title: Loop Quantum Cosmology

abstract:
  Quantum gravity is expected to be necessary in order to understand situations
where classical general relativity breaks down. In particular in cosmology one
has to deal with initial singularities, i.e. the fact that the backward
evolution of a classical space-time inevitably comes to an end after a finite
amount of proper time. This presents a breakdown of the classical picture and
requires an extended theory for a meaningful description. Since small length
scales and high curvatures are involved, quantum effects must play a role. Not
only the singularity itself but also the surrounding space-time is then
modified. One particular realization is loop quantum cosmology, an application
of loop quantum gravity to homogeneous systems, which removes classical
singularities. Its implications can be studied at different levels. Main
effects are introduced into effective classical equations which allow to avoid
interpretational problems of quantum theory. They give rise to new kinds of
early universe phenomenology with applications to inflation and cyclic models.
To resolve classical singularities and to understand the structure of geometry
around them, the quantum description is necessary. Classical evolution is then
replaced by a difference equation for a wave function which allows to extend
space-time beyond classical singularities. One main question is how these
homogeneous scenarios are related to full loop quantum gravity, which can be
dealt with at the level of distributional symmetric states. Finally, the new
structure of space-time arising in loop quantum gravity and its application to
cosmology sheds new light on more general issues such as time.


title: Interacting Dark Energy and the Cosmic Coincidence Problem

abstract:
  The introduction of an interaction for dark energy to the standard cosmology
offers a potential solution to the cosmic coincidence problem. We examine the
conditions on the dark energy density that must be satisfied for this scenario
to be realized. Under some general conditions we find a stable attractor for
the evolution of the Universe in the future. Holographic conjectures for the
dark energy offer some specific examples of models with the desired properties.


title: Linearized Quantum Gravity Using the Projection Operator Formalism

abstract:
  The theory of canonical linearized gravity is quantized using the Projection
Operator formalism, in which no gauge or coordinate choices are made. The ADM
Hamiltonian is used and the canonical variables and constraints are expanded
around a flat background. As a result of the coordinate independence and linear
truncation of the perturbation series, the constraint algebra surprisingly
becomes partially second-class in both the classical and quantum pictures after
all secondary constraints are considered. While new features emerge in the
constraint structure, the end result is the same as previously reported: the
(separable) physical Hilbert space still only depends on the
transverse-traceless degrees of freedom.


title:
Fluctuations of an evaporating black hole from back reaction of its
  Hawking radiation: Questioning a premise in earlier work

abstract:
  This paper delineates the first steps in a systematic quantitative study of
the spacetime fluctuations induced by quantum fields in an evaporating black
hole. We explain how the stochastic gravity formalism can be a useful tool for
that purpose within a low-energy effective field theory approach to quantum
gravity. As an explicit example we apply it to the study of the
spherically-symmetric sector of metric perturbations around an evaporating
black hole background geometry. For macroscopic black holes we find that those
fluctuations grow and eventually become important when considering sufficiently
long periods of time (of the order of the evaporation time), but well before
the Planckian regime is reached. In addition, the assumption of a simple
correlation between the fluctuations of the energy flux crossing the horizon
and far from it, which was made in earlier work on spherically-symmetric
induced fluctuations, is carefully analyzed and found to be invalid. Our
analysis suggests the existence of an infinite amplitude for the fluctuations
of the horizon as a three-dimensional hypersurface. We emphasize the need for
understanding and designing operational ways of probing quantum metric
fluctuations near the horizon and extracting physically meaningful information.


title: The homogeneous and isotropic Weyssenhoff fluid

abstract:
  We consider a Weyssenhoff fluid assuming that the spacetime is homogeneous
and isotropic, therefore being relevant for cosmological considerations of
gravity theories with torsion. In this paper, it is explicitely shown that the
Weyssenhoff fluids obeying the Frenkel condition or the Papapetrou-Corinaldesi
condition are incompatible with the cosmological principle, which restricts the
torsion tensor to have only a vector and an axial vector component. Moreover it
turns out that the Weyssenhoff fluid obeying the Tulczyjew condition is also
incompatible with the cosmological principle. Based on this result we propose
to reconsider a number of previous works that analysed cosmological solutions
of Einstein-Cartan theory, since their spin fluids usually did not obey the
cosmological principle.


title: Poincare gauge gravity: selected topics

abstract:
  In the gauge theory of gravity based on the Poincare group (the semidirect
product of the Lorentz group and the spacetime translations) the mass
(energy-momentum) and the spin are treated on an equal footing as the sources
of the gravitational field. The corresponding spacetime manifold carries the
Riemann-Cartan geometric structure with the nontrivial curvature and torsion.
We describe some aspects of the classical Poincare gauge theory of gravity.
Namely, the Lagrange-Noether formalism is presented in full generality, and the
family of quadratic (in the curvature and the torsion) models is analyzed in
detail. We discuss the special case of the spinless matter and demonstrate that
Einstein's theory arises as a degenerate model in the class of the quadratic
Poincare theories. Another central point is the overview of the so-called
double duality method for constructing of the exact solutions of the classical
field equations.


title: The last orbit of binary black holes

abstract:
  We have used our new technique for fully numerical evolutions of orbiting
black-hole binaries without excision to model the last orbit and merger of an
equal-mass black-hole system. We track the trajectories of the individual
apparent horizons and find that the binary completed approximately one and a
third orbits before forming a common horizon. Upon calculating the complete
gravitational radiation waveform, horizon mass, and spin, we find that the
binary radiated 3.2% of its mass and 24% of its angular momentum. The early
part of the waveform, after a relatively short initial burst of spurious
radiation, is oscillatory with increasing amplitude and frequency, as expected
from orbital motion. The waveform then transitions to a typical `plunge'
waveform; i.e. a rapid rise in amplitude followed by quasinormal ringing. The
plunge part of the waveform is remarkably similar to the waveform from the
previously studied `ISCO' configuration. We anticipate that the plunge
waveform, when starting from quasicircular orbits, has a generic shape that is
essentially independent of the initial separation of the binary.


title:
Generalized Uncertainty Principle, Modified Dispersion Relations and
  Early Universe Thermodynamics

abstract:
  In this paper, we study the effects of Generalized Uncertainty Principle(GUP)
and Modified Dispersion Relations(MDRs) on the thermodynamics of
ultra-relativistic particles in early universe. We show that limitations
imposed by GUP and particle horizon on the measurement processes, lead to
certain modifications of early universe thermodynamics.


title: Ultrarelativistic boost of spinning and charged black rings

abstract:
  We study Aichelburg-Sexl ultrarelativistic limits in higher dimensions. After
reviewing the boost of D>=4 Reissner-Nordstrom black holes as a simple
illustrative example, we consider the case of D=5 black rings, presenting new
results for static charged rings.


title: A quasi-Kinnersley tetrad for the quasi-Kerr metric

abstract:
  Explicit expressions for the quasi-Kinnersley tetrad for the quasi-Kerr
metric are given. These provide a very clear and simple example of the
quasi-Kinnersley tetrad, and may be useful in the future development of a
`quasi-Teukolsky' scheme for the analysis of perturbation equations in
spacetimes which are Petrov type I but in some sense close to type D.


title: The spin-foam-representation of loop quantum gravity

abstract:
  The problem of background independent quantum gravity is the problem of
defining a quantum field theory of matter and gravity in the absence of an
underlying background geometry. Loop quantum gravity (LQG) is a promising
proposal for addressing this difficult task. Despite the steady progress of the
field, dynamics remains to a large extend an open issue in LQG. Here we present
the main ideas behind a series of proposals for addressing the issue of
dynamics. We refer to these constructions as the {\em spin foam representation}
of LQG. This set of ideas can be viewed as a systematic attempt at the
construction of the path integral representation of LQG.
  The {\em spin foam representation} is mathematically precise in 2+1
dimensions, so we will start this chapter by showing how it arises in the
canonical quantization of this simple theory. This toy model will be used to
precisely describe the true geometric meaning of the histories that are summed
over in the path integral of generally covariant theories.
  In four dimensions similar structures appear. We call these constructions
{\em spin foam models} as their definition is incomplete in the sence that at
least one of the following issues remains unclear: 1) the connection to a
canonical formulation, and 2) regularization independence (renormalizability).
In the second part of this chapter we will describe the definition of these
models emphasizing the importance of these open issues. We also discuss the non
standard picture of quantum spacetime that follows from background
independence.


title:
Theoretical foundations for on-ground tests of LISA PathFinder thermal
  diagnostics

abstract:
  This paper reports on the methods and results of a theoretical analysis to
design an insulator which must provide a thermally quiet environment to test on
ground delicate temperature sensors and associated electronics. These will fly
on board ESA's LISA PathFinder (LPF) mission as part of the thermal diagnostics
subsystem of the LISA Test-flight Package (LTP). We evaluate the heat transfer
function (in frequency domain) of a central body of good thermal conductivity
surrounded by a layer of a very poorly conducting substrate. This is applied to
assess the materials and dimensions necessary to meet temperature stability
requirements in the metal core, where sensors will be implanted for test. The
analysis is extended to evaluate the losses caused by heat leakage through
connecting wires, linking the sensors with the electronics in a box outside the
insulator. The results indicate that, in spite of the very demanding stability
conditions, a sphere of outer diameter of the order one metre is sufficient.


title: Planck-scale physics: facts and beliefs

abstract:
  The relevance of the Planck scale to a theory of quantum gravity has become a
worryingly little examined assumption that goes unchallenged in the majority of
research in this area. However, in all scientific honesty, the significance of
Planck's natural units in a future physical theory of spacetime is only a
plausible, yet by no means certain, assumption. The purpose of this article is
to clearly separate fact from belief in this connection.


title: Features of gravity-Yang-Mills hierarchies in d-dimensions

abstract:
  Higher dimensional, direct analogues of the usual d=4 Einstein--Yang-Mills
(EYM) systems are studied. These consist of the gravitational and Yang-Mills
hierarchies in d=4p dimensional spacetimes, both consisting of 2p-form
curvature terms only. Regular and black hole solutions are constructed in
$2p+2\le d \le 4p$, in which dimensions the total mass-energy is finite,
generalising the familiar Bartnik-McKinnon solutions in EYM theory for p=1. In
d=4p, this similarity is complete. In the special case of d=2p+1, just beyond
the finite energy range of d, exact solutions in closed form are found.
Finally, d=2p+1 purely gravitational systems, whose solutions generalise the
static d=3 BTZ solutions, are discussed.


title: A Numerical Approach to Space-Time Finite Elements for the Wave Equation

abstract:
  We study a space-time finite element approach for the nonhomogeneous wave
equation using a continuous time Galerkin method. We present fully implicit
examples in 1+1, 2+1, and 3+1 dimensions using linear quadrilateral,
hexahedral, and tesseractic elements. Krylov solvers with additive Schwarz
preconditioning are used for solving the linear system. We introduce a time
decomposition strategy in preconditioning which significantly improves
performance when compared with unpreconditioned cases.


title: Crossing of the w=-1 barrier in viscous modified gravity

abstract:
  We consider a modified form of gravity in which the action contains a power
alpha of the scalar curvature. It is shown how the presence of a bulk viscosity
in a spatially flat universe may drive the cosmic fluid into the phantom region
(w<-1) and thus into a Big Rip singularity, even if it lies in the quintessence
region (w>-1) in the non-viscous case. The condition for this to occur is that
the bulk viscosity contains the power (2 alpha-1) of the scalar expansion. Two
specific examples are discussed in detail. The present paper is a
generalization of the recent investigation dealing with barrier crossing in
Einstein's gravity: I. Brevik and O. Gorbunova, Gen. Relativ. Grav. 37 (2005)
2039.


title: On the ADM decomposition of the 5-D Kaluza-Klein model

abstract:
  Our purpose is to recast KK model in terms of ADM variables. We examine and
solve the problem of the consistency of this approach, with particular care
about the role of the cylindricity hypothesis. We show in details how the KK
reduction commutes with the ADM slicing procedure and how this leads to a well
defined and unique ADM reformulation. This allows us to consider the
hamiltonian formulation of the model and can be the first step for the Ashtekar
reformulation of the KK scheme. Moreover we show how the time component of the
gauge vector arises naturally from the geometrical constraints of the dynamics;
this is a positive check for the autoconsistency of the KK theory and for an
hamiltonian description of the dynamics which wants to take into account the
compactification scenario: this result enforces the physical meaning of KK
model.


title: Finding and using exact solutions of the Einstein equations

abstract:
  The evolution of the methods used to find solutions of Einstein's field
equations during the last 100 years is described. Early papers used assumptions
on the coordinate forms of the metrics. Since the 1950s more invariant methods
have been deployed in most new papers. The uses to which the solutions found
have been put are discussed, and it is shown that they have played an important
role in the development of many aspects, both mathematical and physical, of
general relativity.


title: Platonic Gravitating Skyrmions

abstract:
  We construct globally regular gravitating Skyrmions, which possess only
discrete symmetries. In particular, we present tetrahedral and cubic Skyrmions.
The SU(2) Skyrme field is parametrized by an improved harmonic map ansatz.
Consistency then requires also a restricted ansatz for the metric. The
numerical solutions obtained within this approximation are compared to those
obtained in dilaton gravity.


title: Embedding of the Kerr-Newman Black Hole Surface in Euclidean Space

abstract:
  We obtain a global embedding of the surface of a rapidly rotating Kerr-Newman
black hole in an Euclidean 4-dimensional space.


title:
Model for a Universe described by a non-minimally coupled scalar field
  and interacting dark matter

abstract:
  In this work it is investigated the evolution of a Universe where a scalar
field, non-minimally coupled to space-time curvature, plays the role of
quintessence and drives the Universe to a present accelerated expansion. A
non-relativistic dark matter constituent that interacts directly with dark
energy is also considered, where the dark matter particle mass is assumed to be
proportional to the value of the scalar field. Two models for dark matter
pressure are considered: the usual one, pressureless, and another that comes
from a thermodynamic theory and relates the pressure with the coupling between
the scalar field and the curvature scalar. Although the model has a strong
dependence on the initial conditions, it is shown that the mixture consisted of
dark components plus baryonic matter and radiation can reproduce the expected
red-shift behavior of the deceleration parameter, density parameters and
luminosity distance.


title:
A model for a non-minimally coupled scalar field interacting with dark
  matter

abstract:
  In this work we investigate the evolution of a Universe consisted of a scalar
field, a dark matter field and non-interacting baryonic matter and radiation.
The scalar field, which plays the role of dark energy, is non-minimally coupled
to space-time curvature, and drives the Universe to a present accelerated
expansion. The non-relativistic dark matter field interacts directly with the
dark energy and has a pressure which follows from a thermodynamic theory. We
show that this model can reproduce the expected behavior of the density
parameters, deceleration parameter and luminosity distance.


title:
Comment on "Self-dual teleparallel formulation of general relativity and
  the positive energy theorem" G. Y. Chee

abstract:
  We give a correct tensor proof of the positive energy problem for the case
including momentum on basis of conditions of existence of the two-to-one
correspondence between the Sen-Witten spinor field and the Sen-Witten
orthonormal frame. These conditions were obtained in our previous publications,
but true significance of our works was not estimated properly by G.Y.Chee, and
these were not correct quoted in his publication. On other hand, the main
result of our work is key argument in favour of geometrical nature of the
Sen-Witten spinor field.


title:
Signatures of Planck-scale interactions in the cosmic microwave
  background?

abstract:
  Based on a rather general low-energy effective action (interacting quantum
fields in classical curved space-times), we calculate potential signatures of
new physics (such as quantum gravity) at ultra-high energies (presumably the
Planck scale) in the anisotropies of the cosmic microwave background. These
Planck-scale interactions create non-Gaussian contributions, where special
emphasis is laid on the three-point function as the most promising observable,
which also allows the discrimination between models violating and those obeying
Lorentz invariance. PACS: 98.80.Cq, 04.62.+v, 98.70.Vc, 98.80.Qc.


title: On the geometry of Killing and conformal tensors

abstract:
  The second order Killing and conformal tensors are analyzed in terms of their
spectral decomposition, and some properties of the eigenvalues and the
eigenspaces are shown. When the tensor is of type I with only two different
eigenvalues, the condition to be a Killing or a conformal tensor is
characterized in terms of its underlying almost-product structure. A canonical
expression for the metrics admitting these kinds of symmetries is also
presented. The space-time cases 1+3 and 2+2 are analyzed in more detail.
Starting from this approach to Killing and conformal tensors a geometric
interpretation of some results on quadratic first integrals of the geodesic
equation in vacuum Petrov-Bel type D solutions is offered. A generalization of
these results to a wider family of type D space-times is also obtained. A
generalization of these results to a wider family of type D space-times is also
obtained.


title: Relativistic Positioning Systems

abstract:
  The theory of relativistic {\em location systems} is sketched. An interesting
class of these systems is that of relativistic {\em positioning systems,} which
consists in sets of four clocks broadcasting their proper time. Among them, the
more important ones are the {\em auto-located positioning systems,} in which
every clock broadcasts not only its proper time but the proper times that it
receives from the other three. At this level, no reference to any exterior
system (the Earth surface, for example) and no synchronization are needed. Some
properties are presented. In the SYPOR project, such a structure is proposed,
eventually anchored to a classical reference system on the Earth surface, as
the best relativistic structure for Global Navigation Satellite Systems.


title: Fermi-transported spinor and Dirac equation in general relativity

abstract:
  The Fermi transport of the Dirac spinor is considered as a generalization of
the parallel transport of this spinor which was introduced by V. Fock and D.
Ivanenko (1929). The possible structure of the new variant of the
general-relativistic Dirac equation based on the Fermi transport is discussed.


title:
A numerical investigation of the stability of steady states and critical
  phenomena for the spherically symmetric Einstein-Vlasov system

abstract:
  The stability features of steady states of the spherically symmetric
Einstein-Vlasov system are investigated numerically. We find support for the
conjecture by Zeldovich and Novikov that the binding energy maximum along a
steady state sequence signals the onset of instability, a conjecture which we
extend to and confirm for non-isotropic states. The sign of the binding energy
of a solution turns out to be relevant for its time evolution in general. We
relate the stability properties to the question of universality in critical
collapse and find that for Vlasov matter universality does not seem to hold.


title: Approximate gravitational field of a rotating deformed mass

abstract:
  A new approximate solution of vacuum and stationary Einstein field equations
is obtained. This solution is constructed by means of a power series expansion
of the Ernst potential in terms of two independent and dimensionless parameters
representing the quadrupole and the angular momentum respectively. The main
feature of the solution is a suitable description of small deviations from
spherical symmetry through perturbations of the static configuration and the
massive multipole structure by using those parameters. This quality of the
solution might eventually provide relevant differences with respect to the
description provided by the Kerr solution.


title: Brane world corrections to Newton's law

abstract:
  We discuss possible variations of the effective gravitational constant with
length scale, predicted by most of alternative theories of gravity and unified
models of physical interactions. After a brief general exposition, we review in
more detail the predicted corrections to Newton's law of gravity in diverse
brane world models. We consider various configurations in 5 dimensions (flat,
de Sitter and AdS branes in Einstein and Einstein-Gauss-Bonnet theories, with
and without induced gravity and possible incomplete graviton localization), 5D
multi-brane systems and some models in higher dimensions. A common feature of
all models considered is the existence of corrections to Newton's law at small
radii comparable with the bulk characteristic length: at such radii, gravity on
the brane becomes effectively multidimensional. Many models contain superlight
perturbation modes, which modify gravity at large scale and may be important
for astrophysics and cosmology.


title: Gravitational Wave Background from Phantom Superinflation

abstract:
  Recently, the early superinflation driven by phantom field has been proposed
and studied. The detection of primordial gravitational wave is an important
means to know the state of very early universe. In this brief report we discuss
in detail the gravitational wave background excited during the phantom
superinflation.


title:
Comparison of Approaches to Quantum Correction of Black Hole
  Thermodynamics

abstract:
  There are several approaches to quantum gravitational corrections of black
hole thermodynamics. String theory and loop quantum gravity, by direct analysis
on the basis of quantum properties of black holes, show that in the
entropy-area relation the leading order correction should be of log-area type.
On the other hand, generalized uncertainty principle(GUP) and modified
dispersion relations(MDRs) provide perturbational framework for such
modifications. Although both GUP and MDRs are common features of all quantum
gravity scenarios, their functional forms are quantum gravity model dependent.
Since both string theory and loop quantum gravity give more reliable solution
of the black hole thermodynamics, one can use their results to test approximate
results of GUP and MDRs. In this paper, we find quantum corrected black hole
thermodynamics in the framework of GUP and MDR and then we compare our results
with string theory solutions. This comparison suggests severe constraints on
the functional form of GUP and MDRs. These constraints may reflect
characteristic features of ultimate quantum gravity theory.


title: Coll Positioning systems: a two-dimensional approach

abstract:
  The basic elements of Coll positioning systems (n clocks broadcasting
electromagnetic signals in a n-dimensional space-time) are presented in the
two-dimensional case. This simplified approach allows us to explain and to
analyze the properties and interest of these relativistic positioning systems.
The positioning system defined in flat metric by two geodesic clocks is
analyzed. The interest of the Coll systems in gravimetry is pointed out.


title: Further Exact Cosmological Solutions to Higher-Order Gravity Theories

abstract:
  We investigate the effect of deviations from general relativity on approach
to the initial singularity by finding exact cosmological solutions to a wide
class of fourth-order gravity theories. We present new anisotropic vacuum
solutions of modified Kasner type and demonstrate the extent to which they are
valid in the presence of non-comoving perfect-fluid matter fields. The infinite
series of Mixmaster oscillations seen in general relativity will not occur in
these solutions, except in unphysical cases.


title: Flat Information Geometries in Black Hole Thermodynamics

abstract:
  The Hessian of either the entropy or the energy function can be regarded as a
metric on a Gibbs surface. For two parameter families of asymptotically flat
black holes in arbitrary dimension one or the other of these metrics are flat,
and the state space is a flat wedge. The mathematical reason for this is traced
back to the scale invariance of the Einstein-Maxwell equations. The picture of
state space that we obtain makes some properties such as the occurence of
divergent specific heats transparent.


title: Charges and the boundary in Chern Simons gravity

abstract:
  In this work a new set of boundary conditions for Chern Simons gravity that
lead to a fully gauge invariant action is analyzed. This particular form of the
action reproduces the standard results of black hole thermodynamics and
determines that the algebra of charges of diffeomorphisms at the horizon be the
Virasoro algebra.


title: The causal set approach to quantum gravity

abstract:
  The ideas of spacetime discreteness and causality are important in several of
the popular approaches to quantum gravity. But if discreteness is accepted as
an initial assumption, conflict with Lorentz invariance can be a consequence.
The causal set is a discrete structure which avoids this problem and provides a
possible history space on which to build a ``path integral'' type quantum
gravity theory. Motivation, results and open problems are discussed and some
comparisons to other approaches are made. Some recent progress on recovering
locality in causal sets is recounted.


title: Einstein Product Metrics in Diverse Dimensions

abstract:
  We use direct products of Einstein Metrics to construct new solutions to
Einstein's Equations with cosmological constant. We illustrate the technique
with three families of solutions having the geometries Kerr/de Sitter X de
Sitter, Kerr/anti-de Sitter X anti-de Sitter and Kerr X Kerr.


title: Generalized theories of gravity and conformal continuations

abstract:
  Many theories of gravity admit formulations in different, conformally related
manifolds, known as the Jordan and Einstein conformal frames. Among them are
various scalar-tensor theories of gravity and high-order theories with the
Lagrangian $f(R)$ where $R$ is the scalar curvature and $f$ an arbitrary
function. It may happen that a singularity in the Einstein frame corresponds to
a regular surface S_trans in the Jordan frame, and the space-time is then
continued beyond this surface. This phenomenon is called a conformal
continuation (CC). We discuss the properties of vacuum static, spherically
symmetric configurations of arbitrary dimension in scalar-tensor and $f(R)$
theories of gravity and indicate necessary and sufficient conditions for the
existence of solutions admitting a CC. Two cases are distinguished, when
S_trans is an ordinary regular sphere and when it is a Killing horizon. Two
explicit examples of CCs are presented.


title:
A minimization problem for the lapse and the initial-boundary value
  problem for Einstein's field equations

abstract:
  We discuss the initial-boundary value problem of General Relativity. Previous
considerations for a toy model problem in electrodynamics motivate the
introduction of a variational principle for the lapse with several attractive
properties. In particular, it is argued that the resulting elliptic gauge
condition for the lapse together with a suitable condition for the shift and
constraint-preserving boundary conditions controlling the Weyl scalar Psi_0 are
expected to yield a well posed initial-boundary value problem for metric
formulations of Einstein's field equations which are commonly used in numerical
relativity.
  To present a simple and explicit example we consider the 3+1 decomposition
introduced by York of the field equations on a cubic domain with two periodic
directions and prove in the weak field limit that our gauge condition for the
lapse and our boundary conditions lead to a well posed problem. The method
discussed here is quite general and should also yield well posed problems for
different ways of writing the evolution equations, including first order
symmetric hyperbolic or mixed first-order second-order formulations. Well posed
initial-boundary value formulations for the linearization about arbitrary
stationary configurations will be presented elsewhere.


title: Some properties of emission coordinates

abstract:
  4 emitters broadcasting an increasing electromagnetic signal generate a
system of relativistic coordinates for the space-time, called emission
coordinates. Their physical realization requires an apparatus similar to the
one of the Global Navigation Satellite Systems (GNSS). Several relativistic
corrections are utilized for the current precisions, but the GNSS are conceived
as classical (Newtonian) systems, which has deep implications in the way of
operating them. The study of emission coordinates is an essential step in order
to develop a fully relativistic theory of positioning systems. This talk
presents some properties of emission coordinates. In particular, we
characterize how any observer sees a configuration of satellites giving a
degenerated system and show that the trajectories of the satellites select a
unique privileged observer at each point and, for any observer, a set of 3
orthogonal spatial axes.


title:
Quantification of electrical charges in Kaluza Klein theory without
  strings of Dirac

abstract:
  It is proposed a formalism of quantification of the electric charges in the
Kaluza Klein theory of five dimensions and a explanation of the cause of the
variation of the electromagnetic fine-structure constant in cosmological
times.There is a formalism that eliminates the problem that impeached the KK
theory to complete the unification of the electromagnetic and the gravitational
fields. The complete unification of these two fields is obtained. The radius of
compactation is determined as 70,238 lengths of Planck.


title: Why Eppley and Hannah's Experiment Isn't

abstract:
  It is shown that Eppley and Hannah's thought experiment establishing that
gravity must be quantized is fatally flawed. The device they propose, even if
built, cannot establish their claims, nor is it plausible that it can be built
with any materials compatible with the values of c, h, and G. Finally the
device, and any reasonable modification of it, would be so massive as to be
within its own Schwarzschild radius--a fatal flaw for any thought experiment.


title:
The Principle of the Fermionic Projector: An Approach for Quantum
  Gravity?

abstract:
  In this short article we introduce the mathematical framework of the
principle of the fermionic projector and set up a variational principle in
discrete space-time. The underlying physical principles are discussed. We
outline the connection to the continuum theory and state recent results. In the
last two sections, we speculate on how it might be possible to describe quantum
gravity within this framework.


title: Application of stochastic resonance in gravitational-wave interferometer

abstract:
  We investigate novel approach, which improves the sensitivity of
gravitational wave (GW) interferometer due to stochastic resonance (SR)
phenomenon, performing in additional nonlinear cavity (NC). The NC is installed
in the output of interferometer before photodetector, so that optical signal
emerging interferometer incidents on the NC and passes through it. Under
appropriate circumstances a specific transformation of noisy signal inside the
NC takes place, which results in the increase of output signal-to-noise ratio
(SNR). As a result optical noisy signal of interferometer becomes less noisy
after passing through the NC. The improvement of SNR is especially effective in
bistable NC for wideband (several hundreds Hz) detection, when chirp GW signal
is detected. Then SNR gain reaches amount ~ 10. When detection bandwidth is
narrowed, the influence of SR mechanism gradually disappears, and SNR gain
tends to 1. SNR gain also tends to 1 when the NC is gradually transformed to
linear cavity. Proposed enhancement of SNR due to the SR is not dependent of
noise type, which is prevalent in interferometer. Particularly proposed
approach is capable to increase signal-to-displacement noise ratio.


title: Accelerating Kerr-Newman black holes in (anti-)de Sitter space-time

abstract:
  A class of exact solutions of the Einstein-Maxwell equations is presented
which describes an accelerating and rotating charged black hole in an
asymptotically de Sitter or anti-de Sitter universe. The metric is presented in
a new and convenient form in which the meaning of the parameters is clearly
identified, and from which the physical properties of the solution can readily
be interpreted.


title: The Cauchy Problem for the Wave Equation in the Schwarzschild Geometry

abstract:
  The Cauchy problem is considered for the scalar wave equation in the
Schwarzschild geometry. We derive an integral spectral representation for the
solution and prove pointwise decay in time.


title:
Massless and massive graviton spectra in anisotropic dilatonic
  braneworld cosmologies

abstract:
  We consider a braneworld model in which an anisotropic brane is embedded in a
dilatonic background. We solve the background solutions and study the behavior
of the perturbations when the universe evolves from an inflationary Kasner
phase to a Minkowski phase. We calculate the massless mode spectrum, and find
that it does not differ from what expected in standard four-dimensional
cosmological models. We then evaluate the spectrum of both light
(ultrarelativistic) and heavy (nonrelativistic) massive modes, and find that,
at high energies, there can be a strong enhancement of the Kaluza-Klein
spectral amplitude, which can become dominant in the total spectrum. The
presence of the dilaton, on the contrary, decrease the relative importance of
the massive modes.


title: Energy Density Associated with the Bianchi Type-II Space-Time

abstract:
  This paper has been removed by arXiv administrators because of overlap with
gr-qc/9910015 and hep-th/0308070. It also has excessive overlap with the
following papers also written by the authors or their collaborators:
gr-qc/0509047, gr-qc/0607110, gr-qc/0602012, gr-qc/0606028, and others.


title: Loop quantum cosmology and the Wheeler-De Witt equation

abstract:
  We present some results concerning the large volume limit of loop quantum
cosmology in the flat homogeneous and isotropic case. We derive the Wheeler-De
Witt equation in this limit. Looking for the action from which this equation
can also be obtained, we then address the problem of the modifications to be
brought to the Friedman's equation of motion and to the equation of motion of
the scalar field, in the classical limit.


title: Torsion induces Gravity

abstract:
  In this work the Poincare-Chern Simons and Anti de Sitter Chern Simons
gravities are studied. For both a solution that can be casted as a black hole
with manifest torsion is found. Those solutions resemble Schwarzschild and
Schwarzschild-AdS solutions respectively.


title: Black hole and cosmological space-times in Born-Infeld-Einstein theory

abstract:
  In this paper I examine black hole and cosmological space-times in
Born-Infeld-Einstein theory with electric and magnetic charges. The field
equations are derived and written in the form $G_{\mu\nu}=-\kappa T_{\mu\nu}$
for spherically symmetric space-times. The energy-momentum tensor is not the
Born-Infeld energy-momentum tensor, but can be obtained from Born-Infeld theory
by letting $a\to ia$, where $a$ is the Born-Infeld parameter. It is shown that
there is a curvature singularity in spherically symmetric space-times at a
nonzero radial coordinate and that, as in Reissner-Nordstrom space-times, there
are zero, one or two horizons. Charged black holes have either two horizons and
a timelike singularity or one horizon with a spacelike, timelike, or null
singularity. Anisotropic cosmological solutions with electric and magnetic
fields are obtained from the spherically symmetric solutions.


title:
Spectral Line Broadening and Angular Blurring due to Spacetime Geometry
  Fluctuations

abstract:
  We treat two possible phenomenological effects of quantum fluctuations of
spacetime geometry: spectral line broadening and angular blurring of the image
of a distance source. A geometrical construction will be used to express both
effects in terms of the Riemann tensor correlation function. We apply the
resulting expressions to study some explicit examples in which the fluctuations
arise from a bath of gravitons in either a squeezed state or a thermal state.
In the case of a squeezed state, one has two limits of interest: a coherent
state which exhibits classical time variation but no fluctuations, and a
squeezed vacuum state, in which the fluctuations are maximized.


title: Coordinates and frames from the causal point of view

abstract:
  Lorentzian frames may belong to one of the 199 causal classes. Of these
numerous causal classes, people are essentially aware only of two of them.
Nevertheless, other causal classes are present in some well-known solutions, or
present a strong interest in the physical construction of coordinate systems.
Here we show the unusual causal classes to which belong so familiar coordinate
systems as those of Lema{\^{\i}}tre, those of Eddington-Finkelstein, or those
of Bondi-Sachs. Also the causal classes associated to the Coll light
coordinates (four congruences of real geodetic null lines) and to the Coll
positioning systems (light signals broadcasted by four clocks) are analyzed.
The role that these results play in the comprehension and classification of
relativistic coordinate systems is emphasized.


title:
On the iterated Crank-Nicolson for hyperbolic and parabolic equations in
  numerical relativity

abstract:
  The iterated Crank-Nicolson is a predictor-corrector algorithm commonly used
in numerical relativity for the solution of both hyperbolic and parabolic
partial differential equations. We here extend the recent work on the stability
of this scheme for hyperbolic equations by investigating the properties when
the average between the predicted and corrected values is made with unequal
weights and when the scheme is applied to a parabolic equation. We also propose
a variant of the scheme in which the coefficients in the averages are swapped
between two corrections leading to systematically larger amplification factors
and to a smaller numerical dispersion.


title:
Hamilton-Jacobi theory for Hamiltonian systems with non-canonical
  symplectic structures

abstract:
  A proposal for the Hamilton-Jacobi theory in the context of the covariant
formulation of Hamiltonian systems is done. The current approach consists in
applying Dirac's method to the corresponding action which implies the inclusion
of second-class constraints in the formalism which are handled using the
procedure of Rothe and Scholtz recently reported. The current method is applied
to the nonrelativistic two-dimensional isotropic harmonic oscillator employing
the various symplectic structures for this dynamical system recently reported.


title: Teleparallel Gravitational Energy in the Gamma Metric

abstract:
  This paper has been removed by arXiv administrators because it plagiarizes
gr-qc/0303034 and gr-qc/0212018.


title: Anti-Newtonian universes do not exist

abstract:
  In a paper by Maartens, Lesame and Ellis (Class. Quant. Grav. 15, 1005) it
was shown that irrotational dust solutions with vanishing electric part of the
Weyl tensor are subject to severe integrability conditions and it was
conjectured that the only such solutions are FLRW spacetimes. In their analysis
the possibility of a cosmological constant Lambda was omitted. The conjecture
is proved, irrespective as to whether Lambda is zero or not, and qualitative
differences with the case of vanishing magnetic Weyl curvature are pointed out.


title: Non-equilibrium Thermodynamics of Spacetime

abstract:
  It has previously been shown that the Einstein equation can be derived from
the requirement that the Clausius relation dS = dQ/T hold for all local
acceleration horizons through each spacetime point, where dS is one quarter the
horizon area change in Planck units, and dQ and T are the energy flux across
the horizon and Unruh temperature seen by an accelerating observer just inside
the horizon. Here we show that a curvature correction to the entropy that is
polynomial in the Ricci scalar requires a non-equilibrium treatment. The
corresponding field equation is derived from the entropy balance relation dS
=dQ/T+dS_i, where dS_i is a bulk viscosity entropy production term that we
determine by imposing energy-momentum conservation. Entropy production can also
be included in pure Einstein theory by allowing for shear viscosity of the
horizon.

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


title: Solar system effects in Schwarzschild--de Sitter spacetime

abstract:
  The Schwarzschild--de Sitter space--time describes the gravitational field of
a spherically symmetric mass in a universe with cosmological constant
$\Lambda$. Based on this space--time we calculate Solar system effects like
gravitational redshift, light deflection, gravitational time delay, Perihelion
shift, geodetic or de Sitter precession, as well as the influence of $\Lambda$
on a Doppler measurement, used to determine the velocity of the Pioneer 10 and
11 spacecraft. For $\Lambda=\Lambda_0 \sim 10^{-52} {\rm m}^{-2}$ the
cosmological constant plays no role for all of these effects, while a value of
$\Lambda \sim - 10^{-37} {\rm m}^{-2}$, if hypothetically held responsible for
the Pioneer anomaly, is not compatible with the Perihelion shift.


title: Time, clocks, parametric invariance and the Pioneer Anomaly

abstract:
  In the context of a parametric theory (with the time being a dynamical
variable) we consider the coupling between the quantum vacuum and the
background gravitation that pervades the universe (unavoidable because of the
universality of gravity). In our model the fourth Heisenberg relation
introduces a possible source of discrepancy between the marches of atomic and
gravitational clocks, which accelerate with respect to one another. This
produces in its turn another discrepancy between the observations, performed
with atomic time, and the theoretical analysis, which uses parametric
astronomical time. Curiously, this approach turns out to be compatible with
current physics; lacking a unified theory of quantum physics and gravitation,
it cannot be discarded {\it a priori}. It happens that this phenomenon has the
same footprint as the Pioneer Anomaly, what suggests a solution to this riddle.
This is because the velocity of the Pioneer spaceship with respect to atomic
time turns out to be slightly smaller that with astronomical time, so that that
the apparent trajectory lags behind the real one. In 1998, after many
unsuccessful efforts to account for this phenomenon, the discoverers suggested
"the possibility that the origin of the anomalous signal is new physics".


title: Radiation Damping in Einstein-Aether Theory

abstract:
  This work concerns the loss of energy of a material system due to
gravitational radiation in Einstein-aether theory-an alternative theory of
gravity in which the metric couples to a dynamical, timelike, unit-norm vector
field. Derived to lowest post-Newtonian order are waveforms for the metric and
vector fields far from a nearly Newtonian system and the rate of energy
radiated by the system. The expressions depend on the quadrupole moment of the
source, as in standard general relativity, but also contain monopolar and
dipolar terms. There exists a one-parameter family of Einstein-aether theories
for which only the quadrupolar contribution is present, and for which the
expression for the damping rate is identical to that of general relativity to
the order worked to here. This family cannot yet be declared observationally
viable, since effects due to the strong internal fields of bodies in the actual
systems used to test the damping rate are not included.


title:
On the impossibility of measuring the general relativistic part of the
  terrestrial acceleration of gravity with superconducting gravimeters

abstract:
  In this paper we very preliminarily investigate the possibility of measuring
the post-Newtonian general relativistic gravitoelectric and gravitomagnetic
components of the acceleration of gravity on the Earth, in continuous regime,
with two absolute measurements at the equator and the south pole with
superconducting gravimeters. The magnitudes of such relativistic effects are
10^-10 m s^-2 and 10^-11 m s^-2, respectively. Unfortunately, the present-day
uncertainties in the Earth's geodetic parameters which enter the classical
Newtonian terms induce systematic errors 1-2 orders of magnitude larger than
the relativistic ones. Moreover, a \sim 1 ngal sensitivity can be reached by
the currently available superconducting gravimeters, but only for relative
measurements.


title: Generalised coherent states and combinatorics of horizon entropy

abstract:
  We calculate the exact degeneracy of states corresponding to the area
operator in the framework of semiclassical loop quantum gravity, using
techniques of combinatorial theory. The degeneracy counting is used to find
entropy of apparent horizons derived from generalised coherent states which
include a sum over graphs. The correction to the entropy is determined as
exponentially decreasing in area.


title: Asymptotic structure of radiation in higher dimensions

abstract:
  We characterize a general gravitational field near conformal infinity (null,
spacelike, or timelike) in spacetimes of any dimension. This is based on an
explicit evaluation of the dependence of the radiative component of the Weyl
tensor on the null direction from which infinity is approached. The behaviour
similar to peeling property is recovered, and it is shown that the directional
structure of radiation has a universal character that is determined by the
algebraic type of the spacetime. This is a natural generalization of analogous
results obtained previously in the four-dimensional case.


title:
(SC)RMI: A (S)emi-(C)lassical (R)elativistic (M)otion (I)integrator, to
  model the orbits of space probes around the Earth and other planets

abstract:
  Today, the motion of spacecrafts is still described according to the
classical Newtonian equations plus the so-called "relativistic corrections",
computed with the required precision using the Post-(Post-)Newtonian formalism.
The current approach, with the increase of tracking precision (Ka-Band Doppler,
interplanetary lasers) and clock stabilities (atomic fountains) is reaching its
limits in terms of complexity, and is furthermore error prone. In the
appropriate framework of General Relativity, we study a method to numerically
integrate the native relativistic equations of motion for a weak gravitational
field, also taking into account small non-gravitational forces. The latter are
treated as perturbations, in the sense that we assume that both the local
structure of space-time is not modified by these forces, and that the
unperturbed satellite motion follows the geodesics of the local space-time. The
use of a symplectic integrator to compute the unperturbed geodesic motion
insures the constancy of the norm of the proper velocity quadrivector. We
further show how this general relativistic framework relates to the classical
one.


title: Fields of accelerated sources: Born in de Sitter

abstract:
  This paper deals thoroughly with the scalar and electromagnetic fields of
uniformly accelerated charges in de Sitter spacetime. It gives details and
makes various extensions of our Physical Review Letter from 2002. The basic
properties of the classical Born solutions representing two uniformly
accelerated charges in flat spacetime are first summarized. The worldlines of
uniformly accelerated particles in de Sitter universe are defined and described
in a number of coordinate frames, some of them being of cosmological
significance, the other are tied naturally to the particles. The scalar and
electromagnetic fields due to the accelerated charges are constructed by using
conformal relations between Minkowski and de Sitter space. The properties of
the generalized `cosmological' Born solutions are analyzed and elucidated in
various coordinate systems. In particular, a limiting procedure is demonstrated
which brings the cosmological Born fields in de Sitter space back to the
classical Born solutions in Minkowski space. In an extensive Appendix, which
can be used independently of the main text, nine families of coordinate systems
in de Sitter spacetime are described analytically and illustrated graphically
in a number of conformal diagrams.


title:
Group field theory formulation of 3d quantum gravity coupled to matter
  fields

abstract:
  We present a new group field theory describing 3d Riemannian quantum gravity
coupled to matter fields for any choice of spin and mass. The perturbative
expansion of the partition function produces fat graphs colored with SU(2)
algebraic data, from which one can reconstruct at once a 3-dimensional
simplicial complex representing spacetime and its geometry, like in the
Ponzano-Regge formulation of pure 3d quantum gravity, and the Feynman graphs
for the matter fields. The model then assigns quantum amplitudes to these fat
graphs given by spin foam models for gravity coupled to interacting massive
spinning point particles, whose properties we discuss.


title: Fermion Mass Generation in de Sitter Space

abstract:
  We study the one-loop radiative corrections for massless fermions in de
Sitter space induced by a Yukawa coupling to a light, nearly minimally coupled
scalar field. We show that the fermions acquire a mass. Next we construct the
corresponding (nonlocal) effective fermionic action, which -- in contrast to
the case of a massive Dirac fermion -- preserves chirality. Nevertheless, the
resulting fermion dynamics is precisely that of a Dirac fermion with a mass
proportional to the expansion rate. Our finding supports the view that an
observer or a test particle responds to a scalar field in inflation by shifting
its energy rather than seeing a thermal bath.


title: Energy Imparted by Gravitational Waves in Inhomogeneous Space-times

abstract:
  This paper has been removed by arXiv administrators because of overlap with
gr-qc/0303034 and others.
  This paper also has excessive overlap with the following papers also written
by the authors or their collaborators: gr-qc/0509047, gr-qc/0601133,
gr-qc/0607126, gr-qc/0606080, gr-qc/0510123, gr-qc/0607115, gr-qc/0606028,
gr-qc/0607103, gr-qc/0509023, gr-qc/0607119, gr-qc/0603044, gr-qc/0511095,
gr-qc/0502031, and others.


title: Generalizing Quantum Mechanics for Quantum Spacetime

abstract:
  Familiar textbook quantum mechanics assumes a fixed background spacetime to
define states on spacelike surfaces and their unitary evolution between them.
Quantum theory has changed as our conceptions of space and time have evolved.
But quantum mechanics needs to be generalized further for quantum gravity where
spacetime geometry is fluctuating and without definite value. This paper
reviews a fully four-dimensional, sum-over-histories, generalized quantum
mechanics of cosmological spacetime geometry. This generalization is
constructed within the framework of generalized quantum theory. This is a
minimal set of principles for quantum theory abstracted from the modern quantum
mechanics of closed systems, most generally the universe. In this
generalization, states of fields on spacelike surfaces and their unitary
evolution are emergent properties appropriate when spacetime geometry behaves
approximately classically. The principles of generalized quantum theory allow
for the further generalization that would be necessary were spacetime not
fundamental. Emergent spacetime phenomena are discussed in general and
illustrated with the example of the classical spacetime geometries with large
spacelike surfaces that emerge from the `no-boundary' wave function of the
universe. These must be Lorentzian with one, and only one, time direction. The
essay concludes by raising the question of whether quantum mechanics itself is
emergent.


title: Cosmological expansion governed by a scalar field from a 5D vacuum

abstract:
  We consider a single field governed expansion of the universe from a five
dimensional (5D) vacuum state. Under an appropiate change of variables the
universe can be viewed in a effective manner as expanding in 4D with an
effective equation of state which describes different epochs of its evolution.
In the example here worked the universe fistly describes an inflationary phase,
followed by a decelerated expansion. Thereafter, the universe is accelerated
and describes a quintessential expansion to finally, in the future, be vacuum
dominated.


title: Two-dimensional approach to relativistic positioning systems

abstract:
  A relativistic positioning system is a physical realization of a coordinate
system consisting in four clocks in arbitrary motion broadcasting their proper
times. The basic elements of the relativistic positioning systems are presented
in the two-dimensional case. This simplified approach allows to explain and to
analyze the properties and interest of these new systems. The positioning
system defined by geodesic emitters in flat metric is developed in detail. The
information that the data generated by a relativistic positioning system give
on the space-time metric interval is analyzed, and the interest of these
results in gravimetry is pointed out.


title: General Theory of Relativity: Will it survive the next decade?

abstract:
  The nature of gravity is fundamental to our understanding of our own solar
system, the galaxy and the structure and evolution of the Universe. Einstein's
general theory of relativity is the standard model that is used for almost
ninety years to describe gravitational phenomena on these various scales. We
review the foundations of general relativity, discuss the recent progress in
the tests of relativistic gravity, and present motivations for high-accuracy
gravitational experiments in space. We also summarize the science objectives
and technology needs for the laboratory experiments in space with laboratory
being the entire solar system. We discuss the advances in our understanding of
fundamental physics anticipated in the near future and evaluate discovery
potential for the recently proposed gravitational experiments.


title: Fourth order gravity: equations, history, and applications to cosmology

abstract:
  The field equations following from a Lagrangian L(R) will be deduced and
solved for special cases. If L is a non-linear function of the curvature
scalar, then these equations are of fourth order in the metric. In the
introduction we present the history of these equations beginning with the paper
of H. Weyl from 1918, who first discussed them as alternative to Einstein's
theory. In the third part, we give details about the cosmic no hair theorem,
i.e., the details how within fourth order gravity with L= R + R^2 the
inflationary phase of cosmic evolution turns out to be a transient attractor.
Finally, the Bicknell theorem, i.e. the conformal relation from fourth order
gravity to scalar-tensor theory, will be shortly presented.


title: Gravitoelectromagnetism in a complex Clifford algebra

abstract:
  A linear vector model of gravitation is introduced in the context of quantum
physics as a generalization of electromagnetism. The gravitoelectromagnetic
gauge symmetry corresponds to a hyperbolic unitary extension of the usual
complex phase symmetry of electromagnetism. The reversed sign for the
gravitational coupling is obtained by means of the pseudoscalar of the
underlying complex Clifford algebra.


title: The Testbed for LISA Analysis Project

abstract:
  The Testbed for LISA Analysis (TLA) Project aims to facilitate the
development, validation and comparison of different methods for LISA science
data analysis, by the broad LISA Science Community, to meet the special
challenges that LISA poses. It includes a well-defined Simulated LISA Data
Product (SLDP), which provides a clean interface between the communities that
have developed to model and to analyze the LISA science data stream; a
web-based clearinghouse (at <http://tla.gravity.psu.edu>) providing SLDP
software libraries, relevant software, papers and other documentation, and a
repository for SLDP data sets; a set of mailing lists for communication between
and among LISA simulators and LISA science analysts; a problem tracking system
for SLDP support; and a program of workshops to allow the burgeoning LISA
science community to further refine the SLDP definition, define specific LISA
science analysis challenges, and report their results. This note describes the
TLA Project, the resources it provides immediately, its future plans, and
invites the participation of the broader community in the furtherance of its
goals.


title:
Some partial solutions of Mathisson-Papapetrou equations in a
  Schwarzschild field

abstract:
  The analytical and numerical solutions of the Mathisson-Papapetrou equations
under the Mathisson-Pirani supplementary condition describing highly
relativistic (ultrarelativistic) motions of a spinning particle in a
Schwarzschild field are investigated. The known condition S/mr<<1, which is
necessary for a test particle, holds on all these solutions. The explicit
expressions for the non-equatorial circular orbits, in particular for the space
boundaries of the region of existence of these orbits, are obtained. The
dynamics of the deviation of a spinning particle from the equatorial
ultrarelativistic circular orbit with r=3M caused by the non-zero initial value
of the radial particle's velocity is studied. It is shown in the concrete cases
that spin can considerable influence the shape of an ultrarelativistic
trajectory, as compared to the corresponding geodesic trajectory, for the short
time, less than the time of one or two revolutions of a spinning particle
around a Schwarzschild mass.


title:
The Barbero connection and its relation to the histories connection
  formalism without gauge fixing

abstract:
  We present a histories version of the connection formalism of general
relativity. Such an approach introduces a spacetime description--a
characteristic feature of the histories approach--and we discuss the extent to
which the usual loop variables are compatible with a spacetime description. In
particular, we discuss the definability of the Barbero connection without any
gauge fixing. Although it is not the pullback of a spacetime connection onto
the three-surface and it does not have a natural spacetime interpretation, this
does not mean that the Barbero connection is not suitable variable for
quantisation; it appears naturally in the formalism even in absence of gauge
fixing. It may be employed therefore, to define loop variables similar to those
employed in loop quantum gravity. However, the loop algebra would have to be
augmented by the introduction of additional variables.


title:
Emergent Gravity and Torsion: String Theory Without String Theory, Why
  the Cosmic Dark Energy Is So Small

abstract:
  A surprisingly simple holographic explanation for the low dark energy density
is suggested. I derive the Einstein-Cartan disclination curvature tetrads and
the physically independent dislocation torsion gap spin connections from an
"M-Matrix" of non-closed Cartan 1-forms made from 8 Goldstone phase 0-forms of
the vacuum ODLRO condensate inflation field in which the non-compact
10-parameter Poincare symmetry group is locally gauged for all invariant matter
field actions. Quantum gravity zero point vacuum fluctuations should be
renormalizable at the spin 1 tetrad level where there is a natural
scale-dependent holographic dimensionless coupling (hG/\zpf/c^3)^1/3 ~
(Bekenstein BITS)^-1/3. The spacetime tetrad rotation coefficients play the
same role as do the Lie algebra structure constants in internal symmetry spin 1
Yang-Mills local gauge theories. This suggests an intuitively pleasing natural
"organizing idea" now missing in superstring theory. It is then clear why
supersymmetry must break in order for our pocket universe to come into being
with a small w = -1 negative pressure zero point exotic vacuum dark energy
density. Just as the Michelson-Morley experiment gave a null result, this model
predicts that the Large Hadron Collider will never find any viable
on-mass-shell dark matter exotic particles able to explain Omega(DM) ~ 0.23 as
a matter of fundamental principle, neither will any other conceivable dark
matter detector because dark matter forming galactic halos et-al is entirely
virtual exotic vacuum w = - 1 with positive irreducibly random quantum zero
point pressure that mimics w = 0 CDM in its gravity lensing and all effects
that we can observe from afar.


title: Quantum mechanics and the cosmological constant

abstract:
  It is shown that in the model [3,4] of quantum mechanics besides probability
amplitudes, the Planck constant and the Fock space, the cosmological constant
also appear in the natural way. The Poisson brackets are generalized for the
case of kinetics.


title:
On the Kaluza-Klein geometrization of the Electro-Weak Model within a
  gauge theory of the 5-dimensional Lorentz group

abstract:
  The geometrization of the Electro-Weak Model is achieved in a 5-dimensional
Riemann-Cartan framework. Matter spinorial fields are extended to 5 dimensions
by the choice of a proper dependence on the extra-coordinate and of a
normalization factor. U(1) weak hyper-charge gauge fields are obtained from a
Kaluza-Klein scheme, while the tetradic projections of the extra-dimensional
contortion fields are interpreted as SU(2) weak isospin gauge fields. SU(2)
generators are derived by the identification of the weak isospin current to the
extra-dimensional current term in the Lagrangian density of the local Lorentz
group. The geometrized U(1) and SU(2) groups will provide the proper
transformation laws for bosonic and spinorial fields. Spin connections will be
found to be purely Riemannian.


title: Wave functions for the Schwarschild black hole interior

abstract:
  Using the Hamiltonian constraint derived by Ashtekar and Bojowald, we look
for pre-classical wave functions in the Schwarzschild interior. In particular,
when solving this difference equation by separation of variables, an inequality
is obtained relating the Immirzi parameter $\gamma$ to the quantum ambiguity
$\delta$ appearing in the model. This bound is violated when we use a natural
value for $\delta$ based on loop quantum gravity together with a recent
proposal for $\gamma$. We also present numerical solutions of the constraint.


title: Binary black hole merger dynamics and waveforms

abstract:
  We study dynamics and radiation generation in the last few orbits and merger
of a binary black hole system, applying recently developed techniques for
simulations of moving black holes. Our analysis of the gravitational radiation
waveforms and dynamical black hole trajectories produces a consistent picture
for a set of simulations with black holes beginning on circular-orbit
trajectories at a variety of initial separations. We find profound agreement at
the level of one percent among the simulations for the last orbit, merger and
ringdown. We are confident that this part of our waveform result accurately
represents the predictions from Einstein's General Relativity for the final
burst of gravitational radiation resulting from the merger of an astrophysical
system of equal-mass non-spinning black holes. The simulations result in a
final black hole with spin parameter a/m=0.69. We also find good agreement at a
level of roughly 10 percent for the radiation generated in the preceding few
orbits.


title:
The periodic standing-wave approximation: eigenspectral computations for
  linear gravity and nonlinear toy models

abstract:
  The periodic standing wave approach to binary inspiral assumes rigid rotation
of gravitational fields and hence helically symmetric solutions. To exploit the
symmetry, numerical computations must solve for ``helical scalars,'' fields
that are functions only of corotating coordinates, the labels on the helical
Killing trajectories. Here we present the formalism for describing linearized
general relativity in terms of helical scalars and we present solutions to the
mixed partial differential equations of the linearized gravity problem (and to
a toy nonlinear problem) using the adapted coordinates and numerical techniques
previously developed for scalar periodic standing wave computations. We argue
that the formalism developed may suffice for periodic standing wave
computations for post-Minkowskian computations and for full general relativity.


title: Accelerated expansion in modified gravity with a Yukawa-like term

abstract:
  We discuss the Palatini formulation of modified gravity including a
Yukawa-like term. It is shown that in this formulation, the Yukawa term offers
an explanation for the current exponential accelerated expansion of the
universe and reduces to the standard Friedmann cosmology in the appropriate
limit. We then discuss the scalar-tensor formulation of the model as a metric
theory and show that the Yukawa term predicts a power-law acceleration at
late-times. The Newtonian limit of the theory is also discussed in context of
the Palatini formalism.


title: Self-restriction of Gravitational Field and its Role in the Universe

abstract:
  It is shown in the article that according to the Relativistic Theory of
Gravitation the gravitational field providing slowing down of the time rate
nevertheless stops itself this slowing down in strong fields. So a physical
tendency of this field to self-restriction of the gravitational potential is
demonstrated. This property of the field leads to a stopping of the collapse of
massive bodies and to the cyclic evolution of the homogeneous and isotropic
Universe.


title: Anisotropic static solutions in modelling highly compact bodies

abstract:
  Einstein field equations for anisotropic spheres are solved and exact
interior solutions obtained. This paper extends earlier treatments to include
anisotropic models which accommodate a wider variety of physically viable
energy densities. Two classes of solutions are possible. The first class
contains the limiting case $\mu\propto r^{-2}$ for the energy density which
arises in many astrophysical applications. In the second class the singularity
at the center of the star is not present in the energy density. The models
presented in this paper allow for increasing and decreasing profiles in the
behavior of the energy density.


title: An exact isotropic solution

abstract:
  The condition for pressure isotropy is reduced to a recurrence equation with
variable, rational coefficients of order three. We prove that this difference
equation can be solved in general. Consequently we can find an exact solution
to the field equations corresponding to a static spherically symmetric
gravitational potential in terms of elementary functions. The metric functions,
the energy density and the pressure are continuous and well behaved which
implies that this solution could be used to model the interior of a
relativistic sphere. The model satisfies a barotropic equation of state in
general which approximates a polytrope close to the stellar centre.


title:
Orbital evolution of a test particle around a black hole: Indirect
  determination of the self force in the post Newtonian approximation

abstract:
  Comparing the corrections to Kepler's law with orbital evolution under a self
force, we extract the finite, already regularized part of the latter in a
specific gauge. We apply this method to a quasi-circular orbit around a
Schwarzschild black hole of an extreme mass ratio binary, and determine the
first- and second-order conservative gravitational self force in a post
Newtonian expansion. We use these results in the construction of the
gravitational waveform, and revisit the question of the relative contribution
of the self force and spin-orbit coupling.


title: A principal component analysis for LISA -- the TDI connection

abstract:
  Data from the Laser Interferometer Space Antenna (LISA) is expected to be
dominated by frequency noise from its lasers. However the noise from any one
laser appears more than once in the data and there are combinations of the data
that are insensitive to this noise. These combinations, called time delay
interferometry (TDI) variables, have received careful study, and point the way
to how LISA data analysis may be performed. Here we approach the problem from
the direction of statistical inference, and show that these variables are a
direct consequence of a principal component analysis of the problem. We present
a formal analysis for a simple LISA model and show that there are eigenvectors
of the noise covariance matrix that do not depend on laser frequency noise.
Importantly, these orthogonal basis vectors correspond to linear combinations
of TDI variables. As a result we show that the likelihood function for source
parameters using LISA data can be based on TDI combinations of the data without
loss of information.


title: The Stability of Lorentzian Space-Time

abstract:
  It is stated in many text books that the any metric appearing in general
relativity should be locally Lorentzian i.e. of the type $\eta_\mn = {\rm diag}
(1,-1,-1,-1)$ this is usually presented as an independent axiom of the theory,
which can not be deduced from other assumptions. In this work we show that the
above assertion is a consequence of a standard stability analysis of the
Einstein \eqs and need not be assumed.


title: New Description of Self-Dual Metrics

abstract:
  We show that Pleba\'nski's equation for self-dual metrics is equivalent to a
pair of equations describing canonical transformations in 2-dimensional phase
spaces. Examples of linearizations of these equations are given.


title:
On inhomogeneous magnetic seed fields and gravitational waves within the
  MHD limit

abstract:
  In this paper we apply second-order gauge-invariant perturbation theory to
investigate the possibility that the non-linear coupling between gravitational
waves (GW) and a large scale inhomogeneous magnetic field acts as an
amplification mechanism in an `almost' Friedmann-Lemaitre-Robertson-Walker
(FLRW) Universe. The spatial inhomogeneities in the magnetic field are
consistently implemented using the magnetohydrodynamic (MHD) approximation,
which yields an additional source term due to the interaction of the magnetic
field with velocity perturbations in the plasma. Comparing the solutions with
the corresponding results in our previous work indicates that, on super-horizon
scales, the interaction with the spatially inhomogeneous field in the dust
regime induces the same boost as the case of a homogeneous field, at least in
the ideal MHD approximation. This is attributed to the observation that the MHD
induced part of the generated field effectively only contributes on scales
where the coherence length of the initial field is less than the Hubble scale.
At sub-horizon scales, the GW induced magnetic field is completely negligible
in relation to the MHD induced field. Moreover, there is no amplification found
in the long-wavelength limit.


title: Gauge/gravity duality

abstract:
  We review the emergence of gravity from gauge theory in the context of
AdS/CFT duality. We discuss the evidence for the duality, its lessons for
gravitational physics, generalizations, and open questions.


title:
The Structure of the Pseudo-Newtonian Force and Potential about a Five
  Dimensional Rotating Black Hole

abstract:
  In this paper we investigate the structure of the pseudo-Newtonian force and
potential about a five dimensional rotating black hole. The conditions for the
force character from an attractive to repulsive are considered. It is also
found that the force will reach a maximum under certain conditions.


title: The number of negative modes of the oscillating bounces

abstract:
  The spectrum of small perturbations about oscillating bounce solutions
recently discussed in the literature is investigated. Our study supports quite
intuitive and expected result: the bounce with N nodes has exactly N
homogeneous negative modes. Existence of more than one negative modes makes
obscure the relation of these oscillating bounce solutions to the false vacuum
decay processes.


title:
Frame dragging, vorticity and electromagnetic fields in axially
  symmetric stationary spacetimes

abstract:
  We present a general study about the relation between the vorticity tensor
and the Poynting vector of the electromagnetic field for axially symmetric
stationary electrovacuum metrics. The obtained expressions allow to understand
the role of the Poynting vector in the dragging of inertial frames. The
particular case of the rotating massive charged magnetic dipole is analyzed in
detail. In addition, the electric and magnetic parts of the Weyl tensor are
calculated and the link between the later and the vorticity is established.
Then we show that, in the vacuum case, the necessary and sufficient condition
for the vanishing of the magnetic part is that the spacetime be static.


title: Testing existence of antigravity

abstract:
  After a brief review of arguments in favor of antigravity (as gravitational
repulsion between matter and antimatter) we present a simple idea for an
experimental test using antiprotons. Different experimental realizations of the
same basic idea are considered


title: Non static local string in higher dimensional gravity

abstract:
  We analyze the space-time structure of local gauge string with a
phenomenological energy momentum tensor, as prescribed by Vilenkin in an
arbitrary number of space-time dimensions with a non-zero cosmological constant
>. A set of solutions of full non-linear Einstein's equations for the interior
region of such a string are presented.


title: Higher dimensional global monopole with cosmological term

abstract:
  We investigate the space-time of a global monopole in a five dimensional
space-time in presence of the cosmological term. Also the gravitational
properties of the monopole solution are discussed.


title:
Validating delta-filters for resonant bar detectors of improved
  bandwidth foreseeing the future coincidence with interferometers

abstract:
  The classical delta filters used in the current resonant bar experiments for
detecting GW bursts are viable when the bandwidth of resonant bars is few Hz.
In that case, the incoming GW burst is likely to be viewed as an impulsive
signal in a very narrow frequency window. After making improvements in the
read-out with new transducers and high sensitivity dc-SQUID, the
Explorer-Nautilus have improved the bandwidth ($\sim 20$ Hz) at the sensitivity
level of $10^{-20}/\sqrt{Hz}$. Thus, it is necessary to reassess this
assumption of delta-like signals while building filters in the resonant bars as
the filtered output crucially depends on the shape of the waveform. This is
presented with an example of GW signals -- stellar quasi-normal modes, by
estimating the loss in SNR and the error in the timing, when the GW signal is
filtered with the delta filter as compared to the optimal filter.


title: A Note on the Instability of Lorentzian Taub-NUT-Space

abstract:
  I show that there are no SU(2)-invariant (time-dependent) tensorial
perturbations of Lorentzian Taub-NUT space. It follows that the spacetime is
unstable at the linear level against generic perturbations. I speculate that
this fact is responsible for so far unsuccessful attempts to define a sensible
thermodynamics for NUT-charged spacetimes.


title:
On the structure of general solution to the equations of shear-free null
  congruences

abstract:
  We demonstrate that the source of a shear-free null congruence is,
generically, a world sheet of a singular string in the complex Minkowski space.
In particular cases the string degenerates into the Newman's point singularity.
We describe also a time-dependent deformation of the Kerr's congruence which
could indicate the unstability of the Kerr (Kerr-Newman) (electro)vacuum
solution. Then we consider a 5D extension of Minkowski space obtained by
complexification of the time coordinate. Singular locus of a shear-free null
congruence on such an extended space-time consists of a (great) number of point
particles (``markeons'') and 2D surfaces -- light-like wave fronts.


title: Late time tails of the massive vector field in a black hole background

abstract:
  We investigate the late-time behavior of the massive vector field in the
background of the Schwarzschild and Schwarzschild-de Sitter black holes. For
Schwarzschild black hole, at intermediately late times the massive vector field
is represented by three functions with different decay law $\Psi_{0} \sim
t^{-(\ell + 3/2)} \sin{m t}$, $\Psi_{1} \sim t^{-(\ell + 5/2)} \sin{m t}$,
$\Psi_{2} \sim t^{-(\ell + 1/2)} \sin{m t}$, while at asymptotically late times
the decay law $\Psi \sim t^{-5/6} \sin{(m t)}$ is universal, and does not
depend on the multipole number $\ell$. Together with previous study of massive
scalar and Dirac fields where the same asymptotically late-time decay law was
found, it means, that the asymptotically late-time decay law $\sim t^{-5/6}
\sin{(m t)}$ \emph{does not depend} also \emph{on the spin} of the field under
consideration. For Schwarzschild-de Sitter black holes it is observed two
different regimes in the late-time decay of perturbations: non-oscillatory
exponential damping for small values of $m$ and oscillatory quasinormal mode
decay for high enough $m$. Numerical and analytical results are found for these
quasinormal frequencies.


title: The Extended ADS/CFT Correspondence

abstract:
  The correspondence between conformal covariant fields in Minkowski's
space-time and isometric fields in the five dimensional anti-deSitter
space-time is extended to a six-dimensional bulk space and its regular
sub-manifolds, so as to include the analysis of evaporating Schwarzschild's
black holes without loss of quantum unitarity.


title: Tetrads in Yang-Mills geometrodynamics

abstract:
  A new set of tetrads is introduced within the framework of SU(2) X U(1)
Yang-Mills field theories in four dimensional Lorentz curved spacetimes. Each
one of these tetrads diagonalizes separately and explicitly each term of the
Yang-Mills stress-energy tensor. Therefore, three pairs of planes also known as
blades, can be defined, and make up the underlying geometrical structure, at
each point. These tetrad vectors are gauge dependent on one hand, and also in
their definition, there is an additional inherent freedom in the choice of two
vector fields. In order to get rid of the gauge dependence, another set of
tetrads is defined, such that the only choice we have to make is for the two
vector fields. A particular choice is made for these two vector fields such
that they are gauge dependent, but the transformation properties of these
tetrads are analogous to those already known for curved spacetimes where only
electromagnetic fields are present. This analogy allows to establish group
isomorphisms between the local gauge group SU(2), and the tensor product of the
groups of local Lorentz tetrad transformations, either on blade one or blade
two. These theorems show explicitly that the local internal groups of
transformations are isomorphic to local spacetime groups of transformations. As
an example of application of these new tetrads, we exhibit three new gauge
invariant objects, and using these objects we show how to diagonalize the
Yang-Mills stress-energy tensor in a gauge invariant way.


title: Brane Universes with Gauss-Bonnet-Induced-Gravity

abstract:
  The DGP brane world model allows us to get the observed late time
acceleration via modified gravity, without the need for a ``dark energy''
field. This can then be generalised by the inclusion of high energy terms, in
the form of a Gauss-Bonnet bulk. This is the basis of the
Gauss-Bonnet-Induced-Gravity (GBIG) model explored here with both early and
late time modifications to the cosmological evolution. Recently the simplest
GBIG models (Minkowski bulk and no brane tension) have been analysed. Two of
the three possible branches in these models start with a finite density
``Big-Bang'' and with late time acceleration. Here we present a comprehensive
analysis of more general models where we include a bulk cosmological constant
and brane tension. We show that by including these factors it is possible to
have late time phantom behaviour.


title:
Problems which are well-posed in a generalized sense with applications
  to the Einstein equations

abstract:
  In the harmonic description of general relativity, the principle part of
Einstein equations reduces to a constrained system of 10 curved space wave
equations for the components of the space-time metric. We use the
pseudo-differential theory of systems which are well-posed in the generalized
sense to establish the well-posedness of constraint preserving boundary
conditions for this system when treated in second order differential form. The
boundary conditions are of a generalized Sommerfeld type that is benevolent for
numerical calculation.


title: The covariance of GPS coordinates and frames

abstract:
  We explore, in the general relativistic context, the properties of the
recently introduced GPS coordinates, as well as those of the associated frames
and coframes. We show that they are covariant, and completely independent of
any observer. We show that standard spectroscopic and astrometric observations
allow any observer to measure (i) the values of the GPS coordinates at his
position, (ii) the components of his [four-]velocity and (iii) the components
of the metric in the GPS frame. This provides to this system an unique value
both for conceptual discussion (no frame dependence) and for practical use
(involved quantities are directly measurable): localisation, motion monitoring,
astrometry, cosmography, tests of gravitation theories. We show explicitly, in
the general relativistic context, how an observer may estimate its position and
motion, and reconstruct the components of the metric. This arises from two main
results: the extension of the velocity fields of the probes to the whole
(curved) spacetime; and the identification of the components of the observer's
velocity in the GPS frame with the (inversed) observed redshifts of the probes.
Specific cases (non relativistic velocities; Minkowski and
Friedmann-Lema\^{i}tre spacetimes; geodesic motions) are studied in details.


title:
Simulation of astrophysical jet using the special relativistic
  hydrodynamics code

abstract:
  This paper describes a multidimensional hydrodynamic code which can be used
for the studies of relativistic astrophysical flows. The code solves the
special relativistic hydrodynamic equations as a hyperbolic system of
conservation laws based on High Resolution Shock Capturing (HRSC) Scheme. Two
standard tests, one of which is the relativistic blast wave tested in our
previous paper\cite{DO1}, and the other is the collision of two
ultrarelativistic blast waves tested in here, are presented to demonstrate that
the code captures correctly and gives solution in the discontinuities,
accurately.
  The relativistic astrophysical jet is modeled for the ultrarelativistic flow
case. The dynamics of jet flowing is then determined by the ambient parameters
such as densities, and velocities of the jets and the momentum impulse applied
to the computational surface. We obtain solutions for the jet structure,
propagation of jet during the time evolution, and variation in the Mach number
on the computational domain at a fixed time.


title: Ricci Flow Gravity

abstract:
  A theory of gravitation is proposed, modeled after the notion of a Ricci
flow. In addition to the metric an independent volume enters as a fundamental
geometric structure. Einstein gravity is included as a limiting case. Despite
being a scalar-tensor theory the coupling to matter is different from
Jordan-Brans-Dicke gravity. In particular there is no adjustable coupling
constant. For the solar system the effects of Ricci flow gravity cannot be
distinguished from Einstein gravity and therefore it passes all classical
tests. However for cosmology significant deviations from standard Einstein
cosmology will appear.


title: Acceleration from M theory and Fine-tuning

abstract:
  The compactification of M theory with time dependent hyperbolic internal
space gives an effective scalar field with exponential potential which provides
a transient acceleration in Einstein frame in four dimensions. Ordinary matter
and radiation are present in addition to the scalar field coming from
compactification. We find that we have to fine-tune the initial conditions of
the scalar field so that our Universe experiences acceleration now. During the
evolution of our Universe, the volume of the internal space increases about 12
times. The time variation of the internal space results in a large time
variation of the fine structure constant which violates the observational
constraint on the variation of the fine structure constant. The large variation
of the fine structure constant is a generic feature of transient acceleration
models.


title:
Periapsis and gravitomagnetic precessions of stellar orbits in Kerr and
  Kerr-de Sitter black hole spacetimes

abstract:
  The exact solution for the motion of a test particle in a non-spherical polar
orbit around a Kerr black hole is derived. Exact novel expressions for frame
dragging (Lense-Thirring effect), periapsis advance and the orbital period are
produced. The resulting formulae, are expressed in terms of Appell's first
hypergeometric function $F_1$, Jacobi's amplitude function, and Appell's $F_1$
and Gau$\ss$ hypergeometric function respectively. The exact expression for
frame dragging is applied for the calculation of the Lense-Thirring effect for
the orbits of S-stars in the central arcsecond of our Galaxy assuming that the
galactic centre is a Kerr black hole, for various values of the Kerr parameter
including those supported by recent observations. In addition, we apply our
solutions for the calculation of frame dragging and periapsis advance for
stellar non-spherical polar orbits in regions of strong gravitational field
close to the event horizon of the galactic black hole, e.g. for orbits in the
central milliarcsecond of our galaxy. Such orbits are the target of the GRAVITY
experiment. We provide examples with orbital periods in the range of 100min -
54 days. Detection of such stellar orbits will allow the possibility of
measuring the relativistic effect of periapsis advance with high precision at
the strong field realm of general relativity. Further, an exact expression for
the orbital period of a test particle in a non-circular equatorial motion
around a Kerr black hole is produced. We also derive exact expressions for the
periapsis advance and the orbital period for a test particle in a non-circular
equatorial motion in the Kerr field in the presence of the cosmological
constant in terms of Lauricella's fourth hypergeometric function $F_D$.


title: Gravitational waves and fundamental physics

abstract:
  I give an overview of the motivations for gravitational-wave research,
concentrating on the aspects related to ``fundamental'' physics.


title:
Superconducting cavity transducer for resonant gravitational radiation
  antennas

abstract:
  Parametric transducers, such as superconducting rf cavities, can boost the
bandwidth and sensitivity of the next generation resonant antennas, thanks to a
readily available technology. We have developed a fully coupled dynamic model
of the system "antenna--transducer" and worked out some estimates of
signal--to--noise ratio and the stability conditions in various experimental
configurations. We also show the design and the prototype of a rf cavity which,
together with a suitable read--out electronic, will be used as a test bench for
the parametric transducer.


title:
Similarities Between Classical Timelike Geodesics in a Naked
  Reissner-Nordstrom Singularity Background and the Behaviour of Electrons in
  Quantum Theory

abstract:
  It is generally assumed that naked singularities must be physically excluded,
as they could otherwise introduce unpredictable influences in their future null
cones. Considering geodesics for a naked Reissner-Nordstrom singularity, it is
found that the singularity is effectively clothed by its repulsive nature.
Regarding electron as naked singularity, the size of the clothed singularity
(electron) turns out to be classical electro-magnetic radius of the electron,
to an observer falling freely from infinity, initially at rest. The size
shrinks for an observer falling freely from infinity, with a positive initial
velocity. For geodetic parameters corresponding to negative energy there are
trapped geodesics. The similarity of this picture with that arising in the
Quantum Theory is discussed.


title: Geometric Constraint in Brane-World

abstract:
  The brane-worlds model was inspired partly by Kaluza-Klein's theory, where
the gravitation and the gauge fields are obtained of a geometry of higher
dimension (bulk). Such a model has been showing positive in the sense of we
find perspectives and probably deep modifications in the physics, such as:
Unification in a scale TeV, quantum gravity in this scale and deviation of
Newton's law for small distances. One of the principles of this model is to
suppose a space-time embedded in a bulk of high dimension. In this note it is
shown, basing on the theorem of Collinson-Szekeres, that the space-time of
Schwarzschild cannot be embedded locally and isometrically in a bulk of five
dimensions with constant curvature,(for example ADS-5). From the point of view
of the semi-Riemannian geometry this last result consists constraints to the
model brane-world.


title:
High energy description of dark energy in an approximate 3-brane
  Brans-Dicke cosmology

abstract:
  We consider a Brans-Dicke cosmology in five-dimensional space-time.
Neglecting the quadratic and the mixed Brans-Dicke terms in the Einstein
equation, we derive a modified wave equation of the Brans-Dicke field. We show
that, at high energy limit, the 3-brane Brans-Dicke cosmology could be
described as the standard one by changing the equation of state. Finally as an
illustration of the purpose, we show that the dark energy component of the
universe agrees with the observations data.


title: Gravitational Lensing by Wormholes

abstract:
  Gravitational lensing by traversable Lorentzian wormholes is a ew possibility
which is analyzed here in the strong field limit. Wormhole solutions are
considered in the Einstein minimally coupled theory and in the brane world
model. The observables in both the theories show significant differences from
those arising in the Schwarzschild black hole lensing. As a corollary, it
follows that wormholes with zero Keplerian mass exhibit lensing properties
which are qualitatively (though not quantitatively) the same as those of a
Schwarzschild black hole. Some special features of the considered solutions are
pointed out.


title: Evolution of Thick Walls in Curved Spacetimes

abstract:
  We generalize our previous thick shell formalism to incorporate any
codimension-1 thick wall with a peculiar velocity and proper thickness bounded
by arbitrary spacetimes. Within this new formulation we obtain the equation of
motion of a spherically symmetric dust thick shell immersed in vacuum as well
as in Friedmann-Robertson-Walker spacetimes.


title:
Twistor algebraic dynamics in complex space-time and physical meaning of
  hidden dimensions

abstract:
  We review the algebraic field theory based completely on a nonlinear
generalization of the CR complex analiticity conditions to the noncommutative
algebra of biquaternions. Any biquaternionic field possesses natural twistor
structure and, in the Minkowski space, gives rise to a shear-free null
congruence of rays and to a set of gauge fields associated with it. In the
paper we develop the algebrodynamical scheme in the complex extension of
Minkowski space-time -- in the full vector space of biquaternion algebra. This
primodial space dynamically reduces to the 6D ``observable'' space-time of the
complex null cone which in turn decomposes into the 4D physical space-time and
the 2D internal ``spin space''. A set of identical point charges
(``duplicons'') -- focal points of the congruence -- arises in the procedure.
Temporal dynamics of individual duplicons is strongly correlated via
fundamental twistor field of the congruence. We briefly discuss some new
notions inevitably arising in the considered algebrodynamical scheme, namely
those of ``complex time'' and of ``evolutionary curve'', as well as their
hypothetical links to the quantum uncertainty phenomenon.


title: See a Black Hole on a Shoestring

abstract:
  The modes of vibration of hanging and partially supported strings provide
useful analogies to scalar fields travelling through spacetimes that admit
conformally flat spatial sections. This wide class of spacetimes includes
static, spherically symmetric spacetimes. The modes of a spacetime where the
scale factor depends as a power-law on one of the coordinates provide a useful
starting point and yield a new classification of these spacetimes on the basis
of the shape of the string analogue. The family of corresponding strings follow
a family of curves related to the cycloid, denoted here as hypercycloids (for
reasons that will become apparent). Like the spacetimes that they emulate these
strings exhibit horizons, typically at their bottommost points where the string
tension vanishes; therefore, hanging strings may provide a new avenue for the
exploration of the quantum mechanics of horizons.


title:
Construction of the second-order gravitational perturbations produced by
  a compact object

abstract:
  Accurate calculation of the gradual inspiral motion in an extreme mass-ratio
binary system, in which a compact-object inspirals towards a supermassive
black-hole requires calculation of the interaction between the compact-object
and the gravitational perturbations that it induces. These metric perturbations
satisfy linear partial differential equations on a curved background spacetime
induced by the supermassive black-hole. At the point particle limit the
second-order perturbations equations have source terms that diverge as
$r^{-4}$, where $r$ is the distance from the particle. This singular behavior
renders the standard retarded solutions of these equations ill-defined. Here we
resolve this problem and construct well-defined and physically meaningful
solutions to these equations. We recently presented an outline of this
resolution [E. Rosenthal, Phys. Rev. D 72, 121503 (2005)]. Here we provide the
full details of this analysis. These second-order solutions are important for
practical calculations: the planned gravitational-wave detector LISA requires
preparation of waveform templates for the expected gravitational-waves.
Construction of templates with desired accuracy for extreme mass-ratio binaries
requires accurate calculation of the inspiral motion including the interaction
with the second-order gravitational perturbations.


title:
Bayesian inference on compact binary inspiral gravitational radiation
  signals in interferometric data

abstract:
  Presented is a description of a Markov chain Monte Carlo (MCMC) parameter
estimation routine for use with interferometric gravitational radiational data
in searches for binary neutron star inspiral signals. Five parameters
associated with the inspiral can be estimated, and summary statistics are
produced. Advanced MCMC methods were implemented, including importance
resampling and prior distributions based on detection probability, in order to
increase the efficiency of the code. An example is presented from an
application using realistic, albeit fictitious, data.