title:
Strong cosmic censorship for surface-symmetric cosmological spacetimes
  with collisionless matter

abstract:
  This paper addresses strong cosmic censorship for spacetimes with
self-gravitating collisionless matter, evolving from surface-symmetric compact
initial data. The global dynamics exhibit qualitatively different features
according to the sign of the curvature $k$ of the symmetric surfaces and the
cosmological constant $\Lambda$. With a suitable formulation, the question of
strong cosmic censorship is settled in the affirmative if $\Lambda=0$ or
$k\le0$, $\Lambda>0$. In the case $\Lambda>0$, $k=1$, we give a detailed
geometric characterization of possible "boundary" components of spacetime; the
remaining obstruction to showing strong cosmic censorship in this case has to
do with the possible formation of extremal Schwarzschild-de Sitter-type black
holes. In the special case that the initial symmetric surfaces are all
expanding, strong cosmic censorship is shown in the past for all $k,\Lambda$.
Finally, our results also lead to a geometric characterization of the future
boundary of black hole interiors for the collapse of asymptotically flat data:
in particular, in the case of small perturbations of Schwarzschild data, it is
shown that these solutions do not exhibit Cauchy horizons emanating from $i^+$
with strictly positive limiting area radius.


title: Canonical wave packets in quantum cosmology

abstract:
  We discuss the construction of wave packets resulting from the solutions of a
class of Wheeler-DeWitt equations in Robertson-Walker type cosmologies, for
arbitrary curvature. We show that there always exists a ``canonical initial
slope" for a given initial wave function, which optimizes some desirable
properties of the resulting wave packet, most importantly good
classical-quantum correspondence. This can be properly denoted as a canonical
wave packet. We introduce a general method for finding these canonical initial
slopes which is generalization of our earlier work.


title:
Higher dimensional spacetimes with a geodesic, shearfree, twistfree and
  expanding null congruence

abstract:
  We present the complete family of higher dimensional spacetimes that admit a
geodesic, shearfree, twistfree and expanding null congruence, thus extending
the well-known D=4 class of Robinson-Trautman solutions. Einstein's equations
are solved for empty space with an arbitrary cosmological constant and for
aligned pure radiation. Main differences with respect to the D=4 case (such as
the absence of type III/N solutions, related to ``violations'' of the
Goldberg-Sachs theorem in D>4) are pointed out, also in connection with other
recent works. A formal analogy with electromagnetic fields is briefly discussed
in an appendix, where we demonstrate that multiple principal null directions of
null Maxwell fields are necessarily geodesic, and that in D>4 they are also
shearing if expanding.


title: Analytical Representation of a Black Hole Puncture Solution

abstract:
  The ``moving puncture'' technique has led to dramatic advancements in the
numerical simulations of binary black holes. Hannam et.al. have recently
demonstrated that, for suitable gauge conditions commonly employed in moving
puncture simulations, the evolution of a single black hole leads to a
well-known time-independent, maximal slicing of Schwarzschild. They construct
the corresponding solution in isotropic coordinates numerically and demonstrate
its usefulness, for example for testing and calibrating numerical codes that
employ moving puncture techniques. In this Brief Report we point out that this
solution can also be constructed analytically, making it even more useful as a
test case for numerical codes.


title: Are moving punctures equivalent to moving black holes?

abstract:
  When simulating the inspiral and coalescence of a binary black-hole system,
special care needs to be taken in handling the singularities. Two main
techniques are used in numerical-relativity simulations: A first and more
traditional one ``excises'' a spatial neighbourhood of the singularity from the
numerical grid on each spacelike hypersurface. A second and more recent one,
instead, begins with a ``puncture'' solution and then evolves the full
3-metric, including the singular point. In the continuum limit, excision is
justified by the light-cone structure of the Einstein equations and, in
practice, can give accurate numerical solutions when suitable discretizations
are used. However, because the field variables are non-differentiable at the
puncture, there is no proof that the moving-punctures technique is correct,
particularly in the discrete case. To investigate this question we use both
techniques to evolve a binary system of equal-mass non-spinning black holes. We
compare the evolution of two curvature 4-scalars with proper time along the
invariantly-defined worldline midway between the two black holes, using
Richardson extrapolation to reduce the influence of finite-difference
truncation errors. We find that the excision and moving-punctures evolutions
produce the same invariants along that worldline, and thus the same spacetimes
throughout that worldline's causal past. This provides convincing evidence that
moving-punctures are indeed equivalent to moving black holes.


title:
Regular second order perturbations of extreme mass ratio black hole
  binaries

abstract:
  We report on the first results of self-consistent second order metric
perturbations produced by a point particle moving in the Schwarzschild
spacetime. The second order waveforms satisfy a wave equation with an effective
source term build up from products of first order metric perturbations and its
derivatives. We have explicitly regularized this source term at the particle
location as well as at the horizon and at spatial infinity.


title: Dark Matter as a Consequence of Nonseparable Lagrangian

abstract: This paper has been withdrawn by the author.


title:
Fulling-Davies-Unruh effect and spontaneous excitation of an accelerated
  atom interacting with a quantum scalar field

abstract:
  We investigate, from the point of view of a coaccelerated frame, the
spontaneous excitation of a uniformly accelerated two-level atom interacting
with a scalar field in a thermal state at a finite temperature $T$ and show
that the same spontaneous excitation rate for the uniformly accelerated atom in
the Minkowski vacuum obtained in the inertial frame can only be recovered in
the coaccelerated frame assuming a thermal bath at the Fulling-Davies-Unruh
temperature $T_{FDU}=a/2\pi$ for what appears to be the Minkowski vacuum to the
inertial observer. Our discussion provides another example of a physical
process different from those examined before in the literature to better
understand the Fulling-Davies-Unruh effect.


title:
Testing frame-dragging with the Mars Global Surveyor spacecraft in the
  gravitational field of Mars

abstract:
  In this paper we propose an interpretation of the time series of the RMS
overlap differences of the out-of-plane part $N$ of the orbit of the Martian
polar orbiter Mars Global Surveyor (MGS) over a time span Delta P of about 5
years (14 November 1999-14 January 2005) in terms of the general relativistic
gravitomagnetic Lense-Thirring effect. It turns out that the average of such a
time series over Delta P, normalized to the predicted Lense-Thirring
out-of-plane mean shift over the same time span, is \mu= 1.0018 +/- 0.0053. A
linear fit of the considered time series over Delta P yields a normalized slope
1.1 +/- 0.3 strengthening our interpretation of the determined MGS out-of-plane
features of motion as due to the gravitomagnetic force of Mars.

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


title:
On the gravitational radiation from the collapse of neutron stars to
  rotating black holes

abstract:
  We provide details and present additional results on the numerical study of
the gravitational-wave emission from the collapse of neutron stars to rotating
black holes in three dimensions. More specifically, we concentrate on the
advantages and disadvantages of the use of the excision technique and on how
alternative approaches to that of excision can be successfully employed.
Furthermore, as a first step towards source-characterization, we present a
systematic discussion of the influence that rotation and different
perturbations have on the waveforms and hence on the energy emitted in
gravitational waves.


title:
General description of Dirac spin-rotation effect with relativistic
  factor

abstract:
  The Mashhoon rotation-spin coupling is studied by means of the parallelism
description of general relativity. The relativistic rotational tetrad is
exploited, which results in the Minkowski metric, and the torsion axial-vector
and Dirac spin coupling will give the Mashhoon rotation-spin term. For the high
speed rotating cases, the tangent velocity constructed by the angular velocity
$\Ome$ multiplying the distance r may exceed over the speed of light c, i.e.,
$\Ome r \ge c$, which will make the relativistic factor $\gamma$ infinity or
imaginary. In order to avoid this "meaningless" difficulty occurred in $\gamma$
factor, we choose to make the rotation nonuniform and position-dependent in a
particular way, and then we find that the new rotation-spin coupling energy
expression is consistent with the previous results in the low speed limit.


title:
General relativistic galvano-gravitomagnetic effect in current carrying
  conductors

abstract:
  The analogy between general relativity and electromagnetism suggests that
there is a galvano-gravitomagnetic effect, which is the gravitational analog of
the Hall effect. This new effect takes place when a current carrying conductor
is placed in a gravitomagnetic field and the conduction electrons moving inside
the conductor are deflected transversally with respect to the current flow. In
connection with this galvano-gravitomagnetic effect, we explore the possibility
of using current carrying conductors for detecting the gravitomagnetic field of
the Earth.


title:
Ohm's Law in General Relativity and Coriolis Force Effects in Rotating
  Conductors

abstract:
  It has been shown that a magnetic field proportional to angular velocity of
rotation $\omega$ arises around a rotating conductor with the radial gradient
of temperature $\nabla_r T$. However, the theoretical value of the
proportionality coefficient $10^{-17} {\rm cm}^{5/2}\cdot {\rm g}^{1/2}\cdot
{\rm deg}^{-1}$ does not coincide with the experimental one $10^{-8} {\rm
cm}^{5/2}\cdot {\rm g}^{1/2}\cdot {\rm deg}^{-1}$ [1]. At least two additional
mechanisms act to produce an observable magnetic field in the experiment [1].
First, the fact that the rotation of the conductor is slowed down and second,
the nonstationarity of the temperature gradient during the measurements
contribute to the vertical magnetic field. The value of the magnetic flux
arising from the azimuthal current induced by the ``Coriolis force'' effect on
the thermoelectric radial current, is in good agreement with the experimental
data [1].


title:
Post-Newtonian gravitational radiation and equations of motion via
  direct integration of the relaxed Einstein equations. IV. Radiation reaction
  for binary systems with spin-spin coupling

abstract:
  Using post-Newtonian equations of motion for fluid bodies that include
radiation-reaction terms at 2.5 and 3.5 post-Newtonian (PN) order O[(v/c)^5]
and O[(v/c)^7] beyond Newtonian order), we derive the equations of motion for
binary systems with spinning bodies, including spin-spin effects. In particular
we determine the effects of radiation-reaction coupled to spin-spin effects on
the two-body equations of motion, and on the evolution of the spins. We find
that radiation damping causes a 3.5PN order, spin-spin induced precession of
the individual spins. This contrasts with the case of spin-orbit coupling,
where there is no effect on the spins at 3.5PN order. Employing the equations
of motion and of spin precession, we verify that the loss of total energy and
total angular momentum induced by spin-spin effects precisely balances the
radiative flux of those quantities calculated by Kidder et al.


title: The dynamics of scalar-tensor cosmology from RS two-brane model

abstract:
  We consider Randall-Sundrum two-brane cosmological model in the low energy
gradient expansion approximation by Kanno and Soda. It is a scalar-tensor
theory with a specific coupling function. We find a first integral of equations
for the A-brane metric and estimate constraints for the dark radiation term. We
perform a complementary analysis of the dynamics of the scalar field (radion)
using phase space methods and examine convergence towards the limit of general
relativity. We find that it is possible to stabilize the radion at a finite
value with suitable negative matter densities on the B-brane.


title: Regularity at space-like and null infinity

abstract:
  We extend Penrose's peeling model for the asymptotic behaviour of solutions
to the scalar wave equation at null infinity on asymptotically flat
backgrounds, which is well understood for flat space-time, to Schwarzschild and
the asymptotically simple space-times of Corvino-Schoen/Chrusciel-Delay. We
combine conformal techniques and vector field methods: a naive adaptation of
the ``Morawetz vector field'' to a conformal rescaling of the Schwarzschild
metric yields a complete scattering theory on Corvino-Schoen/Chrusciel-Delay
space-times. A good classification of solutions that peel arises from the use
of a null vector field that is transverse to null infinity to raise the
regularity in the estimates. We obtain a new characterization of solutions
admitting a peeling at a given order that is valid for both Schwarzschild and
Minkowski space-times. On flat space-time, this allows large classes of
solutions than the characterizations used since Penrose's work. Our results
establish the validity of the peeling model at all orders for the scalar wave
equation on the Schwarzschild metric and on the corresponding
Corvino-Schoen/Chrusciel-Delay space-times.


title: How to Measure the Cosmic Curvature

abstract:
  The conventional method to determine the cosmic curvature is to measure the
total mass density $\Omega_{\rm tot}$. Unfortunately the observational
$\Omega_{\rm tot}$ is closely near the critical value 1. The computation of
this paper shows that $\Omega_{\rm tot}\approx 1$ is an inevitable result for
the young universe independent of the spatial topology. So the mass density is
not a good criterion to determine the cosmic curvature. In this paper, we
derive a new criterion based on the galactic distribution with respect to
redshift $z$, which only depends on the cosmological principle and geometry.
The different type of spatial topology will give different results, then the
case can be definitely determined.


title:
Dynamical stability of fluid spheres in spacetimes with a nonzero
  cosmological constant

abstract:
  The Sturm-Liouville eigenvalue equation for eigenmodes of the radial
oscillations is determined for spherically symmetric perfect fluid
configurations in spacetimes with a nonzero cosmological constant and applied
in the cases of configurations with uniform distribution of energy density and
polytropic spheres. It is shown that a repulsive cosmological constant rises
the critical adiabatic index and decreases the critical radius under which the
dynamical instability occurs.


title:
Escape of photons from two fixed extreme Reissner-Nordstr\"om black
  holes

abstract:
  We study the scattering of light (null geodesics) by two fixed extreme
Reissner-Nordstr\"om black holes, in which the gravitational attraction of
their masses is exactly balanced with the electrostatic repulsion of their
charges, allowing a static spacetime. We identify the set of unstable periodic
orbits that constitute the fractal repeller that completely describes the
chaotic escape dynamics of photons. In the framework of periodic orbit theory,
the analysis of the linear stability of the unstable periodic orbits is used to
obtain the main quantities of chaos that characterize the escape dynamics of
the photons scattered by the black holes. In particular, the escape rate which
is compared with the result obtained from numerical simulations that consider
statistical ensembles of photons. We also analyze the dynamics of photons in
the proximity of a perturbed black hole and give an analytical estimation for
the escape rate in this system.


title: Non-singular inflation with vacuum decay

abstract:
  On the basis of a semi-classical analysis of vacuum energy in an expanding
spacetime, we describe a non-singular cosmological model in which the vacuum
density decays with time, with a concomitant production of matter. During an
infinitely long period we have an empty, inflationary universe, with H \approx
1. This primordial era ends in a fast phase transition, during which H and
\Lambda decrease to nearly zero in a few Planck times, with release of a huge
amount of radiation. The late-time scenario is similar to the standard model,
with the radiation phase followed by a long dust era, which tends
asymptotically to a de Sitter universe, with vacuum dominating again. An
analysis of the redshift-distance relation for supernovas Ia leads to
cosmological parameters in agreement with other current estimations.


title:
Areal Foliation and AVTD Behavior in T^2 Symmetric Spacetimes with
  Positive Cosmological Constant

abstract:
  We prove a global foliation result, using areal time, for T^2 symmetric
spacetimes with a positive cosmological constant. We then find a class of
solutions that exhibit AVTD behavior near the singularity.


title:
Plane waves in a relativistic homogeneous and isotropic elastic
  continuum

abstract:
  Propagation of gravitational and acoustic plane waves in a flat universe
filled with a general relativistic, homogeneous and isotropic, spatially flat
continuum is studied. The continuum is described by analogues of
nonrelativistic characteristics, namely energy per particle, pressure and Lame
coefficients, and considered in the comoving proper-time gauge. For all modes
with the given wave covector, differential equations governing the time
dependence of the amplitudes are derived. In particular, longitudinal acoustic
waves are described, in analogy with the nonrelativistic theory, by two coupled
first-order equations. As an example, plane waves in a stiff ultrarigid
continuum are considered.


title: Big-Rip, Sudden Future, and other exotic singularities in the universe

abstract:
  We discuss exotic singularities in the evolution of the universe motivated by
the progress of observations in cosmology. Among them there are: Big-Rip (BR),
Sudden Future Singularities (SFS), Generalized Sudden Future Singularities
(GSFS), Finite Density Singularities (FD), type III, and type IV singularities.
We relate some of these singularities with higher-order characteristics of
expansion such as jerk and snap. We also discuss the behaviour of pointlike
objects and classical strings on the approach to these singularities.


title: Phantom Dark Energy and its Cosmological Consequences

abstract:
  I discuss the dark energy characterized by the violation of the null energy
condition ($\varrho + p \geq 0$), dubbed phantom. Amazingly, it is admitted by
the current astronomical data from supernovae. We discuss both classical and
quantum cosmological models with phantom as a source of matter and present the
phenomenon called phantom duality.


title: On Energy and Momentum of the Friedman and Some More General Universes

abstract:
  Recently some authors concluded that the energy and momentum of the Fiedman
universes, flat and closed, are equal to zero locally and globally (flat
universes) or only globally (closed universes). The similar conclusion was also
done for more general only homogeneous universes (Kasner and Bianchi type I).
Such conclusions originated from coordinate dependent calculations performed
only in comoving Cartesian coordinates by using the so-called {\it
energy-momentum complexes}. By using new coordinate independent expressions on
energy and momentum one can show that the Friedman and more general universes
{\it needn't be energetic nonentity}.


title: Interacting spinor and scalar fields in Bianchi cosmology

abstract:
  A self-consistent system of interacting spinor and scalar fields is
considered within the scope of Bianchi type VI cosmological model filled with a
perfect fluid. The contribution of the cosmological constant ($\Lambda$-term)
is taken into account as well. Exact self-consistent solutions to the field
equations are obtained for a special choice of spatial inhomogeneity and the
interaction terms of the spinor and scalar fields. It has been found that some
special choice of metric functions can give rise to a singularity-free
solutions independent of the value and sign of the $\Lambda$ term. It is also
shown that the introduction of a positive $\Lambda$, the most widespread kind
of dark energy, leads to the rapid growth of the universe, while the negative
one, corresponding to an additional gravitational energy gives rise to an
oscillatory or non-periodic mode of expansion. The role of the spatial
inhomogeneity in the evolution of the universe is clarified within the scope of
the considered models.


title: Energy and angular momentum densities of stationary gravity fields

abstract:
  We give physical explanations of explicit invariant expressions for the
energy and angular momentum densities of gravitational fields in stationary
space-times. These expressions involve non-locally defined conformal factors.
In certain coordinates these become locally defined in terms of the metric.
These results are derived via expressions for total gravitational potential
energy from the difference between the total energy and the mechanical energy.
The latter involves kinetic energy seen in the frame of static observers.
  When in the axially symmetric case we consider zero angular momentum
observers (who move orthogonally to surfaces of constant time), we find that
the angular momentum they attribute to the gravitational field is solely due to
their motion.


title: The C-metric as a colliding plane wave space-time

abstract:
  It is explicitly shown that part of the C-metric space-time inside the black
hole horizon may be interpreted as the interaction region of two colliding
plane waves with aligned linear polarization, provided the rotational
coordinate is replaced by a linear one. This is a one-parameter generalization
of the degenerate Ferrari-Ibanez solution in which the focussing singularity is
a Cauchy horizon rather than a curvature singularity.


title: Embedding the Schwarzschild Ideal Fluid Metric

abstract:
  Certain semi-Riemannian metrics can be decomposed into a Riemannian part and
an isochronal part. The properties of such metrics are particularly easy to
visualize in a coordinate-free way, using isometric embedding. We present such
an isochronal, isometric embedding of the well known Schwarzschild ideal fluid
metric in an attempt to see what is happening when the pressure becomes
singular.


title: Electrostatics and confinement in Einstein's unified field theory

abstract:
  A way for appending sources at the right-hand sides of the field equations of
Einstein's unified field theory is recalled. Two exact solutions endowed with
point sources in equilibrium are shown, and their physical meaning is
discussed.


title: Heun equation, Teukolsky equation, and type-D metrics

abstract:
  Starting with the whole class of type-D vacuum backgrounds with cosmological
constant we show that the separated Teukolsky equation for zero rest-mass
fields with spin $s=\pm 2$ (gravitational waves), $s=\pm 1$ (electromagnetic
waves) and $s=\pm 1/2$ (neutrinos) is an Heun equation in disguise.


title: Chaotic dynamics around astrophysical objects with nonisotropic stresses

abstract:
  The existence of chaotic behavior for the geodesics of the test particles
orbiting compact objects is a subject of much current research. Some years ago,
Gu\'eron and Letelier [Phys. Rev. E \textbf{66}, 046611 (2002)] reported the
existence of chaotic behavior for the geodesics of the test particles orbiting
compact objects like black holes induced by specific values of the quadrupolar
deformation of the source using as models the Erez--Rosen solution and the Kerr
black hole deformed by an internal multipole term. In this work, we are
interesting in the study of the dynamic behavior of geodesics around
astrophysical objects with intrinsic quadrupolar deformation or nonisotropic
stresses, which induces nonvanishing quadrupolar deformation for the
nonrotating limit. For our purpose, we use the Tomimatsu-Sato spacetime [Phys.
Rev. Lett. \textbf{29} 1344 (1972)] and its arbitrary deformed generalization
obtained as the particular vacuum case of the five parametric solution of Manko
et al [Phys. Rev. D 62, 044048 (2000)], characterizing the geodesic dynamics
throughout the Poincar\'e sections method. In contrast to the results by
Gu\'eron and Letelier we find chaotic motion for oblate deformations instead of
prolate deformations. It opens the possibility that the particles forming the
accretion disk around a large variety of different astrophysical bodies
(nonprolate, e.g., neutron stars) could exhibit chaotic dynamics. We also
conjecture that the existence of an arbitrary deformation parameter is
necessary for the existence of chaotic dynamics.


title:
The Relativistic Generalization of the Gravitational Force for Arbitrary
  Spacetimes

abstract:
  It has been suggested that re-expressing relativity in terms of forces could
provide fresh insights. The formalism developed for this purpose only applied
to static, or conformally static, space-times. Here we extend it to arbitrary
space-times. It is hoped that this formalism may lead to a workable definition
of mass and energy in relativity.


title: Quasi-Maxwell interpretation of the spin-curvature coupling

abstract:
  We write the Mathisson-Papapetrou equations of motion for a spinning particle
in a stationary spacetime using the quasi-Maxwell formalism and give an
interpretation of the coupling between spin and curvature. The formalism is
then used to compute equilibrium positions for spinning particles in the NUT
spacetime.


title: On the thermal boundary condition of the wave function of the Universe

abstract:
  We broaden the domain of application of the recently proposed thermal
boundary condition of the wave function of the Universe, which has been
suggested as the basis of a dynamical selection principle on the landscape of
string solutions.


title:
Gravitational self force on a particle in circular orbit around a
  Schwarzschild black hole

abstract:
  We calculate the gravitational self force acting on a pointlike particle of
mass $\mu$, set in a circular geodesic orbit around a Schwarzschild black hole.
Our calculation is done in the Lorenz gauge: For given orbital radius, we first
solve directly for the Lorenz-gauge metric perturbation using numerical
evolution in the time domain; We then compute the (finite) back-reaction force
from each of the multipole modes of the perturbation; Finally, we apply the
``mode sum'' method to obtain the total, physical self force. The {\em
temporal} component of the self force (which is gauge invariant) describes the
dissipation of orbital energy through gravitational radiation. Our results for
this component are consistent, to within the computational accuracy, with the
total flux of gravitational-wave energy radiated to infinity and through the
event horizon. The {\em radial} component of the self force (which is gauge
dependent) is calculated here for the first time. It describes a conservative
shift in the orbital parameters away from their geodesic values. We thus obtain
the $O(\mu)$ correction to the specific energy and angular momentum parameters
(in the Lorenz gauge), as well as the $O(\mu)$ shift in the orbital frequency
(which is gauge invariant).


title: The Ups and Downs of Cyclic Universes

abstract:
  We investigate homogeneous and isotropic oscillating cosmologies with
multiple fluid components. Transfer of energy between these fluids is included
in order to model the effects of non-equilibrium behavior on closed universes.
We find exact solutions which display a range of new behaviors for the
expansion scale factor. Detailed examples are studied for the exchange of
energy from dust or scalar field into radiation. We show that, contrary to
expectation, it is unlikely that such models can offer a physically viable
solution to the flatness problem.


title:
A signature of quantum gravity at the source of the seeds of cosmic
  structure?

abstract:
  This article reviews a recent work by a couple of colleagues and myself about
the shortcomings of the standard explanations of the quantum origin of cosmic
structure in the inflationary scenario, and a proposal to address them. The
point it that in the usual accounts the inhomogeneity and anisotropy of our
universe seem to emerge from an exactly homogeneous and isotropic initial state
through processes that do not break those symmetries. We argued that some novel
aspect of physics must be called upon to able to address the problem in a fully
satisfactory way. The proposed approach is inspired on Penrose's ideas
regarding an quantum gravity induced, real and dynamical collapse of the wave
function.


title: General Relativistic Thermoelectric Effects in Superconductors

abstract:
  We discuss the general-relativistic contributions to occur in the
electromagnetic properties of a superconductor with a heat flow. The appearance
of general-relativistic contribution to the magnetic flux through a
superconducting thermoelectric bimetallic circuit is shown. A response of the
Josephson junctions to a heat flow is investigated in the general-relativistic
framework. Some gravitothermoelectric effects which are observable in the
superconducting state in the Earth's gravitational field are considered.


title: The inertia of heat and its role in the dynamics of dissipative collapse

abstract:
  The decreasing of the inertial mass density, established in the past for
dissipative fluids, is found to be produced by the ``inertial'' term of the
transport equation. Once the transport equation is coupled to the dynamical
equation one finds that the contribution of the inertial term diminishes the
effective inertial mass and the ``gravitational'' force term, by the same
factor. An intuitive picture, and prospective applications of this result to
astrophysical scenarios are discussed.


title:
Particle vs. future event horizon in interacting holographic dark energy
  model

abstract:
  By choosing the future event horizon as the horizon of the flat FLRW
universe, we show that although the interacting holographic dark energy model
is able to explain the phantom divide line crossing, but the thermodynamics
second law is not respected in this model. We show that if one takes the
particle event horizon as the horizon of the universe, besides describing
$\omega=-1$ crossing in a consistent way with thermodynamics second law, he is
able to determine appropriately the ratio of dark matter to dark energy density
at transition time. In this approach, after the first transition from
quintessence to phantom, there is another transition from phantom to
quintessence phase which avoids the big rip singularity.


title: Bianchi spacetimes in noncommutative phase-space

abstract:
  The effects of noncommutativity in the phase-space of the classical and
quantum cosmology of Bianchi models are investigated. Exact solutions in both
commutative and noncommutative cases are presented and compared. Further, the
Noether symmetries of the Bianchi class A spacetimes are studied in both cases
and similarities and differences are discussed.


title: Detectability of Mode Resonances in Coalescing Neutron Star Binaries

abstract:
  Inspirals of neutron star-neutron star binaries are a promising source of
gravitational waves for gravitational wave detectors like LIGO. During the
inspiral, the tidal gravitational field of one of the stars can resonantly
excite internal modes of the other star, resulting in a phase shift in the
gravitational wave signal. We compute using a Fisher-matrix analysis how large
the phase shift must be in order to be detectable. For a $1.4 M_\odot, 1.4
M_\odot$ binary the result is $\sim 8.1, 2.9$ and 1.8 radians, for resonant
frequencies of $16, 32$ and 64 Hz. The measurement accuracies of the other
binary parameters are degraded by inclusion of the mode resonance effect.


title:
Possible Neutrino-Antineutrino Oscillation Under Gravity and its
  Consequences

abstract:
  We show that under gravity the effective masses for neutrino and antineutrino
are different which opens a possible window of neutrino-antineutrino
oscillation even if the rest masses of the corresponding eigenstates are same.
This is due to CPT violation and possible to demonstrate if the neutrino mass
eigenstates are expressed as a combination of neutrino and antineutrino
eigenstates, as of the neutral kaon system, with the plausible breaking of
lepton number conservation. In early universe, in presence of various lepton
number violating processes, this oscillation might lead to
neutrino-antineutrino asymmetry which resulted baryogenesis from the B-L
symmetry by electro-weak sphaleron processes. On the other hand, for Majorana
neutrinos, this oscillation is expected to affect the inner edge of neutrino
dominated accretion disks around a compact object by influencing the neutrino
sphere which controls the accretion dynamics, and then the related type-II
supernova evolution and the r-process nucleosynthesis.


title:
Thermodynamic Properties of Spherically-Symmetric, Uniformly-Accelerated
  Reference Frames

abstract:
  We aim to study the thermodynamic properties of the spherically symmetric
reference frames with uniform acceleration, including the spherically symmetric
generalization of Rindler reference frame and the new kind of uniformly
accelerated reference frame. We find that, unlike the general studies about the
horizon thermodynamics, one cannot obtain the laws of thermodynamics for their
horizons in the usual approaches, despite that one can formally define an area
entropy (Bekenstein-Hawking entropy). In fact, the common horizon for a set of
uniformly accelerated observers is not always exist, even though the
Hawking-Unruh temperature is still well-defined. This result indicates that the
Hawking-Unruh temperature is only a kinematic effect, to gain the laws of
thermodynamics for the horizon, one needs the help of dynamics. Our result is
in accordance with those from the various studies about the acoustic black
holes.


title:
The Electro-Weak model as low-energy sector of 8-dimensional General
  Relativity

abstract:
  In a Kaluza-Klein background $V^4\otimes S^3$, we provide a way to reproduce,
by the dimensional reduction, a 4-spinor with a SU(2) gauge coupling. Since
additional gauge violating terms cannot be avoided, we compute their order of
magnitude by virtue of the application to the Electro-Weak model.


title: Spinning particles in General Relativity

abstract:
  We analyze the behavior of a spinning particle in gravity, both from a
quantum and a classical point of view. We infer that, since the interaction
between the space-time curvature and a spinning test particle is expected, then
the main features of such an interaction can get light on which degrees of
freedom have physical meaning in a quantum gravity theory with fermions.
Finally, the dimensional reduction of Papapetrou equations is performed in a
5-dimensional Kaluza-Klein background and Dixon-Souriau results for the motion
of a charged spinning body are obtained.


title: Slowly Rotating Homogeneous Stars and the Heun Equation

abstract:
  The scheme developed by Hartle for describing slowly rotating bodies in 1967
was applied to the simple model of constant density by Chandrasekhar and Miller
in 1974. The pivotal equation one has to solve turns out to be one of Heun's
equations. After a brief discussion of this equation and the chances of finding
a closed form solution, a quickly converging series solution of it is
presented. A comparison with numerical solutions of the full Einstein equations
allows one to truncate the series at an order appropriate to the slow rotation
approximation. The truncated solution is then used to provide explicit
expressions for the metric.


title: Braneworlds and Quantum States of Relativistic Shells

abstract:
  We review some applications of relativistic shells that are relevant in the
context of quantum gravity/quantum cosmology. Using a recently developed
approach, the stationary states of this general relativistic system can be
determined in the semiclassical approximation. We suggest that this technique
might be of phenomenological relevance in the context of the brane-world
scenario and we draw a picture of the general set-up and of the possible
developments.


title: Brane world cosmology with Gauss-Bonnet and induced gravity terms

abstract:
  In this thesis we investigate certain cosmological brane world models of the
Randall-Sundrum type. The models are motivated by string theory but we focus on
the phenomenology of the cosmology. Two models of specific interest are the
Dvali-Gabadadze-Porrati (DGP, induced-gravity) model, where the brane action is
modified, and the Gauss-Bonnet model where the bulk action is modified. Both of
these modifications maybe motivated by string theory. We provide a brief review
of Randall-Sundrum models and then consider the Kaluza-Klein modes on Minkowski
and de Sitter branes, in both the two and one brane cases. The spectrum
obtained for the de Sitter branes is a new result. We then consider a
Friedmann-Robertson-Walker brane in order to investigate the cosmological
dynamics on the brane. We present a brief discussion of the DGP and
Gauss-Bonnet brane worlds. We then investigate the Gauss-Bonnet-Induced-Gravity
(GBIG) model where the Gauss-Bonnet (GB) bulk term is combined with the
induced-gravity (IG) brane term of the DGP model. We present a thorough
investigation of cosmological dynamics, in particular focusing on GBIG models
that behave like self-accelerating DGP models at late times but at early times
show the remarkable feature of a finite-temperature Big Bang. We also discuss
the constraints from observations, including ages and Big Bang nucleosynthesis.


title: So you want to stop time

abstract:
  A model of a stasis chamber, slowing the passage of time in its interior down
to arbitrarily small rates relative to the outside world, is considered within
classical general relativity. Since the model is adapted from the
Majumdar-Papapetrou spacetime, interior spatial volumes are expanded by the
same factor (cubed) as the rate of time is reduced. Real and apparent
(gravitational) forces as perceived by an interior observer are altered, but in
opposite ways. Comparison with special-relativistic time dilation shows the use
of such a static stasis chamber to be economical only when the most drastic
slowing of time relative to the outside world is desired (d(tau)/dt < 10^{-20})
or when one wants to avoid spending the time needed to accelerate to
relativistic speeds.


title:
Bounded, asymptotically flat toroidal exteriors for cylindrically
  symmetric spacetimes

abstract:
  A transitional layer matching the asymptotically flat exterior of a charged,
massive toroidal body to an interior with spatially cylindrical symmetry is
described. The changes in the geometry, which by themselves would require an
energy tensor violating the energy conditions of classical general relativity,
are compensated for by an additional strong electric field. Part of its
gradient is consumed, depending on how much the exterior toroid departs from
local cylindrical symmetry; what is left over can be used to effect further
transitions in the cylindrical interior. An example is given of such a
transition creating an angular deficit, allowing an otherwise infinite cosmic
string to be captured within a bounded source.


title:
Mining information from binary black hole mergers: a comparison of
  estimation methods for complex exponentials in noise

abstract:
  The ringdown phase following a binary black hole merger is usually assumed to
be well described by a linear superposition of complex exponentials
(quasinormal modes). In the strong-field conditions typical of a binary black
hole merger, non-linear effects may produce mode coupling. Artificial mode
coupling can also be induced by the black hole's rotation, if the radiation
field is expanded in terms of spin-weighted spherical (rather than spheroidal)
harmonics. Observing deviations from linear black hole perturbation theory
requires optimal fitting techniques to extract ringdown parameters from
numerical waveforms, which are inevitably affected by errors. So far,
non-linear least-squares fitting methods have been used as the standard
workhorse to extract frequencies from ringdown waveforms. These methods are
known not to be optimal for estimating parameters of complex exponentials.
Furthermore, different fitting methods have different performance in the
presence of noise. The main purpose of this paper is to introduce the
gravitational wave community to modern variations of a linear parameter
estimation technique first devised in 1795 by Prony: the Kumaresan-Tufts and
matrix pencil methods. Using "test" damped sinusoidal signals in Gaussian white
noise we illustrate the advantages of these methods, showing that they have
variance and bias at least comparable to standard non-linear least-squares
techniques. Then we compare the performance of different methods on
unequal-mass binary black hole merger waveforms. The methods we discuss should
be useful both theoretically (to monitor errors and search for non-linearities
in numerical relativity simulations) and experimentally (for parameter
estimation from ringdown signals after a gravitational wave detection).


title: The Einstein-Elko system -- Can dark matter drive inflation?

abstract:
  Recently, a spin one half matter field with mass dimension one was
discovered, called Elko spinors. The present work shows how to introduce these
fields into a curved spacetime by the standard covariantisation scheme. After
formulating the coupled Einstein-Elko field equations, the spacetime is assumed
to be homogeneous and isotropic in order to simplify the resulting field
equations. Analytical ghost Elko solutions are constructed which have vanishing
energy-momentum tensor without and with cosmological constant. The cosmological
Elko theory is finally related to the standard scalar field theory with self
interaction that gives rise to inflation and it is pointed out that the Elko
spinors are not only prime dark matter candidates but also prime candidates for
inflation.


title:
Quasinormal modes and late-time tails in the background of Schwarzschild
  black hole pierced by a cosmic string: scalar, electromagnetic and
  gravitational perturbations

abstract:
  We have studied the quasinormal modes and the late-time tail behaviors of
scalar, electromagnetic and gravitational perturbations in the Schwarzschild
black hole pierced by a cosmic string. Although the metric is locally identical
to that of the Schwarzschild black hole so that the presence of the string will
not imprint in the motion of test particles, we found that quasinormal modes
and the late-time tails can reflect physical signatures of the cosmic string.
Compared with the scalar and electromagnetic fields, the gravitational
perturbation decays slower, which could be more interesting to disclose the
string effect in this background.


title:
Influence of Lorentz violation on Dirac quasinormal modes in the
  Schwarzschild black hole spacetime

abstract:
  Using the third-order WKB approximation and monodromy methods, we investigate
the influence of Lorentz violating coefficient $b$ (associated with a special
axial-vector $b_{\mu}$ field) on Dirac quasinormal modes in the Schwarzschild
black hole spacetime. At fundamental overtone, the real part decreases linearly
as the parameter $b$ increases. But the variation of the imaginary part with
$b$ becomes more complex. For the larger multiple moment $k$, the magnitude of
imaginary part increases with the increase of $b$, which means that presence of
Lorentz violation makes Dirac field damps more rapidly. At high overtones, it
is found that the real part of high-damped quasinormal frequency does not tend
to zero, which is quite a different from the symptotic Dirac quasinormal modes
without Lorentz violation.


title:
Cosmologies with variable parameters and dynamical cosmon: implications
  on the cosmic coincidence problem

abstract:
  Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.


title: On a covariant version of Caianiello's Model

abstract:
  Caianiello's derivation of Quantum Geometry through an isometric embedding of
the spacetime ({\bf M},\tilde{g}) in the pseudo-Riemannian structure ({\bf
T^*M},g^*_{AB}) is reconsidered. In the new derivation, a non-linear connection
and the bundle %%@ formalism induce a Lorentzian-type structure in the
4-dimensional manifold {\bf M} that is covariant %%@ under arbitrary local
coordinate transformations in {\bf M}. If models with maximal acceleration are
required to be non-trivial, gravity should be supplied with other interactions
in a unification framework.


title: Vector Field Induced Chaos in Multi-dimensional Homogeneous Cosmologies

abstract:
  We show that in multidimensional gravity vector fields completely determine
the structure and properties of singularity. It turns out that in the presence
of a vector field the oscillatory regime exists for any number of spatial
dimensions and for all homogeneous models. We derive the Poincar\'e return map
associated to the Kasner indexes and fix the rules according to which the
Kasner vectors rotate. In correspondence to a 4-dimensional space time, the
oscillatory regime here constructed overlap the usual
Belinski-Khalatnikov-Liftshitz one.


title: Covariant Description of the Inhomogeneous Mixmaster Chaos

abstract:
  We outline the covariant nature of the chaos characterizing the generic
cosmological solution near the initial singularity. Our analysis is based on a
"gauge" independent ADM-reduction of the dynamics to the physical degrees of
freedom, and shows that the dynamics is isomorphic point by point in space to a
billiard on a Lobachevsky plane. The Jacobi metric associated to the geodesic
flow is constructed and a non-zero Lyapunov exponent is explicitly calculated.
The chaos covariance emerges from the independence of the form of the lapse
function and the shift vector.


title: Classical and Quantum Aspects of the Inhomogeneous Mixmaster Chaoticity

abstract:
  We refine Misner's analysis of the classical and quantum Mixmaster in the
fully inhomogeneous picture; we both connect the quantum behavior to the
ensemble representation, both describe the precise effect of the boundary
conditions on the structure of the quantum states.


title: Evolutionary Quantization of Cosmological Models

abstract:
  We consider a Schr\"odinger quantum dynamics for the gravitational field
associated to a FRW spacetime and then we solve the corresponding eigenvalue
problem. We show that, from a phenomenological point of view, an Evolutionary
Quantum Cosmology overlaps the Wheeler-DeWitt approach. We also show how a so
peculiar solution can be inferred to describe the more interesting case of a
generic cosmological model.


title: Quantum Radiation from Quantum Gravitational Collapse

abstract:
  We study quantum radiation emitted during the collapse of a quantized,
gravitating, spherical domain wall. The amount of radiation emitted during
collapse now depends on the wavefunction of the collapsing wall and the
background spacetime. If the wavefunction is initially in the form of a sharp
wavepacket, the expectation value of the particle occupation number is
determined as a function of time and frequency. The results are in good
agreement with our earlier semiclassical analysis and show that the quantum
radiation is non-thermal and evaporation accompanies gravitational collapse.


title: WKB-type Approximation to Noncommutative Quantum Cosmology

abstract:
  In this work, we develop and apply the WKB approximation to several examples
of noncommutative quantum cosmology, obtaining the time evolution of the
noncommutative universe, this is done starting from a noncommutative quantum
formulation of cosmology where the noncommutativity is introduced by a
deformation on the minisuperspace variables. This procedure gives a
straightforward algorithm to incorporate noncommutativity to cosmology and
inflation.


title:
General Formula for the Momentum Imparted to Test Particles in Arbitrary
  Spacetimes

abstract:
  Ehlers and Kundt have provided an approximate procedure to demonstrate that
gravitational waves impart momentum to test particles. This was extended to
cylindrical gravitational waves by Weber and Wheeler. Here a general, exact,
formula for the momentum imparted to test particles in arbitrary spacetimes is
presented.


title:
Quasinormal Modes and Stability Criterion of Dilatonic Black Hole in 1+1
  and 4+1 Dimensions

abstract:
  We study the stability of black holes that are solutions of the dilaton
gravity derived from string-theoretical models in two and five dimensions
against to scalar field perturbations, using the Quasinormal Modes (QNMs)
approach. In order to find the QNMs corresponding to a black hole geometry, we
consider perturbations described by a massive scalar field non-minimally
coupled to gravity. We find that the QNM's frequencies turn out to be pure
imaginary leading to purely damped modes, that is in agreement with the
literature of dilatonic black holes. Our result exhibits the unstable behavior
of the considered geometry against the scalar perturbations. We consider both
the minimal coupling case, i.e., for which the coupling parameter $\zeta$
vanishes, and the case $\zeta={1/4}$.


title: Classical Dark Matter

abstract:
  Classical particle-like solutions of field equations such as general
relativity, could account for dark matter. Such particles would not interact
quantum mechanically and would have negligible interactions apart from
gravitation. As a relic from the big bang, they would be a candidate for cold
dark matter consistent with observations.


title:
Towards a wave-extraction method for numerical relativity. V. Extracting
  the Weyl scalars in the quasi-Kinnersley tetrad from spatial data

abstract:
  We extract the Weyl scalars $\Psi_0$ and $\Psi_4$ in the quasi-Kinnersley
tetrad by finding initially the (gauge--, tetrad--, and
background--independent) transverse quasi-Kinnersley frame. This step still
leaves two undetermined degrees of freedom: the ratio $|\Psi_0|/|\Psi_4|$, and
one of the phases (the product $|\Psi_0|\cdot |\Psi_4|$ and the {\em sum} of
the phases are determined by the so-called BB radiation scalar). The residual
symmetry ("spin/boost") can be removed by gauge fixing of spin coefficients in
two steps: First, we break the boost symmetry by requiring that $\rho$
corresponds to a global constant mass parameter that equals the ADM mass (or,
equivalently in perturbation theory, that $\rho$ or $\mu$ equal their values in
the no-radiation limits), thus determining the two moduli of the Weyl scalars
$|\Psi_0|, |\Psi_4|$, while leaving their phases as yet undetermined. Second,
we break the spin symmetry by requiring that the ratio $\pi/\tau$ gives the
expected polarization state for the gravitational waves, thus determining the
phases. Our method of gauge fixing--specifically its second step--is
appropriate for cases for which the Weyl curvature is purely electric. Applying
this method to Misner and Brill--Lindquist data, we explicitly find the Weyl
scalars $\Psi_0$ and $\Psi_4$ perturbatively in the quasi-Kinnersley tetrad.


title:
The Laser Astrometric Test of Relativity: Science, Technology, and
  Mission Design

abstract:
  The Laser Astrometric Test of Relativity (LATOR) experiment is designed to
explore general theory of relativity in the close proximity to the Sun -- the
most intense gravitational environment in the solar system. Using independent
time-series of highly accurate measurements of the Shapiro time-delay
(interplanetary laser ranging accurate to 3 mm at 2 AU) and interferometric
astrometry (accurate to 0.01 picoradian), LATOR will measure gravitational
deflection of light by the solar gravity with accuracy of 1 part in a billion
-- a factor ~30,000 better than currently available. LATOR will perform series
of highly-accurate tests in its search for cosmological remnants of scalar
field in the solar system. We present science, technology and mission design
for the LATOR mission.


title: A New Way of Proving Black Hole Rigidity

abstract: This paper has been withdrawn.


title: Stationary Black Holes as Holographs

abstract:
  Smooth spacetimes possessing a (global) one-parameter group of isometries and
an associated Killing horizon in Einstein's theory of gravity are investigated.
No assumption concerning the asymptotic structure is made, thereby, the
selected spacetimes may be considered as generic distorted stationary black
holes. First, spacetimes of arbitrary dimension, $n\geq 3$, with matter
satisfying the dominant energy condition and allowing non-zero cosmological
constant are investigated. In this part complete characterisation of the
topology of the event horizon of ``distorted'' black holes is given. It is
shown that the topology of the event horizon of ``distorted'' black holes is
allowed to possess a much larger variety than that of the isolated black hole
configurations. In the second part, 4-dimensional (non-degenerate) electrovac
distorted black hole spacetimes are considered. It is shown that the spacetime
geometry and the electromagnetic field are uniquely determined in the black
hole region once the geometry of the bifurcation surface and one of the
electromagnetic potentials are specified there. Conditions guaranteeing the
same type of determinacy, in a neighbourhood of the event horizon, on the {\it
domain of outer communication} side are also investigated. In particular, they
are shown to be satisfied in the analytic case.


title:
Scalar gravity: Post-Newtonian corrections via an effective field theory
  approach

abstract:
  The problem of motion in General Relativity has lost its academic status and
become an active research area since the next generation of gravity wave
detectors will rely upon its solution. Here we will show, within scalar
gravity, how ideas borrowed from Quantum Field Theory can be used to solve the
problem of motion in a systematic fashion. We will concentrate in
Post-Newtonian corrections. We will calculate the Einstein-Infeld-Hoffmann
action and show how a systematic perturbative expansion puts strong constraints
on the couplings of non-derivative interactions in the theory.


title: New results at 3PN via an effective field theory of gravity

abstract:
  NRGR, an Effective Field Theory approach to gravity, has emerged as a
powerful tool to systematically compute higher order corrections in the
Post-Newtonian expansion. Here we discuss in somehow more detail the recently
reported new results for the spin-spin gravitational potential at third
Post-Newtonian order.


title: Euler-Poisson-Newton approach in Cosmology

abstract:
  This lecture provides us with Newtonian approaches for the interpretation of
two puzzling cosmological observations that are still discussed subject : a
bulk flow and a foam like structure in the distribution of galaxies. For the
first one, we model the motions describing all planar distortions from Hubble
flow, in addition of two classes of planar-axial distortions with or without
rotation, when spatial distribution of gravitational sources is homogenous.
This provides us with an alternative to models which assume the presence of
gravitational structures similar to Great Attractor as origin of a bulk flow.
For the second one, the model accounts for an isotropic universe constituted by
a spherical void surrounded by a uniform distribution of dust. It does not
correspond to the usual embedding of a void solution into a cosmological
background solution, but to a global solution of fluid mechanics. The general
behavior of the void expansion shows a huge initial burst, which freezes
asymptotically up to match Hubble expansion. While the corrective factor to
Hubble law on the shell depends weakly on cosmological constant at early
stages, it enables us to disentangle significantly cosmological models around
redshift z ~ 1.7. The magnification of spherical voids increases with the
density parameter and with the cosmological constant. An interesting feature is
that for spatially closed Friedmann models, the empty regions are swept out,
what provides us with a stability criterion.


title:
Static Fundamental Solutions of Einstein Equations and Superposition
  Principle in Relativistic Gravityv

abstract:
  We show that Einstein equations are compatible with the presence of massive
point particle idealization and find the corresponding two parameter family of
solutions. They are complete defined by the bare mechanical mass $M>0$ and the
Keplerian mass $m>0$ ($m < M$) of the point source of gravity. The global
analytical properties of these solutions in the complex plane define a unique
preferable radial variable of the one particle problem.
  These new solutions are fundamental solutions of the quasi-linear Einstein
equations. We introduce and discuss a novel nonlinear superposition principle
for solutions of Einstein equations and discover the basic role of the
relativistic analog of the Newton gravitational potential. For the relativistic
potential we introduce a simple quasi-linear superposition principle as a new
physical requirement for the initial conditions for Einstein equations, thus
justifying the instant gravistatic case for N particle system.
  This superposition principle allows us to sketch a new theory of the
gravitational mass defect. In it a specific Mach-like principle for the
Keplerian mass $m$ is valid, i.e. it depends on the mass distribution in the
universe, in contrast to the bare mass $M$, which remains a true constant.
Several basic examples both of discrete and of continuous mass distributions
are considered.


title:
WhiskyMHD: a new numerical code for general relativistic
  magnetohydrodynamics

abstract:
  The accurate modelling of astrophysical scenarios involving compact objects
and magnetic fields, such as the collapse of rotating magnetized stars to black
holes or the phenomenology of gamma-ray bursts, requires the solution of the
Einstein equations together with those of general-relativistic
magnetohydrodynamics. We present a new numerical code developed to solve the
full set of general-relativistic magnetohydrodynamics equations in a dynamical
and arbitrary spacetime with high-resolution shock-capturing techniques on
domains with adaptive mesh refinements. After a discussion of the equations
solved and of the techniques employed, we present a series of testbeds carried
out to validate the code and assess its accuracy. Such tests range from the
solution of relativistic Riemann problems in flat spacetime, over to the
stationary accretion onto a Schwarzschild black hole and up to the evolution of
oscillating magnetized stars in equilibrium and constructed as consistent
solutions of the coupled Einstein-Maxwell equations.


title: Quasiequilibrium black hole-neutron star binaries in general relativity

abstract:
  We construct quasiequilibrium sequences of black hole-neutron star binaries
in general relativity. We solve Einstein's constraint equations in the
conformal thin-sandwich formalism, subject to black hole boundary conditions
imposed on the surface of an excised sphere, together with the relativistic
equations of hydrostatic equilibrium. In contrast to our previous calculations
we adopt a flat spatial background geometry and do not assume extreme mass
ratios. We adopt a Gamma=2 polytropic equation of state and focus on
irrotational neutron star configurations as well as approximately nonspinning
black holes. We present numerical results for ratios of the black hole's
irreducible mass to the neutron star's ADM mass in isolation of
M_{irr}^{BH}/M_{ADM,0}^{NS} = 1, 2, 3, 5, and 10. We consider neutron stars of
baryon rest mass M_B^{NS}/M_B^{max} = 83% and 56%, where M_B^{max} is the
maximum allowed rest mass of a spherical star in isolation for our equation of
state. For these sequences, we locate the onset of tidal disruption and, in
cases with sufficiently large mass ratios and neutron star compactions, the
innermost stable circular orbit. We compare with previous results for black
hole-neutron star binaries and find excellent agreement with third-order
post-Newtonian results, especially for large binary separations. We also use
our results to estimate the energy spectrum of the outgoing gravitational
radiation emitted during the inspiral phase for these binaries.


title: The Cosmology of f(R) Gravity in the Metric Variational Approach

abstract:
  We consider the cosmologies that arise in a subclass of f(R) gravity with
f(R)=R+\mu ^{2n+2}/(-R)^{n} and -1<n<0 in the metric (as opposed to the
Palatini) variational approach to deriving the gravitational field equations.
The calculations of the isotropic and homogeneous cosmological models are
undertaken in the Jordan frame and at both the background and the perturbation
levels. For the former, we also discuss the connection to the Einstein frame in
which the extra degree of freedom in the theory is associated with a scalar
field sharing some of the properties of a 'chameleon' field. For the latter, we
derive the cosmological perturbation equations in general theories of f(R)
gravity in covariant form and implement them numerically to calculate the
cosmic-microwave-background temperature and matter-power spectra of the
cosmological model. The CMB power is shown to reduce at low l's, and the matter
power spectrum is almost scale-independent at small scales, thus having a
similar shape to that in standard general relativity. These are in stark
contrast with what was found in the Palatini f(R) gravity, where the CMB power
is largely amplified at low l's and the matter spectrum is strongly
scale-dependent at small scales. These features make the present model more
adaptable than that arising from the Palatini f(R) field equations, and none of
the data on background evolution, CMB power spectrum, or matter power spectrum
currently rule it out.


title:
Equatorial symmetry/antisymmetry of stationary axisymmetric electrovac
  spacetimes

abstract:
  Two theorems are proved concerning how stationary axisymmetric electrovac
spacetimes that are equatorially symmetric or equatorially antisymmetric can be
characterized correctly in terms of the Ernst potentials $\E$ and $\Phi$ or in
terms of axis-data.


title:
Equatorial symmetry/antisymmetry of stationary axisymmetric electrovac
  spacetimes. II

abstract:
  In this second paper devoted to the equatorial symmetry/antisymmetry of
stationary axisymmetric electrovac spacetimes we show how two theorems proved
in our previous paper (Ernst, Manko and Ruiz 2006 Class. Quantum Grav. 23,
4945) can be utilized to construct exact solutions that are equatorially
symmetric or antisymmetric.


title: Probing Noncommutativity with Inflationary Gravitational Waves

abstract:
  In this paper we study the behaviour of gravitational wave background (GWB)
generated during inflation in the environment of the noncommutative field
approach. From this approach we derive out one additive term, and then we find
that the dispersion relation of the gravitational wave would be modified and
the primordial gravitational wave would obtain an effective mass. Therefore it
breaks lorentz symmetry in local. Moreover, this additive term would a little
raise up the energy spectrum of GWB in low frequency and then greatly suppress
the spectrum at even lower energy scale of which the wave length may be near
the current horizon. Therefore, a sharp peak is formed on the energy spectrum
in the range of low frequencies. This peak should be a key criterion to detect
the possible existence of noncommutativity of space-time in the background of
our universe and a critical test for breaking lorentz symmetry in local field
theory. Adding all possible effects on the evolution of GWB, we give some new
information of the tensor power spectrum and its energy spectrum which may be
probed in the future cosmological observations.


title: Schwarzschild Spacetime without Coordinates

abstract:
  We discuss how to construct the full Schwarzschild (Kruskal-Szekeres)
spacetime in one swoop by using the bundle of orthonormal Lorentz frames and
the Einstein equation without the use of coordinates. We never have to write
down the Kruskal-Szekeres or an equivalent form of the metric.


title: Averaging Problem in Cosmology and Macroscopic Gravity

abstract:
  The Averaging problem in general relativity and cosmology is discussed. The
approach of macroscopic gravity to resolve the problem is presented. An exact
cosmological solution to the equations of macroscopic gravity is given and its
properties are discussed.
  Contents:
  1. Introduction to General Relativity 2. General Relativity -> Relativistic
Cosmology 3. Introduction to Relativistic Cosmology 4. Relativistic Cosmology
-> Mathematical Cosmology 5. Averaging Problem in Relativistic Cosmology 6.
History of the Averaging Problem 7. Averaging and FLRW Cosmologies 8. Averaging
Problem in General Relativity 9. Macroscopic Gravity and General Relativity 10.
Macroscopic Gravity and Cosmology 11. The System of Macroscopic Gravity
Equations 12. An Exact Cosmological Solution


title: Bubbles and Quantum Tunnelling in Inflationary Cosmology

abstract:
  We review a procedure to use semiclassical methods in the quantization of
General Relativistic shells and apply these techniques in some simplified
models of inflationary cosmology. Some interesting open issues are introduced
and the relevance of their solution in the broader context of Quantum Gravity
is discussed.


title: Dirac fermions in de Sitter and anti-de Sitter backgrounds

abstract:
  Starting with a new theory of symmetries generated by isometries in field
theories with spin, one finds the generators of the spinor representation in
backgrounds with a given symmetry. In this manner one obtains a collection of
conserved operators from which one can chose the complete sets of commuting
operators defining quantum modes. In this framework, the quantum modes of the
free Dirac field on de Sitter or anti-de Sitter spacetimes can be completely
derived in static or moving charts. One presents the discrete quantum modes, in
the central static charts of the anti-de Sitter spacetime, whose eigenspinors
can be normalized. The consequence is that the second quantization can be done
in this case in canonical manner. For the free Dirac field on de Sitter
manifolds this can not be done in static charts being forced to consider the
moving ones. The quantum modes of the free Dirac field in these charts are used
for writing down the quantum Dirac field and its one-particle operators.


title:
Approximative analytical solutions of the Dirac equation in
  Schwarzschild spacetime

abstract:
  Approximative analytic solutions of the Dirac equation in the geometry of
Schwarzschild black holes are derived obtaining information about the discrete
energy levels and the asymptotic behavior of the energy eigenspinors.


title: A Spinor Theory of Gravity and the Cosmological Framework

abstract:
  Recently we have presented a new formulation of the theory of gravity based
on an implementation of the Einstein Equivalence Principle distinct from
General Relativity. The kinetic part of the theory - that describes how matter
is affected by the modified geometry due to the gravitational field - is the
same as in General Relativity. However, we do not consider the metric as an
independent field. Instead, it is an effective one, constructed in terms of two
fundamental spinor fields $\Psi$ and $\Upsilon$ and thus the metric does not
have a dynamics of its own, but inherits its evolution through its relation
with the fundamental spinors. In the first paper it was shown that the metric
that describes the gravitational field generated by a compact static and
spherically symmetric configuration is very similar to the Schwarzschild
metric. In the present paper we describe the cosmological framework in the
realm of the Spinor Theory of Gravity.


title:
Quantization of perturbations during inflation in the 1+3 covariant
  formalism

abstract:
  This note derives the analogue of the Mukhanov-Sasaki variables both for
scalar and tensor perturbations in the 1+3 covariant formalism. The possibility
of generalizing them to non-flat Friedmann-Lemaitre universes is discussed.


title:
Nonradial oscillations of slowly and differentially rotating compact
  stars

abstract:
  The equations describing nonradial adiabatic oscillations of differentially
rotating relativistic stars are derived in relativistic slow rotation
approximation. The differentially rotating configuration is described by a
perturbative version of the relativistic j-constant rotation law. Focusing on
the oscillation properties of the stellar fluid, the adiabatic nonradial
perturbations are studied in the Cowling approximation with a system of five
partial differential equations. In these equations, differential rotation
introduces new coupling terms between the perturbative quantites with respect
to the uniformly rotating stars. In particular, we investigate the axisymmetric
and barotropic oscillations and compare their spectral properties with those
obtained in nonlinear hydrodynamical studies. The perturbative description of
the differentially rotating background and the oscillation spectrum agree
within a few percent with those of the nonlinear studies.


title: Binary black holes on a budget: Simulations using workstations

abstract:
  Binary black hole simulations have traditionally been computationally very
expensive: current simulations are performed in supercomputers involving dozens
if not hundreds of processors, thus systematic studies of the parameter space
of binary black hole encounters still seem prohibitive with current technology.
Here we show how the multi-layered refinement level code BAM can be used on
dual processor workstations to simulate certain binary black hole systems. BAM,
based on the moving punctures method, provides grid structures composed of
boxes of increasing resolution near the center of the grid. In the case of
binaries, the highest resolution boxes are placed around each black hole and
they track them in their orbits until the final merger when a single set of
levels surrounds the black hole remnant. This is particularly useful when
simulating spinning black holes since the gravitational fields gradients are
larger. We present simulations of binaries with equal mass black holes with
spins parallel to the binary axis and intrinsic magnitude of S/m^2= 0.75. Our
results compare favorably to those of previous simulations of this particular
system. We show that the moving punctures method produces stable simulations at
maximum spatial resolutions up to M/160 and for durations of up to the
equivalent of 20 orbital periods.


title: Hubble Meets Planck: A Cosmic Peek at Quantum Foam

abstract:
  If spacetime undergoes quantum fluctuations, an electromagnetic wavefront
will acquire uncertainties in direction as well as phase as it propagates
through spacetime. These uncertainties can show up in interferometric
observations of distant quasars as a decreased fringe visibility. The Very
Large Telescope and Keck interferometers may be on the verge of probing
spacetime fluctuations which, we also argue, have repercussions for cosmology,
requiring the existence of dark energy/matter, the critical cosmic energy
density, and accelerating cosmic expansion in the present era.


title: Entropy and anisotropy

abstract:
  We address the problem of defining the concept of entropy for anisotropic
cosmological models. In particular, we analyze for the Bianchi I and V models
the entropy which follows from postulating the validity of the laws of standard
thermodynamics in cosmology. Moreover, we analyze the Cardy-Verlinde
construction of entropy and show that it cannot be associated with the one
following from relativistic thermodynamics.


title: Spin Angular Momentum Imparted by Gravitational Waves

abstract:
  Following the demonstration that gravitational waves impart linear momentum,
it is argued that if they are polarized they should impart angular momentum to
appropriately placed 'test rods' in their path. A general formula for this
angular momentum is obtained and used to provide expressions for the angular
momentum imparted by plane and cylindrical gravitational waves.


title: A dark energy multiverse

abstract:
  We present cosmic solutions corresponding to universes filled with dark and
phantom energy, all having a negative cosmological constant. All such solutions
contain infinite singularities, successively and equally distributed along
time, which can be either big bang/crunchs or big rips singularities.
Classicaly these solutions can be regarded as associated with multiverse
scenarios, being those corresponding to phantom energy that may describe the
current accelerating universe.


title: Quantum-Mechanical Model of Spacetime

abstract:
  We consider a possibility to construct a quantum-mechanical model of
spacetime, where Planck size quantum black holes act as the fundamental
constituents of space and time. Spacetime is assumed to be a graph, where black
holes lie on the vertices. Our model implies that area has a discrete spectrum
with equal spacing. At macroscopic length scales our model reproduces
Einstein's field equation with a vanishing cosmological constant as a sort of
thermodynamical equation of state of spacetime and matter fields. In the low
temperature limit, where most black holes are assumed to be in the ground
state, our model implies the Unruh and the Hawking effects, whereas in the high
temperature limit we find, among other things, that black hole entropy depends
logarithmically on the event horizon area, instead of being proportional to the
area.


title:
Exterior spacetime for stellar models in 5-dimensional Kaluza-Klein
  gravity

abstract:
  It is well-known that Birkhoff's theorem is no longer valid in theories with
more than four dimensions. Thus, in these theories the effective 4-dimensional
picture allows the existence of different possible, non-Schwarzschild,
scenarios for the description of the spacetime outside of a spherical star,
contrary to general relativity in 4D. We investigate the exterior spacetime of
a spherically symmetric star in the context of Kaluza-Klein gravity. We take a
well-known family of static spherically symmetric solutions of the Einstein
equations in an empty five-dimensional universe, and analyze possible stellar
exteriors that are conformal to the metric induced on four-dimensional
hypersurfaces orthogonal to the extra dimension. All these exteriors are
continuously matched with the interior of the star. Then, without making any
assumptions about the interior solution, we prove the following statement: the
condition that in the weak-field limit we recover the usual Newtonian physics
singles out an unique exterior. This exterior is "similar" to Scharzschild
vacuum in the sense that it has no effect on gravitational interactions.
However, it is more realistic because instead of being absolutely empty, it is
consistent with the existence of quantum zero-point fields. We also examine the
question of how would the deviation from the Schwarzschild vacuum exterior
affect the parameters of a neutron star. In the context of a model star of
uniform density, we show that the general relativity upper limit M/R < 4/9 is
significantly increased as we go away from the Schwarzschild vacuum exterior.
We find that, in principle, the compactness limit of a star can be larger than
1/2, without being a black hole. The generality of our approach is also
discussed.


title: Unruh Effect for General Trajectories

abstract:
  We consider two-level detectors coupled to a scalar field and moving on
arbitrary trajectories in Minkowski space-time. We first derive a generic
expression for the response function using a (novel) regularization procedure
based on the Feynmann prescription that is explicitly causal, and we compare it
to other expressions used in the literature. We then use this expression to
study, analytically and numerically, the time dependence of the response
function in various non-stationarity situations. We show that, generically, the
response function decreases like a power in the detector's level spacing, $E$,
for high $E$. It is only for stationary world-lines that the response function
decays faster than any power-law, in keeping with the known exponential
behavior for some stationary cases. Under some conditions the (time dependent)
response function for a non-stationary world-line is well approximated by the
value of the response function for a stationary world-line having the same
instantaneous acceleration, torsion, and hyper-torsion. While we cannot offer
general conditions for this to apply, we discuss special cases; in particular,
the low energy limit for linear space trajectories.


title: Thermodynamic Description of Inelastic Collisions in General Relativity

abstract:
  We discuss head-on collisions of neutron stars and disks of dust ("galaxies")
following the ideas of equilibrium thermodynamics, which compares equilibrium
states and avoids the description of the dynamical transition processes between
them. As an always present damping mechanism, gravitational emission results in
final equilibrium states after the collision. In this paper we calculate
selected final configurations from initial data of colliding stars and disks by
making use of conservation laws and solving the Einstein equations. Comparing
initial and final states, we can decide for which initial parameters two
colliding neutron stars (non-rotating Fermi gas models) merge into a single
neutron star and two rigidly rotating disks form again a final (differentially
rotating) disk of dust. For the neutron star collision we find a maximal energy
loss due to outgoing gravitational radiation of 2.3% of the initial mass while
the corresponding efficiency for colliding disks has the much larger limit of
23.8%.


title:
Relating loop quantum cosmology to loop quantum gravity: Symmetric
  sectors and embeddings

abstract:
  In this paper we address the meaning of states in loop quantum cosmology
(LQC), in the context of loop quantum gravity. First, we introduce a rigorous
formulation of an embedding proposed by Bojowald and Kastrup, of LQC states
into loop quantum gravity. Then, using certain holomorphic representations, a
new class of embeddings, called b-embeddings, are constructed, following the
ideas of [1]. We exhibit a class of operators preserving each of these
embeddings, and show their consistency with the LQC quantization. In the
b-embedding case, the classical analogues of these operators separate points in
phase space. Embedding at the gauge and diffeomorphism invariant level is
discussed briefly in the conclusions.


title: A general class of braneworld wormholes

abstract:
  The brane cosmology scenario is based on the idea that our Universe is a
3-brane embedded in a five-dimensional bulk. In this work, a general class of
braneworld wormholes is explored with $R\neq 0$, where $R$ is the four
dimensional Ricci scalar, and specific solutions are further analyzed. A
fundamental ingredient of traversable wormholes is the violation of the null
energy condition (NEC). However, it is the effective total stress energy tensor
that violates the latter, and in this work, the stress energy tensor confined
on the brane, threading the wormhole, is imposed to satisfy the NEC. It is also
shown that in addition to the local high-energy bulk effects, nonlocal
corrections from the Weyl curvature in the bulk may induce a NEC violating
signature on the brane. Thus, braneworld gravity seems to provide a natural
scenario for the existence of traversable wormholes.


title:
Benefits of Artificially Generated Gravity Gradients for Interferometric
  Gravitational-Wave Detectors

abstract:
  We present an approach to experimentally evaluate gravity gradient noise, a
potentially limiting noise source in advanced interferometric gravitational
wave (GW) detectors. In addition, the method can be used to provide sub-percent
calibration in phase and amplitude of modern interferometric GW detectors.
Knowledge of calibration to such certainties shall enhance the scientific
output of the instruments in case of an eventual detection of GWs. The method
relies on a rotating symmetrical two-body mass, a Dynamic gravity Field
Generator (DFG). The placement of the DFG in the proximity of one of the
interferometer's suspended test masses generates a change in the local
gravitational field detectable with current interferometric GW detectors.


title:
Quantization of massive scalar fields over axis symmetric space-time
  backgrounds

abstract:
  The renormalized mean value of the quantum Lagrangian and the Energy-Momentum
tensor for scalar fields coupled to an arbitrary gravitational field
configuration are analytically evaluated in the Schwinger-DeWitt approximation,
up to second order in the inverse mass value. The cylindrical symmetry
situation is considered. The results furnish the starting point for
investigating iterative solutions of the back-reaction problem related with the
quantization of cylindrical scalar field configurations. Due to the homogeneity
of the equations of motion of the Klein-Gordon field, the general results are
also valid for performing the quantization over either vanishing or
non-vanishing mean field configurations. As an application, compact analytical
expressions are derived here for the quantum mean Lagrangian and
Energy-Momentum tensor in the particular background given by the Black-String
space-time.


title:
Evolution of magnetic component in Yang-Mills condensate dark energy
  models

abstract:
  The evolution of the electric and magnetic components in an effective
Yang-Mills condensate dark energy model is investigated. If the electric field
is dominant, the magnetic component disappears with the expansion of the
Universe. The total YM condensate tracks the radiation in the earlier Universe,
and later it becomes $w_y\sim-1$ thus is similar to the cosmological constant.
So the cosmic coincidence problem can be avoided in this model. However, if the
magnetic field is dominant, $w_y>1/3$ holds for all time, suggesting that it
cannot be a candidate for the dark energy in this case.


title: Evaporating (2+1)-dimensional black strings

abstract:
  We investigate (2+1)-dimensional black strings in the Kaluza-Klein spacetime.
The system is classically stable as long as the horizon size is much larger
than the size of the compact space. Semiclassically, however, the horizon size
shrinks gradually due to the energy loss through the Hawking radiation.
Eventually, the system will enter into the regime of the Gregory-Laflamme
instability and get destabilized. Subsequently, the spherically symmetric black
hole is formed and evaporated in the usual manner. This standard picture may be
altered by the dynamics of the internal space which induced by the Hawking
radiation. We argue that the black string is excised from the Kaluza-Klein
spacetime before the onset of the Gregory-Laflamme instability and therefore
before the evaporation.


title: On the Stability of a class of Modified Gravitational Models

abstract:
  Motivated by the dark energy issue, a minisuperspace approach to the
stability for modified gravitational models in a four dimensional cosmological
setting are investigated. Specifically, after revisiting the $f(R)$ case, $R$
being the Ricci curvature, we present a stability condition around a de Sitter
solution valid for modified gravitational models of generalized Gauss-Bonnet
type $F(R,G,Q)$, $G$ and $Q$ being the Gauss-Bonnet and quadratic Riemann
invariants respectively. A generalization to higher order invariants is
mentioned.


title: Report on the first round of the Mock LISA Data Challenges

abstract:
  The Mock LISA Data Challenges (MLDCs) have the dual purpose of fostering the
development of LISA data analysis tools and capabilities, and demonstrating the
technical readiness already achieved by the gravitational-wave community in
distilling a rich science payoff from the LISA data output. The first round of
MLDCs has just been completed: nine data sets containing simulated
gravitational wave signals produced either by galactic binaries or massive
black hole binaries embedded in simulated LISA instrumental noise were released
in June 2006 with deadline for submission of results at the beginning of
December 2006. Ten groups have participated in this first round of challenges.
Here we describe the challenges, summarise the results, and provide a first
critical assessment of the entries.


title: New Energy Definition for Higher Curvature Gravities

abstract:
  We propose a novel but natural definition of conserved quantities for gravity
models quadratic and higher in curvature. Based on the spatial asymptotics of
curvature rather than of metric, it avoids the GR energy machinery's more
egregious problems--such as zero energy "theorems" and failure in flat
backgrounds -- in this fourth-derivative realm. In D>4, the present expression
indeed correctly discriminates between second derivative Gauss-Bonnet and
generic, fourth derivative, actions.


title:
On the Systematic Errors in the Detection of the Lense-Thirring Effect
  with a Mars Orbiter

abstract:
  We show here that the recent claim of a test of the Lense-Thirring effect
with an error of 0.5% using the Mars Global Surveyor is misleading and the
quoted error is incorrect by a factor of at least ten thousand. Indeed, the
simple error analysis of [1] neglects the role of some important systematic
errors affecting the out-of-plane acceleration. The preliminary error analysis
presented here shows that even an optimistic uncertainty for this measurement
is at the level of, at least, ~ 3026% to ~ 4811%, i.e., even an optimistic
uncertainty is about 30 to 48 times the Lense-Thirring effect. In other words
by including only some systematic errors we obtained an uncertainty almost ten
thousand times larger than the claimed 0.5% error.


title: Quantum gravity and cosmological observations

abstract:
  Quantum gravity places entirely new challenges on the formulation of a
consistent theory as well as on an extraction of potentially observable
effects. Quantum corrections due to the gravitational field are commonly
expected to be tiny because of the smallness of the Planck length. However, a
consistent formulation now shows that key features of quantum gravity imply
magnification effects on correction terms which are especially important in
cosmology with its long stretches of evolution. After a review of the salient
features of recent canonical quantizations of gravity and their implications
for the quantum structure of space-time a new example for potentially
observable effects is given.


title: Gravitational recoil from spinning binary black hole mergers

abstract:
  The inspiral and merger of binary black holes will likely involve black holes
with both unequal masses and arbitrary spins. The gravitational radiation
emitted by these binaries will carry angular as well as linear momentum. A net
flux of emitted linear momentum implies that the black hole produced by the
merger will experience a recoil or kick. Previous studies have focused on the
recoil velocity from unequal mass, non-spinning binaries. We present results
from simulations of equal mass but spinning black hole binaries and show how a
significant gravitational recoil can also be obtained in these situations. We
consider the case of black holes with opposite spins of magnitude $a$
aligned/anti-aligned with the orbital angular momentum, with $a$ the
dimensionless spin parameters of the individual holes. For the initial setups
under consideration, we find a recoil velocity of $V = 475 \KMS a$.
Supermassive black hole mergers producing kicks of this magnitude could result
in the ejection from the cores of dwarf galaxies of the final hole produced by
the collision.


title: Scalar and spinor solutions in the spacetimes of vacuumless defects

abstract:
  We obtain solutions of the Klein-Gordon and Dirac equations in the
gravitational fields of vacuumless defects. We calculate the energy levels and
the current, respectively, in the scalar and spinor cases. In all these
situations we emphasize the role played by the defects on the solutions, energy
and current.


title: The Final Fate of Binary Neutron Stars: What Happens After the Merger?

abstract:
  The merger of two neutron stars usually produces a remnant with a mass
significantly above the single (nonrotating) neutron star maximum mass. In some
cases, the remnant will be stabilized against collapse by rapid, differential
rotation. MHD-driven angular momentum transport eventually leads to the
collapse of the remnant's core, resulting in a black hole surrounded by a
massive accretion torus. Here we present simulations of this process. The
plausibility of generating short duration gamma ray bursts through this
scenario is discussed.


title:
On the Lense-Thirring test with the Mars Global Surveyor in the
  gravitational field of Mars

abstract:
  I discuss some aspects of the recent test of frame-dragging performed by me
by exploiting the Root-Mean-Square (RMS) orbit overlap differences of the
out-of-plane component N of the orbit of the Mars Global Surveyor (MGS)
spacecraft in the gravitational field of Mars. A linear fit of the full time
series of the entire MGS data (4 February 1999-14 January 2005) yields a
normalized slope 1.03 +/- 0.41 (with 95% confidence bounds). Other linear fits
to different data sets confirm the agreement with general relativity. The huge
systematic effects induced by the mismodeling in the martian gravitational
field claimed by some authors are absent in the MGS out-of-plane record. The
non-gravitational forces affect at the same level of the gravitomagnetic one
the in-plane orbital components of MGS, not the out-of-plane one. Moreover,
they experience high-frequency variations which does not matter in the present
case in which secular effects are relevant.


title:
Thermodynamics of Ideal Boson and Fermion Gases in the Static Taub
  Universe

abstract:
  Some thermodynamic quantities of nonrelativistic ideal boson and fermion
gases in the static Taub universe are derived to first order in a small
anisotropy parameter d which measuring the deformation from the spherical
Einstein universe. They are used to investigate the problem of how the
curvature anisotropy affects the thermodynamic behaviors of an ideal gas. It is
found that, when the universe is in the oblate configuration (i.e., d > 0), the
effect of curvature anisotropy is to increase the number of the fraction in the
Bose-Einstein condensation and to decrease the fermion distribution function at
low temperature. When the universe is in the prolate configuration (i.e., d <
0), the effects of curvature anisotropy on the thermodynamic quantities is
contrary to that in the oblate configuration. The density matrix of a two
particle system is evaluated and it is used to define the "statistical
interparticle potential" as an attempt to give a "statistical interpretation"
about the found thermodynamic behaviors. It is found that when the universe is
in the oblate (prolate) configuration the curvature anisotropy will enhance
(reduce) both the "statistic attraction" among the bosons and "statistical
repulsion" among the fermions. It is expected that such a behavior will also be
shown in the relativistic system.


title:
Application of the Hilbert-Huang Transform to the Search for
  Gravitational Waves

abstract:
  We present the application of a novel method of time-series analysis, the
Hilbert-Huang Transform, to the search for gravitational waves. This algorithm
is adaptive and does not impose a basis set on the data, and thus the
time-frequency decomposition it provides is not limited by time-frequency
uncertainty spreading. Because of its high time-frequency resolution it has
important applications to both signal detection and instrumental
characterization. Applications to the data analysis of the ground and space
based gravitational wave detectors, LIGO and LISA, are described.


title: Quantum healing of classical singularities in power-law spacetimes

abstract:
  We study a broad class of spacetimes whose metric coefficients reduce to
powers of a radius r in the limit of small r. Among these four-parameter
"power-law" metrics we identify those parameters for which the spacetimes have
classical singularities as r approaches 0. We show that a large set of such
classically singular spacetimes is nevertheless nonsingular quantum
mechanically, in that the Hamiltonian operator is essentially self-adjoint, so
that the evolution of quantum wave packets lacks the ambiguity associated with
scattering off singularities. Using these metrics, the broadest class yet
studied to compare classical with quantum singularities, we explore the
physical reasons why some that are singular classically are "healed" quantum
mechanically, while others are not. We show that most (but not all) of the
remaining quantum-mechanically singular spacetimes can be excluded if either
the weak energy condition or the dominant energy condition is invoked, and we
briefly discuss the effect of this work on the strong cosmic censorship
hypothesis.


title: Ricci identities in higher dimensions

abstract:
  We explore connections between geometrical properties of null congruences and
the algebraic structure of the Weyl tensor in n>4 spacetime dimensions. First,
we present the full set of Ricci identities on a suitable "null" frame, thus
completing the extension of the Newman-Penrose formalism to higher dimensions.
Then we specialize to geodetic null congruences and study specific consequences
of the Sachs equations. These imply, for example, that Kundt spacetimes are of
type II or more special (like for n=4) and that for odd n a twisting geodetic
WAND must also be shearing (in contrast to the case n=4).


title:
Generalized true- and eccentric-anomaly parametrizations for the
  perturbed Kepler motion

abstract:
  The true- and eccentric-anomaly parametrizations of the Kepler motion are
generalized to quasiperiodic orbits by considering perturbations of the radial
part of kinetic energy as a series in the negative powers of the orbital
radius. A toolbox of methods for averaging observables in terms of the energy
$E$ and angular momentum $L$ is developed. A broad range of systems governed by
the generic Brumberg force, as well as recent applications of the theory of
gravitational radiation involve integrals over a period of motion. These
integrals are evaluated by using the residue theorem. It is shown that the pole
of the integrand is located in the origin and that under certain circumstances
an additional pole emerges.


title: `Mass without mass' from thin shells in Gauss-Bonnet gravity

abstract:
  Five tensor equations are obtained for a thin shell in Gauss-Bonnet gravity.
There is the well known junction condition for the singular part of the stress
tensor intrinsic to the shell, which we also prove to be well defined. There
are also equations relating the geometry of the shell (jump and average of the
extrinsic curvature as well as the intrinsic curvature) to the non-singular
components of the bulk stress tensor on the sides of the thin shell.
  The equations are applied to spherically symmetric thin shells in vacuum. The
shells are part of the vacuum, they carry no energy tensor. We classify these
solutions of `thin shells of nothingness' in the pure Gauss-Bonnet theory.
There are three types of solutions, with one, zero or two asymptotic regions
respectively. The third kind of solution are wormholes. Although vacuum
solutions, they have the appearance of mass in the asymptotic regions. It is
striking that in this theory, exotic matter is not needed in order for
wormholes to exist- they can exist even with no matter.


title:
N-dimensional Vaidya metric with cosmological constant in double-null
  coordinates

abstract:
  A recently proposed approach to the construction of the Vaidya metric in
double-null coordinates for generic mass functions is extended to the
$n$-dimensional $(n>2)$ case and to allow the inclusion of a cosmological
constant. The approach is based on a qualitative study of the null-geodesics,
allowing the description of light-cones and revealing many features of the
underlying causal structure. Possible applications are illustrated by explicit
examples. Some new exact solutions are also presented and discussed. The
results presented here can simplify considerably the study of spherically
symmetric gravitational collapse and mass accretion in arbitrary dimensions.