title:
On the Circular Orbit Approximation for Binary Compact Objects In
  General Relativity

abstract:
  One often-used approximation in the study of binary compact objects (i.e.,
black holes and neutron stars) in general relativity is the instantaneously
circular orbit assumption. This approximation has been used extensively, from
the calculation of innermost circular orbits to the construction of initial
data for numerical relativity calculations. While this assumption is
inconsistent with generic general relativistic astrophysical inspiral phenomena
where the dissipative effects of gravitational radiation cause the separation
of the compact objects to decrease in time, it is usually argued that the
timescale of this dissipation is much longer than the orbital timescale so that
the approximation of circular orbits is valid. Here, we quantitatively analyze
this approximation using a post-Newtonian approach that includes terms up to
order ({Gm/(rc^2)})^{9/2} for non-spinning particles. By calculating the
evolution of equal mass black hole / black hole binary systems starting with
circular orbit configurations and comparing them to the more astrophysically
relevant quasicircular solutions, we show that a minimum initial separation
corresponding to at least 6 (3.5) orbits before plunge is required in order to
bound the detection event loss rate in gravitational wave detectors to < 5%
(20%). In addition, we show that the detection event loss rate is > 95% for a
range of initial separations that include all modern calculations of the
innermost circular orbit (ICO).


title: Creation of massive particles in a tunneling universe

abstract:
  We examine the particle production during tunneling in quantum cosmology. We
consider a minisuperspace model with a massive, conformally coupled scalar
field and a uniform radiation background. In this model, we construct a
semiclassical wave function describing a small recollapsing universe and a
nucleated inflating universe (``tunneling from something''). We find that the
quantum states of the scalar field in both the initial and the nucleated
universe are close to the adiabatic vacuum, the number of created particles is
small, and their backreaction on the metric is negligible. We show that the use
of the semiclassical approximation is justified for this wave function. Our
results imply that the creation of the universe from nothing can be understood
as a limit of tunneling from a small recollapsing universe.


title: Parity violating spin-two gauge theories

abstract:
  Nonlinear covariant parity-violating deformations of free spin-two gauge
theory are studied in n>2 spacetime dimensions, using a linearized frame and
spin-connection formalism, for a set of massless spin-two fields. It is shown
that the only such deformations yielding field equations with a second order
quasilinear form are the novel algebra-valued types in n=3 and n=5 dimensions
already found in some recent related work concentrating on lowest order
deformations. The complete form of the deformation to all orders in n=5
dimensions is worked out here and some features of the resulting new
algebra-valued spin-two gauge theory are discussed. In particular, the internal
algebra underlying this theory on 5-dimensional Minkowski space is shown to
cause the energy for the spin-two fields to be of indefinite sign. Finally, a
Kaluza-Klein reduction to n=4 dimensions is derived, giving a parity-violating
nonlinear gauge theory of a coupled set of spin-two, spin-one, and spin-zero
massless fields.


title: 3D simulations of linearized scalar fields in Kerr spacetime

abstract:
  We investigate the behavior of a dynamical scalar field on a fixed Kerr
background in Kerr-Schild coordinates using a 3+1 dimensional spectral
evolution code, and we measure the power-law tail decay that occurs at late
times. We compare evolutions of initial data proportional to f(r)
Y_lm(theta,phi) where Y_lm is a spherical harmonic and (r,theta,phi) are
Kerr-Schild coordinates, to that of initial data proportional to f(r_BL)
Y_lm(theta_BL,phi), where (r_BL,theta_BL) are Boyer-Lindquist coordinates. We
find that although these two cases are initially almost identical, the
evolution can be quite different at intermediate times; however, at late times
the power-law decay rates are equal.


title:
Null vector fields in spaces with affine connections and metrics.
  Doppler's effect, Hubble's effect, and aberration's effect

abstract:
  The notion of null (isotropic) vector field is considered in spaces with
affine connections and metrics as models of space or space-time. On its basis
the propagation of signals in space-time is considered. The Doppler effect is
generalized for these types of spaces. The notions of standard (longitudinal)
Doppler effect and transversal Doppler effect are introduced. On their grounds,
the Hubble effect and the aberration effect appear as Doppler effects with
explicit forms of the centrifugal (centripetal) and Coriolis velocity vector
fields in spaces with affine connections and metrics. The upper limit of the
value of the general observed shift parameter z, generated by both the effects,
based on the Doppler effects, is found to be z = 1.41. Doppler's effects,
Hubble's effect, and aberration's effect could be used in mechanics of
continuous media and in other classical field theories in the same way as the
standard Doppler effect is used in classical and relativistic mechanics. PACS
numbers: 04.20.Cv; 04.50.+h; 04.40.b; 04.90.+e; 83.10.Bb


title: On the dynamics of Gowdy space times

abstract:
  We study the behavior near the singularity t=0 of Gowdy metrics. We prove
existence of an open dense set of boundary points near which the solution is
smoothly "asymptotically velocity term dominated" (AVTD). We show that the set
of AVTD solutions satisfying a uniformity condition is open in the set of all
solutions. We analyse in detail the asymptotic behavior of "power law"
solutions at the (hitherto unchartered) points at which the asymptotic velocity
equals zero or one. Several other related results are established.


title: A note on the thermodynamics of gravitational radiation

abstract:
  It is shown that linearized gravitational radiation confined in a cavity can
achieve thermal equilibrium if the mean density of the radiation and the size
of the cavity satisfy certain constraints.


title: Weak gravity in DGP braneworld model

abstract:
  We analyze the weak gravity in the braneworld model proposed by
Dvali-Gabadadze-Porrati, in which the unperturbed background spacetime is given
by five dimensional Minkowski bulk with a brane which has the induced Einstein
Hilbert term. This model has a critical length scale $r_c$. Naively, we expect
that the four dimensional general relativity (4D GR) is approximately recovered
at the scale below $r_c$. However, the simple linear perturbation does not work
in this regime. Only recently the mechanism to recover 4D GR was clarified
under the restriction to spherically symmetric configurations, and the leading
correction to 4D GR was derived. Here, we develop an alternative formulation
which can handle more general perturbations. We also generalize the model by
adding bulk cosmological constant and the brane tension.


title: Four Dimensional Spherical Space-Times and String Theory

abstract:
  Some shortcomings in regard to our lack of conceptual understanding of string
theory are displayed and prescription to untangle them is proposed. String
theory should be a fundamental dynamics of four dimensional symmetric
space-times. Properties of the two dimensional equivalent action are studied,
in the hydrodynamic approximation. In the pressureless regime it is conformal
invariant. Correlation of our proposal to 't Hooft work on quantization of
black holes[7] and work on 2D black hole solutions established by Witten [14]
are pointed out as perspectives of the present work.


title: Particle-Like Description in Quintessential Cosmology

abstract:
  Assuming equation of state for quintessential matter: $p=w(z)\rho$, we
analyse dynamical behaviour of the scale factor in FRW cosmologies. It is shown
that its dynamics is formally equivalent to that of a classical particle under
the action of 1D potential $V(a)$. It is shown that Hamiltonian method can be
easily implemented to obtain a classification of all cosmological solutions in
the phase space as well as in the configurational space. Examples taken from
modern cosmology illustrate the effectiveness of the presented approach.
Advantages of representing dynamics as a 1D Hamiltonian flow, in the analysis
of acceleration and horizon problems, are presented. The inverse problem of
reconstructing the Hamiltonian dynamics (i.e. potential function) from the
luminosity distance function $d_{L}(z)$ for supernovae is also considered.


title: A technique for generating new solutions of Einstein's equations

abstract:
  We present a simple technique for generating new solutions of Einstein's
equations using such function transformations that leave the field equations in
the Ernst form. In this context we recover all the known covariant
transformations of Ernst equations and we find the role of the analytic ones.
Finally we obtain a new asymptotically flat solution starting from the Kerr
solutions.


title: A novel derivation for Kerr metric in Papapetrou gauge

abstract:
  We present a simple novel derivation, ab initio, of the equations appropriate
for stationary axisymmetric spacetimes using the Papapetrou form of the metric
(Papapetrou gauge). It is shown that using coordinates which preserve the
Papapetrou gauge three separated solutions of the Ernst equations appear in the
case of Kerr metric. In this context a parameter arises which represents
topological defects induced by an infinite static string along the z axis.
Finally, we discuss a simple solution that may be derived from the Kerr ansatz.


title: Defining perturbations on submanifolds

abstract:
  We study the definition of perturbations in the presence of a submanifold,
like e.g. a brane. In the standard theory of cosmological perturbations, one
compares quantities at the same coordinate points in the non-perturbed and the
perturbed manifolds, identified via a (non-unique) mapping between the two
manifolds. In the presence of a physical submanifold one needs to modify this
definition in order to evaluate perturbations of quantities at the submanifold
location. As an application, we compute the perturbed metric and the extrinsic
curvature tensors at the brane position in a general gauge.


title:
Quasinormal frequencies of Schwarzschild black holes in anti-de Sitter
  spacetimes: A complete study on the asymptotic behavior

abstract:
  We present a thorough analysis for the quasinormal (QN) behavior, associated
with the decay of scalar, electromagnetic and gravitational perturbations, of
Schwarzschild-anti-de Sitter black holes. As it is known the anti-de Sitter
(AdS) QN spectrum crucially depends on the relative size of the black hole to
the AdS radius. There are three different types of behavior depending on
whether the black hole is large, intermediate, or small. The results of
previous works, concerning lower overtones for large black holes, are completed
here by obtaining higher overtones for all the three black hole regimes. There
are two major conclusions that one can draw from this work: First,
asymptotically for high overtones, all the modes are evenly spaced, and this
holds for all three types of regime, large, intermediate and small black holes,
independently of l, where l is the quantum number characterizing the angular
distribution; Second, the spacing between modes is apparently universal, in
that it does not depend on the field, i.e., scalar, electromagnetic and
gravitational QN modes all have the same spacing for high overtones. We are
also able to prove why scalar and gravitational perturbations are isospectral,
asymptotically for high overtones, by introducing appropriate superpartner
potentials.


title: General Relativity from the three dimensional linear group

abstract:
  This letter describes a novel derivation of general relativity by considering
the (non)self-consistency of theories whose Hamiltonians are constraints. The
constraints, from Hamilton's equations, generate the evolution, while the
evolution, in turn, must preserve the constraints. This closure requirement can
be used as a selection mechanism for general relativity starting from a very
simple set of assumptions. The configuration space is chosen to be a family of
$3 \times 3$ positive definite symmetric matrices on some bare 3-manifold. A
general Hamiltonian is constructed on this space of matrices which consists of
a single constraint per space point. It is assumed that this constraint looks
like an energy balance relationship. It will be the sum of a `kinetic' term
which is quadratic and undifferentiated in the momenta, and a `potential' term,
which is any function of the configuration variables. Further, the constraint
must be a scalar under the linear group, the natural symmetry group of the
configuration space. This inexorably leads to the ADM Hamiltonian for general
relativity. Both the space of Riemannian geometries (Wheeler's superspace), and
spacetime are emergent quantities in this analysis.


title: A new general purpose event horizon finder for 3D numerical spacetimes

abstract:
  I present a new general purpose event horizon finder for full 3D numerical
spacetimes. It works by evolving a complete null surface backwards in time. The
null surface is described as the zero level set of a scalar function, that in
principle is defined everywhere. This description of the surface allows the
surface, trivially, to change topology, making this event horizon finder able
to handle numerical spacetimes, where two (or more) black holes merge into a
single final black hole.


title: Nonlocality of Accelerated Systems

abstract:
  The conceptual basis for the nonlocality of accelerated systems is presented.
The nonlocal theory of accelerated observers and its consequences are briefly
described. Nonlocal field equations are developed for the case of the
electrodynamics of linearly accelerated systems.


title: Brane-Universes with Variable G and Lambda

abstract:
  We investigate the cosmological consequences of a brane-world theory which
incorporates time variations in the gravitational coupling G and the
cosmological term Lambda. We analyze in detail the model where (dG/dt)/G ~ H
and Lambda ~ H^2, which seems to be favored by observations. We show that these
conditions single out models with flat space sections. We determine the
behavior of the expansion scale factor, as well as, the variation of G, Lambda
and H for different possible scenarios where the bulk cosmological constant can
be zero, positive or negative. We demonstrate that the universe must
recollapse, if it is embedded in an Anti-de Sitter five-dimensional bulk, which
is the usual case in brane models. We evaluate the cosmological parameters,
using some observational data, and show that we are nowhere near the time of
recollapse. We conclude that the models with zero and negative bulk
cosmological constant agree with the observed accelerating universe, while
fitting simultaneously the observational data for the density and deceleration
parameters. The age of the universe, even in the recollapsing case, is much
larger than in the FRW universe.


title:
Quantum Mechanical Formulation Of Quantum Cosmology For Brane-World
  Effective Action

abstract:
  Canonical quantization of the Brane-World effective action presented by Kanno
and Soda containing higher order curvature invariant terms, has been performed.
It requires introduction of an auxiliary variable. As observed in a series of
publications by Sanyal and Modak, here again we infer that properly chosen
auxiliary variable leads to a Schrodinger like equation where the kinetic part
of a canonical variable disentangles from the rest of the variables giving a
natural quantum mechanical flavour of time. Further, the effective Hamiltonian
turns out to be hermitian, leading to the continuity ewuation. Thus, a quantum
mechanical probability inter pretation is plausible. Finally, the extremization
of the effective potential leads to Einstein's equation and a well behaved
classical solution, which is a desirable feature of the gravitational action
containing higher order curvature invariant terms.


title:
Inflationary Dilatonic de Sitter Universe from cal N = 4 Super Yang -
  Mills Theory Perturbed by Scalars

abstract:
  In this paper a quantum N = 4 super Yang-Mills theory perturbed by
dilaton-coupled scalars, is considered. The induced effective action for such a
theory is calculated on a dilaton-gravitational background using the conformal
anomaly found via AdS/CFT correspondence. Considering such an effective action
(using the large N method) as a quantum correction to the classical gravity
action with cosmological constant we study the effect from dilaton to the scale
factor (which corresponds to the inflationary universe without dilaton). It is
shown that, depending on the initial conditions for the dilaton, the dilaton
may slow down, or accelerate, the inflation process. At late times, the dilaton
is decaying exponentially. At the end of this work, we consider the question
how the perturbation of the solution for the scale factor affects the stability
of the solution for the equations of motion and therefore the stability of the
Inflationary Universe, which could be eternal.


title: Non-minimally coupled scalar fields and isolated horizons

abstract:
  The isolated horizon framework is extended to include non-minimally coupled
scalar fields. As expected from the analysis based on Killing horizons, entropy
is no longer given just by (a quarter of) the horizon area but also depends on
the scalar field. In a subsequent paper these results will serve as a point of
departure for a statistical mechanical derivation of entropy using quantum
geometry.


title:
Stress-Energy Tensor for the Massless Spin 1/2 Field in Static Black
  Hole Spacetimes

abstract:
  The stress-energy tensor for the massless spin 1/2 field is numerically
computed outside and on the event horizons of both charged and uncharged static
non-rotating black holes, corresponding to the Schwarzschild,
Reissner-Nordstrom and extreme Reissner-Nordstr\"om solutions of Einstein's
equations. The field is assumed to be in a thermal state at the black hole
temperature. Comparison is made between the numerical results and previous
analytic approximations for the stress-energy tensor in these spacetimes. For
the Schwarzschild (charge zero) solution, it is shown that the stress-energy
differs even in sign from the analytic approximation. For the
Reissner-Nordstrom and extreme Reissner-Nordstrom solutions, divergences
predicted by the analytic approximations are shown not to exist.


title:
The Sagnac Phase Shift suggested by the Aharonov-Bohm effect for
  relativistic matter beams

abstract:
  The phase shift due to the Sagnac Effect, for relativistic matter beams
counter-propagating in a rotating interferometer, is deduced on the bases of a
a formal analogy with the the Aharonov-Bohm effect. A procedure outlined by
Sakurai, in which non relativistic quantum mechanics and newtonian physics
appear together with some intrinsically relativistic elements, is generalized
to a fully relativistic context, using the Cattaneo's splitting technique. This
approach leads to an exact derivation, in a self-consistently relativistic way,
of the Sagnac effect. Sakurai's result is recovered in the first order
approximation.


title:
Conserved Quantities from the Equations of Motion (with applications to
  natural and gauge natural theories of gravitation)

abstract:
  We present an alternative field theoretical approach to the definition of
conserved quantities, based directly on the field equations content of a
Lagrangian theory (in the standard framework of the Calculus of Variations in
jet bundles). The contraction of the Euler-Lagrange equations with Lie
derivatives of the dynamical fields allows one to derive a variational
Lagrangian for any given set of Lagrangian equations. A two steps algorithmical
procedure can be thence applied to the variational Lagrangian in order to
produce a general expression for the variation of all quantities which are
(covariantly) conserved along the given dynamics. As a concrete example we test
this new formalism on Einstein's equations: well known and widely accepted
formulae for the variation of the Hamiltonian and the variation of Energy for
General Relativity are recovered. We also consider the Einstein-Cartan
(Sciama-Kibble) theory in tetrad formalism and as a by-product we gain some new
insight on the Kosmann lift in gauge natural theories, which arises when trying
to restore naturality in a gauge natural variational Lagrangian.


title:
The binary black-hole dynamics at the third-and-a-half post-Newtonian
  order in the ADM-formalism

abstract:
  We specialize the radiation-reaction part of the Arnowitt-Deser-Misner (ADM)
Hamiltonian for many non-spinning point-like bodies, calculated by Jaranowski
and Schaefer [1], to third-and-a-half post-Newtonian approximation to general
relativity, to binary systems. This Hamiltonian is used for the computation of
the instantaneous gravitational energy loss of a binary to 1PN reactive order.
We also derive the equations of motion, which include PN reactive terms via
Hamiltonian and Euler-Lagrangian approaches. The results are consistent with
the expressions for reactive acceleration provided by Iyer-Will formalism in
Ref. [2] in a general class of gauges.


title:
Torsion nonminimally coupled to the electromagnetic field and
  birefringence

abstract:
  In conventional Maxwell--Lorentz electrodynamics, the propagation of light is
influenced by the metric, not, however, by the possible presence of a torsion
T. Still the light can feel torsion if the latter is coupled nonminimally to
the electromagnetic field F by means of a supplementary Lagrangian of the type
l^2 T^2 F^2 (l = coupling constant). Recently Preuss suggested a specific
nonminimal term of this nature. We evaluate the spacetime relation of Preuss in
the background of a general O(3)-symmetric torsion field and prove by
specifying the optical metric of spacetime that this can yield birefringence in
vacuum. Moreover, we show that the nonminimally coupled homogeneous and
isotropic torsion field in a Friedmann cosmos affects the speed of light.


title:
Spacetime Noncommutativity and Antisymmetric Tensor Dynamics in the
  Early Universe

abstract:
  This paper investigates the possible cosmological implications of the
presence of an antisymmetric tensor field related to a lack of commutatitivity
of spacetime coordinates at the Planck era. For this purpose, such a field is
promoted to a dynamical variable, inspired by tensor formalism. By working to
quadratic order in the antisymmetric tensor, we study the field equations in a
Bianchi I universe in two models: an antisymmetric tensor plus scalar field
coupled to gravity, or a cosmological constant and a free massless
antisymmetric tensor. In the first scenario, numerical integration shows that,
in the very early universe, the effects of the antisymmetric tensor can prevail
on the scalar field, while at late times the former approaches zero and the
latter drives the isotropization of the universe. In the second model, an
approximate solution is obtained of a nonlinear ordinary differential equation
which shows how the mean Hubble parameter and the difference between
longitudinal and orthogonal Hubble parameter evolve in the early universe.


title: Update on gravitational-wave research

abstract:
  The recently assembled laser-beam detectors of gravitational waves are
approaching the planned level of sensitivity. In the coming 1 - 2 years, we may
be observing the rare but powerful events of inspiral and merger of binary
stellar-mass black holes. More likely, we will have to wait for a few years
longer, until the advanced detectors become operational. Their sensitivity will
be sufficient to meet the most cautious evaluations of the strength and event
rates of astrophysical sources of gravitational waves. The experimental and
theoretical work related to the space-based laser-beam detectors is also
actively pursued. The current gravitational wave research is broad and
interesting. Experimental innovations, source modelling, methods of data
analysis, theoretical issues of principle are being studied and developed at
the same time. The race for direct detection of relatively high-frequency waves
is accompanied by vigorous efforts to discover the very low-frequency relic
gravitational waves through the measurements of the cosmic microwave background
radiation. In this update, we will touch upon each of these directions of
research, including the recent data from the Wilkinson Microwave Anisotropy
Probe (WMAP).


title:
Some Consequences of a Generalization to Heisenberg Algebra in Quantum
  Electrodynamics

abstract:
  In this essay it will be shown that the introduction of a modification to
Heisenberg algebra (here this feature means the existence of a minimal
obserlvable length), as a fundamental part of the quantization process of the
electrodynamical field, renders states in which the uncertainties in the two
quadrature components violate the usual Heisenberg uncertainty relation. Hence
in this context it may be asserted that any physically realistic generalization
of the uncertainty principle must include, not only a minimal observable
length, but also a minimal observable momentum.


title: Cold scalar-tensor black holes coupled to a massless scalar field

abstract:
  New four-dimensional black hole solutions of Brans-Dicke equations with a
negative constant $\omega$, coupled to a massless scalar field, are presented.
The temperature of these black holes is zero and the horizon area is infinite.
An astrophysical application is also discussed.


title: Condensation of Planckian Modes and the Inflaton

abstract:
  To confront the transplanckian problem encountered in the backward
extrapolation of the cosmological expansion of the momenta of the modes of
quantum field theory, it is proposed that there is a reservoir, depository of
transplanckian degrees of freedom. These are solicited by the cisplanckian
modes so as to keep their density fixed and the total energy density of vacuum
at a minimum.
  The mechanism is due to mode - reservoir interaction, whereupon virtual
quantum processes give rise to an effective mode-mode attraction. A BCS
condensate results. It has a massless and massy collective excitation, the
latter identified with the inflaton. For an effective non dimensional
mode-reservoir coupling constant, g approx 0.3, the order of magnitude of its
mass is what is required to account for cosmological fluctuations i.e. O(10^-6
-> 10^-5)m_Planck.


title: Gravity's Rainbow

abstract:
  Non-linear special relativity (or doubly special relativity) is a simple
framework for encoding properties of flat quantum space-time. In this paper we
show how this formalism may be generalized to incorporate curvature (leading to
what might be called ``doubly general relativity''). We first propose a dual to
non-linear realizations of relativity in momentum space, and show that for such
a dual the space-time invariant is an energy-dependent metric. This leads to an
energy-dependent connection and curvature, and a simple modification to
Einstein's equations. We then examine solutions to these equations. We find the
counterpart to the cosmological metric, and show how cosmologies based upon our
theory of gravity may solve the ``horizon problem''. We discuss the
Schwarzchild solution, examining the conditions for which the horizon is energy
dependent. We finally find the weak field limit.


title:
A global and non-entropic arrow of time: the double role of the
  energy-momentum tensor

abstract:
  In this paper we adopt a global and non-entropic approach to the problem of
the arrow of time, according to which the arrow of time is an intrinsic
geometrical property of spacetime. Our main aim is to show the double role
played by the energy-momentum tensor in the context of our approach. On the one
hand, the energy-momentum tensor is the intermediate step that permits to turn
the geometrical time-asymmetry of the universe into a local arrow of time
manifested as a time-asymmetric energy flow. On the other hand, the
energy-momentum tensor supplies the basis for deducing the time-asymmetry of
quantum field theory, posed as an axiom in this theory.


title:
Proposal of a second generation of quantum-gravity-motivated
  Lorentz-symmetry tests: sensitivity to effects suppressed quadratically by
  the Planck scale

abstract:
  Over the last few years the study of possible Planck-scale departures from
classical Lorentz symmetry has been one of the most active areas of
quantum-gravity research. We now have a satisfactory description of the fate of
Lorentz symmetry in the most popular noncommutative spacetimes and several
studies have been devoted to the fate of Lorentz symmetry in loop quantum
gravity. Remarkably there are planned experiments with enough sensitivity to
reveal these quantum-spacetime effects, if their magnitude is only linearly
suppressed by the Planck length. Unfortunately, in some quantum-gravity
scenarios even the strongest quantum-spacetime effects are suppressed by at
least two powers of the Planck length, and many authors have argued that it
would be impossible to test these quadratically-suppressed effects. I here
observe that advanced cosmic-ray observatories and neutrino observatories can
provide the first elements of an experimental programme testing the possibility
of departures from Lorentz symmetry that are quadratically Planck-length
suppressed.


title:
Computing gravitational waves from slightly nonspherical stellar
  collapse to black hole: Odd-parity perturbation

abstract:
  Nonspherical stellar collapse to a black hole is one of the most promising
gravitational wave sources for gravitational wave detectors. We numerically
study gravitational waves from a slightly nonspherical stellar collapse to a
black hole in linearized Einstein theory. We adopt a spherically collapsing
star as the zeroth-order solution and gravitational waves are computed using
perturbation theory on the spherical background. In this paper we focus on the
perturbation of odd-parity modes. Using the polytropic equations of state with
polytropic indices $n_p=1$ and 3, we qualitatively study gravitational waves
emitted during the collapse of neutron stars and supermassive stars to black
holes from a marginally stable equilibrium configuration. Since the matter
perturbation profiles can be chosen arbitrarily, we provide a few types for
them. For $n_p=1$, the gravitational waveforms are mainly characterized by a
black hole quasinormal mode ringing, irrespective of perturbation profiles
given initially. However, for $n_p=3$, the waveforms depend strongly on the
initial perturbation profiles. In other words, the gravitational waveforms
strongly depend on the stellar configuration and, in turn, on the ad hoc choice
of the functional form of the perturbation in the case of supermassive stars.


title:
Global existence and asymptotic behaviour in the future for the
  Einstein-Vlasov system with positive cosmological constant

abstract:
  The behaviour of expanding cosmological models with collisionless matter and
a positive cosmological constant is analysed. It is shown that under the
assumption of plane or hyperbolic symmetry the area radius goes to infinity,
the spacetimes are future geodesically complete, and the expansion becomes
isotropic and exponential at late times. This proves a form of the cosmic no
hair theorem in this class of spacetimes.


title:
Future complete Einsteinian space times with U(1) symmetry, the
  unpolarized case

abstract:
  I prove the existence of vacuum $S^{1}$ symmetric Einsteinian, unpolarized,
space times which are complete in the direction of the expansion, for small
initial data, without supposing that the $S^{1}$ orbits are orthogonal to the
3-manifolds, as was done in previous work in collaboration with V. Moncrief.


title: Analogue models for FRW cosmologies

abstract:
  It is by now well known that various condensed matter systems may be used to
mimic many of the kinematic aspects of general relativity, and in particular of
curved-spacetime quantum field theory. In this essay we will take a look at
what would be needed to mimic a cosmological spacetime -- to be precise a
spatially flat FRW cosmology -- in one of these analogue models. In order to do
this one needs to build and control suitable time dependent systems. We discuss
here two quite different ways to achieve this goal. One might rely on an
explosion, physically mimicking the big bang by an outflow of whatever medium
is being used to carry the excitations of the analogue model, but this idea
appears to encounter dynamical problems in practice. More subtly, one can avoid
the need for any actual physical motion (and avoid the dynamical problems) by
instead adjusting the propagation speed of the excitations of the analogue
model. We shall focus on this more promising route and discuss its
practicality.


title: Dynamical system approach to FRW models in higher-order gravity theories

abstract:
  We study the late time evolution of positively curved FRW models with a
scalar field which arises in the conformal frame of the $R+\alpha R^{2}$
theory. The resulted three-dimensional dynamical system has two equilibrium
solutions corresponding to a de Sitter space and an ever expanding closed
universe. We analyze the structure of the first equilibrium with the methods of
the center manifold theory and, for the second equilibrium we apply the normal
form theory to obtain a simplified system, which we analyze with special phase
plane methods. It is shown that an initially expanding closed FRW spacetime
avoids recollapse.


title:
Action based approach to the dynamics of extended bodies in General
  Relativity

abstract:
  We present, for the first time, an action principle that gives the equations
of motion of an extended body possessing multipole moments in an external
gravitational field, in the weak field limit. From the action, the
experimentally observable quantum phase shifts in the wavefunction of an
extended object due to the coupling of its multipole moments with the
gravitational field are obtained. Also, since the theory may be quantized using
the action, the present approach is useful in the interface between general
relativity and quantum mechanics.


title:
Brans-Dicke Cosmology in an anisotropic model when Velocity of Light
  Varies

abstract:
  In this paper, we have studied Brans-Dicke Cosmology in anisotropic
Kantowski-Sachs space-time model; considering variation of the velocity of
light. We have addressed the flatness problem considering both the cases
namely, (i) when the Brans-Dicke scale field $\phi$ is constant (ii) when
$\phi$ varies, specially for radiation dominated era perturbatively and
non-perturbatively and asymptotic behaviour have been studied.


title: A Quintessence Problem in Brans-Dicke Theory with Varying Speed of Light

abstract:
  It is shown that minimally coupled scalar field in Brans-Dicke theory with
varying speed of light can solve the quintessence problem and it is possible to
have a non-decelerated expansion of the present universe with BD-theory for
anisotropic models without any matter.


title: From Kaluza-Klein theory to Campbell-Magaard theorem and beyond

abstract:
  We give a brief review of recent developments in five-dimensional theories of
spacetime and highlight their geometrical structure mainly in connection with
the Campbell-Magaard theorem.


title: Affine Quantum Gravity

abstract:
  A sketch of the affine quantum gravity program illustrates a different
perspective on several difficult issues of principle: metric positivity;
quantum anomalies; and nonrenormalizability.


title: Issues on the cosmological constant

abstract:
  Some issues of the cosmological constant or dark energy are briefly reviewed.
There are an increasing number of observations that constrain the equation of
state of dark energy more stringently and favor the time-independent
cosmological constant. Then a plausible model of dark energy would be a theory
with degenerate perturbative vacua in which its origin is explained by a
nonperturbative effect so that, unlike quintessence, k-essence etc., it is
separable from the perturbative problem why its amplitude is smaller than the
Planckian density by a factor of $\order (10^{-120})$.


title: Initial Conditions for a Universe

abstract:
  In physical theories, boundary or initial conditions play the role of
selecting special situations which can be described by a theory with its
general laws. Cosmology has long been suspected to be different in that its
fundamental theory should explain the fact that we can observe only one
particular realization. This is not realized, however, in the classical
formulation and in its conventional quantization; the situation is even worse
due to the singularity problem. In recent years, a new formulation of quantum
cosmology has been developed which is based on quantum geometry, a candidate
for a theory of quantum gravity. Here, the dynamical law and initial conditions
turn out to be linked intimately, in combination with a solution of the
singularity problem.


title: Time machine (1988--2001)

abstract: A very brief and popular account of the time machine problem.


title: Physics and Initial Data for Multiple Black Hole Spacetimes

abstract:
  An orbiting black hole binary will generate strong gravitational radiation
signatures, making these binaries important candidates for detection in
gravitational wave observatories. The gravitational radiation is characterized
by the orbital parameters, including the frequency and separation at the
inner-most stable circular orbit (ISCO). One approach to estimating these
parameters relies on a sequence of initial data slices that attempt to capture
the physics of the inspiral. Using calculations of the binding energy, several
authors have estimated the ISCO parameters using initial data constructed with
various algorithms. In this paper we examine this problem using conformally
Kerr-Schild initial data. We present convergence results for our initial data
solutions, and give data from numerical solutions of the constraint equations
representing a range of physical configurations. In a first attempt to
understand the physical content of the initial data, we find that the Newtonian
binding energy is contained in the superposed Kerr-Schild background before the
constraints are solved. We examine some deficiencies with the initial data
approach to orbiting binaries, especially touching on the effects of prior
motion and spin-orbital coupling of the angular momenta. Making rough estimates
of these effects, we find that they are not insignificant compared to the
binding energy, leaving some doubt of the utility of using initial data to
predict ISCO parameters. In computations of specific initial-data
configurations we find spin-specific effects that are consistent with
analytical estimates.


title: Deconstructing the Cosmological Constant

abstract:
  Deconstruction provides a novel way of dealing with the notoriously difficult
ultraviolet problems of four-dimensional gravity. This approach also naturally
leads to a new perspective on the holographic principle, tying it to the
fundamental requirements of unitarity and diffeomorphism invariance, as well as
to a new viewpoint on the cosmological constant problem. The numerical
smallness of the cosmological constant is implied by a unique combination of
holography and supersymmetry, opening a new window into the fundamental physics
of the vacuum.


title:
The Pauli Exclusion Principle and SU(2) vs. SO(3) in Loop Quantum
  Gravity

abstract:
  Recent attempts to resolve the ambiguity in the loop quantum gravity
description of the quantization of area has led to the idea that j=1 edges of
spin-networks dominate in their contribution to black hole areas as opposed to
j=1/2 which would naively be expected. This suggests that the true gauge group
involved might be SO(3) rather than SU(2) with attendant difficulties. We argue
that the assumption that a version of the Pauli principle is present in loop
quantum gravity allows one to maintain SU(2) as the gauge group while still
naturally achieving the desired suppression of spin-1/2 punctures. Areas come
from j=1 punctures rather than j=1/2 punctures for much the same reason that
photons lead to macroscopic classically observable fields while electrons do
not.


title: Questions of Stability near Black Hole Critical Points

abstract:
  In this letter, we discuss how thermal fluctuations can effect the stability
of (generally) charged black holes when close to certain critical points. Our
novel treatment utilizes the black hole area spectrum (which is, for
definiteness, taken to be evenly spaced) and makes an important distinction
between fixed and fluctuating charge systems (with these being modeled,
respectively, as a canonical and grand canonical ensemble.) The discussion
begins with a summary of a recent technical paper [gr-qc/0305018]. We then go
on to consider the issue of stability when the system approaches the critical
points of interest. These include the $d$-dimensional analogue of the
Hawking-Page phase transition, a phase transition that is relevant to
Reissner-Nordstrom black holes and various extremal-limiting cases.


title: Quasi-Spherical Gravitational Collapse in Any Dimension

abstract:
  We study the occurrence and nature of naked singularities for a dust model
with non-zero cosmological constant in ($n+2$)-dimensional Szekeres space-times
(which possess no Killing vectors) for $n\geq 2$. We find that central
shell-focusing singularities may be locally naked in higher dimensions but
depend sensitively on the choice of initial data. In fact, the nature of the
initial density determines the possibility of naked singularity in space-times
with more than five dimensions. The results are similar to the collapse in
spherically symmetric Tolman-Bondi-Lema\^{\i}tre space-times.


title: Isotropy of the velocity of light and the Sagnac effect

abstract:
  In this paper, it is shown, using a geometrical approach, the isotropy of the
velocity of light measured in a rotating frame in Minkowski space-time, and it
is verified that this result is compatible with the Sagnac effect. Furthermore,
we find that this problem can be reduced to the solution of geodesic triangles
in a Minkowskian cylinder. A relationship between the problems established on
the cylinder and on the Minkowskian plane is obtained through a local isometry.


title: Noncommutative Cosmology

abstract:
  We show that the cosmological large number coincidence can be interpreted as
giving the filling factor in a Landau problem. The analogy with the Landau
problem leads naturally to the noncommutativity of the gravitational and matter
degrees of freedom. We present a toy model that supports this view. We discuss
some of the physical consequences this noncommutativity implies, like a
different insight into the semiclassical approximation of quantum gravity and a
different tackling of the cosmological constant problem.


title:
Testing a theory of gravity in celestial mechanics: a new method and its
  first results for a scalar theory

abstract:
  A new method of post-Newtonian approximation (PNA) for weak gravitational
fields is presented together with its application to test an alternative,
scalar theory of gravitation. The new method consists in defining a
one-parameter family of systems, by applying a Newtonian similarity
transformation to the initial data that defines the system of interest. This
method is rigorous. Its difference with the standard PNA is emphasized. In
particular, the new method predicts that the internal structure of the bodies
does have an influence on the motion of the mass centers. The translational
equations of motion obtained with this method in the scalar theory are adjusted
in the solar system, and compared with an ephemeris based on the standard PNA
of GR.


title: Unfashionable observations about three dimensional gravity

abstract:
  It is commonly accepted that the study of 2+1 dimensional quantum gravity
could teach us something about the 3+1 dimensional case. The non-perturbative
methods developed in this case share, as basic ingredient, a reformulation of
gravity as a gauge field theory. However, these methods suffer many problems.
Firstly, this perspective abandon the non-degeneracy of the metric and
causality as fundamental principles, hoping to recover them in a certain
low-energy limit. Then, it is not clear how these combinatorial techniques
could be used in the case where matter fields are added, which are however the
essential ingredients in order to produce non trivial observables in a
generally covariant approach. Endly, considering the status of the observer in
these approaches, it is not clear at all if they really could produce a
completely covariant description of quantum gravity. We propose to re-analyse
carefully these points. This study leads us to a really covariant description
of a set of self-gravitating point masses in a closed universe. This approach
is based on a set of observables associated to the measurements accessible to a
participant-observer, they manage to capture the whole dynamic in Chern-Simons
gravity as well as in true gravity. The Dirac algebra of these observables can
be explicitely computed, and exhibits interesting algebraic features related to
Poisson-Lie groupoids theory.


title:
The Phoenix Project: Master Constraint Programme for Loop Quantum
  Gravity

abstract:
  The Hamiltonian constraint remains the major unsolved problem in Loop Quantum
Gravity (LQG). Seven years ago a mathematically consistent candidate
Hamiltonian constraint has been proposed but there are still several unsettled
questions which concern the algebra of commutators among smeared Hamiltonian
constraints which must be faced in order to make progress. In this paper we
propose a solution to this set of problems based on the so-called {\bf Master
Constraint} which combines the smeared Hamiltonian constraints for all smearing
functions into a single constraint. If certain mathematical conditions, which
still have to be proved, hold, then not only the problems with the commutator
algebra could disappear, also chances are good that one can control the
solution space and the (quantum) Dirac observables of LQG. Even a decision on
whether the theory has the correct classical limit and a connection with the
path integral (or spin foam) formulation could be in reach. While these are
exciting possibilities, we should warn the reader from the outset that, since
the proposal is, to the best of our knowledge, completely new and has been
barely tested in solvable models, there might be caveats which we are presently
unaware of and render the whole {\bf Master Constraint Programme} obsolete.
Thus, this paper should really be viewed as a proposal only, rather than a
presentation of hard results, which however we intend to supply in future
submissions.


title: On the vacuum entropy and the cosmological constant

abstract:
  It is generally accepted that the entropy of an asymptotically de Sitter
universe is bounded by the area, in Planck units, of the de Sitter horizon.
Based on an analysis of the entropy associated to the vacuum quantum
fluctuations, we suggest that the existence of such a holographic bound
constitutes a possible explanation for the observed value of the cosmological
constant, theoretically justifying a relation proposed 35 years ago by
Zel'dovich.


title: Non-minimal couplings, quantum geometry and black hole entropy

abstract:
  The black hole entropy calculation for type I isolated horizons, based on
loop quantum gravity, is extended to include non-minimally coupled scalar
fields. Although the non-minimal coupling significantly modifies quantum
geometry, the highly non-trivial consistency checks for the emergence of a
coherent description of the quantum horizon continue to be met. The resulting
expression of black hole entropy now depends also on the scalar field precisely
in the fashion predicted by the first law in the classical theory (with the
same value of the Barbero-Immirzi parameter as in the case of minimal
coupling).


title: Astronomical Tests of the Einstein Equivalence Principle

abstract:
  The Einstein equivalence principle is certainly a key element in the
development of new enhanced theories of gravity. Although being an important
building block in Einstein's general relativity, theoretically predicted
violations of its validity are a main feature in alternative, nonmetric
gravitation theories if they are able to incorporate quantum mechanical
principles.
  We have investigated the metric-affine gauge theory of gravity which predict
that a gravitational field singles out an orthogonal pair of polarization
states of light that propagate with different phase velocities. This
gravity-induced birefringence implies that propagation through a gravitational
field can alter the polarization of light and, so, violates the Einstein
equivalence principle. We use solar and white dwarf polarimetric data to
constrain birefringence induced by the gravitational field of these objects and
set limits on coupling constants required by such theories.


title: The relativistic Sagnac Effect: two derivations

abstract:
  The phase shift due to the Sagnac Effect, for relativistic matter and
electromagnetic beams, counter-propagating in a rotating interferometer, is
deduced using two different approaches. From one hand, we show that the
relativistic law of velocity addition leads to the well known Sagnac time
difference, which is the same independently of the physical nature of the
interfering beams, evidencing in this way the universality of the effect.
Another derivation is based on a formal analogy with the phase shift induced by
the magnetic potential for charged particles travelling in a region where a
constant vector potential is present: this is the so called Aharonov-Bohm
effect. Both derivations are carried out in a fully relativistic context, using
a suitable 1+3 splitting that allows us to recognize and define the space where
electromagnetic and matter waves propagate: this is an extended 3-space, which
we call "relative space". It is recognized as the only space having an actual
physical meaning from an operational point of view, and it is identified as the
'physical space of the rotating platform': the geometry of this space turns out
to be non Euclidean, according to Einstein's early intuition.


title: Reference frames and rigid motions in relativity

abstract:
  A reference frame consists of: a reference space, a time scale and a spatial
metric. The geometric structure induced by these objects in spacetime is
developed. The existence of a class of spatial metrics that are rigid, have
free mobility and can be derived as a slight deformation of the radar metric,
is shown.


title:
Generalized Chaplygin Gas Model: Dark Energy - Dark Matter Unification
  and CMBR Constraints

abstract:
  The generalized Chaplygin gas (GCG) model allows for an unified description
of the recent accelerated expansion of the Universe and the evolution of energy
density perturbations. This dark energy - dark matter unification is achieved
through an exotic background fluid whose equation of state is given by $p = -
A/\rho^{\alpha}$, where $A$ is a positive constant and $0 < \alpha \le 1$.
Stringent constraints on the model parameters can be obtained from recent WMAP
and BOOMERanG bounds on the locations of the first few peaks and troughs of the
Cosmic Microwave Background Radiation (CMBR) power spectrum as well as SNe Ia
data.


title: Quintessential Transition of the Internal Manifold

abstract:
  In this paper, I study the dynamics of the Kaluza-Klein internal manifold
using its ``shape'' and the ``size'' as collective coordinates. The essential
motivation is to be able to explain symmetry breaking in gauge theories through
a transition of the internal manifold from a symmetrical metric space to a less
symmetric one. We find that this may be possible depending on the values of
certain group theoretical parameters. Further, this transition resembles a
``quintessential inflation'' scenario, the feasibility of which however, needs
to be further studied in details.


title:
The role of the equation of state and the space-time dimension in
  spherical collapse

abstract:
  We study the spherically symmetric collapse of a fluid with non-vanishing
radial pressure in higher dimensional space-time. We obtain the general exact
solution in the closed form for the equation of state ($P_r = \gamma \rho$)
which leads to the explicit construction of the root equation governing the
nature (black hole versus naked singularity) of the central singularity. A
remarkable feature of the root equation is its invariance for the three cases:
(${D+1}, {\gamma = -1}$),
  (${D}, {\gamma = 0}$) and (${D - 1}, {\gamma = 1}$) where $D$ is the
dimension of space-time. That is, for the ultimate end result of the collapse,
$D$-dimensional dust, ${D+1}$ - AdS (anti de Sitter)-like and ${D-1}$ - dS-like
are absolutely equivalent.


title: A new form of the C-metric

abstract:
  The usual form of the C-metric has the structure function
G(\xi)=1-\xi^2-2mA\xi^3, whose cubic nature can make calculations cumbersome,
especially when explicit expressions for its roots are required. In this paper,
we propose a new form of the C-metric, with the explicitly factorisable
structure function G(\xi)=(1-\xi^2)(1+2mA\xi). Although this form is related to
the usual one by a coordinate transformation, it has the advantage that its
roots are now trivial to write down. We show that this leads to potential
simplifications, for example, when casting the C-metric in Weyl coordinates.
These results also extend to the charged C-metric, whose structure function can
be written in the new form G(\xi)=(1-\xi^2)(1+r_{+}A\xi)(1+r_{-}A\xi), where
r_{\pm} are the usual locations of the horizons in the Reissner-Nordstrom
solution. As a by-product, we explicitly cast the extremally charged C-metric
in Weyl coordinates.


title: Non-topological soliton in three-dimensional supergravity

abstract:
  We construct numerical solutions for non-topological solitons in
three-dimensional U(1)-gauged ${\cal N}=2$ supergravity. We find the region of
the solutions showing with the BTZ mass, the angular momentum and the magnetic
flux and discuss the relation among the physical parameters for various values
of a cosmological constant.


title: Comment on "Conformally flat stationary axisymmetric metrics"

abstract:
  Garcia and Campuzano claim to have found a previously overlooked family of
stationary and axisymmetric conformally flat spacetimes, contradicting an old
theorem of Collinson. In both these papers it is tacitly assumed that the
isometry group is orthogonally transitive. Under the same assumption, we point
out here that Collinson's result still holds if one demands the existence of an
axis of symmetry on which the axial Killing vector vanishes. On the other hand
if the assumption of orthogonal transitivity is dropped, a wider class of
metrics is allowed and it is possible to find explicit counterexamples to
Collinson's result.


title: The New Planck Scale: Quantized Spin and Charge Coupled to Gravity

abstract:
  In the standard approach to defining a Planck scale where gravity is brought
into the quantum domain, the Schwarzschild gravitational radius is set equal to
the Compton wavelength. However, ignored thereby are the charge and spin, the
fundamental quantized aspects of matter. The gravitational and null-surface
radii of the Kerr-Newman metric are used to introduce spin and charge into a
new extended Planck scale. The fine structure constant appears in the extended
Planck mass and the recent discovery of the $\alpha$ variation with the
evolution of the universe adds further significance. An extended Planck charge
and Planck spin are derived. There is an intriguing suggestion of a connection
with the $\alpha$ value governing high-energy radiation in Z-boson production
and decay.


title: Gravitomagnetic measurement of the angular momentum of celestial bodies

abstract:
  The asymmetry in the time delay for light rays propagating on opposite sides
of a spinning body is analyzed. A frequency shift in the perceived signals is
found. A practical procedure is proposed for evidencing the asymmetry, allowing
for a measurement of the specific angular momentum of the rotating mass. Orders
of magnitude are considered and discussed.


title: On some weak-gravitational effects

abstract:
  Some physically interesting weak-gravitational effects and phenomena are
reviewed and briefly discussed: particle geometric phases due to the
time-dependent spin-rotation couplings, non-inertial gravitational wave in
rotating reference of frame, hyperbolical geometric quantum phases and
topological dual mass as well.


title: Discrete gravity and and its continuum limit

abstract:
  Recently Gambini and Pullin proposed a new consistent discrete approach to
quantum gravity and applied it to cosmological models. One remarkable result of
this approach is that the cosmological singularity can be avoided in a general
fashion. However, whether the continuum limit of such discretized theories
exists is model dependent. In the case of massless scalar field coupled to
gravity with $\Lambda=0$, the continuum limit can only be achieved by fine
tuning the recurrence constant. We regard this failure as the implication that
cosmological constant should vary with time. For this reason we replace the
massless scalar field by Chaplygin gas which may contribute an effective
cosmological constant term with the evolution of the universe. It turns out
that the continuum limit can be reached in this case indeed.


title:
Probing the equation of state of the early universe with a space laser
  interferometer

abstract:
  We propose a method to probe the equation of state of the early universe and
its evolution, using the stochastic gravitational wave background from
inflation. A small deviation from purely radiation dominated universe ($w=
1/3$) would be clearly imprinted on the gravitational wave spectrum
  $\Omega_{GW}(f)$ due to the nearly scale invariant nature of inflationary
generated waves.


title: Simultaneity and the Concept of `Particle'

abstract:
  The history of the particle concept is briefly reviewed, with particular
emphasis on the `foliation dependence' of many particle creation models, and
the possible connection between our notion of particle and our notion of
simultaneity. It is argued that the concept of `radar time' (originally
introduced by Sir Hermann Bondi in his work on k-calculus) provides a
satisfactory concept of `simultaneity' for observers in curved spacetimes. This
is used to propose an observer-dependent particle interpretation, applicable to
an arbitrary observer, depending solely on that observers motion and not on a
choice of coordinates or gauge. This definition is illustrated with application
to non-inertial observers and simple cosmologies, demonstrating its generality
and its consistency with known cases.


title: Loss of coherence from discrete quantum gravity

abstract:
  We show that a recent proposal for the quantization of gravity based on
discrete space-time implies a modification of standard quantum mechanics that
naturally leads to a loss of coherence in quantum states of the type discussed
by Milburn. The proposal overcomes the energy conservation problem of
previously proposed decoherence mechanisms stemming from quantum gravity.
Mesoscopic quantum systems (as Bose--Einstein condensates) appear as the most
promising testing grounds for an experimental verification of the mechanism.


title: `c' is the speed of light, isn't it?

abstract:
  Theories proposing a varying speed of light have recently been widely
promoted under the claim that they offer an alternative way of solving the
standard cosmological problems. Recent observational hints that the fine
structure constant may have varied during over cosmological scales also has
given impetus to these models. In theoretical physics the speed of light, $c$,
is hidden in almost all equations but with different facets that we try to
distinguish. Together with a reminder on scalar-tensor theories of gravity,
this sheds some light on these proposed varying speed of light theories.


title:
Pseudoenergy, Superenergy, and Energy Radiation via Gravitational Waves
  in General Relativity

abstract:
  For slowly spinning matter the rate of energy loss via radiation of
gravitational waves is estimated in General Relativity (GR) within a generally
covariant superenergy approach. This estimation differs from Einstein's
Quadrupole Formula (EQF) by a suppression factor ($\Pi\ll1$). For a symmetric
two-body-like distribution of scalar matter $\Pi$ is estimated to be
$\ll(v/c)^2(r/R)^2$, where $v$ is orbital velocity of the bodies, $c$ -
velocity of light, $r$ - radius of each body, and $R$ -- the inter-body
distance. This contradiction with EQF is briefly discussed.


title:
Optimizing the third-and-a-half post-Newtonian gravitational
  radiation-reaction force for numerical simulations

abstract:
  The gravitational radiation-reaction force acting on perfect fluids at 3.5
post-Newtonian order is cast into a form which is directly applicable to
numerical simulations. Extensive use is made of metric-coefficient changes
induced by functional coordinate transformations, of the continuity equation,
as well as of the equations of motion. We also present an expression
appropriate for numerical simulations of the radiation field causing the worked
out reaction force.


title: Spacetime Embedding Diagrams for Spherically Symmetric Black Holes

abstract:
  We show that it is possible to embed the 1+1 dimensional reduction of certain
spherically symmetric black hole spacetimes into 2+1 Minkowski space. The
spacetimes of interest (Schwarzschild de-Sitter, Schwarzschild anti de-Sitter,
and Reissner-Nordstrom near the outer horizon) represent a class of metrics
whose geometries allow for such embeddings. The embedding diagrams have a
dynamic character which allows one to represent the motion of test particles.
We also analyze various features of the embedding construction, deriving the
general conditions under which our procedure provides a smooth embedding. These
conditions also yeild an embedding constant related to the surface gravity of
the relevant horizon.


title: A Quintessence Problem in Self-interacting Brans-Dicke Theory

abstract:
  A quintessence scalar field in self-interacting Brans-Dicke theory is shown
to give rise to a non-decelerated expansion of the present universe for open,
flat and closed models. Along with providing a non-decelerating solution, it
can potentially solve the flatness problem too.


title:
The Quantum Cosmological Wavefunction at Very Early Times for a
  Quadratic Gravity Theory

abstract:
  The quantum cosmological wavefunction for a quadratic gravity theory derived
from the heterotic string effective action is obtained near the inflationary
epoch and during the initial Planck era. Neglecting derivatives with respect to
the scalar field, the wavefunction would satisfy a third-order differential
equation near the inflationary epoch which has a solution that is singular in
the scale factor limit $a(t)\to 0$. When scalar field derivatives are included,
a sixth-order differential equation is obtained for the wavefunction and the
solution by Mellin transform is regular in the $a\to 0$ limit. It follows that
inclusion of the scalar field in the quadratic gravity action is necessary for
consistency of the quantum cosmology of the theory at very early times.


title: Higher Dimensional Bell-Szekeres Metric

abstract:
  The collision of pure electromagnetic plane waves with collinear polarization
in N-dimensional (N=2+n) Einstein-Maxwell theory is considered. A class of
exact solutions for the higher dimensional Bell-Szekeres metric is obtained and
its singularity structure is examined.


title:
Relativistic Arquimedes law for fast moving bodies and the
  general-relativistic resolution of the "submarine paradox"

abstract:
  We investigate and solve in the context of General Relativity the apparent
paradox which appears when bodies floating in a background fluid are set in
relativistic motion. Suppose some macroscopic body, say, a submarine designed
to lie just in equilibrium when it rests (totally) immersed in a certain
background fluid. The puzzle arises when different observers are asked to
describe what is expected to happen when the submarine is given some high
velocity parallel to the direction of the fluid surface. On the one hand,
according to observers at rest with the fluid, the submarine would contract
and, thus, sink as a consequence of the density increase. On the other hand,
mariners at rest with the submarine using an analogous reasoning for the fluid
elements would reach the opposite conclusion. The general relativistic
extension of the Arquimedes law for moving bodies shows that the submarine
sinks.


title: Vacuum solutions which cannot be written in diagonal form

abstract:
  A vacuum solution of the Einstein gravitational field equation is given that
follows from a general ansatz but fails to follow from it if a certain
symmetric matrix is assumed to be in diagonal form from the beginning.


title:
Influence of general convective motions on the exterior of isolated
  rotating bodies in equilibrium

abstract:
  The problem of describing isolated rotating bodies in equilibrium in General
Relativity has so far been treated under the assumption of the circularity
condition in the interior of the body. For a fluid without energy flux, this
condition implies that the fluid flow moves only along the angular direction,
i.e. there is no convection. Using this simplification, some recent studies
have provided us with uniqueness and existence results for asymptotically flat
vacuum exterior fields given the interior sources. Here, the generalisation of
the problem to include general sources is studied. It is proven that the
convective motions have no direct influence on the exterior field, and hence,
that the aforementioned results on uniqueness and existence of exterior fields
apply equally in the general case.


title:
Axial Torsion-Dirac spin Effect in Rotating Frame with Relativistic
  Factor

abstract:
  In the framework of spacetime with torsion and without curvature, the Dirac
particle spin precession in the rotational system is studied. We write out the
equivalent tetrad of rotating frame, in the polar coordinate system, through
considering the relativistic factor, and the resultant equivalent metric is a
flat Minkowski one. The obtained rotation-spin coupling formula can be applied
to the high speed rotating case, which is consistent with the expectation.


title: Kinematics of discretely self-similar spherically symmetric spacetimes

abstract:
  We summarize the consequences of the twin assumptions of (discrete)
self-similarity and spherical symmetry for the global structure of a spacetime.
All such spacetimes can be constructed from two building blocks, the "fan" and
"splash". Each building block contains one radial null geodesic that is
invariant under the self-similarity (self-similarity horizon).


title: On the gravitational collapse in anti-de Sitter space-time

abstract:
  We study the semiclassical evolution of a self-gravitating thick shell in
Anti-de Sitter space-time. We treat the matter on the shell as made of
quantized bosons and evaluate the back-reaction of the loss of gravitational
energy which is radiated away as a non-adiabatic effect. A peculiar feature of
anti-de Sitter is that such an emission also occurs for large shell radius,
contrary to the asymptotically flat case.


title: Quantum Mechanics of Klein-Gordon-Type Fields and Quantum Cosmology

abstract:
  We formulate the quantum mechanics of the solutions of a Klein-Gordon-type
field equation: (\partial_t^2+D)\psi(t)=0, where D is a positive-definite
operator acting in a Hilbert space \tilde H. We determine all the
positive-definite inner products on the space H of the solutions of such an
equation and establish their physical equivalence. We use a simple realization
of the unique Hilbert space structure on H to construct the observables of the
theory explicitly. In general, there are infinitely many choices for the
Hamiltonian each leading to a different notion of time-evolution in H. Among
these is a particular choice that generates t-translations in H and identifies
t with time whenever D is t-independent. For a t-dependent D, we show that
t-translations never correspond to unitary evolutions in H, and t cannot be
identified with time. We apply these ideas to develop a formulation of quantum
cosmology based on the Wheeler-DeWitt equation for a Friedman-Robertson-Walker
model coupled to a real scalar field. We solve the Hilbert space problem,
construct the observables, introduce a new set of the wave functions of the
universe, reformulate the quantum theory in terms of the latter, reduce the
problem of time to the determination of a Hamiltonian operator acting in
L^2(\R)\oplus L^2(\R), show that the factor-ordering problem is irrelevant for
the kinematics of the quantum theory, and propose a formulation of the
dynamics. Our method allows for a genuine probabilistic interpretation and a
unitary Schreodinger time-evolution.


title:
A note on 4-dimensional traversable wormholes and energy conditions in
  higher dimensions

abstract:
  We show explicitly that traversable wormholes requiring exotic matter in
4-dimensions nevertheless have acceptable stress-tensors obeying reasonable
energy conditions in higher dimensions if the wormholes are regarded as being
embedded in higher dimensional space-times satisfying Einstein's field
equations. From the 4-dimensional perspective, the existence of higher
dimensions may thus facilitate wormhole and time-machine constructions through
access to "exotic matter".


title:
Active gravitational mass and the invariant characterization of
  Reissner-Nordstrom spacetime

abstract:
  We analyse the concept of active gravitational mass for Reissner-Nordstrom
spacetime in terms of scalar polynomial invariants and the Karlhede
classification. We show that while the Kretschmann scalar does not produce the
expected expression for the active gravitational mass, both scalar polynomial
invariants formed from the Weyl tensor, and the Cartan scalars, do.


title: Finding apparent horizons and other two-surfaces of constant expansion

abstract:
  Apparent horizons are structures of spacelike hypersurfaces that can be
determined locally in time. Closed surfaces of constant expansion (CE surfaces)
are a generalisation of apparent horizons. I present an efficient method for
locating CE surfaces. This method uses an explicit representation of the
surface, allowing for arbitrary resolutions and, in principle, shapes. The CE
surface equation is then solved as a nonlinear elliptic equation.
  It is reasonable to assume that CE surfaces foliate a spacelike hypersurface
outside of some interior region, thus defining an invariant (but still
slicing-dependent) radial coordinate. This can be used to determine gauge modes
and to compare time evolutions with different gauge conditions. CE surfaces
also provide an efficient way to find new apparent horizons as they appear e.g.
in binary black hole simulations.


title: Schroedinger's cat and the clock: Lessons for quantum gravity

abstract:
  I review basic principles of the quantum mechanical measurement process in
view of their implications for a quantum theory of general relativity. It turns
out that a clock as an external classical device associated with the observer
plays an essential role. This leads me to postulate a ``principle of the
integrity of the observer''. It essentially requires the observer to be part of
a classical domain connected throughout the measurement process. Mathematically
this naturally leads to a formulation of quantum mechanics as a kind of
topological quantum field theory. Significantly, quantities with a direct
interpretation in terms of a measurement process are associated only with
amplitudes for connected boundaries of compact regions of space-time. I discuss
some implications of my proposal such as in-out duality for states,
delocalization of the ``collapse of the wave function'' and locality of the
description. Differences to existing approaches to quantum gravity are also
highlighted.


title: Cosmological applications of loop quantum gravity

abstract:
  After a brief introduction to classical and quantum gravity we discuss
applications of loop quantum gravity in the cosmological realm. This includes
the basic formalism and recent results of loop quantum cosmology, and a
computation of modified dispersion relations for quantum gravity phenomenology.
The presentation is held at a level which does not require much background
knowledge in general relativity or mathematical techniques such as functional
analysis, so as to make the article accessible to graduate students and
researchers from other fields.


title:
Cylindrically symmetric non-aligned Einstein-Maxwell solutions with
  rotation and pseudorotation of the types O and N

abstract:
  A new solution to the Einstein-Maxwell field equations is presented
describing a cylindrically symmetric homogeneous cosmology. The solution is
conformally flat, it possesses seven Killing vectors of which the timelike one
is rotating and one of the spacelike, pseudorotating. Our solution also admits
a Kerr-Schild form. It is alternatively produced by different electromagnetic
sources some of which represent constant null electromagnetic fields, while the
others, a circularly polarized plane electromagnetic wave (seemingly, a unique
situation in general relativity). The concrete electromagnetic four-potentials
are found from the assumption that they are proportional to the Killing
covectors. The general solution is obtained for timelike and null geodesics.
Finally, we find that this space-time admits closed timelike non-geodesic
lines.


title:
Gyromagnetic ratio of rapidly rotating compact stars in general
  relativity

abstract:
  We numerically calculate equilibrium configurations of uniformly rotating and
charged neutron stars, in the case of insulating material and neglecting the
electromagnetic forces acting on the equilibrium of the fluid. This allows us
to study the behaviour of the gyromagnetic ratio for those objects, when
varying rotation rate and equation of state for the matter. Under the
assumption of low charge and incompressible fluid, we find that the
gyromagnetic ratio is directly proportional to the compaction parameter M/R of
the star, and very little dependent on its angular velocity. Nevertheless, it
seems impossible to have g=2 for these models with low charge-to-mass ratio,
where matter consists of a perfect fluid and where the collapse limit is never
reached.


title:
Extending the first-order post-Newtonian scheme in multiple systems to
  the second-order contributions to light propagation

abstract:
  In this paper, we extend the first-order post-Newtonian scheme in multiple
systems presented by Damour-Soffel-Xu to the second-order contribution to light
propagation without changing the virtueof the scheme on the linear partial
differential equations of the potential and vector potential. The spatial
components of the metric are extended to second order level both in a global
coordinates ($q_{ij}/ c^4$) and a local coordinates ($Q_{ab}/ c^4$). The
equations of $q_{ij}$ (or $Q_{ab}$) are obtained from the field equations.The
relationship between $q_{ij}$ and $Q_{ab}$ are presented in this paper also. In
special case of the solar system (isotropic condition is applied ($q_{ij} =
\delta_{ij} q $)), we obtain the solution of $q$. Finally, a further extension
of the second-order contributions in the parametrized post-Newtonian formalism
is discussed.


title: Generating $G_2$--cosmologies with perfect fluid in dilaton gravity

abstract:
  We present a method for generating exact diagonal $G_2$-cosmological
solutions in dilaton gravity coupled to a radiation perfect fluid and with a
cosmological potential of a special type. The method is based on the symmetry
group of the system of $G_2$-field equations. Several new classes of explicit
exact inhomogeneous perfect fluid scalar-tensor cosmologies are presented.


title: Electromagnetic Generators and Detectors of Gravitational Waves

abstract:
  The renewed serious interest to possible practical applications of
gravitational waves is encouraging. Building on previous work, I am arguing
that the strong variable electromagnetic fields are appropriate systems for the
generation and detection of high-frequency gravitational waves (HFGW). The
advantages of electromagnetic systems are clearly seen in the proposed complete
laboratory experiment, where one has to ensure the efficiency of, both, the
process of generation and the process of detection of HFGW. Within the family
of electromagnetic systems, one still has a great variety of possible
geometrical configurations, classical and quantum states of the electromagnetic
field, detection strategies, etc. According to evaluations performed 30 years
ago, the gap between the HFGW laboratory signal and its level of detectability
is at least 4 orders of magnitude. Hopefully, new technologies of today can
remove this gap and can make the laboratory experiment feasible. The laboratory
experiment is bound to be expensive, but one should remember that a part of the
cost is likely to be reimbursed from the Nobel prize money ! Electromagnetic
systems seem also appropriate for the detection of high-frequency end of the
spectrum of relic gravitational waves. Although the current effort to observe
the stochastic background of relic gravitational waves is focused on the
opposite, very low-frequency, end of the spectrum, it would be extremely
valuable for fundamental science to detect, or put sensible upper limits on,
the high-frequency relic gravitational waves. I will briefly discuss the origin
of relic gravitational waves, the expected level of their high-frequency
signal, and the existing estimates of its detectability.


title:
Relativistic theory of elastic deformable astronomical bodies:
  perturbation equations in rotating spherical coordinates and junction
  conditions

abstract:
  In this paper, the dynamical equations and junction conditions at the
interface between adjacent layers of different elastic properties for an
elastic deformable astronomical body in the first post-Newtonian approximation
of Einstein theory of gravity are discussed in both rotating Cartesian
coordinates and rotating spherical coordinates. The unperturbed rotating body
(the ground state) is described as uniformly rotating, stationary and
axisymmetric configuration in an asymptotically flat space-time manifold.
Deviations from the equilibrium configuration are described by means of a
displacement field. In terms of the formalism of relativistic celestial
mechanics developed by Damour, Soffel and Xu, and the framework established by
Carter and Quintana the post Newtonian equations of the displacement field and
the symmetric trace-free shear tensor are obtained. Corresponding
post-Newtonian junction conditions at interfaces also the outer surface
boundary conditions are presented. The PN junction condition is an extension of
Wahr's one which is a Newtonian junction conditions without rotating.


title: Post Newtonian Rigid Body

abstract:
  In this paper, it is the first time to construct a complete post-Newtonian
(PN) model of a rigid body by means of a new constraint on the mass current
density and mass density. In our PN rigid body model most of relations, such as
spin vector proportional to the angular velocity, the definition on the moment
of inertia tensor, the key relation between the mass quadrupole moment and the
moment of inertia tensor, rigid rotating formulae of mass quadrupole moment and
the moment of inertia tensor, are just the extension of the main relations in
Newtonian rigid body model. When all of $1/c^2$ terms are neglected, the PN
rigid body model and the corresponding formulae reduce to Newtonian version.
The key relation is obtained in this paper for the first time, which might be
very useful in the future application to problems in geodynamics and astronomy.


title: Gravitational lensing by a charged black hole of string theory

abstract:
  We study gravitational lensing by the
Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) charged black hole of
heterotic string theory and obtain the angular position and magnification of
the relativistic images. Modeling the supermassive central object of the galaxy
as a GMGHS black hole, we estimate the numerical values of different
strong-lensing parameters. We find that there is no significant string effect
present in the lensing observables in the strong-gravity scenario.


title: Wormhole solutions in the Randall-Sundrum scenario

abstract:
  In the simplest form of the Randall-Sundrum model, we consider the metric
generated by a static, spherically symmetric distribution of matter on the
physical brane. The solution to the five-dimensional Einstein equations,
obtained numerically, describes a wormhole geometry.


title: Scalar-tensor theories and cosmology

abstract:
  Scalar-tensor theories are the best motivated alternatives to general
relativity and provide a mathematically consistent framework to test the
various observable predictions. They can involve three functions of the scalar
field: (i) a potential (as in "quintessence" models), (ii) a matter-scalar
coupling function (as in "extended quintessence", where it may also be
rewritten as a nonminimal coupling of the scalar field to the scalar
curvature), and (iii) a coupling function of the scalar field to the
Gauss-Bonnet topological invariant. We recall the main experimental constraints
on this class of theories, and underline that solar-system, binary-pulsar, and
cosmological observations give qualitatively different tests. We finally show
that the combination of these data is necessary to constrain the existence of a
scalar-Gauss-Bonnet coupling.


title:
Quantum-gravity-motivated Lorentz-symmetry tests with laser
  interferometers

abstract:
  We consider the implications for laser interferometry of the
quantum-gravity-motivated modifications in the laws of particle propagation,
which are presently being considered in attempts to explain puzzling
observations of ultra-high-energy cosmic rays. We show that there are
interferometric setups in which the Planck-scale effect on propagation leads to
a characteristic signature. A naive estimate is encouraging with respect to the
possibility of achieving Planck-scale sensitivity, but we also point out some
severe technological challenges which would have to be overcome in order to
achieve this sensitivity.


title:
Gauge conditions for binary black hole puncture data based on an
  approximate helical Killing vector

abstract:
  We show that puncture data for quasicircular binary black hole orbits allow a
special gauge choice that realizes some of the necessary conditions for the
existence of an approximate helical Killing vector field. Introducing free
parameters for the lapse at the punctures we can satisfy the condition that the
Komar and ADM mass agree at spatial infinity. Several other conditions for an
approximate Killing vector are then automatically satisfied, and the 3-metric
evolves on a timescale smaller than the orbital timescale. The time derivative
of the extrinsic curvature however remains significant. Nevertheless,
quasicircular puncture data are not as far from possessing a helical Killing
vector as one might have expected.


title:
Constraints on non-minimally coupled curved space electrodynamics from
  astrophysical observations

abstract:
  We study interactions of electro-magnetic fields with the curvature tensor of
the form $\lambda R_{\mu \nu \alpha \beta}F^{\mu \nu}F^{\alpha \beta}$. Such
coupling terms though are invariant under general coordinate transformation and
CPT, however violate the Einstein equivalence principle. These couplings do not
cause any energy dependent dispersion of photons but they exhibit
birefringence. We put constraints on the coupling constant $\lambda$ using
results from solar system radar ranging experiments and millisecond-pulsar
observations. We find that the most stringent constraint comes from pulsar
observations and is given by $ \lambda < 10^{11} cm^2 $ obtained from the
timing of binary pulsar PSR B1534+12.


title: Considerations on the Unruh Effect: Causality and Regularization

abstract:
  This article is motivated by the observation, that calculations of the Unruh
effect based on idealized particle detectors are usually made in a way that
involves integrations along the {\em entire} detector trajectory up to the
infinitely remote {\em future}. We derive an expression which allows
time-dependence of the detector response in the case of a non-stationary
trajectory and conforms more explicitely to the principle of causality, namely
that the response at a given instant of time depends only on the detectors {\em
past} movements. On trying to reproduce the thermal Unruh spectrum we are led
to an unphysical result, which we trace down to the use of the standard
regularization $t\to t-i\eps$ of the correlation function. By consistently
employing a rigid detector of finite extension, we are led to a different
regularization which works fine with our causal response function.


title: Varying Constants

abstract:
  We review some string-inspired theoretical models which incorporate a
correlated spacetime variation of coupling constants while remaining naturally
compatible both with phenomenological constraints coming from geochemical data
(Oklo; Rhenium decay) and with present equivalence principle tests. Barring
unnatural fine-tunings of parameters, a variation of the fine-structure
constant as large as that recently ``observed'' by Webb et al. in quasar
absorption spectra appears to be incompatible with these phenomenological
constraints. Independently of any model, it is emphasized that the best
experimental probe of varying constants are high-precision tests of the
universality of free fall, such as MICROSCOPE and STEP. Recent claims by
Bekenstein that fine-structure-constant variability does not imply detectable
violations of the equivalence principle are shown to be untenable.


title: Pioneer Anomaly and the Helicity-Rotation Coupling

abstract:
  The modification of the Doppler effect due to the coupling of the helicity of
the radiation with the rotation of the source/receiver is considered in the
case of the Pioneer 10/11 spacecraft. We explain why the Pioneer anomaly is not
influenced by the helicity-rotation coupling.


title:
Point-particle limit in a scalar theory of gravitation and the weak
  equivalence principle

abstract:
  A scalar theory with a preferred reference frame is summarized. To test that
theory in celestial mechanics, an "asymptotic" post-Newtonian (PN) scheme has
been developed. This associates a conceptual family of self-gravitating systems
with the given system, in order to have a true small parameter available. The
resulting equations for a weakly-self-gravitating system of extended bodies
include internal-structure effects. The internal-structure influence subsists
at the point-particle limit--a violation of the weak equivalence principle. If
one could develop an "asymptotic" approximation scheme in general relativity
also, this could plausibly be found there also, in a gauge where the PN space
metric would not be "conformally Euclidean".


title: On stationary metrics in five dimensions

abstract:
  It is well-known that the Kerr-metric (rotating black hole in four
dimensions) has Petrov type D. We prove a similar property in five dimensions.
The Myers-Perry metric (rotating black hole in five dimensions) with one
non-zero angular momentum has Petrov type \underline{22}. Conversely, we show
that the Myers-Perry solution is unique within a certain restricted class of
metrics of Petrov type \underline{22}.


title: Non-Linear N-Parameter Spacetime Perturbations: Gauge Transformations

abstract:
  We introduce N-parameter perturbation theory as a new tool for the study of
non-linear relativistic phenomena. The main ingredient in this formulation is
the use of the Baker-Campbell-Hausdorff formula. The associated machinery
allows us to prove the main results concerning the consistency of the scheme to
any perturbative order. Gauge transformations and conditions for gauge
invariance at any required order can then be derived from a generating
exponential formula via a simple Taylor expansion. We outline the relation
between our novel formulation and previous developments.


title:
Can a combination of the conformal thin-sandwich and puncture methods
  yield binary black hole solutions in quasi-equilibrium?

abstract:
  We consider combining two important methods for constructing
quasi-equilibrium initial data for binary black holes: the conformal
thin-sandwich formalism and the puncture method. The former seeks to enforce
stationarity in the conformal three-metric and the latter attempts to avoid
internal boundaries, like minimal surfaces or apparent horizons. We show that
these two methods make partially conflicting requirements on the boundary
conditions that determine the time slices. In particular, it does not seem
possible to construct slices that are quasi-stationary and avoid physical
singularities and simultaneously are connected by an everywhere positive lapse
function, a condition which must obtain if internal boundaries are to be
avoided. Some relaxation of these conflicting requirements may yield a soluble
system, but some of the advantages that were sought in combining these
approaches will be lost.


title: Einstein-Cartan theory as a theory of defects in space-time

abstract:
  The Einstein-Cartan theory of gravitation and the classical theory of defects
in an elastic medium are presented and compared. The former is an extension of
general relativity and refers to four-dimensional space-time, while we
introduce the latter as a description of the equilibrium state of a
three-dimensional continuum. Despite these important differences, an analogy is
built on their common geometrical foundations, and it is shown that a
space-time with curvature and torsion can be considered as a state of a
four-dimensional continuum containing defects. This formal analogy is useful
for illustrating the geometrical concept of torsion by applying it to concrete
physical problems. Moreover, the presentation of these theories using a common
geometrical basis allows a deeper understanding of their foundations.


title: Is the semi-classical analysis valid for extreme black holes?

abstract:
  The surface gravity for the extreme Reissner-Nordstr\"om black hole is zero
suggesting that it has a zero temperature. However, the direct evaluation of
the Bogolubov's coefficients, using the standard semi-classical analysis,
indicates that the temperature of the extreme black hole is ill definite: the
Bogolubov's coefficients obtained by performing the usual analysis of a
collapsing model of a thin shell, and employing the geometrical optical
approximation, do not obey the normalization conditions. We argue that the
failure of the employement of semi-classical analysis for the extreme black
hole is due to the absence of orthonormal quantum modes in the vicinity of the
event horizon in this particular case.


title: A Diagram for Bianchi A-Types

abstract:
  A diagram for Bianchi spaces with vanishing vector of structure constants
(type A in the Ellis-MacCallum classification) illustrates the relations among
their different types under similarity transformations. The Ricci coefficients
and the Ricci tensor are related by a Cremona transformation.


title: Newton's law on an Einstein "Gauss-Bonnet" brane

abstract:
  It is known that Newton's law of gravity holds asymptotically on a flat
"brane" embedded in an anti-de Sitter "bulk" ; this was shown not only when
gravity in the bulk is described by Einstein's theory but also in Einstein
"Lanczos Lovelock Gauss-Bonnet"'s theory. We give here the expressions for the
corrections to Newton's potential in both theories, in analytic form and valid
for all distances. We find that in Einstein's theory the transition from the
1/r behaviour at small r to the 1/r^2 one at large r is quite slow. In the
Einstein Gauss-Bonnet case on the other hand, we find that the correction to
Newton's potential can be small for all r. Hence, Einstein Gauss-Bonnet
equations in the bulk (rather than simply Einstein's) induce on the brane a
better approximation to Newton's law.


title: Aproximacion Discreta de la Relatividad General

abstract:
  These Lecture notes give an introduction to Regge calculus as a discrete
model of General Relativity.


title:
General relativity histories theory I: The spacetime character of the
  canonical description

abstract:
  The problem of time in canonical quantum gravity is related to the fact that
the canonical description is based on the prior choice of a spacelike
foliation, hence making a reference to a spacetime metric. However, the metric
is expected to be a dynamical, fluctuating quantity in quantum gravity. We show
how this problem can be solved in the histories formulation of general
relativity. We implement the 3+1 decomposition using metric-dependent
foliations which remain spacelike with respect to all possible Lorentzian
metrics. This allows us to find an explicit relation of covariant and canonical
quantities which preserves the spacetime character of the canonical
description. In this new construction, we also have a coexistence of the
spacetime diffeomorphisms group, and the Dirac algebra of constraints.


title:
Generally covariant theories: the Noether obstruction for realizing
  certain space-time diffeomorphisms in phase space

abstract:
  Relying on known results of the Noether theory of symmetries extended to
constrained systems, it is shown that there exists an obstruction that prevents
certain tangent-space diffeomorphisms to be projectable to phase-space, for
generally covariant theories. This main result throws new light on the old fact
that the algebra of gauge generators in the phase space of General Relativity,
or other generally covariant theories, only closes as a soft algebra and not a
a Lie algebra.
  The deep relationship between these two issues is clarified. In particular,
we see that the second one may be understood as a side effect of the procedure
to solve the first. It is explicitly shown how the adoption of specific
metric-dependent diffeomorphisms, as a way to achieve projectability, causes
the algebra of gauge generators (constraints) in phase space not to be a Lie
algebra --with structure constants-- but a soft algebra --with structure {\it
functions}.


title: General relativity histories theory II: Invariance groups

abstract:
  We show in detail how the histories description of general relativity carries
representations of both the spacetime diffeomorphisms group and the Dirac
algebra of constraints. We show that the introduction of metric-dependent
equivariant foliations leads to the crucial result that the canonical
constraints are invariant under the action of spacetime diffeomorphisms.
Furthermore, there exists a representation of the group of generalised
spacetime mappings that are functionals of the four-metric: this is a spacetime
analogue of the group originally defined by Bergmann and Komar in the context
of the canonical formulation of general relativity. Finally, we discuss the
possible directions for the quantization of gravity in histories theory.


title:
Parameterization of the Equation of State and the expansion history of
  the very recent universe

abstract:
  Motivated by a nice result shown by E. Linder, a detailed discussion on the
choice of the expansion parameters of the Maclaurin series for the equation of
states of a perfect fluid is presented in this paper. We show that their nice
recent result is in fact a linear approximation to the full Maclaurin series as
a power series of the parameter $y=z/(1+z)$. The power series for the energy
density function, the Hubble parameter and related physical quantities of
interest are also shown in this paper. The method presented here will have
significant application in the precision distance-redshift observations aiming
to map out the recent expansion history of the universe. In addition, a
complete analysis of all known advantageous parameterizations for the equation
of states to high redshift is also presented.


title:
Bounds on the interior geometry and pressure profile of static fluid
  spheres

abstract:
  It is a famous result of relativistic stellar structure that (under mild
technical conditions) a static fluid sphere satisfies the Buchdahl--Bondi bound
2M/R <= 8/9; the surprise here being that the bound is not 2M/R <= 1. In this
article we provide further generalizations of this bound by placing a number of
constraints on the interior geometry (the metric components), on the local
acceleration due to gravity, on various combinations of the internal density
and pressure profiles, and on the internal compactness 2m(r)/r of static fluid
spheres. We do this by adapting the standard tool of comparing the generic
fluid sphere with a Schwarzschild interior geometry of the same mass and radius
-- in particular we obtain several results for the pressure profile (not merely
the central pressure) that are considerably more subtle than might first be
expected.


title:
Some Inhomogeneous Magnetized Viscous Fluid Cosmological Models with
  Varying $\Lambda$

abstract:
  Some cylindrically symmetric inhomogeneous viscous fluid cosmological models
with electro-magnetic field are obtained. To get a solution a supplementary
condition between metric potentials is used. The viscosity coefficient of bulk
viscous fluid is assumed to be a power function of mass density. Without
assumin g any {\it ad hoc} law, we obtain a cosmological constant as a
decreasing function of time. The behaviour of the electro-magnetic field tensor
together with some p hysical aspects of the model are also discussed.


title: Quintessence Problem and Brans-Dicke Theory

abstract:
  It has been shown that Brans-Dicke (BD) theory in anisotropic cosmological
model can alone solve the quintessence problem and we have accelerated
expanding universe without any quintessence matter. Also the flatness problem
has been discussed in this context.