315 research outputs found
Seeing relativity -- I. Ray tracing in a Schwarzschild metric to explore the maximal analytic extension of the metric and making a proper rendering of the stars
We present an implementation of a ray tracing code in the Schwarzschild
metric. We aim at building a numerical code with a correct implementation of
both special (aberration, amplification, Doppler) and general (deflection of
light, lensing, gravitational redshift) relativistic effects so as to simulate
what an observer with arbitrary velocity would see near, or possibly within,
the black hole. We also pay some specific attention to perform a satisfactory
rendering of stars. Using this code, we then show several unexplored features
of the maximal analytical extension of the metric. In particular, we study the
aspect of the second asymptotic region of the metric as seen by an observer
crossing the horizon. We also address several aspects related to the white hole
region (i.e., past singularity) seen both from outside the black hole, inside
the future horizon and inside the past horizon, which gives rise to the most
counter-intuitive effects
Quintessence Model Building
A short review of some of the aspects of quintessence model building is
presented. We emphasize the role of tracking models and their possible
supersymmetric origin.Comment: 14 pages, to appear in the proceedings of the sixth workshop of the
American University of Pari
Reproducing Cosmic Microwave Background anisotropies with mixed isocurvature perturbations
Recently high quality data of the cosmic microwave background anisotropies
have been published. In this work we study to which extent the cosmological
parameters determined by using this data depend on assumptions about the
initial conditions. We show that for generic initial conditions, not only the
best fit values are very different but, and this is our main result, the
allowed parameter range enlarges dramatically.Comment: 4 pages, 5 figures, submitted to PRL; Major changes following
referees suggestions; the allowed cosmological parameter range enlarges
dramaticall
Bounds on isocurvature perturbations from CMB and LSS data
We obtain very stringent bounds on the possible cold dark matter, baryon and
neutrino isocurvature contributions to the primordial fluctuations in the
Universe, using recent cosmic microwave background and large scale structure
data. In particular, we include the measured temperature and polarization power
spectra from WMAP and ACBAR, as well as the matter power spectrum from the 2dF
galaxy redshift survey. Neglecting the possible effects of spatial curvature,
tensor perturbations and reionization, we perform a Bayesian likelihood
analysis with nine free parameters, and find that the amplitude of the
isocurvature component cannot be larger than about 31% for the cold dark matter
mode, 91% for the baryon mode, 76% for the neutrino density mode, and 60% for
the neutrino velocity mode, at 2-sigma, for uncorrelated models. On the other
hand, for correlated adiabatic and isocurvature components, the fraction could
be slightly larger. However, the cross-correlation coefficient is strongly
constrained, and maximally correlated/anticorrelated models are disfavored.
This puts strong bounds on the curvaton model, independently of the bounds on
non-Gaussianity.Comment: 4 pages, 1 figure, some minor corrections; version accepted in PR
Cosmic microwave background constraints on multi-connected spherical spaces
This article describes the Cosmic Microwave Background anisotropies expected
in a closed universe with the topology of a lens space L(p,q) and with density
parameter Omega_0 close to 1. It provides the first simulated maps for such
spaces along with their corresponding power spectra. In spite of our initial
expectations that increasing p (and thus decreasing the size of the fundamental
domain) should suppress the quadrupole, we found just the opposite: increasing
p elevates the relative power of the low multipoles, for reasons that have
since become clear. For Omega_0 = 1.02, an informal ``by eye'' examination of
the simulated power spectra suggests that must be less than 15 for
consistency with WMAP's data, while geometric considerations imply that
matching circles will exist (potentially revealing the multi-connected
topology) only if p > 7. These bounds become less stringent for values of
Omega_0 closer to 1.Comment: 4 pages, 9 figures, to appear in PR
Coherent forward stimulated Brillouin scattering of a spatially incoherent laser beam in a plasma and its effect on beam spray
A statistical model for forward stimulated Brillouin scattering (FSBS) is
developed for a spatially incoherent, monochromatic, laser beam propagating in
a plasma. A threshold for the average power in a speckle is found, well below
the self-focusing one, above which the laser beam spatial incoherence can not
prevent the coherent growth of FSBS. Three-dimensional simulations confirm its
existence and reveal the onset of beam spray above it. From these results, we
propose a new figure of merit for the control of the propagation through a
plasma of a spatially incoherent laser beam.Comment: submitted to PR
Sub-horizon Perturbation Behavior in Extended Quintessence
In the general context of scalar-tensor theories, we consider a model in
which a scalar field coupled to the Ricci scalar in the gravitational sector of
the Lagrangian, is also playing the role of an ``Extended Quintessence'' field,
dominating the energy content of the Universe at the present time. In this
framework, we study the linear evolution of the perturbations in the
Quintessence energy density, showing that a new phenomenon, named here
``gravitational dragging'', can enhance the scalar field density perturbations
as much as they reach the non-linear regime. The possibility of dark energy
clumps formation is thus discussed.Comment: Proceedings of the 5th International UCLA Symposium on Sources and
Detection of Dark Matter and Dark Energy in the Universe (Dark Matter 2002),
Marina del Rey, California, USA, 20-22 February 200
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