alcubierre metric to a standard,
nonstatic metric which is only describable on two different coordinate patches


title: The Energy-Momentum Tensor for the Gravitational Field

abstract:
  The search for the gravitational energy-momentum tensor is often qualified as
an attempt of looking for ``the right answer to the wrong question''. This
position does not seem convincing to us. We think that we have found the right
answer to the properly formulated question. We have further developed the field
theoretical formulation of the general relativity which treats gravity as a
non-linear tensor field in flat space-time. The Minkowski metric is a
reflection of experimental facts, not a possible choice of the artificial
``prior geometry''. In this approach, we have arrived at the gravitational
energy-momentum tensor which is: 1) derivable from the Lagrangian in a regular
prescribed way, 2) tensor under arbitrary coordinate transformations, 3)
symmetric in its components, 4) conserved due to the equations of motion
derived from the same Lagrangian, 5) free of the second (highest) derivatives
of the field variables, and 6) is unique up to trivial modifications not
containing the field variables. There is nothing else, in addition to these 6
conditions, that one could demand from an energy-momentum object, acceptable
both on physical and mathematical grounds. The derived gravitational
energy-momentum tensor should be useful in practical applications.


title:
Fluid accretion onto a spherical black hole: relativistic description
  versus Bondi model

abstract:
  We describe general-relativistically a spherically symmetric stationary fluid
accretion onto a black hole. Relativistic effects enhance mass accretion, in
comparison to the Bondi model predictions, in the case when backreaction is
neglected. That enhancement depends on the adiabatic index and the asymptotic
gas temperature and it can magnify accretion by one order in the
ultrarelativistic regime.


title: On the Papapetrou field in vacuum

abstract:
  In this paper we study the electromagnetic fields generated by a Killing
vector field in vacuum space-times (Papapetrou fields). The motivation of this
work is to provide new tools for the resolution of Maxwell's equations as well
as for the search, characterization, and study of exact solutions of Einstein's
equations. The first part of this paper is devoted to an algebraic study in
which we give an explicit and covariant procedure to construct the principal
null directions of a Papapetrou field. In the second part, we focus on the main
differential properties of the principal directions, studying when they are
geodesic, and in that case we compute their associated optical scalars. With
this information we get the conditions that a principal direction of the
Papapetrou field must satisfy in order to be aligned with a multiple principal
direction of the Weyl tensor in the case of algebraically special vacuum
space-times. Finally, we illustrate this study using the Kerr, Kasner and pp
waves space-times.


title: Kerr-AdS and Kerr-dS solutions revisited

abstract:
  We reconsider the Kerr metric with cosmological term $\Lambda$ imposing the
condition that the angular velocity $\omega$ of the dragging of the inertial
frames vanishes at spatial boundaries. Some properties of the extreme black
holes in the revisited solutions are discussed.


title: Semiclassical Limit and Time in Quantum Cosmology

abstract:
  We propose a method to recover the time variable and the classical evolution
of the Universe from the minisuperspace wave function of the Wheeler-DeWitt
equation. Defining a Hamilton-Jacobi characteristic function $W$ as the
imaginary part of the $\ln \Psi$ we can recover the classical solution, and
quantum corrections. The key idea is to let the energy of the Wheeler-DeWitt
equation vanish only after the semiclassical limit is taken.


title: Wave Functions for Quantum Black Hole Formation in Scalar Field Collapse

abstract:
  We study quantum mechanically the self-similar black hole formation by
collapsing scalar field and find the wave functions that give the correct
semiclassical limit. In contrast to classical theory, the wave functions for
the black hole formation even in the supercritical case have not only incoming
flux but also outgoing flux. From this result we compute the rate for the black
hole formation. In the subcritical case our result agrees with the
semiclassical tunneling rate. Furthermore, we show how to recover the classical
evolution of black hole formation from the wave function by defining the
Hamilton-Jacobi characteristic function as $W = \hbar {\rm Im} \ln \psi$. We
find that the quantum corrected apparent horizon deviates from the classical
value only slightly without any qualitative change even in the critical case.


title: Unmissing the Missing Matter in Neo-Kaluza-Klein Universes

abstract:
  Uncompactified KK universes are so intrinsically connected to the otherwise
only empirically required "missing" Dark Matter (DM), that:
  1) They yield a simple prediction which explains both the enigma of the
extra-dimensions' (XD) unobservability and the enigma of the present DM. The
two enigmas are "annihilated" into the hypothesis of "missing light", or better
of "photonland". This eliminates the very need to hypothesize/search/find
DM-candidates of exceptional/exotic properties to explain their darkness.
  2) An early, spontaneous gravitational XD-collapse of their natural 5D-DM
replaces KK's compactification mechanisms and cylindricity condition, and
(partly?) eliminates the quandary of the radically insufficient density
fluctuations.


title: The observable light deflection angle

abstract:
  The physical deflection angle of a light ray propagating in a space-time
supplied with an asymptotically flat metric has to be expressed in terms of the
impact parameter.


title:
Gravitational Lensing Effects of Fermion-Fermion Stars: I. Strong Field
  Case

abstract:
  We investigate a two-component model for gravitational lenses, i.e., the
fermion-fermion star as a dark matter self-gravitating system made from two
kinds of fermions with different masses. We calculate the deflection angles
varying from arcseconds to even degrees. There is one Einstein ring. In
particular, we find three radial critical curves for radial magnifications and
four or five images of a point source. These are different from the case of the
one-component model such as the fermion stars and boson stars. This is due to
the fermion-fermion star being a two-component concentric sphere. Our results
suggest that any possible observations of the number of images more than 3
could imply a polytropic distribution of the mass inside the lens in the
universe.


title: Quantum corrections for the Reisner-Nordstrom black hole

abstract:
  We calculate the quantum corrections of geometric and thermodynamic
quantities for the Reisner-Nordstrom charged black hole, within the context of
2D spherically symmetric dilaton gravity model. Special attention is payed to
the quantum corrections of the extreme Reisner-Nordstrom solution. We find a
state of the extreme black hole with regular behaviour on the horizon.


title: Lightcone fluctuations in quantum gravity and extra dimensions

abstract:
  We discuss how compactified extra dimensions may have potentially observable
effects which grow as the compactification scale decreases. This arises because
of lightcone fluctuations in the uncompactified dimensions which can result in
the broadening of the spectral lines from distant sources. We analyze this
effect in a five dimensional model, and argue that data from gamma ray burst
sources require the compactification length to be greater than about $10^5$ cm
in this model.


title: Late-Time Dynamics of Scalar Fields on Rotating Black Hole Backgrounds

abstract:
  Motivated by results of recent analytic studies, we present a numerical
investigation of the late-time dynamics of scalar test fields on Kerr
backgrounds. We pay particular attention to the issue of mixing of different
multipoles and their fall-off behavior at late times. Confining ourselves to
the special case of axisymmetric modes with equatorial symmetry, we show that,
in agreement with the results of previous work, the late-time behavior is
dominated by the lowest allowed l-multipole. However the numerical results
imply that, in general, the late-time fall-off of the dominating multipole is
different from that in the Schwarzschild case, and seems to be incompatible
with a result of a recently published analytic study.


title: Vortex Dynamics in Superfluids

abstract:
  Superfluid condensates are known to occur in contexts ranging from laboratory
liquid helium to neutron stars, and are also likely to occur in cosmological
phenomena such as axion fields. In the zero temperature limit, such condensates
are describable at a mesoscopic level by irrotational configurations of simple
relativistic perfect fluid models. The general mechanical properties of such
models are presented here in an introductory review giving special attention to
the dynamics of vorticity flux 2-surfaces and the action principles governing
both individual flow trajetories and the evolution of the system as a whole.
Macroscopic rotation of such a condensate requires the presence of a lattice of
quantised vortex defects, whose averaged tension violates perfect fluid
isotropy. It is shown that for any equation of state (relating the mass density
$\rho$ to the pressure $P$) the mesoscopic perfect fluid model can be extended
in a uniquely simple and natural manner to a corresponding macroscopic model
(in a conformally covariant category) that represents the effects of the vortex
fibration anisotropy. The limiting case of an individual vortex defect is shown
to be describable by a (``global'') string type model with a variable tension
${\cal T}$ (obtained as a function of the background fluid density) whose
``vorton'' (i.e. closed loop equilibrium) states are derived as an exercise.


title: On Applications of Campbell's Embedding Theorem

abstract:
  A little known theorem due to Campbell is employed to establish the local
embedding of a wide class of 4-dimensional spacetimes in 5-dimensional
Ricci-flat spaces. An embedding for the class of n-dimensional Einstein spaces
is also found. The local nature of Campbell's theorem is highlighted by
studying the embedding of some lower-dimensional spaces.


title:
Hawking radiation from dilaton gravity in 1 + 1 dimensions: a
  pedagogical review

abstract:
  Hawking radiation in d=4 is regarded as a well understood quantum theoretical
feature of Black Holes or of other geometric backgrounds with an event horizon.
On the other hand, the dilaton theory emerging after spherical reduction and
generalized dilaton theories only during the last years became the subject of
numerous studies which unveiled a surprisingly difficult situation. Recently we
have found some solution to the problem of Hawking flux in spherically reduced
gravity which has the merit of using a minimal input. It leads to exact
cancellation of negative contributions to this radiative flux, encountered in
other approaches at infinity, so that our result asymptotically coincides with
the one of minimally coupled scalars. The use of an integrated action is
avoided - although we have been able to present also that quantity in a closed
expression. This short review also summarizes and critically discusses recent
activities in this field, including the problem of ``conformal frames'' for the
background and questions which seem to be open in our own approach as well as
in others.


title: Classical Signature Change in the Black Hole Topology

abstract:
  Investigations of classical signature change have generally envisaged
applications to cosmological models, usually a
Friedmann-Lemaitre-Robertson-Walker model. The purpose has been to avoid the
inevitable singularity of models with purely Lorentzian signature, replacing
the neighbourhood of the big bang with an initial, singularity free region of
Euclidean signture, and a signature change. We here show that signature change
can also avoid the singularity of gravitational collapse. We investigate the
process of re-birth of Schwarzschild type black holes, modelling it as a double
signature change, joining two universes of Lorentzian signature through a
Euclidean region which provides a `bounce'. We show that this process is viable
both with and without matter present, but realistic models -- which have the
signature change surfaces hidden inside the horizons -- require non-zero
density. In fact the most realistic models are those that start as a finite
cloud of collapsing matter, surrounded by vacuum. We consider how geodesics may
be matched across a signature change surface, and conclude that the particle
`masses' must jump in value. This scenario may be relevant to Smolin's recent
proposal that a form of natural selection operates on the level of universes,
which favours the type of universe we live in.


title: Qualitative Analysis of Early Universe Cosmologies

abstract:
  A qualitative analysis is presented for a class of homogeneous cosmologies
derived from the string effective action when a cosmological constant is
present in the matter sector of the theory. Such a term has significant effects
on the qualitative dynamics. For example, models exist which undergo a series
of oscillations between expanding and contracting phases due to the existence
of a heteroclinic cycle in the phase space. Particular analytical solutions
corresponding to the equilibrium points are also found.


title: High-Order Contamination in the Tail of Gravitational Collapse

abstract:
  It is well known that the late-time behaviour of gravitational collapse is
{\it dominated} by an inverse power-law decaying tail. We calculate {\it
higher-order corrections} to this power-law behaviour in a spherically
symmetric gravitational collapse. The dominant ``contamination'' is shown to
die off at late times as $M^2t^{-4}\ln(t/M)$. This decay rate is much {\it
slower} than has been considered so far. It implies, for instance, that an
`exact' (numerical) determination of the power index to within $\sim 1 %$
requires extremely long integration times of order $10^4 M$. We show that the
leading order fingerprint of the black-hole electric {\it charge} is of order
$Q^2t^{-4}$.


title: Gravitational waves and the cosmological equation of state

abstract:
  Primordial gravitational waves are amplified during eras when their
wavelengths are pushed outside the cosmological horizon. This occurs in both
inflationary and ``pre-big-bang'' or ``bounce'' cosmologies. The spectrum is
expressed as a normalized energy density per unit logarithmic frequency,
denoted Omega. The spectral index (logarithmic slope) of Omega is simply
related to three properties of the early universe: (i) the gravitons' mean
initial quantum occupation number N(n) (=1/2 for a vacuum state), where n is
the (invariant) conformal frequency of the mode, and (ii) & (iii) the parameter
gamma=p/rho of the cosmological equation of state during the epoch when the
waves left the horizon (gamma=gamma_i) and when they reentered (gamma=gamma_f).
In the case of an inflationary cosmology, the spectral index is equal to
  d(ln N)/d(ln n) + 2(gamma_i + 1)/(gamma_i + 1/3) + 2(gamma_f - 1/3)/(gamma_f
+ 1/3)
  and for bounce cosmologies it is equal to
  d(ln N)/d(ln n) + 4(gamma_i)/(gamma_i + 1/3) + 2(gamma_f - 1/3)/(gamma_f +
1/3)
  These expressions are compared against various more model-specific results
given in the literature.


title:
Statistical Mechanics on Axially-symmetric Space-times with the Killing
  Horizon and Entropy of Rotating Black Holes in Induced Gravity

abstract:
  We develop a method for computing the free-energy of a canonical ensemble of
quantum fields near the horizon of a rotating black hole. We show that the
density of energy levels of a quantum field on a stationary background can be
related to the density of levels of the same field on a fiducial static
space-time. The effect of the rotation appears in the additional interaction of
the "static" field with a fiducial abelian gauge-potential. The fiducial static
space-time and the gauge potential are universal, i.e., they are determined by
the geometry of the given physical space-time and do not depend on the spin of
the field. The reduction of the stationary axially symmetric problem to the
static one leads to a considerable simplification in the study of statistical
mechanics and we use it to draw a number of conclusions. First, we prove that
divergences of the entropy of scalar and spinor fields at the horizon in the
presence of rotation have the same form as in the static case and can be
removed by renormalization of the bare black hole entropy. Second, we
demonstrate that statistical-mechanical representation of the
Bekenstein-Hawking entropy of a black hole in induced gravity is universal and
does not depend on the rotation.


title: Dynamics of Bianchi IX universe with massive scalar field

abstract:
  The dynamics of the Bianchi IX cosmological model with minimally coupled
massive real scalar field is studied. The possibility of non-singular
transition from contraction to expansion is shown. A set of initial conditions
that lead to non-singular solutions is studied numerically.


title:
Gravitational waves in vacuum spacetimes with cosmological constant. I.
  Classification and geometrical properties of non-twisting type N solutions

abstract:
  All non-twisting Petrov-type N solutions of vacuum Einstein field equations
with cosmological constant Lambda are summarized. They are shown to belong
either to the non-expanding Kundt class or to the expanding Robinson-Trautman
class. Invariant subclasses of each class are defined and the corresponding
metrics are given explicitly in suitable canonical coordinates. Relations
between the subclasses and their geometrical properties are analyzed. In the
subsequent paper these solutions are interpreted as exact gravitational waves
propagating in de Sitter or anti-de Sitter spacetimes.


title:
Gravitational waves in vacuum spacetimes with cosmological constant. II.
  Deviation of geodesics and interpretation of non-twisting type N solutions

abstract:
  In a suitably chosen essentially unique frame tied to a given observer in a
general spacetime, the equation of geodesic deviation can be decomposed into a
sum of terms describing specific effects: isotropic (background) motions
associated with the cosmological constant, transverse motions corresponding to
the effects of gravitational waves, longitudinal motions, and Coulomb-type
effects. Conditions under which the frame is parallelly transported along a
geodesic are discussed. Suitable coordinates are introduced and an explicit
coordinate form of the frame is determined for spacetimes admitting a
non-twisting null congruence. Specific properties of all non-twisting type N
vacuum solutions with cosmological constant Lambda (non-expanding Kundt class
and expanding Robinson-Trautman class) are then analyzed. It is demonstrated
that these spacetimes can be understood as exact transverse gravitational waves
of two polarization modes "+" and "x", shifted by pi/4, which propagate "on"
Minkowski, de Sitter, or anti-de Sitter backgrounds. It is also shown that the
solutions with Lambda>0 may serve as exact demonstrations of the cosmic
"no-hair" conjecture in radiative spacetimes with no symmetry.


title: Exact General Relativistic Disks with Magnetic Fields

abstract:
  The well-known ``displace, cut, and reflect'' method used to generate cold
disks from given solutions of Einstein equations is extended to solutions of
Einstein-Maxwell equations. Four exact solutions of the these last equations
are used to construct models of hot disks with surface density, azimuthal
pressure, and azimuthal current. The solutions are closely related to Kerr,
Taub-NUT, Lynden-Bell-Pinault and to a one-soliton solution. We find that the
presence of the magnetic field can change in a nontrivial way the different
properties of the disks. In particular, the pure general relativistic
instability studied by Bicak, Lynden-Bell and Katz [Phys. Rev. D47, 4334, 1993]
can be enhanced or cured by different distributions of currents inside the
disk. These currents, outside the disk, generate a variety of axial symmetric
magnetic fields. As far as we know these are the first models of hot disks
studied in the context of general relativity.


title: The cosmological model with an analytic exit from inflation

abstract:
  A cosmological model of homogeneous and isotropic spatially flat Universe
with gravitating self-interacting scalar field is considered. The exact
solution, admitting an analytical exit from inflationary stage into a radiation
era and a matter dominated epoch, is obtained by virtue of ``fine turning of
the potential'' method. We found that an inflationary stage is supported by
decay of higgs bosons in the framework of the solution obtained. Freidmann's
regim is associated with adiabatical expantion of the Universe, filled by the
matter with special equation of state. Thus we presented the exact solution
which solve the problem of transition from an inflationary to a radiation eras
or long standing `exit' problem.


title: Gravity Role in Classical Electrodynamics of Charged Point Source

abstract:
  This paper deals with the problem of a point-like charged source under the
influence of the external electromagnetic field in terms of perturbation theory
for GR equations. It is obtained that GR, in contrast with the classical
electrodynamics, in linear perturbation theory predicts an unlimited growth of
the dipole perturbation. It is shown that the reason for this unlimited
perturbation growth might be related to the presence of the unstable rotational
perturbation mode. The analysis of the conditions under which this instability
may disappear is performed. The momentum value at which the stability is
reached is estimated. These estimations give the electron spin by the order of
magnitude (when charge value is equal to elementary one).


title:
Scalar Field Cosmologies with Barotropic Matter: Models of Bianchi class
  B

abstract:
  We investigate in detail the qualitative behaviour of the class of Bianchi
type B spatially homogeneous cosmological models in which the matter content is
composed of two non-interacting components; the first component is described by
a barotropic fluid having a gamma-law equation of state, whilst the second is a
non-interacting scalar field (phi) with an exponential potential V=Lambda exp(k
phi). In particular, we study the asymptotic properties of the models both at
early and late times, paying particular attention on whether the models
isotropize (and inflate) to the future, and we discuss the genericity of the
cosmological scaling solutions.


title:
Maxwell-Boltzmann, Bose-Einstein, Fermi-Dirac statistical entropies in a
  D-dimensional stationary axisymmetry space-time

abstract:
  Statistical entropies of a general relativistic ideal gas obeying
Maxwell-Boltzmann, Bose-Einstein and Fermi-Dirac statistics are calculated in a
general axisymmetry space-time of arbitrary dimension. This general formation
can be used to discuss the entropy of a quantum field not only in the flat
space-time but also in a curved space-time. It can also be used to compare the
entropies in different dimensional space-times. Analytical expressions for the
thermodynamic potentials are presented, and their behaviors in the high or low
temperature approximation are discussed. The entropy of a quantum field is
shown to be proportional to the volume of optical space or that of the dragged
optical space only in the high temperature approximation or in the zero mass
case. In the case of a black hole, the entropy of a quantum field at the
Hartle-Hawking temperature is proportional to the horizon "area" if and only if
the horizon is located at the light velocity surface.


title:
Detection strategies for scalar gravitational waves with interferometers
  and resonant spheres

abstract:
  We compute the response and the angular pattern function of an interferometer
for a scalar component of gravitational radiation in Brans-Dicke theory. We
examine the problem of detecting a stochastic background of scalar GWs and
compute the scalar overlap reduction function in the correlation between an
interferometer and the monopole mode of a resonant sphere. While the
correlation between two interferometers is maximized taking them as close as
possible, the interferometer-sphere correlation is maximized at a finite value
of f*d, where `f' is the resonance frequency of the sphere and `d' the distance
between the detectors. This defines an optimal resonance frequency of the
sphere as a function of the distance. For the correlation between the Virgo
interferometer located near Pisa and a sphere located in Frascati, near Rome,
we find an optimal resonance frequency f=590 Hz. We also briefly discuss the
difficulties in applying this analysis to the dilaton and moduli fields
predicted by string theory.


title: Inhomogeneous scalar field solutions and inflation

abstract:
  We present new exact cosmological inhomogeneous solutions for gravity coupled
to a scalar field in a general framework specified by the parameter $\lambda$.
The equations of motion (and consequently the solutions) in this framework
correspond either to low-energy string theory or Weyl integrable spacetime
according to the sign of $\lambda$. We show that different inflationary
behaviours are possible, as suggested by the study of the violation of the
strong energy condition. Finally, by the analysis of certain curvature scalars
we found that some of the solutions may be nonsingular.


title: Energetic Quantum Limit in Large-Scale Interferometers

abstract:
  For each optical topology of an interferometric gravitational wave detector,
quantum mechanics dictates a minimum optical power (the ``energetic quantum
limit'') to achieve a given sensitivity. For standard topologies, when one
seeks to beat the standard quantum limit by a substantial factor, the energetic
quantum limit becomes impossibly large. Intracavity readout schemes may do so
with manageable optical powers.


title: Space-Times Admitting Isolated Horizons

abstract:
  We characterize a general solution to the vacuum Einstein equations which
admits isolated horizons. We show it is a non-linear superposition -- in
precise sense -- of the Schwarzschild metric with a certain free data set
propagating tangentially to the horizon. This proves Ashtekar's conjecture
about the structure of spacetime near the isolated horizon. The same
superposition method applied to the Kerr metric gives another class of vacuum
solutions admitting isolated horizons. More generally, a vacuum spacetime
admitting any null, non expanding, shear free surface is characterized. The
results are applied to show that, generically, the non-rotating isolated
horizon does not admit a Killing vector field and a spacetime is not
spherically symmetric near a symmetric horizon.


title: Duality of Coordinates and Matter Fields in Curved Spacetime

abstract:
  We show that there exists a duality between the local coordinates and the
solutions of the Klein-Gordon equation in curved spacetime in the same sense as
in the Minkowski spacetime. However, the duality in curved spacetime does not
have the same generality as in flat spacetime and it holds only if the system
satisfies certain constraints. We derive these constraints and the basic
equations of duality and discuss the implications in the quantum theory.


title: Negative energy density for a Dirac-Maxwell field

abstract:
  It is well known that there can be negative energy densities in quantum field
theory. Most of the work done in this area has involved free non-interacting
systems. In this paper we show how a quantum state with negative energy density
can be formulated for a Dirac field interacting with an Electromagnetic field.
It will be shown that, for this case, there exist quantum states whose average
energy density over an arbitrary volume is a negative number with an
arbitrarily large magnitude.


title: Strength of the null singularity inside black holes

abstract:
  We study analytically the Cauchy horizon singularity inside
spherically-symmetric charged black holes, coupled to a spherical
self-gravitating, minimally-coupled, massless scalar field. We show that all
causal geodesics terminate at the Cauchy horizon at a null singularity, which
is weak according to the Tipler classification. The singularity is also
deformationally-weak in the sense of Ori. Our results are valid at arbitrary
points along the null singularity, in particular at late retarded times, when
non-linear effects are crucial.


title:
Topology of the Future Chronological Boundary: Universality for
  Spacelike Boundaries

abstract:
  A method is presented for imputing a topology for any chronological set,
i.e., a set with a chronology relation, such as a spacetime or a spacetime with
some sort of boundary. This topology is shown to have several good properties,
such as replicating the manifold topology for a spacetime and replicating the
expected topology for some simple examples of spacetime-with-boundary; it also
allows for a complete categorical characterization, in topological categories,
of the Future Causal Boundary construction of Geroch, Kronheimer, and Penrose,
showing that construction to have a universal property for future-completing
chronological sets with spacelike boundaries. Rigidity results are given for
any reasonable future completion of a spacetime, in terms of the GKP boundary:
In the imputed topology, any such boundary must be homeomorphic to the GKP
boundary (if all points have indecomposable pasts) or to a topological quotient
of a closely related boundary (if boundaries are spacelike). A large class of
warped-product-type spacetimes with spacelike boundaries is examined,
calculating the GKP and other possible boundaries, and showing that the imputed
topology gives expected results; included among these are the Schwarzschild
singularity and those Robertson-Walker singularities which are spacelike.


title: Holography, Quantum Geometry, and Quantum Information Theory

abstract:
  We interpret the Holographic Conjecture in terms of quantum bits (qubits).
N-qubit states are associated with surfaces that are punctured in N points by
spin networks' edges labeled by the spin-1/2 representation of SU(2), which are
in a superposed quantum state of spin "up" and spin "down". The formalism is
applied in particular to de Sitter horizons, and leads to a picture of the
early inflationary universe in terms of quantum computation. A discrete
micro-causality emerges, where the time parameter is being defined by the
discrete increase of entropy. Then, the model is analysed in the framework of
the theory of presheaves (varying sets on a causal set) and we get a quantum
history. A (bosonic) Fock space of the whole history is considered. The Fock
space wavefunction, which resembles a Bose-Einstein condensate, undergoes
decoherence at the end of inflation. This fact seems to be responsible for the
rather low entropy of our universe.

categories: gr-qc astro-ph cond-mat hep-th math-ph math.MP quant-ph


title: Quantum Creation of Topological Black Hole

abstract:
  The constrained instanton method is used to study quantum creation of a
vacuum or charged topological black hole. At the $WKB$ level, the relative
creation probability is the exponential of a quarter sum of the horizon areas
associated with the seed instanton.


title: Quantum Creation of BTZ Black Hole

abstract:
  The constrained instanton method is used to study quantum creation of a $BTZ$
black hole. It is found that the relative creation probability is the
exponential of the negative sum of the entropy associated with the outer and
inner black hole horizons. The quantum creations of the 4- or higher
dimensional versions of the $BTZ$ black hole are also studied.