title: Simplified models of electromagnetic and gravitational radiation damping

abstract:
  In previous work the authors analysed the global properties of an approximate
model of radiation damping for charged particles. This work is put into context
and related to the original motivation of understanding approximations used in
the study of gravitational radiation damping. It is examined to what extent the
results obtained previously depend on the particular model chosen. Comparisons
are made with other models for gravitational and electromagnetic fields. The
relation of the kinetic model for which theorems were proved to certain
many-particle models with radiation damping is exhibited.


title:
Thermal noise in half infinite mirrors with non-uniform loss: a slab of
  excess loss in a half infinite mirror

abstract:
  We calculate the thermal noise in half-infinite mirrors containing a layer of
arbitrary thickness and depth made of excessively lossy material but with the
same elastic material properties as the substrate. For the special case of a
thin lossy layer on the surface of the mirror, the excess noise scales as the
ratio of the coating loss to the substrate loss and as the ratio of the coating
thickness to the laser beam spot size. Assuming a silica substrate with a loss
function of 3x10-8 the coating loss must be less than 3x10-5 for a 6 cm spot
size and a 7 micrometers thick coating to avoid increasing the spectral density
of displacement noise by more than 10%. A similar number is obtained for
sapphire test masses.


title: Charge Effects on Gravitational Wave Detectors

abstract:
  We show that the mean-square displacement of a charged oscillator due to the
zero point oscillations of the radiation field is unique in the sense that it
is very sensitive to the value of the bare mass of the charge. Thus, a
controlled experiment using gravitational wave detectors could lead to a
determination of the electron bare mass and shed some light on quantum
electrodynamic theory. We also speculate that the irregular signals of
non-gravitational origin often observed in gravitational wave bar detectors
could be caused by stray charges and that such charges could also adversely
affect LIGO and other such detectors


title: Signature for the Shape of the Universe

abstract:
  If the universe has a nontrivial shape (topology) the sky may show multiple
correlated images of cosmic objects. These correlations can be couched in terms
of distance correlations. We propose a statistical quantity which can be used
to reveal the topological signature of any Robertson-Walker (RW) spacetime with
nontrivial topology. We also show through computer-aided simulations how one
can extract the topological signatures of flat, elliptic, and hyperbolic RW
universes with nontrivial topology.


title: All Universes Great and Small

abstract:
  If the topology of the universe is compact we show how it significantly
changes our assessment of the naturalness of the observed structure of the
universe and the likelihood of its present state of high isotropy and near
flatness arising from generic initial conditions. We also identify the most
general cosmological models with compact space.


title: p-Adic and Adelic Minisuperspace Quantum Cosmology

abstract:
  We consider the formulation and some elaboration of p-adic and adelic quantum
cosmology. The adelic generalization of the Hartle-Hawking proposal does not
work in models with matter fields. p-Adic and adelic minisuperspace quantum
cosmology is well defined as an ordinary application of p-adic and adelic
quantum mechanics. It is illustrated by a few minisuperspace cosmological
models in one, two and three minisuperspace dimensions. As a result of p-adic
quantum effects and the adelic approach, these models exhibit some discreteness
of the minisuperspace and cosmological constant. In particular, discreteness of
the de Sitter space and its cosmological constant is emphasized.


title: Classical Correspondence of Unruh Effect

abstract:
  Derived from semi-classical quantum field theory in curved spacetime, Unruh
effect was known as a quantum effect. We find that there does exist a classical
correspondence of this effect in electrodynamics. The thermal nature of the
vacuum in correlation function for the uniformly accelerated detector is coming
from the non-linear relationship between the proper time and the propagating
length of the electromagnetic wave. Both the Coulomb field of the detector
itself and the radiation supporting the detector's uniformly accelerating
motion contribute to the non-vanishing vacuum energy. From this observation we
conclude that Unruh temperature experienced by a uniformly accelerated
classical electron has no additional effects to Born's solution for laboratory
observers far away from the classical electron.


title: Exact Inflationary Solution On The Brane

abstract:
  We study the evolution of universe with a single scalar field of constant
potential minimally coupled to gravity in the brane world cosmology.We find an
exact inflationary solution which is not in slow roll.We discuss the limiting
cases of the solution.We show that at late times the solution is asymptotic to
the de Sitter solution independently of the brane tension. For $t\to 0$ the
solution leads to singularity but the nature of the approach to singularity
depends upon the brane tension.


title: Wormhole-generated physical universe

abstract:
  We constructed a model where the central core of the universe is a modified
Gidding-Strominger wormhole and surrounding the core is a Robertson-Walker
Universe with k=0. They are separated by a thin wall which does not allow the
content of the inner core to travel to the outer universe. But this wall allows
the pressure of the inner core to be transferred to the outer physical
universe. Assuming that the fluid density of the physical universe is
practically independent of time, we have calculated the Hubble constant and the
deacceleration parameter, qo, of the physical universe at the present time. The
Hubble constant comes out to be positive, whereas qo becomes negative. The
negative signature of this deacceleration parameter conforms to present
experimental data.


title: Scale factor duality in quintessence models ?

abstract:
  We consider several kinds of quintessence models in the framework of scale
factor duality. We show that this symmetry exists only for a very small number
of quintessence potentials. We then apply the duality transformations found to
several analytical solutions. It turns out that, in some cases, the presence of
the potential allows a smooth connection between the pre- and the post-Big Bang
phases. This may be a first step toward the resolution of the singularity
problem.


title: Black holes are one-dimensional

abstract:
  The holographic principle has revealed that physical systems in 3-D space,
black holes included, are basically two-dimensional as far as their information
content is concerned. This conclusion is complemented by one sketched here: as
far as entropy or information flow is concerned, a black hole behaves as a
one-dimensional channel. We define a channel in flat spacetime in thermodynamic
terms, and contrast it with common entropy emitting systems. A black hole is
more like the former: its entropy output is related to the emitted power as it
would be for a one-dimensional channel, and disposal of an information stream
down a black hole is limited by the power invested in the same way as for a
one-dimensional channel.


title:
Late-time evolution of a self-interacting scalar field in the spacetime
  of dilaton black hole

abstract:
  We investigate the late-time tails of self-interacting (massive) scalar
fields in the spacetime of dilaton black hole. Following the no hair theorem we
examine the mechanism by which self-interacting scalar hair decay. We revealed
that the intermediate asymptotic behavior of the considered field perturbations
is dominated by an oscillatory inverse power-law decaying tail. The numerical
simulations showed that at the very late-time massive self-interacting scalar
hair decayed slower than any power law.


title: Proposal of Experimental Test of General Relativity Theory

abstract:
  On the basis of the relativistic mass-energy concept we found that a proper
mass of a test particle in a gravitational field depends on a potential energy,
hence, a freely falling particle has a varying proper mass. Consequently, a
multitude of freely falling reference frames cannot be regarded as a multitude
of equivalent inertial reference frames. There is a class of experiments, in
which an inner observer can distinguish between the state of free fall in a
gravitational field and the state of free space by detecting the effect of a
proper mass variation. If so, a demonstration of a violation of the Equivalence
Principle is possible. It is shown that a variant of the classical
Pound-Rebka-Snider experiment on a photon frequency shift in a gravitational
field, if conducted in a freely falling laboratory, would be such a test.
  Abbreviation: SRT- the Special Relativity Theory, GRT- the General Relativity
Theory, EP - the Equivalence Principle, PRS - the Pound-Rebka-Snider
(experiment)


title: Brane-world generalizations of the Einstein static universe

abstract:
  A static Friedmann brane in a 5-dimensional bulk (Randall-Sundrum type
scenario) can have a very different relation between the density, pressure,
curvature and cosmological constant than in the case of the general
relativistic Einstein static universe. In particular, static Friedmann branes
with zero cosmological constant and 3-curvature, but satisfying rho>0 and
rho+3p>0, are possible. Furthermore, we find static Friedmann branes in a bulk
that satisfies the Einstein equations but is not Schwarzschild-anti de Sitter
or its specializations. In the models with negative bulk cosmological constant,
a positive brane tension leads to negative density and 3-curvature.


title: On Higher-Dimensional Dynamics

abstract:
  Technical results are presented on motion in N(>4)D manifolds to clarify the
physics of Kaluza-Klein theory, brane theory and string theory. The so-called
canonical or warp metric in 5D effectively converts the manifold from a
coordinate space to a momentum space, resulting in a new force (per unit mass)
parallel to the 4D velocity. The form of this extra force is actually
independent of the form of the metric, but for an unbound particle is tiny
because it is set by the energy density of the vacuum or cosmological constant.
It can be related to a small change in the rest mass of a particle, and can be
evaluated in two convenient gauges relevant to gravitational and quantum
systems. In the quantum gauge, the extra force leads to Heisenberg's relation
between increments in the position and momenta. If the 4D action is quantized
then so is the higher-dimensional part, implying that particle mass is
quantized, though only at a level of 10^{-65} gm or less which is unobservably
small. It is noted that massive particles which move on timeline paths in 4D
can move on null paths in 5D. This agrees with the view from inflationary
quantum field theory, that particles acquire mass dynamically in 4D but are
intrinsically massless. A general prescription for dynamics is outlined,
wherein particles move on null paths in an N(>4)D manifold which may be flat,
but have masses set by an embedded 4D manifold which is curved.


title:
Testing Relativistic Effect of Propagation of Gravity by Very-Long
  Baseline Interferometry

abstract:
  It is shown that the finite speed of gravity affects very-long baseline
interferometric observations of quasars during the time of their line-of-sight
close angular encounter with Jupiter. The next such event will take place in
2002, September 8. The present Letter suggests a new experimental test of
general relativity in which the effect of propagation of gravity can be
directly measured by very-long baseline interferometry as an excess time delay
in addition to the logarithmic Shapiro time delay (Shapiro, I. I., 1964, Phys.
Rev. Lett., 13, 789).

categories: gr-qc astro-ph hep-ph hep-th physics.space-ph


title: A Planck-like problem for quantum charged black holes

abstract:
  Motivated by the parallelism existing between the puzzles of classical
physics at the beginning of the XXth century and the current paradoxes in the
search of a quantum theory of gravity, we give, in analogy with Planck's black
body radiation problem, a solution for the exact Hawking flux of evaporating
Reissner-Nordstrom black holes. Our results show that when back-reaction
effects are fully taken into account the standard picture of black hole
evaporation is significantly altered, thus implying a possible resolution of
the information loss problem.


title:
An analysis of helium primordial nucleosynthesis with a variable
  cosmological coupling

abstract:
  The synthesis of helium in the early Universe depends on many input
parameters, including the value of the gravitational coupling during the period
when the nucleosynthesis takes place. We compute the primordial abundance of
helium as function of the gravitational coupling, using a semi-analytical
method, in order to track the influence of $G$ in the primordial
nucleosynthesis. To be specific, we construct a cosmological model with varying
$G$, using the Brans-Dicke theory. The greater the value of $G$ at
nucleosynthesis period, the greater the abundance of helium predicted. Using
the observational data for the abundance of the primordial helium, constraints
for the time variation of $G$ are established.


title: The Quantum Physics of Black Holes: Results from String Theory

abstract:
  We review recent progress in our understanding of the physics of black holes.
In particular, we discuss the ideas from string theory that explain the entropy
of black holes from a counting of microstates of the hole, and the related
derivation of unitary Hawking radiation from such holes.


title: Regular Type III and Type N Approximate Solutions

abstract:
  New type III and type N approximate solutions which are regular in the linear
approximation are shown to exist. For that, we use complex transformations on
self-dual Robinson-Trautman metrics rather then the classical approach. The
regularity criterion is the boundedness and vanishing at infinity of a scalar
obtained by saturating the Bel-Robinson tensor of the first approximation by a
time-like vector which is constant with respect to the zeroth approximation.


title:
Symmetries and geodesics in (anti-)de Sitter spacetimes with
  nonexpanding impulsive waves

abstract:
  We consider a class of exact solutions which represent nonexpanding impulsive
waves in backgrounds with nonzero cosmological constant. Using a convenient
5-dimensional formalism it is shown that these spacetimes admit at least three
global Killing vector fields. The same geometrical approach enables us to find
all geodesics in a simple explicit form and describe the effect of impulsive
waves on test particles. Timelike geodesics in the axially-symmetric
Hotta-Tanaka spacetime are studied in detail. It is also demonstrated that for
vanishing cosmological constant, the symmetries and geodesics reduce to those
for well-known impulsive pp-waves.


title: Dimensionally Dependent Tensor Identities by Double Antisymmetrisation

abstract:
  Some years ago, Lovelock showed that a number of apparently unrelated
familiar tensor identities had a common structure, and could all be considered
consequences in n-dimensional space of a pair of fundamental identities
involving trace-free (p,p)-forms where 2p >= n$. We generalise Lovelock's
results, and by using the fact that associated with any tensor in n-dimensional
space there is associated a fundamental tensor identity obtained by
antisymmetrising over n+1 indices, we establish a very general 'master'
identity for all trace-free (k,l)-forms. We then show how various other special
identities are direct and simple consequences of this master identity; in
particular we give direct application to Maxwell, Lanczos, Ricci, Bel and
Bel-Robinson tensors, and also demonstrate how relationships between scalar
invariants of the Riemann tensor can be investigated in a systematic manner.


title: The Inverse Scale Factor in Isotropic Quantum Geometry

abstract:
  The inverse scale factor, which in classical cosmological models diverges at
the singularity, is quantized in isotropic models of loop quantum cosmology by
using techniques which have been developed in quantum geometry for a
quantization of general relativity. This procedure results in a bounded
operator which is diagonalizable simultaneously with the volume operator and
whose eigenvalues are determined explicitly. For large scale factors (in fact,
up to a scale factor slightly above the Planck length) the eigenvalues are
close to the classical expectation, whereas below the Planck length there are
large deviations leading to a non-diverging behavior of the inverse scale
factor even though the scale factor has vanishing eigenvalues. This is a first
indication that the classical singularity is better behaved in loop quantum
cosmology.


title: Kinematics of a Spacetime with an Infinite Cosmological Constant

abstract:
  A solution of the sourceless Einstein's equation with an infinite value for
the cosmological constant \Lambda is discussed by using Inonu-Wigner
contractions of the de Sitter groups and spaces. When \Lambda --> infinity,
spacetime becomes a four-dimensional cone, dual to Minkowski space by a
spacetime inversion. This inversion relates the four-cone vertex to the
infinity of Minkowski space, and the four-cone infinity to the Minkowski
light-cone. The non-relativistic limit c --> infinity is further considered,
the kinematical group in this case being a modified Galilei group in which the
space and time translations are replaced by the non-relativistic limits of the
corresponding proper conformal transformations. This group presents the same
abstract Lie algebra as the Galilei group and can be named the conformal
Galilei group. The results may be of interest to the early Universe Cosmology.


title: Photon Decay at the Schwarzschild Horizon

abstract:
  A recent proposal that gravity theory is an emergent phenomenon also entails
the possibility of photon decay near the Schwarzschild event horizon. We
present a possible mechanism for such decay, which utilizes a dimensional
reduction near the horizon.


title: Microlensing by natural wormholes: theory and simulations

abstract:
  We provide an in depth study of the theoretical peculiarities that arise in
effective negative mass lensing, both for the case of a point mass lens and
source, and for extended source situations. We describe novel observational
signatures arising in the case of a source lensed by a negative mass. We show
that a negative mass lens produces total or partial eclipse of the source in
the umbra region and also show that the usual Shapiro time delay is replaced
with an equivalent time gain. We describe these features both theoretically, as
well as through numerical simulations. We provide negative mass microlensing
simulations for various intensity profiles and discuss the differences between
them. The light curves for microlensing events are presented and contrasted
with those due to lensing produced by normal matter. Presence or absence of
these features in the observed microlensing events can shed light on the
existence of natural wormholes in the Universe.


title: Physical Components, Coordinate Components, and the Speed of Light

abstract:
  For generalized coordinate systems, the numerical values of vector and tensor
components do not generally equal the physical values, i.e., the values one
would measure with standard physical instruments. Hence, calculating physical
components from coordinate components is important for comparing experiment
with theory. Surprisingly, however, this calculational method is not widely
known among physicists, and is rarely taught in relativity courses, though it
is commonly employed in at least one other field (applied mechanics.) Different
derivations of this method, ranging from elementary to advanced level, are
presented. The result is then applied to clarify the oftentimes confusing issue
of whether or not the speed of light in non-inertial frames is equal to c.


title: Homogeneous and Isotropic Charge Densities to Accelerate the Universe

abstract:
  Various theoretical obstacles are associated with a homogeneous and isotropic
distribution of ``charge'' which is subject to a repulsive, long-range force.
We show how these can be overcome, for all practical purposes, by the simple
device of endowing the particle which carries the force with a small mass. The
resulting situation may be relevant to a phase of cosmological acceleration
which is triggered by the approach to masslessness of such a force carrier.


title: On the detection of scalar hair

abstract:
  It is shown that the conclusion regarding the existence of a scalar hair for
a black hole in a nonminimally coupled self interacting scalar tensor theory
can be drawn from the that for a scalar field minimally coupled to gravity by
means of a conformal transformation.


title: Induced Matter and Particle Motion in Non-Compact Kaluza-Klein Gravity

abstract:
  We examine generalizations of the five-dimensional canonical metric by
including a dependence of the extra coordinate in the four-dimensional metric.
We discuss a more appropriate way to interpret the four-dimensional
energy-momentum tensor induced from the five-dimensional space-time and show it
can lead to quite different physical situations depending on the interpretation
chosen. Furthermore, we show that the assumption of five-dimensional null
trajectories in Kaluza-Klein gravity can correspond to either four-dimensional
massive or null trajectories when the path parameterization is chosen properly.
Retaining the extra-coordinate dependence in the metric, we show the
possibility of a cosmological variation in the rest masses of particles and a
consequent departure from four-dimensional geodesic motion by a geometric
force. In the examples given, we show that at late times it is possible for
particles traveling along 5D null geodesics to be in a frame consistent with
the induced matter scenario.


title:
Acceleration, streamlines and potential flows in general relativity:
  analytical and numerical results

abstract:
  Analytical and numerical solutions for the integral curves of the velocity
field (streamlines) of a steady-state flow of an ideal fluid with $p = \rho$
equation of state are presented. The streamlines associated with an accelerate
black hole and a rigid sphere are studied in some detail, as well as, the
velocity fields of a black hole and a rigid sphere in an external dipolar field
(constant acceleration field). In the latter case the dipole field is produced
by an axially symmetric halo or shell of matter. For each case the fluid
density is studied using contour lines. We found that the presence of
acceleration is detected by these contour lines. As far as we know this is the
first time that the integral curves of the velocity field for accelerate
objects and related spacetimes are studied in general relativity.


title: R0 Cosmology ?

abstract:
  It is postulated that the action of the FRW-universe is the cosmological term
of Einsteins theory (no curvature term - R0 Cosmology). The expansion equation
emerging from the embedding of this most simple brane world with variable speed
of light is deduced. The universal dimensionless coupling constant of gravity
is addressed. Some implications on the deep problems of cosmology are
discussed.


title: Self-similar collapse of a massless scalar field in three-dimensions

abstract:
  We study an analytical solution to the Einstein's equations in
2+1-dimensions, representing the self-similar collapse of a circularly
symmetric, minimally coupled, massless, scalar field. Depending on the value of
certain parameters, this solution represents the formation of black holes.
Since our solution is asymptotically flat, our black holes do not have the BTZ
space-time as their long time limit. They represent a new family of black holes
in 2+1-dimensions.


title: Quantum Dilaton Gravity in Two Dimensions with Matter

abstract:
  In this thesis special emphasis is put on the quantization of the spherically
reduced Einstein-massless-Klein-Gordon model using a first order approach for
geometric quantities, because phenomenologically it is probably the most
relevant of all dilaton models with matter. After a Hamiltonian BRST analysis
path integral quantization is performed using temporal gauge for the Cartan
variables. Retrospectively, the simpler Faddeev-Popov approach turns out to be
sufficient. It is possible to eliminate all unphysical and geometric quantities
establishing a non-local and non-polynomial action depending solely on the
scalar field and on some integration constants, fixed by suitable boundary
conditions on the asymptotic effective line element.
  Then, attention is turned to the evaluation of the (two) lowest order tree
vertices, explicitly assuming a perturbative expansion in the scalar field
being valid. Each of them diverges, but unexpected cancellations yield a finite
S-matrix element when both contributions are summed. The phenomenon of a
"virtual black hole" -- already encountered in the simpler case of minimally
coupled scalars in two dimensions -- occurs, as the study of the (matter
dependent) metric reveals. A discussion of the scattering amplitude leads to
the prediction of gravitational decay of spherical waves, a novel physical
phenomenon. Several possible extensions conclude this dissertation.


title:
Testing the gravitomagnetic clock effect on the Earth with neutron
  interferometry

abstract:
  The general relativistic gravitomagnetic clock effect consists in the fact
that two point particles orbiting a central spinning object along identical,
circular equatorial geodesic paths, but in opposite directions, exhibit a time
difference in describing a full revolution. It turns out that the particle
rotating in the same sense of the central body is slower than the particle
rotating in the opposite sense. In this paper it is proposed to measure such
effect in an Earth laboratory experiment involving interferometry of slow
neutrons. With a sphere of 2.5 cm radius and spinning at 4.3 x 10^4 rad/s as
central source, and using neutrons with wavelength of 1 Angstrom it should be
possible to obtain, for a given sense of rotation of the central source, a
phase shift of 0.18 rad, well within the experimental sensitivity. By reversing
the sense of rotation of the central body it should be possible to obtain a
0.06 fringe shift.


title:
Bouncing open universes embeddable in a distorted Randall-Sundrum brane
  scenario

abstract:
  In reference gr-qc/0104036 a four-dimensional effective theory of gravity
embeddable in a five-dimensional "distorted" Randall-Sundrum brane scenario was
derived. The present paper is aimed at the application of such a theory to
describe physics in an open Friedmann-Robertson-Walker (Weyl-symmetric)
universe. It is shown that regular bouncing universes arise for a given range
of the free parameter of the theory.


title: New variables for the Lema\^{\i}tre-Tolman-Bondi dust solutions

abstract:
  We re-examine the Lem\^aitre-Tolman-Bondi (LTB) solutions with a dust source
admitting symmetry centers. We consider as free parameters of the solutions the
initial value functions: $Y_i$, $\rho_i$ and $\Ri$, obtained by restricting the
curvature radius, $Y\equiv \sqrt{g_{\theta\theta}}$, the rest mass density,
$\rho$, and the 3-dimensional Ricci scalar of the rest frames, $\R$, to an
arbitrary regular Cauchy hypersurface, $\Ti$, marked by constant cosmic time
($t=t_i$). Using $Y_i$ to fix the radial coordinate and the topology
(homeomorphic class) of $\Ti$, and scaling the time evolution in terms of an
adimensional scale factor $y=Y/Y_i$, we show that the dynamics, regularity
conditions and geometric features of the models are determined by $\rho_i$,
$\Ri$ and by suitably constructed volume averages and contrast functions
expressible in terms of invariant scalars defined in $\Ti$. These quantities
lead to a straightforward characterization of initial conditions in terms of
the nature of the inhomogeneity of $\Ti$, as density and/or curvature
overdensities (``lumps'') and underdensities (''voids'') around a symmetry
center. In general, only models with initial density and curvature lumps evolve
without shell crossing singularities, though special classes of initial
conditions, associated with a simmultaneous big bang, allow for a regular
evolution for initial density and curvature voids. Specific restrictions are
found so that a regular evolution for $t>t_i$ is possible for initial voids. A
step-by-step guideline is provided for using the new variables in the
construction of LTB models and for plotting all relevant quantities.


title: Dilatonic Interpretation of the Quintessence?

abstract:
  We discuss the possibility that "quintessential effects", recently displayed
by large scale observations, may be consistently described in the context of
the low-energy string effective action, and we suggest a possible approach to
the problem of the cosmic coincidence based on the link between the strength of
the dilaton couplings and the cosmological state of our Universe.


title: General relativistic analysis of peculiar velocities

abstract:
  We give a careful general relativistic and (1+3)-covariant analysis of
cosmological peculiar velocities induced by matter density perturbations in the
presence of a cosmological constant. In our quasi-Newtonian approach,
constraint equations arise to maintain zero shear of the non-comoving
fundamental worldlines which define a Newtonian-like frame, and these lead to
the (1+3)-covariant dynamical equations, including a generalized Poisson-type
equation. We investigate the relation between peculiar velocity and peculiar
acceleration, finding the conditions under which they are aligned. In this case
we find (1+3)-covariant relativistic generalizations of well-known Newtonian
results.


title:
Dirac's Observables for the Rest-Frame Instant Form of Tetrad Gravity in
  a Completely Fixed 3-Orthogonal Gauge

abstract:
  We define the {\it rest-frame instant form} of tetrad gravity restricted to
Christodoulou-Klainermann spacetimes. After a study of the Hamiltonian group of
gauge transformations generated by the 14 first class constraints of the
theory, we define and solve the multitemporal equations associated with the
rotation and space diffeomorphism constraints, finding how the cotriads and
their momenta depend on the corresponding gauge variables. This allows to find
quasi-Shanmugadhasan canonical transformation to the class of 3-orthogonal
gauges and to find the Dirac observables for superspace in these gauges.
  The construction of the explicit form of the transformation and of the
solution of the rotation and supermomentum constraints is reduced to solve a
system of elliptic linear and quasi-linear partial differential equations. We
then show that the superhamiltonian constraint becomes the Lichnerowicz
equation for the conformal factor of the 3-metric and that the last gauge
variable is the momentum conjugated to the conformal factor. The gauge
transformations generated by the superhamiltonian constraint perform the
transitions among the allowed foliations of spacetime, so that the theory is
independent from its 3+1 splittings. In the special 3-orthogonal gauge defined
by the vanishing of the conformal factor momentum we determine the final Dirac
observables for the gravitational field even if we are not able to solve the
Lichnerowicz equation. The final Hamiltonian is the weak ADM energy restricted
to this completely fixed gauge.


title: On naked singularities in higher dimensional Vaidya space-times

abstract:
  We investigate the end state of gravitational collapse of null fluid in
higher dimensional space-times. Both naked singularities and black holes are
shown to be developing as final outcome of the collapse. The naked singularity
spectrum in collapsing Vaidya region (4D) gets covered with increase in
dimensions and hence higher dimensions favor black hole in comparison to naked
singularity. The Cosmic Censorship Conjecture will be fully respected for a
space of infinite dimension.


title: Plane symmetric analogue of NUT space

abstract:
  In this article on the basis of a new definition of spacetime symmetry, which
is in accordance with the symmetry of the curvature invariants, we investigate
exact vacuum solutions of Einstein field equations corresponding to both static
and stationary plane symmetric spacetimes using the concepts of the
(1+3)-decomposition or threading formalism. Demanding the presence of a plane
symmetric gravitomagnetic field we find a family of two parameter (m and l)
solutions, every member of which being the plane symmetric analogue of NUT
space.


title:
A comment on a proposal to use neutrons to reveal gravitomagnetic
  effects on Earth

abstract:
  Arguments are presented which show that the recent proposals of using
neutrons to test gravitomagnetic effects on Earth are impracticable.


title: A family of heavenly metrics

abstract:
  This is a corrected and essentially extended version of the previous preprint
arXiv:gr-qc/0105088 v4 (2002) by Y. Nutku and M. Sheftel containing new
results. It is being published now in honor of Y. Nutku's memory. All
responsibility for the additions and changes must be attributed to M. Sheftel.
We present new anti-self-dual exact solutions of the Einstein field equations
with Euclidean and neutral (ultra-hyperbolic) signatures that admit only one
rotational Killing vector. Such solutions of the Einstein field equations are
determined by non-invariant solutions of Boyer-Finley ($BF$) equation. For the
case of Euclidean signature such a solution of the $BF$ equation was first
constructed by Calderbank and Tod. Two years later, Martina, Sheftel and
Winternitz applied the method of group foliation to the Boyer-Finley equation
and reproduced the Calderbank-Tod solution together with new solutions for the
neutral signature. In the case of Euclidean signature we obtain new metrics
which asymptotically locally look like a flat space and have a non-removable
singular point at the origin. In the case of ultra-hyperbolic signature there
exist three inequivalent forms of metric. Only one of these can be obtained by
analytic continuation from the Calderbank-Tod solution whereas the other two
are new.


title:
Some remarks on black hole temperature and the second law of
  thermodynamics

abstract:
  I present a formulation of the second law of thermodynamics in the presence
of black holes which makes use of the efficiency of an ideal machine extracting
heat cyclically from a black hole. The Carnot coefficient is found and it is
shown to be a simple function of the mass.


title:
Cosmological Constant, Quintessence and Expansive Nondecelerative
  Universe

abstract:
  Recent observations of the Universe have led to a conclusion suppressing an
up-to-now supposed deceleration of the Universe caused by attractive
gravitational forces. Contrary, there is a renaissance of the cosmological
member lambda and introduction of enigmatic repulsive dark energy in attempts
to rationalize a would-be acceleration of the Universe expansion. It is
documented that the model of Expansive Nondecelerative Universe is capable to
offer acceptable answers to the questions on the Universe expansion, state
equations of the Universe, the parameter omega, the cosmological member lambda
without any necessity to introduce new strange kinds of matter or energy being
in accord with the fundamental conservation laws and generally accepted
parameters of the Universe.


title:
The statistic thermodynamical entropy of de Sitter space and it's 1-loop
  correction

abstract:
  From the partition function of canonical ensemble we derive the entropy of
the de Sitter space by anti-Wick rotation. And then from the one-loop bubble
$S^2\times S^2$ created from vacuum fluctuation in de Sitter background space,
we obtain the one-loop quantum correction to the entropy of de Sitter space.


title: Black holes in three-dimensional Einstein-Born-Infeld-dilaton theory

abstract:
  The three-dimensional static and circularly symmetric solution of the
Einstein-Born-Infeld-dilaton system is derived. The solutions corresponding to
low energy string theory are investigated in detail, which include black hole
solutions if the cosmological constant is negative and the mass parameter
exceeds a certain critical value. Some differences between the Born-Infeld
nonlinear electrodynamics and the Maxwell electrodynamics are revealed.


title: On the Penrose inequality

abstract:
  The purpose of this letter is to point out an argument which may ultimately
lead to a rigorous proof of the Penrose inequality in the general case. The
argument is a variation of Geroch's original proposal for a proof of the
positive energy theorem which was later adapted by Jang and Wald to apply to
initial data sets containing apparent horizons. The new input is to dispense
with the a priori restriction to an initial data set and to use the
four-dimensional structure of spacetime in an essential way.


title: Computer algebra in gravity

abstract:
  We survey the application of computer algebra in the context of gravitational
theories. After some general remarks, we show of how to check the second
Bianchi-identity by means of the Reduce package Excalc. Subsequently we list
some computer algebra systems and packages relevant to applications in
gravitational physics. We conclude by presenting a couple of typical examples.


title:
Type II Fluid Solutions to Einstein Field Equations in N-Dimensional
  Spherical Spacetimes

abstract:
  A large class of Type II fluid solutions to Einstein field equations in
N-dimensional spherical spacetimes is found, wich includes most of the known
solutions. A family of the generalized collapsing Vaidya solutions with
homothetic self-similarity, parametrized by a constant $\lambda$, is studied,
and found that when $\lambda$ $>$ $\lambda_c(N)$, the collapse always forms
black holes, and when $\lambda$ $<$ $\lambda_c(N)$, it always forms naked
singularities, where $\lambda_c(N)$ is function of the spacetime dimension N
only.


title: Gravitoelectromagnetism: Just a Big Word?

abstract:
  Arguments are made in favor of broadening the scope of the various approaches
to splitting spacetime into a single common framework in which measured
quantities, derivative operations, and adapted coordinate systems are clearly
understood in terms of associated test observer families. This ``relativity of
splitting formalisms" for fully nonlinear gravitational theory has been tagged
with the name "gravitoelectromagnetism" because of the well known analogy
between its linearization and electromagnetism, and it allows relationships
between the various approaches to be better understood and makes it easier to
extrapolate familiarity with one approach to the others. This is important
since particular problems or particular features of those problems in
gravitational theory are better suited to different approaches, and the present
barriers between the proponents of each individual approach sometimes prevent
the best match from occurring.


title: Quantum general invariance and loop gravity

abstract:
  A quantum physical projector is proposed for generally covariant theories
which are derivable from a Lagrangian. The projector is the quantum analogue of
the integral over the generators of finite one-parameter subgroups of the gauge
symmetry transformations which are connected to the identity. Gauge variables
are retained in this formalism, thus permitting the construction of spacetime
area and volume operators in a tentative spacetime loop formulation of quantum
general relativity.


title:
Gravitational waves from inspiralling compact binaries: Energy flux to
  third post-Newtonian order

abstract:
  The multipolar-post-Minkowskian approach to gravitational radiation is
applied to the problem of the generation of waves by the compact binary
inspiral. We investigate specifically the third post-Newtonian (3PN)
approximation in the total energy flux. The new results are the computation of
the mass quadrupole moment of the binary to the 3PN order, and the current
quadrupole and mass octupole to the 2PN order. Wave tails and tails of tails in
the far zone are included up to the 3.5PN order. The recently derived 3PN
equations of binary motion are used to compute the time-derivatives of the
moments. We find perfect agreement to the 3.5PN order with perturbation
calculations of black holes in the test-mass limit for one body. Technical
inputs in our computation include a model of point particles for describing the
compact objects, and the Hadamard self-field regularization. Because of a
physical incompleteness of the Hadamard regularization at the 3PN order, the
energy flux depends on one unknown physical parameter, which is a combination
of a parameter \lambda in the equations of motion, and a new parameter \theta
coming from the quadrupole moment.


title:
Gravitational-Wave Inspiral of Compact Binary Systems to 7/2
  Post-Newtonian Order

abstract:
  The inspiral of compact binaries, driven by gravitational-radiation reaction,
is investigated through 7/2 post-Newtonian (3.5PN) order beyond the quadrupole
radiation. We outline the derivation of the 3.5PN-accurate binary's
center-of-mass energy and emitted gravitational flux. The analysis consistently
includes the relativistic effects in the binary's equations of motion and
multipole moments, as well as the contributions of tails, and tails of tails,
in the wave zone. However the result is not fully determined because of some
physical incompleteness, present at the 3PN order, of the model of
point-particle and the associated Hadamard-type self-field regularization. The
orbital phase, whose prior knowledge is crucial for searching and analyzing the
inspiral signal, is computed from the standard energy balance argument.


title:
Robust statistics for deterministic and stochastic gravitational waves
  in non-Gaussian noise I: Frequentist analyses

abstract:
  Gravitational wave detectors will need optimal signal-processing algorithms
to extract weak signals from the detector noise. Most algorithms designed to
date are based on the unrealistic assumption that the detector noise may be
modeled as a stationary Gaussian process. However most experiments exhibit a
non-Gaussian ``tail'' in the probability distribution. This ``excess'' of large
signals can be a troublesome source of false alarms. This article derives an
optimal (in the Neyman-Pearson sense, for weak signals) signal processing
strategy when the detector noise is non-Gaussian and exhibits tail terms. This
strategy is robust, meaning that it is close to optimal for Gaussian noise but
far less sensitive than conventional methods to the excess large events that
form the tail of the distribution. The method is analyzed for two different
signal analysis problems: (i) a known waveform (e.g., a binary inspiral chirp)
and (ii) a stochastic background, which requires a multi-detector signal
processing algorithm. The methods should be easy to implement: they amount to
truncation or clipping of sample values which lie in the outlier part of the
probability distribution.


title: Area scaling entropies for gravitating systems

abstract:
  The entropy of a spherically symmetric distribution of matter in
self-equilibrium is calculated. When gravitational effects are neglected, the
entropy of the system is proportional to its volume. As effects due to
gravitational self-interactions become more important, the entropy acquires a
correction term and is no longer purely volume scaling. In the limit that the
boundary of the system approaches its event horizon, the total entropy of the
system is proportional to its area. The scaling laws of the system's
thermodynamical quantities are identical to those of a black hole, even though
the system does not possess an event horizon.

categories: gr-qc astro-ph cond-mat.stat-mech hep-th


title: Pure Quantum Solutions of Bohmian Quantum Gravity

abstract:
  In this paper we have investigated the pure quantum solutions of Bohmian
quantum gravity. By pure quantum solution we mean a solution in which the
quantum potential cannot be ignored with respect to the classical potential,
especially in Bohmian quantum gravity we are interested in the case where these
two potentials are equal in their magnitude and in fact their sum is zero. Such
a solutions are obtained both using the perturbation and using the linear field
approximation.


title:
Quasi-Normal Modes of Schwarzschild Anti-De Sitter Black Holes:
  Electromagnetic and Gravitational Perturbations

abstract:
  We study the quasi-normal modes (QNM) of electromagnetic and gravitational
perturbations of a Schwarzschild black hole in an asymptotically Anti-de Sitter
(AdS) spacetime. Some of the electromagnetic modes do not oscillate, they only
decay, since they have pure imaginary frequencies. The gravitational modes show
peculiar features: the odd and even gravitational perturbations no longer have
the same characteristic quasinormal frequencies. There is a special mode for
odd perturbations whose behavior differs completely from the usual one in
scalar and electromagnetic perturbation in an AdS spacetime, but has a similar
behavior to the Schwarzschild black hole in an asymptotically flat spacetime:
the imaginary part of the frequency goes as 1/r+, where r+ is the horizon
radius. We also investigate the small black hole limit showing that the
imaginary part of the frequency goes as r+^2. These results are important to
the AdS/CFT conjecture since according to it the QNMs describe the approach to
equilibrium in the conformal field theory.


title:
Inhomogeneous Cosmological Models Containing Homogeneous Inner
  Hypersurface Geometry. Changes of the Bianchi Type

abstract:
  There are investigated such cosmological models which instead of the usual
spatial homogeneity property only fulfil the condition that in a certain
synchronized system of reference all spacelike sections t = const. are
homogeneous manifolds. This allows time-dependent changes of the BIANCHI type.
Discussing differential geometrical theorems it is shown which of them are
permitted. Besides the trivial case of changing into type I there exist some
possible changes between other types. However, physical reasons like energy
inequalities partially exclude them.


title:
Inhomogeneous Cosmological Models with Flat Slices Generated from the
  Einstein-de Sitter Universe

abstract:
  A family of cosmological models is considered which in a certain synchronized
system of reference possess flat slices t = const. They are generated from the
Einstein-de Sitter universe by a suitable transformation. Under physically
reasonable presumptions these transformed models fulfil certain energy
conditions.


title:
Surface Layers in General Relativity and Their Relation to Surface
  Tensions

abstract:
  For a thin shell, the intrinsic 3-pressure will be shown to be analogous to
-A, where A is the classical surface tension: First, interior and exterior
Schwarzschild solutions will be matched together such that the surface layer
generated at the common boundary has no gravitational mass; then its intrinsic
3-pressure represents a surface tension fulfilling Kelvin's relation between
mean curvature and pressure difference in the Newtonian limit. Second, after a
suitable definition of mean curvature, the general relativistic analogue to
Kelvin's relation will be proven to be contained in the equation of motion of
the surface layer.


title:
On static spherically symmetric solutions of the Bach-Einstein
  gravitational field equations

abstract:
  For field equations of 4th order, following from a Lagrangian `Ricci scalar
plus Weyl scalar', it is shown (using methods of non-standard analysis) that in
a neighbourhood of Minkowski space there do not exist regular static
spherically symmetric solutions. With that (besides the known local expansions
about r = 0 and r = infinity resp.) for the first time a global statement on
the existence of such solutions is given. Finally, this result will be
discussed in connection with Einstein's particle programme.


title: Non-trivial Solutions of the Bach Equation Exist

abstract:
  We show that solutions of the Bach equation exist which are not conformal
Einstein spaces.


title: Conserved quantities in a black hole collision

abstract:
  The Newman-Penrose constants of the spacetime corresponding to the
development of the Brill-Lindquist initial data are calculated by making use of
a particular representation of spatial infinity due to H. Friedrich. The
Brill-Lindquist initial data set represents the head-on collision of two
non-rotating black holes. In this case one non-zero constant is obtained. Its
value is given in terms of the product of the individual masses of the black
holes and the square of a distance parameter separating the two black holes.
This constant retains its value all along null infinity, and therefore it
provides information about the late time evolution of the collision process. In
particular, it is argued that the magnitude of the constants provides
information about the amount of residual radiation contained in the spacetime
after the collision of the black holes.


title:
Inertial control of the mirror suspensions of the VIRGO interferometer
  for gravitational wave detection

abstract:
  In order to achieve full detection sensitivity at low frequencies, the
mirrors of interferometric gravitational wave detectors must be isolated from
seismic noise. The VIRGO vibration isolator, called 'superattenuator', is fully
effective at frequencies above 4 Hz. Nevertheless, the residual motion of the
mirror at the mechanical resonant frequencies of the system are too large for
the interferometer locking system and must be damped. A multidimensional
feedback system, using inertial sensors and digital processing, has been
designed for this purpose. An experimental procedure for determining the
feedback control of the system has been defined. In this paper a full
description of the system is given and experimental results are presented.


title:
New integral equation form of integrable reductions of Einstein
  equations

abstract:
  A new development of the ``monodromy transform'' method for analysis of
hyperbolic as well as elliptic integrable reductions of Einstein equations is
presented. Compatibility conditions for some alternative representations of the
fundamental solutions of associated linear systems with spectral parameter in
terms of a pair of dressing (``scattering'') matrices give rise to a new set of
linear (quasi-Fredholm) integral equations equivalent to the symmetry reduced
Einstein equations. Unlike previously derived singular integral equations
constructed with the use of conserved (nonevolving) monodromy data on the
spectral plane for the fundamental solutions of associated linear systems, the
scalar kernels of the new equations include another kind of functional
parameters -- the evolving (``dynamical'') monodromy data for the scattering
matrices. For hyperbolic reductions, in the context of characteristic initial
value problem these data are determined completely by the characteristic
initial data for the fields. In terms of solutions of the new integral
equations the field components are expressed in quadratures.


title:
Cosmological Implications of a Non-Separable 5D Solution of the Vacuum
  Einstein Field Equations

abstract:
  An exact class of solutions of the 5D vacuum Einstein field equations (EFEs)
is obtained. The metric coefficients are found to be non-separable functions of
time and the extra coordinate $l$ and the induced metric on $l$ = constant
hypersurfaces has the form of a Friedmann-Robertson-Walker cosmology. The 5D
manifold and 3D and 4D submanifolds are in general curved, which distinguishes
this solution from previous ones in the literature. The singularity structure
of the manifold is explored: some models in the class do not exhibit a big
bang, while other exhibit a big bang and a big crunch. For the models with an
initial singularity, the equation of state of the induced matter evolves from
radiation like at early epochs to Milne-like at late times and the big bang
manifests itself as a singular hypersurface in 5D. The projection of comoving
5D null geodesics onto the 4D submanifold is shown to be compatible with
standard 4D comoving trajectories, while the expansion of 5D null congruences
is shown to be in line with conventional notions of the Hubble expansion.


title: The Semiclassical Limit of Loop Quantum Cosmology

abstract:
  The continuum and semiclassical limits of isotropic, spatially flat loop
quantum cosmology are discussed, with an emphasis on the role played by the
Barbero-Immirzi parameter \gamma in controlling space-time discreteness. In
this way, standard quantum cosmology is shown to be the simultaneous limit
\gamma \to 0, j \to \infty of loop quantum cosmology. Here, j is a label of the
volume eigenvalues, and the simultaneous limit is technically the same as the
classical limit \hbar \to 0, l \to \infty of angular momentum in quantum
mechanics. Possible lessons for semiclassical states at the dynamical level in
the full theory of quantum geometry are mentioned.


title:
The Cosmological Constant Problem in Brane--Worlds and Gravitational
  Lorentz Violations

abstract:
  Brane worlds are theories with extra spatial dimensions in which ordinary
matter is localized on a (3+1) dimensional submanifold. Such theories could
have interesting consequences for particle physics and gravitational physics.
In this essay we concentrate on the cosmological constant (CC) problem in the
context of brane worlds. We show how extra-dimensional scenarios may violate
Lorentz invariance in the gravity sector of the effective 4D theory, while
particle physics remains unaffected. In such theories the usual no-go theorems
for adjustment of the CC do not apply, and we indicate a possible explanation
of the smallness of the CC. Lorentz violating effects would manifest themselves
in gravitational waves travelling with a speed different from light, which can
be searched for in gravitational wave experiments.


title: Branes, Charge and Intersections

abstract:
  This is a brief summary of lectures given at the Fourth Mexican School on
Gravitation and Mathematical Physics. The lectures gave an introduction to
branes in eleven-dimensional supergravity and in type IIA supergravities in
ten-dimensions. Charge conservation and the role of the so-called `Chern-Simons
terms' were emphasized. Known exact solutions were discussed and used to
provide insight into the question `Why don't fundamental strings fall off of
D-branes,' which is often asked by relativists. The following is a brief
overview of the lectures with an associated guide to the literature.


title:
Obtention of a Gravitational Force from a Relativistic Solution to
  Binet's Equation: The Schwarzschild's Case

abstract:
  Making use of the classical Binet's equation a general procedure to obtain
the gravitational force corresponding to an arbitrary 4-dimensional spacetime
is presented. This method provides, for general relativistic scenarios,
classics expressions that may help to visualize certain effects that Newton's
theory can not explain. In particular, the force produced by a gravitational
field which source is spherically symmetrical (Schwarzschild's spacetime) is
obtained. Such expression uses a redefinition of the classical reduced mass, in
the limit case it can be reduced to Newton's Universal Law of Gravitation and
it produces two different orbital velocities for test particles that
asimptotically coincide with the Newtonian one. PACS: 04.25.Nx, 95.10.Ce,
95.30.Sf. Keywords: Universal gravitational law, perihelionshift, Schwarzschild
potential, reduced mass.


title: Inhomogeneity and Nonlinear Preheating

abstract:
  We investigated the possibility that nonlinear gravitational effects
influence the preheating era after inflation, using numerical solutions of the
inhomogeneous Einstein field equations. We compared our results to perturbative
calculations and to solutions of the nonlinear field equations in a rigid
(unperturbed) spacetime, in order to isolate gravitational phenomena. We
confirm the broad picture of preheating obtained from the nonlinear field
equations in a rigid background, but find gravitational effects have a
measurable impact on the dynamics. The longest modes in the simulation grow
much more rapidly in the relativistic calculation than with a rigid background.
We used the Weyl tensor to quantify the departure from homogeneity in the
universe. We saw no evidence for the sort of gravitational collapse that leads
to the formation of primordial black holes.


title: On linearized gravity in the Randall-Sundrum scenario

abstract:
  In the literature about the Randall-Sundrum scenario one finds on one hand
that there exist (small) corrections to Newton's law of gravity on the brane,
and on another that the exact (and henceforth linearized) Einstein equations
can be recovered on the brane. The explanation for these seemingly
contradictory results is that the behaviour of the bulk far from the brane is
different in both models. We show that explicitely in this paper.


title: The Kretschmann scalar for 5D Vacua

abstract:
  We present a streamlined calculation of the Kretschmann scalar for 5D Vacua
considered recently by Fukui et al. (gr-qc/0105112).


title:
The Effective Energy-Momentum Tensor in Kaluza-Klein Gravity With Large
  Extra Dimensions and Off-Diagonal Metrics

abstract:
  We consider a version of Kaluza-Klein theory where the cylinder condition is
not imposed. The metric is allowed to have explicit dependence on the "extra"
coordinate(s). This is the usual scenario in brane-world and space-time-matter
theories. We extend the usual discussion by considering five-dimensional
metrics with off-diagonal terms. We replace the condition of cylindricity by
the requirement that physics in four-dimensional space-time should remain
invariant under changes of coordinates in the five-dimensional bulk. This
invariance does not eliminate physical effects from the extra dimension but
separates them from spurious geometrical ones. We use the appropriate splitting
technique to construct the most general induced energy-momentum tensor,
compatible with the required invariance. It generalizes all previous results in
the literature. In addition, we find two four-vectors, J_{m}^{mu} and
J_{e}^{mu}, induced by off-diagonal metrics, that separately satisfy the usual
equation of continuity in 4D. These vectors appear as source-terms in equations
that closely resemble the ones of electromagnetism. These are Maxwell-like
equations for an antisymmetric tensor {F-hat}_{mu nu} that generalizes the
usual electromagnetic one. This generalization is not an assumption, but
follows naturally from the dimensional reduction. Thus, if {F-hat}_{mu nu}
could be identified with the electromagnetic tensor, then the theory would
predict the existence of classical magnetic charge and current. The splitting
formalism used allows us to construct 4D physical quantities from
five-dimensional ones, in a way that is independent on how we choose our
space-time coordinates from those of the bulk.


title: Relic Gravity Waves from Braneworld Inflation

abstract:
  We discuss a scenario in which extra dimensional effects allow a scalar field
with a steep potential to play the dual role of the inflaton as well as dark
energy (quintessence). The post-inflationary evolution of the universe in this
scenario is generically characterised by a `kinetic regime' during which the
kinetic energy of the scalar field greatly exceeds its potential energy
resulting in a `stiff' equation of state for scalar field matter $P_\phi \simeq
\rho_\phi$. The kinetic regime precedes the radiation dominated epoch and
introduces an important new feature into the spectrum of relic gravity waves
created quantum mechanically during inflation. The gravity wave spectrum
increases with wavenumber for wavelengths shorter than the comoving horizon
scale at the commencement of the radiative regime. This `blue tilt' is a
generic feature of models with steep potentials and imposes strong constraints
on a class of inflationary braneworld models. Prospects for detection of the
gravity wave background by terrestrial and space-borne gravity wave
observatories such as LIGO II and LISA are discussed.


title: Interacting spin 0 fields with torsion via Duffin-Kemmer-Petiau theory

abstract:
  Here we study the behaviour of spin 0 sector of the DKP field in spaces with
torsion. First we show that in a Riemann-Cartan manifold the DKP field presents
an interaction with torsion when minimal coupling is performed, contrary to the
behaviour of the KG field, a result that breaks the usual equivalence between
the DKP and the KG fields.
  Next we analyse the case of Teleparallel Equivalent of General Relativity
Weitzenbock manifold, showing that in this case there is a perfect agreement
between KG and DKP fields. The origins of both results are also discussed.


title:
The T-domain and extreme matter phases inside spherically symmetric
  black holes

abstract:
  Black hole interiors (the $T$-domain) are studied here in great detail. Both
the {\em general} and particular $T$-domain solutions are presented including
non-singular ones. Infinitely many local $T$-domain solutions may be modeled
with this scheme. The duality between the $T$ and $R$ domains is presented. It
is demonstrated how generally well behaved $R$-domain solutions will give rise
to exotic phases of matter when collapsed inside the event horizon. However, as
seen by an external observer, the field is simply that of the Schwarzschild
vacuum with well behaved mass term and no evidence of this behaviour may be
observed. A singularity theorem is also presented which is independent of
energy conditions.


title: Some aspects of pre big bang cosmology

abstract:
  This is a summary of a course given at the Fourth Mexican School on
Gravitation and Mathematical Physics on some aspects of PBB cosmology. After
introductory remarks the lectures concentrate on some amusing consequences
derived from the symmetries of the string theory with respect to such classical
concepts as isotropy and homogeneity. The extra dimensions and the symmetries
of the M theory are further applied to show that the classical singularities
might be just physically irrelevant. In the final lecture a model universe is
"produced" from "almost nothing" and it is argued that initial plane waves are
thermodynamically natural state for the universe to emerge from.


title: Einstein Gravity as an emergent phenomenon?

abstract:
  In this essay we marshal evidence suggesting that Einstein gravity may be an
emergent phenomenon, one that is not ``fundamental'' but rather is an almost
automatic low-energy long-distance consequence of a wide class of theories.
Specifically, the emergence of a curved spacetime ``effective Lorentzian
geometry'' is a common generic result of linearizing a classical scalar field
theory around some non-trivial background. This explains why so many different
``analog models'' of general relativity have recently been developed based on
condensed matter physics; there is something more fundamental going on. Upon
quantizing the linearized fluctuations around this background geometry, the
one-loop effective action is guaranteed to contain a term proportional to the
Einstein--Hilbert action of general relativity, suggesting that while classical
physics is responsible for generating an ``effective geometry'', quantum
physics can be argued to induce an ``effective dynamics''. This physical
picture suggests that Einstein gravity is an emergent low-energy long-distance
phenomenon that is insensitive to the details of the high-energy short-distance
physics.


title: Inertia Manipulation through Metric Patching

abstract:
  This paper will present the exact solution for the stress-energy tensor of a
spherical matter shell of finite thickness that will patch together different
metrics at the boundaries of the shell. The choice of vacuum field solutions
for the shell exterior and hollow interior that we make will allow us to
manipulate the inertial state of an object within the shell. The choice will
cause it to be in a state of acceleration with the shell relative to an
external observer for an indefinite time. The stress-energy tensor's solution
results in zero ship frame energy requirements, and only finite stress
requirements, and we show how any WEC violation can be avoided.


title:
Status of Relativity with observer-independent length and velocity
  scales

abstract:
  I have recently shown that it is possible to formulate the Relativity
postulates in a way that does not lead to inconsistencies in the case of
space-times whose structure is governed by observer-independent scales of both
velocity and length. Here I give an update on the status of this proposal,
including a brief review of some very recent developments. I also emphasize the
role that one of the kappa-Poincare' Hopf algebras could play in the
realization of a particular example of the new type of postulates. I show that
the new ideas on Relativity require us to extend the set of tools provided by
kappa-Poincare' and to revise our understanding of certain already available
tools, such as the energy-momentum coproduct.


title:
Observed threshold anomalies as the first hope of a manifestation of
  Planck-length physics

abstract:
  The observations of photons from the BL Lac object Mk501 with energies above
10 TeV and of cosmic rays with energies above the GZK threshold appear to be
inconsistent with conventional theories. Remarkably, among the recent
new-physics proposals of solutions of these threshold paradoxes a prominent
role has been played by proposals based on quantum properties of space-time.
While the experimental evidence (and theory work attempting to interpret it) is
much too preliminary to justify any serious hopes that we might have stumbled
upon the first manifestation of a "quantum gravity", the fact that for the
first time phenomenological models involving quantum-gravity ideas are
competing on level ground with other new-physics proposals clearly marks the
beginning of a new stage of quantum-gravity research. I emphasize one important
aspect of this new phenomenology: combining the determination of the relevant
thresholds with data on the time/energy structure of gamma-ray bursts it is
possible to distinguish between alternative quantum-gravity scenarios. This
point is illustrated focusing on 3 specific scenarios: dispersion-inducing
space-time foam, string-theory-motivated non-commutative space-time, and this
author's recent proposal of a relativistic theory in which the Planck length
has the role of fundamental observer-independent minimum length.


title: Conformal General Relativity

abstract:
  The inflation-free solution of problems of the modern cosmology (horizon,
cosmic initial data, Planck era, arrow of time, singularity,homogeneity, and so
on) is considered in the conformal-invariant unified theory given in the space
with geometry of similarity where we can measure only the conformal-invariant
ratio of all quantities. Conformal General Relativity is defined as the
$SU_c(3)\times SU(2)\times U(1)$-Standard Model where the dimensional parameter
in the Higgs potential is replaced by a dilaton scalar field described by the
negative Penrose-Chernikov-Tagirov action. Spontaneous SU(2) symmetry breaking
is made on the level of the conformal-invariant angle of the dilaton-Higgs
mixing, and it allows us to keep the structure of Einstein's theory with the
equivalence principle. We show that the lowest order of the linearized
equations of motion solves the problems mentioned above and describes the Cold
Universe Scenario with the constant temperature T and z-history of all masses
with respect to an observable conformal time. A new fact is the intensive
cosmic creation of $W,Z$-vector bosons due to their mass singularity. In the
rigid state, this effect is determined by the integral of motion $(m_w^2H_{\rm
hubble})^{1/3}=2.7 K k_B$ that coincides with the CMB temperature and has the
meaning of the primordial Hubble parameter. The created bosons are enough to
consider their decay as an origin of the CMB radiation and all observational
matter with the observational element abundances and the baryon asymmetry.
Recent Supernova data on the relation between the luminosity distance and
redshift (including the point $z=1.7$) do not contradict the dominance of the
rigid state of the dark matter in the Conformal Cosmology.


title:
Machian General Relativity: a possible solution to the Dark Energy
  problem, and a replacement for Big Bang cosmology

abstract:
  Observations of an apparent acceleration in the expansion rate of the
universe, derived from measurements of high-redshift supernovae, have been used
to support the hypothesis that the universe is permeated by some form of dark
energy. We show that an alternative cosmological model, based on a linearly
expanding universe with Omega=1, can fully account for these observations. This
model is also able to resolve the other problems associated with the standard
Big Bang model. A scale-invariant form of the field equations of General
Relativity is postulated, based on the replacement of the Newtonian
gravitational constant by an formulation based explicitly on Mach's principle.
We derive the resulting dynamical equations, and show that their solutions
describe a linearly expanding universe. Potential problems with non-zero
divergencies in the modified field equations are shown to be resolved by
adopting a radically different definition of time, in which the unit of time is
a function of the scale-factor. We observe that the effects of the modified
field equations are broadly equivalent to a Newtonian gravitational constant
that varies both in time and in space, and show that this is also equivalent to
Varying Speed of Light (VSL) theories using a standard definition of time. Some
of the implications of this observation are discussed in relation to Black
Holes, Planck scale phenomena, and the ultimate fate of the Universe.


title: Internal structure of Skyrme black hole

abstract:
  We consider the internal structure of the Skyrme black hole under a static
and spherically symmetric ansatz. $@u8(Be concentrate on solutions with the
node number one and with the "winding" number zero, where there exist two
solutions for each horizon radius; one solution is stable and the other is
unstable against linear perturbation. We find that a generic solution exhibits
an oscillating behavior near the sigularity, as similar to a solution in the
Einstein-Yang-Mills (EYM) system, independently to stability of the solution.
Comparing it with that in the EYM system, this oscillation becomes mild because
of the mass term of the Skyrme field. We also find Schwarzschild-like
exceptional solutions where no oscillating behavior is seen. Contrary to the
EYM system where there is one such solution branch if the node number is fixed,
there are two branches corresponding to the stable and the unstable ones.


title: Approach to the Singularity in Spatially Inhomogeneous Cosmologies

abstract:
  A combination of analytic and numerical methods has yielded a clear
understanding of the approach to the singularity in spatially inhomogeneous
cosmologies. Strong support is found for the longstanding claim by Belinskii,
Khalatnikov, and Lifshitz that the collapse is dominated by local Kasner or
Mixmaster behavior. The Method of Consistent Potentials is used to establish
the consistency of asymptotic velocity term dominance (AVTD) (local Kasner
behavior) in that no terms in Einstein's equations will grow exponentially when
the VTD solution, obtained by neglecting all terms containing spatial
derivatives, is substituted into the full equations. When the VTD solution is
inconsistent, the exponential terms act dynamically as potentials either to
drive the system into a consistent AVTD regime or to maintain an oscillatory
approach to the singularity.


title:
Support for a prosaic explanation for the anomalous acceleration of
  Pioneer 10 and 11

abstract:
  Anderson, et al., find the measured trajectories of Pioneer 10 and 11
spacecraft deviate from the trajectories computed from known forces acting on
them. This unmodelled acceleration can be accounted for by non-isotropic
radiation of spacecraft heat. This explanation was first proposed by Murphy,
but Anderson, et al. felt it could not explain the observed effect. This paper
includes new calculations on the expected magnitude of this effect, based on
the relative emissivities of the different sides of the spacecraft, as
estimated from the known spacecraft construction. The calculations indicate the
proposed effect can account for most, if not all, of the unmodelled
acceleration.


title: Demianski-Newman solution revisited

abstract:
  The derivation of the Demianski-Newman solution within the framework of the
Ernst complex formalism is considered. We show that this solution naturally
arises as a two-soliton specialization of the axisymmetric multi-soliton
electrovacuum metric, and we work out the full set of the corresponding
metrical fields and electromagnetic potentials. Some limits and physical
properties of the DN space-time are briefly discussed.


title: Quantization and entropy of black holes

abstract:
  The spectrum of the area operator for a Schwarzschild black hole in loop
quantum gravity is fixed by the demand that the entropy of a black hole is
maximum. This paper has been withdrawn by the author, due a crucial error in
the derivation.


title:
The intrinsic derivative and centrifugal forces in general relativity:
  I. Theoretical foundations

abstract:
  Everyday experience with centrifugal forces has always guided thinking on the
close relationship between gravitational forces and accelerated systems of
reference. Once spatial gravitational forces and accelerations are introduced
into general relativity through a splitting of spacetime into space-plus-time
associated with a family of test observers, one may further split the local
rest space of those observers with respect to the direction of relative motion
of a test particle world line in order to define longitudinal and transverse
accelerations as well. The intrinsic covariant derivative (induced connection)
along such a world line is the appropriate mathematical tool to analyze this
problem, and by modifying this operator to correspond to the observer
measurements, one understands more clearly the work of Abramowicz et al who
define an ``optical centrifugal force'' in static axisymmetric spacetimes and
attempt to generalize it and other inertial forces to arbitrary spacetimes. In
a companion article the application of this framework to some familiar
stationary axisymmetric spacetimes helps give a more intuitive picture of their
rotational features including spin precession effects, and puts related work of
de Felice and others on circular orbits in black hole spacetimes into a more
general context.


title:
The intrinsic derivative and centrifugal forces in general relativity:
  II. Applications to circular orbits in some familiar stationary axisymmetric
  spacetimes

abstract:
  The tools developed in a preceding article for interpreting spacetime
geometry in terms of all possible space-plus-time splitting approaches are
applied to circular orbits in some familiar stationary axisymmetric spacetimes.
This helps give a more intuitive picture of their rotational features including
spin precession effects, and puts related work of Abramowicz, de Felice, and
others on circular orbits in black hole spacetimes into a more general context.


title: Binary black holes in circular orbits. I. A global spacetime approach

abstract:
  We present a new approach to the problem of binary black holes in the
pre-coalescence stage, i.e. when the notion of orbit has still some meaning.
Contrary to previous numerical treatments which are based on the initial value
formulation of general relativity on a (3-dimensional) spacelike hypersurface,
our approach deals with the full (4-dimensional) spacetime. This permits a
rigorous definition of the orbital angular velocity. Neglecting the
gravitational radiation reaction, we assume that the black holes move on closed
circular orbits, which amounts to endowing the spacetime with a helical Killing
vector. We discuss the choice of the spacetime manifold, the desired properties
of the spacetime metric, as well as the choice of the rotation state for the
black holes. As a simplifying assumption, the space 3-metric is approximated by
a conformally flat one. The problem is then reduced to solving five of the ten
Einstein equations, which are derived here, as well as the boundary conditions
on the black hole surfaces and at spatial infinity. We exhibit the remaining
five Einstein equations and propose to use them to evaluate the error induced
by the conformal flatness approximation. The orbital angular velocity of the
system is computed through a requirement which reduces to the classical virial
theorem at the Newtonian limit.


title:
Binary black holes in circular orbits. II. Numerical methods and first
  results

abstract:
  We present the first results from a new method for computing spacetimes
representing corotating binary black holes in circular orbits. The method is
based on the assumption of exact equilibrium. It uses the standard 3+1
decomposition of Einstein equations and conformal flatness approximation for
the 3-metric. Contrary to previous numerical approaches to this problem, we do
not solve only the constraint equations but rather a set of five equations for
the lapse function, the conformal factor and the shift vector. The orbital
velocity is unambiguously determined by imposing that, at infinity, the metric
behaves like the Schwarzschild one, a requirement which is equivalent to the
virial theorem. The numerical scheme has been implemented using multi-domain
spectral methods and passed numerous tests. A sequence of corotating black
holes of equal mass is calculated. Defining the sequence by requiring that the
ADM mass decrease is equal to the angular momentum decrease multiplied by the
orbital angular velocity, it is found that the area of the apparent horizons is
constant along the sequence. We also find a turning point in the ADM mass and
angular momentum curves, which may be interpreted as an innermost stable
circular orbit (ISCO). The values of the global quantities at the ISCO,
especially the orbital velocity, are in much better agreement with those from
third post-Newtonian calculations than with those resulting from previous
numerical approaches.


title:
General Relativistic Initial Data for the Binary Black Hole / Neutron
  Star System in Quasicircular Orbit

abstract:
  We present an algorithm for solving the general relativistic initial value
equations for a corotating polytropic star in quasicircular orbit with a
nonspinning black hole. The algorithm is used to obtain initial data for cases
where the black hole mass is 1, 3, and 10 times larger than the mass of the
star. By analyzing sequences of constant baryon mass, constant black hole mass
initial data sets and carefully monitoring the numerical error, we find
innermost stable circular orbit (ISCO) configuration for these cases. While
these quasiequilibrium, conformally flat sequences of initial data sets are not
true solutions of the Einstein equations (each set, however, solves the full
initial value problem), and thus, we do not expect the ISCO configurations
found here to be completely consistent with the Einstein equations, they will
be used as convenient starting points for future numerical evolutions of the
full 3+1 Einstein equations.


title: Black Hole Decay Rates in Large Extra Dimensions

abstract:
  We study the evaporation of black holes in space-times with extra dimensions
of size L. We show that the luminosity is greatly damped when the horizon
becomes smaller than L and black holes born with an initial size smaller than L
are almost stable. This effect is due to the dependence of both the occupation
number density of Hawking quanta and the grey-body factor of a black hole on
the dimensionality of space.


title: Deflection of light by the screw dislocation in space-time

abstract:
  We derive the light deflection caused by the screw dislocation in space-time.
The derivation is based on the idea that space-time is a medium which can be
deformed by gravity and that the deformation of space-time is equivalent to the
existence of gravity.


title: Inevitability of Spacetime Singularities in The Canonical Metric

abstract:
  We discuss the question of whether the existence of singularities is an
intrinsic property of 4D spacetime. Our hypothesis is that singularities in 4D
are induced by the separation of spacetime from the other dimensions. We
examine this hypothesis in the context of the so-called canonical or warp
metrics in 5D. These metrics are popular because they provide a clean
separation between the extra dimension and spacetime. We show that the
spacetime section, in these metrics, inevitably becomes singular for some
finite (non-zero) value of the extra coordinate. This is true for all canonical
metrics that are solutions of the field equations in space-time-matter theory.
This is a geometrical singularity in 5D, but appears as a physical one in 4D.
At this singular hypersurface, the determinant of the spacetime metric becomes
zero and the curvature of the spacetime blows up to infinity. These results are
consistent with our hypothesis.


title:
Study of the Magnitude-Redshift Relation for type Ia Supernovae in a
  Model resulting from a Ricci-Symmetry

abstract:
  Models with a dynamic cosmological term \Lambda (t) are becoming popular as
they solve the cosmological constant problem in a natural way. Instead of
considering any ad-hoc assumption for the variation of \Lambda, we consider a
particular symmetry, the contracted Ricci-collineation along the fluid flow, in
Einstein's theory. We show that apart from having interesting properties, this
symmetry does require \Lambda to be a function of the scale factor of the
Robertson-Walker metric. In order to test the consistency of the resulting
model with observations, we study the magnitude-redshift relation for the type
Ia supernovae data from Perlmutter et al. The data fit the model very well and
require a positive non-zero \Lambda and a negative deceleration parameter. The
best-fitting flat model is obtained as \Omega_0 \approx 0.5 with q_0 \approx
-0.2.


title:
Generalized Slow Roll Conditions and the Possibility of Intermediate
  Scale Inflation in Scalar-Tensor Theory

abstract:
  Generalized slow roll conditions and parameters are obtained for a general
form of scalar-tensor theory (with no external sources), having arbitrary
functions describing a nonminimal gravitational coupling F(\phi), a Kahler-like
kinetic function k(\phi), and a scalar potential V(\phi). These results are
then used to analyze a simple toy model example of chaotic inflation with a
single scalar field \phi and a standard Higgs potential and a simple
gravitational coupling function. In this type of model inflation can occur with
inflaton field values at an intermediate scale of roughly 10^{11} GeV when the
particle physics symmetry breaking scale is approximately 1 TeV, provided that
the theory is realized within the Jordan frame. If the theory is realized in
the Einstein frame, however, the intermediate scale inflation does not occur.


title: Nariai metric is the first example of the singularity free model

abstract:
  This is just to point out that the Nariai metric is the first example of the
singularity free expanding perfect fluid cosmological model satisfying the weak
energy condition, $\rho>0, \rho+p=0$. It is a conformally non-flat Einstein
space.


title:
Causal Sets, a Possible Interpretation for the Black Hole Entropy,and
  Related Topics

abstract:
  The Causal Set hypothesis asserts that spacetime, ultimately, is discrete and
its underlying structure is that of a locally finite partial ordered set, and
macroscopic causality reflects a deeper notion of order in terms of which all
the geometrical structure of spacetime must find their ultimate expression.
After reviewing the main aspects of Causal Sets Kinematics, and the recently
developed Stochastic Dynamics. We concentrate on possible implications in the
fields of cosmology and black holes. In the context of black hole, we propose a
possible interpretation of the entropy as the number of links crossing the
horizon.


title:
On a relation between the Bach equation and the equation of
  geometrodynamics

abstract:
  The Bach equation and the equation of geometrodynamics are based on two quite
different physical motivations, but in both approaches, the conformal
properties of gravitation plays the key role. In this paper we present an
analysis of the relation between these two equations and show that the
solutions of the equation of geometrodynamics are of a more general nature. We
show the following non-trivial result: there exists a conformally invariant
Lagrangian, whose field equation generalizes the Bach equation and has as
solutions those Ricci tensors which are solutions to the equation of
geometrodynamics.


title: Boundary conditions in linearized harmonic gravity

abstract:
  We investigate the initial-boundary value problem for linearized
gravitational theory in harmonic coordinates. Rigorous techniques for
hyperbolic systems are applied to establish well-posedness for various
reductions of the system into a set of six wave equations. The results are used
to formulate computational algorithms for Cauchy evolution in a 3-dimensional
bounded domain. Numerical codes based upon these algorithms are shown to
satisfy tests of robust stability for random constraint violating initial data
and random boundary data; and shown to give excellent performance for the
evolution of typical physical data. The results are obtained for plane
boundaries as well as piecewise cubic spherical boundaries cut out of a
Cartesian grid.


title: Comments on quantum cosmology with extrinsic time

abstract:
  The closed de Sitter universe is used to present a way to deal with the
deparametrization and quantization of cosmological models with extrinsic time.


title:
Covariant Formulation of the Invariant Measure for the Mixmaster
  Dynamics

abstract:
  We provide a Hamiltonian analysis of the Mixmaster Universe dynamics showing
the covariant nature of its chaotic behavior with respect to any choice of time
variable. We construct the appropriate invariant measure for the system (which
relies on the existence of an ``energy-like'' constant of motion) without
fixing the time gauge, i.e. the corresponding lapse function. The key point in
our analysis consists of introducing generic Misner-Chitr\'e-like variables
containing an arbitrary function, whose specification allows one to set up the
same dynamical scheme in any time gauge.


title: Cosmic repulsion in presence of matter

abstract:
  A basic problem that confronts the standard cosmological models is the
problem of initial singularity characterised by infinite material density,
infinite temperature and infinite spacetime curvature. The inevitable existence
of such a phase of the universe may be considered to be one of the major
drawbacks of Einstein's field equations. To some extent inflation models
ameliorate this. In the present work we postulate that whenever matter
(radiation) arises in flat spacetime, it introduces curvature and causes a
repulsive interaction to develop. A modified energy momentum tensor is
introduced towards this end, which invokes the temperature, entropy and a
cosmic scalar field. This redefinition of the energy momentum distribution when
applied to the early universe dynamics has the effect of producing a
non-singular initial behaviour. the repulsive interaction introduces some
features of an accelarating universe. In this model all cosmological parameters
are mathematically well behaved. There is no flatness problem. Temperature at
the beginning of this universe is 10^32 kelvin.


title: Black holes with hair

abstract:
  I review some recent results on black holes with hair. I focus on the
magnetically charged black holes in spontaneously broken Yang-Mills-Higgs
theories and on the related self-gravitating magnetic monopoles. Some
implications for black hole thermodynamics are discussed.


title: Viscous Cosmologies with Extra Dimensions

abstract:
  We present an analysis of a n-dimensional vacuum Einstein field equations in
which 4-dimensional space-time which is described by a Friedmann
Robertson-Walker (FRW) metric and that of the extra dimensions by a Kasner type
Euclidean metric. The field equations are interpreted as four dimensional
Einstein equations with effective matter properties. We obtain solutions for
the cases when the effective matter is described by imperfect fluid. We
consider the theories of imperfect fluid given by Eckart, truncated
Israel-Stewart and full Israel-Stewart theories and obtain cosmological
solutions for a flat model of the universe.


title: On classical super-radiance in Kerr-Newman-anti-de Sitter black holes

abstract:
  We study in detail the modes of a classical scalar field on a
Kerr-Newman-anti-de Sitter (KN-AdS) black hole. We construct sets of basis
modes appropriate to the two possible boundary conditions (``reflective'' and
``transparent'') at time-like infinity, and consider whether super-radiance is
possible. If we employ ``reflective'' boundary conditions, all modes are
non-super-radiant. On the other hand, for ``transparent'' boundary conditions,
the presence of super-radiance depends on our definition of positive frequency.
For those KN-AdS black holes having a globally time-like Killing vector, the
natural choice of positive frequency leads to no super-radiance. For other
KN-AdS black holes, there is a choice of positive frequency which gives no
super-radiance, but for other choices there will, in general, be
super-radiance.


title: Topological Lensing in Spherical Spaces

abstract:
  This article gives the construction and complete classification of all
three-dimensional spherical manifolds, and orders them by decreasing volume, in
the context of multiconnected universe models with positive spatial curvature.
It discusses which spherical topologies are likely to be detectable by
crystallographic methods using three-dimensional catalogs of cosmic objects.
The expected form of the pair separation histogram is predicted (including the
location and height of the spikes) and is compared to computer simulations,
showing that this method is stable with respect to observational uncertainties
and is well suited for detecting spherical topologies.


title:
Comment to the paper `Conformal transformations single out a unique
  measure of distance'

abstract:
  We discuss the consequences of the incorrectness [see the Erratum in Phys.
Rev D 49, 1145 (1994)] of that paper and add two related remarks. The scope of
this comment is to encourage further research on: `Which of the conformally
equivalent metrics is the physical one?'


title: Comparing selfinteracting scalar fields and R + R^3 cosmological models

abstract:
  We generalize the well-known analogies between m^2 \phi^2 and R + R^2
theories to include the selfinteraction \lambda \phi^4-term for the scalar
field. It turns out to be the R + R^3 Lagrangian which gives an appropriate
model for it. Considering a spatially flat Friedman cosmological model, common
and different properties of these models are discussed, e.g., by linearizing
around a ground state the masses of the resp. spin 0-parts coincide. Finally,
we prove a general conformal equivalence theorem between a Lagrangian L = L(R),
L'L" \ne 0, and a minimally coupled scalar field in a general potential.


title: On Ellis' programme within fourth order gravity

abstract:
  For the non-tachyonic curvature squared action we show that the expanding
Bianchi-type I models tend to the dust-filled Einstein-de Sitter model for t
tending to infinity if the metric is averaged over the typical oscillation
period. Applying a conformal equivalence between curvature squared action and a
minimally coupled scalar field (which holds for all dimensions > 2) the problem
is solved by discussing a massive scalar field in an anisotropic cosmological
model.


title: The Newtonian limit of fourth-order gravity

abstract: The weak-field slow-motion limit of fourth-order gravity will be discussed.


title:
Simulated gravity without true gravity in asymmetric brane-world
  scenarios

abstract:
  This article investigates asymmetric brane-world scenarios in the limit when
the bulk gravity is negligible. We show that, even when true self gravity is
negligible, local mass concentrations will be subject to a mutual attraction
force which simulates the effect of Newtonian gravity in the non-relativistic
limit. Cosmological and also post-Newtonian constraints are examined.


title:
Perturbation Theory Calculation of the Black Hole Elastic Scattering
  Cross Section

abstract:
  The differential cross section for scattering of a Dirac particle in a black
hole background is found. The result is the gravitational analog of the Mott
formula for scattering in a Coulomb background. The equivalence principle is
neatly embodied in the cross section, which depends only on the incident
velocity, and not the particle mass. The low angle limit agrees with classical
calculations based on the geodesic equation. The calculation employs a
well-defined iterative scheme which can be extended to higher orders. Repeating
the calculation in different gauges shows that our result for the cross section
is gauge-invariant and highlights the issues involved in setting up a sensible
iterative scheme.


title:
Gravitating sphalerons and sphaleron black holes in asymptotically
  anti-de Sitter spacetime

abstract:
  Numerical arguments are presented for the existence of spherically symmetric
regular and black hole solutions of the EYMH equations with a negative
cosmological constant. These solutions approach asymptotically the anti-de
Sitter spacetime. The main properties of the solutions and the differences with
respect to the asymptotically flat case are discussed. The instability of the
gravitating sphaleron solutions is also proven.


title: Probability for Primordial Black Holes in Higher Dimensional Universe

abstract:
  We investigate higher dimensional cosmological models in the semiclassical
approximation with Hartle-Hawking Boundary conditions, assuming a gravitational
action which is described by the scalar curvature with a cosmological constant.
In the framework the probability for quantum creation of an inflationary
universe with a pair of black holes in a multidimensional universe is
evaluated. The probability for creation of a universe with a spatial section
with $S^{1}XS^{D -2}$ topology is then compared with that of a higher
dimensional de Sitter universe with $S^{D -1}$ spatial topology. It is found
that a higher dimensional universe with a product space with primordial black
holes pair is less probable to nucleate when the extra dimensions scale factors
do not vary in an inflating universe.


title: A stationary vacuum solution dual to the Kerr solution

abstract:
  We present a stationary axially symmetric two parameter vacuum solution which
could be considered as ``dual'' to the Kerr solution. It is obtained by
removing the mass parameter from the function of the radial coordinate and
introducing a dimensionless parameter in the function of the angle coordinate
in the metric functions. It turns out that it is in fact the massless limit of
the Kerr - NUT solution.


title: The Many Faces of Gravitoelectromagnetism

abstract:
  The numerous ways of introducing spatial gravitational forces are fit
together in a single framework enabling their interrelationships to be
clarified. This framework is then used to treat the ``acceleration equals
force" equation and gyroscope precession, both of which are then discussed in
the post-Newtonian approximation, followed by a brief examination of the
Einstein equations themselves in that approximation.


title: Are Small Hyperbolic Universes Observationally Detectable?

abstract:
  Using recent observational constraints on cosmological density parameters,
together with recent mathematical results concerning small volume hyperbolic
manifolds, we argue that, by employing pattern repetitions, the topology of
nearly flat small hyperbolic universes can be observationally undetectable.
This is important in view of the facts that quantum cosmology may favour
hyperbolic universes with small volumes, and from the expectation coming from
inflationary scenarios, that $\Omega_0$ is likely to be very close to one.


title: Light Rays at Optical Black Holes in Moving Media

abstract:
  Light experiences a non-uniformly moving medium as an effective gravitational
field, endowed with an effective metric tensor $\tilde{g}^{\mu \nu}=\eta^{\mu
\nu}+(n^2-1)u^\mu u^\nu$, $n$ being the refractive index and $u^\mu$ the
four-velocity of the medium. Leonhardt and Piwnicki [Phys. Rev. A {\bf 60},
4301 (1999)] argued that a flowing dielectric fluid of this kind can be used to
generate an 'optical black hole'. In the Leonhardt-Piwnicki model, only a
vortex flow was considered. It was later pointed out by Visser [Phys. Rev.
Lett. {\bf 85}, 5252 (2000)] that in order to form a proper optical black hole
containing an event horizon, it becomes necessary to add an inward radial
velocity component to the vortex flow. In the present paper we undertake this
task: we consider a full spiral flow, consisting of a vortex component plus a
radially infalling component. Light propagates in such a dielectric medium in a
way similar to that occurring around a rotating black hole. We calculate, and
show graphically, the effective potential versus the radial distance from the
vortex singularity, and show that the spiral flow can always capture light in
both a positive, and a negative, inverse impact parameter interval. The
existence of a genuine event horizon is found to depend on the strength of the
radial flow, relative to the strength of the azimuthal flow. A limitation of
our fluid model is that it is nondispersive.