3,792 research outputs found
Instabilities in dark coupled models and constraints from cosmological data
Coupled dark matter-dark energy systems can suffer from non-adiabatic
instabilities at early times and large scales. In these proceedings, we
consider two parameterizations of the dark sector interaction. In the first one
the energy-momentum transfer 4-vector is parallel to the dark matter 4-velocity
and in the second one to the dark energy 4-velocity. In these cases, coupled
models which suffer from non-adiabatic instabilities can be identified as a
function of a generic coupling Q and of the dark energy equation of state. In
our analysis, we do not refer to any particular cosmic field. We confront then
a viable class of models in which the interaction is directly proportional to
the dark energy density and to the Hubble rate parameter to recent cosmological
data. In that framework, we show that correlations between the dark coupling
and several cosmological parameters allow for a larger neutrino mass than in
uncoupled models.Comment: To be published in the proceedings of the Invisible Universe
International Conference (Paris, 29 June - 3 July 2009
The calibration and flight test performance of the space shuttle orbiter air data system
The Space Shuttle air data system (ADS) is used by the guidance, navigation and control system (GN&C) to guide the vehicle to a safe landing. In addition, postflight aerodynamic analysis requires a precise knowledge of flight conditions. Since the orbiter is essentially an unpowered vehicle, the conventional methods of obtaining the ADS calibration were not available; therefore, the calibration was derived using a unique and extensive wind tunnel test program. This test program included subsonic tests with a 0.36-scale orbiter model, transonic and supersonic tests with a smaller 0.2-scale model, and numerous ADS probe-alone tests. The wind tunnel calibration was further refined with subsonic results from the approach and landing test (ALT) program, thus producing the ADS calibration for the orbital flight test (OFT) program. The calibration of the Space Shuttle ADS and its performance during flight are discussed in this paper. A brief description of the system is followed by a discussion of the calibration methodology, and then by a review of the wind tunnel and flight test programs. Finally, the flight results are presented, including an evaluation of the system performance for on-board systems use and a description of the calibration refinements developed to provide the best possible air data for postflight analysis work
Thermodynamics of noncommutative quantum Kerr black holes
Thermodynamic formalism for rotating black holes, characterized by
noncommutative and quantum corrections, is constructed. From a fundamental
thermodynamic relation, equations of state and thermodynamic response functions
are explicitly given and the effect of noncommutativity and quantum correction
is discussed. It is shown that the well known divergence exhibited in specific
heat is not removed by any of these corrections. However, regions of
thermodynamic stability are affected by noncommutativity, increasing the
available states for which some thermodynamic stability conditions are
satisfied.Comment: 16 pages, 9 figure
Photographing the Nation: Early Female Photographers in Mexico 1870-1930
This analysis examines photographs created by three female photographers in Mexico: Alice Le Plongeon, Sara Castrejón, María Santibáñez, and how their images are reflective of the political and social-cultural environment in which they were created. These images span 1870-1930 and highlight elite conceptions of national identity, the role women played in forming this national narrative. This thesis provides a multidisciplinary approach that examines the photographic composition, the identity, and background of the photographer as well as the context in which the images were created and viewed. Additional primary documents including written publications, journals, and magazine publications have also been utilized when applicable. To contextualize these images to illustrate how they are connected to a larger narrative of self-expression and identity formation, a variety of secondary sources have been utilized. The analysis also focuses on the individual lives of each photographer, their contributions to the field, and how each woman challenged the boundaries of what was perceived to be socially acceptable
Thiemann transform for gravity with matter fields
The generalised Wick transform discovered by Thiemann provides a
well-established relation between the Euclidean and Lorentzian theories of
general relativity. We extend this Thiemann transform to the Ashtekar
formulation for gravity coupled with spin-1/2 fermions, a non-Abelian
Yang-Mills field, and a scalar field. It is proved that, on functions of the
gravitational and matter phase space variables, the Thiemann transform is
equivalent to the composition of an inverse Wick rotation and a constant
complex scale transformation of all fields. This result holds as well for
functions that depend on the shift vector, the lapse function, and the Lagrange
multipliers of the Yang-Mills and gravitational Gauss constraints, provided
that the Wick rotation is implemented by means of an analytic continuation of
the lapse. In this way, the Thiemann transform is furnished with a geometric
interpretation. Finally, we confirm the expectation that the generator of the
Thiemann transform can be determined just from the spin of the fields and give
a simple explanation for this fact.Comment: LaTeX 2.09, 14 pages, no figure
Biases on cosmological parameters by general relativity effects
General relativistic corrections to the galaxy power spectrum appearing at
the horizon scale, if neglected, may induce biases on the measured values of
the cosmological parameters. In this paper, we study the impact of general
relativistic effects on non standard cosmologies such as scenarios with a time
dependent dark energy equation of state, with a coupling between the dark
energy and the dark matter fluids or with non-Gaussianities. We then explore
whether general relativistic corrections affect future constraints on
cosmological parameters in the case of a constant dark energy equation of state
and of non-Gaussianities. We find that relativistic corrections on the power
spectrum are not expected to affect the foreseen errors on the cosmological
parameters nor to induce large biases on them.Comment: 17 pages, 5 figures, one added figure, results of Tab. I revised,
version accepted for publication in PR
The role of shell crossing on the existence and stability of trapped matter shells in spherical inhomogeneous \Lambda-CDM models
We analyse the dynamics of trapped matter shells in spherically symmetric
inhomogeneous \Lambda-CDM models. The investigation uses a Generalised
Lema\^itre-Tolman-Bondi description with initial conditions subject to the
constraints of having spatially asymptotic cosmological expansion, initial
Hubble-type flow and a regular initial density distribution. We discuss the
effects of shell crossing and use a qualitative description of the local
trapped matter shells to explore global properties of the models. Once shell
crossing occurs, we find a splitting of the global shells separating expansion
from collapse into, at most, two global shells: an inner and an outer limit
trapped matter shell. In the case of expanding models, the outer limit trapped
matter shell necessarily exists. We also study the role of shear in this
process, compare our analysis with the Newtonian framework and give concrete
examples using density profile models of structure formation in cosmology.Comment: 17pp 12fig
Wormholes as Basis for the Hilbert Space in Lorentzian Gravity
We carry out to completion the quantization of a Friedmann-Robertson-Walker
model provided with a conformal scalar field, and of a Kantowski-Sachs
spacetime minimally coupled to a massless scalar field. We prove that the
Hilbert space determined by the reality conditions that correspond to
Lorentzian gravity admits a basis of wormhole wave functions. This result
implies that the vector space spanned by the quantum wormholes can be equipped
with an unique inner product by demanding an adequate set of Lorentzian reality
conditions, and that the Hilbert space of wormholes obtained in this way can be
identified with the whole Hilbert space of physical states for Lorentzian
gravity. In particular, all the normalizable quantum states can then be
interpreted as superpositions of wormholes. For each of the models considered
here, we finally show that the physical Hilbert space is separable by
constructing a discrete orthonormal basis of wormhole solutions.Comment: 23 pages (Latex), Preprint IMAFF-RC-04-94, CGPG-94/5-
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