title:
Consistent long distance modification of gravity from inverse powers of
  the curvature

abstract:
  In this paper we study long distance modifications of gravity obtained by
considering actions that are singular in the limit of vanishing curvature. In
particular, we showed in a previous publication that models that include
inverse powers of curvature invariants that diverge for r->0 in the
Schwarzschild geometry, recover an acceptable weak field limit at short
distances from sources. We study then the linearisation of generic actions of
the form L=F[R,P,Q] where P=R_{ab}R^{ab} and Q=R_{abcd}R^{abcd}. We show that
for the case in which F[R,P,Q]=F[R,Q-4P], the theory is ghost free. Assuming
this is the case, in the models that can explain the acceleration of the
Universe without recourse to Dark Energy there is still an extra scalar field
in the spectrum besides the massless spin two graviton. The mass of this extra
excitation is of the order of the Hubble scale in vacuum. We nevertheless
recover Einstein gravity at short distances because the mass of this scalar
field depends on the background in such a way that it effectively decouples
when one gets close to any source. Remarkably, for the values of the parameters
necessary to explain the cosmic acceleration the induced modifications of
gravity are suppressed at the Solar System level but can be important for
systems like a galaxy.


title: Renormalized Stress Tensor for trans-Planckian Cosmology

abstract:
  Finite expressions for the mean value of the stress tensor corresponding to a
scalar field with a generalized dispersion relation in a
Friedman--Robertson--Walker universe are obtained using adiabatic
renormalization. Formally divergent integrals are evaluated by means of
dimensional regularization. The renormalization procedure is shown to be
equivalent to a redefinition of the cosmological constant and the Newton
constant in the semiclassical Einstein equations.


title: Quasinormal Spectrum and Quantization of Charged Black Holes

abstract:
  Black-hole quasinormal modes have been the subject of much recent attention,
with the hope that these oscillation frequencies may shed some light on the
elusive theory of quantum gravity. We study {\it analytically} the asymptotic
quasinormal spectrum of a {\it charged} scalar field in the (charged)
Reissner-Nordstr\"om spacetime. We find an analytic expression for these
black-hole resonances in terms of the black-hole physical parameters: its
Bekenstein-Hawking temperature $T_{BH}$, and its electric potential $\Phi$. We
discuss the applicability of the results in the context of black-hole
quantization. In particular, we show that according to Bohr's correspondence
principle, the asymptotic resonance corresponds to a fundamental area unit
$\Delta A=4\hbar\ln2$.


title: Accurate Evolutions of Orbiting Black-Hole Binaries Without Excision

abstract:
  We present a new algorithm for evolving orbiting black-hole binaries that
does not require excision or a corotating shift. Our algorithm is based on a
novel technique to handle the singular puncture conformal factor. This system,
based on the BSSN formulation of Einstein's equations, when used with a
`pre-collapsed' initial lapse, is non-singular at the start of the evolution,
and remains non-singular and stable provided that a good choice is made for the
gauge. As a test case, we use this technique to fully evolve orbiting
black-hole binaries from near the Innermost Stable Circular Orbit (ISCO)
regime. We show fourth order convergence of waveforms and compute the radiated
gravitational energy and angular momentum from the plunge. These results are in
good agreement with those predicted by the Lazarus approach.


title: Fractional Boundaries for Fluid Spheres

abstract:
  A single Israel layer can be created when two metrics adjoin with no
continuous metric derivative across the boundary. The properties of the layer
depend only on the two metrics it separates. By using a fractional derivative
match, a family of Israel layers can be created between the same two metrics.
The family is indexed by the order of the fractional derivative. The method is
applied to Tolman IV and V interiors and a Schwarzschild vacuum exterior. The
method creates new ranges of modeling parameters for fluid spheres. A thin
shell analysis clarifies pressure/tension in the family of boundary layers.


title:
General Relativity, Maxwell's Electrodynamics, and the Foundations of
  the Quantum Theory of Gravitation and Matter

abstract:
  The formalism of electric - magnetic duality, first pioneered by Reinich and
Wheeler, extends General Relativity to encompass non-Abelian fields. Several
energy Tensors T^uv with non-vanishing trace matter are developed solely as a
function of the field strength tensor F^uv, including the Euler tensor, and
tensors for matter in flux, pressure in flux, and stationary pressure. The
spacetime metric g_uv is not only a solution to the second-order Einstein
equation based on T^uv, but is also constrained by a third-order equation
involving the Bianchi identity together with the gravitational energy
components kappa_u for each T^uv. The common appearance of F^uv in all of the
T^uv and kappa_v makes it possible to obtain quantum solutions for the
spacetime metric, thereby geometrizing quantum physics as a non-linear theory.


title: Thermodynamics of an accelerated expanding universe

abstract:
  We investigate the laws of thermodynamics in an accelerating universe driven
by dark energy with a time-dependent equation of state. In the case we consider
that the physically relevant part of the Universe is that envelopped by the
dynamical apparent horizon, we have shown that both the first law and second
law of thermodynamics are satisfied. On the other hand, if the boundary of the
Universe is considered to be the cosmological event horizon the thermodynamical
description based on the definitions of boundary entropy and temperature breaks
down. No parameter redefinition can rescue the thermodynamics laws from such a
fate, rendering the cosmological event horizon unphysical from the point of
view of the laws of thermodynamics.


title:
On the use of the Kodama vector field in spherically symmetric dynamical
  problems

abstract:
  It is shown that by making use of the Kodama vector field, as a preferred
time evolution vector field, in spherically symmetric dynamical systems
unexpected simplifications arise. In particular, the evolution equations
relevant for the case of a massless scalar field minimally coupled to gravity
are investigated. The simplest form of these equations in the 'canonical gauge'
are known to possess the character of a mixed first order elliptic-hyperbolic
system. The advantages related to the use of the Kodama vector field are
two-folded although they show up simultaneously. First, it is found that the
true degrees of freedom separate. Second, a subset of the field equations
possessing the form of a first order symmetric hyperbolic system for these
preferred degrees of freedom is singled out. It is also demonstrated, in the
appendix, that the above results generalise straightforwardly to the case of a
generic self-interacting scalar field.


title:
Radio Wave 'Messengers' of Periodic Gravitational Radiation and the
  Problem of Gravitationally Induced Nonlinearity in Electrodynamic Systems

abstract:
  We discuss a gravitationally induced nonlinearity in hierarchic systems. We
consider the generation of extremely low-frequency radio waves with a frequency
of the periodic gravitational radiation; the generation is due to an induced
nonlinear self-action of electromagnetic radiation in the vicinity of the
gravitational-radiation source. These radio waves are a fundamentally new type
of response of an electrodynamic system to gravitational radiation. That is why
we here use an unconventional term: radio-wave messengers of periodic
gravitational radiation.


title: Black hole remnants due to GUP or quantum gravity?

abstract:
  Based on the micro-black hole \emph{gedanken} experiment as well as on
general considerations of quantum mechanics and gravity the generalized
uncertainty principle (GUP) is analyzed by using the running Newton constant.
The result is used to decide between the GUP and quantum gravitational effects
as a possible mechanism leading to the black hole remnants of about Planck
mass.


title: Light scattering test regarding the relativistic nature of heat

abstract:
  The dynamic structure factor of a simple relativistic fluid is calculated.
The coupling of acceleration with the heat flux present in Eckart's version of
irreversible relativistic thermodynamics is examined using the
Rayleigh-Brillouin spectrum of the fluid. A modification of the width of the
Rayleigh peak associated to Eckart's picture of the relativistic nature of heat
is predicted and estimated.


title: Quasinormal modes for arbitrary spins in the Schwarzschild background

abstract:
  The leading term of the asymptotic of quasinormal modes in the Schwarzschild
background, omega_n = - i n/2, is obtained in two straightforward analytical
ways for arbitrary spins. One of these approaches requires almost no
calculations. As simply we demonstrate that for any odd integer spin, described
by the Teukolsky equation, the first correction to the leading term vanishes.
Then, this correction for half-integer spins is obtained in a slightly more
intricate way. At last, we derive analytically the general expression for the
first correction for all spins, described by the Teukolsky equation.


title: Scales Set by the Cosmological Constant

abstract:
  The cosmological constant sets certain scales important in cosmology. We show
that Lambda in conjunction with other parameters like the Schwarzschild radius
leads to scales relevant not only for cosmological but also for astrophysical
applications. Of special interest is the extension of orbits and velocity of
test particles traveling over Mpc distances. We will show that there exists a
lower and an upper cut-off on the possible velocities of test particles. For a
test body moving in a central gravitational field Lambda enforces a maximal
value of the angular momentum if we insist on bound orbits of the test body
which move at a distance larger than the Schwarzschild radius.


title: Perturbative Degrees of Freedom in Loop Quantum Gravity: Anisotropies

abstract:
  The relation between an isotropic and an anisotropic model in loop quantum
cosmology is discussed in detail, comparing the strict symmetry reduction with
a perturbative implementation of symmetry. While the latter cannot be done in a
canonical manner, it allows to consider the dynamics including the role of
small non-symmetric degrees of freedom for the symmetric evolution. This serves
as a model for the general situation of perturbative degrees of freedom in a
background independent quantization such as loop quantum gravity, and for the
more complicated addition of perturbative inhomogeneities. While being crucial
for cosmological phenomenology, it is shown that perturbative non-symmetric
degrees of freedom do not allow definitive conclusions for the singularity
issue and in such a situation could even lead to wrong claims.


title: On a class of 2-surface observables in general relativity

abstract:
  The boundary conditions for canonical vacuum general relativity is
investigated at the quasi-local level. It is shown that fixing the area element
on the 2- surface S (rather than the induced 2-metric) is enough to have a well
defined constraint algebra, and a well defined Poisson algebra of basic
Hamiltonians parameterized by shifts that are tangent to and divergence-free on
$. The evolution equations preserve these boundary conditions and the value of
the basic Hamiltonian gives 2+2 covariant, gauge-invariant 2-surface
observables. The meaning of these observables is also discussed.


title: Hermitian Dirac Hamiltonian in time dependent gravitational field

abstract:
  It is shown by a straightforward argument that the Hamiltonian generating the
time evolution of the Dirac wave function in relativistic quantum mechanics is
not hermitian with respect to the covariantly defined inner product whenever
the background metric is time dependent. An alternative, hermitian, Hamiltonian
is found and is shown to be directly related to the canonical field Hamiltonian
used in quantum field theory.


title: Post-Newtonian corrections to the motion of spinning bodies in NRGR

abstract:
  In this paper we include spin and multipole moment effects in the formalism
used to describe the motion of extended objects recently introduced in
hep-th/0409156. A suitable description for spinning bodies is developed and
spin-orbit, spin-spin and quadrupole-spin Hamiltonians are found at leading
order. The existence of tidal, as well as self induced finite size effects is
shown, and the contribution to the Hamiltonian is calculated in the latter. It
is shown that tidal deformations start formally at O(v^6) and O(v^10) for
maximally rotating general and compact objects respectively, whereas self
induced effects can show up at leading order. Agreement is found for the cases
where the results are known.


title:
A duality relation between non-spherical mirror optical cavities and its
  application to gravitational-wave detectors

abstract:
  In this paper, we analytically prove a unique duality relation between the
eigenspectra of paraxial optical cavities with non-spherical mirrors: a
one-to-one mapping between eigenmodes and eigenvalues of cavities deviating
from flat mirrors by $h(\vec{r})$ and cavities deviating from concentric
mirrors by $-h(\vec{r})$, where $h$ need not be a small perturbation. We then
illustrate its application to optical cavities, proposed for advanced
interferometric gravitational-wave detectors, where the mirrors are designed to
support beams with rather flat intensity profiles over the mirror surfaces.
This unique mapping might be very useful in future studies of alternative
optical designs for advanced gravitational waves interferometers or experiments
employing optical cavities with non-standard mirrors.


title: Ultimate gravitational mass defect

abstract:
  We present a new type of gravitational mass defect in which an infinite
amount of matter may be bounded in a zero ADM mass. This interpolates between
effects typical of closed worlds and T-spheres. We consider the Tolman model of
dust distribution and show that this phenomenon reveals itself for a solution
that has no origin on one side but is closed on the other side. The second
class of examples corresponds to smooth gluing T-spheres to the portion of the
Friedmann-Robertson-Walker solution. The procedure is generalized to
combinations of smoothly connected T-spheres, FRW and Schwarzschild metrics. In
particular, in this approach a finite T-sphere is obtained that looks for
observers in two R-regions as the Schwarzschild metric with two different
masses one of which may vanish.


title: On vacuum gravitational collapse in nine dimensions

abstract:
  We consider the vacuum gravitational collapse for cohomogeneity-two solutions
of the nine dimensional Einstein equations. Using combined numerical and
analytical methods we give evidence that within this model the
Schwarzschild-Tangherlini black hole is asymptotically stable. In addition, we
briefly discuss the critical behavior at the threshold of black hole formation.


title: Gravitational lensing by Einstein-Born-Infeld black holes

abstract:
  In this paper, charged black holes in general relativity coupled to
Born-Infeld electrodynamics are studied as gravitational lenses. The positions
and magnifications of the relativistic images are obtained using the strong
deflection limit, and the results are compared with those corresponding to a
Reissner-Nordstrom black hole with the same mass and charge. As numerical
examples, the model is applied to the supermassive Galactic center black hole
and to a small size black hole situated in the Galactic halo.


title: How robust is the entanglement entropy-area relation?

abstract:
  We revisit the problem of finding the entanglement entropy of a scalar field
on a lattice by tracing over its degrees of freedom inside a sphere. It is
known that this entropy satisfies the area law -- entropy proportional to the
area of the sphere -- when the field is assumed to be in its ground state. We
show that the area law continues to hold when the scalar field degrees of
freedom are in generic coherent states and a class of squeezed states. However,
when excited states are considered, the entropy scales as a lower power of the
area. This suggests that for large horizons, the ground state entropy
dominates, whereas entropy due to excited states gives power law corrections.
We discuss possible implications of this result to black hole entropy.


title: Reinventing spacetime on a dynamical hypersurface

abstract:
  In braneworld models, Space-Time-Matter and other Kaluza-Klein theories, our
spacetime is devised as a four-dimensional hypersurface {\it orthogonal} to the
extra dimension in a five-dimensional bulk. We show that the FRW line element
can be "reinvented" on a dynamical four-dimensional hypersurface, which is {\it
not} orthogonal to the extra dimension, without any internal contradiction.
This hypersurface is selected by the requirement of continuity of the metric
and depends explicitly on the evolution of the extra dimension. The main
difference between the "conventional" FRW, on an orthogonal hypersurface, and
the new one is that the later contains higher-dimensional modifications to the
regular matter density and pressure in 4D. We compare the evolution of the
spacetime in these two interpretations. We find that a wealth of "new" physics
can be derived from a five-dimensional metric if it is interpreted on a
dynamical (non-orthogonal) 4D hypersurface. In particular, in the context of a
well-known cosmological metric in $5D$, we construct a FRW model which is
consistent with the late accelerated expansion of the universe, while fitting
simultaneously the observational data for the deceleration parameter. The model
predicts an effective equation of state for the universe, which is consistent
with observations.


title:
Relational Particle Models. I. Reconciliation with standard classical
  and quantum theory

abstract:
  This paper concerns the absolute versus relative motion debate. The Barbour
and Bertotti 1982 work may be viewed as an indirectly set up relational
formulation of a portion of Newtonian mechanics. I consider further direct
formulations of this and argue that the portion in question -- universes with
zero total angular momentum, that are conservative and with kinetic terms that
are (homogeneous) quadratic in their velocities -- is capable of accommodating
a wide range of classical physics phenomena. Furthermore, as I develop in Paper
II, this relational particle model is a useful toy model for canonical general
relativity.
  I consider what happens if one quantizes relational rather than absolute
mechanics, indeed whether the latter is misleading. By exploiting Jacobi
coordinates, I show how to access many examples of quantized relational
particle models and then interpret these from a relational perspective. By
these means, previous suggestions of bad semiclassicality for such models can
be eluded. I show how small (particle number) universe relational particle
model examples display eigenspectrum truncation, gaps, energy interlocking and
counterbalanced total angular momentum. These features mean that these small
universe models make interesting toy models for some aspects of closed universe
quantum cosmology. While, these features do not compromise the recovery of
reality as regards the practicalities of experimentation in a large universe
such as our own.


title:
Relational Particle Models. II. Use as toy models for quantum
  geometrodynamics

abstract:
  Relational particle models are employed as toy models for the study of the
Problem of Time in quantum geometrodynamics. These models' analogue of the thin
sandwich is resolved. It is argued that the relative configuration space and
shape space of these models are close analogues from various perspectives of
superspace and conformal superspace respectively. The geometry of these spaces
and quantization thereupon is presented. A quantity that is frozen in the scale
invariant relational particle model is demonstrated to be an internal time in a
certain portion of the relational particle reformulation of Newtonian
mechanics. The semiclassical approach for these models is studied as an
emergent time resolution for these models, as are consistent records
approaches.


title:
On the recovery of geometrodynamics from two different sets of first
  principles

abstract:
  The conventional spacetime formulation of general relativity may be recast as
a dynamics of spatial 3-geometries (geometrodynamics). Furthermore,
geometrodynamics can be derived from first principles. I investigate two
distinct sets of these: (i) Hojman, Kucha\v{r} and Teitelboim's, which
presuppose that the spatial 3-geometries are embedded in spacetime. (ii) The
3-space approach of Barbour, Foster, \'{O} Murchadha and Anderson in which the
spatial 3-geometries are presupposed but spacetime is not. I consider how the
constituent postulates of the conventional approach to relativity emerge or are
to be built into these formulations. I argue that the 3-space approach is a
viable description of classical physics (fundamental matter fields included),
and one which affords considerable philosophical insight because of its
`relationalist' character. From these assumptions of less structure, it is also
interesting that conventional relativity can be recovered (albeit as one of
several options).
  However, contrary to speculation in the earlier 3-space approach papers, I
also argue that this approach is not selective over which sorts of fundamental
matter physics it admits. In particular, it does not imply the equivalence
principle.


title: The present universe in the Einstein frame, metric-affine R+1/R gravity

abstract:
  We study the present, flat isotropic universe in 1/R-modified gravity. We use
the Palatini (metric-affine) variational principle and the Einstein
(metric-compatible connected) conformal frame. We show that the energy density
scaling deviates from the usual scaling for nonrelativistic matter, and the
largest deviation occurs in the present epoch. We find that the current
deceleration parameter derived from the apparent matter density parameter is
consistent with observations. There is also a small overlap between the
predicted and observed values for the redshift derivative of the deceleration
parameter. The predicted redshift of the deceleration-to-acceleration
transition agrees with that in the \Lambda-CDM model but it is larger than the
value estimated from SNIa observations.


title:
Checking the variability of the gravitational constant with binary
  pulsars

abstract:
  The most precise measurements are done at present by timing of radiopulsars
in binary systems with two neutron stars. The timing measurements of the
Taylor-Hulse pulsar B1913+16 gave the most precise results on testing of
general relativity (GR), finding implicit proof of existence of gravitational
waves. We show that available results of existing measurements, obtained to the
year 1993, in combination with the results of the Mariner 10 in (1992), give
the boundaries for the variation of the gravitational constant ${\dot G}/{G}$
inside the limits $(-0.6 \div +2)\cdot 10^{-12}$ year{$^{-1}$}.


title: Teaching General Relativity

abstract:
  This Resource Letter provides some guidance on issues that arise in teaching
general relativity at both the undergraduate and graduate levels. Particular
emphasis is placed on strategies for presenting the mathematical material
needed for the formulation of general relativity.


title: The white wall, a gravitational mirror

abstract:
  We describe the exact solution of Einstein's equation corresponding to a
static homogenous distribution of matter with plane symmetry lying below $z=0$.
We study the geodesics in it and we show that this simple spacetime exhibits
very curious properties. In particular, it has a repelling singular boundary
and all geodesics bounce off it.


title:
Well-posed constrained evolution of 3+1 formulations of General
  Relativity

abstract:
  We present an analysis of well-posedness of constrained evolution of 3+1
formulations of GR. In this analysis we explicitly take into account the energy
and momentum constraints as well as possible algebraic constraints on the
evolution of high-frequency perturbations of solutions of Einstein's equations.
In this respect, our approach is principally different from standard analyses
of well-posedness of free evolution in general relativity. Our study reveals
the existence of subsets of the linearized Einstein's equations that control
the well-posedness of constrained evolution. It is demonstrated that the
well-posedness of ADM, BSSN and other 3+1 formulations derived from ADM by
adding combinations of constraints to the right-hand-side of ADM and/or by
linear transformation of the dynamical ADM variables depends entirely on the
properties of the gauge. For certain classes of gauges we formulate conditions
for well-posedness of constrained evolution. This provides a new basis for
constructing stable numerical integration schemes for a classical
Arnowitt--Deser--Misner (ADM) and many other 3+1 formulations of general
relativity.


title: The Existence of Godel, Einstein and de Sitter Universes

abstract:
  We determine the general conditions for the existence of Godel, Einstein
static, and de Sitter universes in gravity theories derived from a Lagrangian
that is an arbitrary function of the scalar curvature and Ricci and Riemann
curvature invariants. Explicit expressions for the solutions are found in terms
of the parameters defining the Lagrangian. We also determine the conditions on
the Lagrangian of the theory under which time-travel is allowed in the Godel
universes.


title: Quantum Gravity as a Deformed Topological Quantum Field Theory

abstract:
  It is known that the Einstein-Hilbert action with a positive cosmological
constant can be represented as a perturbation of the SO(4,1) BF theory by a
symmetry-breaking term quadratic in the B field. Introducing fermionic matter
generates additional terms in the action which are polynomial in the tetrads
and the spin connection. We describe how to construct the generating functional
in the spin foam formalism for a generic BF theory when the sources for the B
and the gauge field are present. This functional can be used to obtain a path
integral for General Relativity with matter as a perturbative series whose the
lowest order term is a path integral for a topological gravity coupled to
matter.


title: Rotating figures of equilibrium in General Relativity

abstract:
  A generalization of the notion of surfaces of revolution in the spaces of
General Relativity is presented. We apply this definition to the case of
Carter's family [A] of solutions and we study the Kerr's metric with respect
the above mentioned foliation.


title: Brane Cosmology from Heterotic String Theory

abstract:
  We consider brane cosmologies within the context of five-dimensional actions
with O(a') higher curvature corrections. The actions are compatible with bulk
string amplitude calculations from heterotic string theory. We find wrapped
solutions that satisfy the field equations in an approximate but acceptable
manner given their complexity, where the internal four-dimensional scale factor
is naturally inflating, having an exponential De-Sitter form. The temporal
dependence of the metric components is non-trivial so that this metric cannot
be factored as in a conformally flat case. The effective Planck mass is finite
and the brane solutions localize four-dimensional gravity, while the
four-dimensional gravitational constant varies with time. The Hubble constant
can be freely specified through the initial value of the scalar field, to
conform with recent data.


title: Spin Foam Models from the Tetrad Integration

abstract:
  We describe a class of spin foam models of four-dimensional quantum gravity
which is based on the integration of the tetrad one-forms in the path integral
for the Palatini action of General Relativity. In the Euclidian gravity case
this class of models can be understood as a modification of the Barrett-Crane
spin foam model. Fermionic matter can be coupled by using the path integral
with sources for the tetrads and the spin connection, and the corresponding
state sum is based on a spin foam where both the edges and the faces are
colored independently with the irreducible representations of the spacetime
rotations group.


title:
How soliton-anti soliton di quark pairs signify an Einstein constant
  dominated cosmology, and lead to new inflationary cosmology physics

abstract:
  We review the results of a model of how nucleation of a new universe occurs,
assuming a di quark identification for soliton-anti soliton constituent parts
of a scalar field. Initially, we employ a false vacuum potential system;
however ,when cosmological expansion is dominated by the Einstein cosmological
constant at the end of chaotic inflation, the initial di quark scalar field is
not consistent with respect to a semi classical consistency conditions we
analyze as the potential changes to the chaotic inflationary potential utilized
by Guth. We use Scherrer's derivation of a sound speed being zero during
initial inflationary cosmology, and obtain a sound speed approaching unity as
the slope of the scalar field moves away from a thin wall approximation. All
this is to aid in a data reconstruction problem of how to account for the
initial origins of CMB due to dark matter since effective field theories as
presently constructed require a cut off value for applicability of their
potential structure. This is often at the cost of, especially in early universe
theoretical models, of clearly defined baryogenesis, and of a well defined
mechanism of phase transitions.


title: The need for dark matter in galaxies

abstract:
  Cooperstock and Tieu have proposed a model to account for galactic rotation
curves without invoking dark matter. I argue that no model of this type can
work.


title: Gauge invariance and the detection of gravitational radiation

abstract:
  The detection of gravitational radiation raises some subtle issues having to
do with the coordinate invariance of general relativity. This paper explains
these issues and their resolution by using an analogy with the Aharonov-Bohm
effect of quantum mechanics.


title:
Effective gauge group of pure loop quantum gravity is SO(3): New
  estimate of the Immirzi parameter

abstract:
  We argue that the effective gauge group for {\it pure} four-dimensional loop
quantum gravity(LQG) is SO(3) (or $SO(3,C)$) instead of SU(2) (or $SL(2,C)$).
As a result, links with half-integer spins in spin network states are not
realized for {\it pure} LQG, implying a modification of the spectra of area and
volume operators. Our observations imply a new value of $\gamma \approx 0.170$
for the Immirzi parameter which is obtained from matching the
Bekenstein-Hawking entropy to the number of states from LQG calculations.
Moreover, even if the dominant contribution to the entropy is not assumed to
come from configurations with the minimum spins, the results of both pure LQG
and the supersymmetric extension of LQG can be made compatible when only
integer spins are realized for the former, while the latter also contains
half-integer spins, together with an Immirzi parameter for the supersymmetric
case which is twice the value of the SO(3) theory. We also verify that the
$-{1/2}$ coefficient of logarithmic correction to the Bekenstein-Hawking
entropy formula is robust, independent of whether only integer, or also
half-integer spins, are realized.


title: Quasinormal modes of a black hole surrounded by quintessence

abstract:
  Using the third-order WKB approximation, we evaluate the quasinormal
frequencies of massless scalar field perturbation around the black hole which
is surrounded by the static and spherically symmetric quintessence. Our result
shows that due to the presence of quintessence, the scalar field damps more
rapidly. Moreover, we also note that the quintessential state parameter
$\epsilon$ (the ratio of pressure $p_q$ to the energy density $\rho_q$) play an
important role for the quasinormal frequencies. As the state parameter
$\epsilon$ increases the real part increases and the absolute value of the
imaginary part decreases. This means that the scalar field decays more slowly
in the larger $\epsilon$ quintessence case.


title: On a macroscopic traversable spacewarp in practice

abstract:
  A design of a configuration for violation of the averaged null energy
condition (ANEC) and consequently other classic energy conditions (CECs), is
presented. The methods of producing effective exotic matter (EM) for a
traversable wormhole (TW) are discussed. Also, the approaches of less necessity
of TWs to EM are considered. The result is, TW and similar structures; i.e.,
warp drive (WD) and Krasnikov tube are not just theoretical subjects for
teaching general relativity (GR) or objects only an advanced civilization would
be able to manufacture anymore, but a quite reachable challenge for current
technology. Besides, a new compound metric is introduced as a choice for
testing in the lab.


title:
Proof of the (local) angular momemtum-mass inequality for axisymmetric
  black holes

abstract:
  We prove that for any vacuum, maximal, asymptotically flat, axisymmetric
initial data for Einstein equations close to extreme Kerr data, the inequality
$\sqrt{J} \leq m$ is satisfied, where $m$ and $J$ are the total mass and
angular momentum of the data. The proof consists in showing that extreme Kerr
is a local minimum of the mass.


title: Quantization of simple parametrized systems

abstract:
  I study the canonical formulation and quantization of some simple
parametrized systems, including the non-relativistic parametrized particle and
the relativistic parametrized particle. Using Dirac's formalism I construct for
each case the classical reduced phase space and study the dependence on the
gauge fixing used. Two separate features of these systems can make this
construction difficult: the actions are not invariant at the boundaries, and
the constraints may have disconnected solution spaces. The relativistic
particle is affected by both, while the non-relativistic particle displays only
by the first. Analyzing the role of canonical transformations in the reduced
phase space, I show that a change of gauge fixing is equivalent to a canonical
transformation. In the relativistic case, quantization of one branch of the
constraint at the time is applied and I analyze the electromagenetic
backgrounds in which it is possible to quantize simultaneously both branches
and still obtain a covariant unitary quantum theory. To preserve unitarity and
space-time covariance, second quantization is needed unless there is no
electric field. I motivate a definition of the inner product in all these cases
and derive the Klein-Gordon inner product for the relativistic case. I
construct phase space path integral representations for amplitudes for the BFV
and the Faddeev path integrals, from which the path integrals in coordinate
space (Faddeev-Popov and geometric path integrals) are derived.


title: Differential Structures - the Geometrization of Quantum Mechanics

abstract:
  The usual quantization of a classical space-time field does not touch the
non-geometrical character of quantum mechanics. We believe that the deep
problems of unification of general relativity and quantum mechanics are rooted
in this poor understanding of the geometrical character of quantum mechanics.
In Einstein's theory gravitation is expressed by geometry of space-time, and
the solutions of the field equation are invariant w.r.t. a certain equivalence
class of reference frames. This class can be characterized by the differential
structure of space-time. We will show that matter is the transition between
reference frames that belong to different differential structures, that the set
of transitions of the differential structure is given by a Temperley-Lieb
algebra which is extensible to a $C^{*}$-algebra comprising the field operator
algebra of quantum mechanics and that the state space of quantum mechanics is
the linear space of the differential structures. Furthermore we are able to
explain the appearance of the complex numbers in quantum theory. The strong
relation to Loop Quantum Gravity is discussed in conclusion.


title: Non-Abelian Einstein-Born-Infeld-Dilaton Cosmology

abstract:
  The non-abelian Einstein-Born-Infeld-Dilaton theory, which rules the dynamics
of tensor-scalar gravitation coupled to a $su(2)$-valued gauge field ruled by
Born-Infeld lagrangian, is studied in a cosmological framework. The microscopic
energy exchange between the gauge field and the dilaton which results from a
non-universality of the coupling to gravity modifies the usual behaviour of
tensor-scalar theories coupled to matter fluids. General cosmological
evolutions are derived for different couplings to gravitation and a comparison
to universal coupling is highlighted. Evidences of cosmic acceleration are
presented when the evolution is interpreted in the Jordan physical frame of a
matter respecting the weak equivalence principle. The importance for the
mechanism of cosmic acceleration of the dynamics of the Born-Infeld gauge
field, the attraction role of the matter fluid and the non-universality of the
gravitational couplings is briefly outlined.


title: A new look at the Plebanski-Demianski family of solutions

abstract:
  The Plebanski-Demianski metric, and those that can be obtained from it by
taking coordinate transformations in certain limits, include the complete
family of space-times of type D with an aligned electromagnetic field and a
possibly non-zero cosmological constant. Starting with a new form of the line
element which is better suited both for physical interpretation and for
identifying different subfamilies, we review this entire family of solutions.
Our metric for the expanding case explicitly includes two parameters which
represent the acceleration of the sources and the twist of the repeated
principal null congruences, the twist being directly related to both the
angular velocity of the sources and their NUT-like properties. The
non-expanding type D solutions are also identified. All special cases are
derived in a simple and transparent way.


title: BBO and the Neutron-Star-Binary Subtraction Problem

abstract:
  The Big Bang Observer (BBO) is a proposed space-based gravitational-wave (GW)
mission designed primarily to search for an inflation-generated GW background
in the frequency range 0.1-1 Hz. The major astrophysical foreground in this
range is gravitational radiation from inspiraling compact binaries. This
foreground is expected to be much larger than the inflation-generated
background, so to accomplish its main goal, BBO must be sensitive enough to
identify and subtract out practically all such binaries in the observable
universe. It is somewhat subtle to decide whether BBO's current baseline design
is sufficiently sensitive for this task, since, at least initially, the
dominant noise source impeding identification of any one binary is confusion
noise from all the others. Here we present a self-consistent scheme for
deciding whether BBO's baseline design is indeed adequate for subtracting out
the binary foreground. We conclude that the current baseline should be
sufficient. However if BBO's instrumental sensitivity were degraded by a factor
2-4, it could no longer perform its main mission. It is impossible to perfectly
subtract out each of the binary inspiral waveforms, so an important question is
how to deal with the "residual" errors in the post-subtraction data stream. We
sketch a strategy of "projecting out" these residual errors, at the cost of
some effective bandwidth. We also provide estimates of the sizes of various
post-Newtonian effects in the inspiral waveforms that must be accounted for in
the BBO analysis.


title:
Approximate Equations of Motion for Compact Spinning Bodies in General
  Relativity

abstract:
  Approximate equations are derived for the motion of a gyroscope on the
earth's gravitational field using the Einstein, Infeld, Hoffmann surface
integral method. This method does not require a knowledge of the
energy-momentum-stress tensor associated with the gyroscope and uses only its
exterior field for its characterization. The resulting equations of motion
differ from those of previous derivations.


title: The stability of modified gravity models

abstract:
  Conditions for the existence and stability of de Sitter space in modified
gravity are derived by considering inhomogeneous perturbations in a
gauge-invariant formalism. The stability condition coincides with the
corresponding condition for stability with respect to homogeneous
perturbations, while this is not the case in scalar-tensor gravity. The
stability criterion is applied to various modified gravity models of the early
and the present universe.


title: On the Energy-Momentum in Closed Universes

abstract:
  This paper has been removed by arXiv administrators because it plagiarizes
gr-qc/0011027, "Viscous cosmologies in scalar-tensor theories for Kasner type
metrics," by M. Cataldo, S. del Campo and P. Salgado; gr-qc/0301022, "A
generalisation of the Heckmann-Schucking cosmological solution," by I.M.
Khalatnikov and A.Y. Kamenshchik; and gr-qc/0303034, "The energy of the
universe in teleparallel gravity," by T. Vargas.


title:
Consistent discretization and canonical classical and quantum Regge
  calculus

abstract:
  We apply the ``consistent discretization'' technique to the Regge action for
(Euclidean and Lorentzian) general relativity in arbitrary number of
dimensions. The result is a well defined canonical theory that is free of
constraints and where the dynamics is implemented as a canonical
transformation. This provides a framework for the discussion of topology change
in canonical quantum gravity. In the Lorentzian case, the framework appears to
be naturally free of the ``spikes'' that plague traditional formulations. It
also provides a well defined recipe for determining the measure of the path
integral.


title: Gravastar Solutions with Continuous Pressures and Equation of State

abstract:
  We study the gravitational vacuum star (gravastar) configuration as proposed
by other authors in a model where the interior de Sitter spacetime segment is
continuously extended to the exterior Schwarzschild spacetime. The multilayered
structure in previous papers is replaced by a continuous stress-energy tensor
at the price of introducing anisotropy in the (fluid) model of the gravastar.
Either with an ansatz for the equation of state connecting the radial $p_r$ and
tangential $p_t$ pressure or with a calculated equation of state with
non-homogeneous energy/fluid density, solutions are obtained which in all
aspects satisfy the conditions expected for an anisotropic gravastar. Certain
energy conditions have been shown to be obeyed and a polytropic equation of
state has been derived. Stability of the solution with respect to possible
axial perturbation is shown to hold.


title:
Late-time tail of a coupled scalar field in the background of a black
  hole with a global monopole

abstract:
  Using the technique of spectral decomposition, we investigated the late-time
tails of massless and massive coupled scalar fields in the background of a
black hole with a global monopole. We found that due to existence of the
coupling between the scalar and gravitational fields, the massless scalar field
decay faster at timelike infinity $i_+$, and so does the massive one in the
intermediate late time. But the asymptotically late-time tail for the massive
scalar field is not affected and its decay rate still is $t^{-5/6}$.


title: Fermions as sources of accelerated regimes in cosmology

abstract:
  In this work it is investigated if fermionic sources could be responsible for
accelerated periods during the evolution of a universe where a matter field
would answer for the decelerated period. The self-interaction potential of the
fermionic field is considered as a function of the scalar and pseudo-scalar
invariants. Irreversible processes of energy transfer between the matter and
gravitational fields are also considered. It is shown that the fermionic field
could behave like an inflaton field in the early universe and as dark energy
for an old universe.


title:
Non Abelian gauge symmetries induced by the unobservability of
  extra-dimensions in a Kaluza-Klein approach

abstract:
  In this work we deal with the extension of the Kaluza-Klein approach to a
non-Abelian gauge theory; we show how we need to consider the link between the
n-dimensional model and a four-dimensional observer physics, in order to
reproduce fields equations and gauge transformations in the four-dimensional
picture. More precisely, in fields equations any dependence on
extra-coordinates is canceled out by an integration, as consequence of the
unobservability of extra-dimensions. Thus, by virtue of this extra-dimensions
unobservability, we are able to recast the multidimensional Einstein equations
into the four-dimensional Einstein-Yang-Mills ones, as well as all the right
gauge transformations of fields are induced. The same analysis is performed for
the Dirac equation describing the dynamics of the matter fields and, again, the
gauge coupling with Yang-Mills fields are inferred from the multidimensional
free fields theory, together with the proper spinors transformations.


title: Angular momentum-mass inequality for axisymmetric black holes

abstract:
  The inequality $\sqrt{J}\leq m$ is proved for vacuum, asymptotically flat,
maximal and axisymmetric data close to extreme Kerr data. The physical
significance of this inequality and its relation to the standard picture of the
gravitational collapse are discussed.


title: The Extreme Distortion of Black Holes due to Matter

abstract:
  A highly accurate computer program is used to study axially symmetric and
stationary spacetimes containing a Black Hole surrounded by a ring of matter.
It is shown that the matter ring affects the properties of the Black Hole
drastically. In particular, the absolute value of the ratio of the Black Hole's
angular momentum to the square of its mass not only exceeds one, but can be
greater than ten thousand (|J|/M^2 > 10^4). Indeed, the numerical evidence
suggests that this quantity is unbounded.


title:
Gravitational wave extraction from an inspiraling configuration of
  merging black holes

abstract:
  We present new techniqes for evolving binary black hole systems which allow
the accurate determination of gravitational waveforms directly from the wave
zone region of the numerical simulations. Rather than excising the black hole
interiors, our approach follows the "puncture" treatment of black holes, but
utilzing a new gauge condition which allows the black holes to move
successfully through the computational domain. We apply these techniques to an
inspiraling binary, modeling the radiation generated during the final plunge
and ringdown. We demonstrate convergence of the waveforms and good conservation
of mass-energy, with just over 3% of the system's mass converted to gravitional
radiation.


title: Entanglement system, Casimir energy and black hole

abstract:
  We investigate the connection between the entanglement system in Minkowski
spacetime and the black hole using the scaling analysis. Here we show that the
entanglement system satisfies the Bekenstein entropy bound. Even though the
entropies of two systems are the same form, the entanglement energy is
different from the black hole energy. Introducing the Casimir energy of the
vacuum energy fluctuations rather than the entanglement energy, it shows a
feature of the black hole energy. Hence the Casimir energy is more close to the
black hole than the entanglement energy. Finally, we find that the entanglement
system behaves like the black hole if the gravitational effects are included
properly.


title: Analogue model for quantum gravity phenomenology

abstract:
  So called "analogue models" use condensed matter systems (typically
hydrodynamic) to set up an "effective metric" and to model curved-space quantum
field theory in a physical system where all the microscopic degrees of freedom
are well understood. Known analogue models typically lead to massless minimally
coupled scalar fields. We present an extended "analogue space-time" programme
by investigating a condensed-matter system - in and beyond the hydrodynamic
limit - that is in principle capable of simulating the massive Klein-Gordon
equation in curved spacetime. Since many elementary particles have mass, this
is an essential step in building realistic analogue models, and an essential
first step towards simulating quantum gravity phenomenology. Specifically, we
consider the class of two-component BECs subject to laser-induced transitions
between the components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. Furthermore our model suggests
constraints on quantum gravity phenomenology in terms of the "naturalness
problem" and "universality issue".


title:
Asymptotic quasinormal modes of a coupled scalar field in the
  Gibbons-Maeda dilaton spacetime

abstract:
  Adopting the monodromy technique devised by Motl and Neitzke, we investigate
analytically the asymptotic quasinormal frequencies of a coupled scalar field
in the Gibbons-Maeda dilaton spacetime. We find that it is described by $
e^{\beta \omega}=-[1+2\cos{(\frac{\sqrt{2\xi+1}}{2} \pi)}]-e^{-\beta_I
\omega}[2+2\cos{(\frac{\sqrt{2\xi+1}}{2}\pi)}]$, which depends on the structure
parameters of the background spacetime and on the coupling between the scalar
and gravitational fields. As the parameters $\xi$ and $\beta_I$ tend to zero,
the real parts of the asymptotic quasinormal frequencies becomes $T_H\ln{3}$,
which is consistent with Hod's conjecture. When $\xi={91/18} $, the formula
becomes that of the Reissner-Nordstr\"{o}m spacetime.


title: Quantum field theory and its symmetry reduction

abstract:
  The relation between symmetry reduction before and after quantization of a
field theory is discussed using a toy model: the axisymmetric Klein-Gordon
field. We consider three possible notions of symmetry at the quantum level:
invariance under the group action, and two notions derived from imposing
symmetry as a system of constraints a la Dirac, reformulated as a first class
system. One of the latter two turns out to be the most appropriate notion of
symmetry in the sense that it satisfies a number of physical criteria,
including the commutativity of quantization and symmetry reduction. Somewhat
surprisingly, the requirement of invariance under the symmetry group action is
not appropriate for this purpose. A generalization of the physically selected
notion of symmetry to loop quantum gravity is presented and briefly discussed.


title: Spherically Symmetric Quantum Geometry: Hamiltonian Constraint

abstract:
  Variables adapted to the quantum dynamics of spherically symmetric models are
introduced, which further simplify the spherically symmetric volume operator
and allow an explicit computation of all matrix elements of the Euclidean and
Lorentzian Hamiltonian constraints. The construction fits completely into the
general scheme available in loop quantum gravity for the quantization of the
full theory as well as symmetric models. This then presents a further
consistency check of the whole scheme in inhomogeneous situations, lending
further credence to the physical results obtained so far mainly in homogeneous
models. New applications in particular of the spherically symmetric model in
the context of black hole physics are discussed.


title: Regular phantom black holes

abstract:
  For self-gravitating, static, spherically symmetric, minimally coupled scalar
fields with arbitrary potentials and negative kinetic energy (favored by the
cosmological observations), we give a classification of possible regular
solutions to the field equations with flat, de Sitter and AdS asymptotic
behavior. Among the 16 presented classes of regular rsolutions are traversable
wormholes, Kantowski-Sachs (KS) cosmologies beginning and ending with de Sitter
stages, and asymptotically flat black holes (BHs). The Penrose diagram of a
regular BH is Schwarzschild-like, but the singularity at $r=0$ is replaced by a
de Sitter infinity, which gives a hypothetic BH explorer a chance to survive.
Such solutions also lead to the idea that our Universe could be created from a
phantom-dominated collapse in another universe, with KS expansion and
isotropization after crossing the horizon. Explicit examples of regular
solutions are built and discussed. Possible generalizations include $k$-essence
type scalar fields (with a potential) and scalar-tensor theories of gravity.


title: Quantum-Corrected Black Hole Thermodynamics in Extra Dimensions

abstract:
  Bekenstein-Hawking formalism of black hole thermodynamics should be modified
to incorporate quantum gravitational effects. Generalized Uncertainty
Principle(GUP) provides a suitable framework to perform such modifications. In
this paper, we consider a general form of GUP to find black hole thermodynamics
in a model universe with large extra dimensions. We will show that black holes
radiate mainly in the four-dimensional brane. Existence of black holes remnants
as a possible candidate for dark matter is discussed.


title:
Eigenvalues and eigenfunctions of spin-weighted spheroidal harmonics in
  four and higher dimensions

abstract:
  Spin-weighted spheroidal harmonics are useful in a variety of physical
situations, including light scattering, nuclear modeling, signal processing,
electromagnetic wave propagation, black hole perturbation theory in four and
higher dimensions, quantum field theory in curved space-time and studies of
D-branes. We first review analytic and numerical calculations of their
eigenvalues and eigenfunctions in four dimensions, filling gaps in the existing
literature when necessary. Then we compute the angular dependence of the
spin-weighted spheroidal harmonics corresponding to slowly-damped quasinormal
mode frequencies of the Kerr black hole, providing numerical tables and
approximate formulas for their scalar products. Finally we present an
exhaustive analytic and numerical study of scalar spheroidal harmonics in (n+4)
dimensions.


title:
Gravitational radiation, vorticity and the electric and magnetic part of
  Weyl tensor

abstract:
  The electric and the magnetic part of the Weyl tensor, as well as the
invariants obtained from them, are calculated for the Bondi vacuum metric. One
of the invariants vanishes identically and the other only exhibits
contributions from terms of the Weyl tensor containing the static part of the
field. It is shown that the necessary and sufficient condition for the
spacetime to be purely electric is that such spacetime be static. It is also
shown that the vanishing of the electric part implies Minkowski spacetime.
Unlike the electric part, the magnetic part does not contain contributions from
the static field. Finally a speculation about the link between the vorticity of
world lines of observers at rest in a Bondi frame, and gravitational radiation,
is presented.


title: On the geometry of cosmological model building

abstract:
  This article analyzes the present anomalies of cosmology from the point of
view of integrable Weyl geometry. It uses P.A.M. Dirac's proposal for a weak
extension of general relativity, with some small adaptations. Simple models
with interesting geometrical and physical properties, not belonging to the
Friedmann-Lema\^{\i}tre class, are studied in this frame. Those with positive
spatial curvature (Einstein-Weyl universes) go well together with observed mass
density $\Omega_m$, CMB, supernovae Ia data, and quasar frequencies. They
suggest a physical role for an equilibrium state of the Maxwell field proposed
by I.E. Segal in the 1980s (Segal background) and for a time invariant
balancing condition of vacuum energy density. The latter leads to a surprising
agreement with the BF-theoretical calculation proposed by C. Castro (2002).


title:
Magnetized black holes and black rings in the higher dimensional dilaton
  gravity

abstract:
  In this paper we consider magnetized black holes and black rings in the
higher dimensional dilaton gravity. Our study is based on exact solutions
generated by applying a Harrison transformation to known asymptotically flat
black hole and black ring solutions in higher dimensional spacetimes. The
explicit solutions include the magnetized version of the higher dimensional
Schwarzschild-Tangherlini black holes, Myers-Perry black holes and five
dimensional (dipole) black rings. The basic physical quantities of the
magnetized objects are calculated. We also discuss some properties of the
solutions and their thermodynamics. The ultrarelativistic limits of the
magnetized solutions are briefly discussed and an explicit example is given for
the $D$-dimensional magnetized Schwarzschild-Tangherlini black holes.


title:
Off-diagonal coefficients of the DeWitt-Schwinger and Hadamard
  representations of the Feynman propagator

abstract:
  Having in mind applications to gravitational wave theory (in connection with
the radiation reaction problem), stochastic semiclassical gravity (in
connection with the regularization of the noise kernel) and quantum field
theory in higher-dimensional curved spacetime (in connection with the Hadamard
regularization of the stress-energy tensor), we improve the DeWitt-Schwinger
and Hadamard representations of the Feynman propagator of a massive scalar
field theory defined on an arbitrary gravitational background by deriving
higher-order terms for the covariant Taylor series expansions of the
geometrical coefficients -- i.e., the DeWitt and Hadamard coefficients -- that
define them.


title: Considerations of a $k=+1$ Varluminopic Cosmology

abstract:
  Every relativistic particle has 4-speed equal to $c$, since $g_{\mu \nu}
\frac{dx^\mu}{d\tau} \frac{dx^\nu}{d\tau} = c^2$. With the choice of $k = +1$
in the FRW metric, the cosmological scale factor $a(t)$ has the natural
interpretation of the radius of the sphere $S^3_a = \{x \in \mathbb{R}^4 : (x,
x) = a^2\}$. Thus, a particle at rest in the cosmological frame has 4-speed
equal to $\frac{da}{dt}$. This leads us to infer that $\dot a = c$, which
respresents a simple kinematic constraint linking the speed of light to the
cosmological scale factor. This drastically changes the $k=+1$ picture from a
closed deaccelerating universe to an open accelerating universe, settles the
horizon problem, and provides for a new cosmological model more appealing to
our natural intuition. In this paper we shall consider ramifications of this
model.


title: Reality and causality in quantum gravity modified electrodynamics

abstract:
  We present a general description of the propagation properties of quantum
gravity modified electrodynamics characterized by constitutive relations up to
second order in the correction parameter. The effective description corresponds
to an electrodynamics in a dispersive and absorptive non-local medium, where
the Green functions and the refraction indices can be explicitly calculated.
The reality of the electromagnetic field together with the requirement of
causal propagation in a given referrence frame leads to restrictions in the
form of such refraction indices. In particular, absorption must be present in
all cases and, contrary to the usual assumption, it is the dominant aspect in
those effective models which exhibit linear effects in the correction parameter
not related to birefringence. In such a situation absorption is linear while
propagation is quadratical in the correction parameter.


title: Superselection Sectors and General Covariance.I

abstract:
  This paper is devoted to the analysis of charged superselection sectors in
the framework of the locally covariant quantum field theories. We shall analize
sharply localizable charges, and use net-cohomology of J.E. Roberts as a main
tool. We show that to any 4-dimensional globally hyperbolic spacetime it is
attached a unique, up to equivalence, symmetric tensor $\Crm^*-$category with
conjugates (in case of finite statistics); to any embedding between different
spacetimes, the corresponding categories can be embedded, contravariantly, in
such a way that all the charged quantum numbers of sectors are preserved. This
entails that to any spacetime is associated a unique gauge group, up to
isomorphisms, and that to any embedding between two spacetimes there
corresponds a group morphism between the related gauge groups. This form of
covariance between sectors also brings to light the issue whether local and
global sectors are the same. We conjecture this holds that at least on simply
connected spacetimes. It is argued that the possible failure might be related
to the presence of topological charges. Our analysis seems to describe theories
which have a well defined short-distance asymptotic behaviour.


title: CR Structures and Asymptotically Flat Space-Times

abstract:
  We discuss the unique existence, arising by analogy to that in algebraically
special space-times, of a CR structure realized on null infinity for any
asymptotically flat Einstein or Einstein-Maxwell space-time.


title:
Clifford bundle formulation of BF gravity generalized to the standard
  model

abstract:
  The structure and dynamics of the standard model and gravity are described by
a Clifford valued connection and its curvature.


title: Does the Schwarzschild black hole really exist?

abstract:
  We use the Kruskal time coordinate T to define the initial time. By this way,
it naturally divides the stable study into one connected with the two regions:
the white-hole-connected region and the black-hole-connected region. The union
of the two regions covers the Schwarzschild space-time (r>2m). We also obtain
the very reasonable conclusion: the white-hole-connected region is instable;
whereas the black-hole-connected region is stable. If we take the instability
with caution and seriousness, it might be not unreasonable to regard that the
Schwarzschild black hole might be instable too.


title: Global aspects of accelerating and rotating black hole space-times

abstract:
  The complete family of exact solutions representing accelerating and rotating
black holes with possible electromagnetic charges and a NUT parameter is known
in terms of a modified Plebanski-Demianski metric. This demonstrates the
singularity and horizon structure of the sources but not that the complete
space-time describes two causally separated black holes. To demonstrate this
property, the metric is first cast in the Weyl-Lewis-Papapetrou form. After
extending this up to the acceleration horizon, it is then transformed to the
boost-rotation-symmetric form in which the global properties of the solution
are manifest. The physical interpretation of these solutions is thus clarified.


title: Derivation of the Raychaudhuri Equation

abstract:
  As a homage to A K Raychaudhuri, I derive in a straightforward way his famous
equation and also indicate the problems he was last engaged in.


title: Spin Gauge Theory of Gravity in Clifford Space

abstract:
  A theory in which 16-dimensional curved Clifford space (C-space) provides a
realization of Kaluza-Klein theory is investigated. No extra dimensions of
spacetime are needed: "extra dimensions" are in C-space. We explore the spin
gauge theory in C-space and show that the generalized spin connection contains
the usual 4-dimensional gravity and Yang-Mills fields of the U(1)xSU(2)xSU(3)
gauge group. The representation space for the latter group is provided by
16-component generalized spinors composed of four usual 4-component spinors,
defined geometrically as the members of four independent minimal left ideals of
Clifford algebra.


title: Matter Inheritance Symmetries of Spherically Symmetric Static Spacetimes

abstract:
  In this paper we discuss matter inheritance collineations by giving a
complete classification of spherically symmetric static spacetimes by their
matter inheritance symmetries. It is shown that when the energy-momentum tensor
is degenerate, most of the cases yield infinite dimensional matter inheriting
symmetries. It is worth mentioning here that two cases provide finite
dimensional matter inheriting vectors even for the degenerate case. The
non-degenerate case provides finite dimensional matter inheriting symmetries.
We obtain different constraints on the energy-momentum tensor in each case. It
is interesting to note that if the inheriting factor vanishes, matter
inheriting collineations reduce to be matter collineations already available in
the literature. This idea of matter inheritance collineations turn out to be
the same as homotheties and conformal Killing vectors are for the metric
tensor.


title: A twin paradox for 'clever' students

abstract:
  We discuss the twin paradox or the clock paradox under the small velocity
approximation of special relativity. In this paper the traveller twin of the
standard twin parable sets out with a non-relativistic speed for the trip
leaving behind the stay-at-home one on earth and continues up to a distance and
finally returns home with the same speed when the siblings can compare their
ages or their initially synchronized wrist watches. The common knowledge that
at small velocities the length contraction and time dilation effects of special
relativity can be ignored so that the world becomes essentially classical,
tends to lead to a paradox in connection with the twin problem, which a
"clever" student eventually discovers. After discussing and resolving the
issue, some more related paradoxes have been presented. The resolution of all
these problems provides some additional insight into the century old paradox.


title: Anisotropically Inflating Universes

abstract:
  We show that in theories of gravity that add quadratic curvature invariants
to the Einstein-Hilbert action there exist expanding vacuum cosmologies with
positive cosmological constant which do not approach the de Sitter universe.
Exact solutions are found which inflate anisotropically. This behaviour is
driven by the Ricci curvature invariant and has no counterpart in the general
relativistic limit. These examples show that the cosmic no-hair theorem does
not hold in these higher-order extensions of general relativity and raises new
questions about the ubiquity of inflation in the very early universe and the
thermodynamics of gravitational fields.


title: Curvaton reheating in tachyonic inflationary models

abstract:
  The curvaton reheating in a tachyonic inflationary universe model with an
exponential potential is studied. We have found that the energy density in the
kinetic epoch, has a complicated dependencies of the scale factor. For
different scenarios, the temperature of reheating is computed. These
temperature result to be analogous to those obtained in the standard case of
the curvaton scenario.


title:
Explicit Fermi Coordinates and Tidal Dynamics in de Sitter and Goedel
  Spacetimes

abstract:
  Fermi coordinates are directly constructed in de Sitter and Goedel spacetimes
and the corresponding exact coordinate transformations are given explicitly.
The quasi-inertial Fermi coordinates are then employed to discuss the dynamics
of a free test particle in these spacetimes and the results are compared to the
corresponding generalized Jacobi equations that contain only the lowest-order
tidal terms. The domain of validity of the generalized Jacobi equation is thus
examined in these cases. Furthermore, the difficulty of constructing explicit
Fermi coordinates in black-hole spacetimes is demonstrated.


title: On the Physical Properties of the Plane Symmetric Self-Similar Solution

abstract:
  This paper discusses some of the physical properties of plane symmetric
self-similar solutions of the first kind (i.e., homothetic solutions). We are
interested in calculating the expansion, the acceleration, the rotation, the
shear tensor, the shear invariant, and the expansion rate (given by
Raychaudhuri's equation). We check these properties both in co-moving and
non-co-moving coordinates (only in the radial direction). Further, the
singularity structure of such solutions will be explored. This analysis
provides some interesting features of self-similar solutions.


title: Rotating "Black Holes" with Holes in the Horizon

abstract:
  Kerr-Schild solutions of the Einstein-Maxwell field equations, containing
semi-infinite axial singular lines, are investigated.
  It is shown that axial singularities break up the black hole, forming holes
in the horizon. As a result, a tube-like region appears which allows matter to
escape from the interior without crossing the horizon. It is argued that axial
singularities of this kind, leading to very narrow beams, can be created in
black holes by external electromagnetic or gravitational excitations and may be
at the origin of astrophysically observable effects such as jet formation.


title: Braneworld with Induced Axial Symmetry

abstract:
  We take arbitrary gravitational perturbations of a 5d spacetime and reduce it
to the form an axially symmetric warped braneworld. Then, we write the filed
equations for the linearized gravity perturbations. We obtain the equations
that describes the graviton, gravivector and the graviscalar fluctuations and
analyse the effects of the Schr\"odinger potentials that appear in these
equations.


title: Perturbations around black holes

abstract:
  Perturbations around black holes have been an intriguing topic in the last
few decades. They are particularly important today, since they relate to the
gravitational wave observations which may provide the unique fingerprint of
black holes' existence. Besides the astrophysical interest, theoretically
perturbations around black holes can be used as testing grounds to examine the
proposed AdS/CFT and dS/CFT correspondence.


title: Non-abelian Yang-Mills in Kundt spacetimes

abstract:
  We present new exact solutions of the Einstein-Yang-Mills system. The
solutions are described by a null Yang-Mills field in a Kundt spacetime. They
generalize a previously known solution for a metric of $pp$ wave type. The
solutions are formally of Petrov type III.


title:
A Morse-theoretical analysis of gravitational lensing by a Kerr-Newman
  black hole

abstract:
  Consider, in the domain of outer communication of a Kerr-Newman black hole, a
point (observation event) and a timelike curve (worldline of light source).
Assume that the worldline of the source (i) has no past end-point, (ii) does
not intersect the caustic of the past light-cone of the observation event, and
(iii) goes neither to the horizon nor to infinity in the past. We prove that
then for infinitely many positive integers k there is a past-pointing lightlike
geodesic of (Morse) index k from the observation event to the worldline of the
source, hence an observer at the observation event sees infinitely many images
of the source. Moreover, we demonstrate that all lightlike geodesics from an
event to a timelike curve in the domain of outer communication are confined to
a certain spherical shell. Our characterization of this spherical shell shows
that in the Kerr-Newman spacetime the occurrence of infinitely many images is
intimately related to the occurrence of centrifugal-plus-Coriolis force
reversal.


title:
Binary neutron stars: Equilibrium models beyond spatial conformal
  flatness

abstract:
  Equilibria of binary neutron stars in close circular orbits are computed
numerically in a waveless formulation: The full Einstein-relativistic-Euler
system is solved on an initial hypersurface to obtain an asymptotically flat
form of the 4-metric and an extrinsic curvature whose time derivative vanishes
in a comoving frame. Two independent numerical codes are developed, and
solution sequences that model inspiraling binary neutron stars during the final
several orbits are successfully computed. The binding energy of the system near
its final orbit deviates from earlier results of third post-Newtonian and of
spatially conformally flat calculations. The new solutions may serve as initial
data for merger simulations and as members of quasiequilibrium sequences to
generate gravitational wave templates, and may improve estimates of the
gravitational-wave cutoff frequency set by the last inspiral orbit.


title:
Can Solar System observations tell us something about the cosmological
  constant?

abstract:
  In this note we show that the latest determinations of the residual Mercury's
perihelion advance, obtained by accounting for almost all known Newtonian and
post-Newtonian orbital effects, yields only very broad constraints on the
cosmological constant. Indeed, from \delta\dot\omega=-0.0036 + - 0.0050
arcseconds per century one gets -2 10^-34 km^-2 < Lambda < 4 10^-35 km^-2. The
currently accepted value for Lambda, obtained from many independent
cosmological and large-scale measurements, amounts to almost 10^-46 km^-2.


title: Orbital effects of Sun's mass loss and the Earth's fate

abstract:
  I calculate the classical effects induced by an isotropic mass loss of a body
on the orbital motion of a test particle around it. By applying my results to
the phase in which the radius of the Sun, already moved to the Red Giant Branch
of the Hertzsprung-Russell Diagram, will become as large as 1.20 AU in about 1
Myr, I find that the Earth's perihelion position on the fixed line of the
apsides will increase by about 0.22-0.25 AU (for \dot M/M = 2 x 10^-7 yr^-1);
other researchers point towards an increase of 0.37-0.63 AU. Mercury will be
destroyed already at the end of the Main Sequence, while Venus should be
engulfed in the initial phase of the Red Giant Branch phase; the orbits of the
outer planets will increase by 1.2-7.5 AU. Simultane- ous long-term numerical
integrations of the equations of motion of all the major bodies of the solar
system, with the inclusion of a mass-loss term in the dynamical force models as
well, are required to check if the mutual N-body interactions may substantially
change the picture analytically outlined here, especially in the Red Giant
Branch phase in which Mercury and Venus may be removed from the integration.


title: Asymptotic silence-breaking singularities

abstract:
  We discuss three complementary aspects of scalar curvature singularities:
asymptotic causal properties, asymptotic Ricci and Weyl curvature, and
asymptotic spatial properties. We divide scalar curvature singularities into
two classes: so-called asymptotically silent singularities and non-generic
singularities that break asymptotic silence. The emphasis in this paper is on
the latter class which have not been previously discussed. We illustrate the
above aspects and concepts by describing the singularities of a number of
representative explicit perfect fluid solutions.


title: The Fermion Self-Energy during Inflation

abstract:
  We compute the one loop fermion self-energy for massless Dirac + Einstein in
the presence of a locally de Sitter background. We employ dimensional
regularization and obtain a fully renormalized result by absorbing all
divergences with BPHZ counterterms. An interesting technical aspect of this
computation is the need for a noninvariant counterterm owing to the breaking of
de Sitter invariance by our gauge condition. Our result can be used in the
quantum-corrected Dirac equation to search for inflation-enhanced quantum
effects from gravitons, analogous to those which have been found for massless,
minimally coupled scalars.


title:
Scalar and tensorial topological matter coupled to (2+1)-dimensional
  gravity:A.Classical theory and global charges

abstract:
  We consider the coupling of scalar topological matter to (2+1)-dimensional
gravity. The matter fields consist of a 0-form scalar field and a 2-form tensor
field. We carry out a canonical analysis of the classical theory, investigating
its sectors and solutions. We show that the model admits both BTZ-like
black-hole solutions and homogeneous/inhomogeneous FRW cosmological
solutions.We also investigate the global charges associated with the model and
show that the algebra of charges is the extension of the Kac-Moody algebra for
the field-rigid gauge charges, and the Virasoro algebrafor the diffeomorphism
charges. Finally, we show that the model can be written as a generalized
Chern-Simons theory, opening the perspective for its formulation as a
generalized higher gauge theory.


title: KKW Analysis for the Dyadosphere of a Charged Black Hole

abstract:
  The Keski-Vakkuri, Kraus and Wilczek (KKW) analysis is used to compute the
temperature and entropy in the dyadosphere of a charged black hole solution.
For our purpose we choose the dyadosphere region of the Reissner-Nordstrom
black hole solution. Our results show that the expressions of the temperature
and entropy in the dyadosphere of this charged black hole are not the Hawking
temperature and the Bekenstein-Hawking entropy, respectively.


title: Momentum constraint relaxation

abstract:
  Full relativistic simulations in three dimensions invariably develop runaway
modes that grow exponentially and are accompanied by violations of the
Hamiltonian and momentum constraints. Recently, we introduced a numerical
method (Hamiltonian relaxation) that greatly reduces the Hamiltonian constraint
violation and helps improve the quality of the numerical model. We present here
a method that controls the violation of the momentum constraint. The method is
based on the addition of a longitudinal component to the traceless extrinsic
curvature generated by a vector potential w_i, as outlined by York. The
components of w_i are relaxed to solve approximately the momentum constraint
equations, pushing slowly the evolution toward the space of solutions of the
constraint equations. We test this method with simulations of binary neutron
stars in circular orbits and show that effectively controls the growth of the
aforementioned violations. We also show that a full numerical enforcement of
the constraints, as opposed to the gentle correction of the momentum relaxation
scheme, results in the development of instabilities that stop the runs shortly.


title: Magnetized Tolman-Bondi Collapse

abstract:
  We investigate the gravitational implosion of magnetized matter by studying
the inhomogeneous collapse of a weakly magnetized Tolman-Bondi spacetime. The
role of the field is analyzed by looking at the convergence of neighboring
particle worldlines. In particular, we identify the magnetically related
stresses in the Raychaudhuri equation and use the Tolman-Bondi metric to
evaluate their impact on the collapsing dust. We find that, despite the low
energy level of the field, the Lorentz force dominates the advanced stages of
the collapse, leading to a strongly anisotropic contraction. In addition, of
all the magnetic stresses, those that resist the collapse are found to grow
faster.


title:
Detecting a gravitational-wave background with next-generation space
  interferometers

abstract:
  Future missions of gravitational-wave astronomy will be operated by
space-based interferometers, covering very wide range of frequency. Search for
stochastic gravitational-wave backgrounds (GWBs) is one of the main targets for
such missions, and we here discuss the prospects for direct measurement of
isotropic and anisotropic components of (primordial) GWBs around the frequency
0.1-10 Hz. After extending the theoretical basis for correlation analysis, we
evaluate the sensitivity and the signal-to-noise ratio for the proposed future
space interferometer missions, like Big-Bang Observer (BBO), Deci-Hertz
Interferometer Gravitational-wave Observer (DECIGO) and recently proposed
Fabry-Perot type DECIGO. The astrophysical foregrounds which are expected at
low frequency may be a big obstacle and significantly reduce the
signal-to-noise ratio of GWBs. As a result, minimum detectable amplitude may
reach h^2 \ogw = 10^{-15} \sim 10^{-16}, as long as foreground point sources
are properly subtracted. Based on correlation analysis, we also discuss
measurement of anisotropies of GWBs. As an example, the sensitivity level
required for detecting the dipole moment of GWB induced by the proper motion of
our local system is closely examined.


title: Search for gravitational wave bursts in LIGO's third science run

abstract:
  We report on a search for gravitational wave bursts in data from the three
LIGO interferometric detectors during their third science run. The search
targets subsecond bursts in the frequency range 100-1100 Hz for which no
waveform model is assumed, and has a sensitivity in terms of the
root-sum-square (rss) strain amplitude of hrss ~ 10^{-20} / sqrt(Hz). No
gravitational wave signals were detected in the 8 days of analyzed data.


title: Relic gravitons on Kasner branes

abstract:
  We present a simple model in which a brane with anisotropic metric is
embedded in an AdS bulk. We discuss the localization of the massless mode and
the amplification of both the massless and the massive modes on the branes,
paying particular attention to the normalization of the perturbed action and to
the evaluation of the effective coupling constant that controls the amplitude
of the spectrum. In the model under investigation there is no mass gap between
massless and massive modes, and the massive modes can be amplified, with
mass-dependent amplitudes.


title: Local effects of global gravitation

abstract:
  In what respect does terrestrial physics reflect the two unique features of
the global gravitational field: its infinite range and its equivalence to
spacetime curvature? We quote the evidence that true irreversibility, i.e the
growth of the Boltzmann entropy of any finite system, is the consequence of the
global state of the gravitation dominated and expanding universe. Moreover, as
another example, we calculate the effect of global expansion and of the
gravitational potential observed in our Local Group on spacetime metric in
terms of curvature. Surprisingly, we find an energy density which is in
numerical agreement with the purely quantum theoretical result for the Casimir
energy density containing Planck's constant.


title: Factor ordering in standard quantum cosmology

abstract:
  The Wheeler-DeWitt equation of Friedmann models with a massless quantum field
is formulated with arbitrary factor ordering of the Hamiltonian constraint
operator. A scalar product of wave functions is constructed, giving rise to a
probability interpretation and making comparison with the classical solution
possible. In general the bahaviour of the wave function of the model depends on
a critical energy of the matter field, which, in turn, depends on the chosen
factor ordering. By certain choices of the ordering the critical energy can be
pushed down to zero.


title:
An analytic model for the transition from decelerated to accelerated
  cosmic expansion

abstract:
  We consider the scenario where our observable universe is devised as a
dynamical four-dimensional hypersurface embedded in a five-dimensional bulk
spacetime, with a large extra dimension, which is the {\it generalization of
the flat FRW cosmological metric to five dimensions}. This scenario generates a
simple analytical model where different stages of the evolution of the universe
are approximated by distinct parameterizations of the {\it same} spacetime. In
this model the evolution from decelerated to accelerated expansion can be
interpreted as a "first-order" phase transition between two successive stages.
The dominant energy condition allows different parts of the universe to evolve,
from deceleration to acceleration, at different redshifts within a narrow era.
This picture corresponds to the creation of bubbles of new phase, in the middle
of the old one, typical of first-order phase transitions. Taking $\Omega_{m} =
0.3$ today, we find that the cross-over from deceleration to acceleration
occurs at $z \sim 1-1.5 $, regardless of the equation of state in the very
early universe. In the case of primordial radiation, the model predicts that
the deceleration parameter "jumps" from $q \sim + 1.5$ to $q \sim - 0.4$ at $z
\sim 1.17$. At the present time $q = - 0.55$ and the equation of state of the
universe is $w = p/\rho \sim - 0.7 $, in agreement with observations and some
theoretical predictions.


title: The adiabatic evolution of orbital parameters in the Kerr spacetime

abstract:
  We investigate the adiabatic orbital evolution of a point particle in the
Kerr spacetime due to the emission of gravitational waves. In the case that the
timescale of the orbital evolution is enough smaller than the typical timescale
of orbits, the evolution of orbits is characterized by the change rates of
three constants of motion, the energy $E$, the azimuthal angular momentum $L$,
and the Carter constant $Q$. For $E$ and $L$, we can evaluate their change
rates from the fluxes of the energy and the angular momentum at infinity and on
the event horizon according to the balance argument. On the other hand, for the
Carter constant, we cannot use the balance argument because we do not know the
conserved current associated with it. %and the corresponding conservation law.
Recently, Mino proposed a new method of evaluating the averaged change rate of
the Carter constant by using the radiative field. In our previous paper we
developed a simplified scheme for practical evaluation of the evolution of the
Carter constant based on the Mino's proposal. In this paper we describe our
scheme in more detail, and derive explicit analytic formulae for the change
rates of the energy, the angular momentum and the Carter constant.


title: Recent results on the search for continuous sources with LIGO and GEO600

abstract:
  An overview of the searches for continuous gravitational wave signals in LIGO
and GEO 600 performed on different recent science runs and results are
presented. This includes both searching for gravitational waves from known
pulsars as well as blind searches over a wide parameter space.


title: Towards the Scale Invariant Cosmology

abstract:
  An argument is made to show that the singularity in the General Theory of
Relativity (GTR) is the expression of a non-Machian feature. It can be avoided
with a scale-invariant dynamical theory, a property lacking in GTR. It is
further argued that the global non-conservation of energy in GTR also results
from the lack of scale-invariance and the field formulation presented by
several authors can only resolve the problem in part. A truly scale-invariant
theory is required to avoid these two problems in a more consistent approach


title: Testing numerical relativity with the shifted gauge wave

abstract:
  Computational methods are essential to provide waveforms from coalescing
black holes, which are expected to produce strong signals for the gravitational
wave observatories being developed. Although partial simulations of the
coalescence have been reported, scientifically useful waveforms have so far not
been delivered. The goal of the AppleswithApples (AwA) Alliance is to design,
coordinate and document standardized code tests for comparing numerical
relativity codes. The first round of AwA tests have now being completed and the
results are being analyzed. These initial tests are based upon periodic
boundary conditions designed to isolate performance of the main evolution code.
Here we describe and carry out an additional test with periodic boundary
conditions which deals with an essential feature of the black hole excision
problem, namely a non-vanishing shift. The test is a shifted version of the
existing AwA gauge wave test. We show how a shift introduces an exponentially
growing instability which violates the constraints of a standard harmonic
formulation of Einstein's equations. We analyze the Cauchy problem in a
harmonic gauge and discuss particular options for suppressing instabilities in
the gauge wave tests. We implement these techniques in a finite difference
evolution algorithm and present test results. Although our application here is
limited to a model problem, the techniques should benefit the simulation of
black holes using harmonic evolution codes.


title:
Bose-Einstein Condensate and Spontaneous Breaking of Conformal Symmetry
  on Killing Horizons II

abstract:
  In a previous paper (hep-th/0407256) local scalar QFT (in Weyl algebraic
approach) has been constructed on degenerate semi-Riemannian manifolds
$\bS^1\times \Sigma$ corresponding to the extension of Killing horizons by
adding points at infinity to the null geodesic forming the horizon. It has been
proved that the theory admits a natural representation of $PSL(2,\bR)$ in terms
of $*$-automorphisms and this representation is unitarily implementable if
referring to a certain invariant state $\lambda$. Among other results it has
been proved that the theory admits a class of inequivalent algebraic (coherent)
states $\{\lambda_\zeta\}$, with $\zeta\in L^2(\Sigma)$, which break part of
the symmetry, in the sense that each of them is not invariant under the full
group $PSL(2,\bR)$ and so there is no unitary representation of whole group
$PSL(2,\bR)$ which leaves fixed the cyclic GNS vector. These states, if
restricted to suitable portions of $\bM$ are invariant and extremal KMS states
with respect a surviving one-parameter group symmetry. In this paper we clarify
the nature of symmetry breakdown. We show that, in fact, {\em spontaneous}
symmetry breaking occurs in the natural sense of algebraic quantum field
theory: if $\zeta \neq 0$, there is no unitary representation of whole group
$PSL(2,\bR)$ which implements the $*$-automorphism representation of
$PSL(2,\bR)$ itself in the GNS representation of $\lambda_\zeta$ (leaving fixed
or not the state).


title:
Adiabatic Particle Production with Decaying $\Lambda$ and Anisotropic
  Universe

abstract:
  Here we study an anisotropic model of the universe with constant energy per
particle. A decaying cosmological constant and particle production in an
adiabatic process are considered as the sources for the entropy. The
statefinder parameters $\{r,s\}$ are defined and their behaviour are analyzed
graphically in some cases.


title: Relativistic analysis of the LISA long range optical links

abstract:
  The joint ESA/NASA LISA mission consists in three spacecraft on heliocentric
orbits, flying in a triangular formation of 5 Mkm each side, linked by infrared
optical beams. The aim of the mission is to detect gravitational waves in a low
frequency band. For properly processing the science data, the propagation
delays between spacecraft must be accurately known. We thus analyse the
propagation of light between spacecraft in order to systematically derive the
relativistic effects due to the static curvature of the Schwarzschild spacetime
in which the spacecraft are orbiting with time-varying light-distances. In
particular, our analysis allows to evaluate rigorously the Sagnac effect, and
the gravitational (Einstein) redshift.


title: Dynamics of $k$-essence

abstract:
  There are a number of mathematical theorems in the literature on the dynamics
of cosmological models with accelerated expansion driven by a positive
cosmological constant $\Lambda$ or a nonlinear scalar field with potential $V$
(quintessence) which do not assume homogeneity and isotropy from the beginning.
The aim of this paper is to generalize these results to the case of $k$-essence
models which are defined by a Lagrangian having a nonlinear dependence on the
kinetic energy. In particular, Lagrangians are included where late time
acceleration is driven by the kinetic energy, an effect which is qualitatively
different from anything seen in quintessence models. A general criterion for
isotropization is derived and used to strengthen known results in the case of
quintessence.


title: Scaling of cosmic string loops

abstract:
  We study the spectrum of loops as a part of a complete network of cosmic
strings in flat spacetime. After a long transient regime, characterized by
production of small loops at the scale of the initial conditions, it appears
that a true scaling regime takes over. In this final regime the characteristic
length of loops scales as $0.1 t$, in contrast to earlier simulations which
found tiny loops. We expect the expanding-universe behavior to be qualitatively
similar. The large loop sizes have important cosmological implications. In
particular, the nucleosynthesis bound becomes $G\mu \lesssim 10^{-7}$, much
tighter than before.


title:
Indications for a preferred reference frame from an ether-drift
  experiment

abstract:
  We present a fully model-independent analysis of the extensive observations
reported by a recent ether-drift experiment in Berlin. No a priori assumption
is made on the nature of a hypothetical preferred frame. We find a remarkable
consistency with an Earth's cosmic motion exhibiting an average declination
angle |\gamma|\sim 43^o and with values of the RMS anisotropy parameter
(1/2-\beta+\delta) that are one order of magnitude larger than the presently
quoted ones. This might represent the first modern indication for a preferred
frame and for a non-zero anisotropy of the speed of light.


title: Spin networks, quantum automata and link invariants

abstract:
  The spin network simulator model represents a bridge between (generalized)
circuit schemes for standard quantum computation and approaches based on
notions from Topological Quantum Field Theories (TQFT). More precisely, when
working with purely discrete unitary gates, the simulator is naturally modelled
as families of quantum automata which in turn represent discrete versions of
topological quantum computation models. Such a quantum combinatorial scheme,
which essentially encodes SU(2) Racah--Wigner algebra and its braided
counterpart, is particularly suitable to address problems in topology and group
theory and we discuss here a finite states--quantum automaton able to accept
the language of braid group in view of applications to the problem of
estimating link polynomials in Chern--Simons field theory.


title: Asymptotic charges in 3d gravity with torsion

abstract:
  We discuss some new developments in three-dimensional gravity with torsion,
based on Riemann-Cartan geometry. Using the canonical approach, we study the
structure of asymptotic symmetry, clarify its fundamental role in defining the
gravitational conserved charges, and explore the influence of the asymptotic
structure on the black hole entropy.


title: Cauchy-perturbative matching revisited: tests in spherical symmetry

abstract:
  During the last few years progress has been made on several fronts making it
possible to revisit Cauchy-perturbative matching (CPM) in numerical relativity
in a more robust and accurate way. This paper is the first in a series where we
plan to analyze CPM in the light of these new results.
  Here we start by testing high-order summation-by-parts operators, penalty
boundaries and contraint-preserving boundary conditions applied to CPM in a
setting that is simple enough to study all the ingredients in great detail:
Einstein's equations in spherical symmetry, describing a black hole coupled to
a massless scalar field. We show that with the techniques described above, the
errors introduced by Cauchy-perturbative matching are very small, and that very
long term and accurate CPM evolutions can be achieved. Our tests include the
accretion and ring-down phase of a Schwarzschild black hole with CPM, where we
find that the discrete evolution introduces, with a low spatial resolution of
\Delta r = M/10, an error of 0.3% after an evolution time of 1,000,000 M. For a
black hole of solar mass, this corresponds to approximately 5 s, and is
therefore at the lower end of timescales discussed e.g. in the collapsar model
of gamma-ray burst engines.
  (abridged)


title:
Rebounce and Black hole formation in a Gravitational Collapse Model with
  Vanishing Radial Pressure

abstract:
  We examine spherical gravitational collapse of a matter model with vanishing
radial pressure and non-zero tangential pressure. It is seen analytically that
the collapsing cloud either forms a black hole or disperses depending on values
of the initial parameters which are initial density, tangential pressure and
velocity profile of the cloud. A threshold of black hole formation is observed
near which a scaling relation is obtained for the mass of black hole, assuming
initial profiles to be smooth. The similarities in the behaviour of this model
at the onset of black hole formation with that of numerical critical behaviour
in other collapse models are indicated.


title: On the Riemann Extension of the G\"odel Space-Time metric

abstract:
  Some properties of the G\"odel space-time metric and its Riemann extension
are studied


title:
New, efficient, and accurate high order derivative and dissipation
  operators satisfying summation by parts, and applications in
  three-dimensional multi-block evolutions

abstract:
  We construct new, efficient, and accurate high-order finite differencing
operators which satisfy summation by parts. Since these operators are not
uniquely defined, we consider several optimization criteria: minimizing the
bandwidth, the truncation error on the boundary points, the spectral radius, or
a combination of these. We examine in detail a set of operators that are up to
tenth order accurate in the interior, and we surprisingly find that a
combination of these optimizations can improve the operators' spectral radius
and accuracy by orders of magnitude in certain cases. We also construct
high-order dissipation operators that are compatible with these new finite
difference operators and which are semi-definite with respect to the
appropriate summation by parts scalar product. We test the stability and
accuracy of these new difference and dissipation operators by evolving a
three-dimensional scalar wave equation on a spherical domain consisting of
seven blocks, each discretized with a structured grid, and connected through
penalty boundary conditions.