title:
On explicit solutions to the stationary axisymmetric Einstein-Maxwell
  equations describing dust disks

abstract:
  We review explicit solutions to the stationary axisymmetric Einstein-Maxwell
equations which can be interpreted as disks of charged dust. The disks of
finite or infinite extension are infinitesimally thin and constitute a surface
layer at the boundary of an electro-vacuum. The Einstein-Maxwell equations in
the presence of one Killing vector are obtained by using a projection
formalism. The SU(2,1) invariance of the stationary Einstein-Maxwell equations
can be used to construct solutions for the electro-vacuum from solutions to the
pure vacuum case via a so-called Harrison transformation. It is shown that the
corresponding solutions will always have a non-vanishing total charge and a
gyromagnetic ratio of 2. Since the vacuum and the electro-vacuum equations in
the stationary axisymmetric case are completely integrable, large classes of
solutions can be constructed with techniques from the theory of solitons. The
richest class of physically interesting solutions to the pure vacuum case due
to Korotkin is given in terms of hyperelliptic theta functions. The Harrison
transformed hyperelliptic solutions are discussed.


title: Cosmic Acceleration from Elementary Interactions

abstract:
  It is possible to generate an accelerated period of expansion from reasonable
potentials acting between the universe particle constituents. The pressure of
primordial nucleons interacting via a simple nuclear potential is obtained via
Mayer's cluster expansion technique. The attractive part of the potential
engenders a negative pressure and may therefore be responsible for the cosmic
acceleration.


title:
Bifurcation and fine structure phenomena in critical collapse of a
  self-gravitating $\sigma$-field

abstract:
  Building on previous work on the critical behavior in gravitational collapse
of the self-gravitating SU(2) $\sigma$-field and using high precision numerical
methods we uncover a fine structure hidden in a narrow window of parameter
space. We argue that this numerical finding has a natural explanation within a
dynamical system framework of critical collapse.


title: Best chirplet chain: near-optimal detection of gravitational wave chirps

abstract:
  The list of putative sources of gravitational waves possibly detected by the
ongoing worldwide network of large scale interferometers has been continuously
growing in the last years. For some of them, the detection is made difficult by
the lack of a complete information about the expected signal. We concentrate on
the case where the expected GW is a quasi-periodic frequency modulated signal
i.e., a chirp. In this article, we address the question of detecting an a
priori unknown GW chirp. We introduce a general chirp model and claim that it
includes all physically realistic GW chirps. We produce a finite grid of
template waveforms which samples the resulting set of possible chirps. If we
follow the classical approach (used for the detection of inspiralling binary
chirps, for instance), we would build a bank of quadrature matched filters
comparing the data to each of the templates of this grid. The detection would
then be achieved by thresholding the output, the maximum giving the individual
which best fits the data. In the present case, this exhaustive search is not
tractable because of the very large number of templates in the grid. We show
that the exhaustive search can be reformulated (using approximations) as a
pattern search in the time-frequency plane. This motivates an approximate but
feasible alternative solution which is clearly linked to the optimal one.
[abridged version of the abstract]


title: Gravitacional field of a global defect

abstract:
  Global topological defects described by real scalar field in (3,1) dimensions
coupled to gravity are analyzed. We consider a class of scalar potentials with
explicit dependence with distance, evading Derrick's theorem and leading to
defects with spherical symmetry. The analysis shows that the defects have
finite energy on flat space, contrary to the observed for the global monopole.
With the aim to study the gravitational field produced by such defects, after
an {\it Ansatz} for the static metric with spherical symmetry, we obtain the
coupled system of Einstein and field equations. On the Newtonian approximation,
we numerically find that the defects have a repulsive gravitational field. This
field is like one generated by a negative mass distributed on a spherical
shell. In the weak gravity regime a relation between the Newtonian potential
and one of the metric coefficients is obtained. The numerical analysis in this
regime leads to a spacetime with a deficit solid angle on the core of the
defect.


title: Naturalness in emergent spacetime

abstract:
  Effective field theories (EFTs) have been widely used as a framework in order
to place constraints on the Planck suppressed Lorentz violations predicted by
various models of quantum gravity. There are however technical problems in the
EFT framework when it comes to ensuring that small Lorentz violations remain
small -- this is the essence of the "naturalness" problem. Herein we present an
"emergent" space-time model, based on the "analogue gravity'' programme, by
investigating a specific condensed-matter system that is in principle capable
of simulating the salient features of an EFT framework with Lorentz violations.
Specifically, we consider the class of two-component BECs subject to
laser-induced transitions between the components, and we show that this model
is an example for Lorentz invariance violation due to ultraviolet physics.
Furthermore our model explicitly avoids the "naturalness problem", and makes
specific suggestions regarding how to construct a physically reasonable quantum
gravity phenomenology.


title: Generalized second law in phantom dominated universe

abstract:
  We study the conditions of validity of generalized second law in phantom
dominated era.


title: Causal structures in the four dimensional Euclidean space

abstract:
  It is shown that in the 4d Euclidean space there are two causal structures
defined by the temporal field. One of them is well-known Minkowski spacetime.
In this case the gravitational potential (the positive definite Riemann metric)
and temporal field satisfy the Einstein equations with trivial energy-momentum
tensor. However, in the case of the second causal structure the gravitational
potential and temporal field should be connected with some nontrivial
energy-momentum tensor. We consider the simplest case with energy-momentum
tensor of the real scalar field and derive exact solution of the field
equations. It can be viewed as the ground to consider the second causal
structure on the equal footing with the Minkowski spacetime, i.e., as an object
interesting from the physical point of view, especially in the framework of the
field theory.


title: A combinatorial approach to discrete geometry

abstract:
  We present a paralell approach to discrete geometry: the first one introduces
Voronoi cell complexes from statistical tessellations in order to know the mean
scalar curvature in term of the mean number of edges of a cell. The second one
gives the restriction of a graph from a regular tessellation in order to
calculate the curvature from pure combinatorial properties of the graph.
  Our proposal is based in some epistemological pressupositions: the
macroscopic continuous geometry is only a fiction, very usefull for describing
phenomena at certain sacales, but it is only an approximation to the true
geometry. In the discrete geometry one starts from a set of elements and the
relation among them without presuposing space and time as a background.


title: Cylindrically Symmetric-Static Brans-Dicke-Maxwell Solutions

abstract:
  We present static cylindrically symmetric electrovac solutions in the
framework of the Brans-Dicke theory and show that our solution yields some of
the well-known solutions for special values of the parameters of the resulting
metric functions.


title: Spin networks in quantum gravity

abstract:
  This is a review paper about one of the approaches to unify Quantum Mechanics
and the theory of General Relativity. Starting from the pioneer work of Regge
and Penrose other scientists have constructed state sum models, as Feymann path
integrals, that are topological invariant on the triangulated Riemannian
surfaces, and that in the continuous limit become the Hilbert-Einstein action.


title: Geometric Analysis and General Relativity

abstract:
  This article discusses methods of geometric analysis in general relativity,
with special focus on the role of "critical surfaces" such as minimal surfaces,
marginal surface, maximal surfaces and null surfaces.


title: Remarks on the WDW equation

abstract:
  We show a kind of converse to some results of Hall and Reginatto on exact
uncertainty related to the Schroedinger and Wheeler-deWitt equations. Some
survey material on statistical geometrodynamics is also sketched.


title: Relativistic MHD and black hole excision: Formulation and initial tests

abstract:
  A new algorithm for solving the general relativistic MHD equations is
described in this paper. We design our scheme to incorporate black hole
excision with smooth boundaries, and to simplify solving the combined Einstein
and MHD equations with AMR. The fluid equations are solved using a finite
difference Convex ENO method. Excision is implemented using overlapping grids.
Elliptic and hyperbolic divergence cleaning techniques allow for maximum
flexibility in choosing coordinate systems, and we compare both methods for a
standard problem. Numerical results of standard test problems are presented in
two-dimensional flat space using excision, overlapping grids, and elliptic and
hyperbolic divergence cleaning.


title:
Classical Solutions in Einstein's Gravity and Study of some Collapsing
  Models

abstract:
  My research work can be classified into two parts namely, (i) Cosmological
phenomena with varying speed of light and (ii) Gravitational collapse and black
holes.
  We have investigated several cosmological phenomena when velocity of light
varies with time. We have considered the variation of light as power-law in
time and have studied the Brans-Dicke cosmology, Quintessence problem and
others. We have also examined whether some cosmological problems can be solved
using it for anisotropic space-time model.
  In brane scenario we have studied some cosmological implications namely, the
Cosmic No-Hair Conjecture (CNHC) and variable gravity theory. In CNHC we have
shown that in realistic situation no more extra conditions are needed in brane
scenario.
  In gravitational collapse, we have mainly concentrated to spherical collapse
for dust distribution. We have considered both four and higher dimensional
space-time and have examined the possibility for formation of black hole or
naked singularity. In marginally bound case we have found naked singularity is
possible only for four and five dimension, while for non-marginally bound case
the possibility of black hole increases with the increase in the dimension of
the space-time.
  In quasi-spherical gravitational collapse for dust distribution, we have
(n+2) dimensional Szekeres space-time and have examined the possibility for
formation of black hole or naked singularity. In marginally bound case, we have
found that possibility of naked singularity depends on dimension of the
space-time.


title:
Asymptotically null slices in numerical relativity: mathematical
  analysis and spherical wave equation tests

abstract:
  We investigate the use of asymptotically null slices combined with stretching
or compactification of the radial coordinate for the numerical simulation of
asymptotically flat spacetimes. We consider a 1-parameter family of coordinates
characterised by the asymptotic relation $r\sim R^{1-n}$ between the physical
radius $R$ and coordinate radius $r$, and the asymptotic relation $K\sim
R^{n/2-1}$ for the extrinsic curvature of the slices. These slices are
asymptotically null in the sense that their Lorentz factor relative to
stationary observers diverges as $\Gamma\sim R^{n/2}$. While $1<n\le 2$ slices
intersect $\scri$, $0< n\le 1$ slices end at $i^0$. We carry out numerical
tests with the spherical wave equation on Minkowski and Schwarzschild
spacetime. Simulations using our coordinates with $0<n\le 2$ achieve higher
accuracy at lower computational cost in following outgoing waves to very large
radius than using standard $n=0$ slices without compactification. Power-law
tails in Schwarzschild are also correctly represented.


title: Radiating black hole solutions in Einstein-Gauss-Bonnet gravity

abstract:
  In this paper, we find some new exact solutions to the Einstein-Gauss-Bonnet
equations. First, we prove a theorem which allows us to find a large family of
solutions to the Einstein-Gauss-Bonnet gravity in $n$-dimensions. This family
of solutions represents dynamic black holes and contains, as particular cases,
not only the recently found Vaidya-Einstein-Gauss-Bonnet black hole, but also
other physical solutions that we think are new, such as, the Gauss-Bonnet
versions of the Bonnor-Vaidya(de Sitter/anti-de Sitter) solution, a global
monopole and the Husain black holes. We also present a more general version of
this theorem in which less restrictive conditions on the energy-momentum tensor
are imposed. As an application of this theorem, we present the exact solution
describing a black hole radiating a charged null fluid in a Born-Infeld
nonlinear electrodynamics.


title: Lorentz Gauge Gravity and Induced Effective Theories

abstract:
  We develop the gauge approach based on the Lorentz group to the gravity with
torsion. With a Lagrangian quadratic in curvature we show that the
Einstein-Hilbert action can be induced from a simple gauge model due to quantum
corrections of torsion via formation of a gravito-magnetic condensate. An
effective theory of cosmic knots at Planckian scale is proposed.


title: Supersymmetry Breaking Casimir Warp Drive

abstract:
  Within the framework of brane-world models it is possible to account for the
cosmological constant by assuming supersymmetry is broken on the 3-brane but
preserved in the bulk. An effective Casimir energy is induced on the brane due
to the boundary conditions imposed on the compactified extra dimensions. It
will be demonstrated that modification of these boundary conditions allows a
spacecraft to travel at any desired speed due to a local adjustment of the
cosmological constant which effectively contracts/expands space-time in the
front/rear of the ship resulting in motion potentially faster than the speed of
light as seen by observers outside the disturbance.


title: Charged particles' tunnelling from the Kerr-Newman black hole

abstract:
  In this letter, Parikh-Wilczek tunnelling framework, which treats Hawking
radiation as a tunnelling process, is extended, and the emission rate of a
charged particle tunnelling from the Kerr-Newman black hole is calculated. The
emission spectrum takes the same functional form as that of uncharged particles
and consists with an underlying unitary theory but deviates from the pure
thermal spectrum. Moreover, our calculation indicates that the emission process
is treated as a reversible process in the Parikh-Wilczek tunnelling framework,
and the information conservation is a natural result of the first law of black
hole thermodynamics.


title: Lower Neutrino Mass Bound from SN1987A Data and Quantum Geometry

abstract:
  A lower bound on the light neutrino mass $m_\nu$ is derived in the framework
of a geometrical interpretation of quantum mechanics. Using this model and the
time of flight delay data for neutrinos coming from SN1987A, we find that the
neutrino masses are bounded from below by $m_\nu\gtrsim 10^{-4}-10^{-3}$eV, in
agreement with the upper bound $m_\nu\lesssim$ $({\cal O}(0.1) - {\cal O} (1))$
eV currently available. When the model is applied to photons with effective
mass, we obtain a lower limit on the electron density in intergalactic space
that is compatible with recent baryon density measurements.


title: Bulk Viscous Cosmological Models in General Relativity

abstract:
  A new class of exact solutions of Einstein's field equations with bulk
viscous fluid for an LRS Bianchi type-I spacetime is obtained by using
deceleration parameter as variable. The value of Hubble's constant $H_{0}$ is
found to be less than one for non-flat model and is equal to 1.5 for flat model
which are of the physical interest. Some physical and geometric properties of
the models are also discussed.


title: Axially symmetric solution in Teleparallel Theory of Gravitation

abstract:
  An exact solution has an axial symmetry is obtained in the teleparallel
theory of gravitation. The associated metric has the structure function
G(xi)=1-xi^2-2mA(xi)^3. The cubic nature of the structure function can make
calculations cumbersome. Using a coordinate transformation we get a tetrad that
its associated metric has the structure function in a factorisable form. This
new form has the advantage that its roots are now trivial to write down. The
singularities of the obtained tetrad are studied. Using another coordinate
transformation we get a tetrad that its associated metric gives the
Schwarzschild spacetime. Calculate the energy content of this tetrad we get a
meaningless result!


title:
Light propagation and gravitational lensing on the Weyl-like spacetime
  in scalar-tensor theories of gravity

abstract:
  We study light propagation and gravitational lensing in scalar-tensor
theories of gravity by using a static, axisymmetric exterior solution. The
solution has asymptotic flatness properties and is reduced to Voorhees's one in
the case of a constant scalar field. Our studies are done by using a technique
of the conformal transformation such that their results are independent of
details of scalar-tensor theories. For some specific cases, we analytically
obtain a deflection angle of the light path and find that it can become
negative. The appearance of a negative deflection angle indicates
``reflection'' of a light path, and we investigate under which conditions the
light reflection occurs. As for the optical scalars, the Weyl source-term shows
significantly different properties compared with that in the Schwarzschild
spacetime. We therefore classify a space of the model parameters into four
distinct regions on the basis of the qualitative properties of the Weyl
source-term and find a close relationship between this classification and the
occurrence of the light reflection. We finally solve the null geodesic
equations and the optical scalar equations numerically. We find that a picture
of the thin lens is applicable and give a simple analytic model for the optical
scalars. As for the properties of gravitational lensing, the deflection angle
and the image distortion rate are obtained as functions of the impact
parameter. Again, we find a close relationship between their qualitative
properties and the classification above.


title: Generalized Chaplygin gas model, supernovae and cosmic topology

abstract:
  In this work we study to which extent the knowledge of spatial topology may
place constraints on the parameters of the generalized Chaplygin gas (GCG)
model for unification of dark energy and dark matter. By using both the
Poincar\'e dodecahedral and binary octahedral spaces as the observable spatial
topologies, we examine the current type Ia supernovae (SNe Ia) constraints on
the GCG model parameters. We show that the knowledge of spatial topology does
provide additional constraints on the $A_s$ parameter of the GCG model but does
not lift the degeneracy of the $\alpha$ parameter.


title: Spin-1 gravitational waves. Theoretical and experimental aspects

abstract:
  Exact solutions of Einstein field equations invariant for a non-Abelian
2-dimensional Lie algebra of Killing fields are described. Physical properties
of these gravitational fields are studied, their wave character is checked by
making use of covariant criteria and the observable effects of such waves are
outlined. The possibility of detection of these waves with modern detectors,
spherical resonant antennas in particular, is sketched.


title:
On gravitational-wave spectroscopy of massive black holes with the space
  interferometer LISA

abstract:
  Newly formed black holes are expected to emit characteristic radiation in the
form of quasi-normal modes, called ringdown waves, with discrete frequencies.
LISA should be able to detect the ringdown waves emitted by oscillating
supermassive black holes throughout the observable Universe. We develop a
multi-mode formalism, applicable to any interferometric detectors, for
detecting ringdown signals, for estimating black hole parameters from those
signals, and for testing the no-hair theorem of general relativity. Focusing on
LISA, we use current models of its sensitivity to compute the expected
signal-to-noise ratio for ringdown events, the relative parameter estimation
accuracy, and the resolvability of different modes. We also discuss the extent
to which uncertainties on physical parameters, such as the black hole spin and
the energy emitted in each mode, will affect our ability to do black hole
spectroscopy.


title: Scalar Perturbation Spectra in an Emergent Cosmological Island

abstract:
  The possibility that the universe may have a fundamental and positive
cosmological constant has motivated an interesting cosmological model, in which
initially the universe is in a cosmological constant sea, then the local
quantum fluctuations violating the null energy condition create some islands
with matter and radiation, which under certain conditions might corresponds to
our observable universe. We in this note study the perturbation spectra of
scalar fields not affecting the evolution of background during the fluctuation.
We will examine whether they can be interesting, and responsible for the
structure formation of observable universe.


title: Quantum gravitational proton decay at high temperature

abstract:
  One of the most important challenges of contemporary physics is to find
experimental signatures of quantum gravity. It is expected that quantum
gravitational effects lead to proton decay but on time scales way beyond what
is of any relevance to experiments. At non-zero temperatures there are reasons
to believe that the situation is much more favourable. We will argue that at
the temperatures and densities reached at present and future fusion facilities
there is a realistic possibility that proton decay could be detectable.


title:
IR Renormalisation of General Effective Actions and Hawking Flux in 2D
  Gravity Theories

abstract:
  The infrared problem of the effective action in 2D is discussed in the
framework of the Covariant Perturbation Theory. The divergences are regularised
by a mass and the leading term is evaluated up to the third order of
perturbation theory. A summation scheme is proposed which isolates the
divergences from the finite part of the series and results in a single term.
The latter turns out to be equivalent to the coupling to a certain classical
external field. This suggests a renormalisation by factorisation.


title:
The variation of the gravitational constant inferred from the Hubble
  diagram of Type Ia supernovae

abstract:
  We consider a cosmological model with a variable gravitational constant, G,
based on a scalar-tensor theory. Using the recent observational data for the
Hubble diagram of type Ia supernovae (SNeIa) we find a phenomenological
expression describing the variation of G. The corresponding variation of the
fine structure constant \alpha within multidimensional theories is also
computed and is shown not to support known constraints on \Delta \alpha /
\alpha.


title: Fermionic Quasinormal Spectrum of the Kerr Black Hole

abstract:
  We study {\it analytically} the asymptotic quasinormal spectrum of fermionic
fields in the Kerr spacetime. We find an analytic expression for these
black-hole resonances in terms of the black-hole physical parameters: its
Bekenstein-Hawking temperature $T_{BH}$, and its horizon's angular velocity
$\Omega$, which is valid in the asymptotic limit $1 \ll \omega_I \ll \omega_R$.
It is shown that according to Bohr's correspondence principle, the emission of
a Rarita-Schwinger quantum ($s=3/2$) corresponds to a fundamental black-hole
area change $\Delta A=4\hbar \ln 2$, while the emission of a Weyl neutrino
field ($s=1/2$) corresponds to an adiabatic quantum transition with $\Delta
A=0$.


title: Supersymmetric Lorentz invariant deformations of superspaces

abstract:
  Lorentz invariant supersymmetric deformations of superspaces based on Moyal
star product parametrized by Majorana spinor $\lambda_{a}$ and Ramond
grassmannian vector $\psi_{m}=-{1\over 2}(\bar\theta\gamma_{m}\lambda)$ in the
spinor realization \cite{VZ} are proposed. The map of supergravity background
into composite supercoordinates: $(B^{-1}_{mn}, \Psi^{a}_{m}, C_{ab})
\leftrightarrow (i\psi_{m}\psi_{n}, \psi_{m}\lambda^{a},
\lambda_{a}\lambda_{b})$ valid up to the second order corrections in
deformation parameter $h$ and transforming the background dependent Lorentz
noninvariant (anti)commutators of supercoordinates into their invariant Moyal
brackets is revealed. We found one of the deformations to depend on the axial
vector $\psi_{1m}={1/2}(\bar\theta\gamma_{m}\gamma_{5}\lambda)$ and to vanish
for the $\theta$ components with the same chiralities. The deformations in the
(super)twistor picture are discussed.


title: Over the Rainbow: Numerical Relativity beyond Scri+

abstract:
  This is a study of the behavior of wave equations in conformally compactified
spacetimes suited to the use of computational boundaries beyond Scri+. There
light cones may be adjusted for computational convenience and/or Scri+ may be
approximated by a "proto-Scri" spacelike hypersurface just outside a de Sitter
horizon. One expects a numerical implementation to excise the physically
unnecessary universe somewhat beyond the outer horizon. As an entry level
example I study forms of the Maxwell equations and causal relations for an
outer boundary in that example.


title:
Note on the relationship between the speed of light and gravity in the
  bi-metric theory of gravity

abstract:
  Relationship between the speed of gravity c_g and the speed of light c_e in
the bi-metric theory of gravity is discussed. We reveal that the speed of light
is a function of the speed of gravity which is a primary fundamental constant.
Thus, experimental measurement of relativistic bending of light propagating in
time-dependent gravitational field directly compares the speed of gravity
versus the speed of light and tests if there is any aether associated with the
gravitational field considered as a transparent `medium' with the constant
refraction index.


title: Event and Apparent Horizon Finders for 3+1 Numerical Relativity

abstract:
  Event and apparent horizons are key diagnostics for the presence and
properties of black holes. In this article I review numerical algorithms and
codes for finding event and apparent horizons in numerically-computed
spacetimes, focusing on calculations done using the 3+1 ADM formalism.
  There are 3 basic algorithms for finding event horizons, based respectively
on integrating null geodesics \emph{forwards} in time, integrating null
geodesics \emph{backwards} in time, and integrating null \emph{surfaces}
backwards in time. The last of these is generally the most efficient and
accurate.
  There are a large number of apparent-horizon finding algorithms, with
differing trade-offs between speed, robustness, accuracy, and ease of
programming. In axisymmetry, shooting algorithms work well and are fairly easy
to program. In slices with no continuous symmetries, Nakamura et al.'s
algorithm and elliptic-PDE algorithms are fast and accurate, but require good
initial guesses to converge. In many cases Schnetter's "pretracking" algorithm
can greatly improve an elliptic-PDE algorithm's robustness. Flow algorithms are
generally quite slow, but can be very robust in their convergence.


title:
Coupling of Radial and Axial non-Radial Oscillations of Compact Stars:
  Gravitational Waves from first-order Differential Rotation

abstract:
  We investigate the non-linear coupling between radial and non-radial
oscillations of static spherically symmetric neutron stars as a possible
mechanism for the generation of gravitational waves that may lead to observable
signatures. In this paper we concentrate on the axial sector of the non-radial
perturbations. By using a multi-parameter perturbative framework we introduce a
complete description of the non-linear coupling between radial and axial
non-radial oscillations; we study the gauge invariant character of the
associated perturbative variables and develop a computational scheme to evolve
the non-linear coupling perturbations in the time domain. We present results of
simulations corresponding to different physical situations and discuss the
dynamical behaviour of this non-linear coupling. Of particular interest is the
occurrence of signal amplifications in the form of resonance phenomena when a
frequency associated with the radial pulsations is close to a frequency
associated with one of the axial w-modes of the star. Finally, we mention
possible extensions of this work and improvements towards more astrophysically
motivated scenarios.


title: On the differential geometry of curves in Minkowski space

abstract:
  We discuss some aspects of the differential geometry of curves in Minkowski
space. We establish the Serret-Frenet equations in Minkowski space and use them
to give a very simple proof of the fundamental theorem of curves in Minkowski
space. We also state and prove two other theorems which represent Minkowskian
versions of a very known theorem of the differential geometry of curves in
tridimensional Euclidean space. We discuss the general solution for torsionless
paths in Minkowki space. We then apply the four-dimensional Serret-Frenet
equations to describe the motion of a charged test particle in a constant and
uniform electromagnetic field and show how the curvature and the torsions of
the four-dimensional path of the particle contain information on the
electromagnetic field acting on the particle.


title: Supersymmetry and the Supergravity Landscape

abstract:
  In the recent times a lot of effort has been devoted to improve our knowledge
about the space of string theory vacua (``the landscape'') to find statistical
grounds to justify how and why the theory selects its vacuum. Particularly
interesting are those vacua that preserve some supersymmetry, which are always
supersymmetric solutions of some supergravity theory. After an general
introduction to how the pursuit of unification has lead to the vacuum selection
problem, we are going to review some recent results on the problem of finding
all the supersymmetric solutions of a supergravity theory applied to the
N=4,d=4 supergravity case.


title: Gauge Theory of Gravity and Supergravity

abstract:
  We present a formulation of gravity in terms of a theory based on complex
SU(2) gauge fields with a general coordinate invariant action functional
quadratic in the field strength. Self-duality or anti-self-duality of the field
strength emerges as a constraint from the equations of motion of this theory.
This in turn leads to Einstein gravity equations for a dilaton and an axion
conformally coupled to gravity for the self-dual constraint. The analysis has
also been extended to N=1 and 2 super Yang-Mills theory of complex SU(2) gauge
fields. This leads, besides other equations of motion, to
self-duality/anti-self-duality of generalized supercovariant field-strengths.
The self-dual case is then shown to yield as its solutions $N = 1, 2$
supergravity equations respectively.


title: Local cosmic string and C-field

abstract:
  We investigate a local cosmic string with a phenomenological energy momentum
tensor as prescribed by Vilenkin, in presence of C-field . The solutions of
full nonlinear Einstein's equations for exterior and interior regions of such a
string are presented.


title: Black hole entropy in 3D gravity with torsion

abstract:
  The role of torsion in quantum three-dimensional gravity is investigated by
studying the partition function of the Euclidean theory in Riemann-Cartan
spacetime. The entropy of the black hole with torsion is found to differ from
the standard Bekenstein-Hawking result, but its form is in complete agreement
with the first law of black hole thermodynamics.


title: Gravitational Effects of Varying Alpha Strings

abstract:
  We study spatial variations of the fine-structure constant in the presence of
static straight cosmic strings in the weak-field approximation in Einstein
gravity. We work in the context of a generic Bekenstein-type model and consider
a gauge kinetic function linear in the scalar field. We determine an analytical
form for the scalar field and the string metric at large distances from the
core. We show that the gravitational effects of $\alpha$-varying strings can be
seen as a combination of the gravitational effects of global and local strings.
We also verify that at large distances to the core the space-time metric is
similar to that of a global string. We study the motion of test particles
approaching from infinity and show that photons are scattered to infinity while
massive particles are trapped in bounded trajectories. We also calculate an
overall limit on the magnitude of the variation of $\alpha$ for a GUT string,
by considering suitable cosmological constraints coming from the Equivalence
Principle.


title: Quantum field theory in curved spacetime

abstract:
  An encyclopedia article on mathematical aspects of quantum field theory in
curved spacetime. Section titles are: Introduction and preliminaries;
Construction of *-algebra for a real linear scalar field on globally hyperbolic
spacetimes and some general theorems; Particle creation and the limitations of
the particle concept; Theory of the stress-energy tensor; Hawking and Unruh
effects; Non-globally hyperbolic spacetimes and the time-machine question;
Other related topics and some warnings.


title:
Global existence of solutions for the Einstein-Boltzmann system with
  cosmological constant in the Robertson-Walker space-time for arbitrarily
  large initial data

abstract:
  We prove a global in time existence theorem for the initial value problem for
the Einstein-Boltzmann system, with positive cosmological constant and
arbitrarily large initial data, in the spatially homogeneous case, in a
Robertson-Walker space-time.


title: The Dirac equation in Rindler space: A pedagogical introduction

abstract:
  A pedagogical introduction to the Dirac equation for massive particles in
Rindler space is presented. The spin connection coefficients are explicitly
derived using techniques from general relativity. We then apply the
Lagrange-Green identity to greatly simplify calculation of the inner products
needed to normalize the states. Finally, the Bogolubov coefficients relating
the Rindler and Minkowski modes are derived in an intuitive manner. These
derivations are useful for students interested in learning about quantum field
theory in a curved space-time.


title: Cosmology in Six Dimensions

abstract:
  We discuss cosmological models in six-dimensional spacetime. For
codimension-1 branes, we consider a (4+1) braneworld model and discuss its
cosmological evolution. For codimension-2 branes, we consider an infinitely
thin conical braneword model in the presence of an induced gravity term on the
brane and a Gauss-Bonnet term in the bulk. We discuss the cosmological
evolution of isotropic and anisotropic matter on the brane. We also briefly
discuss cosmological models in six-dimensional supergravity.


title:
Cosmology with variable parameters and effective equation of state for
  Dark Energy

abstract:
  A cosmological constant, Lambda, is the most natural candidate to explain the
origin of the dark energy (DE) component in the Universe. However, due to
experimental evidence that the equation of state (EOS) of the DE could be
evolving with time/redshift (including the possibility that it might behave
phantom-like near our time) has led theorists to emphasize that there might be
a dynamical field (or some suitable combination of them) that could explain the
behavior of the DE. While this is of course one possibility, here we show that
there is no imperative need to invoke such dynamical fields and that a variable
cosmological constant (including perhaps a variable Newton's constant too) may
account in a natural way for all these features.


title:
Fermions in Ashtekar-Barbero Connections Formalism for Arbitrary Values
  of the Immirzi Parameter

abstract:
  The Ashtekar-Barbero-Immirzi formulation of General Relativity is extended to
include spinor matter fields. Our formulation applies to generic values of the
Immirzi parameter and reduces to the Ashtekar-Romano-Tate approach when the
Immirzi parameter is taken equal to the imaginary unit. The dynamics of the
gravity-fermions coupled system is described by the Holst plus Dirac action
with a non-minimal coupling term. The non-minimal interaction together with the
Holst modification to the Hilbert-Palatini action reconstruct the Nieh-Yan
invariant, so that the effective action coming out is the one of
Einstein-Cartan theory with a typical Fermi-like interaction term: in spite of
the presence of spinor matter fields, the Immirzi parameter plays no role in
the classical effective dynamics and results to be only a multiplicative factor
in front of a total divergence.
  We reduce the total action of the theory to the sum of dynamically
independent Ashtekar-Romano-Tate actions for self and anti-self dual
connections, with different weights depending on the Immirzi parameter. This
allows to calculate the constraints of the complete theory in a simple way, it
is only necessary to realize that the Barbero-Immirzi connection is a weighted
sum of the self and anti-self dual Ashtekar connections. Finally the obtained
constraints for the separated action result to be polynomial in terms of the
self and anti-self dual connections, this could have implications in the
inclusion of spinor matter in the framework of non-perturbative quantum
gravity.


title:
Einstein--Maxwell--Dilaton metrics from three--dimensional
  Einstein--Weyl structures

abstract:
  A class of time dependent solutions to $(3+1)$ Einstein--Maxwell-dilaton
theory with attractive electric force is found from Einstein--Weyl structures
in (2+1) dimensions corresponding to dispersionless Kadomtsev--Petviashvili and
$SU(\infty)$ Toda equations. These solutions are obtained from time--like
Kaluza--Klein reductions of $(3+2)$ solitons.


title:
On the Measurement of the Lense-Thirring effect Using the Nodes of the
  LAGEOS Satellites in reply to "On the reliability of the so-far performed
  tests for measuring the Lense-Thirring effect with the LAGEOS satellites" by
  L. Iorio

abstract:
  In this paper, we provide a detailed description of our recent analysis and
determination of the frame-dragging effect obtained using the nodes of the
satellites LAGEOS and LAGEOS 2, in reply to the paper "On the reliability of
the so-far performed tests for measuring the Lense-Thirring effect with the
LAGEOS satellites" by L. Iorio


title:
Einstein-Maxwell and Einstein-Proca theory from a modified gravitational
  action

abstract:
  A modified gravitational action is considered which involves the quantity
$F_{\mu\nu}=\partial_{\mu}\Gamma_{\nu}-\partial_{\nu}\Gamma_{\mu}$, where
$\Gamma_{\mu}=\Gamma^{\alpha}_{\mu\alpha}$. Since $\Gamma_{\mu}$ transforms
like a U(1) gauge field under coordinate transformations terms such as
$F^{\mu\nu}F_{\mu\nu}$ are invariant under coordinate transformations. If such
a term is added to the usual gravitational action the resulting field
equations, obtained from a Palatini variation, are the Einstein-Proca
equations. The vector field can be coupled to point charges or to a complex
scalar density of weight $ie$, where $e$ is the charge of the field. If this
scalar density is taken to be $g^{-ie/2}$ and the overall factor of the scalar
density Lagrangian takes on a particular value the resulting field equations
are the Einstein-Maxwell equations.


title:
N-spheres in general relativity: regular black holes without apparent
  horizons, static wormholes with event horizons and gravastars with a
  tube-like core

abstract:
  We consider a way to avoid black hole singularities by gluing a black hole
exterior to an interior with a tube-like geometry consisting of a direct
product of two-dimensional AdS, dS, or Rindler spacetime with a two-sphere of
constant radius. As a result we obtain a spacetime with either "cosmological"
or "acceleration" (event) horizons but without an apparent horizon. The inner
region is everywhere regular and supported by matter with the vacuum-like
equation of state $p_{r}+\rho =0$ where $p_{r}=T_{r}^{r}$ is the longitudinal
pressure, $\rho =-T_{0}^{0}$ is the energy density, $T_{\mu}^{\nu}$ is the
stress-energy tensor. When the surface of gluing approaches the horizon,
surface stresses vanish, while $p_{r}$ may acquire a finite jump on the
boundary. Such composite spacetimes accumulate an infinitely large amount of
matter inside the horizon but reveal themselves for an external observer as a
sphere of a finite ADM mass and size. If the throat of the inner region is
glued to two black hole exteriors, one obtains a wormhole of an arbitrarily
large length. Wormholes under discussion are static but not traversable, so the
null energy condition is not violated. In particular, they include the case
with an infinite proper distance to the throat. We construct also gravastars
with an infinite tube as a core and traversable wormholes connected by a finite
tube-like region.


title:
The relation between Hawking radiation via tunnelling and the laws of
  black hole thermodynamics

abstract:
  In Parikh and Wilczek's original works, the laws of black hole thermodynamics
are not referred and it seems that there is no relation between Hawking
radiation via tunnelling and the laws of black hole thermodynamics in their
works. However, taking examples for the R-N black hole and the Kerr black hole,
we find that they are correlated and even consistent if the tunnelling process
is a reversible process.


title:
Adiabatic Expansion for Metric Perturbation and the condition to solve
  the Gauge Problem for Gravitational Radiation Reaction Problem

abstract:
  We examine the adiabatic approximation in the study of a relativistic
two-body problem with the gravitational radiation reaction. We recently pointed
out that the usual metric perturbation scheme using a perturbation of the
stress-energy tensor may not be appropriate for study of the dissipative
dynamics of the bodies due to the radiation reaction.
  We recently proposed a possible approach to solve this problem with a linear
black hole perturbation. This paper proposes a non-linear generalization of
that method for a general application of this problem. We show that, under a
specific gauge condition, the method actually allows us to avoid the gauge
problem.


title:
Quantum corrections to the Schwarzschild metric and reparametrization
  transformations

abstract:
  Quantum corrections to the Schwarzschild metric generated by loop diagrams
have been considered by Bjerrum-Bohr, Donoghue, and Holstein (BHD) [Phys. Rev.
D68, 084005 (2003)], and Khriplovich and Kirilin (KK) [J. Exp. Theor. Phys. 98,
1063 (2004)]. Though the same field variables in a covariant gauge are used,
the results obtained differ from one another. The reason is that the different
sets of diagrams have been used. Here we will argue that the quantum
corrections to metric must be independent of the choice of field variables,
i.e., must be reparametrization invariant. Using simple reparametrization
transformation, we will show that the contribution considered by BDH, is not
invariant under it. Meanwhile the contribution of the complete set of the
diagrams, considered by KK, satisfies the requirement of the invariance.


title: On the mean anomaly and the Lense-Thirring effect

abstract:
  In this brief note we reply to the authors of a recent preprint in which an
alleged explicit proposal of using the mean anomaly of the LAGEOS satellites to
measure the general relativistic Lense-Thirring effect in the gravitational
field of the Earth is attributed to the present author.


title:
On the relation between ADM and Bondi energy-momenta III -- perturbed
  radiative spatial infinity

abstract:
  In a vacuum spacetime equipped with the Bondi's radiating metric which is
asymptotically flat at spatial infinity including gravitational radiation ({\bf
Condition D}), we establish the relation between the ADM total energy-momentum
and the Bondi energy-momentum for perturbed radiative spatial infinity. The
perturbation is given by defining the "real" time the sum of the retarded time,
the Euclidean distance and certain function $f$.


title: Warped products and black holes

abstract:
  We apply the warped product spacetime scheme to the
Banados-Teitelboim-Zanelli black holes and the Reissner-Nordstr\"om-anti-de
Sitter black hole to investigate their interior solutions in terms of warped
products. It is shown that there exist no discontinuities of the Ricci and
Einstein curvatures across event horizons of these black holes.


title:
Isotropy, shear, symmetry and exact solutions for relativistic fluid
  spheres

abstract:
  The symmetry method is used to derive solutions of Einstein's equations for
fluid spheres using an isotropic metric and a velocity four vector that is
non-comoving. Initially the Lie, classical approach is used to review and
provide a connecting framework for many comoving and so shear free solutions.
This provides the basis for the derivation of the classical point symmetries
for the more general and mathematicaly less tractable description of Einstein's
equations in the non-comoving frame. Although the range of symmetries is
restrictive, existing and new symmetry solutions with non-zero shear are
derived. The range is then extended using the non-classical direct symmetry
approach of Clarkson and Kruskal and so additional new solutions with non-zero
shear are also presented. The kinematics and pressure, energy density, mass
function of these solutions are determined.


title:
A generic relation between baryonic and radiative energy densities of
  stars

abstract:
  By using elementary astrophysical concepts, we show that for any
self-luminous astrophysical object, the ratio of radiation energy density
inside the body (rho_r) and the baryonic energy density (rho_0) may be crudely
approximated, in the Newtonian limit, as rho_r/rho_0 ~ GM/Rc^2, where G is
constant of gravitation, c is the speed of light, M is gravitational mass, and
R is the radius of the body. The key idea is that radiation quanta must move
out in a diffusive manner rather than free stream inside the body of the star.
When one would move to the extreme General Realtivistic case i.e., if the
surface gravitational redshift, z >> 1, it is found that, rho_r/rho_0 ~ (1+z).
Previous works on gravitational collapse, however, generally assumed
rho_r/rho_0 << 1. On the other hand, actually, during continued general
relativistic gravitational collapse to the Black Hole state (z --> infty), the
collapsing matter may essentially become an extremely hot fireball a la the
very early universe even though the observed luminosity of the body as seen by
a faraway observer, L^\infty ~ (1+z)^{-1} --> 0 as z --> infty, and the
collapse might appear as ``adiabatic''.


title: Unequal Mass Binary Black Hole Plunges and Gravitational Recoil

abstract:
  We present results from fully nonlinear simulations of unequal mass binary
black holes plunging from close separations well inside the innermost stable
circular orbit with mass ratios q = M_1/M_2 = {1,0.85,0.78,0.55,0.32}, or
equivalently, with reduced mass parameters $\eta=M_1M_2/(M_1+M_2)^2 = {0.25,
0.248, 0.246, 0.229, 0.183}$. For each case, the initial binary orbital
parameters are chosen from the Cook-Baumgarte equal-mass ISCO configuration. We
show waveforms of the dominant l=2,3 modes and compute estimates of energy and
angular momentum radiated. For the plunges from the close separations
considered, we measure kick velocities from gravitational radiation recoil in
the range 25-82 km/s. Due to the initial close separations our kick velocity
estimates should be understood as a lower bound. The close configurations
considered are also likely to contain significant eccentricities influencing
the recoil velocity.


title: An Explicit Isochronal, Isometric Embedding

abstract:
  Certain semi-Riemannian metrics may be decomposed into a Riemannian part and
an isochronal part. We use this idea and an idea of Kasner to construct a
manifold in 6+1 Minkowski space with a well known metric. The full embedding we
display is isochronal which simplifies visualizing the properties of the
manifold.


title: Constrained Mechanics and Noiseless Subsystems

abstract:
  Many theories are formulated as constrained systems. We provide a mechanism
that explains the origin of physical states of a constrained system by a
process of selection of noiseless subsystems when the system is coupled to an
external environment. Effectively, physical states that solve all the
constraints are selected by a passive error correction scheme which has been
developed in the context of quantum information processing. We apply this
mechanism to several constrained theories including the relativistic particle,
electromagnetism, and quantum gravity, and discuss some interesting (and
speculative) implications on the problem of time and the status of symmetries
in nature.


title:
Gravitomagnetic resonant excitation of Rossby modes in coalescing
  neutron star binaries

abstract:
  In coalescing neutron star binaries, r-modes in one of the stars can be
resonantly excited by the gravitomagnetic tidal field of its companion. This
post-Newtonian gravitomagnetic driving of these modes dominates over the
Newtonian tidal driving previously computed by Ho and Lai. To leading order in
the tidal expansion parameter R/r (where R is the radius of the neutron star
and r is the orbital separation), only the l=2, |m|= 1 and |m| = 2 r-modes are
excited. The tidal work done on the star through this driving has an effect on
the evolution of the inspiral and on the phasing of the emitted gravitational
wave signal. For a neutron star of mass M, radius R, spin frequency f_spin,
modeled as a Gamma =2 polytrope, with a companion also of mass M, the
gravitational wave phase shift for the m=2 mode is
(0.1radians)(R/10km)^4(M/1.4M_sun)^{-10/3}(f_spin/100Hz)^{2/3} for optimal spin
orientation. For canonical neutron star parameters this phase shift will likely
not be detectable by gravitational wave detectors such as LIGO, but if the
neutron star radius is larger it may be detectable if the signal-to-noise ratio
is moderately large. For neutron star - black hole binaries, the effect is
smaller; the phase shift scales as companion mass to the -4/3 power for large
companion masses. The net energy transfer from the orbit into the star is
negative corresponding to a slowing down of the inspiral. This occurs because
the interaction reduces the spin of the star, and occurs only for modes which
satisfy the Chandrasekhar-Friedman-Schutz instability criterion.


title: A new algorithm for anisotropic solutions

abstract:
  We establish a new algorithm that generates a new solution to the Einstein
field equations, with an anisotropic matter distribution, from a seed isotropic
solution. The new solution is expressed in terms of integrals of an isotropic
gravitational potential; and the integration can be completed exactly for
particular isotropic seed metrics. A good feature of our approach is that the
anisotropic solutions necessarily have an isotropic limit. We find two examples
of anisotropic solutions which generalise the isothermal sphere and the
Schwarzschild interior sphere. Both examples are expressed in closed form
involving elementary functions only.


title:
Measurement of the mechanical loss of a cooled reflective coating for
  gravitational wave detection

abstract:
  We have measured the mechanical loss of a dielectric multilayer reflective
coating (ion-beam sputtered SiO$_2$ and Ta$_2$O$_5$) in cooled mirrors. The
loss was nearly independent of the temperature (4 K $\sim$ 300 K), frequency,
optical loss, and stress caused by the coating, and the details of the
manufacturing processes. The loss angle was $(4 \sim 6) \times 10^{-4}$. The
temperature independence of this loss implies that the amplitude of the coating
thermal noise, which is a severe limit in any precise measurement, is
proportional to the square root of the temperature. Sapphire mirrors at 20 K
satisfy the requirement concerning the thermal noise of even future
interferometric gravitational wave detector projects on the ground, for
example, LCGT.


title: Gauge Invariant Treatment of the Energy Carried by a Gravitational Wave

abstract:
  Even though the energy carried by a gravitational wave is not itself gauge
invariant, the interaction with a gravitational antenna of the gravitational
wave which carries that energy is. It therefore has to be possible to make some
statements which involve the energy which are in fact gauge invariant, and it
is the objective of this paper to provide them. In order to develop a gauge
invariant treatment of the issues involved, we construct a specific action for
gravitational fluctuations which is gauge invariant to second perturbative
order. Then, via variation of this action, we obtain an energy-momentum tensor
for perturbative gravitational fluctuations around a general curved background
whose covariant conservation condition is also fully gauge invariant to second
order. Contraction of this energy-momentum tensor with a Killing vector of the
background conveniently allows us to convert this covariant conservation
condition into an ordinary conservation condition which is also gauge invariant
through second order. Then, via spatial integration we are able to obtain a
relation involving the time derivative of the total energy of the fluctuation
and its asymptotic spatial momentum flux which is also completely gauge
invariant through second order. It is only in making the simplification of
setting the asymptotic momentum flux to zero that one would actually lose
manifest gauge invariance, with only invariance under those particular gauge
transformations which leave the asymptotic momentum flux zero then remaining.
However, if one works in an arbitrary gauge where the asymptotic momentum flux
is non-zero, the gravitational wave will then deliver both energy and momentum
to a gravitational antenna in a completely gauge invariant manner, no matter
how badly behaved at infinity the gauge function might be.


title: A darkless space-time

abstract:
  In cosmology it has become usual to introduce new entities as dark matter and
dark energy in order to explain otherwise unexplained observational facts.
Here, we propose a different approach treating spacetime as a continuum endowed
with properties similar to the ones of ordinary material continua, such as
internal viscosity and strain distributions originated by defects in the
texture. A Lagrangian modeled on the one valid for simple dissipative phenomena
in fluids is built and used for empty spacetime. The internal "viscosity" is
shown to correspond to a four-vector field. The vector field is shown to be
connected with the displacement vector field induced by a point defect in a
four-dimensional continuum. Using the known symmetry of the universe, assuming
the vector field to be divergenceless and solving the corresponding
Euler-Lagrange equation, we directly obtain inflation and a phase of
accelerated expansion of spacetime. The only parameter in the theory is the
"strength" of the defect. We show that it is possible to fix it in such a way
to also quantitatively reproduce the acceleration of the universe. We have
finally verified that the addition of ordinary matter does not change the
general behaviour of the model.


title:
Massive particles in acoustic space-times emergent inertia and passive
  gravity

abstract:
  I show that massive-particle dynamics can be simulated by a weak, spherical,
external perturbation on a potential flow in an ideal fluid. The effective
Lagrangian is of the form mc^2L(U^2/c^2), where U is the velocity of the
particle relative to the fluid and c the speed of sound. This can serve as a
model for emergent relativistic inertia a la Mach's principle with m playing
the role of inertial mass, and also of analog gravity where it is also the
passive gravitational mass. m depends on the particle type and intrinsic
structure, while L is universal: For D dimensional particles L is proportional
to the hypergeometric function F(1,1/2;D/2;U^2/c^2). Particles fall in the same
way in the analog gravitational field independent of their internal structure,
thus satisfying the weak equivalence principle. For D less or equal 5 they all
have a relativistic limit with the acquired energy and momentum diverging as U
approaches c. For D less or equal 7 the null geodesics of the standard acoustic
metric solve our equation of motion. Interestingly, for D=4 the dynamics is
very nearly Lorentzian. The particles can be said to follow the geodesics of a
generalized acoustic metric of a Finslerian type that shares the null geodesics
with the standard acoustic metric. In vortex geometries, the ergosphere is
automatically the static limit. As in the real world, in ``black hole''
geometries circular orbits do not exist below a certain radius that occurs
outside the horizon. There is a natural definition of antiparticles; and I
describe a mock particle vacuum in whose context one can discuss, e.g.,
particle Hawking radiation near event horizons.


title:
Science, Technology and Mission Design for the Laser Astrometric Test Of
  Relativity

abstract:
  The Laser Astrometric Test Of Relativity (LATOR) is a Michelson-Morley-type
experiment designed to achieve a major improvement in the accuracy of the tests
of relativistic gravity in the solar system. By using a combination of
independent time-series of gravitational deflection of light in the immediate
proximity to the Sun, along with measurements of the relativistic time delay on
interplanetary scales (to a precision respectively better than 0.1 picoradians
and 1 cm), LATOR will measure the key post-Newtonian Eddington parameter \gamma
with accuracy of one part in a billion - a factor of 30,000 improvement
compared to the present best result, Cassini's 2003 test. LATOR's primary
measurement pushes to unprecedented accuracy the search for cosmologically
relevant scalar-tensor modifications of gravity by looking for a remnant scalar
field in today's solar system. We present a comprehensive discussion of the
science objectives, proposed technology, mission and optical designs, as well
as the expected performance of this fundamental physics experiment in space.


title: Darwin Meets Einstein: LISA Data Analysis Using Genetic Algorithms

abstract:
  This work presents the first application of the method of Genetic Algorithms
(GAs) to data analysis for the Laser Interferometer Space Antenna (LISA). In
the low frequency regime of the LISA band there are expected to be tens of
thousands galactic binary systems that will be emitting gravitational waves
detectable by LISA. The challenge of parameter extraction of such a large
number of sources in the LISA data stream requires a search method that can
efficiently explore the large parameter spaces involved. As signals of many of
these sources will overlap, a global search method is desired. GAs represent
such a global search method for parameter extraction of multiple overlapping
sources in the LISA data stream. We find that GAs are able to correctly extract
source parameters for overlapping sources. Several optimizations of a basic GA
are presented with results derived from applications of the GA searches to
simulated LISA data.


title: Relic gravitational waves and the cosmic accelerated expansion

abstract:
  The possibility of reconstructing the whole history of the scale factor of
the Universe from the power spectrum of relic gravitational waves (RGWs) makes
the study of these waves quite interesting. First, we explore the impact of a
hypothetical era -right after reheating- dominated by mini black holes and
radiation that may lower the spectrum several orders of magnitude. Next, we
calculate the power spectrum of the RGWs taking into account the present stage
of accelerated expansion and an hypothetical second dust era. Finally, we study
the generalized second law of gravitational thermodynamics applied to the
present era of accelerated expansion of the Universe.


title: Quantum inequalities for massless spin-3/2 field in Minkowski spacetime

abstract:
  Quantum inequalities have been established for various quantum fields in both
flat and curved spacetimes. In particular, for spin-3/2 fields, Yu and Wu have
explicitly derived quantum inequalities for massive case. Employing the similar
method developed by Fewster and colleagues, this paper provides an explicit
formula of quantum inequalities for massless spin-3/2 field in four-dimensional
Minkowski spacetime.


title: Harmonic Initial-Boundary Evolution in General Relativity

abstract:
  Computational techniques which establish the stability of an
evolution-boundary algorithm for a model wave equation with shift are
incorporated into a well-posed version of the initial-boundary value problem
for gravitational theory in harmonic coordinates. The resulting algorithm is
implemented as a 3-dimensional numerical code which we demonstrate to provide
stable, convergent Cauchy evolution in gauge wave and shifted gauge wave
testbeds. Code performance is compared for Dirichlet, Neumann and Sommerfeld
boundary conditions and for boundary conditions which explicitly incorporate
constraint preservation. The results are used to assess strategies for
obtaining physically realistic boundary data by means of Cauchy-characteristic
matching.


title: Tilted Bianchi VII_0 cosmologies -- the radiation bifurcation

abstract:
  We derive the late-time behaviour of tilted Bianchi VII_0 cosmologies with an
irrotational radiation fluid as source, and give the asymptotic form of the
general solution as $t \to +\infty$, making comparisons with the dust-filled
models. At first sight the radiation-filled models appear to approximate the
flat FL model at late times, since the Hubble-normalized shear and the tilt
tend to zero and the density parameter tends to one. The Hubble-normalized Weyl
curvature diverges, however, indicating that physically significant anisotropy
remains. We also discuss the influence of a cosmological constant on this
phenomenon.


title: Horizons, Constraints, and Black Hole Entropy

abstract:
  Black hole entropy appears to be ``universal''--many independent
calculations, involving models with very different microscopic degrees of
freedom, all yield the same density of states. I discuss the proposal that this
universality comes from the behavior of the underlying symmetries of the
classical theory. To impose the condition that a black hole be present, we must
partially break the classical symmetries of general relativity, and the
resulting Goldstone boson-like degrees of freedom may account for the
Bekenstein-Hawking entropy. In particular, I demonstrate that the imposition of
a ``stretched horizon'' constraint modifies the algebra of symmetries at the
horizon, allowing the use of standard conformal field theory techniques to
determine the asymptotic density of states. The results reproduce the
Bekenstein-Hawking entropy without any need for detailed assumptions about the
microscopic theory.


title: Hyperbolic heat equation in Kaluza's magnetohydrodynamics

abstract:
  This paper shows that a hyperbolic equation for heat conduction can be
obtained directly using the tenets of linear irreversible thermodynamics in the
context of the five dimensional space-time metric originally proposed by T.
Kaluza back in 1922. The associated speed of propagation is slightly lower than
the speed of light by a factor inversely proportional to the specific charge of
the fluid element. Moreover, consistency with the second law of thermodynamics
is achieved. Possible implications in the context of physics of clusters of
galaxies of this result are briefly discussed.


title: Can Gravitons Be Detected?

abstract:
  Freeman Dyson has questioned whether any conceivable experiment in the real
universe can detect a single graviton. If not, is it meaningful to talk about
gravitons as physical entities? We attempt to answer Dyson's question and find
it is possible concoct an idealized thought experiment capable of detecting one
graviton; however, when anything remotely resembling realistic physics is taken
into account, detection becomes impossible, indicating that Dyson's conjecture
is very likely true. We also point out several mistakes in the literature
dealing with graviton detection and production.


title: On horizon constraints and Hawking radiation

abstract:
  Questions about black holes in quantum gravity generally presuppose the
presence of a horizon. Recently Carlip has shown that enforcing an initial data
surface to be a horizon leads to the correct form for the Bekenstein-Hawking
entropy of the black hole. Requiring a horizon also constitutes fixed
background geometry, which generically leads to non-conservation of the matter
stress tensor at the horizon. In this work, I show that the generated matter
energy flux for a Schwarzschild black hole is in agreement with the first law
of black hole thermodynamics, $8 \pi G \Delta Q = \kappa \Delta A$.


title:
Matching Scherrer's k essence argument with alterations of Di Quark
  scalar fields permitting an eventual comological constant dominated
  inflationary expansion

abstract:
  We previously showed that we can use di quark pairs as a model of how
nucleation of a new universe occurs. We now can construct a model showing
evolution from a dark matter dark energy mix to a pure cosmological constant
cosmology due to changes in the slope of the resulting scalar field,using much
of Scherrer's k-essence model.This same construction permits a use of the speed
of sound,in k-essence models evolving from zero to one. Having the sound speed
eventually reach unity permits matching conventional cosmological observations
in the aftermath of change of slope of a di quark pair generated scalar field
during the nucleation process of a new universe. These results are consistent
with applying Bunyi and Hu's semi classical criteria for cosmological
potentials to indicate a phase transition alluded to by Dr. Edward Kolbs model
of how the initial degrees of freedom declined from over 100 to something
approaching what we see today in flat space cosmology.


title:
Improved initial data for black hole binaries by asymptotic matching of
  post-Newtonian and perturbed black hole solutions

abstract:
  We construct approximate initial data for non-spinning black hole binary
systems by asymptotically matching the 4-metrics of two tidally perturbed
Schwarzschild solutions in isotropic coordinates to a resummed post-Newtonian
4-metric in ADMTT coordinates. The specific matching procedure used here
closely follows the calculation in gr-qc/0503011, and is performed in the so
called buffer zone where both the post-Newtonian and the perturbed
Schwarzschild approximations hold. The result is that both metrics agree in the
buffer zone, up to the errors in the approximations. However, since isotropic
coordinates are very similar to ADMTT coordinates, matching yields better
results than in the previous calculation, where harmonic coordinates were used
for the post-Newtonian 4-metric. In particular, not only does matching improve
in the buffer zone, but due to the similarity between ADMTT and isotropic
coordinates the two metrics are also close to each other near the black hole
horizons. With the help of a transition function we also obtain a global smooth
4-metric which has errors on the order of the error introduced by the more
accurate of the two approximations we match. This global smoothed out 4-metric
is obtained in ADMTT coordinates which are not horizon penetrating. In
addition, we construct a further coordinate transformation that takes the
4-metric from global ADMTT coordinates to new coordinates which are similar to
Kerr-Schild coordinates near each black hole, but which remain ADMTT further
away from the black holes. These new coordinates are horizon penetrating and
lead, for example, to a lapse which is everywhere positive on the t=0 slice.
Such coordinates may be more useful in numerical simulations.


title: Electromagnetic field with constraints and Papapetrou equation

abstract:
  It is shown that geometric optical description of electromagnetic wave with
account of its polarization in curved space-time can be obtained
straightforwardly from the classical variational principle for electromagnetic
field. For this end the entire functional space of electromagnetic fields must
be reduced to its subspace of locally plane monochromatic waves. We have
formulated the constraints under which the entire functional space of
electromagnetic fields reduces to its subspace of locally plane monochromatic
waves. These constraints introduce variables of another kind which specify a
field of local frames associated to the wave and contain some congruence of
null-curves. The Lagrangian for constrained electromagnetic field contains
variables of two kinds, namely, a congruence of null-curves and the field
itself. This yields two kinds of Euler-Lagrange equations. Equations of first
kind are trivial due to the constraints imposed. Variation of the curves yields
the Papapetrou equations for a classical massless particle with helicity 1.


title: Gravitational-Wave Detector Dulkyn

abstract:
  The article deals with the results of the research on the realization of the
scientific and technical project Dulkyn for detection of low frequency periodic
gravitational radiation from the double relativistic astrophysical objects,
done by the Scientific Centre of Gravitational-Wave Research Dulkyn of the
Academy of Sciences of the Tatarstan Republic (SC GWR Dulkyn AS RT), and the
Federal State Unitary Enterprise and R and D Corporation called State Institute
of Applied Optics (FSUE NPO GIPO).


title:
Role of Modified Chaplygin Gas as a Dark Energy Model in Collapsing
  Spherically Symmetric Cloud

abstract:
  In this work, gravitational collapse of a spherical cloud, consists of both
dark matter and dark energy in the form of modified Chaplygin gas is studied.
It is found that dark energy alone in the form of modified Chaplygin gas forms
black hole. Also when both components of the fluid are present then the
collapse favors the formation of black hole in cases the dark energy dominates
over dark matter. The conclusion is totally opposite to the usually known
results.


title: Relic gravitational waves in the expanding Universe

abstract:
  In this thesis, we have reviewed how the expansion of the Universe amplifies
the quantum vacuum fluctuations, and how the relic GWs spectrum is related with
the scale factor. We have evaluated the spectrum in different scenarios: a
model of expanding universe with an era dominated by mini black holes and
radiation right after the inflation and a model with an era of accelerated
expansion right after the radiation era. Next, we have applied the generalized
second law of thermodynamics to the latter model. Finally, we have extended the
GSL study to a single stage universe model dominated by dark energy (either
phantom or not).


title:
Four-dimensional Einstein spaces on Six-dimensional Ricci-flat base
  space

abstract:
  The examples of ten-dimensional vacuum Einstein spaces composed of
four-dimensional Einstein spaces and six-dimensional Ricci-flat base space
defined by the solutions of the Korteveg de-Vries equation are constructed.


title: Geometrization of the electro-weak model bosonic component

abstract:
  In this work we develop a geometrical unification theory for gravity and the
electro-weak model in a Kaluza-Klein approach; in particular, from the
curvature dimensional reduction Einstein-Yang-Mills action is obtained. We
consider two possible space-time manifolds: 1)$V^{4}\otimes S^{1}\otimes S^{2}$
where isospin doublets are identified with spinors; 2) $V^{4}\otimes
S^{1}\otimes S^{3}$ in which both quarks and leptons doublets can be recast
into the same spinor, such that the equal number of quark generations and
leptonic families is explained. Finally a self-interacting complex scalar field
is introduced to reproduce the spontaneous symmetry breaking mechanism; in this
respect, at the end we get an Higgs fields whose two components have got
opposite hypercharges.


title:
Formalism for testing theories of gravity using lensing by compact
  objects. II: Probing Post-Post-Newtonian metrics

abstract:
  We study gravitational lensing by compact objects in gravity theories that
can be written in a Post-Post-Newtonian (PPN) framework: i.e., the metric is
static and spherically symmetric, and can be written as a Taylor series in m/r,
where m is the gravitational radius of the compact object. Working invariantly,
we compute corrections to standard weak-deflection lensing observables at first
and second order in the ratio of the angular gravitational radius to the
angular Einstein ring radius of the lens. We show that the first-order
corrections to the total magnification and centroid position vanish universally
for gravity theories that can be written in the PPN framework. This arises from
some surprising, fundamental relations among the lensing observables in PPN
gravity models. We derive these relations for the image positions,
magnifications, and time delays. A deep consequence is that any violation of
the universal relations would signal the need for a gravity model outside the
PPN framework (provided that some basic assumptions hold). In practical terms,
the relations will guide observational programs to test general relativity,
modified gravity theories, and possibly the Cosmic Censorship conjecture. We
use the new relations to identify lensing observables that are accessible to
current or near-future technology, and to find combinations of observables that
are most useful for probing the spacetime metric. We give explicit applications
to the Galactic black hole, microlensing, and the binary pulsar J0737-3039.


title: Helicity-Rotation-Gravity Coupling for Gravitational Waves

abstract:
  The consequences of spin-rotation-gravity coupling are worked out for linear
gravitational waves. The coupling of helicity of the wave with the rotation of
a gravitational-wave antenna is investigated and the resulting modifications in
the Doppler effect and aberration are pointed out for incident high-frequency
gravitational radiation. Extending these results to the case of a
gravitomagnetic field via the gravitational Larmor theorem, the rotation of
linear polarization of gravitational radiation propagating in the field of a
rotating mass is studied. It is shown that in this case the linear polarization
state rotates by twice the Skrotskii angle as a consequence of the spin-2
character of linear gravitational waves.


title:
What do the orbital motions of the outer planets of the Solar System
  tell us about the Pioneer anomaly?

abstract:
  In this paper we investigate the effects that an anomalous acceleration as
that experienced by the Pioneer spacecraft after they passed the 20 AU
threshold would induce on the orbital motions of the Solar System planets
placed at heliocentric distances of 20 AU or larger as Uranus, Neptune and
Pluto. It turns out that such an acceleration, with a magnitude of 8.74\times
10^-10 m s^-2, would affect their orbits with secular and short-period signals
large enough to be detected according to the latest published results by E.V.
Pitjeva, even by considering errors up to 30 times larger than those released.
The absence of such anomalous signatures in the latest data rules out the
possibility that in the region 20-40 AU of the Solar System an anomalous force
field inducing a constant and radial acceleration with those characteristics
affects the motion of the major planets.


title:
The Local Effects of Cosmological Variations in Physical 'Constants' and
  Scalar Fields II. Quasi-Spherical Spacetimes

abstract:
  We investigate the conditions under which cosmological variations in physical
`constants' and scalar fields are detectable on the surface of local
gravitationally-bound systems, such as planets, in non-spherically symmetric
background spacetimes. The method of matched asymptotic expansions is used to
deal with the large range of length scales that appear in the problem. We
derive a sufficient condition for the local time variation of the scalar fields
driving variations in 'constants' to track their large-scale cosmological
variation and show that this is consistent with our earlier conjecture derived
from the spherically symmetric problem. We perform our analysis with spacetime
backgrounds that are of Szekeres-Szafron type. They are approximately
Schwarzschild in some locality and free of gravitational waves everywhere. At
large distances, we assume that the spacetime matches smoothly onto a Friedmann
background universe. We conclude that, independent of the details of the
scalar-field theory describing the varying `constant', the condition for its
cosmological variations to be measured locally is almost always satisfied in
physically realistic situations. The very small differences expected to be
observed between different scales are quantified. This strengthens the proof
given in our previous paper that local experiments see global variations by
dropping the requirement of exact spherical symmetry. It provides a rigorous
justification for using terrestrial experiments and solar system observations
to constraint or detect any cosmological time variations in the traditional
`constants' of Nature in the case where non-spherical inhomogeneities exist.


title: Exact solutions to the Chandrasekhar Page angular equation

abstract:
  Exact solutions are found for the Chandrasekhar Page angular equation which
results when the Dirac equation in a Kerr Newman space time is separated into
its radial and angular parts. The solutions turn out to be remarkably simple in
form while satisfying the asymptotic conditions deduced earlier. The
eigenvalues are found to be the square root of the total angular momentum as
first found by Dirac for flat space; supplemented by a term which is the
product of the mass of the Dirac particle times the specific angular momentum
of the black hole. The additional contribution is what is expected from frame
dragging.


title: Angular momentum and conservation laws for dynamical black holes

abstract:
  Black holes can be practically located (e.g. in numerical simulations) by
trapping horizons, hypersurfaces foliated by marginal surfaces, and one desires
physically sound measures of their mass and angular momentum. A generically
unique angular momentum can be obtained from the Komar integral by demanding
that it satisfy a simple conservation law. With the irreducible (Hawking) mass
as the measure of energy, the conservation laws of energy and angular momentum
take a similar form, expressing the rate of change of mass and angular momentum
of a black hole in terms of fluxes of energy and angular momentum, obtained
from the matter energy tensor and an effective energy tensor for gravitational
radiation. Adding charge conservation for generality, one can use Kerr-Newman
formulas to define combined energy, surface gravity, angular speed and electric
potential, and derive a dynamical version of the so-called "first law" for
black holes. A generalization of the "zeroth law" to local equilibrium follows.
Combined with an existing version of the "second law", all the key quantities
and laws of the classical paradigm for black holes (in terms of Killing or
event horizons) have now been formulated coherently in a general dynamical
paradigm in terms of trapping horizons.


title: Wave Mechanics and General Relativity: A Rapprochement

abstract:
  Using exact solutions, we show that it is in principle possible to regard
waves and particles as representations of the same underlying geometry, thereby
resolving the problem of wave-particle duality.


title: Energy Distribution associated with Static Axisymmetric Solutions

abstract:
  This paper has been addressed to a very old but burning problem of energy in
General Relativity. We evaluate energy and momentum densities for the static
and axisymmetric solutions. This specializes to two metrics, i.e., Erez-Rosen
and the gamma metrics, belonging to the Weyl class. We apply four well-known
prescriptions of Einstein, Landau-Lifshitz, Papaterou and M$\ddot{o}$ller to
compute energy-momentum density components. We obtain that these prescriptions
do not provide similar energy density, however momentum becomes constant in
each case. The results can be matched under particular boundary conditions.


title: Gravitational field energy contribution to the neutron star mass

abstract:
  Neutron stars are discussed as laboratories of physics of strong
gravitational fields. The mass of a neutron star is split into matter energy
and gravitational field energy contributions. The energy of the gravitational
field of neutron stars is calculated with three different approaches which give
the same result. It is found that up to one half of the gravitational mass of
maximum mass neutron stars is comprised by the gravitational field energy.
Results are shown for a number of realistic equations of state of neutron star
matter.


title: Classical dynamics and stability of collapsing thick shells of matter

abstract:
  We study the collapse towards the gravitational radius of a macroscopic
spherical thick shell surrounding an inner massive core. This overall
electrically neutral macroshell is composed by many nested delta-like massive
microshells which can bear non-zero electric charge, and a possibly non-zero
cosmological constant is also included. The dynamics of the shells is described
by means of Israel's (Lanczos) junction conditions for singular hypersurfaces
and, adopting a Hartree (mean field) approach, an effective Hamiltonian for the
motion of each microshell is derived which allows to check the stability of the
matter composing the macroshell. We end by briefly commenting on the quantum
effects which may arise from the extension of our classical treatment to the
semiclassical level.


title: Rotating Black Holes in Metric-Affine Gravity

abstract:
  Within the framework of metric-affine gravity (MAG, metric and an independent
linear connection constitute spacetime), we find, for a specific gravitational
Lagrangian and by using {\it prolongation} techniques, a stationary axially
symmetric exact solution of the vacuum field equations. This black hole
solution embodies a Kerr-deSitter metric and the post-Riemannian structures of
torsion and nonmetricity. The solution is characterized by mass, angular
momentum, and shear charge, the latter of which is a measure for violating
Lorentz invariance.


title: Axisymmetric Numerical Relativity

abstract:
  This thesis is concerned with formulations of the Einstein equations in
axisymmetric spacetimes which are suitable for numerical evolutions. We develop
two evolution systems based on the (2+1)+1 formalism. The first is a
(partially) constrained scheme with elliptic gauge conditions arising from
maximal slicing and conformal flatness. The second is a strongly hyperbolic
first-order formulation obtained by combining the (2+1)+1 formalism with the Z4
formalism. A careful study of the behaviour of regular axisymmetric tensor
fields enables us to cast the equations in a form that is well-behaved on the
axis. Further topics include (non)uniqueness of solutions to the elliptic
equations arising in constrained schemes, and comparisons between various
boundary conditions used in numerical relativity. The numerical implementation
is applied to adaptive evolutions of nonlinear Brill waves, including twist.


title: The Equivalence Principle as a Probe for Higher Dimensions

abstract:
  Higher-dimensional theories of the kind which may unify gravitation with
particle physics can lead to significant modifications of general relativity.
In five dimensions, the vacuum becomes non-standard, and the Weak Equivalence
Principle becomes a geometrical symmetry which can be broken, perhaps at a
level detectable by new tests in space.


title: Simulation of Acoustic Black Hole in a Laval Nozzle

abstract:
  A numerical simulation of fluid flows in a Laval nozzle is performed to
observe formations of acoustic black holes and the classical counterpart to
Hawking radiation under a realistic setting of the laboratory experiment. We
determined the Hawking temperature of the acoustic black hole from obtained
numerical data. Some noteworthy points in analyzing the experimental data are
clarified through our numerical simulation.


title: Bogoliubov's Integrals of Motion in Quantum Cosmology and Gravity

abstract:
  Quantum Cosmology and Gravity are formulated here as the primary and
secondary quantizations of the energy constraints by analogy with the
historical formulation of quantum field theory. New fact is that both the
Universe and its matter are created from stable vacuum obtained by the
Bogoliubov-type transformation just as it is in the theory of quantum
superfluid liquid. Such the Quantum Gravity gives us possibility to explain
topical problems of cosmology by the cosmological creation of universes and
particles from vacuum.


title: Quadratic field equations on the brane

abstract:
  It is shown that four dimensional vacuum Einstein solutions simply embedded
in five dimensions obey the Gauss-Bonnet-Einstein field equations:
$G_{ab}+\alpha GB_{ab}+\delta^{55}_{ab}\alpha\exp(-2\chi/\sqrt{\alpha})GB_4=0$
and the Pauli-Einstein equations $G_{ab}-3\alpha P_{ab}/5=0$, and the
Bach-Einstein equations $B_{ab}=0$. General equations are calculated for which
these and similar results follow. It is briefly argued that such field
equations could be significant on large distance scales.


title: Constructing an interval of Minkowski space from a causal set

abstract:
  A criticism sometimes made of the causal set quantum gravity program is that
there is no practical scheme for identifying manifoldlike causal sets and
finding embeddings of them into manifolds. A computational method for
constructing an approximate embedding of a small manifoldlike causal set into
Minkowski space (or any spacetime that is approximately flat at short scales)
is given, and tested in the 2 dimensional case. This method can also be used to
determine how manifoldlike a causal set is, and conversely to define scales of
manifoldlikeness.


title:
Moller Energy-Momentum Prescription for a Locally Rotationally Symmetric
  Space-Time

abstract:
  This paper has inappropriate amounts of overlap with the following papers
also written by the authors or their collaborators: gr-qc/0608050,
gr-qc/0509047, gr-qc/0505078, gr-qc/0603108, gr-qc/0512080, gr-qc/0511030,
gr-qc/0606022, gr-qc/0603027, gr-qc/0603063, gr-qc/0607116, and others.


title: Laser Ranging Delay in the Bi-Metric Theory of Gravity

abstract:
  We introduce a linearized bi-metric theory of gravity with two metrics. The
metric g_{ab} describes null hypersurfaces of the gravitational field while
light moves on null hypersurfaces of the optical metric \bar{g}_{ab}.
Bi-metrism naturally arises in vector-tensor theories with matter being
non-minimally coupled to gravity via long-range vector field. We derive
explicit Lorentz-invariant solution for a light ray propagating in space-time
of the bi-metric theory and disentangle relativistic effects associated with
the two metrics. This anlysis can be valuable for future spaceborne laser
missions ASTROD and LATOR dedicated to map various relativistic gravity
parameters in the solar system to unparalleled degree of accuracy.


title:
Parameter estimation of compact binaries using the inspiral and ringdown
  waveforms

abstract:
  We analyze the problem of parameter estimation for compact binary systems
that could be detected by ground-based gravitational wave detectors.
  So far this problem has only been dealt with for the inspiral and the
ringdown phases separately. In this paper, we combine the information from both
signals, and we study the improvement in parameter estimation, at a fixed
signal-to-noise ratio, by including the ringdown signal without making any
assumption on the merger phase. The study is performed for both initial and
advanced LIGO and VIRGO detectors.


title: Possible physical universes

abstract:
  The purpose of this paper is to discuss the various types of physical
universe which could exist according to modern mathematical physics. The paper
begins with an introduction that approaches the question from the viewpoint of
ontic structural realism. Section 2 takes the case of the 'multiverse' of
spatially homogeneous universes, and analyses the famous Collins-Hawking
argument, which purports to show that our own universe is a very special member
of this collection. Section 3 considers the multiverse of all solutions to the
Einstein field equations, and continues the discussion of whether the notions
of special and typical can be defined within such a collection.


title: Plane waves in metric-affine gravity

abstract:
  We describe plane-fronted waves in the Yang-Mills type quadratic
metric-affine theory of gravity. The torsion and the nonmetricity are both
nontrivial, and they do not belong to the triplet ansatz.


title:
Dynamical response of the "GGG" rotor to test the Equivalence Principle:
  theory, simulation and experiment. Part I: the normal modes

abstract:
  Recent theoretical work suggests that violation of the Equivalence Principle
might be revealed in a measurement of the fractional differential acceleration
$\eta$ between two test bodies -of different composition, falling in the
gravitational field of a source mass- if the measurement is made to the level
of $\eta\simeq 10^{-13}$ or better. This being within the reach of ground based
experiments, gives them a new impetus. However, while slowly rotating torsion
balances in ground laboratories are close to reaching this level, only an
experiment performed in low orbit around the Earth is likely to provide a much
better accuracy.
  We report on the progress made with the "Galileo Galilei on the Ground" (GGG)
experiment, which aims to compete with torsion balances using an instrument
design also capable of being converted into a much higher sensitivity space
test.
  In the present and following paper (Part I and Part II), we demonstrate that
the dynamical response of the GGG differential accelerometer set into
supercritical rotation -in particular its normal modes (Part I) and rejection
of common mode effects (Part II)- can be predicted by means of a simple but
effective model that embodies all the relevant physics. Analytical solutions
are obtained under special limits, which provide the theoretical understanding.
A simulation environment is set up, obtaining quantitative agreement with the
available experimental data on the frequencies of the normal modes, and on the
whirling behavior. This is a needed and reliable tool for controlling and
separating perturbative effects from the expected signal, as well as for
planning the optimization of the apparatus.


title:
Variability of signal to noise ratio and the network analysis of
  gravitational wave burst signals

abstract:
  The detection and estimation of gravitational wave burst signals, with {\em a
priori} unknown polarization waveforms, requires the use of data from a network
of detectors. For determining how the data from such a network should be
combined, approaches based on the maximum likelihood principle have proven to
be useful. The most straightforward among these uses the global maximum of the
likelihood over the space of all waveforms as both the detection statistic and
signal estimator. However, in the case of burst signals, a physically
counterintuitive situation results: for two aligned detectors the statistic
includes the cross-correlation of the detector outputs, as expected, but this
term disappears even for an infinitesimal misalignment. This {\em two detector
paradox} arises from the inclusion of improbable waveforms in the solution
space of maximization. Such waveforms produce widely different responses in
detectors that are closely aligned. We show that by penalizing waveforms that
exhibit large signal-to-noise ratio (snr) variability, as the corresponding
source is moved on the sky, a physically motivated restriction is obtained that
(i) resolves the two detector paradox and (ii) leads to a better performing
statistic than the global maximum of the likelihood. Waveforms with high snr
variability turn out to be precisely the ones that are improbable in the sense
mentioned above. The coherent network analysis method thus obtained can be
applied to any network, irrespective of the number or the mutual alignment of
detectors.


title: Considerations on the excitation of black hole quasinormal modes

abstract:
  We provide some considerations on the excitation of black hole quasinormal
modes (QNMs) in different physical scenarios. Considering a simple model in
which a stream of particles accretes onto a black hole, we show that resonant
QNM excitation by hyperaccretion requires a significant amount of fine-tuning,
and is quite unlikely to occur in nature. Then we summarize and discuss present
estimates of black hole QNM excitation from gravitational collapse, distorted
black holes and head-on black hole collisions. We emphasize the areas that, in
our opinion, are in urgent need of further investigation from the point of view
of gravitational wave source modeling.


title: Brane-world cosmology

abstract:
  Brane-world models, where observers are restricted to a brane in a
higher-dimensional spacetime, offer a novel perspective on cosmology. I discuss
some approaches to cosmology in extra dimensions and some interesting aspects
of gravity and cosmology in brane-world models.


title:
Hawking Radiation from Acoustic Black Holes, Short Distance and
  Back-Reaction Effects

abstract:
  Using the action principle we first review how linear density perturbations
(sound waves) in an Eulerian fluid obey a relativistic equation: the d'Alembert
equation. This analogy between propagation of sound and that of a massless
scalar field in a Lorentzian metric also applies to non-homogeneous flows. In
these cases, sound waves effectively propagate in a curved four-dimensional
''acoustic'' metric whose properties are determined by the flow. Using this
analogy, we consider regular flows which become supersonic, and show that the
acoustic metric behaves like that of a black hole. The analogy is so good that,
when considering quantum mechanics, acoustic black holes should produce a
thermal flux of Hawking phonons.
  We then focus on two interesting questions related to Hawking radiation which
are not fully understood in the context of gravitational black holes due to the
lack of a theory of quantum gravity. The first concerns the calculation of the
modifications of Hawking radiation which are induced by dispersive effects at
short distances, i.e., approaching the atomic scale when considering sound. We
generalize existing treatments and calculate the modifications caused by the
propagation near the black hole horizon. The second question concerns
backreaction effects. We return to the Eulerian action, compute second order
effects, and show that the backreaction of sound waves on the fluid's flow can
be expressed in terms of their stress-energy tensor. Using this result in the
context of Hawking radiation, we compute the secular effect on the background
flow.


title:
Constant scalar curvature hypersurfaces in extended Schwarzschild
  space-time

abstract:
  We present a class of spherically symmetric hypersurfaces in the
  Kruskal extension of the Schwarzschild space-time. The hypersurfaces have
constant negative scalar curvature, so they are hyperboloidal in the regions of
space-time which are asymptotically flat.


title: Improved Hough search for gravitational wave pulsars

abstract:
  We describe an improved version of the Hough transform search for continuous
gravitational waves from isolated neutron stars assuming the input to be short
segments of Fourier transformed data. The method presented here takes into
account possible non-stationarities of the detector noise and the amplitude
modulation due to the motion of the detector. These two effects are taken into
account for the first stage only, i.e. the peak selection, to create the
time-frequency map of our data, while the Hough transform itself is performed
in the standard way.


title: Quantum Hamiltonian for gravitational collapse

abstract:
  Using a Hamiltonian formulation of the spherically symmetric gravity-scalar
field theory adapted to flat spatial slicing, we give a construction of the
reduced Hamiltonian operator. This Hamiltonian, together with the null
expansion operators presented in an earlier work, form a framework for studying
gravitational collapse in quantum gravity. We describe a setting for its
numerical implementation, and discuss some conceptual issues associated with
quantum dynamics in a partial gauge fixing.


title: Numerical analysis of backreaction in acoustic black holes

abstract:
  Using methods of Quantum Field Theory in curved spacetime, the first order in
hbar quantum corrections to the motion of a fluid in an acoustic black hole
configuration are numerically computed. These corrections arise from the non
linear backreaction of the emitted phonons. Time dependent (isolated system)
and equilibrium configurations (hole in a sonic cavity) are both analyzed.


title: Back-reaction effects in acoustic black holes

abstract:
  Acoustic black holes are very interesting non-gravitational objects which can
be described by the geometrical formalism of General Relativity. These models
can be useful to experimentally test effects otherwise undetectable, as for
example the Hawking radiation. The back-reaction effects on the background
quantities induced by the analogue Hawking radiation could be the key to
indirectly observe it. We briefly show how this analogy works and derive the
backreaction equations for the linearized quantum fluctuations in the
background of an acoustic black hole. A first order in hbar solution is given
in the near horizon region. It indicates that acoustic black holes, unlike
Schwarzschild ones, get cooler as they radiate phonons. They show remarkable
analogies with near-extremal Reissner-Nordstrom black holes.