title: High-order gauge-invariant perturbations of a spherical spacetime

abstract:
  We complete the formulation of a general framework for the analysis of
high-order nonspherical perturbations of a four-dimensional spherical spacetime
by including a gauge-invariant description of the perturbations. We present a
general algorithm to construct these invariants and provide explicit formulas
for the case of second-order metric perturbations. We show that the well-known
problem of lack of invariance for the first-order perturbations with l=0,1
propagates to increasing values of l for perturbations of higher order, owing
to mode coupling. We also discuss in which circumstances it is possible to
construct the invariants.


title:
Quasinormal ringing of acoustic black holes in Laval nozzles: Numerical
  simulations

abstract:
  Quasinormal ringing of acoustic black holes in Laval nozzles is discussed.
The equation for sounds in a transonic flow is written into a
Schr\"{o}dinger-type equation with a potential barrier, and the quasinormal
frequencies are calculated semianalytically. From the results of numerical
simulations, it is shown that the quasinormal modes are actually excited when
the transonic flow is formed or slightly perturbed, as well as in the real
black hole case. In an actual experiment, however, the purely-outgoing boundary
condition will not be satisfied at late times due to the wave reflection at the
end of the apparatus, and a late-time ringing will be expressed as a
superposition of "boxed" quasinormal modes. It is shown that the late-time
ringing damps more slowly than the ordinary quasinormal ringing, while its
central frequency is not greatly different from that of the ordinary one. Using
this fact, an efficient way for experimentally detecting the quasinormal
ringing of an acoustic black hole is discussed.


title: Quasinormal mode characterization of evaporating mini black holes

abstract:
  According to recent theoretical developments, it might be possible to produce
mini black holes in the high energy experiments in the LHC at CERN. We propose
here a model based on the $n$-dimensional Vaidya metric in double null
coordinates for these decaying black holes. The associated quasinormal modes
are considered. It is shown that only in the very last instants of the
evaporation process the stationary regime for the quasinormal modes is broken,
implying specific power spectra for the perturbations around these mini
black-holes. From scattered fields one could recover, in principle, the black
hole parameters as well as the number of extra dimensions. The still mysterious
final fate of such objects should not alter significantly our main conclusions.


title:
Black holes and Rindler superspace: classical singularity and quantum
  unitarity

abstract:
  Canonical quantization of spherically symmetric initial data which is
appropriate to classical interior black hole solutions in four dimensions is
carried out and solved exactly without gauge fixing the remaining kinematic
Gauss Law constraint. The resultant mini-superspace manifold and arena for
quantum geometrodynamics is two-dimensional, of signature (+, -), non-singular,
and can in fact be identified precisely with the first Rindler wedge. The
associated Wheeler-DeWitt equation with evolution in intrinsic superspace time
can be formulated as a free massive Klein-Gordon equation; and the
Hamilton-Jacobi semiclassical limit of plane wave solutions can be matched
precisely to the interiors of Schwarzschild black holes. Furthermore, classical
black hole horizons and singularities correspond to the boundaries of the
Rindler wedge. Exact wavefunctions of the first-order-in-superspace intrinsic
time Dirac equation are also considered. Precise correspondence between
Schwarzschild black holes and free particle mechanics in superspace is noted.
Another intriguing observation is that, despite the presence of classical
horizons and singularities, hermiticity of the Dirac hamiltonian operator for
the mini-superspace, and thus unitarity of the quantum theory, is equivalent to
an appropriate boundary condition which must be satisfied by the quantum
states. This boundary condition holds for quite generic quantum wavepackets of
energy eigenstates, but fails for the usual Rindler fermion modes which are
precisely eigenstates with zero uncertainty in energy.


title:
The Chevreton Tensor and Einstein-Maxwell Spacetimes Conformal to
  Einstein Spaces

abstract:
  In this paper we characterize the source-free Einstein-Maxwell spacetimes
which have a trace-free Chevreton tensor. We show that this is equivalent to
the Chevreton tensor being of pure-radiation type and that it restricts the
spacetimes to Petrov types \textbf{N} or \textbf{O}. We prove that the trace of
the Chevreton tensor is related to the Bach tensor and use this to find all
Einstein-Maxwell spacetimes with a zero cosmological constant that have a
vanishing Bach tensor. Among these spacetimes we then look for those which are
conformal to Einstein spaces. We find that the electromagnetic field and the
Weyl tensor must be aligned, and in the case that the electromagnetic field is
null, the spacetime must be conformally Ricci-flat and all such solutions are
known. In the non-null case, since the general solution is not known on closed
form, we settle with giving the integrability conditions in the general case,
but we do give new explicit examples of Einstein-Maxwell spacetimes that are
conformal to Einstein spaces, and we also find examples where the vanishing of
the Bach tensor does not imply that the spacetime is conformal to a $C$-space.
The non-aligned Einstein-Maxwell spacetimes with vanishing Bach tensor are
conformally $C$-spaces, but none of them are conformal to Einstein spaces.


title: Three-geometry and reformulation of the Wheeler-DeWitt equation

abstract:
  A reformulation of the Wheeler-DeWitt equation which highlights the role of
gauge-invariant three-geometry elements is presented. It is noted that the
classical super-Hamiltonian of four-dimensional gravity as simplified by
Ashtekar through the use of gauge potential and densitized triad variables can
furthermore be succinctly expressed as a vanishing Poisson bracket involving
three-geometry elements. This is discussed in the general setting of the
Barbero extension of the theory with arbitrary non-vanishing value of the
Immirzi parameter, and when a cosmological constant is also present. A proposed
quantum constraint of density weight two which is polynomial in the basic
conjugate variables is also demonstrated to correspond to a precise simple
ordering of the operators, and may thus help to resolve the factor ordering
ambiguity in the extrapolation from classical to quantum gravity. Alternative
expression of a density weight one quantum constraint which may be more useful
in the spin network context is also discussed, but this constraint is
non-polynomial and is not motivated by factor ordering. The article also
highlights the fact that while the volume operator has become a preeminient
object in the current manifestation of loop quantum gravity, the volume element
and the Chern-Simons functional can be of equal significance, and need not be
mutually exclusive. Both these fundamental objects appear explicitly in the
reformulation of the Wheeler-DeWitt constraint.


title: Binary black hole mergers: large kicks for generic spin orientations

abstract:
  We present results from several simulations of equal mass black holes with
spin. The spin magnitudes are $S/m^2=0.8$ in all cases, but we vary the spin
orientations arbitrarily, in and outside the orbital plane. We find that in all
but one case the final merged black hole acquires a kick of more than 1000
km/s, indicating that kicks of this magnitude are likely to be generic and
should be expected for mergers with general spin orientations. The maximum kick
velocity we find is 2500 km/s and occurs for initial spins which are
anti-aligned in the initial orbital plane.


title:
The quadratic spinor Lagrangian, axial torsion current, and
  generalizations

abstract:
  We show that the Einstein-Hilbert, the Einstein-Palatini, and the Holst
actions can be derived from the Quadratic Spinor Lagrangian (QSL), when the
three classes of Dirac spinor fields, under Lounesto spinor field
classification, are considered. To each one of these classes, there corresponds
a unique kind of action for a covariant gravity theory. In other words, it is
shown to exist a one-to-one correspondence between the three classes of
non-equivalent solutions of the Dirac equation, and Einstein-Hilbert,
Einstein-Palatini, and Holst actions. Furthermore, it arises naturally, from
Lounesto spinor field classification, that any other class of spinor field
(Weyl, Majorana, flagpole, or flag-dipole spinor fields) yields a trivial
(zero) QSL, up to a boundary term. To investigate this boundary term we do not
impose any constraint on the Dirac spinor field, and consequently we obtain new
terms in the boundary component of the QSL. In the particular case of a
teleparallel connection, an axial torsion 1-form current density is obtained.
New terms are also obtained in the corresponding Hamiltonian formalism. We then
discuss how these new terms could shed new light on more general
investigations.


title: Scalar graviton and the modified black holes

abstract:
  Under the explicit violation of the general covariance to the unimodular one,
the effect of the emerging scalar graviton on the static spherically symmetric
metrics is studied. The main results are three-fold. First, there appears the
two-parametric family of such metrics, instead of the one-parametric black-hole
family in General Relativity (GR). Second, there may exist the one-parametric
subfamily describing a pure gravitational object, the ``graviball'', missing in
GR. Third, in a simplifying assumption, all the metrics possess the correct
Newton's limit as in GR.


title:
Snyder's Model -- de Sitter Special Relativity Duality and de Sitter
  Gravity

abstract:
  Between Snyder's quantized space-time model in de Sitter space of momenta and
the \dS special relativity on \dS-spacetime of radius $R$ with Beltrami
coordinates, there is a one-to-one dual correspondence supported by a minimum
uncertainty-like argument. Together with Planck length $\ell_P$, $R\simeq
(3/\Lambda)^{1/2}$ should be a fundamental constant. They lead to a
dimensionless constant $g{\sim\ell_PR^{-1}}=(G\hbar c^{-3}\Lambda/3)^{1/2}\sim
10^{-61}$. These indicate that physics at these two scales should be dual to
each other and there is in-between gravity of local \dS-invariance
characterized by $g$. A simple model of \dS-gravity with a gauge-like action on
umbilical manifolds may show these characters. It can pass the observation
tests and support the duality.


title:
Initial Singularity, Lambda-Problem and Crossing the Phantom Divide in
  Scale Invariant TMT Model

abstract:
  In the framework of the scale invariant model of the Two Measures Field
Theory (TMT), we study the dilaton-gravity sector in the context of spatially
flat FRW cosmology. The scale invariance is spontaneously broken due to the
intrinsic features of the TMT dynamics. If no fine tuning is made, the
effective $\phi$-Lagrangian $p(\phi,X)$ depends quadratically upon the kinetic
term $X$. Hence TMT represents an explicit example of the effective k-essence
resulting from first principles without any exotic term in the underlying
action intended for obtaining this result. Depending of the choice of regions
in the parameter space (but without fine tuning), TMT exhibits interesting
outputs for cosmological dynamics, for example: a) Absence of initial
singularity of the curvature while its time derivative is singular. This is a
sort of "sudden" singularities studied by Barrow on purely kinematic grounds.
b) Power law inflation in the subsequent stage of evolution which ends with a
graceful exit into the state with zero cosmological constant (CC). c)
Possibility of resolution of the old CC problem. From the point of view of TMT,
it becomes clear why the old CC problem cannot be solved (without fine tuning)
in conventional field theories; d) There is a wide range of the parameters such
that in the late time universe: the equation-of-state w=p/\rho <-1; w
asymptotically (as t\to\infty) approaches -1 from below; $\rho$ approaches a
constant, the smallness of which does not require fine tuning of dimensionfull
parameters.


title:
Reply to "Can gravitational dynamics be obtained by diffeomorphism
  invariance of action?"

abstract:
  In a previous work we showed that, in a suitable setting, one can use
diffeomorphism invariance in order to derive gravitational field equations from
boundary terms of the gravitational action. Standing by our results we reply
here to a recent comment questioning their validity.


title: Tangent Euler Top in General Relativity

abstract:
  We define the tangent Euler top in General Relativity through a constrained
Lagrangian on the orthonormal frame bundle. The corresponding motions are
studied to various degrees of approximation, the lowest of which is shown to
yield the Mathisson-Papapetrou equations.


title: Where are the black hole entropy degrees of freedom ?

abstract:
  Understanding the area-proportionality of black hole entropy (the `Area Law')
from an underlying fundamental theory has been one of the goals of all models
of quantum gravity. A key question that one asks is: where are the degrees of
freedom giving rise to black hole entropy located? Taking the point of view
that entanglement between field degrees of freedom inside and outside the
horizon can be a source of this entropy, we show that when the field is in its
ground state, the degrees of freedom near the horizon contribute most to the
entropy, and the area law is obeyed. However, when it is in an excited state,
degrees of freedom far from the horizon contribute more significantly, and
deviations from the area law are observed. In other words, we demonstrate that
horizon degrees of freedom are responsible for the area law.


title:
Hydrodynamic Flow as Congruence of Geodesic Lines in Riemannian
  Space-Time

abstract:
  It is shown that small elements of perfect fluid in adiabatic processes move
along geodesic lines of a Riemannian space-time.

categories: gr-qc astro-ph math-ph math.MP physics.flu-dyn


title: Talking About Singularities

abstract:
  We discuss some aspects of recent research as well as more general issues
about motivation, useful methods and open problems in the field of cosmological
singularities. In particular, we review some of the approaches to the general
area and include discussions of the method of asymptotic splittings,
singularity and completeness theorems and the use of the Bel-Robinson energy to
prove completeness theorems and classify cosmological singularities.


title:
Nonlinear spinor field in Bianchi type-I Universe filled with viscous
  fluid: numerical solutions

abstract:
  We consider a system of nonlinear spinor and a Bianchi type I gravitational
fields in presence of viscous fluid. The nonlinear term in the spinor field
Lagrangian is chosen to be $\lambda F$, with $\lambda$ being a self-coupling
constant and $F$ being a function of the invariants $I$ an $J$ constructed from
bilinear spinor forms $S$ and $P$. Self-consistent solutions to the spinor and
BI gravitational field equations are obtained in terms of $\tau$, where $\tau$
is the volume scale of BI universe. System of equations for $\tau$ and $\ve$,
where $\ve$ is the energy of the viscous fluid, is deduced. This system is
solved numerically for some special cases.


title:
Use and Abuse of the Fisher Information Matrix in the Assessment of
  Gravitational-Wave Parameter-Estimation Prospects

abstract:
  The Fisher-matrix formalism is used routinely in the literature on
gravitational-wave detection to characterize the parameter-estimation
performance of gravitational-wave measurements, given parametrized models of
the waveforms, and assuming detector noise of known colored Gaussian
distribution. Unfortunately, the Fisher matrix can be a poor predictor of the
amount of information obtained from typical observations, especially for
waveforms with several parameters and relatively low expected signal-to-noise
ratios (SNR), or for waveforms depending weakly on one or more parameters, when
their priors are not taken into proper consideration. In this paper I discuss
these pitfalls; show how they occur, even for relatively strong signals, with a
commonly used template family for binary-inspiral waveforms; and describe
practical recipes to recognize them and cope with them.
  Specifically, I answer the following questions: (i) What is the significance
of (quasi-)singular Fisher matrices, and how must we deal with them? (ii) When
is it necessary to take into account prior probability distributions for the
source parameters? (iii) When is the signal-to-noise ratio high enough to
believe the Fisher-matrix result? In addition, I provide general expressions
for the higher-order, beyond--Fisher-matrix terms in the 1/SNR expansions for
the expected parameter accuracies.


title:
A possible quantum probability increase of the cylindrical gravitational
  field

abstract:
  As known, the cylindrical gravitational field (wave) have been canonically
quantized and its wave function, as the quantum one, interpreted in probability
terms. We show in this work, using quantum Zeno methods, that this probability
may be substantially increased and even approach unity. For that we first show,
in detailed manner, that the cylindrical gravitational field may be discussed
in the commutation number representation. We also discuss this field in the
transverse-traceless (TT) gauge and calculate the related trapped surface.


title: The Schwarzschild or Kerr Solutions in 3 Space and N Time Dimensions

abstract:
  In this note the Schwarzschild and Kerr solutions are constructed for 3 space
and $N$ time dimensions. Solutions, by construction, possesses symmetry with
respect to rotations in time volume.


title: Quantum-gravity phenomenology, Lorentz symmetry, and the SME

abstract:
  Violations of spacetime symmetries have recently been identified as promising
signatures for physics underlying the Standard Model. The present talk gives an
overview over various topics in this field: The motivations for
spacetime-symmetry research, including some mechanisms for Lorentz breaking,
are reviewed. An effective field theory called the Standard-Model Extension
(SME) for the description of the resulting low-energy effects is introduced,
and some experimental tests of Lorentz and CPT invariance are listed.


title: Two Examples of Circular Motion for Introductory Courses in Relativity

abstract:
  The circular twin paradox and Thomas Precession are presented in a way that
makes both accessible to students in introductory relativity courses. Both are
discussed by examining what happens during travel around a polygon and then in
the limit as the polygon tends to a circle. Since relativistic predictions
based on these examples can be verified in experiments with macroscopic objects
such as atomic clocks and the gyroscopes on Gravity Probe B, they are
particularly convincing to introductory students.


title: Black string and velocity frame dragging

abstract:
  We investigate velocity frame dragging with the boosted Schwarzschild black
string solution and the boosted Kaluza-Klein bubble solution, in which a
translational symmetry along the boosted $z$-coordinate is implemented. The
velocity frame dragging effect can be nullified by the motion of an observer
using the boost symmetry along the $z-$coordinate if it is not compact.
However, in spacetime with the compact $z-$coordinate, we show that the effect
cannot be removed since the compactification breaks the global Lorentz boost
symmetry. As a result, the comoving velocity is dependent on $r$ and the
momentum parameter along the $z-$coordinate becomes an observer independent
characteristic quantity of the black string and bubble solutions. The dragging
induces a spherical ergo-region around the black string.


title: A Note on General Covariant Stability Theory

abstract:
  In the present work we suggest a general covariant theory which can be used
to study the stability of any physical system treated geometrically. Stability
conditions are connected to the magnitude of the deviation vector. This theory
is a modification of an earlier joint work, by the same authors, concerning
stability. A comparison between the present work and the earlier one is given.
The suggested theory can be used to study the stability of planetary orbits,
astrophysical configurations and cosmological models.


title:
Numerical simulations of gravitational collapse in Einstein-aether
  theory

abstract:
  We study gravitational collapse of a spherically symmetric scalar field in
Einstein-aether theory (general relativity coupled to a dynamical unit timelike
vector field). The initial value formulation is developed, and numerical
simulations are performed. The collapse produces regular, stationary black
holes, as long as the aether coupling constants are not too large. For larger
couplings a finite area singularity occurs. These results are shown to be
consistent with the stationary solutions found previously.


title: The equivalence principle in Kaluza-Klein gravity

abstract:
  In four-dimensional general relativity the spacetime outside of an isolated
spherical star is described by a unique line element, which is the
Schwarzschild metric. As a consequence, the "gravitational" mass and the
"inertial" mass of a star are equal to each other. However, theories that
envision our world as embedded in a larger universe, with more than four
dimensions, permit a number of possible non-Schwarzschild 4D exteriors, which
typically lead to {\it different} masses, violating the weak equivalence
principle of ordinary general relativity. Therefore, the question arises of
whether the violation of this principle, i.e., the equality of gravitational
and inertial mass, is a necessary consequence of the existence of extra
dimensions. In this paper, in the context of Kaluza-Klein gravity in 5D, we
show that the answer to this question is negative. We find a one-parameter
family of asymptotically flat non-Schwarzschild static exteriors for which the
inertial and gravitational masses are equal to each other, and equal to the
Deser-Soldate mass. This family is consistent with the Newtonian weak-field
limit as well as with the general-relativistic Schwarzschild limit. Thus, we
conclude that the existence of an extra dimension, and the corresponding
non-Schwarzschild exterior, does not necessarily require different masses.
However, to an observer in 4D, it does affect the motion of test particles in
4D, which is a consequence of the departure from the usual $(4D)$ law of
geodesic motion.


title: Searching Exact Solutions for Compact Stars in Braneworld: a conjecture

abstract:
  In the context of the braneworld, a method to find consistent solutions to
Einstein's field equations in the interior of a spherically symmetric, static
and non uniform stellar distribution with Weyl stresses is developed. This
method, based in the fact that any braneworld stellar solution must have the
general relativity solution as a limit, produces a constraint which reduces the
degrees of freedom on the brane. Hence the non locality and non closure of the
braneworld equations can be overcome. The constraint found is physically
interpreted as a necessary condition to regain general relativity, and a
particular solution for it is used to find an exact and physically acceptable
analytical internal solution to no-uniform stellar distributions on the brane.
It is shown that such an exact solution is possible due to the fact that bulk
corrections to pressure, density and a metric component are a null source of
anisotropic effects on the brane. A conjecture is proposed about the
possibility of finding physically relevant exact solutions to non-uniform
stellar distributions on the brane.


title: Holographic Foam, Dark Energy and Infinite Statistics

abstract:
  Quantum fluctuations of spacetime give rise to quantum foam, and black hole
physics dictates that the foam is of holographic type. Applied to cosmology,
the holographic model requires the existence of dark energy which, we argue, is
composed of an enormous number of inert ``particles'' of extremely long
wavelength. These "particles" necessarily obey infinite statistics in which all
representations of the particle permutation group can occur. For every boson or
fermion in the present observable universe there could be $\sim 10^{31}$ such
"particles". We also discuss the compatibility between the holographic
principle and infinite statistics.


title: New directions in Background Independent Quantum Gravity

abstract:
  We discuss the meaning of background independence in quantum theories of
gravity where geometry and gravity are emergent and illustrate the
possibilities using the framework of quantum causal histories.


title:
Relativity theory does not imply that the future already exists: a
  counterexample

abstract:
  It is often said that the relativistic fusion of time with space rules out
genuine change or ``becoming''. I offer the classical sequential growth models
of causal set theory as counterexamples.


title: Does Locality Fail at Intermediate Length-Scales

abstract:
  If quantum gravity implies a fundamental spatiotemporal discreteness, and if
its ``laws of motion'' are compatible with the Lorentz transformations, then
physics cannot remain local. One might expect this nonlocality to be confined
to the fundamental discreteness scale, but I will present evidence that it
survives at much lower energies, yielding for example a nonlocal equation of
motion for a scalar field propagating on an underlying causal set.


title: Stability of Closed Timelike Curves in Goedel Universe

abstract:
  We study, in some detail, the linear stability of closed timelike curves in
the Goedel metric. We show that these curves are stable. We present a simple
extension (deformation) of the Goedel metric that contains a class of closed
timelike curves similar to the ones associated to the original Goedel metric.
This extension correspond to the addition of matter whose energy-momentum
tensor is analyzed. We find the conditions to have matter that satisfies the
usual energy conditions. We study the stability of closed timelike curves in
the presence of usual matter as well as in the presence of exotic matter
(matter that does satisfy the above mentioned conditions). We find that the
closed timelike curves in Goedel universe with or whithout the inclusion of
regular or exotic matter are also stable under linear perturbations. We also
find a sort of structural stability.


title:
(Non)commutative isotropization in Bianchi I with Barotropic perfect
  fluid and $\Lambda$ Cosmological

abstract:
  We present the classical solutions to the Einstein field equations derived
using the WKB-like and Hamilton procedures. The investigation is carried out in
the commutative and noncommutative scenario for the Bianchi type I cosmological
model coupled to barotropic perfect fluid and $\lambda $ Cosmological for two
different gauges. Noncommutativity is achieved by modifying the symplectic
structure considering that all minisuperspace variables $\rm q^i$ does not
commute and by a deformation between all the minisuperspace variables. In the
gauge N=1, it is possible to obtain that the anisotropic parameter $\rm
\beta_{\pm nc}$ tend to a constant curvature for large period of time
considering different values in the noncommutative parameters $\theta$ and
cosmological term. However, this behavior give the idea that is necessary
introduce other class of matter in the models, for to have a real
isotropization in the model, such as dark energy or dark matter.


title: Scale covariant gravity and equilibrium cosmologies

abstract:
  Causal structure, inertial path structure and compatibility with quantum
mechanics demand no full Lorentz metric, but only an integrable Weyl geometry
for space time (Ehlers/Pirani/Schild 1972, Audretsch e.a. 1984). A proposal of
(Tann 1998,,Drechsler/Tann 1999) for a minimal coupling of the Hilbert-Einstein
action to a scale covariant scalar vacuum field $\phi$ (weight -1) plus (among
others) a
  Klein-Gordon action term opens the access to a scale covariant formulation of
gravity. The ensuing scale covariant K-G equation specifies a natural scale
gauge vacuum gauge). Adding other natural assumptions for gauge conditions (in
particular Newton gauge, with unchanging Newton constant) the chosen Ansatz
leads to a class of Weyl geometric
  Robertson-Walker solutions of the Einstein equation, satisfying $a''a+a'^2=
const$, analogous to the Friedmann-Lemaitre equation but with completely
different dynamical properties ($a$ the warp function in Riemann gauge). The
class has an asymptotically attracting 1-parameter subfamily of extremely
simple space-time geometries with an isotropic Robertson-Walker fluid as source
of the Einstein equation, discussed as Weyl universes elsewhere (Scholz 2005).
  Under the assumption of a heuristic gravitational self energy binding Ansatz
for the fluid, equilibrium solutions arise, in stark contrast to classical
(semi-Riemannian) cosmology. Weyl universes agree very well with a variety of
empirical data from observational cosmology, in particular supernovae
luminosities and quasar data.


title:
The Schwarzschild-de Sitter solution in five-dimensional general
  relativity briefly revisited

abstract:
  We briefly revisit the Schwarzschild-de Sitter solution in the context of
five-dimensional general relativity. We obtain a class of five-dimensional
solutions of Einstein vacuum field equations into which the four-dimensional
Schwarzschild-de Sitter space can be locally and isometrically embedded. We
show that this class of solutions is well-behaved in the limit of lambda
approaching zero. Applying the same procedure to the de Sitter cosmological
model in five dimensions we obtain a class of embedding spaces which are
similarly well-behaved in this limit. These examples demonstrate that the
presence of a non-zero cosmological constant does not in general impose a rigid
relation between the (3+1) and (4+1)-dimensional spacetimes, with degenerate
limiting behaviour.


title: The principle of equivalence and projective structure in space-times

abstract:
  This paper discusses the extent to which one can determine the space-time
metric from a knowledge of a certain subset of the (unparametrised) geodesics
of its Levi-Civita connection, that is, from the experimental evidence of the
equivalence principle. It is shown that, if the space-time concerned is known
to be vacuum, then the Levi-Civita connection is uniquely determined and its
associated metric is uniquely determined up to a choice of units of
measurement, by the specification of these geodesics. It is further
demonstrated that if two space-times share the same unparametrised geodesics
and only one is assumed vacuum then their Levi-Civita connections are again
equal (and so the other metric is also a vacuum metric) and the first result
above is recovered.


title: Child universes UV regularization?

abstract:
  It is argued that high energy density excitations, responsible for UV
divergences in quantum field theories, including quantum gravity, are likely to
be the source of child universes which carry them out of the original space
time. This decoupling prevents these high UV excitations from having any
influence on physical amplitudes. Child universe production could therefore be
responsible for UV regularization in quantum field theories which takes into
account gravitational effects. Also child universe production in the last
stages of black hole evaporation, the prediction of absence of tranplanckian
primordial perturbations, connection to the minimum length hypothesis and in
particular connection to the maximal curvature hypothesis are discussed.


title:
The Spatial Averaging Limit of Covariant Macroscopic Gravity - Scalar
  Corrections to the Cosmological Equations

abstract:
  It is known that any explicit averaging scheme of the type essential for
describing the large scale behaviour of the Universe, must necessarily yield
corrections to the Einstein equations applied in the Cosmological setting. The
question of whether or not the resulting corrections to the Einstein equations
are significant, is still a subject of debate, partly due to possible
ambiguities in the averaging schemes available. In particular, it has been
argued in the literature that the effects of averaging could be gauge
artifacts. We apply the formalism of Zalaletdinov's Macroscopic Gravity (MG)
which is a fully covariant and nonperturbative averaging scheme, in an attempt
to construct gauge independent corrections to the standard
Friedmann-Lemaitre-Robertson-Walker (FLRW) equations. We find that whereas one
cannot escape the problem of dependence on \emph{one} gauge choice -- which is
inherent in the assumption of large scale homogeneity and isotropy -- it is
however possible to construct \emph{spacetime scalar} corrections to the
standard FLRW equations. This partially addresses the criticism concerning the
corrections being gauge artifacts. For a particular initial choice of gauge
which simplifies the formalism, we explicitly construct these scalars in terms
of the underlying inhomogeneous geometry, and incidentally demonstrate that the
formal structure of the corrections with this gauge choice is identical to that
of analogous corrections derived by Buchert in the context of spatial averaging
of scalars.


title: Charged black holes and unusual wormholes in scalar-tensor gravity

abstract:
  We consider static, spherically symmetric, electrically or/and magnetically
charged configurations of a minimally coupled scalar field with an arbitrary
potential $V(\phi)$ in general relativity. Using the inverse problem method, we
obtain a four-parameter family of asymptotically dS, flat and AdS solutions,
including those with naked singularities and both extreme and non-extreme
black-hole (BH) solutions. The parameters are identified as the asymptotic
cosmological constant, an arbitrary length scale, mass and charge. In all
asymptotically flat BH solutions, the potential $V(\phi)$ is partly negative,
in accord with Bekenstein and Mayo's no-hair theorem. The well-known conformal
mapping extends the BH solutions to Jordan's pictures of a general class of
scalar-tensor theories (STT) of gravity under the condition that the nonminimal
coupling function $f(\phi)$ is everywhere positive. Relaxing the latter
condition and assuming $f=0$ at some value of $\phi$, we obtain wormhole
solutions in a particular subclass of STT. In such solutions, the double
horizon of an extreme BH in Einstein's picture maps into the second spatial
infinity in Jordan's. However, at this second infinity, the effective
gravitational coupling infinitely grows.


title: Asymptotically Shear-free and Twist-free Null Geodesic Congruences

abstract:
  We show that, though they are rare, there are asymptotically flat space-times
that possess null geodesic congruences that are both asymptotically shear- free
and twist-free (surface forming). In particular, we display the class of
space-times that possess this property and demonstrate how these congruences
can be found. A special case within this class are the Robinson- Trautman
space-times. In addition, we show that in each case the congruence is isolated
in the sense that there are no other neighboring congruences with this dual
property.


title: Periodic Gravitational Waves From Small Cosmic String Loops

abstract:
  We consider a population of small, high-velocity cosmic string loops. We
assume the typical length of these loops is determined by the gravitational
radiation scale and use the results of \cite{Polchinski:2007rg} which pointed
out their highly relativistic nature. A study of the gravitational wave
emission from such a population is carried out. The large Lorentz boost
involved causes the lowest harmonics of the loops to fall within the frequency
band of the LIGO detector. Due to this feature the gravitational waves emitted
by such loops can be detected in a periodic search rather than in burst or
stochastic analysis.
  It is shown that, for interesting values of the string tension
($10^{-10}\lsim G\mu\lsim 10^{-8}$) the detector can observe loops at
reasonably high redshifts and that detection is, in principle, possible. We
compute the number of expected observations produced by such a process. For a
10 hour search we find that this number is of order $O(10^{-4})$. This is a
consequence of the low effective number density of the loops traveling along
the line of sight. However, small probabilities of reconnection and longer
observation times can improve the result.


title: Gravitational perturbations of an ultrastatic wormhole

abstract:
  This paper has been withdrawn by the author due to a crucial sign error in
equation 1


title: Statefinder Parameters for Tachyon Dark Energy Model

abstract:
  In this paper we study the statefinder parameters for the tachyon dark energy
model. There are two kinds of stable attractor solutions in this model. The
statefinder diagrams characterize the properties of the tachyon dark energy
model. Our results show that the evolving trajectories of the attractor
solutions lie in the total region and pass through the LCDM fixed point, which
is different from other dark energy model.


title: Parametrization of Tachyon Field

abstract:
  We assume that universe is dominated by non-relativistic matter and tachyon
field and reconstruct the potential of tachyon field directly from the
effective equation of state (EOS) of dark energy. We apply the method to four
known parametrization of equation of state and discuss the general features of
the resulting potentials.


title:
Reconstructing the Equation of State for Dark Energy In the Double
  Complex Symmetric Gravitational Theory

abstract:
  We propose to study the accelerating expansion of the universe in the double
complex symmetric gravitational theory (DCSGT). The universe we live in is
taken as the real part of the whole spacetime ${\cal M}^4_C(J)$ which is double
complex. By introducing the spatially flat FRW metric, not only the double
Friedmann Equations but also the two constraint conditions $p_J=0$ and $J^2=1$
are obtained. Furthermore, using parametric $D_L(z)$ ansatz, we reconstruct the
$\omega^{'}(z)$ and $V(\phi)$ for dark energy from real observational data. We
find that in the two cases of $J=i,p_J=0$ and $J=\epsilon,p_J\neq 0$, the
corresponding equations of state $\omega^{'}(z)$ remain close to -1 at present
($z=0$) and change from below -1 to above -1. The results illustrate that the
whole spacetime, i.e. the double complex spacetime ${\cal M}^4_C(J)$, may be
either ordinary complex ($J=i,p_J=0$) or hyperbolic complex
($J=\epsilon,p_J\neq 0$). And the fate of the universe would be Big Rip in the
future.


title: Note on non-metric gravity

abstract:
  We discuss a class of alternative gravity theories that are specific to four
dimensions, do not introduce new degrees of freedom, and come with a physical
motivation. In particular we sketch their Hamiltonian formulation, and their
relation to some earlier constructions.


title: Classification of spacelike surfaces in spacetime

abstract:
  A classification of 2-dimensional surfaces imbedded in spacetime is
presented, according to the algebraic properties of their shape tensor. The
classification has five levels, and provides among other things a refinement of
the concepts of trapped, umbilical and extremal surfaces, which split into
several different classes. The classification raises new important questions
and opens many possible new lines of research. These, together with some
applications and examples, are briefly considered.


title: Loop quantum gravity and Planck-size black hole entropy

abstract:
  The Loop Quantum Gravity (LQG) program is briefly reviewed and one of its
main applications, namely the counting of black hole entropy within the
framework is considered. In particular, recent results for Planck size black
holes are reviewed. These results are consistent with an asymptotic linear
relation (that fixes uniquely a free parameter of the theory) and a logarithmic
correction with a coefficient equal to -1/2. The account is tailored as an
introduction to the subject for non-experts.


title:
Trans-Planckian physics and signature change events in Bose gas
  hydrodynamics

abstract:
  We present an example of emergent spacetime as the hydrodynamic limit of a
more fundamental microscopic theory. The low-energy, long-wavelength limit in
our model is dominated by collective variables that generate an effective
Lorentzian metric. This system naturally exhibits a microscopic mechanism
allowing us to perform controlled signature change between Lorentzian and
Riemannian geometries. We calculate the number of particles produced from a
finite-duration Euclidean-signature event, where we take the position that to a
good approximation the dynamics is dominated by the evolution of the linearized
perturbations, as suggested by Calzetta and Hu [Phys. Rev. A 68 (2003) 043625].
We adapt the ideas presented by Dray et al. [Gen. Rel. Grav. 23 (1991) 967],
such that the field and its canonical momentum are continuous at the
signature-change event.
  We investigate the interplay between the underlying microscopic structure and
the emergent gravitational field, focussing on its impact on particle
production in the ultraviolet regime. In general, this can be thought of as the
combination of trans-Planckian physics and signature-change physics. Further we
investigate the possibility of using the proposed signature change event as an
amplifier for analogue "cosmological particle production" in condensed matter
experiments.


title:
A fast stroboscopic spectral method for rotating systems in numerical
  relativity

abstract:
  We present a numerical technique for solving evolution equations, as the wave
equation, in the description of rotating astrophysical compact objects in
comoving coordinates, which avoids the problems associated with the light
cylinder. The technique implements a fast spectral matching between two domains
in relative rotation: an inner spherical domain, comoving with the sources and
lying strictly inside the light cylinder, and an outer inertial spherical
shell. Even though the emphasis is placed on spectral techniques, the matching
is independent of the specific manner in which equations are solved inside each
domain, and can be adapted to different schemes. We illustrate the strategy
with some simple but representative examples.


title: Time scales in LISA

abstract:
  The LISA mission is a space interferometer aiming at the detection of
gravitational waves in the [$10^{-4}$,$10^{-1}$] Hz frequency band. In order to
reach the gravitational wave detection level, a Time Delay Interferometry (TDI)
method must be applied to get rid of (most of) the laser frequency noise and
optical bench noise. This TDI analysis is carried out in terms of the
coordinate time corresponding to the Barycentric Coordinate Reference System
(BCRS), TCB, whereas the data at each of the three LISA stations is recorded in
terms of each station proper time. We provide here the required proper time
versus BCRS time transformation. We show that the difference in rate of station
proper time versus TCB is of the order of $5 10^{-8}$. The difference between
station proper times and TCB exhibits an oscillatory trend with a maximum
amplitude of about $10^{-3}$ s.


title:
Quantum-corrected ultraextremal horizons and validity of WKB in massless
  limit

abstract:
  We consider quantum backreaction of the quantized scalar field with an
arbitrary mass and curvature coupling on ultraextremal horizons. The problem is
distinguished in that (in contrast to non-extremal or extremal black holes) the
WKB approximation remains valid near $r_{+}$ (which is the radius of the
horizon) even in the massless limit. We examine the behavior of the
stress-energy tensor of the quantized field near $r_{+}$ and show that
quantum-corrected objects under discussion do exist. In the limit of the large
mass our results agree with previous ones known in literature.


title: Causality and the speed of sound

abstract:
  A usual causal requirement on a viable theory of matter is that the speed of
sound be at most the speed of light. In view of various recent papers querying
this limit, the question is revisited here. We point to various issues
confronting theories that violate the usual constraint.


title: Cosmography: Extracting the Hubble series from the supernova data

abstract:
  We perform a number of inter-related cosmographic fits to the legacy05 and
gold06 supernova datasets. We pay particular attention to the influence of both
statistical and systematic uncertainties, and also to the extent to which the
choice of distance scale and manner of representing the redshift scale affect
the cosmological parameters. While the "preponderance of evidence" certainly
suggests an accelerating universe, we would argue that (based on the supernova
data) this conclusion is not currently supported "beyond reasonable doubt". As
part of the analysis we develop two particularly transparent graphical
representations of the redshift-distance relation -- representations in which
acceleration versus deceleration reduces to the question of whether the graph
slopes up or down.
  Turning to the details of the cosmographic fits, three issues in particular
concern us: First, the fitted value for the deceleration parameter changes
significantly depending on whether one performs a chi^2 fit to the luminosity
distance, proper motion distance, angular diameter distance, or other suitable
distance surrogate. Second, the fitted value for the deceleration parameter
changes significantly depending on whether one uses the traditional redshift
variable z, or what we shall argue is on theoretical grounds an improved
parameterization y=z/(1+z). Third, the published estimates for systematic
uncertainties are sufficiently large that they certainly impact on, and to a
large extent undermine, the usual purely statistical tests of significance. We
conclude that the case for an accelerating universe is considerably less
watertight than commonly believed.


title: Quantum gravitational optics: the induced phase

abstract:
  The geometrical approximation of the extended Maxwell equation in curved
spacetime incorporating interactions induced by the vacuum polarization effects
is considered. Taking into account these QED interactions and employing the
analogy between eikonal equation in geometrical optics and Hamilton-Jacobi
equation for the particle motion, we study the phase structure of the modified
theory. There is a complicated, local induced phase which is believed to be
responsible for the modification of the classical picture of light ray. The
main features of QGO could be obtained through the study of this induced phase.
We discuss initial principles in conventional and modified geometrical optics
and compare the results.


title:
Interacting spinor and scalar fields in Bianchi type-I Universe filled
  with viscous fluid: exact and numerical solutions

abstract:
  We consider a self-consistent system of spinor and scalar fields within the
framework of a Bianchi type I gravitational field filled with viscous fluid in
presence of a $\Lambda$ term. Exact self-consistent solutions to the
corresponding spinor, scalar and BI gravitational field equations are obtained
in terms of $\tau$, where $\tau$ is the volume scale of BI universe. System of
equations for $\tau$ and $\ve$, where $\ve$ is the energy of the viscous fluid,
is deduced. Some special cases allowing exact solutions are thoroughly studied.


title: Why does gravitational radiation produce vorticity?

abstract:
  We calculate the vorticity of world--lines of observers at rest in a
Bondi--Sachs frame, produced by gravitational radiation, in a general Sachs
metric. We claim that such an effect is related to the super--Poynting vector,
in a similar way as the existence of the electromagnetic Poynting vector is
related to the vorticity in stationary electrovacum spacetimes.


title: Exact Friedmann Solutions in Higher-Order Gravity Theories

abstract:
  We find the general behaviour of homogeneous and isotropic cosmological
models in some fourth-order theories of gravity. Explicit, exact, general
solutions are given for both empty universes and those filled with a perfect
fluid. For the vacuum case, solutions are found with closed, open and flat
geometries, whilst the perfect fluid solutions are all spatially flat. Both
early and late-time limits are studied, and attractor behaviour towards simple
power-law expansion is identified. Multiple solutions to the same theories,
with the same matter content and topology are found. It is shown that these
solutions exhibit great variety in their evolution.


title: Apparent horizon formation in the head-on collision of gyratons

abstract:
  The gyraton model describes a gravitational field of an object moving with
the velocity of light which has finite energy and spin distributed during some
finite time interval $L$. A gyraton may be considered as a classical toy model
for a quantum wave packet of high-energy particles with spin. In this paper we
study a head-on collision of two gyratons and black hole formation in this
process. The goal of this study is to understand the role of the gravitational
spin-spin interaction in the process of mini-black-hole formation in particle
collisions. To simplify the problem we consider several gyraton models with
special profiles of the energy and spin density distribution. For these models
we study the apparent horizon (AH) formation on the future edge of a spacetime
region before interaction. We demonstrate that the AH forms only if the energy
duration and the spin are smaller than some critical values, while the length
of the spin distribution should be at least of the order of the system
gravitational radius. We also study gravitational spin-spin interaction in the
head-on collision of two gyratons under the assumption that the values of
gyraton spins are small. We demonstrate that the metric in the interaction
region for such gyratons depends on the relative helicities of incoming
gyratons, and the collision of gyratons with oppositely directed spins allows
the AH formation in a larger parameter region than in the collision of the
gyratons with the same direction of spins. Some applications of the obtained
results to the mini-black-hole production at the Large Hadron Collider in TeV
gravity scenarios are briefly discussed.


title: The causal ladder and the strength of K-causality. II

abstract:
  Hawking's stable causality implies Sorkin and Woolgar's K-causality. The work
investigates the possible equivalence between the two causality requirements,
an issue which was first considered by H. Seifert and then raised again by R.
Low after the introduction of K-causality. First, a new proof is given that a
spacetime is stably causal iff the Seifert causal relation is a partial order.
It is then shown that given a K-causal spacetime and chosen an event, the light
cones can be widened in a neighborhood of the event without spoiling
K-causality. The idea is that this widening of the light cones can be continued
leading to a global one. Unfortunately, due to some difficulties in the
inductive process the author was not able to complete the program for a proof
as originally conceived by H. Seifert. Nevertheless, it is proved that if
K-causality coincides with stable causality then in any K-causal spacetime the
K future coincides with the Seifert future. Explicit examples are provided
which show that the K^+ future may differ from the Seifert relation in causal
spacetimes.


title: Improved outer boundary conditions for Einstein's field equations

abstract:
  In a recent article, we constructed a hierarchy B_L of outer boundary
conditions for Einstein's field equations with the property that, for a
spherical outer boundary, it is perfectly absorbing for linearized
gravitational radiation up to a given angular momentum number L. In this
article, we generalize B_2 so that it can be applied to fairly general
foliations of spacetime by space-like hypersurfaces and general outer boundary
shapes and further, we improve B_2 in two steps: (i) we give a local boundary
condition C_2 which is perfectly absorbing including first order contributions
in 2M/R of curvature corrections for quadrupolar waves (where M is the mass of
the spacetime and R is a typical radius of the outer boundary) and which
significantly reduces spurious reflections due to backscatter, and (ii) we give
a non-local boundary condition D_2 which is exact when first order corrections
in 2M/R for both curvature and backscatter are considered, for quadrupolar
radiation.


title: A new theorem for black holes

abstract:
  A new theorem for black holes is established. The mass of a black hole
depends on where the observer is. The horizon mass theorem states that for all
black holes: neutral, charged or rotating, the horizon mass is always twice the
irreducible mass observed at infinity. The horizon mass theorem is crucial for
understanding the occurrence of Hawking radiation. Without black hole
radiation, the Second Law of Thermodynamics is lost.


title: Uniqueness of de Sitter space

abstract:
  All inextendible null geodesics in four dimensional de Sitter space dS^4 are
complete and globally achronal. This achronality is related to the fact that
all observer horizons in dS^4 are eternal, i.e. extend from future infinity
scri^+ all the way back to past infinity scri^-. We show that the property of
having a null line (inextendible achronal null geodesic) that extends from
scri^- to scri^+ characterizes dS^4 among all globally hyperbolic and
asymptotically de Sitter spacetimes satisfying the vacuum Einstein equations
with positive cosmological constant. This result is then further extended to
allow for a class of matter models that includes perfect fluids.


title: A no-go theorem for polytropic spheres in Palatini f(R) gravity

abstract:
  Non-vacuum static spherically-symmetric solutions in Palatini f(R) gravity
are examined. It is shown that for generic choices of f(R), there are
commonly-used equations of state for which no satisfactory physical solution of
the field equations can be found within this framework, apart from in the
special case of General Relativity, casting doubt on whether Palatini f(R)
gravity can be considered as giving viable alternatives to General Relativity.


title: A note on cosmology in a brane model

abstract:
  We study some aspects of cosmology in a five-dimensional model with matter,
radiation and cosmological constant on the four-dimensional brane(s) and
without matter in the bulk. The action of the model does not contain explicit
curvature terms on the the brane(s). We obtain solution of the generalized
Friedman equation as a function of dimensionless ratio of the scales $b^2 =
\frac{\mu M^2_{pl}}{M^3}$ ($\mu$ is the scale in the warp factor in the 5D
metric which is taken of order $10^{3\div 4}GeV$, $M\geq\mu$ is the $5D$
fundamental scale). We assume that there is a hierarchy between 4D and 5D
scales. For $b^2 =O(1)$ the age of the Universe is found comparable, but below
the current experimental value, for $b^2\gg 1$ it is obtained much smaller than
the experimental bound. Because time dependence of temperature of the Universe
in the 5D model is different from that in the standard cosmology, the abundance
of ${}^4$He produced in the primordial nucleosynthesis is obtained about three
times more than in the standard cosmology.


title: The symplectic 2-form and Poisson bracket of null canonical gravity

abstract:
  It is well known that free (unconstrained) initial data for the gravitational
field in general relativity can be identified on an initial hypersurface
consisting of two intersecting null hypersurfaces. Here the phase space of
vacuum general relativity associated with such an initial data hypersurface is
defined; a Poisson bracket is defined, via Peierls' prescription, on
sufficiently regular functions on this phase space, called ``observables''; and
a bracket on initial data is defined so that it reproduces the Peierls bracket
between observables when these are expressed in terms of the initial data. The
brackets between all elements of a free initial data set are calculated
explicitly. The bracket on initial data presented here has all the
characteristics of a Poisson bracket except that it does not satisfy the Jacobi
relations (even though the brackets between the observables do).
  The initial data set used is closely related to that of Sachs. However, one
significant difference is that it includes a ``new'' pair of degrees of freedom
on the intersection of the two null hypersurfaces which are present but quite
hidden in Sachs' formalism. As a step in the calculation an explicit expression
for the symplectic 2-form in terms of these free initial data is obtained.


title: Loop quantization of spherically symmetric midi-superspaces

abstract:
  We quantize the exterior of spherically symmetric vacuum space-times using a
midi-superspace reduction within the Ashtekar new variables. Through a partial
gauge fixing we eliminate the diffeomorphism constraint and are left with a
Hamiltonian constraint that is first class. We complete the quantization in the
loop representation. We also use the model to discuss the issues that will
arise in more general contexts in the ``uniform discretization'' approach to
the dynamics.


title:
Horizon news function and quasi-local energy-momentum flux near black
  hole

abstract:
  From the 'quasi-local' definition of horizons, e.g. isolated horizon and
dynamical horizon, the consequence quasi-local energy-momentum near horizons
can be observed by using the idea of frame alignment. In particular, we find
the horizon news function from the asymptotic expansion near horizons and use
this to describe the gravitational flux and change of mass of a black hole.


title: Decoherence in Quantum Gravity: Issues and Critiques

abstract:
  An increasing number of papers have appeared in recent years on decoherence
in quantum gravity at the Planck energy. We discuss the meaning of decoherence
in quantum gravity starting from the common notion that quantum gravity is a
theory for the microscopic structures of spacetime, and invoking some generic
features of quantum decoherence from the open systems viewpoint. We dwell on a
range of issues bearing on this process including the relation between
statistical and quantum, noise from effective field theory, the meaning of
stochasticity, the origin of non-unitarity and the nature of nonlocality in
this and related contexts. To expound these issues we critique on two
representative theories: One claims that decoherence in quantum gravity scale
leads to the violation of CPT symmetry at sub-Planckian energy which is used to
explain today's particle phenomenology. The other uses this process in place
with the Brownian motion model to prove that spacetime foam behaves like a
thermal bath.


title:
An Evidence Based Search Method For Gravitational Waves From Neutron
  Star Ring-downs

abstract:
  The excitation of quadrupolar quasi-normal modes in a neutron star leads to
the emission of a short, distinctive, burst of gravitational radiation in the
form of a decaying sinusoid or `ring-down'. We present a Bayesian analysis
method which incorporates relevant prior information about the source and known
instrumental artifacts to conduct a robust search for the gravitational wave
emission associated with pulsar glitches and soft $\gamma$-ray repeater flares.
Instrumental transients are modelled as sine-Gaussian and their evidence, or
marginal likelihood, is compared with that of Gaussian white noise and
ring-downs via the `odds-ratio'. Tests using simulated data with a noise
spectral density similar to the LIGO interferometer around 1 kHz yield 50%
detection efficiency and 1% false alarm probability for ring-down signals with
signal-to-noise ratio $\rho=5.2$. For a source at 15 kpc this requires an
energy of $1.3\times 10^{-5}M_{\astrosun}c^2$ to be emitted as gravitational
waves.


title: Closed timelike geodesics in a gas of cosmic strings

abstract:
  We find a class of solutions of Einstein's field equations representing
spacetime outside a spinning cosmic string surrounded by a gas of non-spinning
cosmic strings, and show that there exist closed timelike geodesics in this
spacetime.


title:
A mathematical analysis of the evolution of perturbations in a modified
  Chaplygin gas model

abstract:
  One approach in modern cosmology consists in supposing that dark matter and
dark energy are different manifestations of a single `quartessential' fluid.
Following such idea, this work presents a study of the evolution of
perturbations of density in a flat cosmological model with a modified Chaplygin
gas acting as a single component. Our goal is to obtain properties of the model
which can be used to distinguish it from another cosmological models which have
the same solutions for the general evolution of the scale factor of the
universe, without the construction of the power spectrum. Our analytical
results, which alone can be used to uniquely characterize the specific model
studied in our work, show that the evolution of the density contrast can be
seen, at least in one particular case, as composed by a spheroidal wave
function. We also present a numerical analysis which clearly indicates as one
interesting feature of the model the appearence of peaks in the evolution of
the density constrast.


title: On Mach's principle: Inertia as gravitation

abstract:
  In order to test the validity of Mach's principle, we calculate the action of
the entire universe on a test mass in its rest frame, which is an acceleration
${\bf g}^*$. We show the dependence of the inertia principle on the lapse and
the shift. Using the formalism of linearized gravitation, we obtain the
non-relativistic limit of ${\bf g}^*$ in terms of two integrals. We follow then
two approaches. In the first one, these integrals are calculated in the actual
time section $t=t_0$ up to the distance $R_U=ct_0$. In the more exact and
satisfactory second approach, they are calculated over the past light cone
using the formalism of the retarded potentials. The aim is to find whether the
acceleration $\dot{\bf v}$ in the LHS of Newton's second law can be interpreted
as a reactive acceleration, in other words, as minus the acceleration of
gravity ${\bf g}^*$ in the rest frame of the accelerated particle ({\it i. e.}
to know whether or not ${\bf g}^*=-\dot{\bf v}$). The results strongly support
Mach's idea since the reactive acceleration for $\Omega_\Lambda =0.7$ turns out
to be about ${\bf g}^*=-1.1 \dot{\bf v}$, in the first approach, and about
${\bf g}^*= -0.7 \dot{\bf v}$, in the second. These results depend little on
$\Omega_\Lambda$ if $\Omega_\Lambda<0.9$. Even considering the approximations
and idealizations made during the calculations, we deem these results as
interesting and encouraging.


title: Particle creation in (2+1) circular dust collapse

abstract:
  We investigate the quantum particle creation during the circularly symmetric
collapse of a 2+1 dust cloud, for the cases when the cosmological constant is
either zero or negative. We derive the Ford-Parker formula for the 2+1 case,
which can be used to compute the radiated quantum flux in the geometric optics
approximation. It is shown that no particles are created when the collapse ends
in a naked singularity, unlike the 3+1 case. When the collapse ends in a BTZ
black hole, we recover the expected Hawking radiation.


title:
A Theoretical Construction of Thin Shell Wormhole from Tidal Charged
  Black hole

abstract:
  Recently, Dadhich et al [ Phys.Lett.B 487, 1 (2000)] have discovered a black
hole solution localized on a three brane in five dimensional gravity in the
Randall-Sundrum scenario. In this article, we develop a new class of thin shell
wormhole by surgically grafting above two black hole spacetimes. Various
aspects of this thin wormhole are also analyzed.


title:
Dynamical coherent states and physical solutions of quantum cosmological
  bounces

abstract:
  A new model is studied which describes the quantum behavior of transitions
through an isotropic quantum cosmological bounce in loop quantum cosmology
sourced by a free and massless scalar field. As an exactly solvable model even
at the quantum level, it illustrates properties of dynamical coherent states
and provides the basis for a systematic perturbation theory of loop quantum
gravity. The detailed analysis is remarkably different from what is known for
harmonic oscillator coherent states. Results are evaluated with regard to their
implications in cosmology, including a demonstration that in general quantum
fluctuations before and after the bounce are unrelated. Thus, even within this
solvable model the condition of classicality at late times does not imply
classicality at early times before the bounce without further assumptions.
Nevertheless, the quantum state does evolve deterministically through the
bounce.


title:
Constraint Damping in First-Order Evolution Systems for Numerical
  Relativity

abstract:
  A new constraint suppressing formulation of the Einstein evolution equations
is presented, generalizing the five-parameter first-order system due to Kidder,
Scheel and Teukolsky (KST). The auxiliary fields, introduced to make the KST
system first-order, are given modified evolution equations designed to drive
constraint violations toward zero. The algebraic structure of the new system is
investigated, showing that the modifications preserve the hyperbolicity of the
fundamental and constraint evolution equations. The evolution of the
constraints for pertubations of flat spacetime is completely analyzed, and all
finite-wavelength constraint modes are shown to decay exponentially when
certain adjustable parameters satisfy appropriate inequalities. Numerical
simulations of a single Schwarzschild black hole are presented, demonstrating
the effectiveness of the new constraint-damping modifications.


title:
Charged-rotating black holes and black strings in higher dimensional
  Einstein-Maxwell theory with a positive cosmological constant

abstract:
  We present arguments for the existence of charged, rotating black holes in
$d=2N+1$ dimensions, with $d\geq 5$ with a positive cosmological constant.
These solutions posses both, a regular horizon and a cosmological horizon of
spherical topology and have $N$ equal-magnitude angular momenta. They approach
asymptotically the de Sitter spacetime background. The counterpart equations
for $d=2N+2$ are investigated, by assuming that the fields are independant of
the extra dimension $y$, leading to black strings solutions. These solutions
are regular at the event horizon. The asymptotic form of the metric is not the
de Sitter form and exhibit a naked singularity at finite proper distance.


title:
The Maxwell field on the Schwarzschild spacetime: behaviour near spatial
  infinity

abstract:
  The behaviour of the Maxwell field near one of the spatial infinities of the
Schwarzschild solution is analysed by means of the transport equations implied
by the Maxwell equations on the cylinder at spatial infinity. Initial data for
the Maxwell equations will be assumed to be expandable in terms of powers of a
coordinate $\rho$ measuring the geodesic distance to spatial infinity (in the
conformal picture) and such that the highest possible spherical harmonics at
order $p$ are $2^p$-polar ones. It is shown that if the $2^p$-polar harmonics
at order $p$ in the initial data satisfy a certain regularity condition then
the solutions to the transport equations at orders $p$ and $p+1$ are completely
regular at the critical sets where null infinity touches spatial infinity. On
the other hand, the solutions to the transport equations of order $p+2$
contain, in general, logarithmic singularities at the critical sets. It is
expected that the ideas and techniques developed in the analysis of this
problem can be employed to discuss the more challenging case of the transport
equations implied by the conformal Einstein equations, and thus provide a way
of constructing a proof of a certain rigidity conjecture concerning the
developments of conformally flat initial data sets.


title: Stability of Closed Timelike Geodesics

abstract:
  The existence and stability under linear perturbations of closed timelike
geodesics (CTGs) in Bonnor-Ward spacetime is studied in some detail. Regions
where the CTG exist and are linearly stable are exhibited.


title: Einstein's Apple: His First Principle of Equivalence

abstract:
  After a historical discussion of Einstein's 1907 principle of equivalence, a
homogeneous gravitational field in Minkowski spacetime is constructed. It is
pointed out that the reference frames in gravitational theory can be understood
as spaces with a flat connection and torsion defined through teleparallelism.
This kind of torsion was introduced by Einstein in 1928. The concept of torsion
is discussed through simple examples and some historical observations.


title:
A gravitational explanation for quantum theory - non-time-orientable
  manifolds

abstract:
  Spacetime manifolds that are not time orientable play a key role in a
gravitational explanation of quantum theory. Such manifolds allow topology
change, but also have fascinating additional properties such as net charge from
source-free equations and spin half transformation properties. It is shown how
the logical structure of propositions and the probabilities of quantum theory
arise from such acausal space times


title: Non Commutative Bianchi type II Quantum Cosmology

abstract:
  In this paper we present the noncommutative Bianchi Class A cosmological
models coupled to barotropic perfect fluid. The commutative and noncommutative
quantum solution to the Wheeler-DeWitt equation for any factor ordering, to the
anisotropic Bianchi type II cosmological model are found, using a stiff fluid
($\gamma=1$). In our toy model, we introduce noncommutative scale factors, is
say, we consider that all minisuperspace variables $\rm q^i$ does not commute,
so the simplectic structure was modified.


title:
Binary system delays and timing noise in searches for gravitational
  waves from known pulsars

abstract:
  The majority of fast millisecond pulsars are in binary systems, so that any
periodic signal they emit is modulated by both Doppler and relativistic
effects. Here we show how well-established binary models can be used to account
for these effects in searches for gravitational waves from known pulsars within
binary systems. A seperate issue affecting certain pulsar signals is that of
timing noise and we show how this, with particular reference to the Crab
pulsar, can be compensated for by using regularly updated timing ephemerides.


title:
Casimir effect for concentric spheres in de Sitter spacetime with
  signature change

abstract:
  We revisit the Casimir effect for two concentric spherical shells in de
Sitter background with a new geometric configuration, namely Euclidean
signature between and Lorentzian signature outside the spheres with different
cosmological constants, for a massless scalar field satisfying Dirichlet
boundary conditions on the spheres. It is shown that an extra constant pressure
emerges due to this signature changing configuration. Some interesting aspects
of this extra term are then discussed.


title: Signature change, vacuum condensation and cosmological constant

abstract:
  We proposes an alternative model of duality symmetry, based on the previously
obtained divergence theory, including an scalar field, an internal vector and a
metric signature. At some small scale an effective scalar field equation has
appeared whose potential acts like a Higgs one, where the metric signature
plays the role of an order parameter. Non-vanishing Vacuum condensation of this
Higgs field occurs once a signature change from Euclidean to Lorentzian is
formed. The mass scale of Higgs field excitations around this vacuum may
contribute, in the Lorentzian sector, to the cosmological constant, in
agreement with observations.


title:
A fast search strategy for gravitational waves from low-mass X-ray
  binaries

abstract:
  We present a new type of search strategy designed specifically to find
continuously emitting gravitational wave sources in known binary systems based
on the incoherent sum of frequency modulated binary signal sidebands. The
search pipeline can be divided into three stages: the first is a wide
bandwidth, F-statistic search demodulated for sky position. This is followed by
a fast second stage in which areas in frequency space are identified as signal
candidates through the frequency domain convolution of the F-statistic with an
approximate signal template. For this second stage only precise information on
the orbit period and approximate information on the orbital semi-major axis are
required apriori. For the final stage we propose a fully coherent Markov chain
monte carlo based follow up search on the frequency subspace defined by the
candidates identified by the second stage. This search is particularly suited
to the low-mass X-ray binaries, for which orbital period and sky position are
typically well known and additional orbital parameters and neutron star spin
frequency are not. We note that for the accreting X-ray millisecond pulsars,
for which spin frequency and orbital parameters are well known, the second
stage can be omitted and the fully coherent search stage can be performed. We
describe the search pipeline with respect to its application to a simplified
phase model and derive the corresponding sensitivity of the search.


title:
Gravitational waves during inflation in presence of a decaying
  cosmological parameter from a 5D vacuum theory of gravity

abstract:
  We study gravitational waves generated during the inflationary epoch in
presence of a decaying cosmological parameter on a 5D geometrical background
which is Riemann flat. Two examples are considered, one with a constant
cosmological parameter and the second with a decreasing one.


title: Non-Minimally Coupled Scalar Field and Ashtekar Variables

abstract:
  The non-minimal coupling of a scalar field is considered in the framework of
Ashtekar's new variables formulation of gravity. A first order action
functional for this system is derived in which the field variables are a tetrad
field, and an SL(2,C) connection, together with the scalar field. The tetrad
field and the SL(2,C) connection are related to the Ashtekar variables for the
vacuum case by a conformal transformation. A canonical analysis shows that for
this coupling the equations of Ashtekar's formulation of canonical gravity are
non-polynomial in the scalar field. (to be published in Phys. Rev. D)


title: Remarks on Pure Spin Connection Formulations of Gravity

abstract:
  In the derivation of a pure spin connection action functional for gravity two
methods have been proposed. The first starts from a first order lagrangian
formulation, the second from a hamiltonian formulation. In this note we show
that they lead to identical results for the specific cases of pure gravity with
or without a cosmological constant.


title: Nonsymmetric Gravity Theories: Inconsistencies and a Cure

abstract:
  Motivated by the apparent dependence of string $\sigma$--models on the sum of
spacetime metric and antisymmetric tensor fields, we reconsider gravity
theories constructed from a nonsymmetric metric. We first show that all such
"geometrical" theories homogeneous in second derivatives violate standard
physical requirements: ghost-freedom, absence of algebraic inconsistencies or
continuity of degree-of-freedom content. This no-go result applies in
particular to the old unified theory of Einstein and its recent avatars.
However, we find that the addition of nonderivative, ``cosmological'' terms
formally restores consistency by giving a mass to the antisymmetric tensor
field, thereby transmuting it into a fifth-force-like massive vector but with
novel possible matter couplings. The resulting macroscopic models also exhibit
``van der Waals''-type gravitational effects, and may provide useful
phenomenological foils to general relativity.


title:
One Formulation for both Lineal Gravities through a Dimensional
  Reduction

abstract:
  The two lineal gravities --- based on the de Sitter group or a central
extension of the Poincar\'e group in 1+1 dimensions --- are shown to derive
classically from a unique topological gauge theory. This one is obtained after
a dimensional reduction of a Chern--Simons model, which describes pure gravity
in 2+1 dimensions, the gauge symmetry being given by an extension of ISO(2,1).


title: Degenerate Extensions of General Relativity

abstract:
  General relativity has previously been extended to incorporate degenerate
metrics using Ashtekar's hamiltonian formulation of the theory. In this letter,
we show that a natural alternative choice for the form of the hamiltonian
constraints leads to a theory which agrees with GR for non-degenerate metrics,
but differs in the degenerate sector from Ashtekar's original degenerate
extension. The Poisson bracket algebra of the alternative constraints closes in
the non-degenerate sector, with structure functions that involve the {\it
inverse} of the spatial triad. Thus, the algebra does {\it not} close in the
degenerate sector. We find that it must be supplemented by an infinite number
ofsecondary constraints, which are shown to be first class (although their
explicit form is not worked out in detail). All of the constraints taken
together are implied by, but do not imply, Ashtekar's original form of
constraints. Thus, the alternative constraints give rise to a different
degenerate extension of GR. In the corresponding quantum theory, the single
loop and intersecting loop holonomy states found in the connection
representation satisfy {\it all} of the constraints. These states are therefore
exact (formal) solutions to this alternative degenerate extension of quantum
gravity, even though they are {\it not} solutions to the usual vector
constraint.


title: The Spin Holonomy Group In General Relativity

abstract:
  It has recently been shown by Goldberg et al that the holonomy group of the
chiral spin-connection is preserved under time evolution in vacuum general
relativity. Here, the underlying reason for the time-independence of the
holonomy group is traced to the self-duality of the curvature 2-form for an
Einstein space. This observation reveals that the holonomy group is
time-independent not only in vacuum, but also in the presence of a cosmological
constant. It also shows that once matter is coupled to gravity, the
"conservation of holonomy" is lost. When the fundamental group of space is
non-trivial, the holonomy group need not be connected. For each homotopy class
of loops, the holonomies comprise a coset of the full holonomy group modulo its
connected component. These cosets are also time-independent. All possible
holonomy groups that can arise are classified, and examples are given of
connections with these holonomy groups. The classification of local and global
solutions with given holonomy groups is discussed.


title: Comments on Theoretical Problems in Nonsymmetric Gravitational Theory

abstract:
  Damour, Deser and McCarthy have claimed that the nonsymmetric gravitational
theory (NGT) is untenable due to curvature coupled ghost modes and bad
asymptotic behavior. This claim is false for it is based on a physically
inaccurate treatment of wave propagation on a curved background and an
incorrect method for extracting asymptotic behavior. We show that the flux of
gravitational radiation in NGT is finite in magnitude and positive in sign.


title: The No-Hair Theorem for the Abelian Higgs Model

abstract:
  We consider the general procedure for proving no-hair theorems for static,
spherically symmetric black holes. We apply this method to the abelian Higgs
model and find a proof of the no-hair conjecture that circumvents the
objections raised against the original proof due to Adler and Pearson.


title:
Gravitational Waves from an Axi-symmetric Source in the Nonsymmetric
  Grav. Theory

abstract:
  We examine gravitational waves in an isolated axi--symmetric reflexion
symmetric NGT system. The structure of the vacuum field equations is analyzed
and the exact solutions for the field variables in the metric tensor are found
in the form of expansions in powers of a radial coordinate. We find that in the
NGT axially symmetric case the mass of the system remains constant only if the
system is static (as it necessarily is in the case of spherical symmetry). If
the system radiates, then the mass decreases monotonically and the energy flux
associated with waves is positive.


title: The Dirac Equation Is Separable On The Dyon Black Hole Metric

abstract:
  Using the tetrad formalism, we carry out the separation of variables for the
massive complex Dirac equation in the gravitational and electromagnetic field
of a four-parameter (mass, angular momentum, electric and magnetic charges)
black hole.


title: Centrifugal force in Kerr geometry

abstract:
  We have obtained the correct expression for the centrifugal force acting on a
particle at the equatorial circumference of a rotating body in the locally
non-rotating frame of the Kerr geometry. Using this expression for the
equilibrium of an element on the surface of a slowly rotating Maclaurin
spheroid, we obtain the expression for the ellipticity (as discussed earlier by
Abramowicz and Miller) and determine the radius at which the ellipticity is
maximum.


title: The Interpretation of Quantum Cosmological Models

abstract:
  We consider the problem of extracting physical predictions from the wave
function of the universe in quantum cosmological models. We state the features
of quantum cosmology an interpretational scheme should confront. We discuss the
Everett interpretation, and extensions of it, and their application to quantum
cosmology. We review the steps that are normally taken in the process of
extracting predictions from solutions to the Wheeler-DeWitt equation for
quantum cosmological models. Some difficulties and their possible resolution
are discussed. We conclude that the usual wave function-based approach admits
at best a rather heuristic interpretation, although it may in the future be
justified by appeal to the decoherent histories approach.


title: Thermodynamics of Two-Dimensional Black-Holes

abstract:
  We explore the thermodynamics of a general class of two dimensional dilatonic
black-holes. A simple prescription is given that allows us to compute the mass,
entropy and thermodynamic potentials, with results in agreement with those
obtained by other methods, when available.


title: Generation of seed perturbations from Quantum Cosmology

abstract:
  The origin of seed perturbations in the Universe is studied within the
framework of a specific minisuperspace model. It is shown that the `creation'
of the Universe as a result of a quantum transition from a flat empty spacetime
would lead to a flat FLRW (Friedmann Lema\^\i tre Robertson-Walker) Universe
with weak inhomogeneous perturbations at large wavelengths. The power spectrum
of these perturbations is found to be scale invariant at horizon crossing
(i.e., the Harrison-Zeldovich spectrum). It is also recognised that the seed
perturbations generated in our model would be generically of the isocurvature
kind.


title: The $D\to 2$ Limit of General Relativity

abstract:
  A method for taking the $D\to 2$ limit of D-dimensional general relativity is
constructed, yielding a two-dimensional theory which couples gravitation to
conserved stress-energy. We show how this theory is related to those obtained
via an alternative dimensional reduction approach.


title: Nonlinear Noise in Cosmology

abstract:
  This paper derives and analyzes exact, nonlocal Langevin equations
appropriate in a cosmological setting to describe the interaction of some
collective degree of freedom with a surrounding ``environment.'' Formally,
these equations are much more general, involving as they do a more or less
arbitrary ``system,'' characterized by some time-dependent potential, which is
coupled via a nonlinear, time-dependent interaction to a ``bath'' of
oscillators with time-dependent frequencies. The analysis reveals that, even in
a Markov limit, which can often be justified, the time dependences and
nonlinearities can induce new and potentially significant effects, such as
systematic and stochastic mass renormalizations and state-dependent ``memory''
functions, aside from the standard ``friction'' of a heuristic Langevin
description. One specific example is discussed in detail, namely the case of an
inflaton field, characterized by a Landau-Ginsburg potential, that is coupled
quadratically to a bath of scalar ``radiation.'' The principal conclusion
derived from this example is that nonlinearities and time-dependent couplings
do {\em not} preclude the possibility of deriving a fluctuation-dissipation
theorem, and do {\em not} change the form of the late-time steady state
solution for the system, but {\em can} significantly shorten the time scale for
the approach towards the steady state.


title: Stochastic Inflation:The Quantum Phase Space Approach

abstract:
  In this paper a quantum mechanical phase space picture is constructed for
coarse-grained free quantum fields in an inflationary Universe. The appropriate
stochastic quantum Liouville equation is derived. Explicit solutions for the
phase space quantum distribution function are found for the cases of power law
and exponential expansions. The expectation values of dynamical variables with
respect to these solutions are compared to the corresponding cutoff regularized
field theoretic results (we do not restrict ourselves only to $\VEV{\F^2}$).
Fair agreement is found provided the coarse-graining scale is kept within
certain limits. By focusing on the full phase space distribution function
rather than a reduced distribution it is shown that the thermodynamic
interpretation of the stochastic formalism faces several difficulties (e.g.,
there is no fluctuation-dissipation theorem). The coarse-graining does not
guarantee an automatic classical limit as quantum correlations turn out to be
crucial in order to get results consistent with standard quantum field theory.
Therefore, the method does {\em not} by itself constitute an explanation of the
quantum to classical transition in the early Universe. In particular, we argue
that the stochastic equations do not lead to decoherence.


title: There is no $R^3 X S^1$ vacuum gravitational instanton

abstract:
  Gravitational instantons, solutions to the euclidean Einstein equations, with
topology $R^3 XS^1$ arise naturally in any discussion of finite temperature
quantum gravity. This Letter shows that all such instantons (irrespective of
their interior behaviour) must have the same asymptotic structure as the
Schwarzschild instanton. Using this, it can be shown that if the Ricci tensor
of the manifold is non-negative it must be flat. One special case is when the
Ricci tensor vanishes; hence one can conclude that there is no nontrivial
vacuum gravitational instanton. This result has uses both in quantum and
classical gravity. It places a significant restriction on the instabilities of
hot flat space. It also can be used to show that any static vacuum Lorentzian
Kaluza-Klein solution is flat.


title: Trapped surfaces and spherical closed cosmologies

abstract:
  This article gives necessary and sufficient conditions for the formation of
trapped surfaces in spherically symmetric initial data defined on a closed
manifold. Such trapped surfaces surround a region in which there occurs an
enhancement of matter over the average. The conditions are posed directly in
terms of physical variables and show that what one needs is a relatively large
amount of excess matter confined to a small volume. The expansion of the
universe and an outward flow of matter oppose the formation of trapped
surfaces; an inward flow of matter helps. The model can be regarded as a
Friedmann-Lema\^\i tre-Walker cosmology with localized spherical
inhomogeneities. We show that the total excess mass cannot be too large.


title: The Entropy of the Gravitational Field

abstract:
  We derive a formula for the nonequilibrium entropy of a classical stochastic
field in terms of correlation functions of this field. The formalism is then
applied to define the entropy of gravitational perturbations (both
gravitational waves and density fluctuations). We calculate this entropy in a
specific cosmological model (the inflationary Universe) and find that on scales
of interest in cosmology the entropy in both density perturbations and
gravitational waves exceeds the entropy of statistical fluctuations of the
microwave background. The nonequilibrium entropy discussed here is a measure of
loss of information about the system. We discuss the origin of the entropy in
our cosmological models and compare the definition of entropy in terms of
correlation functions with the microcanonical definition in quantum statistical
mechanics.


title: Massive Analogue of Ashtekar-CJD Action

abstract:
  The action of Ashtekar gravity have been found by Cappovilla, Jacobson and
Dell. It does not depend on the metric nor the signature of the space-time. The
action has a similar structure as that of a massless relativistic particle. The
former is naturally generalized by adding a term analogous to a mass term of
the relativistic particle. The new action possesses a constant parameter
regarded as a kind of a cosmological constant. It is interesting to find a
covariant Einstein equation from the action. In order to do it we will examine
how the geometrical quantities are determined from the non-metric action and
how the Einstein equation follows from it.


title:
Comparison of the Sachs-Wolfe Effect for Gaussian and Non-Gaussian
  Fluctuations

abstract:
  A consequence of non-Gaussian perturbations on the Sachs-Wolfe effect is
studied. For a particular power spectrum, predicted Sachs-Wolfe effects are
calculated for two cases: Gaussian (random phase) configuration, and a specific
kind of non-Gaussian configuration. We obtain a result that the Sachs-Wolfe
effect for the latter case is smaller when each temperature fluctuation is
properly normalized with respect to the corresponding mass fluctuation ${\delta
M\over M}(R)$. The physical explanation and the generality of the result are
discussed.


title: Quantum Gravity, the Origin of Time and Time's Arrow

abstract:
  The local Lorentz and diffeomorphism symmetries of Einstein's gravitational
theory are spontaneously broken by a Higgs mechanism by invoking a phase
transition in the early Universe, at a critical temperature $T_c$ below which
the symmetry is restored. The spontaneous breakdown of the vacuum state
generates an external time and the wave function of the Universe satisfies a
time dependent Schrodinger equation, which reduces to the Wheeler-deWitt
equation in the classical regime for $T < T_c$, allowing a semi-classical WKB
approximation to the wave function. The conservation of energy is spontaneously
violated for $T > T_c$ and matter is created fractions of seconds after the big
bang, generating the matter in the Universe. The time direction of the vacuum
expectation value of the scalar Higgs field generates a time asymmetry, which
defines the cosmological arrow of time and the direction of increasing entropy
as the Lorentz symmetry is restored at low temperatures.


title: Results on exact solutions of low energy string theory

abstract:
  A family of solutions to low energy string theory is found. These solutions
represent waves traveling along "extremal black strings"


title: Transformations and BRST-Charges in 2+1 Dimensional Gravitation

abstract:
  Canonical transformations relating the variables of the ADM-, Ashtekar's and
Witten's formulations of gravity are computed in 2+1~dimensions. Three
different forms of the BRST-charge are given in the 2+1 dimensional Ashtekar
formalism, two of them using Ashtekar's form of the constraints and one of them
using the forms suggested by Witten. The BRST-charges are of different rank.


title: A Quasilocal Test of the Finiteness of the Universe

abstract:
  The Cosmological Principle states that the universe is both homogeneous and
isotropic. This, alone, is not enough to specify the global geometry of the
spacetime. If we were able to measure both the Hubble constant and the energy
density we could determine whether the universe is open or closed.
Unfortunately, while some agreement exists on the value of the Hubble constant,
the question of the energy density seems quite intractable. This Letter
describes a possible way of avoiding this difficulty and shows that if one
could measure the rate at which light-rays emerging from a surface expand, one
might well be able to deduce whether the universe is closed.


title: The Loss of Unitarity in the Vicinity of a Time Machine

abstract:
  We construct the propagator of a non-relativistic non-interacting particle in
a flat spacetime in which two regions have been identified. This corresponds to
the simplest "time machine". We show that while completeness is lost in the
vicinity of the time machine it holds before the time machine appears and it is
recovered afterwards. Unitarity, however, is not satisfied anywhere. We discuss
the implications of these results and their relationship to the loss of
unitarity in black hole evaporation.


title: Asymptotic Behavior of the $T^3 \times R$ Gowdy Spacetimes

abstract:
  We present new evidence in support of the Penrose's strong cosmic censorship
conjecture in the class of Gowdy spacetimes with $T^3$ spatial topology.
Solving Einstein's equations perturbatively to all orders we show that
asymptotically close to the boundary of the maximal Cauchy development the
dominant term in the expansion gives rise to curvature singularity for almost
all initial data. The dominant term, which we call the ``geodesic loop
solution'', is a solution of the Einstein's equations with all space
derivatives dropped. We also describe the extent to which our perturbative
results can be rigorously justified.


title: Semiclassical Effects and the Onset of Inflation

abstract:
  We present a class of exact solutions to the constraint equations of General
Relativity coupled to a Klein - Gordon field, these solutions being isotropic
but not homogeneous. We analyze the subsequent evolution of the consistent
Cauchy data represented by those solutions, showing that only certain special
initial conditions eventually lead to successfull Inflationary cosmologies. We
argue, however, that these initial conditions are precisely the likely outcomes
of quantum events occurred before the inflationary era.


title: Continuous Family of Einstein-Yang-Mills Wormholes

abstract:
  It is shown that for some particular value of the cosmological constant
depending on the gauge coupling constant a continuous one-parameter family of
Einstein-Yang-Mills wormholes exists which interpolates between the instanton
and the gravitating meron solutions. In contradistinction with the previously
known solutions the topological charge of these wormholes is not quantized. For
all of them the contribution of gravity to the action exactly cancels that of
the gauge field.


title: Euclidean Maxwell-Einstein Theory

abstract:
  After reviewing the context in which Euclidean propagation is useful we
compare and contrast Euclidean and Lorentzian Maxwell-Einstein theory and give
some examples of Euclidean solutions.


title: Detection, Measurement and Gravitational Radiation

abstract:
  Here I examine how to determine the sensitivity of the LIGO, VIRGO, and LAGOS
gravitational wave detectors to sources of gravitational radiation by
considering the process by which data are analyzed in a noisy detector. By
constructing the probability that the detector output is consistent with the
presence of a signal, I show how to (1) quantify the uncertainty that the
output contains a signal and is not simply noise, and (2) construct the
probability distribution that the signal parameterization has a certain value.
  From the distribution and its mode I determine volumes $V(P)$ in parameter
space such that actual signal parameters are in $V(P)$ with probability $P$. If
we are {\em designing} a detector, or determining the suitability of an
existing detector for observing a new source, then we don't have detector
output to analyze but are interested in the ``most likely'' response of the
detector to a signal. I exploit the techniques just described to determine the
``most likely'' volumes $V(P)$ for detector output corresponding to the source.
Finally, as an example, I apply these techniques to anticipate the sensitivity
of the LIGO and LAGOS detectors to the gravitational radiation from a perturbed
Kerr black hole.


title:
The Modular Group, Operator Ordering, and Time in (2+1)-Dimensional
  Gravity

abstract:
  A choice of time-slicing in classical general relativity permits the
construction of time-dependent wave functions in the ``frozen time''
Chern-Simons formulation of $(2+1)$-dimensional quantum gravity. Because of
operator ordering ambiguities, however, these wave functions are not unique. It
is shown that when space has the topology of a torus, suitable operator
orderings give rise to wave functions that transform under the modular group as
automorphic functions of arbitrary weights, with dynamics determined by the
corresponding Maass Laplacians on moduli space.