5,865 research outputs found
The central structure of Broad Absorption Line QSOs: observational characteristics in the cm-mm wavelength domain
Accounting for ~20% of the total QSO population, Broad Absorption Line QSOs
are still an unsolved problem in the AGN context. They present wide troughs in
the UV spectrum, due to material with velocities up to 0.2 c toward the
observer. The two models proposed in literature try to explain them as a
particular phase of the evolution of QSOs or as normal QSOs, but seen from a
particular line of sight.
We built a statistically complete sample of Radio-Loud BAL QSOs, and carried
out an observing campaign to piece together the whole spectrum in the cm
wavelength domain, and highlight all the possible differences with respect to a
comparison sample of Radio-Loud non-BAL QSOs. VLBI observations at high angular
resolution have been performed, to study the pc-scale morphology of these
objects. Finally, we tried to detect a possible dust component with
observations at mm-wavelengths.
Results do not seem to indicate a young age for all BAL QSOs. Instead a
variety of orientations and morphologies have been found, constraining the
outflows foreseen by the orientation model to have different possible angles
with respect to the jet axis
The Speciality Index as invariant indicator in the BKL Mixmaster Dynamics
The speciality index, which has been mainly used in Numerical Relativity for
studying gravitational waves phenomena as an indicator of the special or
non-special Petrov type character of a spacetime, is applied here in the
context of Mixmaster cosmology, using the Belinski-Khalatnikov-Lifshitz map.
Possible applications for the associated chaotic dynamics are discussed
Are braneworlds born isotropic?
It has recently been suggested that an isotropic singularity may be a generic
feature of brane cosmologies, even in the inhomogeneous case. Using the
covariant and gauge-invariant approach we present a detailed analysis of linear
perturbations of the isotropic model which is a past attractor in
the phase space of homogeneous Bianchi models on the brane. We find that for
matter with an equation of state parameter , the dimensionless
variables representing generic anisotropic and inhomogeneous perturbations
decay as , showing that the model is asymptotically stable
in the past. We conclude that brane universes are born with isotropy naturally
built-in, contrary to standard cosmology. The observed large-scale homogeneity
and isotropy of the universe can therefore be explained as a consequence of the
initial conditions if the brane-world paradigm represents a description of the
very early universe.Comment: Changed to match published versio
Are Simple Real Pole Solutions Physical?
We consider exact solutions generated by the inverse scattering technique,
also known as the soliton transformation. In particular, we study the class of
simple real pole solutions. For quite some time, those solutions have been
considered interesting as models of cosmological shock waves. A coordinate
singularity on the wave fronts was removed by a transformation which induces a
null fluid with negative energy density on the wave front. This null fluid is
usually seen as another coordinate artifact, since there seems to be a general
belief that that this kind of solution can be seen as the real pole limit of
the smooth solution generated with a pair of complex conjugate poles in the
transformation. We perform this limit explicitly, and find that the belief is
unfounded: two coalescing complex conjugate poles cannot yield a solution with
one real pole. Instead, the two complex conjugate poles go to a different
limit, what we call a ``pole on a pole''. The limiting procedure is not unique;
it is sensitive to how quickly some parameters approach zero. We also show that
there exists no improved coordinate transformation which would remove the
negative energy density. We conclude that negative energy is an intrinsic part
of this class of solutions.Comment: 13 pages, 3 figure
Two-parameter non-linear spacetime perturbations: gauge transformations and gauge invariance
An implicit fundamental assumption in relativistic perturbation theory is
that there exists a parametric family of spacetimes that can be Taylor expanded
around a background. The choice of the latter is crucial to obtain a manageable
theory, so that it is sometime convenient to construct a perturbative formalism
based on two (or more) parameters. The study of perturbations of rotating stars
is a good example: in this case one can treat the stationary axisymmetric star
using a slow rotation approximation (expansion in the angular velocity Omega),
so that the background is spherical. Generic perturbations of the rotating star
(say parametrized by lambda) are then built on top of the axisymmetric
perturbations in Omega. Clearly, any interesting physics requires non-linear
perturbations, as at least terms lambda Omega need to be considered. In this
paper we analyse the gauge dependence of non-linear perturbations depending on
two parameters, derive explicit higher order gauge transformation rules, and
define gauge invariance. The formalism is completely general and can be used in
different applications of general relativity or any other spacetime theory.Comment: 22 pages, 3 figures. Minor changes to match the version appeared in
Classical and Quantum Gravit
Averaging in Cosmology
In this paper we discuss the effect of local inhomogeneities on the global
expansion of nearly FLRW universes, in a perturbative setting. We derive a
generic linearized averaging operation for metric perturbations from basic
assumptions, and we explicify the issue of gauge invariance. We derive a gauge
invariant expression for the back-reaction of density inhomogeneities on the
global expansion of perturbed FLRW spacetimes, in terms of observable
quantities, and we calculate the effect quantitatively. Since we do not adopt a
comoving gauge, our result incorporates the back-reaction on the metric due to
scalar velocity and vorticity perturbations. The results are compared with the
results by other authors in this field.Comment: 24 pages, Latex, accepted for publication in Phys. Rev.
5- n-Alkylresorcinol Profiles in Different Cultivars of Einkorn, Emmer, Spelt, Common Wheat, and Tritordeum
Gauge invariant perturbations around symmetry reduced sectors of general relativity: applications to cosmology
We develop a gauge invariant canonical perturbation scheme for perturbations
around symmetry reduced sectors in generally covariant theories, such as
general relativity. The central objects of investigation are gauge invariant
observables which encode the dynamics of the system. We apply this scheme to
perturbations around a homogeneous and isotropic sector (cosmology) of general
relativity. The background variables of this homogeneous and isotropic sector
are treated fully dynamically which allows us to approximate the observables to
arbitrary high order in a self--consistent and fully gauge invariant manner.
Methods to compute these observables are given. The question of backreaction
effects of inhomogeneities onto a homogeneous and isotropic background can be
addressed in this framework. We illustrate the latter by considering
homogeneous but anisotropic Bianchi--I cosmologies as perturbations around a
homogeneous and isotropic sector.Comment: 39 pages, 1 figur
Diffractive charm photoproduction at HERA ep-collider
The cross section of the -meson diffractive photoproduction at the HERA
collider has been calculated in the framework of perturbatively motivated model
for the different kinematic regions. The camparison between the different
Pomeron models has been performed.Comment: 9 pages, 3 figure
- âŠ